Submitted to: Journal of Food Safety
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/2/2007
Publication Date: 2/1/2008
Citation: Renye Jr, J.A., Somkuti, G.A., Vallejo-Cordoba, B., Van Hekken, D.L., Gonzalez-Cordova, A. 2008. Characterization of the microflora isolated from queso fresco raw milk and pasteurized milk. Journal of Food Safety. 28(1):59-75. Interpretive Summary: Queso Fresco is one of the most popular Hispanic soft-style cheeses available today. Production of traditional raw milk Queso Fresco is not permitted within the U.S. since the final product spoils prior to the 60-day holding period required for all raw milk cheeses. Queso Fresco produced in the U.S. is made with pasteurized milk and commercial starter bacteria, but it does not possess exactly the same texture and flavor of the raw milk cheeses. The rising demand for authentic Hispanic-style cheeses within the U.S. has increased the need for the development of a pasteurized milk cheese with the qualities of the traditional raw milk Queso Fresco. We have analyzed six commercial brands of Queso Fresco from Mexico in order to identify the different bacteria present in the final product. From the bacteria identified, it is thought that three species could serve as starter bacteria to produce a Queso Fresco using pasteurized milk with the quality traits of traditional raw milk cheeses.
Technical Abstract: This study characterized the indigenous microflora present in four raw milk (RM) samples and two pasteurized milk (PM) samples of Queso Fresco from Mexico. As expected the microbial load was high for both the raw and pasteurized milk cheeses, ranging between log 7.31 and 8.98 CFU g-1 on plate count agar. The highest colony counts were obtained on M17 and MRS agars, used for the selection of streptococci and lactobacilli. Sequence analysis of the 16S rRNA genes from isolates obtained on these media identified Lactococcus lactis ssp. lactis, in four samples, as the only lactic acid bacteria (LAB) traditionally used in cheese starter cultures. Lactobacillus species were not identified in any samples. Growth on selective media also suggested that Leuconostoc species were present in all cheeses and sequence analysis confirmed the isolates to be L. mesenteroides. A high concentration of coliforms, enterococci and coagulase-positive staphylococci were identified in all RM cheeses. The numbers of coliform species showed a 2 log10 reduction in both PM samples but the number of presumptive coagulase-positive staphylococci was reduced in only one sample (3 log10 reduction). The number of enterococci remained high in both PM samples. Sequence analysis identified Enterococcus faecium as the only enterococcal species isolated from all cheeses. These results suggest that L. lactis, L. mesenteroides and E. faecium may act as a specific starter culture for the production of traditional Queso Fresco.