Effects of nisin on Staphylococcus aureus count and physicochemical properties of Minas Frescal cheese

The aim of this work was to evaluate the effects of nisin on in vitro and in situ Staphylococcus aureus counts. For in vitro experiment, milk was inoculated with 5.0 log cfu·mL−1 of S. aureus and nisin was added at concentrations of 0, 100, 200, 400, and 500 IU mL−1. The main effect of the bacteriocin was lag phase extension from 0 h, for 0 and 100 IU·mL−1 to 8 h, when 200, 400, and 500 IU·mL−1 of nisin were used; however, log phase was not affected. Microbial growth rate was found to be exponential and around 0.11 log cfu·mL−1·h−1 for all treatments. For in situ experiments, 0, 400, and 500 IU·mL−1 of nisin were directly added to pasteurized milk previously inoculated with 5.0 log cfu·g−1 of S. aureus. Milk, curd, and whey were analyzed to S. aureus counts. Nisin at concentration of 500 IU·mL−1 was able to reduce S. aureus count in curd and whey, demonstrating nisin partition between both phases. Throughout storage at 4°C, S. aureus count increased for all treatments, but the bacterial grew slower when nisin was added in both concentrations, maintaining S. aureus count about 1.5 log cycles lower than the control, despite abusive initial S. aureus count. Therefore, nisin seems to play an important role in reducing S. aureus initial count in cheese made with highly contaminated milk. Nisin showed potential to be used as an additional, important hurdle to improve Minas Frescal cheese safety, without replacing good manufacturing practices.