Effect of lemon balm and spearmint extracts on the survival of S. aureus in goat’s raw milk cheese

Previous investigation has revealed that lemon balm and spearmint hydroethanolic extracts present high inhibitory capacity against S. aureus (SA). Raw milk cheeses have shown moderate prevalence of (SA), imposing a safety issue. Thus, this study aimed to evaluate the antimicrobial effect of lemon balm and spearmint extracts against SA in goat’s raw milk cheeses during maturation.

Lemon balm and spearmint extracts were obtained using ethanol 70% (v/v) as solvent in a shaking water bath. Milk was inoculated with SA to reach 5 log CFU/g and 1% (w/w) of each extract was added to the curd, while a non-inoculated control was kept. Cheeses were kept in a chamber at 10 °C/98% RH for 15 days. Water activity, pH and SA counts were determined at specific days.

For every treatment, a log-decay function with tail in differential form as primary model (with varying D-value), coupled to a secondary model Bigelow equation of D-value as a function of pH (with parameters logD* at pH 7.0 and zpH) was adjusted.

The dynamic models adequately fitted the survival curves with root mean square errors of 0.1172 and 0.0633, for spearmint and lemon balm, respectively, producing significant parameter estimates. The parameter logD* was affected by the addition of extracts (0.621 [SE=0.061] for spearmint; 1.189 [SE=0.200] for lemon balm) versus the controls (0.932 [SE=0.166] and 0.996 [SE=0.056]); whereas zpH tended to be higher with the addition of extracts (3.172 [SE=0.660] for spearmint; 2.339 [SE=0.835] for lemon balm). The addition of plant extracts significantly decreased the time to achieve one log reduction, which in practical terms corresponded to up to 1.36 log CFU/g reduction by the end of maturation.

Using a dynamic model, this work characterised SA survival parameters in raw milk cheese and demonstrated that lemon balm and spearmint extracts can be used to control SA during maturation.