ISOLATED SOYBEAN PROTEIN (ISP)
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Isolated soybean proteins, or soybean protein isolates as they are also called, are the most concentrated form of commercially available soybean protein products. They contain over 90% protein, on a moisture free basis.
Soy protein isolates have been known and produced for industrial purposes, mainly as adhesives for the paper coating industry, well before World War II. ISP's for food use, however, have been developed only in the early fifties.
The basic principles of ISP production are simple. Using defatted soy flour or flakes as the starting material, the protein is first solubilized in water. The solution is separated from the solid residue. Finally, the protein is precipitated from the solution, separated and dried. In the production of ISP for food use, in contrast to ISP for industrial use, care is taken to minimize chemical modification of the proteins during processing. Obviously,the sanitary requirements are also much more demanding.
Being almost pure protein, ISP can be made to be practically free of objectionable odour, flavour, colour, anti-nutritional factors and flatulence. Furthermore, the high protein concentration provides maximum formulation flexibility when ISP's are incorporated into food products. These and other advantages have been the source of highly optimistic forecasts regarding the widespread use of ISP. Although the volume of production increased and although several production facilities have been erected in the U.S.A., Europe, Japan, India and Brazil, the tonnage figures are far from those predicted when food grade ISP was first marketed.
The principal reasons for this situation are: the relatively high production cost (see below), nutritional and regulatory limitations , the inability of ISP-based texturized products to compete with texturized soy flour and texturized SPC, and finally,the competition of other abundant "isolated proteins", particularly casein and caseinates. Nevertheless, it should be noted that many novel isolated proteins,such as those obtained from cottonseed, peanuts, fish, squid etc. have been much less successful than ISP. Many of these did not reach the stage of commercial production.
Although actual trade figures are not disclosed, the growth in sales of concentrates and isolates is said to be, at present, stronger than that of flours.
ISP can be further modified and processed into more sophisticated products. These include: spun fibres from ISP as an ingredient for muscle food analogs, proteinates and enzyme modified ISP.
The cost of isolated soybean proteins is five to seven times higher than that of defatted soy flour. On an equal protein weight basis the cost ratio of these two products is nearly 3:1. The main reasons for the added cost will become evident from the description of the manufacturing methods for ISP.
6.2 Defintion, composition, types
The specification of the Association of American Feed Control Officials, Inc. (AAFCO) defines ISP is as follows:
"Soy Protein Isolate is the major proteinaceous fraction of soybeans prepared from dehulled soybeans by removing the majority of non-protein components and must contain not less than 90% protein on a moisture-free basis." (from '90 Soya Bluebook).
There are no official standard definitions or specifications for the various types of isolates. ISP is bought and sold on the basis of specifications formulated by the manufacturer or the user.
The typical composition of an isolated soy protein is shown in Table 6-1.
Table 6.1 Typical composition of ISP
Source: Kolar et al. (1985)
The conventional procedure for ISP production is based on protein solubilization at neutral or slightly alkaline pH, and precipitation by acidification to the isoelectric region, near pH 4.5. The resulting product is "isoelectric ISP". It has low solubility in water and limited functional activity. Different "proteinates" can be produced by resuspending isoelectric ISP in water, neutralizing with different bases and spray-drying the resulting solution or suspension. According to the base used for neutralization sodium, potassium, ammonium or calcium "proteinates" are produced. The first three are highly soluble in water, producing solutions with very high viscosities, foaming, emulsification and gel forming properties. Calcium proteinate has low solubility. Low-solubility (inert) ISP's are used where the formulation calls for a high level of protein incorporation without excessive viscosity of other functional contributions.
Since spray-drying is the common drying method in the production of ISP, the primary physical form of ISP in commerce, is that of fine powders. Structured forms, such as granules, spun fibres and other fibrous forms are made by further processing. These forms will be discussed in a separate chapter, dealing with texturized products.
6.3 Production processes6-3-1 The conventional process
This is the process commonly described in the literature and suggested by suppliers of equipment and complete plants. Exact processing conditions and the type of equipment used may vary from plant to plant.
An outline of the process is given in Fig.30.
a- Starting material: Dehulled, defatted, edible grade white flakes or meal with the highest possible protein solubility index are used. Although the rate of protein extraction from finely ground flour would be faster, flakes permit easier separation after extraction. In batch extraction, particle size has no effect on protein extraction yield, if extraction time is over 30 minutes.
b- Protein extraction: The flakes are mixed with the extraction medium in agitated, heated vessels. The extraction medium is water to which an alkali such as sodium hydroxide, lime, ammonia or tri-basic sodium phosphate has been added, so as to bring the Ph to neutral to slightly alkaline reaction. Under these conditions, the majority of the proteins go into solution. The sugars and other soluble substances are also dissolved.
* Alkalinity: More protein can be extracted at higher pH. However, the extracted proteins may undergo undesirable chemical modifications in strongly alkaline solutions. These include protein denaturation and chemical changes in amino acids. Excessively high pH also favours protein-carbohydrate interaction (Maillard reaction) which results in the formation of dark pigments and in loss of nutritive value. Furthermore, proteins precipitated from highly alkaline media tend to retain too much water, and do not settle well. In practice, the range between pH 7.5 and pH 9.0 is most commonly preferred.
One of the chemical reactions of amino acids in alkaline media has attracted particular attention. That is the destruction of the amino acid cystine, with the formation of dehydroalanine. In addition to the nutritional implications resulting from the loss of cystine, there might be also a toxicological aspect to consider. Dehydroalanine can react with free epsilon-amino groups of lysine, to produce lysinoalanine. This compound has been found to cause kidney lesions in rats under certain experimental conditions. The toxicity of lysinoalanine for man is still an open question.
* Extraction time: The course of nitrogen extraction from white flakes , using 0.03 molar calcium hydroxide as extractant is shown in Fig. 31. The amount of nitrogen extracted under these conditions increased steadily during the first 30 minutes and reached a nearly constant level after 45 minutes. The extraction time in industrial operation is, probably, in the order of 1 hour.
* Temperature: Protein extraction yield is considerably increased by raising the temperature, up to 80°C.
* Solid/liquid ratio: Protein extraction yield is improved as the quantity of liquid medium used to extract a given weight of flakes is increased. After extraction and separation by filtration or centrifugation, the extracted flakes retain a considerable proportion of extract, about 2.5 times the weight of solid. In single-stage batch extraction, if the more liquid is used for extraction, the protein concentration in the extract is lower and the quantity of protein associated with the retained portion of the extract is smaller. On the other hand, larger volumes of liquid have to be handled per unit weight of protein produced. This means larger extraction vessels, centrifuges etc. and a larger volume of "whey" for disposal.
The choice of a solid/liquid ratio for extraction is, therefore, a matter of economical optimization. The ratios used in industry range apparently between 1:10 and 1:20.
* Heat treatment history of the meal: The NSI value of the starting material is the most important factor affecting isolation yield. (Fig. 32)
Figure 31: The Course of Soy Protein Extraction
* Agitation: As in any extraction operation, agitation increases the rate of protein solubilization. However, within the practical values of extraction time for batch operations (about one hour), little is gained by increasing the turbulence beyond that provided by moderate agitation. Furthermore, strong agitation causes excessive flake disintegration, increases the proportion of fine particles in the extract , rendering solid/liquid separation more difficult. Moderate agitation can be defined as any mixing operation that would keep the flakes in suspension within the extraction medium.
c- Solid-liquid separation after extraction:
The extract contains considerable amount of fine particles of extracted
flour, the elimination of which, prior to precipitation, is necessary in order
to obtain a "curd" of acceptable purity.
Table 6-2 shows the effect of fine solids separation on the purity of the final product.
Table 6.2 Effect of extract
clarification on the purity of ISP curd
Mesh # of sieve used for separation of solids
%Protein (N x 6.25) in dry curd
Source: Cogan et al. (1967)
In industrial scale operation, it may prove convenient to carry out the extract clarification process in two steps: screening (vibrating screen, rotary screen or the like) to separate most of the solids, followed by centrifugal clarification of the extract. The wet solids can be pressed to remove as much entrapped extract as possible. All these operations can also be carried out in one step, using decanter centrifuges. A flow diagram of decanter-based process for the production of ISP is shown in Fig. 33.
Figure 33: ISP Production Using Decanter Centrifuges (Courtesy of Alfa-Laval)
d- Extract treatment: The clarified extract can be treated so as to remove certain impurities, thus improving the blandness, colour and nutritional quality and modifying the functional properties of the final product. Extract treatment may include: ion exchange to remove phytate and reduce the ash content, treatment with activated carbon to remove phenolic substances, ultrafiltration for concentration and removal of low molecular weight components etc. Although such processes have been suggested in the literature it is not known whether they are practised in the industrial production of ISP. The use of membrane processes for extract purification and concentration have been reported to be industrially applied in Europe and Japan. (Elias, 1979).
e- Precipitation: The protein is precipitated from the extract by bringing the pH down to the isoelectric region. The type of acid used or the temperature of precipitation do not affect the yield or purity of precipitated protein.
f- Separation and washing of the curd: The precipitated protein (curd) is separated from the supernatant (whey) by filtration or centrifugation. Desludger or decanter centrifuges can be used for this purpose. The curd must be washed in order to remove residues of whey solubles. This can be done by resuspending the curd in water and re-centrifuging, or continuously on a rotary or belt filter. Thorough washing is most important for the obtention of high purity ISP.
g- Drying: The usual method for drying the washed curd is spray-drying.
6-3-2 Problems in conventional processing
a: Process losses: The conventional process separates the soy solids into three fractions: extraction residue, curd (ISP) and whey.
Extraction residue (okara) is the insoluble solid material left behind after extraction and separated from the extract by filtration or decanting. It represents approximately 40% of the solids in the raw material and carries away 15% of the protein entering the process. It is usually pressed, dried and sold as a by-product of ISP manufacture. It can be used as a protein source for animal feeding rations or as a source of dietary fibre in human nutrition. It has been also used in food products for its exceptional water adsorbing capacity.
Whey is the liquid supernatant, after the protein is precipitated from the extract. It contains the sugars and the nitrogenous substances not precipitated by acidification.
Approximately 25% of the dry matter of the raw material and 10% of its nitrogen content is found in this fraction. Early investigations ( Hackler et al. 1963) indicated that soybean "whey" may be toxic to animals. This finding has been reconfirmed often since then. Furthermore, ISP whey is a highly diluted stream, containing 1 to 3% solids depending on the solvent:flake ratio used for extraction. Concentration and drying of ISP whey would be too costly. ISP whey is , therefore, a waste stream of the isolation process.
The curd is the precipitate obtained by acidification of the extract. After washing and drying, it becomes the final product: isoelectric ISP. It contains 75% of the protein of the starting material. Nearly 3 tons of defatted soybean are needed to produce one ton of protein isolate.
This low yield explains, to a large extent, the relatively high cost of ISP.
b: Quality: ISP obtained by the conventional process contains several types of impurities ( e.g. phytates and phenolic substances) which may somewhat impair its functional,sensory and nutritional quality. More complete dehulling of the beans , thorough extract clarification and repeated washing of the curd reduce the impurities but does not eliminate them completely.6-3-3 Alternative processes
Several alternative processes for the isolation of soy protein have been reported in the literature. These include:
a: Solubilization of the soy proteins in the salt solutions (salting-in) followed by precipitation by dilution with water.
b: Precipitation from the extract at near-boiling temperature, using calcium salts ( as in the production of Tofu).
c: Ultrafiltration of the extract so as to remove the low molecular weight components of the whey , leaving a concentrated solution of protein which may be spray-dried.
d: Physical separation of the intact protein bodies from very finely ground soy flour by density fractionation (flotation).
e: Purification of the extract by ultrafiltration, filtration through activated carbon and ion exchange, in order to increase curd purity.
6.4 Utilization6-4-1 Meat products
In this paragraph, only the use of non-texturized ISP and proteinates will be discussed. It should be remembered, however, that the major application of ISP in connection with meat and related product is based on the use of texturized ISP, in one form or another, to replace meat. This application will be dealt with in a separate chapter.
In emulsion type sausages, such as frankfurters and bologna, ISP and proteinates are used for their moisture and fat binding properties and as emulsion stabilizers. Typical usage levels are 1% to 4% on a prehydrated basis. The use of ISP in these products permits reducing the proportion of expensive meat in the formulation, without reducing the protein content or sacrificing eating quality.
Methods for incorporating soy protein products into whole muscle meat have been developed recently. Isolated soybean protein is dispersed in specially formulated meat curing brines and injected into whole muscle using stitch pumps. It is also possible to incorporate the protein by surface application of the protein containing brine, followed by massaging or tumbling, as practised in the cured meat industry. Typical brine formulations contain salt, sugars, phosphates, nitrite and/or ascorbic acid.6-4-2 Seafood products
The most important of application in this category is the use of ISP in fish sausage and surimi based restructured fish products in Japan. Surimi is extensively washed, minced fish flesh.6-4-3 Cereal products
ISP is sometimes used instead of, or in combination with isolates and soy flour, in the formulation of milk replacer mixtures in bakery products. ISP has been used for protein fortification of pasta and specialty bread. In these applications, the high protein content and blandness of ISP are clear advantages.6-4-4 Dairy-type products
Soybean protein isolates are used in non-dairy coffee whiteners, liquid whipped toppings, emulsified sour cream or cheese dressings, non dairy frozen deserts etc. The basis for these applications is, demand for non-non-dairy (all-vegetarian, cholesterol-free, allergen-free) food products, as well as economy.
Imitation cheeses have been produced from isolated soy proteins, with or without milk whey components. The types of cheeses which can be produced include soft, semi-soft, surface-cultured (imitation Camembert) and ripened hard cheeses.6-4-5 Infant formulas
Infant formulas where milk solids have been replaced by soy products are well established commercial products. ISP is the preferred soy ingredient, because of its blandness, absence of flatus-producing sugars and negligible fibre content.The principal market for these products are lactose-intolerant babies. However, soy protein based dietetic formulas are finding increasing use in geriatric and post-operative feeding as well as in weight reduction programs.6-4-6 Other uses
Partially hydrolysed soy proteins possess good foam stabilization properties and can be used as whipping agents in combination with egg albumen or whole eggs in confectionery products and deserts.
Isolated soybean protein has been shown to be an effective spray-drying aid in fruit purees. In this application, it can replace maltodextrins, with the advantage of contributing protein to the final product. A nutritious "shake" base was produced by spray-drying ripe banana puree containing up to 20% ISP on dry matter basis. (Mizrahi et al.,1967).REFERENCES
Cogan U., A. Yaron, Z. Berk and S. Mizrahi (1967)
Isolation of Soybean Protein: Effect of Processing Conditions on Yield and Purity. J. Am. Oil Chem. Soc. 44: 321
Elias S. (1979)
Food Eng. 51(10), 81
Kolar C.W., S.H. Richert, C.D. Decker, F.H. Steinke and R.J.
Van der Zanden (1985)
Isolated Soy Protein in "New Protein Foods", A.M. Altschul and H.L. Wilke eds. Academic Press Inc. Orlando, Florida.
Mizrahi S., Z. Berk and U. Cogan (1967)
Isolated Soybean Protein as a Banana Spray-drying Aid. Cereal Sci. Today 12: 322
Morr C.V. (1990)
Current Status of Soy Protein Functionality in Food Systems J.Amer. Oil Chem. Soc. 67: 265
Soy Protein Council (1987)
"Soy Protein Products" Soy Protein Council, Washington, D.C.
Soya Bluebook (1990)
Soyatech Inc. Bar Harbor, ME.
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