UTILIZATION OF SOYBEANS
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2.1 Utilization options for soybeans
The objective of this chapter is to present a concise overview of the present situation of soybean utilization. Only food uses will be considered. A detailed discussion on the soybean based protein foods and the industrial processes for their manufacture will be the subject of the subsequent chapters.
The amazing number and diversity of utilization alternatives have earned the soybean the title of " the wonder bean". Active research and development efforts continue to expand the already extensive list of soybean products.
The utilization options for soybeans can be divided into two groups: those based on the utilization of the whole seed and those which start with the fractionation of the soybean into oil and meal (oil mill operations). An estimate of the quantitative distribution of the world soybean production according to these main routes of utilization is given in Figure 5. A more detailed, but by no means complete utilization chart is shown in Figure 6.
Figure 5: Main Routes of Soybean Utilization
( From Soya Technology Systems, 1987 )
Figure 6: Soybean Utilization Chart
2.2 Whole bean utilization
Roasted whole soybeans and their flour are used as ingredients of traditional confectionery products and snacks in China, Japan, Korea and Indonesia. Immature whole green soybeans are also consumed as a vegetable. However, mature dry soybeans are seldom used as cooked legume ( such as navy beans, black beans, chick peas or lentils) even in the traditional areas of soybean consumption. The reason for this may have been the persistent bitterness and "green beany taste" of soybeans, the low starch content, the relatively low water adsorption (swelling) capacity, long cooking time and poor digestibility. It is interesting to note that consumption of dry soybeans as a cooked legume is more frequent in western countries than in the Orient. The quantities in question are very small and mainly connected with health food eating tendencies. On the other hand, germinated beans and soybean sprouts, long known in Oriental cookery, are quickly becoming standard vegetable items in western countries as well.
All the traditional routes of utilization of soybeans as food involve some sort of processing or fractionation, intended to overcome the disadvantages mentioned above. Examples of such processes are:
a- Extraction with water as in the production of soymilk.
b- Precipitation from water extracts as in the preparation of soybean curd (tofu).
c- Fermentation processes as in the elaboration of soybean paste (miso), soysauce (sho-yu) , tempeh, fermented soybean curd (su-fu) etc.
Full-fat soy flour, with different degrees of heat treatment, may be considered as the modern, industrial version of the oriental "soybean powder". It is made from dehulled whole beans and used mainly in bakery and dietetic foods. Very finely ground full-fat soybean flour is being offered as an alternative to spray-dried soymilk.
Growing interest in soybean foods in the West has led to the application of modern technology to the manufacture of some of these traditional products ( e.g. novel processes for soymilk production) and the development of totally new products, such as tofu-based ice-cream and soybean yogurt.
2.3 The oil mill route
This option starts with the separation of the soybeans into two fractions: oil and meal. There are, basically,two process alternatives to achieve this purpose: pressing and solvent extraction. Each one of the fractions is then further processed to yield a multitude of products and by-products, with practically no waste. Since oil meal operations are often the starting point in the preparation of soybean protein products, they will be reviewed in some detail in Chapter 3. The processes and products associated with the oil fraction will be described here in some detail. Soybean protein products which branch-off from the meal fraction will be just mentioned here for the sake of completeness and taken up in detail in specific chapters to follow.
2-3-1 Utilization of the oil fraction:
a- Oil refining: The preparation of marketable soybean oil for human consumption from crude soybean oil requires a series of operations known as " refining ". Several alternative technologies are available for each one of these operations. Each one can be carried out in batchwise, continuous or semi-continuous fashion.
The first step in refining crude soybean oil is the removal of the phospholipids, or "degumming". Degumming is necessary in order to prevent the separation and settling of gums (sticky, viscous oil-water emulsions stabilized by the phospholipids) during transportation and storage of crude oil, to reduce oil losses in the subsequent phases of refining and to avoid excessive darkening of the oil in the course of high-temperature deodorization. Crude oil is mixed thoroughly with a small amount of water and an acid ( usually phosphoric acid ). "Gums" are formed and precipitated, carrying in the emulsion a certain amount of oil. They are separated by centrifugation, dried under vacuum and bleached. The resulting product consists approximately of 50% phospholipids and 50% oil and has the consistency of honey.
The phospholipid fraction may be separated from practically all the oil by a series of solvent extraction and precipitation processes. Oil free soybean phospholipids are solid. All these by-products of the degumming process are known as "soybean lecithin" and sold under different trade-names and in a variety of quality grades. The principal quality parameters for commercial lecithins are: phospholipid content ( measured as percent acetone insolubles ), free acidity, non-lipid impurities ( measured as hexane insolubles ), viscosity and colour. For certain applications requiring an extremely bland lecithin, the phospholipids are separated from the crude soybean oil fraction, purified and then redissolved in any desired type of refined oil. Lecithins are mainly used for their activity at the interface between fats and hydrophillic phases. They act as emulsifiers in sauces and salad dressings, as viscosity reducers and stabilizers in chocolate, as anti-spattering agents in margarine, as pan release agents in bakery and confectionery , as dough improvers and staling retardants in bread, as wetting agents in instant food powders etc. They also have some antioxidant property.
Degumming is usually carried out at the extraction plant, even if the subsequent steps of refining are performed elsewhere. Whenever further processing of the crude gums is not economically feasible, due to insufficient plant scale or insufficient market demand, the crude gums can be added-back to the meal, increasing the bulk and caloric value of the latter.
There are two major types of processes for refining degummed oil. They differ in the way the free fatty acids are removed. In the "chemical " or "caustic" refining process,the most common process applied to soybean oil, the fatty acids are neutralized with alkali (sodium hydroxide and sodium carbonate ) to form salts ( soaps ) soluble in water. Treatment with caustic solutions also removes residues of phospholipids not removed by degumming and results in some degree of bleaching due to the destruction of some of the pigments or their adsorption by the heavy phase.
The resulting aqueous soap solution, known as "soap stock" is removed from the neutralized oil by centrifugation. The amount of alkali to be added is calculated according to the free fatty acid content of the oil plus a slight excess (about 0.1%).
Crude soybean oil contains typically 0.3 to 0.7% free fatty acids. After neutralization, the oil is thoroughly mixed with hot soft water to remove traces of soap (washing ), then centrifuged again and dried by heating under vacuum,in preparation to the next step, bleaching. Soap stock can be used for making soap or it can be converted back to fatty acids by treating with a strong mineral acid. The crude mixture of fatty acids obtained, known as "acidified soap stock" can be used as a caloric component in animal feed or for the manufacture of distilled fatty acids. In the "physical refining" process, less commonly applied to soybean oil, fatty acids are removed by steam distillation under high vacuum, simultaneously achieving deodorization. Oil for physical refining must be degummed more thoroughly than in the case of alkali refining process.
The next step of refining is "bleaching'. Its purpose is to remove the yellow-orange carotenoid pigments and the green chlorophyll of the oil. The extent of bleaching depends on market requirements. The market in the U.S.A. requires almost water-clear appearance while somewhat darker colour may be perfectly acceptable or even preferred in other markets. Bleaching is carried out by treating the oil with solid adsorbents such as Fuller's earth or activated carbon or both. The pigments and some other impurities are adsorbed on the solid surface and removed by filtration. In order to prevent oxidation, the process is carried out under vacuum. Continuous "in-flow" bleaching processes are available.
The last refining operation is "deodorization". It consists in the removal of odorous substances by steam distillation under high vacuum and at temperatures in the range of 2500 C. Typically,the deodorizer is a vertical cylindrical vessel with internal baffles and other devices to ensure exposure of a large surface area of oil and intimate contact between the oil and steam. At the end of the stripping process, the oil must be cooled while still under vacuum to prevent oxidation. Citric acid is usually added in order to chelate any metal ions which may catalyze peroxide formation. In modern deodorizers, all the parts in contact with oil are made of stainless steel to prevent such metal contamination. While the main objective of deodorization is the removal of odour-bearing compounds such as aldehydes, ketones and hydrocarbons, other substances such as sterols and tocopherols are also distilled off. In physical refining, this operation is responsible for the removal of free fatty acids. All these substances may be recovered from the deodorizer condensate stream, if necessary.
b- Further processing and utilization of refined soybean oil: Freshly refined soybean oil is practically odourless and bland. However, objectionable off-flavour described as "green, grassy,fishy" is known to develop quickly if the oil is heated (as in cooking and frying), or stored under conditions which expose it to light and oxygen or permits contamination with certain metals such as copper and iron.
This type of flavour deterioration has been called "flavour reversion", expressing the thought that it brings back the off-flavours of crude oil. Although this has been shown to be false, the term of "flavour reversion" is still used sometimes, when referring to the flavour deterioration of refined soybean oil. The process is apparently triggered by the oxidation of the unsaturated fatty acids and most particularly that of linolenic acid. Unlike oxidative rancidity, flavour reversion occurs at very low levels of oxidation and is not retarded appreciably by antioxidants. It can be retarded by minimizing exposure to oxygen ( bottling under nitrogen ) and to light ( opaque containers, dark glass bottles).
Another method of flavour stabilization is the reduction of the linolenic acid content by selective hydrogenation, followed by chilling ( winterization ) to remove the high melting point saturated fatty acids formed. The partially hydrogenated- winterized soybean oil is perfectly suitable as an all-purpose (salad and cooking) oil. The crystalline fraction separated after chilling is known as "soybean stearin" and used in different solidified fats.
More complete hydrogenation of soybean oil is the basis for the manufacture of shortenings, margarines and tailor-made fats used by various food industries.2-3-2 Utilization of the meal fraction:
a- Soybean meal as animal feedstuff: By far the largest portion of the soybean oil meal and cake production is used as a protein source in animal feed. Although the terms "meal" and "cake" are often used interchangeably, meal refers to the product of solvent extraction, while cake is the product resulting from expeller pressing of soybeans. The different types of soybean meals are characterized mainly by their protein content and the extent of heat treatment applied in their production to inactivate anti-nutritional factors. If the soybeans are extracted without dehulling,or if the hulls are added back after extraction, the meal will contain about 44% protein. Meals produced from dehulled beans contains approximately 50% protein.
The extent of heat treatment or toasting is measured in terms of residual urease activity or as the solubility of the protein under specified conditions ( Nitrogen Solubility Index NDI, or Protein Dispersibility Index PDI ).
The optimal degree of toasting depends on the final application. Thus, meal for poultry rations must be toasted much more thoroughly than meal for use in cattle feeds. Considerable efforts have been made to develop in vitro laboratory tests capable of predicting the nutritional performance of soybean meal in feed rations. The most widely used methods are: urease activity, trypsin inhibitor, dye-binding, fluorescence, protein solubility in water or alkali and available lysine. All these tests refer to the heat treatment history of the meal.
b- Defatted soybean flours and grits: These products, intended for human consumption, are essentially soybean meal which has been ground to the appropriate mesh size. The starting material is dehulled beans and strict sanitary requirements are applied to processing, storage and packaging conditions, in order to secure the microbiological quality of the final product ( e.g. total microbial count ). In addition, a large variety of products, differing in their lipid content are produced by adding-back soybean oil and/or lecithin to defatted flour or grits at specified levels (refatting).
c- Soybean protein concentrates: Products containing about 70% protein are prepared from defatted meal by selective extraction of the soluble carbohydrates (sugars). Extraction with aqueous alcohol is the most common process, but other methods of production are available. The concentrates are essentially bland.
d- Soybean protein isolates: Even higher concentrations of protein, in the order of 96%, are obtained by selective solubilization of the protein ( e.g. alkaline extraction ), followed by purification of the extract and precipitation of the protein ( e.g.by acidification to the isoelectric point ). Isoelectric isolates are insoluble in water and have practically no functional features. They can be converted to sodium, potassium or calcium proteinates by dissolving isoelectric protein in the appropriate base and spray-drying the solution. Sodium and potassium proteinates are water soluble. They are used mainly for their functional properties, such as emulsification or foaming. One of the by-products of the protein isolation process, the insoluble residue, is also commercialized for its remarkable water absorption capacity and as a source of dietary fibre.
e- Extrusion-textured soybean protein: If defatted soybean flour with a specific moisture content is subjected to high shearing forces at high temperature in an extruder, a product with a peculiar laminar structure is obtained. After hydration, this product presents an elastic and chewy texture resembling that of meat. The product is known as "textured soybean protein" or "textured vegetable protein" (TVP).
TVP with higher protein content is made by extrusion of soybean protein concentrates.
f- Spun fibres of soybean protein: The well established technologies for making synthetic fibres can be applied to soybean protein. Isolated soybean protein is dissolved in strong alkali and the solution is allowed to age until it has the consistency of honey. The viscous liquid, known as "dope" is then injected into an acid bath, whereby the protein precipitates in the form of fine fibres. The fibres are stretched, washed and collected as bundles. Spun fibres of soybean protein are used in the manufacture of a variety of meat analogs, to which they impart the fibrous aspect and bite of animal muscle.REFERENCES
Erickson,D.R., E.H.Pryde, O.L.Brekke, T.L.Mounts and R.A.Falb
"Handbook of Soy Oil Processing and Utilization"American Soybean Association, St. Louis, MO.
Snyder, H.E. and T.W.Kwon (1987)
"Soybean Utilization" Van Nostrand Reinhold Company, New York
Soy Protein Council (1987)
"Soy Protein Products" Soy Protein Council, Washington, DC
Watanabe, D.J. and A. Kishi (1984)
"The Book of Soybeans"
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