Studies on the processing and properties of soymilk. 3. Factors affecting concentration of soymilk and its stability during heat sterilisation

Soymilk that has been efficiently heat treated to destroy the trypsin inhibitor is difficult to concentrate because its viscosity increases exponentially with increase of solids content. This imposes a concentration limit of around 17–18% total solids due to the formation of gel-like structure. The possible factors which contribute to this gel formation during concentration were investigated. A sulphydryl-disulphide interchange reaction is mostly responsible for the increase in viscosity of the concentrate up to about 16% total solids but above this concentration other forces predominated. The possibility of gelation through the intermodular cross-linkages by calcium ion was eliminated. Sodium sulphite and N-ethylmaleimide are effective in reducing this viscosity increase to some extent but when the concentrate is sterilised sodium sulphite ceases to have any effect on the viscosity and with N-ethylmaleimide the viscosity of the sterilised concentrate is about 50% of the unstabilised sample. Fore-warming by heating to 115°C for 5 min causes an increase in viscosity during concentration but effectively stabilises the product against further increases in viscosity on sterilisation. Final viscosity of the forewarmed product after sterilisation is about 75% of the unstabilised sample. The conditions necessary for the production of good quality canned soymilk concentrate are presented.