Novel conditions for tofu and pehtze preparation to overcome bacterial contamination in pehtze

Sufu is one of the many soy food products. It is prepared when tofu is fermented with a strain of fungus, producing pehtze, which is then allowed to ripen with various dressing mixtures. Although pasteurization is applied to most commercial sufu, salt-tolerant pathogenic bacteria such as Bacillus spp. and Staphylococcus aureus are still detected in marketed sufu products. This study proposed new conditions for tofu and pehtze preparation to reduce bacterial contamination in pehtze, which would then be more suitable for using in further sufu production. At first, seven isolates of Rhizopus spp. were screened from fermented tofu obtained from a sufu manufacturer in Thailand. Rhizopus sp. KUPR4 strain was the only one out of strains containing protease activity at pH 4.0. The lipase activity of the Rhizopus sp. KUPR4 strain was as good as that of the reference R. oligosporus (TISTR 3001) at pH 4.0 (13.74 unit/ml). Rhizopus sp. KUPR4 strain was thus suggested for using in further acidic pehtze preparation. For protocol optimization of tofu production, different coagulants were tested, including: 1.0% w/v calcium sulfate (C-tofu); 0.5% w/v acetic acid (A-tofu); 1.0% w/v magnesium sulfate (M-tofu); and a combination of coagulum from 1.0% w/v calcium sulfate mixed with coagulum from 0.5% w/v acetic acid in a ratio of 1:1 w/w (AC-tofu). AC-tofu presented the most appropriate quality in terms of smoothness and hardness, and contained the lowest moisture content. Its pH was in a moderately acidic range (pH 5.0) in which Rhizopus sp. KUPR4 could grow very well at 33°C for 20 - 24 h or at 40°C for 16 - 18 h. Acidic AC-tofu was incubated with Rhizopus sp. KUPR4 during pehtze preparation under the above conditions, and then a microbiological contamination assay was performed. After 24 h incubation, pehtze prepared from AC-tofu (pH 5.09) exhibited <10 CFU/g of B. cereus; whereas >105 CFU/g of B. cereus was detected on 24-h-incubated pehtze prepared from C-tofu (pH 5.95), which is similar to the conditions for the preparation of pehtze used in typical sufu manufacturing. From the perspective of food safety, the use of AC-tofu with Rhizopus sp. KUPR4 in pehtze production is suggested to reduce the microbial contamination of sufu. The Rhizopus sp. KUPR4 strain was genetically analyzed and verified to be Rhizopus microsporus var. microsporus.