Hurdle factors minimizing growth of <em>Listeria monocytogenes</em> while counteracting <em>in situ</em> antilisterial effects of a novel nisin A-producing <em>Lactococcus lactis</em> subsp. <em>cremoris</em> costarter in thermized cheese milks

Abstract

The capacity of growth, survival, and adaptive responses of an artificial contamination of a three-strain L. monocytogenes cocktail in factory-scale thermized (65 °C, 30 s) Graviera cheese milk (TGCM) was evaluated. Bulk TGCM samples for inoculation were sequentially taken from the cheese making vat before process initiation (CN-LM) and after addition of a commercial starter culture (CSC), the CSC plus the nisin A-producing (NisA+) costarter strain Lactococcus lactis subsp. cremoris M78 (CSC + M78), and all ingredients with the rennet last (CSC + M78-RT). Additional treatments included Listeria -inoculated TGCM samples coinoculated with the NisA+ costarter strain M78 in the absence of the CSC or with the CSC in previously sterilized TGCM to inactivate the background microbiota (CSC-SM). All cultures were incubated at 37 to 42 °C for 6 h, followed by additional 66 h at 22 °C, and 48 h at 12 °C after addition of 2% edible salt. L. monocytogenes failed to grow and declined in all CSC-inoculated treatments after 24 h. In contrast, the pathogen increased by 3.34 and 1.46 log units in the CN-LM and the CSC-SM treatments, respectively, indicating that the background microbiota or the CSC alone failed to suppress it, but they did so synergistically. Supplementation of the CSC with the NisA+ strain M78 did not deliver additional antilisterial effects, because the CSC Streptococcus thermophilus reduced the growth prevalence rates and counteracted the in situ NisA+ activity of the costarter. In the absence of the CSC, however, strain M78 predominated and caused the strongest in situ nisin-A mediated effects, which resulted in the highest listerial inactivation rates after 24 to 72 h at 22 °C. In all TGCM treatments, however, L. monocytogenes displayed a “tailing” survival (1.63 to 1.96 log CFU/mL), confirming that this pathogen is exceptionally tolerant to cheese-related stresses, and thus, can't be easily eliminated.

Effects of curd heating with or without previous milk pasteurisation on the microbiological quality and safety of craft‐made ‘Pasta Filata’ Kashkaval cheese curds

“…lactis ) starter LAB selected for growth on M17 agar at 42 °C and M17 and MRS agars at 22 and 30 °C, respectively (Noutsopoulos et al . ; Samelis and Kakouri ), were noted (Table ). However, the populations of thermophilic Lb.…”

“…; Noutsopoulos et al . ; Samelis and Kakouri ). In the absence of active commercial starters and depending on the maturation temperature, indigenous lactococci and enterococci dominate in Greek raw or thermised milks and fresh curds (Lianou et al .…”

“…In the absence of active commercial starters and depending on the maturation temperature, indigenous lactococci and enterococci dominate in Greek raw or thermised milks and fresh curds (Lianou et al . ; Samelis and Kakouri ). Nonstarter mesophilic lactobacilli, mainly Lb.…”

“…; Lianou et al . ; Samelis and Kakouri ) and may reach high levels or even predominate in Pasta Filata raw milk cheeses, like Kashkaval (Pappa et al . ), and in naturally matured, hard cheeses from thermised milk (Vandera et al .…”

“…The strong competitive growth and rapid acidification capacity of commercial S. thermophilus starters in fresh cheese curds made of pasteurised or thermised milk during fermentation followed by a shift towards the prevalence of mesophilic Lc. lactis starters during maturation at reduced temperatures are well documented (Beresford et al 2001;Noutsopoulos et al 2017;Samelis and Kakouri 2018). In the absence of active commercial starters and depending on the maturation temperature, indigenous lactococci and enterococci dominate in Greek raw or thermised milks and fresh curds (Lianou et al 2017;Samelis and Kakouri 2018).…”