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 degrees 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 10(6) CFU ml(-1) of a cocktail mixture of three strains of L innocua. Part of the nisin treated samples were heat treated at 63 degrees 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 degrees 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 degrees C. Nisin at 500 IU ml(-1) had a marginal inhibitory activity against L. innocua. However, nisin at 1000 and 1500 IU ml(-1)l resulted in a more than 2 log(10) reduction in L innocua count and the effect was more prominent at 10 degrees C. In comparison, the combination of nisin (1000 or 1500 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 degrees 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. (C) 2011 Elsevier Ltd. All rights reserved.