Application of atmospheric cold plasma for inactivating enteric pathogenic bacteria on shredded coconut and molds on shredded mozzarella and mild cheddar cheese

Abstract

Over the course of the past decade, there has been a shift in consumer demand for less processed and more “natural” foods devoid of synthetic preservatives. In this regard, the food industry has begun to explore using more natural food additives and non-thermal antimicrobial treatments for improving the microbial safety and shelf-life of foods. This thesis reports on the utilization of high voltage atmospheric cold plasma (HVACP) two enteric pathogens and spoilage molds on shredded coconut (dried) and shredded cheese, respectively. In the first study, the efficacy of HVACP against Salmonella enterica and Escherichia coli in shredded coconut was evaluated. Five-strain mixtures of each pathogen were used to inoculate samples of coconut. The samples were then exposed to dielectric barrier discharge HVACP (70kV) at varying treatment times. The Escherichia coli-inoculated samples were treated in 1-minute intervals for 1 to 5 minutes, while the Salmonella-inoculated samples were treated for 3 to15 minutes in 3-minute intervals. Based on the decimal reduction time (D-value) for each pathogen, Escherichia coli exhibited a significantly higher resistance to HVACP compared to Salmonella enterica. The use of the thin agar layer (TAL) method for recovery of pathogen survivors indicated the presence of sub-lethally injured cells in the surviving populations of both pathogens. The effect of HVACP on the quality of the shredded coconut was also assessed, and changes in product color and texture were minimal. In the second study, the efficacy of HVACP (80 kV) against spoilage molds namely, Aspergillus flavus and Penicillium commune, in shredded mozzarella and mild cheddar cheese, was investigated. Blocks of organic mozzarella and cheddar cheese devoid of added preservatives were shredded, and an anticaking agent (Vitacel) added to half of the samples. Batches of the shredded cheese were spot-inoculated with conidial suspensions of Aspergillus flavus and Penicillium commune. The inoculated samples were then exposed to HVACP (80kV) for 4 to16 minutes at 4-minute intervals. After a 16-minute treatment, numbers of viable Aspergillus flavus were reduced by 1.78 log10 CFU/g on cheddar cheese and 2.41 log10 CFU/g on mozzarella cheese. Under those same treatment conditions, numbers of viable Penicillium commune were reduced by 2.73 and 2.03 log10 CFU/g on cheddar cheese and mozzarella cheese, respectively. Lower D-values were observed for both molds in cheddar and mozzarella cheeses with added Vitacel compared to cheese without that anticaking agent; however, only the lower D-values for the molds in cheddar cheese were of statistical significance (P<0.05). In HVACP –treated cheese with added Vitacel, sub-lethal injury was detected in the survivors of Aspergillus flavus (on cheddar) and Penicillium commune (on mozzarella). HVACP (80 kV, 12.0 min) did not produce visible changes in color of cheddar or mozzarella cheese under the conditions tested in the present study. However, treated cheddar and mozzarella cheeses exhibited increased firmness, cohesiveness and resilience compared to the control (P < 0.05). HVACP (80 kV, 12 min) prevented the appearance of mold in aerobically stored (4 ᵒC) mozzarella cheese for up to 25 days (Penicillium commune) and 35 days (Aspergillus flavus). In contrast, under those same treatment (HVACP at 80 kV, 12 min) and product storage conditions (aerobic, 4 ᵒC), no molds were observed on cheddar cheese throughout storage (42 days). The results of the present study indicate that HVACP can inactivate foodborne pathogens Salmonella enterica and Escherichia coli on shredded coconut, and spoilage molds (Aspergillus flavus and Penicillium commune) on shredded cheddar and mozzarella cheese. Thus, HVACP has good potential for enhancing the microbial safety of shredded coconut, and extending the microbial shelf-life of shredded cheddar and mozzarella cheese during refrigerated (4 ᵒC) storage under aerobic conditions.