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Flavours, spices and edible gums: opportunities for integrated agroforestry systems

Joseph Hulse
International Development Consultant
1628 Featherston Drive
Ottawa, Ontario, Canada KIG 6P2

Abstract

Natural flavours, spices, other aromatics and polysaccharide gums (PSGs) share several common features. They are extracted from tree bark, sap, roots, fruits, flowers and seeds of a diversity of plant species, then processed, preserved and enhanced to preserve them and to raise their market value. The price of refined extracts can be up to 20 times the price of the harvested raw material. Aromatics and PSGs are used by related industrial companies: food, pharmaceutical, toiletries and cosmetics.

While these natural biologicals are being challenged by various synthetics, demand for flavours, spices and PSGs is rising as (1) Europeans and North Americans purchase more highly spiced foods; (2) ageing populations consume more pharmaceuticals and cosmetics; and (3) Asian, African and Latin American urban populations with rising incomes diversify their diets, expending more money on commercially processed foods. As a consequence, industrial demand for a reliable supply of aromatics and PSGs, consistent in quality and stable in price, is rising and will continue to do so.

Herein lies an opportunity for agroforestry systems that combine tree crops, bushes, vines and ground plants, each of which synthesizes a high-value flavour, aromatic or PSG. Such extractives should be processed to acceptable commercial quality by a rural industry established close to the area of production. Particular opportunities exist for semi-arid regions and for ecologies threatened by desertification.

Introduction

Natural flavours, spices, other aromatics and polysaccharide gums (PSGs) share many attributes and common features. They are mainly secondary metabolites of various woody and non-woody plants and originate in bark, sap, stems, leaves, roots, flowers and seeds. Aromatics and PSGs may be isolated or concentrated by simple physical processes, such as cleaning, grinding and screening and by extraction with aqueous, polar or non-polar solvents. These compounds find their predominant markets among the same industries: in foods, pharmaceuticals, cosmetics and toiletries. Most aromatics and many natural PSGs are harvested in tropical and subtropical ecosystems. Almost all are exported as dried tissue or crude extracts, to be refined by industries in more affluent countries. Prices realized for the refined products are many times greater than the prices paid to producers and gatherers of the raw material. The producing countries thus at present derive little of the added-value benefit or the related employment opportunities.

Aromatics and PSGs derived from wild species are less consistent in yield, composition and essential properties than those from cultivated plants.Many suitable species are adaptable to dryland ecologies; therefore, agroforestry systems that combine compatible woody and non-woody plants that produce aromatics and PSGs, integrated with contiguous extraction and refining facilities, could be of particular benefit to poor communities in the arid and semi-arid tropics.

Flavours and spices

The brief observations that follow seek to be indicative and illustrative of opportunities for flavours and spices, not a definitive, exhaustive account of all technical and economic possibilities. Natural flavours from woody and non-woody plants include sugars that occur in nectars and sweet saps, such as the African sugar bush, Protea mellifera Thunb., and essential components of virtually all edible fruits and aromatic herbs that stimulate taste buds and olfactory organs. As illustrated by the extensive industrial use of flavours extracted from citrus fruits, opportunities to utilize other tree fruit extracts appear almost limitless and will expand as food processing industries increase and diversify in Asia, Africa and Latin America. As an example, the processing of citrus fruits yields cold-pressed essential oils from the peel; molasses and dried pulp for animal feed from the residue; and monocyclic terpene and limonene, which may inhibit the onset of some epidermal tumours and oral cancers.

Spices and aromatic herbs

`Spice' derives from the Latin species aromatica. Earliest classifications were related to their use: in perfumes, for incense and embalming. Today, any distinction between spices and other aromatic plants is arbitrary. The US Food and Drug Administration describes a spice as `an aromatic vegetable substance, in whole, broken or ground form . . . whose function is for seasoning rather than nutritional'. Others have defined spices as aromatic herbs from tropical ecologies, all others being `culinary herbs'. Spices and aromatic herbs may also be classified as:

Pungent spices including the fruits of Capsicum spp-cayenne, chillies, tabasco-and Piper nigrum L.

Aromatic fruits and seeds such as nutmeg and mace, respectively the seed and the aril of Myristica fragrans Houtt.; allspice, the dried berries of Pimenta dioica (L.) Merr.; mustard seed Sinapis alba L.; and clove the dried flower bud of Syzigium aromaticum(L.) Merr. & Perry

Aromatic bark including cinnamon and cassia from, respectively, Cinnamonum verum J. Presl. and Cinnamonum aromanticum Nees.

Aromatic leaves, which include bay (Laurus nobilis L.), West Indian bay (Pimenta racemosa (Miller) J. Moore) and Californian bay (Umbellularia california) (Hook & Arn.) Nutt.

Trade in spices and aromatic herbs

For many centuries whole, unground spices have been shipped to European and other importers to be recleaned, ground and sold as ground spices, singly or in mixtures. More than 50% are consumed by secondary processors, a proportion that will increase with rising demand for processed foods. Prices paid by North American and European consumers for packaged ground spices are often 15 to 20 times the price paid to developing country producers. The constraints to grinding before shipping are several, including assurance of quality standards required by food regulatory agencies, food industries and consumers in importing countries and the prevention of loss of essential aromatics by oxidation and volatilization during shipment. These constraints are illustrated by export statistics: the US imports about 15 000 tonnes per year of unground cinnamon and cassia but less than 300 tonnes of the ground products; India exports about 40 000 tonnes of whole pepper and less than 100 tonnes of the ground spice; 99% of cloves are shipped whole and virtually none as ground spice. Similar patterns are evident among most other aromatics exported from less developed countries.

Processing and quality

Spices and aromatic herbs are marketed in three forms:

large tissue fragments (whole seeds, flower buds or leaves)

ground particles of the above

extracted essential oils and oleoresins

Essential aromatics remain relatively intact in whole spices if protected from excessive moisture, high temperatures, infection, infestation and other contamination. Aromatic herbs and spices suffer rapid oxidation when ground in the air, a process accelerated by above-normal temperatures. Tropical environments and frictional heat from grinding, which accelerate oxidation and flavour degradation, can be offset by grinding in the presence of inert refrigerants such as liquid nitrogen, liquid or solid carbon dioxide.

Modern food and cosmetic industries are more disposed to using extracted essential oils or oleoresins rather than whole or ground spices. Production of essential oils and oleoresins begins with clean whole seeds or other whole plant tissues. Essential oils are constituents of all aromatic plants. They may be extracted by steam or dry distillation and `rectified' by subsequent fractional distillation. Efficiently processed, packaged and stored, essential oils are uniform and stable in quality and free from microbial and other contaminants.They are oxidized if exposed to air and unfavourably high temperatures. Oleoresins are extracted by organic solvents. They consist of essential oils, various resins and non-volatile oils, and fatty acids. Their composition is dependent upon genotype, conditions of cultivation, the solvent and conditions of extraction. Efficiently processed, packed and stored, oleoresins are uniform, with close resemblance to the original flavour, free from contaminants, have a long storage life, and, being highly concentrated, are less expensive to ship and transport than whole spices. Those in food industries, such as bakers, meat processors, canners and frozen food and dry-mix producers, buy essential oils or oleoresins, mixed or compounded with other materials, from intermediate processors to facilitate weighing and ease of mixing and blending.

Since extracted essential oils and oleoresins converted to free-flowing particles are stable, highly concentrated and less cumbersome and expensive to transport than whole spices, there would seem to be as yet unrealized opportunities for some producing countries to add value by processing and exporting stabilized extractives. During such processing, certain standards would need to be fulfilled. Standards for extractives are based upon the weight of oleoresin that is equivalent in odour and flavour to 100 kg of freshly ground whole spice. For example, from allspice, so named because the flavour suggests a blend of cinnamon, cloves, nutmeg and pepper, 2.4 kg of essential oil or 5.0 kg of oleoresin are equivalent to 100 kg fresh ground spice (see table 1).

Table 1. Standards for extractives

Aromatic plant Essential oil

(%)

Weight (kg) oleoresin =

100 kg of raw material

Bayleaf

Cinnamon

Cloves

Mace

Nutmeg

30%

1-3%

17% (mainly eugenol)

10-15% (88% monoterpene)

16% (mainly erimysisticin)

5.0 of leaf

2.0 of bark

6.0 of ground flower bud

7.0 of ground aril

6.0 of ground kernel

The above numerical estimates are indicative and relate to good quality and efficiently processed materials. All vary according to species and genotypic source (wild or cultivated) and their environment. In addition to the woody species, there are many other plants that synthesize extractable aromatic secondary metabolites. These are also used by food industries and are plants that could be cultivated in agroforestry systems. Piper nigrum is a perennial climbing vine that could be cultivated among other woody species. Capsicum annuum L. and Capiscum frutescens L., which respectively give paprika and chillies, are in worldwide and growing demand. Their genotypes adapt to many environments. In common with the polysaccharide gums, discussed below, spice and other aromatic herb plants provide the raw materials used in many highly profitable and expanding food and cosmetic industries. It is later suggested how, through agroforestry integrated with efficient extractives processing facilities, tropical countries with suitable ecologies and technological capabilities could benefit both economically and socially.

Polysaccharide gums

Origin, nature and uses

PSGs are present in virtually all plants; however, the difficulties of extraction can be industrially uneconomic. Commercially economic sources include:

Exudates and extracts from the bark, sap, leaves, fruits, seeds, roots, rhizomes of various woody and non-woody plants

Metabolites of microbial fermentations

Metabolites of aquatic macro-algae

Organic derivatives of starch and cellulose

PSGs display a wide variety of desirable functional properties, determined by species, origin and genotype, all of which influence their molecular composition and structure. Though several are extracted from wild species, in general yields are higher and the desirable properties are more consistent from systematically cultivated species.

In some connotations, `industrial gum' includes a wide range, from hydrophobic hydrocarbons with high molecular weight, such as rubber and the latex `chicle' from Manilkara zapota (L.) P. Royen used in chewing gum, to resinous saps from evergreens. However, the most common commercial usage of `gum' relates to an assortment of plant and microbial polysaccharides and their derivatives that are miscible with hot or cold water to form viscous solutions or gelatinous dispersions at relatively low concentrations. Criteria that determine industrial choice and selection include the specific properties required, cost, reliability of supply and consistency of quality.

Total annual global industrial use of PSGs, including modified starches and celluloses, is valued at about USD 2 thousand million, with roughly half being consumed by US industries. Total US industrial consumption is estimated at 1.13 million tonnes with US food and feed industries using roughly 280 thousand tonnes valued at USD 375 million. Consumption of PSGs is increasing throughout European industries and can be expected to rise among the rapidly industrializing nations of Asia, Africa and Latin America. A particular attraction of most PSGs is that they can truly be described as `natural products' from renewable biological sources. Because of their wide availability from natural and synthetic sources, PSGs used in the food industry are generally regarded as safe by the US and other food and drug regulatory authorities.

Health benefits

Of all the natural polysaccharides, only starches are fully hydrolysed in the digestive tract. Enzymes in the upper gastrointestinal tract hydrolyse starch and carbohydrates derived from hydrolysed starch. PSGs and cellulose and its derivatives pass relatively unchanged into the large intestine, since neither upper gastrointestinal enzymes nor stomach acids exert any appreciable effect on cellullose or PSGs. Of recent interest is the ability of some PSGs to mimic solid saturated fats in textural and mouth-feel properties. As a result they are increasingly used to provide desirable eating qualities in low fat and low-calorie foods. Also PSGs with high solubility increase faecal bulk and function as soluble fibre to reduce blood plasma cholesterol. Attributes such as these will expand their utility and and raise consumption by food processing industries.

Commercial gums

Exudates

PSGs from exudates were the earliest used, being the most readily accessible from trees and shrubs by tapping or incising the bark. Exudates come mostly from wild species, which display considerable variability in functional properties within and among species. Trees cultivated in plantations from selected genotypes give higher yields of PSGs with more consistent properties.

Most plant families include species that exude gums, although only a few are economically useful. The most widely used PSG exudates include arabic (Acacia senegal (L.) Willd.), ghatti (Anogeissus latifolia (DC) Wallich ex Guillemin & Perrottet), karaya (Sterculia urens Roxb.), and tragacanth (Astralagus gummifer (Lab)).

Gum arabic. An exudate of Acacia senegal and other Acacia species, gum arabic is the most extensively used of the natural exudates (see also Seif el Din & Zarroug, this volume). Its first reported uses, about 3000 BC, were as an adhesive in hieroglyphic paints and in the embalming of Egyptian mummies. The main producing countries are Sudan, Nigeria, Senegal and Mauritania; however none refine the gum to food commercial quality. The market price of high-quality refined gum arabic, USD 15-25 per kg, is 8 to 10 times the price paid for the crude exudate. From wild species, gum yield per tree varies from 20 to 2000 g per year. Yields, consistency and profitability are significantly improved by integrated systematic selection, cultivation, tapping and processing. Acacia trees can be cultivated in plantations and the gum can be processed and refined locally.

Coincident spray drying encapsulates flavour, spice oils and oleoresins to produce dry particles suitable as ingredients of dry soup, sauce, dessert and cake mixes. Its gels form the bases of many varieties of pastilles, gum drops, jelly candies, cough lozenges and other confections high in sucrose. It acts as a stabilizer in marshmallows, caramels and nougats; as a protective coating for nuts and a glaze for baked bread and cakes; in toppings and icings for cakes; and foam on beer. It is extensively used as an emulsion stabilizer and binder in a wide assortment of lotions, emollient creams, lotions and other cosmetics.

Gum karaya. Second in commercial importance to gum arabic is this exudate of a bushy tree, Sterculia urens, which grows on dry, rocky soils in India. Total annual production of the gum is about 4500 tonnes, the average yield per tree about 4.5 kg year-1 with a market price for high-grade gum of roughly USD 5 per kg. Its industrial applications are similar to those of gum arabic, being used in many food, pharmaceutical and toiletry products. It stabilizes salad dressings, cheese spreads and ground meat; prevents ice crystal growth in frozen foods; and improves the stability of whipped egg albumen and other protein foams.

Gum ghatti. Anogeissus latifolia, which produces this exudate, grows in deciduous forests in India and the Indian sub-continent, often in the same areas as Sterculia urens. World production is roughly 1000 tonnes; market price of graded quality is about USD 3 per kg. Its commercial applications are similar to those of gum arabic, that is, it is used as a stabilizer for syrup emulsions and as a carrier for oil-soluble vitamins.

Gum tragacanth. This gum from Astralagus gummifer was in trade at the time of Theophrastus (300 BC). The quality varies among different Astralagus species, which grow mainly in the semi-arid, mountainous regions of Iran, Syria and Turkey. After incision, the gum exudes as flakes or ribbons. Although gum tragacanth has attractive properties, international trade has declined as the price has increased, largely influenced by rising labour costs in the countries of origin. Harvesting, handling and grading of exudates from wild species, is highly labour intensive. Silvicultural experiments in Arizona indicate that yields and quality can be improved and labour costs reduced in plantations with 25 000 trees per hectare, spaced 60 cm apart, and by incising the bark with a battery-operated hand drill. The USA, the largest importer (700 tonnes year-1), recognizes five grades, which ranging in price from USD 30 to 90 per kg. In foods it is an effective emulsion stabilizer at low pH, a useful property in French, Italian and Roquefort salad dressings. It is also a stabilizer of frozen foods that contain acid fruits, of acid fruit centres in chocolates and of vitamin C in beverages. It provides body and `mouth-feel' to low-calorie mayonnaise, salad dressings and sauces. Its capacity to form thick, viscous gels provides an important pharmaceutical application in spermicidal jellies. It facilitates absorption of steroids and oil-soluble vitamins and acts as an emulsifier for cod liver oil. In toiletries it is a useful humectant in toothpaste, skin and hair creams.

Extracts -gums from seeds and leaves

The most important sources of PSGs, other than starch and cellulose, are the seeds of the locust bean, guar, tara, tamarind, quince and psyllium. After seed extracts, the most extensively used PSGs are pectin and pectic acid derivatives extracted mainly from fruit pulp as well as agar, alginates and carrageenans extracted from various seaweeds.

Extracted gums from genetically modified microorganisms and chemically modified cellulose, starches and seaweed extracts are gradually superseding exudates because of the variability among exudate PSGs from wild species. Standards of quality for all food grade exudates and other PSGs are specified by the Joint FAO/WHO Expert Committees on Food Additives and in the Food Chemicals Codex of the US National Academy of Sciences.

Guar gum. The gum is produced by grinding the endosperm of the decorticated seeds of a leguminous plant, Cyanopsis tetragonolobus (L.) Taubert, an annual that grows in arid and semi-arid areas in India. Breeding and agronomic studies in Oklahoma have raised yields. US food processors use about 7000 tonnes per year as a stabilizer and thickener and as a humectant in baked cereals. Guar gum complements the viscoelastic properties of hydrated wheat flour gluten, a property of value to developing countries who seek to reduce importation of wheats by producing fermented bread from composite flours containing significant proportions of non-gluten-forming cereals. Its high viscosity favours its use in cosmetics and toiletries, particularly in hair conditioners and shampoos, and in latex paints.

Locust bean (carob) gum. First known as an adhesive in Egyptian mummy bindings and, biblically, as the desert food of John the Baptist, locust bean gum is obtained from the ground endosperm of the seeds of the legume Ceretonia siliqua (L.), which grows naturally around the Mediterranean. A large tree can produce over 1 tonne of pods, each weighing 15-40 g. World production is about 500 000 tonnes of pods.

Annual use by US food industries approximates 1500 tonnes. It is used as a stabilizer, binder and thickener in processed cheese, in toppings and syrups for ice cream, frozen desserts; in processed meats and heat-processed canned foods; and to improve sheeting in pastry doughs for tortillas and pizzas. It has special applications in sizing, printing, dyeing and fabric finishing in textile industries.

Psyllium seed gum. This is obtained from the seeds of Plantago, which grows around the Mediterranean and is cultivated in France, Spain and Italy. High PSG content has been found in Indian cultivars. It is a mucilaginous gum used for many centuries as a remedy for intestinal disorders, as a mild laxative, and to alleviate irritation of mucous membranes.

Quince seed gum. The gum is extracted from the seeds of a deciduous bushy tree, Cydonia oblonga Miller; the fruit pulp is converted into preserves and marmalades. The gum properties are particularly suited as an ingredient in hair lotions used to set artificial waves and curls.

Tamarind gums. The gums come from the seeds of the leguminous Tamarindus indicus (L.), cultivated in India. The pulp of the mature pods is used in curries and chutneys and is a good source of vitamin C. Seed coats are removed by parching. The dehulled seeds are then dry-milled to produce the PSG. The powdered gum disperses in hot and cold water; its maximum viscosity is reached when it is boiled. Viscosity of the hot paste is over 5 times that of maize starch at 5% concentration. PSG produced from the seeds can in part replace pectin in jams and jellies and acts as a stabilizer or thickener in various foods. It acts as a binder in pharmaceutical tablets, as a humectant and emulsifier in cosmetics, and as a sizing agent for jute and cotton fibres.

Tara (huarango)-Peruvian carob gum. The gum is produced from milled seeds of a shrub species of Caesalpina, which is extensively grown and processed in Peru. Its properties and uses are similar to those of locust bean.

Fenugreek gum. The gum comes from the seeds of an annual, Trigonella foenum-graecum L., which, when cultivated, yields 900 kg ha-1 an-1. It is used in curries and as an essential ingredient of imitation maple syrup. The oil also contains the pharmaceutically useful steroid diosgenin.

Aloe gum. This gum is extracted from the leaves of Aloe species, for example, A. vera L. Burm., grown as a plantation crop in Texas at about 20 000 plant ha-1, and in Central America and the Caribbean. Aloe gum yields approximate to 70 kg plant-1 an-1. It is used in cosmetics as a skin lotion and emollient, and in pharmaceuticals as a therapeutic for skin lesions.

Chia gum. This gum from the seeds of Salvia hispanica L. displays exceptional mucilaginous properties at low aqueous concentration. The seed also contains 35% of good quality oil and 25% protein. The roasted seed was historically a popular food among Mexican Amerindians.

Okra gum. The gum can be extracted from the pods of Abelmoschus esculentus (L.) Moench, which when cultivated is mostly harvested when immature and cooked as vegetable `gombo'. The PSG displays uniquely mucilaginous properties with the ability to be whipped to stable foams. Mixed with egg white, okra gum produces commercially superior confectionery products.

Yellow mustard gum. A product of recent commercial origin, this gum is produced from yellow mustard seeds (Sinapis alba L.). It has wide application as a binder for ground meats and in bread coatings for fried chicken and fish. It also provides antioxidants which, being of natural origin are not classed as `additives'.

Pectins

Pectins exist in all higher plants, particularly as structural materials of soft fruits and fleshy roots. Most common commercial sources are citrus peel and apple pomace, both by-products of fruit juice manufacture. From citrus peel (about 30%) pectin is isolated by acid aqueous extraction, followed by filtration, concentration and precipitation with ethanol, methanol or isopropanol.

Although most widely used in jams and jellies, pectins also stabilize fermented and acidified milks, fruit yogurts and related foods. They act as protective colloids in cosmetic emulsions and surgical dressings.

Pectic substances are extractable from many tropical plants. An interesting source is the jujuba tree (Ziziphus mauritiana Lam. /Z. jujuba Miller), indigenous to Africa, which is highly resistant to drought and salinity. The fruit, like a small plum, has a long post-harvest shelf life, can be eaten fresh, dried or candied and is high in vitamin C (about 75 mg per 100 g); when underripe it is ideal for jams, jellies and chutneys. The leaves contain functionally useful vegetable tannins. The jujuba is only one of many arboreal species that deserve systematic study as components of agroforestry systems that can produce extractable PSGs and other commercial products.

Starch

Starch and many derivatives may be considered as PSGs in that they are polysaccharide polymers that produce viscous solutions or gels. Principal sources are maize, other cereal grains, cassava, roots, tubers, rhizomes and stems from species of the Palmae family. Starch is composed of two principal components: amylose, a straight chain, and amylopectin, a branched chain of glucopyranosyl units. Each starch source can be identified by the shape and size of its granules, by its amylose-to-amylopectin ratio, and by its pattern of swelling and gelatinization when it is hydrated and heated.

Starch granules are released from cleaned vegetative tissue, roots or seeds by maceration or grinding, then are extracted with clean water. The suspended starch granules gradually settle by sedimentation, a process accelerated by centrifuging. The wet starch is dewatered, dried, milled and screened to predetermined particle size. From whatever source, starch can be extracted and processed by either labour-intensive or highly mechanized processes. Cleaning, grinding, macerating, dewatering, screening and packing can be by hand or by machine. Drying can be by insolation or by heating with fuel under vacuum or moving hot air. Labour-intensive processes are least expensive but are also less efficiently productive. For example, cassava roots processed by manual labour, using gravity sedimentation and sun drying, will yield about 12-15% starch, while a modern mechanized process will yield 25% starch proportionate to the root source. Furthermore, starch derived from selected cultivated plants harvested close to the processing factory is more consistent in yield and quality than that from wild or unselected crop types. A study of alternative production systems for sago starch from the palm Metroxylon sagu Rottb/M. rumphi C. Martius has indicated that intensive cultivation will yield 25 t starch ha-1 an-1. More primitive systems produce barely 5 tonnes ha-1 an-1. Starchy rhizomes suitable for agroforestry systems in appropriate ecologies include the various `arrowroots' from species of Tacca, Canna, Curcuma and Maranta, several of which can be cultivated to yield over 30 t ha-1 an-1 with starch contents between 20 and 30%.

Starch can be chemically converted to an immense diversity of derivatives, with properties to suit various industrial requirements. Thin boiling starches are used by laundries, for warp sizing in textiles, and in paper and paper board manufacture. Starches oxidized by hypochlorite have many applications in paper and textile processing. Dextrins from thermal treatment are widely used as adhesives.

Cellulose and lignocellulose

Lignocellulose is the most abundant yet most profligately wasted natural polymer. Woody tissue of crystalline cellulose is surrounded by a matrix of xylan and lignin. The thermoplastic lignin can be converted to a wide range of useful phenolic substances, such as ethyl vanilla, important in food flavours as the synthetic substitute for vanilla. Xylan is a PSG used as a binder and emulsifier in various food, cosmetic and pharmaceutical products.

Cellulose derivatives

Though indigestible by non-ruminants, pure cellulose and several derivatives are technologically important in food products. PSGs of industrial importance are the cellulose alkyl ethers: methyl, hydroxyethylmethyl-, hydroxypropylmethyl-, and carboxymethyl-cellulose. Annual global production of methylcellulose derivatives, by over 40 chemical companies, is in excess of 100 000 tonnes (table 2).

Table 2: Consumption and value of cellulose derivatives in the USA.

  Tonnes (`000) USD million
CMC 42 125
Hydroxyethylmethyl 22 96
Methyl 12 55
Microcrystalline 6 27
Other 2 10
Total 84 313

The first industrial cellulose PSG was sodium carboxymethylcellulose, made in Germany in World War I as a substitute for gelatin. The earliest extensive use of CMC was in the 1930s in synthetic detergents.

Though not digestible, those used in food are non-toxic and generally regarded as safe by the US Food and Drug Administration. PSGs derived from methylcellulose are used in low calorie and dietary foods to replace lipid and carbohydrate, as well as in salad dressings, syrups, preserves and jellies. They effectively supplement hydrated wheat flour gluten in fermented breads. In other baked and frozen foods they retain moisture, stabilize doughs, batters, decorative icings and fillings. They stabilize emulsions in non-dairy whipped toppings; act as binders in extruded foods; and reduce fat absorption in fried meats, fish and vegetables. They can be converted to clear, flexible films, which provide effective barriers to lipids and are useful as packages for fatty foods. Agricultural applications include coatings for eggs, binders for animal feeds, fertilizers and water dispersible pesticides, and as adhesives to bind pesticides to seeds.

Recommendations

The harvesting of raw materials from which aromatics and PSGs are commercially extracted and refined provides employment and income for rural people in tropical and subtropical countries. As most refining takes place in North America, Europe and other affluent regions, producers and gatherers of the raw materials receive the added value from processing, which in most instances is substantial.

Growth in demand is apparent among industries and their consumers in North America and Europe, where increased consumption of processed foods in general and of exotic highly flavoured foods in particular is clearly evident. A progressive growth in demand can be predicted among the economically advancing nations of Asia and Latin America where expanding urban populations with rising disposable incomes will purchase more industrially processed foods, as well as more cosmetics and toiletries. Ageing populations on all continents may be expected to consume more pharmaceutical products. Present yields of PSGs from plant cell and tissue culture are generally too low to be economic; therefore, cultivation is the better alternative. Tissue culture of explants from Acacia senegal produce gums that are different in their properties from gums extracted from the original tree.

Industries that process foods, pharmaceuticals, cosmetics and toiletries are the dominant users of refined aromatics and PSGs. These industries require refined materials that are reliable in supply and consistent in quality and essential properties, all of which are most efficiently provided from selected, cultivated species and from chemical and biosynthetics.

Tropical and subtropical sources, particularly of PSGs from wild species, are increasingly challenged by products of chemical and biosynthesis whose essential properties are more controlled and consistent.

Aromatics and PSGs from agroforestry

Now that it has been noted that chemical and biosynthetics can offer consistent quality, a trend in consumer preference for `natural' over `synthetic' is evident in Europe and North America. The expanding future demand for aromatics and PSGs, particularly by Asian and Latin American industries that serve local consumers, could and should be satisfied from production and processing facilities established within the regions. To continue to export dried and crude extracts from Asia, Africa and Latin America, then to reimport refined extracts from Europe or North America, constitutes economic colonialism. There are several countries of Asia, Africa and Latin America where desirable species could be selected, genetically improved and cultivated in agroforestry systems, and where there exists the scientific and technological capability to maintain and control processing facilities to produce refined extracts of essential and consistent quality.

Herein lies an as yet unrealized opportunity for agroforestry systems. Each system, designed for a prevalent environment, would combine compatible woody and non-woody perennials and annuals, each genotype being a selected productive source of a commercially valuable aromatic or PSG metabolite. The natural flavours, aromatics and PSGs of commerce are derived from tall trees and bushy trees, from climbing vines, from perennial and annual plants of various heights and phenotypic conformations. It should be possible, therefore, to design systems that are highly productive and make the most efficient use of land, water and other essential resources. It is suggested that attention be given first to dryland areas, because an impressive number of species producing aromatics or PSGs are indigenous or adaptable to semi-arid ecological niches. The desert reclamation programme in Egypt has demonstrated the advantages offered by agroforestry systems in arid environments.

A factory equipped to extract and refine essential flavours, aromatics and PSGs could be established adjacent to and integrated with each agroforestry production area. The processing and refining of flavours, aromatics and PSGs employ extractive technologies based upon similar basic principles. As described above, there are common industrial markets for refined aromatics and PSGs. Two or more PSGs are often used in combination to provide a broader spectrum of utility than is possible from a single substance. PSGs are widely employed as protective colloids and as carriers for aromatics. There is therefore logical reason to cultivate suitable species and to extract and refine aromatics and PSGs in neighbouring locations.

The feasibility of what is proposed, particularly for poor dryland areas, may be questioned on several counts, in particular the need for adequate clean water to extract PSGs, and for chemical solvents to extract aromatic oleoresins. Water used for PSG extraction can be microbially purified.

There is a modern alternative, called supercritical gas extraction, which has many advantages over organic solvents for the extraction of essential oils and oleoresins from aromatics. Though the ability of supercritical gases to act as solvents was known early in this century, only as recently as the 1970s was supercritical carbon dioxide first used industrially to extract the essential oils of hops and for the decaffeination of coffee. Extraction with supercritical carbon dioxide does not entail economic difficulties with solvent recovery nor potential toxicological hazards of solvent residues in the aromatic extracts. High-pressure components suitable for relatively small-scale processing units are now available from several industrial engineering companies.

Conclusions

As demands for industrially processed foods, pharmaceuticals, cosmetics and toiletries increase on every continent, so will the consumption of flavours, aromatics and PSGs. PSGs and, in the near term to a lesser extent, aromatics of natural origin, will be in competition with chemical and biosynthetics. Countries that continue to rely on exports of crude extracts and dried tissues of wild species will be at an increasing disadvantage. A growing number of nations, until recently classified as `developing', are capable of cultivating species from which aromatics and PSGs can be extracted and possess the latent scientific and technological resources to establish agroforestry production systems integrated with modern technologies for extraction, refinement and quality control of commercially acceptable aromatics and PSGs. It is believed that particular opportunities exist in regions susceptible to recurrent drought and future desertification.

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