Behavior of artificial listerial contamination in model Greek Graviera cheeses manufactured with the indigenous nisin A‐producing strain Lactococcus lactis subsp. cremoris M104 as costarter culture

Abstract

AbstractGrowth of Listeria monocytogenes during processing of traditional Greek Graviera cheese is inhibited, but the pathogen may survive ripening. Therefore, this study used Lactococcus lactis subsp. cremoris M104, a nisin A‐producing (NisA+) raw milk isolate, to enhance inactivation of a nonpathogenic Listeria cocktail contaminated in model Graviera mini cheeses. Cheeses were manufactured from thermized milk with a commercial starter culture (CSC) or the CSC plus strain M104 (CSC + M104), ripened at 18 °C and 90% RH for 20 days and stored at 4 °C in vacuum for 60 days. Listeria populations declined 10‐fold in all fermenting cheeses but then survived with little death during ripening and storage. NisA+ M104 colonies were prevalent and the nisA gene was detected, but nisin activity was weak to undetectable, in all CSC + M104 cheeses. Thus, Graviera cheese processing should be optimized to ensure nisA gene expression by strain M104 at levels sufficient to increase inactivation of L. monocytogenes.Practical applicationsThis study provided important preliminary evidence that the in situ NisA+ antilisterial activity of L. lactis subsp. cremoris M104 during Graviera cheese fermentation increased by reducing the curd cooking temperature from 48 °C to 42 °C. This indicated the feasibility to increase nisin production and activity by suitably modifying selected technological factors during traditional cooked hard cheese processing. Apart from its bioprotective potential, this NisA+ L. lactis subsp. cremoris genotype (represented by the indigenous strains M104 and M78) possesses desirable sugar fermentation capabilities and enzymatic activities similar to those of the best performing industrial L. lactis or L. cremoris strains in commercial starters (Parapouli et al., ). Since 2015 this novel lactococcal genotype has been applied as hand‐made, costarter culture for the production of commercial Greek Graviera cheeses characterized by specific flavor characteristics and improved textural properties than counterpart cheeses produced with CSCs only (Pappas Bros. collaborating cheese plant, Epirus; private communication).