Major ecological shifts within the dominant nonstarter lactic acid bacteria in mature Greek Graviera cheese as affected by the starter culture type

Traditional Greek Graviera cheese is often produced from thermized milk to control undesirable bacterial contaminants. Since thermization also reduces the desirable lactic acid bacteria (LAB) microbiota of raw milk, natural undefined or commercially defined starters are used. This study evaluated effects of the type of starter added to bulk thermized milk on the microbiology of mature (day-90) Graviera cheese. Cheeses produced with a natural starter culture (NSC) in non-concentrated yogurt-like form or a commercial starter culture (CSC) containing Streptococcus thermophilus and various Lactococcus lactis strains in concentrated freeze-dried form, were analyzed microbiologically, and 200 LAB isolates (100 from each type of cheese) were identified. The LAB microbiota of the mature CSC-cheeses was dominated by nonstarter strains of Lactobacillus paracasei and Lb. plantarum whereas indigenous Enterococcus faecium and E. durans strains of high phenotypic and genotypic diversity predominated in the respective NSC-cheeses. Populations of enterococci in CSC-cheeses were subdominant by 10 to 100-fold compared with those in NSC-cheeses; E. faecium was the most frequently isolated Enterococcus species from the mature CSC-cheeses. Sporadic or no isolates of other LAB species, including the commercial S. thermophilus and Lc. lactis starter strains in the CSC-cheeses and the natural S. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus starter strains plus indigenous Lactococcus, Leuconostoc and E. faecalis in the NSC-cheeses, were detected. In conclusion, the replacement of the NSC with the CSC controlled growth of dairy enterococci in favor of mesophilic nonstarter lactobacilli during ripening. While safety concerns associated with the inefficiency of NSCs to prevent outgrowth of indigenous enterococci suggest that CSCs should be preferred by traditional Greek Graviera cheese processors, panel sensory evaluations showed that the NSC-ripened cheeses were of slightly lower appearance but of occasionally higher flavor scores than the CSC-ripened cheeses. •Graviera cheeses made with natural (NSC) or commercial (CSC) starter were compared.•Dairy enterococci, mainly E. faecium, predominated in NSC-ripened cheeses.•Mesophilic lactobacilli, mainly Lb. paracasei, predominated in CSC-ripened cheeses.•The NSC failed to control growth of enterococci in thermized cheese milks.•Both cheese (NSC and CSC) types were of similarly high sensorial qualities.