Survival and growth of foodborne microorganisms in processed and individually wrapped cheese slices
The objectives of the research reported here were to determine the growth, survival, or inactivation of selected microorganisms on individually wrapped processed cheese (IWC) slices stored at 5°C and 22°C, and to compare quality indices. IWC slices were spot-inoculated with foodborne pathogenic bacteria (Listeria monocytogenes, Staphylococcus aureus, and Salmonella spp.), spoilage bacteria (Pseudomonas spp. and Lactobacillus spp.), and spoilage molds (Penicillium spp. and Cladosporium spp.). Each bacterium was inoculated at 105 CFUs/g for determination of growth, survival, or inactivation. Molds were inoculated at 102 spores per gram and observed for growth. Fifty percent of the inoculated product samples were held at 5°C (to simulate refrigeration), and the other 50 percent were held at 22°C (to simulate ambient temperature) throughout shelf life. Samples taken on days 0, 3, 7, 10, 14, and 28 and after 2, 3, 6, and 9 months, and were evaluated for surviving cells (by means of appropriate selective media), color (with the cheese color guide), and lipid oxidation (by means of peroxide values). Bacterial inactivation was observed in all conditions. At 14 days, a 5-log reduction was observed for Listeria monocytogenes and Salmonella, while a 3-log reduction was observed for Staphylococcus aureus. For Pseudomonas spp. and Lactobacillus spp., a 2-log reduction was observed within 3 days, with an additional 1-log reduction noted after several months. Mold levels showed no change during the first several weeks of storage. At 84 days, mold levels decreased at 5°C, but they showed growth at 22°C, to approximately 105 CFUs/g. Visual color was evaluated on a 10-point National Cheese Institute scale. During storage at 5°C or 22°C, color became darker and values increased from 4 to 5 and 4 to 7, respectively. Higher peroxide values were also obtained for the samples held at 22°C versus 5°C. From a microbiological standpoint, pathogenic and spoilage bacteria were unable to grow in this product; however, long-term storage at 22°C led to lower product quality and mold growth.