Behaviour of Staphylococcus aureus during sufu production at laboratory scale

Sufu is a Chinese soybean cheese, obtained by fungal fermentation of soybean curd (tofu) with Actinomucor elegans to yield pehtze, followed by enzymatic maturation in brine solution containing 12-16% (w/w) of NaCl for 2-6 months. After equilibration, the final product, sufu, usually contains half of the NaCl concentration of the brine used. We studied the behaviour (survival, growth, formation of enterotoxins) of several strains of Staphylococcus aureus in a laboratory-scale simulation of the production of sufu. Of 15 S. aureus strains tested, strains No. 1, 3, 4, 5, 8, 9, and 10 showed good growth in sterile Brain Heart Infusion broth with high (6-12% w/w) NaCl concentration, demonstrating their halotolerance. S. aureus growth was not negatively influenced by A. elegans, and no interaction between the mould and the bacterial strains was observed during pehtze preparation. S. aureus strains No. 3 and 5 were selected for maturation experiments because they grew up to 8.9-9.5 log cfu/g and they produced their typical enterotoxin in all substrates tested. With the objective of creating a worst-case scenario, halotolerant enterotoxin producing S. aureus strains were inoculated at several stages of the sufu model system. Levels of 6% and 12% of NaCl in the brine solution were still too low to inhibit their growth and enterotoxin production. Even 18% of NaCl in the brine solution could not inhibit their growth, but enterotoxin production was prevented. Brine solution with 24% of NaCl inhibited growth as well as enterotoxin production. In conclusion, a salt concentration of 9% in the final product will be a minimum safeguard against S. aureus enterotoxin formation.