Microbiological safety and quality of commercial sufu – a Chinese fermented soybean food

Abstract

In this study, the microbiological safety and quality of commercial sufu were investigated. Twenty-three samples of three different types of sufu were obtained, mainly in China and some in The Netherlands. Chemical parameters analysed included moisture, pH, free amino N, NaCl, ethanol, sucrose, glucose, and fructose. Concentrations of NaCl, ethanol, glucose and fructose varied from 6.2%, 0.5%, 0% and 0% to 14.8%, 6.3%, 6.2% and 4.8%, respectively. Microbiological analyses were done for total count of mesophilic aerobic bacteria (TMAB), bacterial endospores, total count of halotolerant bacteria at 10% (THB10) and at 17.5% NaCl (THB17.5), lactic acid bacteria (LAB), fungi, Enterobacteriaceae, and the following pathogens: Bacillus cereus, Clostridium perfringens, Staphylococcus aureus and Listeria monocytogenes. High levels (>10 5 CFU/g) of TMAB and bacterial endospores were found in most samples, and 85% of TMAB was identified as Gram-positive. Considerable levels (10 5 and 10 7 CFU/g) of LAB were detected in two samples of white sufu, and isolates of LAB were identified as most probably Lb. casei. One-third of the samples contained less then 10 3 CFU/g B. cereus, but three samples had over 10 5 CFU/g indicating potential hazard to consumers. All samples had less than 10 3 CFU/g C. perfringens, except sample R11 (∼10 5 CFU/g). S. aureus could not be detected in any of the samples tested since the competitive microflora (usually bacilli) disturbed typical features on the selective medium used; however S. aureus enterotoxin A was detected in some of the white and grey sufu samples. Fungi, Enterobacteriaceae, and L. monocytogenes were not detected in any of the samples. Based on these results, a microbiological guideline for safe commercial sufu is proposed.