Evaluation of the Microbiological Safety of Tempeh Made from Unacidified Soybeans

Abstract

Studies were done to evaluate the safety of tempeh made from unacidifed soybeans and inoculated with different bacterial pathogens. Pathogens were added to either the soybeans before fermentation by Rhizopus oligosporus or the tempeh after fermentation and steaming. In the latter method, the inoculated products were incubated at several different temperatures (5, 10, 15 and 25°C). Clostridium botulinum (types A and/or B) toxin was produced in 2 d during the fermentation and within 5 d at 25°C or 4 wk at 15°C in tempeh inoculated and incubated in vacuum packages after fermentation and steaming. Staphylococcus aureus grew very well (>6-log₁₀ CFU/g increase) in 2 d during the fermentation, and grew from ca. 10³ CFU/g to 10⁸ CFU/g in 7 d at 25°C and 21 d at 15°C in tempeh inoculated after fermentation and steaming. Staphylococcal enterotoxins were detected in some of these samples. Salmonella typhimurium also grew well during the fermentation (>6-log₁₀ CFU/g increase in 1 d), but grew relatively slowly at 25 and 15°C in tempeh inoculated after fermentation and steaming. Yersinia enterocolitica grew very well (>6-log₁₀ CFU/g increase) in 1 d during the fermentation, and also grew well in tempeh inoculated after fermentation and steaming, with a >6 log₁₀ CFU/g increase in 2 d at 25 or 15°C and 5 d at 10°C. Results of these studies indicate the need for maintaining: (a) a high level of sanitary practices during production and (b) good refrigeration (≤5°C) of the product following fermentation until it is used.