Skim milk powder supplementation affects lactose utilization, microbial survival and biotransformation of isoflavone glycosides to isoflavone aglycones in soymilk by Lactobacillus

Four probiotic bacteria, Lactobacillus acidophilus 4461, L. acidophilus 4962, Lactobacillus casei 290 and L. casei 2607, were used for fermentation of soymilk (SM) prepared from soy protein isolate (SPI) supplemented with skim milk powder (SMP) (SSM). Soymilk and reconstituted skim milk (RSM) were used as controls. Lactose utilization in SSM by these probiotic organisms ranged from 14.97 to 18.15 mg/ml, compared to 14.12–16.06 mg/ml for RSM. The pH in SSM dropped to 4.07–4.29 compared to 6.15–6.36 for SM and 4.10–4.96 for RSM. The microbial viable counts were also significantly enhanced by up to 0.98 log CFU/ml by the supplementation of SMP to SM. The biotransformation level of isoflavone glycosides (IG) to isoflavone aglycones (IA) in SSM ranged from 81.4% to 85.1%, which was 13.9–19.0% higher than that for SM, after 24 h of incubation. Most of IG in SSM was completely converted to IA, except malonyl glycitin and malonyl genistin. At the end of the incubation, IA comprised up to 76.8% of total isoflavones in SSM.