Since the 1960s, soy protein products have been used as nutritional and functional food ingredients in every food category available to the consumer. Soy protein products are an ideal source of some of the essential amino acids used to complement cereal proteins. At present, soy proteins are more versatile than many other food proteins in various worldwide nutrition programs.
Dry soybean contains 36 percent protein, 19 percent oil, 35 percent carbohydrate (17% as dietary fibre), 5 percent minerals and several other components including vitamins.
Advances in soy ingredient technology have resulted in products that can perform many functions in foods such as emulsification, binding and texture. Soy protein product acceptance has grown because of their functional properties, abundance and low cost. The excellent nutritional value of soy protein products has recently been recognized by both the Food and Drug Administration (FDA) and the United States Department of Agriculture’s (USDA) School Lunch Program.
Classification of Soy Protein Products
Soy protein products fall into three major groups. These groups are based on protein content, and range from 40 > 90 percent. All three basic soy protein product groups (except full-fat and partially defatted extruded-expelled flours) are derived from defatted flakes. They include: soy flours and grits; soy protein concentrates; and soy protein isolates.
The quality of protein is judged by several indices; Protein Digestibility Corrected Amino Acid Score (PDCAAS) is one of them. PDCAAS is based on amino acid requirement for humans and their ability to digest it. The quality of soy protein as against other protein sources is furnished in Table 1. The protein content and energy contributed by high protein soy ingredients are depicted in Table 2. Some of the functional properties of soy proteins in food system is provided in Table 3. A serving of soy milk, texturized soy protein and soy flour in a muffin confers 7g, 12 g and 15 g of soy protein respectively.
Soyflours and Grits: Soy flours and grits (40-54% protein content) are made by grinding and screening soybean flakes either before or after removal of the oil. Soy flours and grits may vary in fat content, particle size, and degree of heat treatment. They are also produced in lecithinated or defatted forms. The degree of heat treatment creates varying levels of water dispersibility and enzyme activity qualities that can be useful in tailoring functionality in many food applications.
Soy Protein Concentrates (SPC): SPC are prepared from dehulled and defatted soybeans by removing most of the water-soluble, non-protein constituents. They contain at least 65 percent protein (N x 6.25) on dry matter basis. SPC are produced by processes, i.e. acid leaching (at ~pH 4.5), extracting with aqueous alcohol (60–90%), and denaturing the protein with moist heat before extraction with water. Neutralized concentrates prepared by acid leaching have a higher water-soluble protein content than those prepared by alcohol leach or heat denaturation techniques. Low water-soluble SPC (aqueous alcohol extraction) when heat treated by steam injection or jet cooking have increased solubility and functionality.
Soy Protein Isolates (SPI): Isolates are the most highly refined soy protein products commercially available. They contain > 90 percent protein on dry matter basis. Soy isolates are prepared from dehulled and defatted soybeans. The production of SPI involves solubilising the protein and carbohydrate at neutral or alkaline pH and the recovery of the solubilised protein, separation and optionally washing and neutralization before drying. Both gelling and non-gelling varieties are available, as well as of varying grades of viscosity. SPI are associated with excellent emulsification, moisture retention and adhesiveness. As a protein with superior essential amino acid balance, it is finding a wide range of applications in healthy foods, including ones certified as Food for Specific Health Use (FOSHU) in Japan.
Textured Soy Protein (TSP)
Soy flour is processed through either a single – or double-screw extruder to impart structure, such as fibre or chunk for use as a food ingredient. TSPs are frequently made to resemble beef, pork, seafood, or poultry in structure and appearance when hydrated.
Thinly sliced beef textured one is used in ‘tsukudani’, and beef-on-rice bowls. Due to its numerous functions, including oil and moisture retention, TSP can help preserve the flavour of processed foods, such as hamburgers and Chinese dumplings, as well as enhance and stabilize their quality. Textured SPCs are processed (single-or twin-screw extruder) from either traditional concentrate or acid-leached concentrate.
The potential application of soy protein ingredients in the food system is delineated in Table 4.
Specialty Soy Foods and Ingredients
Partially hydrolyzed soy protein products are products obtained by cleavage of the protein by proteolytic enzymes obtained from animal, plant and microbial peptidases (viz., pepsin, papain, bromelain) to reduce the molecular weight of proteins (i.e. between 3,000 and 5,000 daltons). Molecular weight reduction improves whipping properties and acid solubility. Fully hydrolyzed proteins used as flavouring agents can be prepared from soy grits by acid hydrolysis.
Applications of Soy-based Protein in Food Products
Functional properties of soy proteins of significance in food systems include foaming, whitening, fat and water absorption, crust formation, gelation, viscosity, solubility etc.
Cereals: The quality of the protein is improved in soy–cereal mixtures because soy protein is a rich source of lysine, the first limiting essential amino acid in most cereal proteins.
Dairy Products: An increase in SPI concentration (25-100% substitution of SMP) led to an increase in the viscosity and hardness of ice cream and produced darker colour in relation to control ice cream; the melting resistance of ice cream tended to decrease. The sensory scores of ice creams made using 50 percent SMP substitution with SPI and control product were comparable.
Bakery and Pasta Products: Soy protein ingredients are important in determining the quality of the product as well as facilitating such processing requirements as in improving machinability of cookie dough. Soy flours with high Nitrogen Solubility Index (NSI) or Protein Dispersibility Index (PDI) are used in bakery and cereal products and are added directly to the dough. Flours with a PDI of around 60 are commonly used for improving water-binding capacity (1 to 2% in bread, 10% in waffles and pancakes). Soy protein isolates have superior foaming ability compared to soy flour and concentrates. The capacity of proteins to form stable foams with gas by forming impervious protein films is an important property for angel and sponge cakes.
The protein content of the biscuits increased from 5.0 to 14.2 percent with an increase in the substitution level of wheat flour with soybean flour up to 25 percent. The biscuits prepared blended flour (wheat flour: soy flour, 9:1 w/w) could provide the needed daily nutrient requirement.
Meat, Poultry and Sea Foods: About 55 percent of the functional soy protein produced annually is used in processed muscle foods, including meat, poultry and sea food. The largest area of domestic food utilization of soy proteins is in emulsified meats (frankfurters) and coarse ground meats (ground beef patties). The primary function of the protein is to improve the dimensional stability of the patties, preserve the structural integrity of the ground meat pieces during thermo processing and help to retain meat juices (decrease cooking losses). Depending on the protein ingredient used and the meat product, usage level of SPI may range from 1 to 4 percent. Many emulsified meat formulations containing soy protein products have excellent eye appeal, good texture, no off-flavours, while maintaining good nutritional quality. There is substantial savings due to reduced cooking losses and greater yields. SPI and functional (dispersible) concentrates are the most effective soy ingredients used in emulsion-type meats.TSP products are used as meat extenders in comminute meat products such as patties, fillings, meat sauces, meat balls, etc. Up to 30 percent of the meat can be replaced by hydrated TSPs, without loss of eating quality.
Infant Formula: SPI is the preferred ingredient in infant formulas where milk solids are replaced. This is owing to the bland taste, absence of flatus-producing sugars and negligible fibre content.
Soy Proteins and Human Health
Calorie Control: Obesity is a genuine health concern in the minds of an informed populace. Soy protein products can make a significant contribution to weight reduction, mainly by providing essential high quality protein in a concentrated form for specially designed, low-calorie/high nutrient density meals.
Coronary Heart Disease: Mean intakes of 47 g/d (i.e.from 17 to 124 g), of isolated or textured soy protein resulted in significant reduction in total cholesterol by 9.3 percent, LDL-cholesterol by 12.9 percent and triglycerides by 10.5 percent, with insignificant change in HDL cholesterol levels, compared with animal protein.
Anti-ageing: Many protein hydrolysates have been used in cosmetic formulations for decades because of their ability to stimulate skin firmness, tone and elasticity as well as counteract skin aging. During aging, the epidermis and dermis become thin; an efficient anti-aging product should be able to stimulate the metabolism of senescent fibroblasts and keratinocytes in order to increase the quantity of extracellular matrix components (viz., collagen, glycosaminoglycans). The soy biopeptide induced a significant increase in glycosaminoglycan synthesis – both in vitro and in vivo after 1month treatment; it also stimulated collagen synthesis.
Cancer: Supplementation with soy protein decreased the markers of cancer development and progression in prostate cells including prostate-specific antigen (PSA), testosterone, and androgen receptor in persons having prostate cancer or in men at high risk for developing advanced prostate cancer.
Soybean protein is one of the least expensive sources of dietary protein. Soybean protein is considered to be a good substitute for animal protein, and their nutritional profile except for sulfur amino acids is almost similar to that of animal protein. The concentrated soy proteins namely SPC, SPI and even the TVPs have inherent functional properties that can change the physico-chemical as well as textural property of foods in which they are applied. Increasing the protein content of low-protein foods can be one good means of tackling protein malnutrition. Specialty soy proteins, especially bioactive peptides originating from soy protein can serve as nutraceuticals.
* Komal Patel is Assistant Professor, Dairy Technology Department.
Jana AH is Professor and Head, Dairy Technology Department, SMC College of Dairy Science, Anand,
Professor and Head is Assistant Professor, Food Processing Technology Department, College of Food Processing Technology and Bio-Energy, Anand
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