Inactivation of Foodborne Pathogens in Low Water Activity Food by Thermal and Non-Thermal Treatments

Abstract

The objectives of this study were ( i ) to investigate the effect of thermal treatment on inactivation of foodborne pathogens in low water activity foods, ( ii ) to evaluate the antimicrobial effect of non-thermal treatment against foodborne pathogens in foods, and ( iii ) to evaluate the antimicrobial effect of the combination treatment of non-thermal and thermal treatments against foodborne pathogens in low water activity foods. To investigate the effect of thermal treatment on inactivation of foodborne pathogens in low aw foods, peanut butter inoculated with Salmonella Senftenberg, Salmonella Typhimurium and Salmonella Tennessee were treated with a 915 MHz microwave with 2, 4 and 6 kW. Then acid and peroxide values and color changes were determined after 5 min of microwave heating. Six kW 915 MHz microwave treatment for 5 min reduced these three Salmonella serovars by 3.24 to 4.26 log CFU/g. Four kW 915 MHz microwave processing for 5 min reduced these Salmonella serovars by 1.14 to 1.48 log CFU/g. Microwave treatment did not affect acid, peroxide, or color values of peanut butter. To evaluate the influence of aw of samples on the antimicrobial effect of 915 MHz microwave heating, peanut butter inoculated with Escherichia coli O157:H7, S. Typhimurium and Listeria monocytogenes in peanut butter (0.3, 0.4 and 0.5 aw) were treated with a 915 MHz microwave with 2, 4, and 6 kW for up to 5 min. Six kW 915 MHz microwave treatment for 5 min reduced these three pathogens by 1.97 to > 5.17 log CFU/g. Four kW 915 MHz microwave processing for 5 min reduced these pathogens by 0.41 to 1.98 log CFU/g. Weibull and Log-Linear + Shoulder models were used to describe the survival curves of three pathogens because they exhibited shouldering behavior. td (decimal reduction time) and t5d (the time required for a 5 log reduction) values were calculated based on the Weibull and Log-Linear + Shoulder models. Generally, increased aw resulted in shorter t5d values of pathogens, but not shorter td values. To evaluate the influence of microwave frequency and scale of samples on the antimicrobial effect of microwave heating, peanut butter with 4 different scales (50, 100, 200 and 400 g) were treated with a 2,450 MHz or 915 MHz microwave up to center temperature reached 100°C. When peanut butter samples were treated with 2,450 MHz, increasing peanut butter scale resulted in slow temperature increasing. But, in the case of 915 MHz, increasing peanut butter scale did not affect center temperature increasing. E. coli O157:H7 and S. Typhimurium inoculated peanut butter were treated with 2,450 MHz for 160 s or 915 MHz for 90 s. A 2,450 MHz microwave heating resulted in different inactivation level at different site and large standard deviation level at same site. But, 915 MHz microwave heat treated peanut butter sample showed much faster inactivation of foodborne pathogens and there were no site dependence or large standard deviation at same position. Furthermore, 915 MHz microwave heating did not affect acid, peroxide and color values. But microwave system did not suitable for almonds pasteurization. To find suitable control intervention for almonds, the effect of packaging type on inactivation of foodborne pathogens by dry heat was investigated. Almonds inoculated with Salmonella Enteritidis PT 30, S. Typhimurium or S. Senftenberg were treated with dry heat under open, ambient-sealed or vacuum-sealed packaging to evaluate how the packaging type influences the antimicrobial effect of dry heat. Color changes were determined after 1 h of dry heating. Salmonella populations were reduced following this sequence: open < ambient sealed < vacuum-sealed heating. Subjecting the three types of packaged almonds to dry heat did not affect color values. Moisture contents of ambient-sealed and open heat treated samples were reduced significantly (P < 0.05). To investigate the effect of non-thermal treatment on inactivation of foodborne pathogens in foods, the antimicrobial effect of the combined treatment of ozone and pH against three foodborne pathogens in apple juice has been evaluated. Apple juice (pH 3.0, 4.0 and 5.0) inoculated with the three pathogens were treated with gaseous ozone (3.0 l/min flow rate and 2.0-3.0 g/m3) for up to 4 min. Ozone treatment (4 min) of pH 3.0 apple juice resulted in > 5.36 log CFU/ml reduction of E. coli O157:H7. Ozone treatment of pH 4.0 and 5.0 apple juice for 4 min reduced this pathogen by 5.12 log CFU/ml and 1.86 log CFU/ml, respectively. The combination of low pH and ozone showed a great antimicrobial effect in apple juice. S. Typhimurium and L. monocytogenes showed a reduction trend similar to E. coli O157:H7. There were no significant changes of color values when apple juice was treated with ozone, except for b values. Among all ozone treated samples, there were no significant differences in total phenolic contents. To increase the efficacy of hydrogen peroxide vapor, the antimicrobial effect of vacuumed hydrogen peroxide vapor was evaluated. Black and red pepper inoculated with E. coli O157:H7 and S. Typhimurium were subjected to 0.5 ml of 10 to 50% vacuumed hydrogen peroxide vapor for 1 min, and color change was evaluated after treatment. Pathogen populations decreased with increasing hydrogen peroxide concentration. Fifty percent vacuumed hydrogen peroxide vapor treatment decreased E. coli O157:H7 and S. Typhimurium populations in black pepper > 5.34 and 4.52 log CFU/g, respectively, and 3.01 and 2.36 log CFU/g for E. coli O157:H7 and S. Typhimurium in red pepper, respectively, without causing color change. To investigate the effect of the combined treatment of thermal and non-thermal treatment on inactivation of foodborne pathogens in low aw foods, the antimicrobial effect of the combined treatment of mild heat and ozone against two foodborne pathogens in apple juice concentrates has been evaluated. Four types of apple juice concentrates (12, 18, 36, 72 °Brix) inoculated with pathogens were subjected to ozone (3.0 l/min flow rate and 2.0-3.0 g/m3 concentration) and heat treatment (25, 45, and 50°C) simultaneously for 20, 40 and 60 s. Heat treatment alone (25, 45, and 50°C) for 1 min reduced populations of E. coli O157:H7 by 0 to 4.18 log CFU/ml in four types of apple juice concentrates. The combination of ozone and heat treatment for 1 min at 25 and 45°C reduced E. coli O157:H7 by 0.93 to 3.87 log CFU/ml and below the detection limit (> 1.0 log CFU/ml) at 50°C. A similar tendency was observed for S. Typhimurium. In several instances, results showed a synergistic effect of ozone and heat treatment. Color values were not changed during ozone and heat treatment. In all ozone treated samples, the concentration of residual ozone was reduced to under acceptable levels (< 0.4 mg/l). As another available hurdle combination, the efficacy of vacuum-sealed dry heat combined with vacuumed hydrogen peroxide vapor for decontaminating seeds was investigated. Alfalfa seeds inoculated with three pathogens were subjected to 30% 1 ml of vacuumed hydrogen peroxide vapor and vacuum or open-dry heat at 73°C. Distilled water was used as a control. Vacuum-dry heat effectively reduced three pathogens in alfalfa seeds but open-dry heat showed less inactivation of pathogens. Sequential treatment of 30% 1 ml vacuumed hydrogen peroxide vapor (1 min) + Vacuum-dry heat (2 h) reduced these three pathogens by under the detection limit (1.0 log CFU/g). Sequential treatment did not affect the germination of alfalfa seeds. In conclusion, the results of this study are helpful for the food industry to control pathogens in foods, especially low water activity foods. Non-thermal, thermal or combination of non-thermal and thermal treatment may suggest alternatives to currently used decontamination methods without quality changes. Also, pathogens inactivation studies, focus on low water activity foods, could facilitate preventing foodborne outbreaks due to pathogen contaminated low water activity foods.