For many years, the newly developed soy protein products did not make much progress in occupying a central position in the global protein nutrition picture. The first processed soy protein products were mainly flours or powders which had to be "concealed" in existing foods such as bread, pasta or beverages. The objective of a great part of the research effort was to render these powders sufficiently flavourless and white, and to counteract any change in the accepted characteristics of the "host" food caused by the incorporation of soy protein products at nutritionally and economically significant levels. A breakthrough in the utilization volume occurred in the 1960s, when textured soy protein products of acceptable quality became increasingly available.

Applied to soy protein products, the terms "texturization or texturing" mean the development of a physical structure which will provide, when eaten, a sensation of eating meat. Meat "texture" is a complex concept comprising visual aspect (visible fibres), chewiness, elasticity, tenderness and juiciness. The principal physical elements of meat which create the texture complex are: the muscle fibres and the connective tissue.

A voluminous patent and research literature on vegetable protein texturization has accumulated.( See e.g.Gutcho, 1977). In fact, a meat analog based on wheat gluten was being used for institutional feeding already before the start of our century. A concept of a soy protein based chewy gel and processes for its production have been described in several patents in the late 1950s. ( e.g. Anson and Pader 1957). These inventions produced homogeneous, isotropic ( unoriented, of equal structure in all directions) gels, which had only one of the elements of meat texture: chewiness. They had limited commercial success.

The more successful approaches to soy product texturization can be classified in two categories. The first approach tries to assemble a heterogeneous structure comprising a certain amount of protein fibres within a matrix of binding material. The fibres are produced by a "spinning" process, similar to that used for the production of synthetic fibres for the textile industry. The second approach converts the soy material into a hydratable, laminar, chewy mass without true fibres. Two different processes can be used to produce such a mass: thermoplastic extrusion and steam texturization.

It should be noted that the term "meat" is used here in the wide sense of "flesh food", and includes not only red meat but also poultry, fish and seafood.

The starting material for spun fibres is isolated soybean protein. In contrast, extrusion or steam texturized soy products can be made from flour, concentrate or isolated protein.

7.2 Spun-fibre based texturizationn

The process for the production of textured soy products containing spun protein fibres was first described in a 1954 patent issued to Boyer. Since then many additions to and modifications of the basic concept have been suggested. The basic flow-diagram of the process is shown in Fig. 34.

The first part of the flow diagram describes the steps for the production of isoelectric isolated soybean protein. These steps can be omitted if commercial ISP is used as the starting material. A concentrated protein solution is prepared by adding alkali to the ISP slurry. The solution , containing approximately 20% protein at pH 12 to pH 13 is "aged" ( to permit unfolding of the protein molecules) until its viscosity rises to the consistency of honey (50,000 to 100,000 centipoise).This viscous concentrated protein solution is technically known as "dope".

The next step is the transformation of the dope into distinct, stretched fibres (spinning) by coagulating fine jets of the solution in an acid bath.The "dope" is pumped into the coagulating bath through a spinneret, which is a plate with thousands of fine holes (about 75 microns in diameter). The bath contains a solution of phosphoric acid and salt, maintained at pH of about 2.5. As the jet of "dope" contacts the acid medium, the oriented protein molecules are suddenly coagulated and form a fibre. The fibres are picked up as a "tow" and stretched to enhance molecular orientation and increase fibre strength. Stretching reduces the diameter of the fibre well below that of the holes on the spinneret.

The tows of fibre pass through a step of washing, to remove excess acidity and salt. The subsequent operations depend on the final product. Soy protein fibres are only one ingredient of the meat-like structure. The other ingredients include fat, binders, colouring and flavouring additives etc. The nature of these ingredients, the proportion of fibres and their orientation in the binder matrix depend on the type of flesh food to be imitated. The binder matrix contains heat-coagulable components, commonly egg albumen and the final structure is usually stabilized by thermal setting.

Spun fibre based textured soy products have been used as "total" meat analogs (i.e. to replace meat totally) and as meat extenders ( i.e. to replace part of the meat in ground meat, patties etc.) Some of the products have been used in institutional feeding (hospitals) and in school lunch programs.

The main shortcoming of spun fibre type texturized products is their cost. In the first place, the process requires an expensive starting material: isolated soybean protein. Furthermore,t he process in itself is also costly, both in initial capital investment and in running expenses.

Today, there are very few producers of spun soy protein fibres and textured products containing them. The most successful spun fibre based meat analog has been the imitation bacon chip. This is a shelf-stable low-moisture product with the bite, chewiness and flavour of fried or roasted bacon bits and is used extensively in salads, snacks and garnishes. At present, however, this product too faces the competition of imitation bacon made by the less expensive extrusion texturization technique.

7.3 Extrusion texturization

Extrusion has been long used as a central unit operation in the plastic polymer industry. Their use for continuous pressure-cooking of flours and particulate feed materials has been advocated in the 1950s. A decade later, Mc.Anelly (1964) described a process for the production of spongy, elastic particles from soy flour. A mixture of defatted flour and water was extruded through a food grinder. The extruded strands were heat-set in an autoclave, chopped, leached with hot water and dried. Although this invention can be considered as the forerunner of the extrusion texturization processes, the breakthrough in this field was the disclosure of a continuous cooking-extrusion process, for which a patent was awarded to Atkinson in 1970. In this process, defatted soy flour containing a certain amount of water is passed through a high-pressure extruder-cooker to produce an expanded, porous, somewhat oriented structure described as "pleximellar". Although devoid of true fibres, the product possessed the textural characteristics of chewiness and elasticity, and was deemed to imitate meat in this respect. Extrusion texturized soy flour soon became an established food ingredient known as TVP ( Textured Vegetable Protein ) or TSP (Textured Soy Protein).

The extruder consists basically of a sturdy screw or worm rotating inside a cylindrical barrel (Fig. 35). The barrel can be smooth or grooved. The screw configuration is such that the free volume delimited by one screw flight and the inside surface of the barrel decreases gradually as one goes from one end of the screw shaft to the other.

As a result of this configuration, the material is compressed as it is conveyed forward by the rotating screw. Screws having different compression ratios are used for different applications. The barrel is usually equipped with a number of sections of steam heated jackets or induction heating elements or cooling jackets. A narrow orifice or "die" is fitted at the exit end of the barrel. The shape of the die opening determines the shape of the extruded product.

Defatted soy flour with a high protein solubility index is first conditioned with live steam, before entering the extruder proper. Well controlled conditioning is essential for good texturization and product uniformity. The moisture content of the feed is very important. A moisture level of about 20-25% is used for texturization. The conditioned flour usually assumes the form of small spheres.

The flour-water mixture is next fed into the extruder and picked up by the screw. As it advances along the barrel, it is rapidly heated by the action of friction as well as the energy supplied by the heating elements around the barrel. The high pressures attained through the comression mechanism explained above permits heating to 150-180°C. This rapid "pressure cooking" process transforms the mass into a thermoplastic "melt", hence the name of "themoplastic extrusion" by which the process isalso known. The directional shear forces causes some alignment of the high molecular weight component while the proteins undergo extensive heat denaturation. The sudden release of pressure causes instant evaporation of some of the water and "puffing". The result is a porous, laminar structure. Puffing and therefore porosity can be controlled by monitoring melt temperature at the die. If a dense product is desired, the melt is cooled at the final section of the barrel, just before entering the die.

The extrudate is cut continuously by a rotating knife as it emerges from the die. It may be dried and sold as a shelf-stable product, or it can be hydrated, flavoured, mixed with other ingredients, shaped and marketed, usually, as a frozen food.

While texturizing the soy material, extrusion cooking also provides the heat treatment necessary to reduce the microbial load and to inactivate the trypsin inhibitor. It should be noted that, despite the high temperatures in the extruder, trypsin inhibitor inactivation may be incomplete, due to the relatively short processing time.

The so-called low-cost extruders which have been mentioned in connection with the continuous heat treatment of full fat soy flour or corn-soy-milk (CSM) food supplements are not suitable for texturization. These extruders work with low-moisture feeds and provide heat mainly by friction. The extrusion-cooking machines used for texturization are more sophisticated and expensive. Recently, double-screw food extruders have been replacing the older single-screw models in food processing applications. In double-screw extruders a considerable part of the mixing and friction-heating effect takes place between the screws. The shafts can be fitted with interchangeable screw elements, providing different processing profiles along the extruder.

Extrusion texturized soy flour has been called "the first generation TVP". Being made of flour, it has the composition and flavour of heat treated soy flour. The flavour is intensified by retorting. It contains the sugars of soy flour and presents the problem of flatulence. Usage directions usually prescribe a reconstitution step of soaking in water and pressing to remove the soluble components. More recently, processes have been developed for the texturization of soy protein concentrates. Textured concentrates (second generation TVPs) are now widely available.

Table 7-1 compares the characteristics of texturized soy products, according to the starting materials from which they are made.

Source: Campbell (1981)

Since nothing is removed or added in extrusion texturization, the composition of texturized products , on a dry matter basis, is essentially the same as that of the starting material. Shelf stable dry products are usually marketed at a moisture level of 8%. Texturized soy products made from concentrate do not need to be leached and can be used directly, after proper hydration.

7.4 Steam texturization

Several processes have been described in the patent literature for texturizing soy protein by thermal coagulation coupled with some form of shear induced orientation to provide a fibrous-like structure. In one of these processes, patented by Stromer and Beck (1973), moistened soy flour is fed continuously into a pressurized reactor where it meets high pressure steam (at about 7-8 atmospheres). The thick mass flows, under the action of pressure, through a cylindrical barrel the discharge end of which is open to the atmosphere. The process was sold to one of the leading manufacturers of soy protein products and commercially applied for some time. According to Snyder and Kwon (1987), it is no longer being used.

7.5 Utilization

7-5-1 Meat extenders

The principal use of texturized soy protein products is as a meat extender in comminuted meat product such as patties, fillings, meat sauces, meat balls etc. In such products, as much as 30% of the meat can be replaced by hydrated texturized soy products without loss of eating quality. The cost of textured soy flour is approximately 0.60 U.S. Dollars per kilogram. About 3.5 kilograms of hydrated base is obtained from each kilogram of textured flour. Thus, the cost of meat replacement is only 17 cents of a dollar for each kilogram of meat saved. Furthermore, textured soy products offer not only economic savings but also certain types of product improvement. Their ability to absorb water and fat results in increased product juiciness and permits the use of meat with higher fat content.

Ground beef extended with TVP has been used extensively in school lunch programs, with good results.

The property of TVP to withstand cooking in a retort (retortability, retort stability) is relevant to its use in canned luncheon meat, meat loaf and similar products.

7-5-2 Meat analogs

Chunks of extrusion texturized soy protein products and spun fibre based preparations are marketed as "imitation meat" or "meat analogs". The market for these products was, at first, limited to the relatively small sector of vegetarians. Recently there is a marked trend to reduce the consumption of red meat, associated with the demand for low-cholesterol foods. At the same time, the industry has been successful in developing more attractive meat analogs made from rehydrated textured soy proteins, alone or in combination with wheat gluten. These products are marketed as flavoured, fully prepared, frozen ready-to eat entrées. The present marketing strategy for meat analogs is to present them to the public as new, high quality products, and not as inexpensive substitutes for meat. So far, this strategy seems to be successful. The market for these sophisticated (and by no means inexpensive) products is rapidly expanding, particularly in Western Europe.

7-5-3 Other applications

Imitation bacon bits based on texturized soy protein products have been mentioned earlier. The price range for this product is 1.50 to 2 U.S. Dollars per kilogram.

A pasta product containing texturized soy protein granules is being offered on the retail market, in addition to its use in institutional feeding.

REFERENCES

Anson, M.L. and M. Pader (1957)
U.S. Patent 2,813,025. 

Atkinson, W.T. (1970)
U.S. Patent 3,488,770.

Boyer, R.A. (1954)
U.S. Patent 2,682,466.

Campbell, M.F. (1981)
Processing and Product Characteristics for Textured Soy Flours, Concentrates and Isolates. J. Amer. Oil Chem. Soc. 58: 336

CARGILL INC. (1991)
Commercial Communication Cargill, Incorporated, Cedar Rapids, Iowa

Gutcho, M.H. (1977)
"Textured Protein Processing" Noyes Data Corp. Pea Ridge, N.J.

Horan, F.A. (1974)
Meat Analogs in "New Protein Foods", Vol. 1A, 367. M. Altschul, Ed. Academic Press, New York

Horan, F.A. (1976)
Meat Analogs, a Supplement in "New Protein Foods", Vol. 2, 260 , M. Altschul, Ed. Academic Press, New York

McAnelly, J.K. (1964)
U.S. Patent 3,142,571.

Snyder, H.E. and T.W. Kwon (1987)
"Soybean Utilization" Van Nostrand Reinhold Co. New York

Strommer, P.K. and C.I. Beck (1973)
U.S. Patent 3,754,926.