Characterization of the microflora isolated from queso fresco made from raw and pasteurized milk

This study characterized the microflora present in four Queso Fresco-style cheeses made from raw milk (RM) and two cheeses made from pasteurized milk (PM) obtained directly from six different producers in Mexico. Microbial isolates were initially analyzed on 8 different selective media and further analyzed by 16S rRNA gene sequence analysis. Sequence analysis identified Lactococcus lactis ssp. lactis as an isolate growing on M17 agar in four cheese samples. L. lactis ssp. lactis was the only lactic acid bacterium identified which is traditionally used as a cheese starter culture. Sequence analysis of isolates from MRS medium supplemented with vancomycin identified Leuconostoc mesenteroides in all six cheese samples. A high number of coliforms, enterococci and coagulase-positive staphylococci were identified in all RM cheeses. The numbers of total coliform species were lower in both PM samples (by 2 log₁₀), but the numbers of staphylococci were lower in only one sample (by 3 log₁₀). The numbers of enterococci were high in both PM samples, and sequence analysis identified Enterococcus faecium as the only enterococcal species isolated from all six cheeses. These results suggest that L. lactis ssp. lactis, L. mesenteroides and E. faecium are species that may contribute to the organoleptic qualities of Queso Fresco-style cheeses analyzed in this study. This work was performed to define the natural microflora present in traditional-style Queso Fresco in order to identify bacterial species that may contribute to the organoleptic qualities of the cheese. Identification and enumeration of the "generally regarded as safe" (GRAS) lactic acid bacteria may allow for the development of new culture blends for the production of Queso Fresco using pasteurized milk, while maintaining the desired qualities of the raw milk varieties. In addition, bacteria recovered from the raw milk cheeses may be novel strains offering advantages over commercially available starter and adjunct LAB currently being used to make Queso Fresco. The work also identified potential pathogens and spoilage bacteria associated with traditional-style Queso Fresco, stressing the importance in developing a desirable pasteurized milk cheese.