Changes in the Microbial Composition of Raw Milk Induced by Thermization Treatments Applied Prior to Traditional Greek Hard Cheese Processing

Abstract

The microbiological quality, safety, and composition of mixtures of ewe's and goat's milk (90:10) used for cheesemaking were evaluated before and after thermization at 60 and 67 degrees C for 30 s. Such mild thermal treatments are commonly applied to reduce natural contaminants of raw milk before processing for traditional hard Greek cheeses. Raw milk samples had an average total bacterial count of 7.3 log CFU/ml; most of these bacteria were lactic acid bacteria (LAB) and pseudomonads. The LAB flora of raw milk was dominated by enterococci (40.8%), followed by lactococci (20.4%), leuconostocs (18.4%), and mesophilic lactobacilli (10.2%). Enterococcus faecalis (30.1%) and Enterococcus faecium (13.7%) were the most common LAB isolates, followed by Enterococcus durans, Lactococcus lactis subsp. lactis, Lactobacillus plantarum, and Leuconostoc lactis. Thermization at 60 degrees C for 30 s was effective for reducing raw milk contamination by enterobacteria (5.1 log CFU/ml), coagulase-positive staphylococci (3.3 log CFU/ml), and Listeria (present in 25-ml samples) to safe levels, but it also reduced mesophilic lactococci, leuconostocs, lactobacilli, and selected enterococci (72.0%) in thermized milk. Thermization at 67 degrees C for 30 s had a major inactivation effect on all bacterial groups. Two nisin-producing L. lactis subsp. lactis strains (M78 and M104) were isolated from raw milk, but neither nisin-producing nor other bacteriocin-producing LAB strains were isolated from thermized milk. Thus, thermization treatments control harmful bacteria but also may have a negative impact on milk quality by reducing desirable LAB and the biodiversity of raw milk bacteria overall, inactivating potentially protective LAB strains and enhancing the ability of potentially pathogenic enterococci to grow in fresh cheese curds.

Comparison of Culture Methods for Isolation of Nontuberculous Mycobacteria from Surface Waters

“…Thus, it seems that the concentrations of nontarget microorganisms do not explain the finding that addition of PANTA was not sufficient to allow enumeration of NTM in surface water. In contrast, the higher diversity of microorganisms in surface water (21,55) than in raw milk cheeses (34) could explain the finding that addition of PANTA was not sufficient to allow enumeration of NTM in surface water.…”

Characterization of a Wild, Novel Nisin A-Producing Lactococcus Strain with an L. lactis subsp. cremoris Genotype and an L. lactis subsp. lactis Phenotype, Isolated from Greek Raw Milk

“…The phenotypic taxonomy within subspecies lactis of the Nis ϩ L. lactis isolates M78 and M104 (35) was reconfirmed by testing them for cell morphology, gas production from glucose, arginine hydrolysis, growth at 15°C and 45°C, growth at 4% and 6.5% NaCl, growth on kanamycin esculin azide agar, and survival at 60°C for 30 min under the same culture conditions as those used with the reference strains of L. lactis included in Table 1. The media and methods used were as described by Samelis et al (35). Moreover, the entire sugar fermentation patterns and main enzymatic activities profiles of the wild strains compared to those of the L. lactis reference strains were determined by the use of API 50 CHL and API ZYM kits (bioMérieux, Marcy l'Etoile, Lyon, France), respectively.…”

“…In accordance with those findings, in 2009, in the course of a research project on traditional European foods (www.truefood.eu), we reported the isolation from Greek raw milk of two wild L. lactis isolates, M78 and M104, typical of the subspecies lactis phenotype, which were of great interest due to their strong ability to produce nisin in synthetic media but also in milk and cheese; both isolates were genotypically confirmed to belong to L. lactis and to be genotypically distant from L. garvieae (35), whereas these first molecular results were not able to discriminate them on the subspecies level. Since then, the expression of nisin genes in L. lactis M78 in a cheese-like medium has been quantified by quantitative realtime PCR (qRT-PCR) (36), and its novel nisin A sequence has been published (GenBank accession number HM219853) and compared to other nisin sequences (37).…”

“…Since then, the expression of nisin genes in L. lactis M78 in a cheese-like medium has been quantified by quantitative realtime PCR (qRT-PCR) (36), and its novel nisin A sequence has been published (GenBank accession number HM219853) and compared to other nisin sequences (37). L. lactis M104, which showed nearly the same phenotype as L. lactis M78 (35), has also been evaluated as a costarter and/or protective adjunct in pilot traditional Greek cheese trials (38,39).…”

Survival of pathogens in "Graviera Kritis" cheese made with raw and pasteurized milk

“…Decrease of Listeria monocytogenes, below the level of 100cfu/g at the 4 th day after salting in the present study, is attributed to the drop in the pH (from 6.27 to 5.3) in combination with the decreasing of moisture from 67.1% to 41.92% of cheese. Other studies showed that neither Listeria monocytogenes nor enterotoxigenic staphylococcus aureus could grow in the cheese core during ripening and on the cheese surface post-ripening (Samelis et al 2009b, Giannou et al 2009), but survival and not inactivation of Listeria monocytogenes at levels below 100cfu/g occur in the core of traditional Greek Graviera cheese during fermentation, ripening and storage (Samelis et al 2009a). The antilisterial effect of Lactic Acid Bacteria (LAB) has been documented by Rodriguez et al (2000) in hard cheeses from raw milk, as well as in Graviera from raw and thermized milk (Samelis et al 2009b, Giannou et al 2009, Samelis et al 2010.…”