by
Dennis J. McHugh
Department of Chemistry, University College
University of New South Wales
Australian Defence Force Academy
Campbell, ACT 2600, Australia
SOURCES
Most of the large brown seaweeds are potential sources of alginate. The properties of the alginate varies from one species to another, so the choice of which seaweeds to harvest is based on both the availability of particular species and the properties of the alginate that they contain. The main commercial sources are species of Ascophyllum, Durvillaea, Ecklonia, Laminaria, Lessonia, Macrocystis, Sargassum and Turbinaria. Of these the most important are Laminaria, Macrocystis and Ascophyllum.
Macrocystis is harvested on the west coast of North America, from the Monterey peninsula in central California to the middle of the west coast of Baja California. It has been estimated that the USA harvests about 150 000 t (wet) and Mexico about 40 000 t (wet) per year (ITC, 1981). Nereocystis grows north of the Macrocystis beds but the two overlap and some Nereocystis is also harvested incidentally. Macrocystis was harvested on the east coast of Tasmania, Australia, from 1964-73, but the quantities available were insufficient to sustain an alginate industry.
Laminaria species are harvested principally in Scotland, Ireland, Norway, France, China, Japan and Korea. However in the Asian countries Laminaria is very popular as a food and in Japan and Korea the resulting higher price makes it an expensive raw material for alginate production. In Japan, only the material that is unsuitable for food is used for alginate extraction; since this is insufficient to sustain the alginate industry, other sources have to be found. The situation in China is different; here the cultivation of Laminaria japonica has been very successful, reaching about one million tonnes of wet seaweed annually. About two-thirds of this is used as food and the surplus is available for alginate production; while the cost of cultivated Laminaria is higher than the harvested wild material, the Chinese are able to absorb these higher costs and use the cultivated product for alginate extraction.
Laminaria hyperborea grows on rocky seabeds, usually at depths from 2-15 m, and the upright habit of the plant leads to the use of the phrase, "forests of hyperborea". Stipes that have been cast up by winter storms are collected in France, Ireland, Norway and Scotland. The Norwegian alginate producer, Protan A/S, has developed its own method of harvesting this plant, trawling it with specially built boats that are easily manoeuvred.
Laminaria digitata is found on either side of the low water mark and is usually harvested by hand when the plants are exposed at low tide. It is collected in France, Norway and Scotland but the quantities are small in comparison with Laminaria hyperborea. In France it is harvested using a small boat and a hydraulic arm fitted with a hook device at the end. This is lowered into the bed of Laminaria digitata and rotated so that the weed wraps around it. The arm is then raised to the surface, bringing the seaweed with it.
Ascophyllum nodosum grows in the intertidal zone. It has been harvested by hand in Scotland and Ireland for more than a century. Various attempts have been made to mechanize harvesting but the most successful appears to be that developed by Protan in Norway. This is a nozzle with cutters inside that cut and pump the seaweed through a large diameter pipe into a net bag on a shallow-draught water jet-propelled vessel; the operation is carried out at high tide and the bags can be left floating for later collection. It is also harvested in the southern parts of Nova Scotia.
Durvillaea Lessonia and Ecklonia are used to a lesser extent. Durvillaea antarctica from Chile and Durvillaea potatorum from Australia are used by alginates producers in the UK and USA. In 1985 Chile exported about 390 t and the current exports from Australia are about 3 000 t per annum.
Lessonia is collected in Chile where it is cast up after storms; in 1978 Chile exported 2 045 t of which 1 313 t went to Japan and the remainder to the USA and Canada (ITC, 1981). By 1985 the total export had increased to 5 810 t but no details of countries of destination are available. The North American alginate producers use it to supplement the supply of Macrocystis; the Japanese industry relies mainly on imported seaweeds so Lessonia is one of the primary raw materials.
Ecklonia cava grows in deep water (up to 20 m) and is harvested by divers in both Japan and Korea. Eisenia bicyclis grows in a similar location and is collected along with the Ecklonia in Japan. In Japan, divers find it more profitable to collect the higher priced red seaweeds so the quantities of Ecklonia available are fairly small. Ecklonia that has been cast up by storms is collected in Korea and South Africa; in Korea it is used by the local alginate producer, the South African material is all exported. The Korean industry also uses waste Undaria that is unsuitable for food uses, just as the Japanese industry uses similar waste from Laminaria species.
The alginate obtained from Sargassum and Turbinaria frequently has a poor viscosity so these species are used only when the above colder water species are not available. However recent reports (Shyamali, de Silva and Kumar, 1984; Wedlock, Fasihuddin and Phillips, 1986) on the structure of alginates from warm-water Sargassum and Turbinaria indicate they could be very useful in applications requiring the formation of strong gels (they have a low M/G ratio, see next section). The Indian industry is based on Sargassum that grows in the south (the coasts of Kerala and Tamil Nadu states); the species which grow in the north (Gujarat state) gives a low viscosity alginate, unsuitable for the main Indian market of textile printing; Turbinaria is used only when supplies of Sargassum are unavailable. The Philippines has large resources of Sargassum but this is exported mainly to Japan for use in animal feeds and fertilizers.
PRICES OF SEAWEEDS
There is limited trading in brown seaweeds because many alginate producers harvest the raw material themselves (e.g., Macrocystis, Laminaria hyperborea, Laminaria digitata, Ascophyllum). The dried seaweeds that are traded vary in moisture content and alginate content and this will be reflected in prices. For example air-dried stipes of Laminaria hyperborea contain about 35% moisture while air-dried samples of Lessonia, Durvillaea and Laminaria japonica can vary from 15-20% moisture. A dried tonne of Lessonia could be expected to yield about 140 kg of alginate; some corresponding figures for other seaweeds are, Ascophyllum 120 kg; Laminaria japonica 170 kg; Durvillaea 240 kg. The following are some examples of f.o.b. prices for air-dried seaweed per tonne: Chilean Lessonia US$ 150; South African Ecklonia US$ 250; Australian Durvillaea US$ 400; Chinese Laminaria japonica US$ 500-700; UK Ascophyllum US$ 350. To allow a comparison of the relative costs of these seaweeds, the prices have all be converted to US dollars at the exchange rates current in June 1987. However these prices can show considerable variation from year to year with the fluctuation of the exchange rate of US dollars versus the currency of country of origin of the seaweed.
STRUCTURE OF ALGINIC ACID
Alginic acid is a linear polymer based on two monomeric units, b -D-mannuronic acid and a -L-guluronic acid. The classical Haworth formulas for these monomers are shown in Figure 1, while Figure 2 illustrates the chair formulas, which give a clearer picture of the three-dimensional arrangement of the molecules.
Figure 1 Classical formulas of the two monomeric units of alginic acid
Figure 2 Formulas in Figure 1 expressed as chair forms
Figure 3 C1 and 1C forms of the tetrahydropyran ring
The basic structure of each monomer is the tetrahydropyran ring and this has two possible chair forms, C1 and 1C (Figure 3). b -D-mannuronic acid assumes the C1 form; in the other form, 1C, there would be steric interaction between the axial -COOH on C-5 and the axial -OH on C-3; the C1 form has these groups in the equatorial positions and so is more stable. For similar reasons, a -L-guluronic acid assumes the 1C form rather than the C1 form (Penman and Sanderson, 1972; Atkins et al., 1973, 1973a).
The alginate polymer is formed by joining these monomers at the C-1 and C-4 positions. An ether-oxygen bridge joins the carbon at the 1-position in one molecule to the 4-position of another molecule. It has been shown that the polymer chain is made up of three kinds of regions or blocks. The G blocks contain only units derived from L-guluronic acid (Figure 4), the M blocks are based entirely on D-mannuronic acid (Figure 5) and the MG blocks consist of alternating units from D-mannuronic acid and L-guluronic acid (Haug, Larsen and Smidsrod, 1966, 1974; Grasdalen, Larsen and Smidsrod, 1981).
Note the differing shapes of the M blocks and G blocks. Because an M block is formed from equatorial groups at C-1 and C-4, it is a relatively straight polymer, like a flat ribbon. However the G block is formed from axial groups at both C-1 and C-4 so the resulting chain is buckled; the importance of this buckled shape will be apparent later when the formation of gels from alginate solutions is discussed.
So an alginate molecule can be regarded as a block copolymer containing M, G, and MG blocks, the proportion of these blocks varying with the seaweed source. However Larsen (1981) warns that this is an idealized structure which is at best an approximation of the actual situation.
It has been shown that the physical properties of alginates depend on the relative proportion of the three types of blocks (Haug, Larsen and Smidsrod, 1967; Penman and Sanderson, 1972; Smidsrod and Haug, 1972; Smidsrod, Haug and Whittington, 1972). For example formation of gels, by addition of calcium ions, involves the G blocks so the higher the proportion of these, the greater the gel strength; solubility of alginate in acid depends on the proportion of MG blocks present. The industrial utilization of any particular alginate will depend on its properties and therefore on its uronic acid composition so it has become important to have some measure of the relative proportions of the uronic acids. Various methods have been developed to measure the ratio of mannuronic acid to guluronic acid (the M/G ratio) in a sample of alginic acid (Annison, Cheetham and Couperwhite, 1983, and twelve references cited therein). Some examples of M/G ratios are shown in Table 1.
Even more useful, but more difficult to obtain, is a measure of the M, G and MG blocks in a sample and methods have been developed to achieve this (Haug, Larsen and Smidsrod, 1966, 1974; Penman and Sanderson, 1972; Grasdalen, Larsen and Smidsrod, 1979; Morris, Rees and Thom, 1980). Some examples are shown in Table 2.
Table 1 Percentages of mannuronic acid and guluronic acid, and M/G ratios, of alginic acid from various commercial brown seaweedsa)
|
|
|
Mannuronic acid (%) |
Guluronic acid (%) |
M/G ratio |
|
|
Ascophyllum rodosum |
(1) |
|
|
1.56 |
|
|
(2) |
|
|
1.29 |
||
|
(3) |
64.5 |
35.5 |
1.82 |
||
|
(3) |
|
|
1.10b) |
||
|
(4) |
60.0 |
40.0 |
1.5 |
||
|
(5) |
|
|
1.85b) |
||
|
Ecklonia cava, |
fronds |
(6) |
|
|
2.64-3.08c) |
|
stipes |
(6) |
|
|
1.39-2.91c) |
|
|
Laminaria digitata |
(1) |
|
|
1.45 |
|
|
(4) |
|
|
1.63 |
||
|
(3) |
53.7 |
46.3 |
1.16 |
||
|
(3) |
|
|
1.58 |
||
|
(3) |
59.0 |
41.0 |
1.43b) |
||
|
Laminaria hyperborea |
(2) |
38.3 |
61.7 |
0.62 |
|
|
|
fronds |
(1) |
|
|
1.35 |
|
fronds |
(3) |
56.0 |
44.0 |
1.28 |
|
|
stipes |
(1) |
|
|
0.65 |
|
|
stipes |
(2) |
|
|
0.40 |
|
|
stipes |
(4) |
30.0 |
70.0 |
0.43 |
|
|
stipes |
(3) |
|
|
0.37 |
|
|
stipes |
(3) |
|
|
0.46b) |
|
|
Laminaria japonica |
(7) |
69.3 |
30.7 |
2.26b) |
|
|
|
basal part |
(7) |
|
|
2.34-3.18c) |
|
apical part |
(7) |
|
|
1.61-2.02c) |
|
|
Macrocystis pyrifera, |
|
|
|
|
|
|
|
Australian |
(2) |
|
|
1.38 |
|
American |
(3) |
61.0 |
39.0 |
1.56 |
|
|
frond |
(7) |
|
|
1.52 |
|
|
stipe |
(7) |
50.5 |
49.5 |
1.02 |
|
|
air bladder |
(7) |
|
|
1.41 |
|
|
Undaria pinnatifida |
(7) |
|
|
1.45-2.65c) |
|
a) Unless otherwise stated, the alginic acid samples were prepared in the laboratory from the appropriate seaweed.
b) A sample of alginic acid made from a commercial alginate.
c) The range shows seasonal variation during one year.(1) Haug, Larsen and Smidsrod, 1974;
(2) Penman and Sanderson, 1972;
(3) Haug and Larsen, 1962;
(4) Grasdalen, Larsen and Smidsrod, 1979;
(5) Haug, 1964, p. 108;
(6) Kim, 1984;
(7) Ji, et al., 1984
The M/G ratio of alginate has been altered, on a laboratory scale, by treating it with "mannuronan C-5 epimerase", and enzyme isolated from the soil bacterium, Azotobacter vinelandii. This enzyme converts mannuronic acid residues into guluronic acid residues in the polymer chain, and the resulting alginate forms stronger gels (Larsen and Haug, 1971; Skjak-Braek, 1984). The method has not been applied on an industrial scale.
The alginate of greatest industrial importance is the sodium salt. Uses are also found for the potassium, ammonium and calcium salts, as well as alginic acid itself. The only synthetic derivative of alginic acid to find wide use, and acceptance as a food additive, is propylene glycol alginate. This is formed by reacting propylene oxide with moist alginic acid (Steiner, 1947; Steiner and McNeely, 1950; Kelco 1952; Pettitt and Noto, 1973; Noto and Pettitt, 1980). Esterification occurs at the carboxylic acid groups on the alginate chain, mainly with the primary hydroxyl group of propylene glycol. Depending on reaction conditions, such as reaction temperature and ratios of propylene oxide to alginic acid, varying degrees of esterification can be achieved. A product with about 60-70% esterification is satisfactory for most purposes but up to about 90% esterification can be achieved and this type of product (80-90%) is useful in very acidic, short term applications.
EXTRACTION PROCESSES
INTRODUCTION
Alginic acid was first discovered by Stanford (1881). An excellent history of the evolution of the alginate industry has been written by Booth (1975). He traces a path from Stanford's successful exploitation of crude extracts to the failure by F.C. Thornley, in Orkney about 1923, to establish a briquette business based on using alginate as a binder for anthracite dust. Thornley moved to San Diego and by 1927 his company was producing alginate for use in sealing cans. After some difficulties the company changed its name to Kelp Products Corp. and in 1929 it was reorganized as Kelco Company. Production in the United Kingdom was established in the period 1934-1939 and in Norway after World War II. It is estimated that there are 17 factories in 9 different countries (ITC, 1981), excluding the People's Republic of China. The two largest producers, Kelco Company in USA and Alginate Industries Ltd in UK, have been acquired by Merck and Co. Inc., USA; these combined companies produce about 70% of the world's alginate. The next largest producer is Protan A/S of Norway, followed by companies in Japan and France (ITC, 1981). Production in China is increasing and is now 7 000-8 000 tonnes per annum.
Table 2 Percentages of the three principal types of block structures in alginic acid, prepared from various commercial brown seaweeds
|
Alginate from |
|
Polymannuronic segments |
Polyguluronic segments |
Mixed segments |
|
|
Ascophyllum nodosum |
(1) |
35.0 |
13.0 |
52.0 |
|
|
(2) |
38.4 |
20.7 |
41.0 |
||
|
(3) |
37.8 |
21.4 |
40.8 |
||
|
(4) |
40.0 |
20.0 |
40.0 |
||
|
Laminaria digitata |
(1) |
43.0 |
23.0 |
34.0 |
|
|
(4) |
49.0 |
25.0 |
26.0 |
||
|
Laminaria hyperborea |
(2) |
20.3 |
49.3 |
30.4 |
|
|
|
(3) |
23.1 |
43.3 |
33.7 |
|
|
fronds |
(1) |
43.0 |
31.0 |
26.0 |
|
|
stipes |
(1) |
15.0 |
60.0 |
25.0 |
|
|
stipes |
(2) |
18.7 |
58.6 |
22.7 |
|
|
stipes |
(3) |
22.0 |
64.2 |
13.8 |
|
|
stipes |
(4) |
26.0 |
66.0 |
8.0 |
|
|
Laminaria japonica |
(5) |
36.0 |
14.0 |
50.0 |
|
|
Macrocystis pyrifera |
(2) |
40.6 |
17.7 |
41.7 |
|
|
(3) |
36.5 |
18.5 |
45.0 |
||
(1) Haug, Larsen and Smidsrod, 1974;
(2) Penman and Sanderson, 1972;
(3) Morris, Rees and Thom, 1980;
(4) Grasdalen, Larsen and Smidsrod, 1979;
(5) Ji, et al., 1984
Some of the early patents still provide useful basic information about alginate extraction (Thornley and Walsh, 1931; dark and Green, 1936; Green, 1936; Le Gloahec and Herter, 1938; Le Gloahec, 1939) as does work published by the former Institutes of Seaweed Research in Scotland and Norway (Black and Woodward, 1954; Haug, 1964) and more recently by Braud, Debroise, and Pérez (1977). Processes used in Japan have been described by Okazaki (1971).
The minimal requirements for the profitable operation of an alginate extraction plant have been estimated by Moss and Doty (1987). They discuss the minimal seaweed input, colloid output and capital investment needed; they also list estimates of production costs. This analysis is made for agar and carrageenan as well as alginate.
PROCESSES
The chemistry of the processes used to make sodium alginate from brown seaweeds is relatively simple. The difficulties of the processes arise from the physical separations which are required, such as the need to filter slimy residues from viscous solutions or to separate gelatinous precipitates which hold large amounts of liquid within their structure and which resist both filtration and centrifugation.
Processes for the manufacture of sodium alginate from brown seaweeds fall into two categories. Figure 6 is a diagram of the processes, simplified to show their essential difference. In one, the principal intermediates are calcium alginate and alginic acid. In the other, no calcium alginate is formed, only alginic acid.
The advantage of the first process is that calcium alginate can be precipitated in a fibrous form which can be readily separated; it can then be converted into alginic acid which is still fibrous and can also be readily separated. A further advantage of this process is that some calcium alginate can be allowed to remain in the final sodium alginate produced. This gives the manufacturer another method of controlling the viscosity of the final product, as discussed later in the "Properties" section.
The second process does save one step, the formation of calcium alginate, but it also has some disadvantages. When alginic acid is precipitated in this process, it forms a gelatinous precipitate which is very difficult to separate and the overall losses of alginic acid are generally greater than in the former process. The removal of liquid ("dewatering") from within the gel structure of the separated alginic acid also presents difficulties in this second process. The water content in the dewatered alginic acid is often high, so that alcohol must be used as a solvent for the conversion to sodium alginate. This usually makes the process more expensive unless the alcohol recovery rate is very good, and this is not easy to achieve.
Figure 6 Production of sodium alginate
For each process the principles, methods and problems are discussed below.
CALCIUM ALGINATE PROCESS
1. SIZE REDUCTION OF RAW MATERIAL
The raw material may be fresh, dried, or from silage. The last is seaweed which, when fresh, is chopped into small pieces and treated with a dilute formalin solution; this wet weed can be stored in cool concrete containers for several months. Dried weed is rehydrated by soaking for several hours. Several producers do not break up the seaweed into smaller pieces. However, reduction to small pieces, preferably 5-10 mm square, has two advantages. The first is that in the following treatments with formalin, acid and alkali, these reagents will obviously penetrate the seaweed more thoroughly and more rapidly if it has been broken up in this way. The second advantage is that the seaweed can be transported much more readily, by pumping it as a...rray in water.
The size reduction can be done in two stages, the first using equipment to chop the weed into pieces about 20 mm square (for example a forage harvester or a Rietz Prebreaker). This is used as the feed for a second machine such as a Rietz Vertical Disintegrator fitted with an appropriately sized screen, dependent on the seaweed being used. The product from a Rietz Disintegrator is a slurry of weed and water. The water may be separated using a centrifuge or a rotary drum screen.
2. ACID TREATMENT
In brown seaweeds alginic acid is present mainly as the calcium salt of alginic acid, although magnesium, potassium and sodium salts may also be present. Figure 6 shows that the first major aim of the process is to convert the insoluble calcium and magnesium salts into soluble sodium alginate. If the seaweed is treated with alkali (usually sodium carbonate) then the process necessary for extraction is an ion exchange.
Ca(Alg)2 + 2Na+ ® 2NaAlg + Ca++
However it has been shown that a more efficient extraction is obtained by first treating the seaweed with dilute mineral acid (Haug, 1964).
Ca(Alg)2 + 2H+ ® 2HAlg + Ca++
HAlg + Na+ ® NaAlg + H+
The calcium alginate is converted to alginic acid and this is more readily extracted with alkali than the original calcium alginate; extraction can even be completed at a pH less than 7 (Haug, 1964). At the same time the mineral acid removes all the acid-soluble phenolic compounds. The removal of phenolic compounds is important because (a) they form brown oxidation/polymerisation products with alkali and are largely responsible for a brown discolouration which occurs during alkaline extraction, (b) they cause a loss of viscosity of alginate during alkaline extraction. Pretreatment (i.e., before alkaline extraction) of the seaweed with acid gives a more efficient extraction, a less coloured product and reduced loss of viscosity during extraction, because less of the phenolic compounds are present. Clark and Green (1936) were the first to use this pretreatment.
In practice, the seaweed is stirred with 0.1M sulfuric acid or hydrochloric acid for 30 minutes; temperatures used range from room temperature to about 50°C depending on the seaweed used. Little degradation of alginate occurs with most species of seaweed at temperatures up to 40-50°C. The slurry of seaweed and acid can be separated on a rotary drum screen. The acid-treated weed is usually green and the solid is more free flowing than the untreated material.
3. FORMALDEHYDE TREATMENT
It has been found that acid pretreatment does not remove all phenolic compounds and discoloration still occurs during alkaline extraction. The discoloration can be further reduced by pretreatment with formaldehyde. This process, first used by Le Gloahec (1939), was more thoroughly investigated by Haug (1964) who found that the phenolic compounds and formaldehyde react to give insoluble products, so that no phenolic groups are available for polymerization to dark coloured products during the alkaline extraction.
In practice, the seaweed is stirred with water containing 0.1-0.4% commercial formalin solution, usually at room temperature. Higher temperatures, to about 50°C, can be used but they do not always give a better result. If the particle size of the seaweed has been reduced, as previously described, a reaction time of 15-30 minutes is sufficient. The required concentration of formaldehyde depends on the seaweed being used; some experimentation is necessary to obtain the best conditions for a particular raw material. After treatment, the seaweed is separated using a rotary drum screen and the solids are used in the alkaline extraction.
4. ALKALINE EXTRACTION
In this step, the purpose is to convert the alginate to a soluble form so that it can be removed from the rest of the seaweed. This step can also be used to control the viscosity of the final product. Higher temperatures and longer extraction times lead to breakdown of uronic acid chains and consequent lower viscosities for the sodium alginate. Green (1936) patented a process which used no heating in the alkaline extraction and obtained very high viscosity alginates. The value of producing very high viscosity alginate is debatable; the dried product (usually 10% moisture) is much more prone to breakdown and loss of viscosity, on storage from 6-12 months, than a medium viscosity alginate. Some manufacturers therefore produce medium (and lower) viscosity alginates and for applications requiring very high viscosity, they ensure their product contains sufficient calcium ions to produce the necessary viscosity (see "Properties" section for effect of calcium ions on viscosity). Usually sodium carbonate (soda ash) is used as the alkali because of its low cost; less is required if the seaweed has been given an acid pretreatment.
In practice, seaweed is stirred in a tank with the sodium carbonate solution (about 1.5%) at temperatures from 50-95°C for 1-2 hours. For weed that has undergone size reduction and acid pretreatment, 2 hours at 50°C will generally give good extraction with little degradation of alginate. The time can be reduced by using higher temperatures, usually with some loss of viscosity in the final product. When lower viscosity alginate is desired, the balance of high temperatures versus time can be used to control the viscosity. If whole weed or large pieces are used for the extraction, longer times will be necessary for complete extraction. As extraction proceeds, the extract becomes thicker and may have the consistency of a heavy porridge when complete.
5. SEPARATION OF INSOLUBLE SEAWEED RESIDUE
A. FLOTATION
The dissolved sodium alginate must now be separated from the alkali-insoluble seaweed residue, which is mainly cellulose. The residue is usually slimy and finely divided. It rapidly clogs filter cloths; uneconomical quantities of precoat material, such as kieselguhr or perlite, must be added to achieve reasonable rates of filtration. Some of the residue can be removed using centrifuges but the clarity of the resulting solution is usually poor. The major portion of the insoluble residue is usually removed by a flotation process, based on that originally described by Le Gloahec (1938). The extract is diluted with 4-6 times its volume of water, to produce a suitable viscosity, about 25-100 cps. Then a small quantity of flocculant is added, air is forced into the liquid, and it is left to stand for several hours. The fine particles of insoluble residue form flocs and are raised to the surface by the rising air bubbles which adhere to the flocs. The residue is scraped from the surface and the clarified liquor, beneath it, is drawn off. The dilution of the original extract must be such as to give a viscosity which allows the particles to rise within an acceptable processing time. This is a very economical and effective method of clarification but the resulting solution is still cloudy. It may require no further clarification if the final product is a technical grade alginate where clarity and colour are not important. However for food grade alginates, a filtration step is usually necessary as well. Because the bulk of the insoluble residue has been removed, it is now economical to use a precoat filter. For very high grade products a second filtration may be used.
In practice, the exact procedure and conditions will vary with the type of seaweed being used since the nature of the insoluble residue will vary. Dilution of the alkali extract is best done by in-line mixing, as is the addition of flocculant. The air can be drawn in via centrifugal-type pumps further down the same line and the diluted, aerated extract is pumped into large holding tanks. The cellulose residue usually has a negative charge so cationic flocculants are used, such as the polyacrylamides available from Allied Colloids under the trade name of Magnafloc. Most suppliers of flocculants have a range of anionic, nonionic and cationic products and provide advice on how to evaluate them for particular applications. The flocculant expedites the removal of very fine particles which would otherwise rise too slowly, being too small for air bubbles to attach themselves; the flocculant brings these fine particles together in larger flocs which air bubbles are more likely to encounter and lift.
In a continuous process, the residue can be continually scraped from the surface as the clarified liquor is removed from the lower part of the tank. In a batch process, many holding tanks are used and the clarified liquor is usually drawn off near the bottom of the tank leaving the residue which is washed out and collected separately. The residue sometimes contains significant amounts of soluble alginate which make it economical to attempt recovery. In this case the residue may be mixed with water again, aerated and let stand to separate.
The process can be carried out without the addition of flocculants but it is not as efficient and the product may be more difficult to filter, requiring either further dilution or heating (up to 50°C); however heating such large volumes can become a significant cost factor.
B. FILTRATION
Any insoluble residue remaining after flotation will be carried through the remaining stages of the process and will appear in the final product. For a final product with good clarity, filtration is required. Because the residue is very fine, filter cloths will rapidly block. The best method is to use a rotary precoat vacuum filter. In this, the rotating drum of the filter is coated with a 2-3 cm layer of precoat material, preferably perlite (an expanded or "puffed" lava, principally aluminium alkali silicate) because it gives a more porous medium than diatomaceous earth (kieselguhr) and so does not block as easily. During filtration, a blade on the rotary filter continually removes the top surface of the precoat, so that a clean filter surface is always available. After 9-10 hours, most of the precoat has been removed by the scraper, filtration is stopped, and a new layer of precoat is deposited. Great care is necessary in selecting the appropriate grade of perlite and the correct cloth to support the precoat medium. Most manufacturers of precoat rotary filters, such as Eimco/Envirotech and Dorr-Oliver, have the facilities to conduct appropriate tests to assist customers or potential customers.
For a very high clarity final product, a second filtration is sometimes used. Usually the quantities involved are smaller and a normal filter press is used. However, a filter aid is still necessary and at this stage a less porous one like diatomaceous earth is suitable. It is usually stirred into the solution to be filtered.
Some manufacturers, particularly those using the alginic acid process, use filters of fine mesh metal or terylene (120 to 200 mesh) to try to clarify the dilute extract obtained from the flotation separation. This is not very successful for most seaweeds although a reasonable result is obtained with some types of Ecklonia. The particles of insoluble residue which remain after flotation are generally too fine to be retained on such sieves so there is little improvement in clarity. This type of filtration may be useful in producing an improved technical grade but, for most seaweeds, the clarity will not be sufficient for food grades.
Some manufacturers have tried to use centrifuges, instead of filtration, to clarify the dilute extract from the flotation separation. Like the use of sieves described above, this may give a better technical grade product but the clarity is insufficient for food grades. One manufacturer, processing Macrocystis, tried to use centrifuges instead of the flocculation-flotation process. The capital outlay was considerable and the result was poor; the solution discharged from the centrifuge had poor clarity and had to be given a flocculation-flotation treatment before it could be economically filtered using precoated filters. Centrifuge performance was improved by extensive dilution of the alkaline extract but the large volumes to be handled and the increased water requirements made it impractical.
6. PRECIPITATION OF CALCIUM ALGINATE
The sodium alginate must be recovered in solid form, once its solution has been separated from the residual seaweed. Evaporation is not practical, the solution is too dilute. The alginate can be precipitated as its calcium salt or as alginic acid, either of which must later be converted to sodium alginate. In this process, calcium alginate is precipitated.
By careful addition of the sodium alginate solution to a calcium chloride solution, calcium alginate can be precipitated in the form of fibres. This fibrous calcium alginate can be readily separated on a metal screen, washed with water and when treated with dilute mineral acid, the Ca++ ions are exchanged for H+ ions, and it yields fibrous alginic acid. This alginic acid can be dewatered using a screw press. Some seaweeds give better fibrous calcium alginate than others; Laminaria gives long fibres which are easier to handle than the short fibres obtained from Ascophyllum.
In practice, it is necessary to add the dilute extract to the calcium chloride solution (about 10%); if the reverse is done, a gel will be obtained instead of fibres. The degree of mixing is important; too little will give a gel-type precipitate while too much may cause excessive breaking up of the fibres, making it difficult to retain them on the metal screen used for separation. The precipitation may be done batchwise in tanks or continuously using an in-line mixer. Operator skill and experience are necessary to obtain consistent results. The fibrous calcium alginate can be separated by running the suspension over a metal screen.
7. BLEACHING
Depending on the seaweed used as raw material, the earlier pretreatments with acid and formalin may ensure a sufficiently pale colour in the final product. However for some food and higher grades, bleaching can be used to improve the colour and odour of the final product, if that is necessary. It is best done at this stage, rather than later, because calcium alginate is more resistant to degradation (loss of viscosity) than alginic acid (Thornley and Walsh, 1934). Usually a sufficient quantity of sodium hypochlorite solution (12%) is added to a suspension of the calcium alginate in water. The quantity of hypochlorite required will vary according to the seaweed used and the effectiveness of the pretreatments with acid and formalin. When a suitably coloured solid is obtained, it is again separated on a metal screen.
8. CONVERSION OF CALCIUM ALGINATE TO ALGINIC ACID
The purpose of this step is to obtain a fibrous alginic acid which can be, readily separated and dewatered. This requires an ion exchange (Ca++ ® H+) in the calcium alginate and this is achieved by stirring it in a dilute mineral acid, such as HCl.
In practice, this is done by a three-step countercurrent conversion. Three tanks are used. The calcium alginate is added to the first tank which contains acid previously used in the second tank. After stirring for 30 minutes the solid (now a mixture of alginic acid and calcium alginate) is separated on a screen and the liquid is discarded. The solid is fed to the second tank which contains acid previously used in the third tank. The stirring and separation are repeated and the solid is fed to the third tank which contains unused dilute HCl (0.5M). After stirring and separation, the solid, now alginic acid, is washed once with water. The pH in the three conversion tanks should be adjusted if necessary so that it is always less than pH2.
It can be seen that thorough treatment will produce an alginic acid free of calcium ions. However if calcium is required in the final product (to increase its viscosity) then this can be achieved by varying the conditions of this conversion step and limiting the amount of ion exchange which occurs.
9. DEWATERING THE ALGINIC ACID
The chief advantage of this calcium alginate process is that water can be squeezed from the resulting fibrous alginic acid with relative ease. (This is in contrast to the gel type of alginic acid which results from addition of acid to sodium alginate solution, in the alginic acid process). A screw press, such as the Rietz horizontal continuous S-Press, is often suitable for this squeezing and dewatering. The alginic acid is fed into the rotating graduated-pitch screw and it is compressed by the screw and fixed resistor bars; a screened cone at the end, which rotates at a different speed from the screw, completes the compression and ensures adequate discharge of solids from the press. The dewatered product should contain at least 25% solids if it is to be used in the paste conversion of the next step.
10. CONVERSION OF ALGINIC ACID TO SODIUM ALGINATE
The sodium alginate from the original alkaline extract has now been purified and concentrated in the form of solid alginic acid. This must be converted to solid sodium alginate. Water or an alcohol is used as the 'solvent' with quite different results. In the calcium alginate process, water is usually used. The use of alcohol is described in the alginic acid process.
In practice, the dewatered alginic acid, usually containing greater than 25% solids, is mixed with solid alkali, normally sodium carbonate, in a mixer suitable for blending heavy pastes. The sodium alginate goes into solution, as it forms, in the small amount of water present, giving a heavy paste. However if the original alginic acid had less than 25% solids, the resulting paste may be too fluid. The neutralization can be readily controlled and the product is homogeneous. The reaction can be heated to 50°C if necessary. This paste is forced through small holes and the extrusions are chopped into pellets which are dried. They can be dried on trays in a hot-air oven. On a large scale, it is better to use a fluid-bed dryer fitted with a vibrating tilted screen so that the pellets, continuously fed in, vibrate down the screen and out, as the hot air blows up through the screen. The dried pellets (about 10% moisture) can be milled to an appropriate particle size, usually about 60 mesh (250 microns).
ALGINIC ACID PROCESS
Stages 1. to 5. are identical to the same stages in the calcium alginate process.
6. BLEACHING
Treatment with sodium hypochlorite (12% solution) is best done under alkaline conditions, there is less degradation of the alginate chains, so it is sometimes added to the clarified/filtered alkaline extract (Henkel, 1964; Okazaki, 1971). However the volumes of solution are very large at this stage, because of the dilutions which were necessary in the separation of the insoluble residue in stage 5, so some manufacturers find it more economical to add the hypochlorite in the final step of the process, the conversion of alginic acid to sodium alginate, which is frequently done using sodium hydroxide and an alcohol solvent. The quantity of hypochlorite used is kept to a minimum for economic reasons and will depend on the colour of the original seaweed used for the process, the effectiveness of the acid and formalin pretreatments, and the amount of insoluble, coloured solids which remain after stage 5.
7. PRECIPITATION OF ALGINIC ACID
The clarified sodium alginate extract is treated with dilute mineral acid, usually HCl or H2SO4, at room temperature and a gelatinous precipitate of alginic acid forms which cannot be filtered, it simply blocks any filter medium. It can be removed from the liquid by flotation. Usually a slight excess of sodium carbonate is left in the alkaline extraction filtrate so that the acid addition releases carbon dioxide gas; this becomes incorporated in the alginic acid gel which forms simultaneously. The gas lifts the alginic acid to the surface. The alginic acid gel becomes firmer if a layer several inches thick is allowed to build up on the surface. Once such a layer has formed, the alginic acid is scraped off. For successful separation, this alginic acid precipitate must be treated gently (a) so that it does not break up into fine pieces and (b) so that the carbon dioxide is not lost from its structure. Losses of alginic acid in the drain liquors can be severe if the flotation is not carried out carefully. The alginic acid precipitate must be left in contact with the mineral acid for sufficient time to allow it to react with any sodium alginate solution occluded in the precipitate formed.
In practice the mineral acid (such as 5% sulphuric acid) and alginate solution may be gently mixed in-line, with a pH controller metering the acid addition to give a final pH of 1.5-2.0; the mixture is left to stand for about 60 minutes in a rubber-lined tank, allowing completion of reaction and flotation of the alginic acid. Le Gloahec and Herter (1938) describe a procedure, variations of which are still used; the principle is to mix the acid (usually a small volume) and the alginate solution (usually a large volume) as they flow over an inclined baffle so that good, gentle mixing is achieved with a minimum of occlusion of the alkaline sodium alginate solution. Sometimes the mixture is allowed to flow along a sloping, long, baffled channel, to allow further mixing and reaction, before running into the tank used for flotation. After the alginic acid is removed from the flotation tank, the remaining solution is run to waste but its content of alginic acid must be periodically monitored to ensure that losses at this stage are minimal.
8. DEWATERING ALGINIC ACID
This is undoubtedly the most difficult stage of the alginic acid process. The alginic acid gel obtained from flotation contains only 1-2% solids; its conversion to sodium alginate would give a viscous solution whereas a solid, or a paste which could be dried to a solid, is required. Some of the water can be removed from the gel by (a) pressing or squeezing, (b) centrifuging, (c) mixing with an alcohol.
(a) The consistency of the gel scraped from the flotation tank is too soft to allow the use of a screw press. The consistency can also vary from hour to hour and day to day; it may be quite firm, or it may be soft or sloppy. Some manufacturers, particularly those using a squeezing process, hold it for 1-2 hours in large containers of coarse filter cloth; this allows some of the liquid to drain away. The material can then be shovelled into smaller nylon/terylene filter cloth bags, which are closed and stacked between the plates of a hydraulic press. Pressure is applied and liquid is squeezed from the gel. The alginic acid, now about 20% solids, is removed from the bags and may be used for the next stage, conversion to sodium alginate, if alcohol is to be used as the solvent; if water is to be the solvent, the squeezing process is repeated to give a product with about 25-30% solids (see stage 10 of the calcium alginate process). This process is labour intensive and only economical in some countries. Another method of squeezing is to use a sequence of pairs of rollers, with a steadily decreasing clearance between each pair. Two continuous belts of filter cloth feed the gel between the rollers where it is squeezed and the liquid escapes through the belt. This system requires careful control of the consistency of the feed material; if the gel is too soft it can be squeezed out the sides of the belts.
(b) Some manufacturers take the gel straight from the Cop of the flotation tank and place it in basket-type centrifuges with filter cloth liners on the inside. Centrifuging can increase the solids content to 7-8%; this is suitable if alcohol is to be used in the conversion process but this material is also sufficiently firm for it to be further dewatered using a screw press.
(c) Dewatering the alginic acid from flotation by mixing with an alcohol (usually methanol or ethanol) is suitable as a laboratory procedure but is uneconomic on a large scale because of the costs (i) of recovery of the alcohol and (ii) of the alcohol lost, since recovery is rarely complete. Sometimes, however, alcohol is used to further dewater the alginic acid obtained after squeezing.
10. CONVERSION OF ALGINIC ACID TO SODIUM ALGINATE
Alginic acid with 25% or more solids can be converted, using water as the solvent, by the paste method described in the calcium alginate process. Most manufacturers using the alginic acid process tend to use alcohol as the solvent for conversion because the water content of the alginic acid is high.
The alginic acid is suspended in alcohol (methanol or ethanol) at room temperature and a strong sodium hydroxide solution (40%) is added. The exchange of H+ and Na+ is slow since neither the alginic acid nor the sodium alginate is appreciably soluble. It is not easy to obtain a homogeneous neutralization because this depends on how well the alkali can penetrate the particles of alginic acid. The reaction is followed by taking samples of the solid and measuring the pH of its aqueous solution. The pH of the alcohol solvent is not a measure of the degree of the conversion. When the sodium alginate has pH 6, the suspension can be filtered or centrifuged.
Usually the sodium alginate produced is in the form of fine hair-like fibres, because that was the form of the original alginic acid precipitate. The thickness of these fibres imposes limitations on the maximum particle size which can be obtained after grinding. This is one of the disadvantages of the alcohol conversion method. Fibres and fine powders of sodium alginate are difficult to dissolve because neither disperse easily in water. Ease of solubility is a very important factor for customer acceptance. A more granular or coarse powder, which dissolves more readily, can be obtained from grinding the pellets obtained from the paste method of conversion.
The solid sodium alginate from the filter is sometimes squeezed in a screw press, especially if the alginic acid was only 7-8% solids. This removes most of the residual alcohol-water before it is sent to the dryer. It is dried to about 10% moisture and milled to appropriate particle sizes, according to the intended application. The drying should be in a system equipped for the recovery of the alcohol vapour. This recovered alcohol can be added to the alcoholic filtrate which is distilled and recycled. Alcohol is a relatively expensive raw material and the economical production of alginate using alcohol conversion is very dependent on having a high percentage recovery of alcohol.
An alternate method of alcohol conversion, suitable if the alginic acid has about 20% solids or more, is to place the alginic acid in a paste-type mixer, add strong sodium hydroxide solution and then just enough alcohol to allow the mixing of the wet fibrous solid. When the reaction is complete (10-15 minutes, pH 5.5-6.0) the wet fibrous solid is drained, squeezed in a hydraulic or screw press and then dried. Much smaller quantities of alcohol are used in this method but its recovery is still an important economic factor.
GENERAL
1. WATER
Alginate factories must be established near an adequate water supply since the requirements are high and can range from 1 000-1 500 m3 per tonne of final product. The water should be clear and free from any colloidal clay and suspended matter. The presence of calcium and magnesium ions can cause formation of the corresponding alginate salts which will cause difficulties in some stages of the process. Interference from these ions can be prevented by pretreatment of the water with ion-exchange resins. Bacterial levels should be low, particularly for the production of food and pharmaceutical grades.
2. BACTERIAL CONTAMINATION OF EQUIPMENT
Many brown seaweeds have a natural flora of bacteria which possess enzymes, alginate lysases, capable of breaking down the alginate molecule. These bacteria inevitably enter the processing equipment with the seaweed so precautions must be taken to ensure they do not proliferate, otherwise there will be serious losses in the viscosity of the alginate produced. Seaweed and process residues must not be allowed to accumulate, especially in concrete tanks which have a porous surface and are difficult to clean.
DERIVATIVES
SALTS
Sodium alginate is the main form of alginate in use. Smaller quantities of alginic acid and the ammonium, calcium, potassium and triethanolamine salts are also produced. Calcium alginate and alginic acid are made during the calcium alginate process for making sodium alginate; each can be removed at the appropriate stage, and after thorough washing, can be dried and milled. The other salts are made by neutralization of moist alginic acid with the appropriate base; sufficient water or alcohol can be added to keep the material at a workable consistency and it is processed as described for the paste-conversion method in the calcium alginate process. However the triethanolamine salt is hygroscopic and is best dried in thin layers and then milled.
PROPYLENE GLYCOL ALGINATE
Propylene glycol alginate was first prepared by Steiner (1947) but a better method was published in a more informative patent by Steiner and McNeely (1950). It is made by the reaction of propylene oxide with alginic acid and the carboxylic acid groups on the uronic acid chains are esterified. The original patent (Steiner, 1947) gave a product with a pH of about 3 but it had poor viscosity stability, both in the solid state and in solution. However when a partially neutralized alginic acid was used, the reaction was accelerated and yielded a more stable product with a pH 3.8-4.6; under these conditions less hydrolysis of both the alginic acid and propylene oxide occurred (Steiner and McNeely, 1950). This patent gives a useful description of the process.
Partially neutralized alginic acid can be made by reacting 5-20% of the carboxylic acid groups with an alkali and the product has a pH 3.5-5.5. Ammonia is used for the neutralization because the excess is easily removed, but some methods for using sodium carbonate or sodium phosphate are also described. The alginic acid and alkaline reagent are mixed using repeated passes through a hammer mill and by passing a current of warm, dry air through the mill, the moisture content can be adjusted to 45-55%. This moisture content gave the best reaction rate for esterification with the least hydrolysis of the propylene oxide. The fibrous alginic acid and gaseous propylene oxide, in a mole ratio of about 1:3, are mixed in a pressure vessel at 45-60°C. The reaction takes 8 hours at 50°C and gives a product with about 80% of the carboxyl groups esterified and pH 3.9.
An improvement in the above process was reported by Pettitt and Noto (1973). They were able to reduce the reaction time to 2-3 hours, mainly by removing any inert gas such as air from the reaction vessel. The air is either removed by vacuum or purged by a gas flow of propylene oxide. The alginic acid is partially neutralized to 8-22%, as described by Steiner and McNeely (1950), but its solids content is more carefully controlled at 65-78%; if the solids fall below 65%, the increased moisture leads to excessive formation of propylene glycol and if it rises above 78% the reaction rate becomes very slow. The reaction can be run from 60-100°C with reaction times of 2-3 hours, to give a product (approximately 80% mole ester, pH 3.8-4.6) which has good viscosity stability at room temperature for several months.
More recently Noto and Pettitt (1980) described a process using liquid propylene oxide mixed with partially neutralized alginic acid in a pressure vessel. Good esterification is achieved even with a very low neutralization (0.4%) of the alginic acid and the latter can contain as little as 20% solids, although most of the examples cited contain 34% solids. This means much less drying of the alginic acid is necessary before use in the reaction. The reactions were generally at 75-85°C and required only 2 hours.
PROPERTIES
The following discussion centres on those properties of alginates which are particularly relevant to their uses. Information about other properties, and sometimes more detailed information about the properties discussed here, can be found in Kelco (1976), McDowell (1977), Cottrell and Kovacs (1980) and King (1983).
The commercial products of most interest are sodium alginate, propylene glycol alginate and alginic acid; smaller quantities are used of the potassium, ammonium, calcium and triethanolamine salts as well as mixed salts of sodium and calcium.
STABILITY-SOLID ALGINATES
The degree of polymerization (DP) of an alginate is a measure of the average molecular weight of the molecules and is the number of uronic acid units per average chain. DP and molecular weight relate directly to the viscosity of alginate solutions; loss of viscosity on storage is a measure of the extent of depolymerisation of the alginate.
Sodium alginate is produced in various grades, usually described as low, medium and high viscosity alginates (this refers to the viscosity of its 1% aqueous solution).
Alginates with a high DP are less stable than those with a low DP. Low viscosity sodium alginates (up to about 50 mPa.s) have been stored at 10-20°C with no observable change in 3 years. Medium viscosity sodium alginates (up to about 400 mPa.s) show a 10% loss at 25°C and 45% loss at 33°C after one year, and higher viscosity alginates are less stable.
Propylene glycol alginates show about 40% loss in viscosity after a year at 25°C and also become less soluble. Ammonium alginate is generally less stable than any of the above. Alginic acid is the least stable of the products and any long chain material degrades to shorter chains within a few months at ambient temperatures. However short chain material is stable and alginic acid with a DP of about 40 units of uronic acid per chain will show very little change over a year at 20°C. However the main use of alginic acid, as a disintegrant in pharmaceutical tablets, depends on its ability to swell when wetted and this is not affected by changes in DP.
The commercial alginates should therefore be stored in a cool place, 25°C or lower, since elevated temperatures can cause significant depolymerization which affects the commercially useful properties such as viscosity and gel strength. They usually contain 10-13% moisture and the rate of depolymerization increases as the proportion of moisture is increased, so the storage area should be dry.
STABILITY - ALGINATE SOLUTIONS
The monovalent cation salts [Na+, K+, NH4+, (CH2OH)3NH+] of alginic acid, and its propylene glycol ester, dissolve in water but alginic acid and the calcium salt do not. Neutral solutions of low to medium viscosity alginates can be kept at 25°C for several years, without appreciable viscosity loss, as long as a suitable microbial preservative is added. Solutions of highly polymerized alginates will lose viscosity at room temperature within a year and to achieve high, stable viscosities it is better to add calcium ions to a solution of an alginate with a moderate DP. All solutions of alginate will depolymerize more rapidly as the temperature is raised. Alginates are most stable in the range of ph 5-9 (McDowell, 1977). Small amounts of calcium greatly increase the stability of sodium alginate solutions. Propylene glycol alginate solutions are stable at room temperature from pH 3-4; below pH 2 and above pH 6 they will lose viscosity quickly even at room temperature (McNeely and Pettitt, 1973).
Since alginate solutions contain a polysaccharide anion, they cannot be mixed with cations which will combine with this anion to give an insoluble product. Alginate solutions are incompatible with most divalent and trivalent cations, with quaternary ammonium salts such as those used as bactericides, with acids strong enough to cause precipitation of alginic acid, with strong alkalis which lead to a gradual breakdown of the polysaccharide chains. The compatibility of a more specific list of substances, which are likely to be used with alginates, is discussed by Kelco (1976).
SOLUBILITY
A. PHYSICAL FACTORS
When powders of soluble alginates are wetted with water, the hydration of particles results in each having a tacky surface. Unless some precautions are taken, the particles will rapidly stick together resulting in clumps which are very slow to completely hydrate and dissolve. Particle size and type affect solubility behaviour.
Coarse particles are usually preferred because they are easier to disperse and keep separate, even though they are slower to hydrate and dissolve. Fine particles will dissolve more rapidly but there is more risk of them clumping together; this risk is less if the alginate is diluted with another powder such as a sugar. Alginates made using an alcohol conversion step (described in "Extraction Processes" section) often have fibrous particles; these usually hydrate more rapidly than granular particles (resulting from paste conversion) but tend to wind around each other and are more difficult to disperse.
The quantity of the soluble alginates which will dissolve in water is limited by the physical nature of the solutions rather than actual solubility. As the concentration of alginate increases the solution passes through stages of a viscous liquid to a thick paste; at this point it becomes very difficult to disperse further alginate successfully.
B. CHEMICAL FACTORS
It is more difficult to dissolve alginate in water if the water contains compounds which compete with the alginate for the water necessary for its hydration. The presence of sugars, starches or proteins in the water will reduce the rate of hydration and longer mixing times will be necessary. Salts of monovalent cations (such as NaCl) have a similar effect at levels above about 0.5%. All of these substances are best added after the alginate has been hydrated and dissolved. The presence of small quantities of many polyvalent cations inhibits the hydration of alginates and larger quantities cause precipitation. Sodium alginate is difficult to dissolve in hard water and milk because both contain calcium ions; these ions must first be sequestered with a complexing reagent such as sodium hexametaphosphate or ethylenediamine tetraacetic acid (EDTA). Propylene glycol alginate (preferably 80-85% esterified) is less affected by calcium ions and can be used in milk. Acidic conditions also inhibit hydration and when the pH is less than 4.0 it is better to use propylene glycol alginate which remains soluble down to about pH 2.
Alginates are insoluble in water-miscible solvents such as alcohols and ketones. Aqueous solutions (1%) of most alginates will tolerate the addition of 10-20% of these solvents; propylene glycol alginate tolerates 20-40% while up to 65% ethanol can be added to triethanolamine alginate without causing precipitation. The presence of such solvents in water, before dissolving the alginate, will hinder hydration.
C. DISSOLVING ALGINATES
Most alginate manufacturers provide detailed information on how best to dissolve alginates. The advice of Kelco (1976) is typical; they suggest methods based on (a) high-shear mixing, (b) dry-mix dispersion, (c) liquid-mix dispersion.
In high-shear mixing, the principle is to prevent the clumping together of the particles, which become tacky as soon as the surface is hydrated. Powdered alginate is slowly poured into the upper part of a vortex created in the water by high speed stirrer; the stirrer blades must remain submerged to avoid too much aeration. If some clumps do form, the shear should be sufficient to break them up. For large scale mixing, Kelco sell a funnel attached to a mixing aspirator; a fast flow of water through the aspirator sucks in alginate powder from the funnel, mixes and wets it, and then discharges it into a well agitated tank of water.
Dry-mix dispersion can be used when a formulation requires both alginate and other dry ingredients such as sugars, starches, etc. The dry powders are mixed thoroughly so that the alginate particles are diluted and separated by the other ingredients. This mixture is slowly added to well stirred water, preferably with a vortex as before, and the other ingredients, often in a ratio of 5:1 to 10:1, help to keep the alginate particles apart as they are wetted.
An even more efficient method of diluting the alginate particles is to use liquid-mix dispersion in which they are wetted with a non-solvent. This can be either a water-miscible non-aqueous liquid (such as ethanol or glycerol) or a water-immiscible liquid (such as a vegetable oil). Enough liquid is needed to give a pourable slurry and this is poured into the water, well agitated as before. The particles are dispersed and the rate of hydration, and solution, will depend on the time taken for the non-solvent liquid to diffuse from the surface of the particles.
D. PRESERVATION OF SOLUTIONS
Microorganisms will grow in solutions of commercial alginates because they usually contain sufficient nitrogenous compounds and salts. Bacterial or mould growth may cause depolymerization and loss of viscosity of the alginate as well as contamination and spoiling of any product in which the alginate is used. Food and cosmetic products are protected by their traditional preservatives such as sorbic acid, potassium sorbate, benzoic acid, sodium benzoate and the methyl or ethyl ester of p-hydroxybenzoic acid. For other uses less expensive, and sometimes more effective, preservatives are available such as formaldehyde and sodium pentachlorophenate and other phenol derivatives. Compounds of copper and zinc, and quaternary ammonium salts, should not be used because they will react with the alginate.
VISCOSITY
Many of the uses of alginates depend on their thickening effect, their ability to increase the viscosity of aqueous systems using relatively low concentrations. At the concentrations used in most applications, the viscosity behaviour of alginate solutions is pseudoplastic, the solution flows more readily the more it is stirred or pumped (the viscosity decreases as the rate of shear increases). This effect is reversible except at very high rates of shear (Glicksman, 1969). It is most marked with high molecular weight alginates, with sodium alginate solutions which contain calcium ions, and with propylene glycol alginate above 1% concentration; some of these solutions can also be thixotropic, that is they show a time-dependent chinning at constant shear rate and their recovery to the initial viscosity is time dependent. Reproducible viscosity measurements are made using a rotational type of viscometer, such as the Brookfield Synchro-Lectric.
Several factors influence the viscosity of alginate solutions.
A. MOLECULAR WEIGHT
The higher the molecular weight of a soluble alginate, the greater the viscosity of its solution. Manufacturers can control the molecular weight (degree of polymerization, DP) by varying the severity of the extraction conditions and they offer products ranging from 10-1 000 mPa.s (1% solution) with a DP range of 100-1 000 units. Sodium alginate of viscosity 200-400 mPa.s, "medium viscosity", probably finds the widest application.
B. CONCENTRATION
There is no simple relationship between concentration and viscosity for alginate solutions but McDowell (1960,1977) found a useful empirical equation which applied to a wide variety of alginates over a range of at least a hundredfold change in viscosity:
where a is a constant related to the DP of the alginate, b is a constant for a particular type of alginate. Graphs or tables of viscosity versus concentration are available from manufacturers for their particular products (Kelco, 1976; McDowell, 1977; Protan, 1986) and some typical figures are shown in Table 3.
C. TEMPERATURE
Viscosity decreases as temperature increases, at a rate of about 2.5% per degree Celsius (Figure 7). Viscosity usually returns to a little less than the original value on cooling. However if alginate solutions are kept above 50°C for several hours, depolymerization may occur giving a permanent loss of viscosity.
Alginate solutions can be frozen and thawed without change of viscosity, as long as they are free of calcium (less than 0.5%); if calcium is present the viscosity will increase and a gel may even form and these changes will not reverse.
D. pH
The viscosity of alginate solutions is unaffected over the range of pH 5-11. Below pH 5, the free -COO- ions in the chain start to become protonated, to -COOH, so the electrostatic repulsion between chains is reduced, they are able to come closer and form hydrogen bonds, producing higher viscosities (King, 1983). When the pH is further reduced, a gel will form, usually between pH 3-4; however if the alginate contains residual calcium this gelation may occur about pH 5. If the pH is reduced quickly from pH 6 to pH 2, a gelatinous precipitate of alginic acid will form. Above pH 11, slow depolymerization occurs on storage of alginate solutions, giving a fall in viscosity.
Figure 7 Viscosity in 1% solutions at different temperatures. (Source: Protan, 1986a)
Propylene glycol alginate has fewer -COO- ions and is less affected by increasing acidity. Its solutions remain unchanged to about pH 3; below this value precipitation and gel formation occur. Above pH 6.7, hydrolysis of the ester groups occurs slowly with consequent loss of viscosity.
E. CALCIUM IONS
The presence of low concentrations of calcium ions in an alginate solution will increase its viscosity and larger amounts will cause the formation of a gel. The addition of Ca++ is therefore a way of increasing the viscosity of a solution without having to increase either the amount of alginate dissolved or the molecular weight of the alginate being used. It also allows the flow properties of solutions to be adjusted (reduced) by adding sequestering agents such as calgon and EDTA. The disadvantage is that alginate solutions with calcium ions show a greater loss of viscosity with stirring (are more shear sensitive) than alginates with no calcium. As the concentration of calcium ions and viscosity increase, the solutions change from pseudoplastic to thixotropic, that is they take some time to recover their original viscosity after being stirred. The way in which calcium reacts with alginate is discussed in the later section on "Gels".
Most commercial alginates made by the calcium alginate process contain residual quantities of calcium; for example the usual food grades of sodium alginate from Kelco contain 1.2% calcium and in special low-calcium grades this is reduced to 0.2% (Kelco, 1976). 1.2% calcium represents 17% substitution of calcium for sodium in the sodium alginate and this is sufficient to increase the viscosity; thickened, flowable solutions result from 7-20% substitution by calcium ion while gels form with about 30%. There is a region of calcium addition, just before gel formation, where very thick solutions result which are thixotropic. The range of calcium concentration over which this occurs is much greater for alginates with a high content of mannuronic acid (high M/G ratio). Alginates with a high guluronic acid content show a more abrupt transition from solution to gel. The effect of calcium on the viscosity of a calcium-containing alginate can be estimated by measuring the viscosity of a solution before and after the addition of a sequestering agent, such as sodium hexametaphosphate, which removes the calcium ions in a complexed phosphate ion.
Alginates made by the alginic acid process contain negligible amounts of calcium so that if an increase in viscosity is required at a fixed alginate concentration, a small amount of a sparingly soluble calcium salt such as calcium sulfate or calcium citrate may be added. The effect of calcium on the viscosity of an alginate is difficult to predict and is usually found by experimentation. It will depend on the uronic acid composition and degree of polymerization of the alginate; alginates with higher molecular weights and/or higher M/G ratios give greater viscosity changes (McDowell, 1960). The way in which the solution is made also affects the final viscosity; for example a 1% solution prepared by dilution of a 3% solution will differ in viscosity from a 1% solution prepared directly; the type and duration of the stirring used in preparation will also affect the result.
Propylene glycol alginate with 85% of the carboxylic acid groups esterified is hardly affected by the presence of calcium ions. At the other extreme, 60% or less esterification gives an ester which behaves similarly to sodium alginate except that thixotropic effects are much more evident.
GELS
The polysaccharides derived from seaweeds - alginates, agars, carrageenans and furcelleran - can all be induced to form gels under certain conditions. A better understanding of the structure of these gels has developed in recent years and useful reviews have been written by Rees (1972) and Morris (1985).
Solutions of alginate will react with many di- and trivalent cations to form gels; the gels will form at room temperature, or any temperature up to 100°C, and they do not melt when heated. They find applications in various industries, particularly when calcium is used as the divalent ion. Alginate solutions will also form gels if they are carefully acidified; these gels are generally softer than calcium gels and, unlike calcium gels, give the feel of melting in the mouth so they find many applications in the food industry.
CALCIUM GELS
Those who are seriously interested in formulating calcium gels should refer to the work by King (1983, pp. 141-173) which is the most thorough discussion available of the variables which should be considered, the practical systems commonly used and some examples of actual applications.
Calcium has found greatest popularity as the divalent ion for gel formation because its salts are cheap, readily available and non-toxic. If a calcium chloride solution is stirred into an alginate solution, a precipitate of calcium alginate results; it may be stringy or gelatinous. To obtain a smooth gel, the calcium must be released slowly into the alginate solution. This is done by using a calcium salt with a low solubility (such as calcium citrate) which slowly releases calcium ions. An alternate method is to use a calcium salt which is practically insoluble in neutral solution but dissolves as the pH falls (such as dicalcium phosphate); when an acid of low solubility (such as adipic acid) is added, it gradually lowers the pH, calcium ions are released and a gel forms. The time needed for a gel to form can be controlled by the solubilities of the calcium salt and acid, their particle size and the operating temperature. Retarding agents can also be used, such as sequestrants which complex the calcium ions and make them unavailable until all the sequestrant has reacted; when a dry powder product contains both alginate and a calcium salt, the addition of sufficient sequestrant will delay the availability of calcium ions until the alginate is dispersed and hydrated. Details of calcium salts, acids and sequestrants which are used have been discussed by Littlecott (1982) and more detail by King (1983).
The gel strength depends on the source (algal species) of the alginate, the concentration of alginate, its degree of polymerization and the calcium concentration. Alginates from different seaweeds can have differing ratios of mannuronic acid to guluronic acid in their structures and different proportions of M, G and MG blocks (see "Structure of alginic acid" section). This ratio, and the way in which the acids are distributed in the alginate chains, have a marked effect on gel formation and gel strength. Alginates with a high proportion of G blocks form rigid gels; they form fairly suddenly as calcium ion concentration is steadily increased. The opposite holds for alginates with mainly M blocks; they form gradually and are softer and more elastic. This behaviour is related to the molecular structure of the gels.
The early hypotheses for gel formation was that calcium ions displaced hydrogen ions on the carboxylic acid groups of adjacent chains and formed simple ionic bridges between the chains. Rees (1969) argued why that was unlikely and later he put forward the "egg-box model" (Grant et al., 1973), now generally accepted. This requires the cooperative mechanism of binding, of two or more chains, shown in Figure 8. The buckled chain of guluronic acid units is shown as a two-dimensional analogue of a corrugated egg-box with interstices in which the calcium ions may pack and be coordinated. "The analogy is that the strength and selectivity of cooperative binding is determined by the comfort with which 'eggs' of the particular size may pack in the 'box', and with which the layers of the box pack with each other around the eggs" (Grant et al., 1973). The model can be extended to be three-dimensional. While calcium helps to hold the molecules together, their polymeric nature and their aggregation bind the calcium more firmly; this has been termed "cooperative binding". The structure of the guluronic acid chains (Figure 4) gives distances between carboxyl and hydroxyl groups which allow a high degree of coordination of the calcium. The strontium ion is larger and even more firmly bound; magnesium ions are smaller and are not held, so magnesium alginate does not form gels (McDowell, 1977).
Figure 8 Gel formation via G blocks: egg box model
Propylene glycol alginate, with a low degree of esterification (below 60%) and a high degree of polymerization, can form soft gels with calcium salts. As the degree of esterification is raised to about 85%, it is hardly affected by calcium (McDowell, 1977). It is also more tolerant of calcium at lower pH values (Steiner and McNeely, 1950).
ACID GELS
The structure of these gels has not been studied as comprehensively as calcium gels, probably because they are more limited in their application. A steadily increasing number of carboxyl ions on the alginate chains become protonated as the pH falls, reducing the electrical repulsion between chains. The chains can then move closer together which allows hydrogen bonding to be more effective. At first this produces a higher viscosity and eventually, at pH 3.5-4.0, a gel forms. Small amounts of calcium (less than 0.01%) must be present; the reason is not known.
King (1983) has outlined the useful characteristics of acid gels. For equivalent alginate concentrations, acid gels have about half the strength of calcium gels and they do not show any syneresis. This softness, when combined with their feeling of melting in the mouth, means they are useful in some food applications where they can imitate the effect of gelatin; calcium gels, even soft ones, still feel lumpy in the mouth. They can be made so that they can be stirred or pumped and then reset to a gel, a valuable property when manufacturing some processed foods. However acid gels are not stable when heated and become softer with time even at room temperature, as the alginic acid depolymerizes; they are stable for about a year if refrigerated (5°C). Propylene glycol alginate is not suitable for making acid gels.
FILM FORMATION
Alginates can be made into two types of film which have different properties: water-soluble films (usually from sodium alginate) and oil-soluble films (usually from calcium alginate).
Water-soluble films can be made by evaporation of a solution of alginate or by extrusion of an alginate solution into a non-solvent which mixes with water, such as acetone or ethanol. These films are impervious to grease, fats and waxes but allow water vapour to pass through. They are brittle when dry but can be plasticized with glycerol, sorbitol or urea. They have good non-stick properties and are useful as mould release agents, for example in the manufacture of fibreglass plastics. Where a high solids film is needed, a very low viscosity alginate can be used. Self-supporting films need greater strength and require the use of higher viscosity alginates with a greater degree of polymerization. Triethanolamine alginate is used to form soft flexible films.
Water-insoluble films can be made by treating a water-soluble film with a di- or trivalent cation (Ca++ is the most frequently used one) or with acid. They can also be made by extrusion of a solution of a soluble alginate into a bath of a calcium salt. Some alginates, such as zinc alginate, are soluble in excess ammonia solution; if the NH3 is evaporated from a film of such a solution, an insoluble film of zinc alginate remains. These films of insoluble alginate are not water-repellent and will swell on prolonged exposure to water.
FILAMENT FORMATION
If a solution of sodium alginate is forced through fine holes into a solution of a calcium salt, filaments of calcium alginate will be formed. Much research went into the development of alginate yarns (Steiner and McNeely, 1954; Maass, 1959) but they are not resistant to alkaline soaps. This, plus the relative cost of alginate and the development of many synthetic fibres, led to a loss of interest until quite recently when a new commercial product appeared. Made in the United Kingdom, it is a bandage-type material which is used as a dressing on wounds. When it comes into contact with sodium salts in the body fluids, some of the calcium ions are exchanged for sodium ions so that a thin soft gel forms at the interface of the dressing and the wound and the dressing never sticks to the wound.
GENERAL COLLOIDAL PROPERTIES
General colloidal properties is the term used to explain why alginate is successful in some applications where the reasons are not fully understood and where the alginate has been chosen on an empirical basis. These applications have been discussed by McDowell (1960) and Leigh (1979). They include the use of: sodium alginate, as a flocculant, as a suspending agent, and as a stabilizer in ice cream; propylene glycol alginate, in acidic frozen products such as ice sherberts, in fruit squash containing fruit solids, and in stabilizing beer foam.
SAFETY IN FOODS
The Food Chemical Codex gives specifications for alginic acid, its propylene glycol ester and its ammonium, calcium, potassium and sodium salts. These four salts have been granted GRAS status (generally recognized as safe) in the USA and propylene glycol alginate has been approved as a food additive for use as an emulsifier, stabilizer or thickener. The joint Expert Committee of Food Additives of the Food and Agriculture Organization of UN/World Health Organization has also issued specifications for alginates and recommended an Acceptable Daily Intake, for alginic acid salts of 50 mg per kg body weight per day, for propylene glycol alginate of 25 mg/kg/day. King (1983) has listed 39 countries which permitted alginate salts as at January 1982; three of them had not approved the propylene glycol ester. Food additive laws differ from country to country, even from state to state within a country, and are constantly being revised. Therefore users must acquaint themselves with the latest information in their relevant countries and cannot rely on the information given here and in the references cited.
USES
Earlier reviews on the uses of alginates include those by Steiner and McNeely (1954), Maass (1959), McDowell (1960), Glicksman (1969a), McNeely and Pettitt (1973), Cottrell and Kovacs (1980), and King (1983).
Not all reviews cover all uses; for example while most give lengthy treatments to food uses, few say much about textile printing and paper applications. Reviews which are more specific to a particular use are listed in the following subsections. The main uses of alginates are shown in Table 4.
TEXTILE PRINTING
In textile printing, alginates are used as thickeners for the paste containing the dye. These pastes may be applied to the fabric by either screen or roller printing equipment. An excellent review by Hilton (1969) discusses the role of the thickener in the printing of fabrics and the advantages/disadvantages of sodium alginate in different printing processes. Alginates became, important thickeners with the advent of reactive dyes which combine chemically with cellulose at its hydroxyl groups. Many of the standard thickeners, such as starch, also react with these dyes and this leads to lower colour yields and sometimes insoluble products which are not easily washed out and which can result in a fabric with poor handle. Alginates react minimally with reactive dyes, they wash out of the finished textile readily and are the best thickeners for these dyes. They are also used with other types of dyes.
The viscosity of the paste can be varied according to the application and the equipment. Thick pastes with short flow characteristics are useful when the extent of penetration into the fabric must be limited but thinner pastes with long flow are required for fine-patterned prints. For alginates containing small quantities of calcium, viscosity can be controlled by adding sequestering agents such as polyphosphates. However these pastes are more likely to lose viscosity as shear rate increases and a paste which is less shear sensitive can be made using a high concentration of a lower viscosity alginate. This latter kind of paste is especially useful for printing disperse dyes on synthetic fibres. Most alginate manufacturers can supply basic recipes for the different types of dyes and printing processes (for example, Protan, 1985) which are a useful starting point; the quantities of alginate can vary from 1.5% of high viscosity alginate to 5% of low viscosity alginate.
Table 3 Variation of viscosity (mPa.s) with concentration for sodium alginate solutions at 20°C
|
Type of alginate |
Concentration |
||||
|
1% |
1.5% |
2% |
3% |
4% |
|
|
Very low viscosity |
10 |
20 |
45 |
130 |
350 |
|
Low viscosity |
20 |
60 |
180 |
650 |
2200 |
|
Medium viscosity |
350 |
1800 |
6000 |
not |
measurable |
|
High viscosity |
800 |
4000 |
9000 |
not |
measurable |
Table 4 Principal uses of alginate
|
End-uses |
Percentage of the quantity of total demand |
|
Textile printing |
50 |
|
Food |
30 |
|
Paper |
6 |
|
Welding rods |
5 |
|
Pharmaceuticals |
5 |
|
Others |
4 |
Source: ITC (1981).
Alginates are normally incompatible with cationic dyes. However Racciato (1979) has reported that premixing the cationic dye with selected surfactants before addition to the thickener will allow the use of many cationic dyes. He claims that compatibility with almost every cationic dye can be obtained if either xanthan gum or algin is used. In the printing of cotton cloths using reactive dyes, Prelini (1982) suggests ways of obtaining good colour value and of avoiding colour bleeding, using alginates and other thickeners. Rompp, Axon and Thompson (1983), discuss the use of alginates with reactive dyes on cotton, viscose rayon and cotton-synthetic blend fabrics.
Ramakrishnan (1981) deals with the principles of reactive printing and the problems which arise in rotary printing machines, and the use of sodium alginate in the processing. Obenski (1984) has discussed the US printed fabric market, the use of alginates and guar gum as thickening agents and their share of the US market.
General reviews of thickeners in printing pastes, which include the use of alginates, have been made by Christie (1976), Shenai and Saraf (1981), Narkar (1982) and Teli, Shah and Sinha (1986).
Reviews dealing more specifically with the use of alginates in textile printing can be found in Ornaf (1969), Hilton (1972), Iwahashi (1975), Shah (1975), Balassa (1977), Khairoowala and Afrin (1984), Hebeish et al. (1986) and Teli and Chiplunkar (1986).
FOODS
Alginates have a long history of use in foods and these uses are based mainly on their thickening, gelling and general colloidal properties. Thickening is useful in sauces, syrups and toppings for ice cream, etc., pie fillings (it reduces moisture retention by the pastry), cake mixes (it thickens the batter aids moisture retention), and canned meat and vegetables (it can give either temporary or delayed-action Chickening). Gel formation leads to uses in instant milk desserts and jellies, bakery filling cream, fruit pies, animal foods and reformed fruit. General colloidal properties are difficult to define but are illustrated by the results obtained by adding sodium alginate to ice cream and water ices, or propylene glycol alginate to stabilize beer foam or the suspended solids in fruit drinks (Leigh, 1979). Details of these and other applications can be found in some of the more recent reviews which have been written by Glicksman (1969a), McNeely and Pettitt (1973), McDowell (1975), Lawrence (1976), Cottrell and Kovacs (1980), Littlecott (1982), King (1983) and Sime (1984); their content is summarized below.
McNeely and Pettitt (1973) is well referenced to the general literature and much of the material is still useful. McDowell (1975) classifies uses according to the relevant alginate property such as thickening, gelling, film formation and stabilizing; it is a general review of value to a new user of alginates. Lawrence (1976) surveyed those US patents since the early 1960's that deal with gums for edible purposes and he includes a lengthy section on alginates. The four most recent reviews are all written by personnel from Kelco, the largest world manufacturer of alginate. Littlecott (1982) gives a good explanation of how to form food gels with alginates and provides many examples and formulations. Sime (1984) also deals with food gelling systems and is a useful addition to Littlecott's review; after discussing the general principles, he gives details for making reformed pimiento strips for olives, and structured fruits from fruit puree. Cottrell and Kovacs (1980) relate food applications to properties of alginate and give a wide variety of sample formulations for various food products; they give few literature references.
The review by King (1983) is excellent; he draws on the material used by Cottrell and Kovacs for describing the properties of alginates and then provides a thorough and well documented description of the food uses of alginates up to 1981-82.
One of the more recent developments is the use of alginates in restructured meat products. The US Department of Agriculture approved the use of alginate as a binder in these products last year (September, 1986) and this should lead to a new market for alginate. Restructuring is the process of taking flaked, sectioned or chunked meat and binding the pieces to resemble intact cuts of meat. The final products can be shaped as nuggets, roasts, loaves and steaks. Until now most restructured products have been sold frozen or cooked, so they could retain their shape. With the use of binders, the restructured products can be sold fresh or raw. The binder is a powder of sodium alginate, calcium carbonate, lactic acid and calcium lactate. When mixed with the raw meat, they form a calcium alginate gel which binds the meat. This binding mixture can be used to replace the sodium chloride and phosphate salts commonly used, thereby reducing the sodium level in the restructured products. Up to 1% sodium alginate is permitted. A patent has been assigned to the developers of the process, Colorado State University Research Foundation (Schmidt and Means, 1986), the inventors have published the information separately (Means and Schmidt, 1986) and the application is outlined by Andres (1987).
Alginates have been used for other re-formed foods. Morimoto (1984) patented a process for making shrimp or crabmeat analogue products using alginate and proteins such as soy protein concentrate or sodium caseinate. A mixture of the two is extruded into a calcium chloride bath to form edible fibres which are then frozen, thawed, chopped, coated with sodium alginate and formed in an appropriately shaped mould. After further freezing and thawing a product analogous to natural shrimp is obtained. Wylie (1976) described the manufacture of analogue fish fillets (sole) using minced white fish and a calcium alginate gel; the products could be grilled or cooked with sauce. A meat substitute has been formed from an aqueous mixture of protein and alginate by a process of freezing, slicing, gelling and heat setting; a well defined fibre structure results (Shenouda, 1983).
The principles used for making structured fruit products have been extended to making structured potato products such as croquettes and french fries (Anon., 1983). A synthetic potato skin shell containing alginate can be filled with mashed potato and browned to produce 'baked potatoes' in the fast-food market (Ooraikul and Aboagye, 1986). A patent by Cox (1982) for forming simulated, shaped, edible products includes the production of caviar and cottage cheese as examples.
There has been an increased interest in the use of alginate-pectin mixed gels with potential for use in jams, fruit flans and mayonnaise (Thom et al., 1982; Toft, 1982; Morris and Chilvers, 1984). New dessert gels from alginate have been reviewed by Kelco (1983) while Protan (1986a) has discussed alginates as stabilizers in bakery creams, jams and jellies.
The reasons for the effectiveness of alginate as a stabilizer in ice cream have never been fully understood; Muhr and Blanshard (1984) have studied the mechanism for the reduction of crystal growth but their work is not yet conclusive.
A moisture barrier which allows breaded or batter-covered products to come in contact with a sauce or filling can be made using a coating of soluble alginate which is then treated with calcium chloride (Earle and McKee, 1986). Alginates are being used to make improved rice pasta and vegetable pasta (Hsu, 1985, 1985a). Calcium alginate can be formed as a fibrous precipitate and is used to simulate the texture of natural fruit and vegetables (Anon., 1980).
IMMOBILIZED BIOCATALYSTS
Many commercial chemical syntheses and conversions are best carried out using biocatalysts such as enzymes or whole cells. Examples are (a) the use of enzymes for the conversion of glucose (40% of the sweetness of sucrose) to fructose (about 150% of the sweetness of sucrose), the production of L-amino acids for use in foodstuffs, the synthesis of new penicillins after hydrolysis of penicillin G, (b) the use of whole cells to promote the conversion of starch to ethanol, for beer brewing, for the continuous production of yoghurt. To carry out such processes on a moderate to large scale, the biocatalysts need to be in a concentrated form and to be recoverable from the process for reuse. This can be achieved by "immobilizing" the enzymes or cells; they can be fixed to the surface of an insoluble solid or entrapped in a polymeric material. In the 1970s many single enzymes were isolated, immobilized and used, but more recently it has been found that it is easier, more economical and often more effective to immobilize whole cells, which contain multi-enzyme systems. An added advantage of immobilizing cells is the increased stability often found; it is not unusual for a half-life of one day for ordinary suspended cells, to be increased to 30 days for immobilized or resting cells. A good introduction to the reasons for using biocatalysts and for their immobilization is given by Tramper (1985); he also describes the use of alginate for immobilization. Bucke and Wiseman (1981) give a more detailed background and review the developments to the early 1980s.
Alginate gels have proved to be a very successful medium for entrapping biocatalysts, especially when formed as beads of gel. The cell suspension is mixed with sodium alginate solution (2-4%) and this is extruded as drops into calcium chloride solution (0.05-0.1M). An immediate skin forms around the drop and as calcium ions gradually diffuse inwards, a gel forms. The size of the beads can be regulated from the size of the needle or nozzle, usually 0.2-1.0 mm but up to 5 mm. The fresh beads can be separated and used or they can be dried; drying increases their strength and reduces their ability to swell so they contain, when rewetted, more cells per unit volume.
Alginates which form strong gels are best for this application; the alginate should therefore contain a high proportion of guluronic acid, such as that extracted from the stipes of Laminaria hyperborea, a species particularly abundant in the cold waters of Norway. There have been recent reports of high guluronic acid contents in seaweeds from warmer waters, Sargassum species from Sabah, Malaysia (Wedlock, Fasihuddin and Phillips, 1986) and Sargassum, Turbinaria and Cystoseira from Sri Lanka (Shyamali, de Silva and Savitri Kumar, 1984).
Details of methods are available from manufacturers (such as Protan, 1987) or from the literature (Dallyn, Falloon and Bean, 1977; Klein and Wagner, 1982; Klein, Stock and Vorlop, 1983; Tanaka, Matsumura and Veliky, 1984; Rehg, Dorger and Chau, 1986).
Johansen and Flink (1985, 1986, 1986a, 1986b) have applied the internal gelation principles, developed for food gels, to immobilization techniques using yeast cells for their studies. Sodium alginate, an insoluble calcium salt and D-glucono-1,5-lactone are mixed in water; the lactone slowly hydrolyses, lowers the pH, releases calcium ions and gelation occurs gradually, from within the solution. The resulting immobilizates have particles of higher strength, with at least equal fermentation rates, when compared to externally gelled material. Rochefort, Rehg and Chau (1986) have stabilized calcium alginate gels by washing with 0.1M aluminium nitrate. Burns, Kvesitadze and Graves (1985), produced dried spheres of calcium alginate containing magnetite and found they have good potential as a support for enzyme immobilization.
Cell immobilization with alginate can be done under mild conditions with little loss of activity of the cells and the activity is often stable for extended periods of time. Temperatures can be 0-100°C and the pH neutral but any buffers used must not contain citrate or phosphate. These anions will remove calcium ions from the gel and can lead to its breakdown, although Birnbaum et al. (1981), have developed methods for stabilizing alginate gels in phosphate-containing media. The cell-gel entrapment can be done under sterile conditions and the alginate gel is stable (0-100°C) and non-toxic. The cells can be recovered if necessary be adding a sequestering agent for the calcium ions (such as polyphosphate or EDTA); once the calcium ions are removed from the gel, its structure is lost and it changes to a liquid with the cells suspended in it.
The number of processes in which alginate has been used for cell or enzyme immobilization, on laboratory and larger scales, has increased dramatically in the last few years. Good sources of these works are journals such as Biotechnology Letters, Biotechnology and Bioengineering or the abstracts available from data bases such as Biobusiness (Dialog Information Services) and Current Biotechnology Abstracts (Pergamon Infoline). Some examples are:
(a) production of ethanol from starch (McGhee, Carr and St. Julian, 1984);(b) beer brewing with immobilized yeast (Onaka et al., 1985);
(c) production of citric acid (Lim and Choi, 1986);
(d) continuous yoghurt production (Prevost, Divies and Rousseau, 1985);
(e) fermentation to produce butanol and isopropanol (Schoutens et al., 1986);
(f) continuous acetone-butanol production (Frick and Schuegerl, 1986);
(g) pilot-plant production of prednisolone from hydrocortisone (Kloosterman and Lilly, 1986);
(h) glycerol production from the marine alga, Dunaliella tertiolecta (Grizeau and Navarro, 1986).
PAPER
Until the late 1950s the main use for alginate in the paper industry was in surface sizing. Its addition to the normal starch sizing gives a smooth continuous film and a surface with less fluffing. The oil resistance of alginate films give a size with better oil resistance so an improved gloss is obtained with high gloss inks. If papers or boards are to be waxed, alginate in the size will keep the wax mainly at the surface. The quantity of alginate used is usually 5-10% of the weight of starch in the size.
Alginate is also used in starch adhesives for making corrugated boards because it stabilizes the viscosity of the adhesive and allows control of its rate of penetration. One percent sodium alginate, based on the weight of starch used, is usually sufficient.
Cottrell and Kovacs (1980) give examples of formulations for a kraft lineboard sizing and for corrugating adhesives. An improved sizing with alginate has been obtained by using a paper containing 5-25% of calcium carbonate filler; the calcium alginate film which forms gives better solvent resistance and lower Bendtsen porosity. The alginate is blended with 6-20 parts of starch or it may be combined with polyvinyl alcohol (Kelco, 1985).
Paper coating methods and equipment have developed significantly since the late 1950s as the demand for a moderately priced coated paper for high quality printing. Trailing blade coating equipment runs at 1 000 metres per minute or more so the coating material, usually clay plus a synthetic latex binder, must have consistent rheological properties under the conditions of coating. Up to 1% alginate will prevent change in viscosity of the coating suspension under the high shear conditions where it contacts the roller. The alginate also helps to control water loss from the coating suspension into the paper, between the point where the coating is applied and the point where the excess is removed by the trailing blade. The viscosity of the coating suspension must not be allowed to increase by loss of water into the paper because this leads to uneven removal by the trailing blade and streaking of the coating. Medium to high viscosity alginates are used, at a rate of 0.4-0.8% of the clay solids. A new modified form of sodium alginate has been reported to be more effective than existing alginate and results in lower processing costs (Yin and Grishaber, 1979).
A useful discussion of the evolution of paper coating methods and the use of alginate has been prepared by Blood (1968). Other more general discussions of coating, which also refer to the use of alginates, have been published by Paper (Anon., 1976), Bergmann and Hunger (1978), and Delaplace, Laraillet and Isoard (1985).
Sergeant (1981) has prepared a general review of the applications of alginates in paper converting. He has also described the use of zinc ammonium alginate as a flame retardant in paper (Sergeant, 1980).
WELDING RODS
Coatings are applied to welding rods or electrodes to act as a flux and to control the conditions in the immediate vicinity of the weld, such as temperature or oxygen and hydrogen availability. The dry ingredients of the coating are mixed with sodium silicate (water glass) which gives some of the plasticity necessary for extrusion of the coating onto the rod and which also acts as the binder for the dried coating on the rod. However the wet silicate has no binding action and does not provide sufficient lubrication to allow effective and smooth extrusion. An additional lubricant is needed, and a binder which will hold the damp mass together before extrusion and maintain the shape of the coating on the rod during drying and baking. Alginates are used to meet these requirements.
Soluble alginates (sodium or potassium) are used in coatings on welding rods which are dried at moderate temperatures and in which the alginate remains after drying; this includes organic-type coatings with a high content of cellulosic material and mineral coatings of the "acid" type. Soluble alginates can be used in basic or low-hydrogen rods but calcium alginate, sometimes with a proportion of sodium alginate added, gives much better results. This is related to the high temperatures used to dry these rods (400-450°C) which produces low moisture contents so that only very low hydrogen levels are found in the deposited weld metals. Soluble alginates swell when wetted and as the water is driven out completely in this high-temperature drying, the alginates will contract and cracks will develop in the coating. When calcium alginate is mixed with sodium silicate, a small amount of sodium alginate forms around each particle of calcium alginate. This mixture is thixotropic, its viscosity is lowered when extrusion pressure is applied; it therefore acts as a good binder and extrusion lubricant. During the drying process, because the calcium alginate did not previously swell very much, it does not shrink appreciably and a more uniform coating results.
The quantities of alginates used are very dependent on the type of welding rod being coated and the extrusion equipment being used. For soluble alginates it may be 0.4-1.2% for low-hydrogen welding rods and 0.15-0.25% for acid and organic types. For the thixotropic alginates, manufacturers often find it more effective to use a mixture of calcium alginate and sodium alginate with a total alginate content of 0.4-0.6% for low hydrogen electrodes. Alginate manufacturers are the best source of information for using alginates in welding rod applications, for example Protan (1984).
PHARMACEUTICAL
Alginic acid is insoluble in water but swells when placed in water. This property makes it a useful disintegrating agent in tablets. It is more expensive than the traditional disintegrating agent, starch, but its overall addition to the cost of the tablet is still usually very low. It is a better disintegrant than starch so less is required. It can be added during the granulating process, rather than as a powder after granulation, so the processing is easier. The mechanical strength of the final tablet is greater, compared to using starch.
Sodium alginate is used in some liquid medicines to increase viscosity and improve the suspension of solids. Propylene glycol alginate can improve the stability of emulsions. Capsules containing sodium alginate and calcium carbonate are used to protect inflamed areas near the entrance to the stomach. The acidity of the stomach causes formation of insoluble alginic acid and carbon dioxide; the alginic acid rises to the top of the stomach contents and forms a protective layer.
Very useful dental impression compounds are based on alginate cold-setting gels; some recent examples can be found in Pellico (1983) and Scheuble and Munsch (1983). Alginates are the basis of many slimming or diet foods, particularly biscuits; alginic acid swells in the stomach and fills it so that the dieter no longer feels hungry; the body cannot assimilate the alginic acid so no calories are absorbed.
These uses have been discussed in earlier reviews such as McNeely and Pettitt (1973).
OTHER USES
MEDICAL DRESSINGS
Courtaulds (UK) has patented a wound dressing which combines aspects of alginate filament formation with those of spunbonding to produce a good quality staple fibre (Aldred and Mosely, 1983). This fibre can be easily processed into nonwoven fabrics. The sodium calcium alginate fibres are useful as haemostatic wound dressings which can be absorbed by body fluids, as the calcium in the fibre is exchanged for sodium from the body fluids (Burrow and Welch, 1983).
A new "biopaper" has been made by the Japan Institute of Industrial Research (1985). The papers, made from alginic acid or a mixture of the acid and its calcium salt, are expected to be of value for bandages and similar medical uses where the haemostatic properties of alginates are useful. Bioactive papers have been made from staple fibres in which enzymes have been entrapped (Kobayashi, 1986; Kobayashi and Matsuo, 1986; Kobayashi, Matsuo and Kawakatsu, 1986).
CONTROLLED RELEASE OF CHEMICALS
This use has some similarities to the methods used for immobilization of cells. In this application the cells in the alginate gel beads are replaced by materials having biological activity, such as biological and chemical herbicides. The rate of release of the herbicides into soil or water can be controlled by the properties of the gel beads; the beads can be air dried and become hard granules. By incorporating air into the beads they can be made to float. The patent suggests their use for herbicides, pesticides, algicides and most biologically active substances (Connick, 1983; Connick, Lee and Rawson, 1984).
An alginate-clay mixture and calcium ions have been used to encapsulate microorganisms (chiefly fungi) which have potential to control plant diseases (Fravel et al., 1985). A sustained release system for pharmaceuticals, using calcium alginate beads, has been reported by Badwan et al. (1985).
BINDERS FOR FISH FEEDS
The worldwide growth in aquaculture has led Protan (1978) to investigate the use of alginate as a binder in fish feeds, especially moist feed made from fresh waste fish mixed with various dry components. Alginate binding can lower consumption by up to 40% and pollution of culture ponds is sharply reduced. More recent technical information is available from the authors.
CONFECTIONERY
Alginate gels find a small application in confectionery. Recently the incorporation of fruit pulp has been suggested and a method for making Turkish delight is described (Anon., 1983a).
RELEASE AGENTS
The poor adhesion of films of alginate to many surfaces, together with their insolubility in non-aqueous solvents, have led to their use as mould release agents, originally for plaster moulds and later in the forming of fibreglass plastics. Sodium alginate also makes a good coating for anti-tack paper which is used as a release agent in the manufacture of synthetic resin decorative boards (Cheetham, 1976; Sumitomo Bakelite, 1981). Films of calcium alginate, formed in situ on a paper, have been used to separate decorative laminates after they have been formed in a hot-pressing system (Jaisle and Bunkowski, 1981; Jaisle and Schiermeier, 1981).
MARKETING
There are difficulties and costs in the marketing of seaweed colloids, such as alginate, agar and carrageenan, which are not always apparent to those outside the industry. In some markets one colloid may compete with another, in others one might be the only real choice. They must all compete, in at least some of their uses, with plant gums (such as guar and locust bean) and cellulose derivatives (such as CMC and methyl cellulose) which are often cheaper. It is important to realize that price may not be the determining factor in a buyer's choice of a seaweed colloid; quality and its reproducibility from one batch to another may be more important. Frequently a buyer uses less than 1% of the colloid in his product so a 20% price difference may be inconsequential in the total cost of his product. Many a buyer of seaweed colloids, satisfied with one particular brand or grade, will, despite a higher price, stay with it because the risks of changing may not seem to be worth the saving. So in seaweed colloids, those brands already established in the market often hold a very strong, entrenched position. To dislodge them, a marketing group should include a strong technical team which can run tests and trials to convince the buyer of the equivalence of the new product; sometimes this requires a detailed knowledge of the buyer's industry. In promoting new sales, the colloid producer may have to provide complete formulations and technical know-how to potential buyers. Therefore selling costs of the seaweed colloids can be high and account must be taken of this by the potential producer.
The buyers of alginates fall into two groups. The first is a number of large buyers who know exactly what they want and who require little servicing because they have their own resources. This group includes those specialty gum companies who service smaller users by preparing their own blends of seaweed colloids and other colloids, according to the requirements of a particular customer.
The second and larger group are the smaller users who need some technical service support. Frequently this group yields more profitable sales in the long term because they may be sold specifically formulated products at a premium price and they are generally more reluctant to change to a competitor's product. On the other hand it takes more time and expense to establish such sales. Many of the major producers have such specialty products, shown by the large range of products listed by them.
Alginate manufacturers usually sell direct to the major markets but in minor markets it is more economical to sell through an agent, leaving the task of market penetration to him but providing technical support where necessary. Agents need to have an appreciation of the application of colloids and a knowledge of the client's industry. This ideal might be achieved by an agent selling a variety of chemicals to just one industry, like the food industry, but an agent who deals principally in colloids over a range of industries, which is a not uncommon situation, usually needs more backup from the producer. Large wholesalers/agents may buy from the producer and resell; otherwise they operate on a commission of 5-15%, depending partly on the degree of assistance required from the producer.
The world market for alginates is estimated at 20 000-24 000 tonnes per year (Kjemi, February 1986; Inf.Chim., (265), October 1985) which is similar to estimates made in 1980 (ITC, 1981). Demand for sodium alginate is steady with plentiful supplies.
Three principal grades are available but there are variations of viscosity, and many specially formulated mixtures with additives, within each grade. The highest grades meet the requirements of the National Formulary (USA), food grades generally meet the quality standards of the Food Chemicals Codex (USA) and technical grades vary considerably in their colour and water-insoluble solids (such as cellulose). Other countries or groups (EEC) have similar specifications to the NF and FCC.
Prices have shown little change between 1986 and 1987 and lie in the following ranges: Sodium alginate: pharmaceutical (NF) grade, US$ 6-7 per pound; food (FCC) grade, US$ 3-5 per pound; technical grade, US$ 2.50-3.50 per pound. Propylene glycol alginate, US$ 6-7 per pound (Chemical Marketing Reporter, 27 January 1986, 11 August 1986, 23 February 1987).
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