Characterization of the microflora of industrial Mexican cheeses produced without added chemical preservatives

This study characterized the microflora of three Mexican fresh cheeses with (Oaxaca) or without starter culture (Panela and cottage cheese). The cheeses were produced without chemical preservatives from pasteurized milk in a single plant at industrial-scale in Mexico and stored for 20 days at 10°C, simulating high market handling temperatures not suitable for consumption. Microbiological counts were determined on PC agar and 6 selective media and randomly selected isolates were identified by biochemical or molecular methods. Lactic acid bacteria were predominant at levels of log 5 to 8 cfu/g cheese on KFS, MRS, and M17 agar. Identification confirmed enterococci and lactobacilli on KFS agar and MRS agar, respectively. On M17 agar, Streptococcus thermophilus, Lactococcus sp., and Leuconostoc sp. were observed as most prevalent isolates. Beyond, typical contaminants such as Enterobacteriaceae and fungi (yeasts and molds) were determined at levels of log 3 to 7 cfu/g cheese with a predominance of Enterobacter amnigenus and Klebsiella oxytoca, Candida guilliermondii and Candida tropicalis, and Penicillium sp. No Listeria were found in any of the tested cheeses. These contaminants potentially involved in the spoilage of fresh non-preserved Mexican cheeses may serve as targets for the development of new biopreservation methods