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Among plant-based foods, soy is fairly unique, because the protein in soy products is considered one of the most complete proteins. Acceptable in almost all diets, soy proteins contain virtually no cholesterol, are lactose-free and relatively low in saturated fat. Easily digested by humans, these proteins may provide a number of health benefits, reduce the cost of food production and impart functional benefits to numerous food products. The increased acceptance of soy protein is due to its versatility and functionality in food applications. Soy’s high protein content makes it a valuable component for formulated foods.
Functional properties of proteins can be defined as physic-chemical properties and their interactions with other food components. Important in determining the quality of the final product, these functional properties also assist in facilitating processing (e.g., improved “machinability”), as in the case of cookie dough. To utilize soy ingredients effectively, food processors should have detailed information on the methods of preparation and processing of soy products, because these affect the composition and functional properties of the component proteins (Kinselle, 1979).
From a functional point of view, soy flour, soy concentrates and soy isolates are the best way to add soy protein to different food systems. Soy protein also can enhance the textural and nutritional properties of different food and beverage formulations. Currently, more than 20 different soy protein ingredients (other than soybean oil and its products) are commercially available for food uses.
Some of the ingredients are: 1) roasted soy nuts; 2) enzyme-active, full-fat soy flour and grits; 3) enzyme-inactive, full-fat soy flour and grits, also called toasted, full-fat soy flour and grits or heat-treated, full-fat soy flour; 4) extruder-processed, full-fat soy flour and grits (enzyme-inactive); 5) enzyme-inactive, low-fat soy flour or grits; 6) enzyme-active, defatted flake/soy flour (90 Protein Dispersibility Index [PDI]); 7) defatted soy flake/flour with 70PDI; 8) defatted soy flake/flour with 20PDI; 9) lecithinated soy flour; 10) textured soy flour; 11) refatted soy flour; 12) soy concentrates; 13) textured soy concentrates; 14) soy isolates; 15) soy germ; 16) soy isoflavones mechanically extracted; 17) soy isoflavones chemically extracted; 18) fiber from hulls; 19) soy fiber from cotyledon; and 20) organic soy flour and concentrates.
All of these ingredients are being used in different food systems for either nutritional or functional reasons. For example, soy concentrates manufactured from an aqueous-alcohol wash will have low solubility and low PDI (10-15). This type of soy concentrate is recommended for extrusion purposes. Even though PDI is low, these products retain most of the functional properties of higher PDI materials. In contrast, acid-/water-washed soy protein concentrate will have high solubility (PDI>80) and is used for specialty applications.
Function of Soy Ingredients in Food FormulationTo utilize soy protein in formulations, food processors must understand the soy protein’s functional properties, as they will greatly affect the processing, textural and nutritional qualities of the finished products. If improperly processed, soy protein can negatively affect the taste, texture or physical appearance of the finished product. For example, if too much soy flour is added to bread, the bread loaf may shrink, and the texture could become gummy and dense.
Manufacturers choose soy ingredients for specific food applications. In terms of flavoring, food processors need to keep in mind that flavors should be compatible with soy protein. For example, brown ingredients work well in soy beverages, such as those featuring chocolate and caramel. Taste is the most important factor when using soy protein in food and beverage formulations, and a small amount of salt can help reduce aftertaste in soy beverages. Food formulators should work with a flavor company to identify appropriate flavors and tastes before using soy protein in their products.
Soy ingredients have many functional properties in food systems. A few will be discussed in detail in this article.
SolubilityProtein solubility is a measure of the percent of total protein soluble in water under controlled conditions, and it is a measure of the degree of heat treatment to which the soy flake has been subjected. Protein solubility is closely related to the functional properties needed for bakery food applications.
Several methods are used in determining protein solubility, the main tests being PDI, Nitrogen Solubility Index (NSI) and the Protein Solubility Index (PSI). Each of these tests indicates the percentage of total soluble nitrogen in water, with a range of values from 0-100 (Dubois, 1980). PDI decreases with higher levels of heat treatment.
Solubility is one of the most basic physical properties of proteins and a prime requirement for any functional application. To obtain optimum functionality in uses where gelation, solubility, emulsifying activity, foaming and lipoxygenase activity are required, a highly soluble protein is needed. Soluble protein preparations are also easier to incorporate into foods than less-soluble alternatives. Proteins with low-solubility indices are limited in their functional properties and uses.
The solubility of soy protein is significantly affected by treatments used during its production. Heat treatment, especially moist heat, rapidly insolubilizes soy protein. However, heat treatment is necessary to desolventize, to inactivate anti-nutrient compounds and to improve the flavor of soy flours. Non-heated soy flours, while possessing high lipoxygenase activity, have a bitter, beany flavor that limits their applications.
To compromise between enzyme activity, flavor quality and solubility criteria, processors produce defatted soy flours with a range of solubilities. Concentrates and isolates are prepared from minimum heat-treated flours and generally possess good solubility. Most soy protein suppliers provide different ranges of soy flour, concentrates and isolates, based upon their solubility and application.
Similar to whey proteins, processes such as agglomeration, lecithination and changes in the protein solubility will improve soy protein dispersibility in dry-mix applications. When formulating dry blends of beverages (low- and high-acid beverages) with soy protein, one must consider the different specifications for mouthfeel, viscosity, dispersibility, density, etc. Specifically, lower solubility will improve the dispersibility of the protein. In a liquid beverage, the soy protein must be properly hydrated to achieve the desired results. Agglomeration makes soy protein highly soluble and also makes it possible to develop a new generation of soy-based ingredients.
The more dispersible types of soy flours (with a high NSI or PDI) are used in bakery and cereal products—by adding them directly to the dough (Endres, 2001). Enzyme-active soy flour has a minimum water solubility of 70% (Pringle, 1974). Soy flours with minimum heat treatments (80PDI) show high lipoxygenase activity and are used at 0.5% to bleach flour and improve the flavor of bread. Flours with 60PDI are most commonly used in breads (1-2%), and waffles and pancakes (10%), where they markedly improve water-binding in these products (Kinselle, 1979; Hettiarachchy and Kalapathy, 1997).
Emulsification and FoamingThe ability of protein to aid in the formation and stabilization of emulsions is critical for many food applications, including meat sausages and cake batters. In general, the emulsifying capacity of soy protein products is enhanced by rising solubility. Accordingly, soy proteins progressively reduce interfacial tension as concentration is increased (Kinselle, 1979; Hettiarachchy and Kalapathy, 1997).
Foaming, the capacity of proteins to build stable foams with gas by forming impervious protein films, is an important property in some food applications, including beverages, as well as angel and sponge cakes. Soy protein exhibits foaming properties closely correlated to solubility. Studies have shown differences among the foaming properties of various soy protein products and that soy isolates are superior to soy flours and concentrates (Kinselle, 1979; Hettiarachchy and Kalapathy, 1997).
Gelation and Water-binding CapacityProtein gels consist of a three-dimensional network in which water is trapped. Gels are characterized by relatively high viscosity, plasticity and elasticity. While soy flour and concentrates form soft, fragile gels, soy isolates form firm, hard and resilient gels. Protein gelation is concentration-dependent; a minimum of 8% protein concentration is necessary for soy isolate to form a gel.
The general procedure for producing a soy protein gel involves heating the protein solution at 80-90°C for 30 minutes, followed by cooling at 4°C. The ability of a gel structure to provide a matrix to hold water, fat, flavor, sugar and other food additives is very useful in a variety of food products, such as making chicken or ham analogs from textured soy protein and fibrous soy protein (Kinselle, 1979; Hettiarachchy and Kalapathy, 1997).
Bound water includes all hydration water and some water loosely associated with protein molecules, following centrifugation. The amount of bound water generally ranges from 30-50g/100g protein.
Soy isolate has the highest water-binding capacity (about 35g/100g), because it has the highest protein content among soy protein products (Hettiarachchy and Kalapathy, 1997). Soy concentrates contain polysaccharides, which absorb a significant amount of water. Processing conditions can vary the amount of water that can be absorbed. In fact, these conditions can be varied to influence how tightly the water is bound by the protein in the finished food product (Endres, 2001).
Soy proteins differ considerably from wheat proteins in their chemical composition, as well as in physical properties (such as their total lack of elasticity). Adding soy proteins to wheat flour thus dilutes the gluten proteins and the starch. On the other hand, soy proteins exhibit a strong “binding power” that provides some resistance to dough expansion; the effect is somewhat proportional to the level of soy flour employed. This can be partially overcome by increasing the amount of water used in dough making and by a longer proofing time.
The binding power of soy flour is closely related to its high water-absorption capacity, which in the case of the defatted product, is equivalent to 110% by weight. Hence, soy flour will absorb an amount of water equal to its weight when mixed with wheat flour to normal dough consistency. With full-fat flour, however, no measurable increase in dough absorption results from normal usage levels of the soy product (Pyler, 1988).
Water-holding CapacityWater-holding capacity is a measure of trapped water that includes both bound and hydrodynamic water. Water-holding capacities of soy flour, concentrate and isolate were reported as 2.6, 2.75 and 6.25g/g of solids, respectively. Soy flours with PDI of 15, 55 and 70 had 209g, 307g and 308g water/g flour water-holding capacities, in that order.
The water-holding capacity of protein is very important because it affects texture, juiciness and taste. Also, the ability of soy protein to bind and retain water enhances shelflife in bakery products (Kinselle, 1979; Hettiarachchy and Kalapathy, 1997). All soy protein concentrates, regardless of the process used, have certain fat-and water-holding characteristics (Endres, 2001).
Color ControlSoy flour is rich in the enzyme lipoxygenase, which plays a major role in its bleaching action. It contains a type-1 lipoxygenase (LOX-1) and a type-2 lipoxygenase (LOX-2). Earlier studies have shown that purified LOX-2, but not LOX-1, bleaches flour pigments. Lipoxygenase oxidizes carotenoid and chlorophyll pigments in flour to their colorless form, which results in a bleaching action to whiten flour and bread crumbs.
However, since LOX-2 is also able to oxidize fatty acids while achieving a whiter flour, one risks the development of rancidity. Therefore, the amount of enzyme-active flour is generally restricted to approximately 1% of the flour’s weight.
Examples of Textured Soy Products Available to Food ProcessorsA variety of textured soy products are available for food applications. For example, textured soy protein is available in chuck, flake or minced forms, ranging from 2-30mm in size. Food processors can find these products made from soy flour (50% protein) or from soy concentrates (65% protein).
Textured soy protein made from flour can absorb 2.5 times the water, whereas textured soy protein from concentrate is able to hold 4.5 times the water. Also, these textured products can be white, caramel or red in color, lending themselves to a variety of applications in chicken, beef or mutton-style dishes.
The main applications of textured soy proteins are vegetarian soy protein-based sausage and savory stewed meat analogs. Other textured products available include structured meat analogs made from soy concentrates. The layered, meat-like structure suits diced and stewed meat analogs.
Typically, the meat analog is dried after extrusion, resulting in a very shelf-stable product. The product is rehydrated by the food processor or by the end user and usually flavored in the process of rehydration. The most important properties of these analogs are their meat-like appearance, texture and cooking characteristics.
Another recent, textured vegetable protein is fibrous soy protein. The muscle-like, fibrous structure of this product makes it good for restructured soy-based, meat-style products, including jerky. Typically, this product is dried after extrusion, resulting in a very shelf-stable product. It is rehydrated by the food processor and flavored in the restructuring process. Similar to those made using the fiber spinning process, this product is much more economical.
An example of this product is the soy-based, chicken-flavored product. It can be prepared by steaming for a few minutes, barbecuing, stir-frying, deep-frying or stewing. The most recent textured soy protein product to appear in the market is the high-moisture meat analog. It is 12mm thick X 80mm wide, available in layered or fibrous structure and has a meat-like composition of 60-70% moisture, 2-5% oil and 10-15% protein. After manufacturing, it must be frozen or retort packaged. With a size and shape similar to a cut of fresh meat, it can be used in vegetarian pot pies, fettuccine or barbecued products.
Another textured product available to food processors is textured soy flour, which offers not only the proven economies of vegetable protein, but also opportunities to increase product juiciness and yield. When hydrated and combined with ground meat, textured soy flour is similar in appearance, fibrous structure and chewiness to cooked meat.
Textured soy flour can quickly absorb up to three times its weight in water and absorb natural fats and juices from meat. This means meat with a higher fat content may be used with less grease-disposal problems. Extruded soy flour can reduce fat content in processed meat formulations. Textured soy flour can replace as much as 30% of the raw meat formerly used, saving in final ingredient costs as well as improving product yields.
In summary, advances in the formulation of soy ingredients have contributed to increased sales of finished food products containing soy protein. The most important factors a food company should consider when adding soy are taste, color, functionality and cost.
In general, when formulating foods with different soy protein ingredients, food processors need to understand the chemical and physical properties (water absorption, solubility, dispersibility, viscosity, gelation and emulsion capacity) of the soy ingredients for specific foods. With that understanding, food companies can manufacture very successful food products using soy protein ingredients.
References:1. Dubois, D.K. Soy Products in Bakery Foods, Technical Bulletin, Volume II, Issue 9, American Institute of Baking, Manhattan, 1980.
2. Endres, J.G. Soy Protein Products: Characteristics, Nutritional Aspects, and Utilization, AOAC Press, Champaign, 2001, chap. 5, 6.
3. Hettiarachchy, N. and U. Kalapathy. “Soybean Protein Products,” in Soybeans: Chemistry, Technology, and Utilization, Liu, K., Ed., Chapman & Hall, New York, 1997, chap. 8.
4. Kinselle, J.E. Functional Properties of Soy Proteins, J. Am. Oil Chemists’ Soc., 56, 242, 1979.
5. Pringle, W. Full Fat Soy Flour, J. Am. Oil Chemists’ Soc., 51, 74A, 1974.
6. Pyler, E.J. Baking Science and Technology, Sosland Publishing Co., Merriam, 1988.