Metabolism of oligosaccharides and aldehydes and production of organic acids in soymilk by probiotic bifidobacteria

Four strains of Bifidobacterium were assessed for α-galactosidase activity in MRS broth using p-nitrophenyl-α-d-galactopyranoside as substrate. Bifidobacteria were then used to ferment soymilk prepared from soy protein isolate (SPI), with and without d-glucose and l-cysteine supplementation. Measurements of pH and the quantification of oligosaccharides, organic acids and aldehydes in soymilk were done after 0, 12, 24, 36 and 48 h of incubation. The presence of d-raffinose stimulated the α-galactosidase activity of B. longum BB536 (BB536) and B. pseudolongum 20099 (BP20099). In soymilk, oligosaccharides and aldehydes were effectively metabolized by Bifidobacterium; d-glucose and l-cysteine supplementation enhanced the hydrolysis of raffinose and stachyose. BB536 and BP20099 degraded a significantly greater level of oligosaccharides and aldehydes than B. animalis Bb-12 and B. longum 1941 (P < 0.05). Raffinose and stachyose were completely metabolized by BB536 after 48 h. In soymilk without any supplementation, hexanal and pentanal were not detected after 12 h of fermentation with BB536 and BP20099. BB536 was the highest producer of organic acids, with an average acetic acid/l(+)-lactic acid ratio of 0.7.