A laboratory study was made to develop a simple and economic model method for the systematic determination of functional properties of $quot;Soy Protein Isolates (SPI)$quot; prepared from defatted soybean meal. These are required to evaluate and to predict how SPI may
behave in specific systems and such proteins can be used to simulate or replace conventional proteins.
Data concerning the effects of pH, salt concentration, temperature, and protein concentration on the functional properties which include solubility, heat denaturation, gel forming capacity, emulsifying capacity, and foaming capacity are presented. The results are as follows:
1) The yield of SPI from defatted soybean meal increased to 83.9 % as the soybean meal was extracted with 0.02 N NaOH.
2) The suitable viscocity of a dope solution for spinning fiber was found to be 60 Poises by using syringe needle (0.3 ㎜) with 15 % SPI in 0.6 % NaOH.
3) Heat caused thickening and gelation in concentration of 8 % with a temperature threshold of 70 ℃. At 8∼12 % protein concentration, gel was formed within 10∼30 min at 70∼100 ℃. It was, however, disrupted rapidly at 125 ℃ of overheat treatment. The gel was firm, resilient and self-supporting at protein concentration of 14 % and less susceptible to disruption of overheating.
4) The emulsifying capacity (EC) of SPI was correlated positively to the solubility of protein at μ=0. At pH of the isoelectric point of SPI (pH 4.6), EC increased as concentration of sodium chloride increased. Using model system(mixing speed: 12,000 r.p.m., oil addition rate: 0.9 ㎖/sec, and temperature : 20±1℃), the maximum EC of SPI was found to be 47.2 ㎖ of oil/100 ㎎ protein, at the condition of pH 8.7 and u=0.6. The milk casein had greater EC than SPI at lower ionic strength while the EC of SPI was the same as milk casein at higher ionic strength.
5) The shaking test was used in determining the foam-ability of proteins. Progressively increasing SPI concentration up to 5 % indicated that the maximum protein concentration for foaming capacity was 2 %. Sucrose reduced foam expansion slightly but enhanced foam stability. The results of comparing milk casein and egg albumin were that foaming properties of SPI were the same as egg albumin, and better than milk casein, particularly in foam stability.
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