The influence of environmental pH on biological activity of Bifidobacterium longum CRL 849 grown in MRS-raffinose was evaluated. At pH 6.0, 5.5 and 5.0, raffinose was completely consumed by this microorganism, showing different consumption rates at each pH value (between 3.03 and 0.76 mmol l(-1) h(-1)). At pH 4.5, the growth was lowest. The removal of raffinose was due to the alpha-galactosidase (alpha-gal) activity of this bifidobacteria, which was highest at pH 6.0-5.5 (1,280-1,223 mU ml(-1)). The production of beta-glucosidase (beta-glu) showed a similar pattern to alpha-gal activity with major values. The yield of organic acids produced during raffinose consumption was also highest at pH 6.0-5.5. The results of this study will allow the selection of the optimum growth conditions of B. longum CRL 849, with elevated levels of alpha-gal to be used in the reduction of nondigestible alpha-oligosaccharide in soy products and beta-glu activities involved in isoflavone conversion to bioactive forms when used as starter culture.