Control of pathogenic bacteria in Queso Fresco by using generally recognized as safe ingredients

Abstract

Soft-style fresh Hispanic cheeses like “queso fresco” are generally prepared through minimal processing and recent outbreaks have shown that they can be a vehicle of transmission of L. monocytogenes, Salmonella, and E. coli O157:H7. Currently there are no available treatments to reduce post-pasteurization contamination. The goal of this research was to identify effective queso fresco antimicrobial treatments using available Generally Recognized as Safe (GRAS) ingredients. These antimicrobial agents included nisin (Nisaplin®), caprylic acid, trans-cinnamaldehyde, monolaurin, carvacrol, eugenol, levulinic acid, orange-terpenes, d-limonene, eucalyptol, thymol, and clove essential oils. Batches of queso fresco were inoculated with mixtures of approximately 104 CFU/g of different L. monocytogenes, Salmonella, or E. coli O157:H7 strains/serovars and treated with different combinations of antimicrobials. Twenty five strains of L. monocytogenes, 28 strains of S. enterica, and 24 strains of E. coli O157:H7 were tested. Inoculated queso fresco samples were stored at 4˚C or 8˚C for 3 weeks and the bacterial count was determined every other day by direct plating onto PALCAM, bismuth sulfate, or sorbitol MacConkey agar. All nisin and caprylic acid combinations were effective to decrease survival and growth of all L. monocytogenes and caused at least 4 log CFU/g reductions by the end of storage compared to controls. The levels of most strain mixtures were markedly reduced immediately after treatment and their numbers remained below 103 CFU/ g during the experiments. Combinations of nisin and caprylic acid were only effective to reduce at least 2 log CFU/g Salmonella and E. coli O157:H7 if the caprylic acid concentration was greater than 2 g/kg and 0.7 g/kg respectively. Treatments that included cinnamaldehyde (1 to 2 g/kg) reduced Salmonella and E. coli O157:H7 counts almost 3 and 5 log CFU/g respectively. The combination of nisin, caprylic acid, and cinnamaldehyde was the only antimicrobial treatment able to noticeably inhibit the growth of L. monocytogenes, Salmonella, and E. coli O157:H7 in queso fresco. This combination was also tested to evaluate the impact on the quality and sensory attributes of the cheese. For the quality and sensory analysis, treated cheese samples (sample A containing 0.5 g/kg nisin, 0.4 g/kg caprylic acid, and 0.3 g/kg cinnamaldehyde, sample B containing 0.5 g/kg nisin, 0.4 g/kg caprylic acid, and 0.6 g/kg cinnamaldehyde) were stored over a four week period and tested for lactic acid bacteria, aerobic plate count, and psychrotrophic plate counts. Treated queso fresco samples stored for 4 weeks had similar numbers of lactic acid bacteria, aerobic plate counts, and psychrotrophic plate counts to those of control samples. These results indicated that the treatments had no effect on the growth of potential background spoilage microorganisms. A consumer liking test was used to evaluate sensory aspects of treated samples. A total of 4 samples were tested including control, treated sample A (0.5 g/kg nisin, 0.4 g/kg caprylic acid, and 0.3 g/kg cinnamaldehyde), treated sample B (0.5 g/kg nisin, 0.4 g/kg caprylic acid, and 0.6 g/kg cinnamaldehyde), and a commercial sample. The cheese containing 0.6 g/kg of cinnamaldehyde was the least acceptable of all samples, including a commercial sample, and the control sample was rated highest for moderately liking. The panelists were able to note the presence of off-flavors in both treated samples without significant differences between them. All cheese samples, however, were rated between the levels of dislike slightly and like moderately, indicating a certain level of liking for all cheese samples. The combination of nisin, caprylic acid, and cinnamaldehyde was able to control L. monocytogenes, Salmonella, and E. coli O157:H7 in queso fresco with relatively little impact on quality and sensory characteristics of the cheese. This antimicrobial combination could eventually be used to manufacture safer queso fresco.