Microbial α‐Galactosidase for Soymilk Processing

Abstract

ABSTRACT Several microorganisms isolated from soil enriched with soybean meal were screened for their ability to produce α‐galactosidase (α‐D‐galactoside galactohydrolase, E.C. 3.2.1.22). Soybean carbohydrates, after fermentation with Saccharomyces cerevisiae , were used as carbon source for an effective screening. Fructose‐free soy oligosaccharides were effective as inducers of the enzyme whereas normal soy oligosaccharides, including sucrose, were not. With this carbon source, invertase production was insignificant and the enzymes present liberated galactose from oligosaccharides known to be present in soybeans. Among the microorganisms studied, Cladosporium cladosporioides (Fres.) de Vr. had the highest α‐galactosidase activity. The intracellular enzyme had an optimum pH around 7.0 on the artificial substrate p‐nitrophenyl‐α‐D‐galactopyranoside (PNPG) but a pH optimum of 5.0 with melibiose. At 6.3 (the natural pH of soymilk) the relative activity on the two substrates was 90%. The enzyme was relatively thermostable, with no detectable decrease in activity when held for more than 16 hr at 47°C or 2 hr at 60°C. The enzyme liberated galactose from melibiose, raffinose, and stachyose and eliminated raffinose‐like sugars from soymilk, specially stachyose, in a few hours.