Inactivation of Listeria innocua in brined white cheese by a combination of nisin and heat

The objective of this study was to investigate the effect of nisin alone and in combination with heat (63 °C/5 min) on the inactivation of Listeria innocua in white cheese. Nisin was added at different concentrations (500, 1000, and 1500 IU ml⁻¹) to pasteurized milk before curd formation. The curd was soaked for 24 h in 10% solution of brine containing ca 10⁶ CFU ml⁻¹ of a cocktail mixture of three strains of L. innocua. Part of the nisin treated samples were heat treated at 63 °C/5 min. Total mesophilic count (TMC), L. innocua survivors and changes in the pH of white cheese were monitored each 2 d for a period of 12 d of storage at 4 or 10 °C. Nisin at 500 IU ml⁻¹ did not diminish TMC in white cheese compared to the control. The combination of heat and nisin (1000 or 1500 IU ml⁻¹) exhibited a bacteriostatic effect on TMC throughout the storage period at 4 or 10 °C. Nisin at 500 IU ml⁻¹ had a marginal inhibitory activity against L. innocua. However, nisin at 1000 and 1500 IU ml⁻¹l resulted in a more than 2 log₁₀ reduction in L. innocua count and the effect was more prominent at 10 °C. In comparison, the combination of nisin (1000 or1500 IU ml⁻¹) and heat treatment exhibited a synergistic inhibitory activity against L. innocua, where a complete elimination of the organism was accrued after 6 and 8 d of storage at 10 and 4 °C. Therefore, nisin and heat combination could be used as a prudent hurdle to preclude the growth of Listeria in white cheese, especially under the condition of abused refrigeration conditions.