Investigation of Staphylococcus aureus growth and enterotoxin production during artisanal kefir fermentation

The behavior of two enterotoxigenic strains of Staphylococcus aureus (264 and 137) at two inoculation levels (ca. 2.3 and 4.2 log10 CFU/mL) was evaluated during artisanal kefir manufacture using three kefir grains (‘A’, ‘C’ and ‘K’) at two different grain-to-milk inoculation ratios [1 and 5% (w/v)]. Enumeration of S. aureus, lactic acid bacteria and yeasts was performed at different time points during milk fermentation at 25 °C, whereas enterotoxin production was evaluated at the end of fermentation (24 h post-inoculation). Enterotoxin was detected in four trials conducted using the 1% kefir grain-to-milk ratio and the higher S. aureus inoculum. Enterotoxin production was not detected in any of the trials conducted either with the lower S. aureus inoculum, the 5% grain-to-milk ratio, or kefir grain ‘C’. The results indicate that the growth and enterotoxin production of S. aureus during artisanal kefir-making can be influenced by all the factors studied, i.e. by the initial level of S. aureus contamination, the type of kefir grain used and the ratio of the kefir grain added to the milk. Artisanal kefir-makers should adhere to good hygiene practices and use the 5% grain-to-milk ratio scheme to minimize the risk of enterotoxin production.