Growth of Bacillus cereus in soyabean tempeh

Abstract

Bacillus cereus strains isolated from retail samples of tempeh were added in levels of approx. 10 5 cfu/g to soaking soyabeans intended for tempeh manufacture. Their growth was prevented and tempeh of good quality was obtained when the soaking resulted in acidification of the beans to pH 4.85, but B. cereus could grow well (approx. 10 8 cfu/g) in tempeh and cause its spoilage when the soaking did not cause acidification (bean pH 5.70–6.00). Acidification of soyabeans by addition of lactic acid prior to fungal fermentation required pH ≤ 4.4 to prevent B. cereus growth; at this pH, tempeh of good quality was obtained. Acidification of soyabeans with addition of acetic acid inhibited at pH ≤ 5.5 both B. cereus and Rhizopus oligosporus, the mould responsible for tempeh formation; hence, no tempeh could be obtained. Mixed inocula of R. oligosporus and either Lactobacillus plantarum, L. casei ssp. alactosus or L. fermentum produced tempeh of excellent quality. However, mixed inocula containing lactobacilli were not able to prevent B. cereus growth and subsequent spoilage when added to unacidified soyabeans of pH 6.6. This indicates that soyabean acidification, either by biological or by chemical means, is essential to inhibit B. cereus growth during the tempeh fermentation.