Ultrafiltration studies of foods: Part 1—The removal of undesirable components in soymilk and the effects on the quality of the spray-dried powder

Abstract

The use of ultrafiltration for the removal of low molecular weight anti-nutritional factors, in particular the oligosaccharides, raffinose and stachyose, and phytic acid, was re-examined. Using a membrane having a 20 000 molecular weight cut-off, the rejection rates of the oligosaccharides and phytic acid increased during ultrafiltration. At 60% water removal over 80% of each of the oligosaccharides was removed. In the case of phytic acid, however, only 50% could be removed. This could be because phytic acid exists as phytates or associated with native protein by salt linkages, and thus complete, or near complete, removal of this would be difficult to achieve even if multiple-stage ultrafiltration (UF) is employed. As would be expected, considerable amounts (50%) of the acid are detected in the soybean soak water. Thus, the actual amount of phytic acid present in the UF concentrate is only about one-third that originally present in the bean. Spray-drying studies were also carried out on UF soymilk concentrates. The nitrogen solubility index (NSI) of the spray-dried powder improved with per cent water removal during UF and also by the addition of sucrose to the concentrate prior to spray drying. There is also very little, if any, detectable diffierence in the taste and flavour.