Soya protein isolate production by various methods.
Sunley, Nigel Crispin.
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The concentrated protein fractions of soyabeans, known as soya protein isolate, was produced by three different methods from the same raw material namely defatted soya flakes. Extraction of the soluble fraction of the raw material is common to all three methods. A study was therefore undertaken to optimise the extraction process conditions in terms of time, temperature, pH, extraction time, extraction volume and raw material particle size, thereby maximising yields of soluble material. The three different methods, namely isoelectric precipitation, ultrafiltration and swollen gel technology were then used to separate the soluble and non-soluble protein fractions. Both the isoelectric and ultrafiltration methods gave good yields of finished product, with the ultrafiltration process giving the better overall yield, but the swollen gel method gave disappointing results and was not feasible in practice. Functional properties of the products from the isoelectric and ultrafiltration methods were compared and found to be broadly similar although different in certain respects from those of commercial soya isolates. Levels of the anti-nutritional factors trypsin inhibitor and phytate in products from the three processes were determined and the substantial differences observed in trypsin inhibitor levels were further investigated. Determination of lysinoalanine levels was also attempted but the results obtained were unsatisfactory. Amino acid composition and polyacrylamide gel electrophoresis were used to compare the chemical composition of products from the three processes. The comparative economics of the isoelectric and ultrafiltration processes for large scale production of soya protein isolates were evaluated, taking into account the comparative efficiencies of the two processes as determined during the study. It was established that, while the isoelectric process initially appears more economical, it may be possible to modify the ultrafiltration process in such a manner as to make it more economical than the isoelectric process. Overall figures however indicate that the manufacture of soya protein isolate in South Africa is not currently a viable economic proposition, due to high raw material costs.