Since 1960s, soybean has been recognised as rich source of unsaturated vegetable oil. Soybean oil is one of the leading vegetable oils around the world and mostly used for cooking in Asia. It has been reported the presence of 37% protein, 18% oil, 15% soluble carbohydrate, 15% insoluble carbohydrate and 14% moisture in dried soybean. Although, it contains 37% protein but was mostly used for edible oil. Recently, soybean is also used as protein source in the preparation of various food products. Notably, 98% soy protein is digestible in human digestive system.
Amino acids in soy protein
It is reported that soybean contains maximum amino acids among available oil seeds. Except methionine and tryptophane, all other essential amino acids are abundantly found in soy protein, which can be added in fortification or enrichment of food products for a healthy diet as well functional benefits. Besides, soy protein is good complement to any cereal protein because of it’s high amount of lysine content. Interestingly, unique proteins for gluten viz gliadin and glutenin are completely absent in soybean protein, therefore it can be recommended in the diet for celiac patients.
Preparation of soy protein
‘How will we get soy protein?’ - it is a major question. After extraction of oil from soybean, the remaining material is proteinaceous, which is called as defatted flakes. However, we can get soy protein in three forms like flour or grit, soy protein concentrate and soy protein isolate in which protein ranges from 50% to over 90%. Usually, soy flour contains less amount of protein compared to soy protein concentrate and soy protein isolate.
Flour is prepared by grinding defatted soybean flakes. Particle size of flour is 0.157mm or more fine but grit is coarser than flour. Both of flour and grit are least refined form of soy protein and protein content ranges from 40% to 54% depending on fat, carbohydrate and ash content. Residual lipids and other minor components like saponins and isoflavones cause bad smell of soybean flour and grits. Meanwhile, half of carbohydrate present in flour and grit is polysaccharide, making them insoluble in water.
Soy protein concentrate is more refined than flour and grit and contains more than 70% protein. It is also prepared from defatted flakes or flour by removing oligosaccharide, ash and some of minor components. There are three easy methods of removing unwanted components in the preparation of concentrate. First method involving the washing of defatted flakes with 60 to 80% aqueous alcohol. Sugars and other components are being dissolved and removed through washing while protein and polysaccharide are insoluble in alcohol. Concentrate is then dried to get soy protein concentrate. Second method involves acid leaching at pH 4.5, which is isoelectric point for proteins, therefore sugars are being washed away. After neutralisation of wet protein concentrate, it is dried. Third method is applying moist heat to flacks, which makes denaturation of proteins. Then sugars and other components are removed by water washing. The concentrate has less flavour than flour or grit because of removal of some flavour constituents during the preparation of concentrates.
Most refined form of soy protein is isolate, contains more than 90% protein. Soy protein isolates are prepared by complete removal of polysaccharide, oligosaccharide and other low molecular weight components resulted in protein concentrates. After moist heat treatment, concentrate is washed in water having pH of 7 to 8.5. The insoluble residues namely, polysaccharide and residual protein are separated by washing at pH 4.5. Precipitated proteins are then separated by centrifugation or filtration. Separated proteins are neutralised and dried for getting isolates. Isolates contain almost 95% protein and 2-5% ash and minor constituents.
Consequently, both of soy concentrate and isolate are rich source of protein with sufficient amount of amino acid ‘lysine.’
Use of soy protein
Nowadays, several products are being developed by incorporating soy protein including infant formulation, cheese, drinks, miso, tofu, salami and vegetarian meat substitutes. There are several health benefits of the consumption of soy food products like lowering of blood glucose level, preventing obesity, constipation, cancer, kidney disease and obesity. This protein can be used with the combination of other protein source such as milk, meat and cereal grains. Furthermore, soy protein can be used in the production of value-added baked products.
Soy protein and baked products
Modern technology is helping us to produce sufficient amount of cereals to meet energy requirement for growing population. Most of the cereals have low protein content with imbalanced essential amino acid composition. As a result, cereal grains are not sufficient to meet protein requirement for satisfactory growth of infants, children and to maintain body regulating process. Therefore, protein supplement is desirable to meet the essential amino acids requirement. In this regard, addition of soy protein in the development of value-added bakery products will be effective way to fulfil the lacking of essential amino acids of cereals. In bakery industry, soy protein can be used with cereals for several economical, functional and nutritional reasons. Considering the high price of soy protein concentrate and isolates, defatted soy flour can be widely used as a partial replacement of milk powder. Meanwhile, soy protein concentrate or isolate can be applied to make special bakery products.
Effects of soy protein on textural characteristics of bakery products
Nowadays, several bakers use soy flour as replacement of skim milk. It improves water absorption and textural characteristics of dough, consequently keeps the dough tenderer as does skim milk powder. It helps to keep bread fresh by retaining moisture during baking process. Moreover, soy protein improves the crust colour and makes more tasty than white bread. The same functional properties of soy proteins have been reported for cookies, crackers, biscuits, pancakes, sweet pastry, and snacks. But soy flour with low baking temperature may add impart flavour to baked products.
It has been also studied that protein quality of white bread is equal to 3% soy flour. The protein content of ordinary bread ranges from 9 to 11%. But special bread with 15 to 18% protein content can be prepared by incorporating soy concentrate and isolates with wheat flour. But formulation of special bread requires using lipid emulsifier. Otherwise, high level of soy protein would results in low loaf volume and poor crumb characteristics. High amount of soy protein incorporation brings dramatic change in protein quality of baked products. We can change Protein Efficiency Ratio from 0.7 to 1.95 by adding 12% soy flour. So, soy-fortified wheat flour can be used in developing value-added bakery products for any feeding programme and specially, school feeding programme.
Conclusion
In addition, value-added products have a great demand to people of any class. Functional foods that include soy protein comprise a growing product category. We always look for a product with ingredients that will help control unwanted health symptoms as well as boost immunity. Hence, we can develop our bakery products by incorporating soy protein that will help in reducing Protein Energy Malnutrition (PEM).
