5.1 Introduction

Edible soybean protein concentrates are relatively new products. Their availability as commercial products dates from 1959. In the last 30 years or so, these versatile products have become important ingredients, well accepted by many food industries. In many applications, they simply replace soy flours. In others, they have specific functions which cannot be performed by soy flours.

Historically,the need for the development of soybean protein concentrates stemmed primarily from two considerations: to increase protein concentration and to improve flavour.

It is very difficult to avoid the occurrence of the green-beany flavour of soybeans in untoasted full-fat or defatted soy flour, prepared in the conventional way. Beany flavour is one of the major objectionable characteristics, limiting the use of conventional soy flours. One of the objectives of the further processing of flours into concentrates is to extract the particular components which are responsible for the bitterness and beany taste.

As shown in the previous chapter, the maximum level of protein content in soy flour, even after nearly complete removal of hulls and oil, is about 55% (moisture-free basis). In certain applications, such as in meat products, a soybean protein ingredient with a higher percentage of protein is often preferable.

Soybean protein concentrates normally cost 2 to 2.5 times more than defatted soy flour. Considering the relative protein contents of these two products , the cost per unit weight of protein is about 80% higher in the concentrate.

The starting material for the production of soy protein concentrates is dehulled, defatted soybean meal with high protein solubility (white flakes). The concentration of protein is increased by removing most of the soluble non-protein constituents. These constituents are primarily soluble carbohydrates (mono, di and oligosaccharides), but also some low molecular weight nitrogenous substances and minerals. Normally, 750 kilograms of soybean protein concentrate are obtained from one metric ton of defatted soybean flakes.

There are three major methods for extracting these components in a selective manner, without solubilizing the major protein fractions. These are not different methods for manufacturing the same product, but each method produces a different type of concentrate, with distinct characteristics and specific uses. These methods are known as:

* The aqueous alcohol wash process
* The acid wash process
* Heat denaturation/water wash process

5.2 Defintion, compostion, types

The Association of American Feed Control Officials, Inc. (AAFCO), specifies soy protein concentrates as follows:

" 84.12: Soy Protein Concentrate is prepared from high quality sound, clean, dehulled soybean seeds by removing most of the oil and water soluble non-protein constituents and must contain not less than 70% protein on a moisture free basis."
( from the '89 Soya Bluebook.)

Following is the composition of a typical food-grade soy protein concentrate ( SOLCON, made by Solbar Hatzor Ltd.) as specified by the manufacturer:

As explained above, there are three basic types of soy protein concentrates, distinguished according to the method used for extraction of the non-protein solubles. All three types have basically the following proximate composition, on a moisture-free basis:

Soy protein concentrates are further characterized by their protein solubility index. Soy proteins are rendered insoluble by each of the three extraction processes. However,it is possible to increase the solubility of the protein in the concentrate by further processing, for example by neutralization of acid washed concentrate with alkali. Concentrates made by heat denaturation/water leaching processes are irreversibly denatured and darker in colour. Alcohol-wash concentrate has a low NSI values (10 to 15%) due to denaturation of the protein by the aqueous alcohol. The molecular changes in the proteins caused by alcohols are, however, different from those resulting from heat denaturation. Thus, alcohol-wash concentrate retains most of the functional properties ( slurry viscosity, emulsification power etc.) despite its low protein solubility as determined by the standard NSI or NDI tests.

The dispersibility and functionality of alcohol-wash concentrates can be increased by steam injection or jet-cooking, and improved further by high-shear homogenization. ( Soy Protein Council 1987).

Much of the characteristic beany flavour is also usually removed by the extraction process. Soybean protein concentrates are relatively bland. The flatus-producing oligosaccharides of soybean flour,raffinose and stachyose, are also efficiently removed by the solvents used in the production of concentrates.

Soy protein concentrates are marketed in various forms: granular, flour and spray dried. In addition, texturized concentrates are also available. These texturized products will be discussed in a separate chapter.

Since some low molecular weight proteins are also extracted along with the sugars, the amino acid composition of the concentrates may differ slightly from that of the original flour. (Table 5-1).

Source: Campbell et al. (1985)

5.3 Production processes

5-3-1 The aqueous alcohol wash process

The process is based on the ability of aqueous solutions of lower aliphatic alcohols ( methanol, ethanol and isopropyl alcohol) to extract the soluble sugar fraction of defatted soy flour without solubilizing its proteins. The optimal concentration of alcohol for this process is about 60% by weight.

The theory of solvent extraction (see para. 3-2-4)is applicable to the extraction of defatted soy flour with aqueous alcohol.

Starting with defatted white flakes as raw material, the process consists of the following steps: Liquid-solid extraction, removal and recovery of the solvent from the liquid extract, removal and recovery of the solvent from the extracted flakes, drying and grinding of the flakes.

a- Solid-liquid extraction: This can be carried out batchwise or continuously. Continuous extraction is justified for relatively large scale operations. According to Campbell et al.(1985), continuous processes are employed for plants with typical capacities over 5,000 tons per year. Unlike oilseed crushing industries, smaller plants are not uncommon in this branch. The batch process is, therefore, rather widely applied. The methods and types of equipment used are essentially similar to those encountered in oil extraction plants: horizontal belt and basket extractors, stationary and rotary cell extractors etc. In the case of alcohol extraction, the solvents are quite volatile and flammable. Adequate precautions for the prevention of fire and explosion are necessary.

The reason for using high-NSI white flakes as the starting material is not necessarily related to the objective of obtaining a product with high protein solubility.( As explained above, this would not help anyhow , due to the different type of protein denaturation caused by the alcohol.) The principal reason for preferring this type of raw material is due to the fact that the percentage of extractable soluble sugars in white flakes is higher than in toasted meal. Toasting renders the sugars less soluble by binding them to proteins (Maillard reaction) or by caramelization. As a result of this type of condensation reactions, the sugars are no longer extractable by the solvent and they remain in the product, lowering the protein concentration in it. Furthermore, the darker colour of concentrates made from overheated meal is also objectionable, and their nutritional value is lower (lower lysine availability.)

b- Removal and recovery of the solvent from the liquid extract: The alcohols are removed from the liquid extract by evaporation and rectified by distillation. They are then brought to the proper concentration and recycled through the extractor. The distillation residue is an aqueous solution of the sugars and other solubles. It is concentrated to the consistency of honey and sold as " soy molasses". Typically, soy molasses contain 50% total soluble solids. These solids consist of carbohydrates (60%), proteins and other nitrogenous substances (10%), minerals (10%), fats and lipoids (20%). It is mainly used as a caloric ingredient and as a binding agent in animal feeds.

c- Desolventizing the solids: After extraction, the solvent saturated flakes are desolventized . The methods are essentially the same as for the removal of hexane from soybean meal flakes. Flash desolventizing, using superheated vapours of the alcohol-water mixture can be applied to protein concentrates. Any excess water left in the flakes after desolventizing is removed by hot air drying.

d- Grinding: The methods and equipment used to grind soy protein concentrate flakes are essentially the same as those employed in the production of soy flours (see Section 4-3-1).

5-3-2 The acid-wash process

This process is based on the pH-dependence of the solubility of soybean proteins,discussed in Section 1-6-2. It will be recalled that the majority of soybean proteins exhibit minimum solubility at pH 4.2 to 4.5 (isoelectric region). Therefore, it is possible to extract the sugars, without solubilizing the majority of the proteins, using, as a solvent, water to which an acid has been added so as to keep the pH at the isoelectric region.

The acid-wash process has the obvious advantage of using a non-flammable, non-explosive, non-toxic and inexpensive solvent: water. To a certain extent, this is also the disadvantage of the process. Separation of the solid from the solvent is more difficult and less complete, due to the fact that the flakes absorb considerable quantities of water and swell. Gravity draining is not suitable for efficient solid-extract separation. Rotary vacuum filters or decanting centrifuges must be used instead.

A batch process using horizontal decanting centrifuges is shown in Fig. 29. Defatted soy flakes or flour are mixed with acidified water in an agitated vessel. The slurry is then fed to the decanter centrifuge which separates the extracted solids from the extract (whey). The solids are discharged continuously at approximately 30% dry matter content. The solids can be dried at this stage, to yield an "isoelectric" concentrate of low protein solubility. If a more functionally active, neutral concentrate is desired, the isoelectric solid cake is resuspended in water and the acidity is neutralized. A second step of centrifugal separation gives a cake of neutral concentrate with a protein content of 75% on dry matter basis. This cake also retains about 70% water, by weight.

The cake is usually wet-milled to a fine slurry and spray dried. The protein solubility of the neutralized product is quite high, giving NSI values above 60%, provided that white flakes were used as the starting material.

The liquid extract containing sugars ,minerals, the protein fractions which are soluble at pH 4.5, and other soluble components is usually known as "whey", in analogy to the process of cheese making. Unlike cheese whey, however, soy whey has no use and must be discarded as waste. The reasons for not using soy whey for animal feeding will be discussed in the next chapter, dealing with isolated soybean protein.

5-3-3 Heat denaturation/ water extraction process

In this process, the proteins of defatted soy meal are first rendered insoluble by thermal denaturation, using humid heat. The heat-treated meal is extracted with hot water, which dissolves the sugars.

Since the process is based on solid-liquid extraction using water as the solvent, the operations and equipment systems are essentially similar to those used in the acid-wash process, discussed in the previous paragraph.

5.4 Utilization

5-4-1 Basic considerations

Just as with soy flours, soy protein concentrates are used in food products for their nutritional characteristics or for their functional properties or for both.

Nutritionally, the attractive features of concentrates include: their high protein content, the near-absence of anti-tryptic and other anti-nutritional factors, the absence of flatulence and the substantial "dietary fibre" content. The nutritional value of the protein in the concentrates of different types, expressed as Protein Efficiency Ratio (PER) is slightly lower than that of soy flour protein. (Table 5-2). This is probably due to the slight fractionation effect of the extraction process, mentioned above.

(*) The PER values corrected to: casein = 2.5
Source: Soy Protein Council (1987)

The most important functional characteristics of soy protein concentrates are: water binding (water adsorption) capacity, fat binding capacity and emulsification properties.

5-4-2 Use in bakery products

Unless higher protein fortification levels are necessary, there is no special reason for using soy protein concentrates in bakery products. Nutritionally and functionally, soy flours do the same job, more economically.

5-4-3 Meat products

This area probably represents the most important application of soy protein concentrates in the food industry. SCP is used mostly in comminuted meat, poultry and fish products ( patties, emulsion type sausages, fish sticks etc.) to increase water ant fat retention. The nutritional contribution of soy protein in low-meat, high-fat, low-cost products may also be significant. Typical usage levels, on moisture-free basis, are: 5-10% in patties, 2-8% in chili, 2-12% in meatballs, 3.5% max. in sausages, 5-10% in fish sticks. (Campbell et al. 1985).

5-4-4 Other uses

Soybean protein concentrates have been used as stabilized dispersions in milk-like beverages and simulated dairy products such as sour cream analog. Campbell et al.(1985) present a formula for a milk-like beverage, suggested by A.E. Staley Mfg. Co., producers of the soy protein concentrate and the corn syrup solids components in the formula. The formula and directions for the preparation of the beverage are given below:

Formula for "Soy Concentrate Milk":

The SCP is hydrated with water in a high-shear mixer, then all other ingredients, except the fat are added and mixed thoroughly. The mixture is heated to 65-70^oC. The fat (apparently a hydrogenated, well deodorized oil) and flavouring agents are added. The mixture is homogenized, cooled and packaged.

Non-dairy coffee whiteners can also be made, using the same principle, but different ingredients and proportions.

ALFA-LAVAL, (1990)
Commercial Communication Alfa-Laval Sharpless Ltd., Camberley, U.K.

Campbell, M.F.; C.W. Kraut; W.C. Yackel and Ho Seung Yang (1985).
"Soy Protein Concentrates", in "New Protein Foods", A.M. Altschul and H.L. Wilke, Editors. Vol. 5, p. 301. Academic Press, Inc. Orlando, Florida.

Ohren, J.A. (1981)
Process and Product Characteristics for Soya Concentrates and Isolates. J. Amer. Oil Chem. Soc. 58: 333

Sair, L. (1959)
U.S.Patent 2,881,076

SOLBAR HATZOR (1991)
Commercial Communication Solbar Hatzor Ltd., Ashdod, Israel.

Soy Protein Council (1987)
"Soy Protein Products, Characteristics, Nutritional Aspects and Utilization."Soy Protein Council, Washington DC

Soya Bluebook (1989)
Soyatech, Inc. Bar Harbor, ME.

Waggle, D.H., C.D. Decker and C.W. Colar (1981)
Soya Products in Meat, Poultry and Seafood J. Amer. Oil Chem. Soc. 58: 341