Inactivation of Salmonella, Escherichia coli and Bacillus cereus spores on foods using superheated steam

Abstract

The consumption of spices and nuts has been increased due to the public favoring healthy and fresh foods. Since nuts and spices are prone to deteriorating quality during cooking and sterilization procedure, they are usually consumed as raw material or treated with minimal subsequent sterilization processes. However, these foods are often exposed to foodborne pathogens such as Salmonella during the harvesting, which is a serious hazard for the food safety or due to the low moisture contents during the drying and pulverizing processes promoting the forming of strains of Bacillus spores, some foods are easy to get damages in quality from thermal treatment. Thus, the red peppers were sterilized to prevent the formation of spores from the vegetative Bacillus cells. Based on the previous study that the lab-scale sterilization using superheated steam (SHS) was conducted on black peppercorns, pecans, and almonds, the SHS equipment was improved for the industrial applications as a scale-up study in consideration of increasing the treatment amount of samples and changing the direction of SHS flow in order to reduce heat loss. In this study, the applicability of the modified equipment was verified as a sterilization apparatus on food. Salmonella inoculated on black peppercorns, pecans, and almonds was decontaminated below to detection limit (1 log CFU/g) by 100°C saturated steam, 140 and 180°C superheated steam. And E. coli, and Salmonella inoculated on red pepper were achieved below to detection limit (0.6 log CFU/g) within 10 s by 120, 150, and 180°C SHS, and total aerobic bacteria was reduced within 20, 30, and 50 s by 120, 150, and 180°C SHS, respectively. Due to SHS treatment at a high temperature for a short time, capsaicin content did not change. Recently, instant foods are more demanded as the number of singles and dual income households increase. Garlic, one of the main ingredients in instant food, is easily contaminated with Bacillus, which allows heat-resistant spores to form and be present in garlics at the level of 2-3 log CFU/g even after washing step. For retort foods containing garlic, sterilization is done at 121.1°C. However, the time for an additional processing is necessary owing to deal with heat-resistant bacteria and Bacillus spores including Bacillus cereus strain in the garlic, causing the quality loss of the foods. For this reason, B. cereus spores were pretreated with SHS to reduce the total time for subsequent retort processing and minimize the quality deterioration of foods. The study for inactivating the B. cereus spores by SHS was conducted with garlics cloves and sliced garlics, and germinants were used to induce the spore germination in order to increase the efficiency of the SHS sterilization process. As a result, the treatment of SHS at 180°C on garlic cloves inactivated the contamination of the spores by 3 log CFU/g within 90 s. Inactivation of the spores on garlic cloves was more efficient than the sterilization on garlic slices, because garlic slices were dried up faster during the SHS treatment. Effects of the germination inducers were investigated using a germinant buffer with L- alanine, inosine (as famously known as germination inducers), and disodium 5-inosinate (IMP). The Ala+IMP buffer was applied to real food sample with spores inoculated. The sample treated with the germinant buffer was about 0.5 log CFU/g more effective to reduce the spores than the sample without germinant buffer treatment. As a result, a synergistic effect of Ala+IMP was confirmed. The inactivation pattern of B. cereus spores by SHS on garlic was fitted by the Weibull model and tR-value calculated from the Weibull model was tended to decrease as temperature increased. Through pre-treatment sterilization of spores in garlic, total retort processing time could be reduced which minimizes food quality loss.