II. HEAT TREATMENT OF SOYMILK AND CULTURE ACTIVITY1

Abstract

Soymilk was given different heat treatments and then evaluated as a substrate for acid production by lactic acid bacteria. Unheated soymilk elicited optimal or nearly optimal activity from most test cultures. Heating the medium to 60 C resulted in increased acid formation by Streptococcus and Leuconostoc species and in a reduction of acid production by Pediococcus cerevisiae and Lactobacillus species. Extended heating of soymilk at 60 C reduced its suitability as a substrate for acid development by lactic acid bacteria. Acid formation by all cultures was minimal in soymilk heated at 80 C from <1 to 60 min. Responses in soymilk heated at 100 C for short durations were similar to those obtained when soymilk was heated at 80 C. More severe heating at 100 or 120 C progressively improved the quality of soymilk as a substrate. Inhibitory effects noted when soymilk was heated at 80 C coincided with development of a markedly higher concentration of sulfhydryls and/or toxic volatile sulfides in the medium during heating. Beneficial effects of more severe heating were attributed to expulsion of sulfides, a concurrent decrease in concentration of sulfhydryls, and a decrease in the oxidation-reduction potential of the medium.