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Sources of Variability in the Protein Solubility Assay for Soybean Meal

Poultry Science Division, University of Georgia Athens, Georgia 30602; Phone: (706) 542-7690

Correspondence: Egerton Whittle

A series of experiments was conducted to evaluate some factors that might influence the protein solubility (PS) of soybean meal (SBM) in 0.2% (35.7mM) potassium hydroxide. Particle size, fat content of soybeans, and the duration of protein extraction were evaluated. Commercial SBM was heated in an autoclave for 15 minutes (to simulate heat processing), then ground, and various particle sizes obtained. SBM fractions retained on 8, 14, 20, 45, 48, 60, 80, 100, and 200 mesh gave PS values of 34, 44, 51, 57, 58, 59, 58, and 60% respectively. Raw soybean samples that contained from 0.46 to 9.93% fat in Trail 1, gave PS values ranging from 66% to 71% with a mean of 68%. In Trial 2, the samples contained from 0.17 to 13.25% fat, and gave PS values between 77% and 89%, with a mean of 79%. SBM samples extracted for 10, 20, 25, 30 and 40 minutes had PS of 66, 76, 78, 81, and 81% in Trial 1, and 63, 73, 79, 80, and 81%, in Trial 2, respectively. Variation in particle size and duration of extraction therefore affect the outcome of the PS test. However, fat content does not influence the assay results, indicating that the PS test can be used to evaluate the degree of heating of full-fat soybeans. Standardization of the PS assay is necessary to minimize variability of results within and between laboratories.

Key Words: protein solubility • particle size • fat content • duration of extraction • soybean meal