Pasteurized whole, 2%, and skim milk were inoculated with Listeria monocytogenes Scott A and treated with high-voltage pulsed electric field (PEF). The effects of milk composition (fat content) and PEF parameters (electric field strength, treatment time, and treatment temperature) on the inactivation of the bacterium were studied. No significant differences were observed in the inactivation of L. monocytogenes Scott A in three types of milk by PEF treatment. With treatment at 25 degrees C, 1- to 3-log reductions of L. monocytogenes were observed. PEF lethal effect was a function of field strength and treatment time. Higher field strength or longer treatment time resulted in a greater reduction of viable cells. A 4-log reduction of the bacterium was obtained by increasing the treatment temperature to 50 degrees C. Results indicate that the use of a high-voltage PEF is a promising technology for inactivation of foodborne pathogens.
The time required to cool size 2B (3.43 to 3.75-cm-diameter) pickling cucumbers by a commercial spray-type hydrocooler to less than 9°C was about 18 min at typical initial fruit temperatures of 25 to 29°C. During this period, the fruit was exposed to the recycled water, which reached relatively high populations of bacteria (106 to 107 colony forming unites [CFU]/g total aerobes and 105 to 106 CFU/g total Enterobacteriaceae) during a typical day's operation. These numbers exceeded those present on the unwashed fruit, depending upon the volume of fruit previously cooled. Residual chlorine dioxide at 1.3 ppm was found to optimally control (2 to 6 log-cycles reduction) the numbers of bacteria. At 0.95 ppm chlorine dioxide, the numbers of bacteria in the water were relatively static, while at 2.8 and 5.1 ppm the odor of chlorine dioxide became excessive. The bacterial populations in/on the cucumbers were not greatly influenced by chlorine dioxide, even at 5.1 ppm. Apparently, microorganisms on or in the fruit were protected from the chlorine dioxide. Thus, the use of chlorine dioxide in hydrocooling water of cucumbers seems to be an effective means of controlling microbial build-up in the water, but has little effect upon microorganisms on or in the fruit.
In this study, the adhesion of bacteria to fresh cucumber surfaces in aqueous suspension was shown to be dependent on time of incubation, inoculum species and concentration, and temperature. The adhesion of bacteria to the fruit in wash water was less extensive at lower temperatures and shorter exposure times. Various species of bacteria were adsorbed to cucumber surfaces in the following relative order: Salmonella Typhimurium > Staphylococcus aureus > Lactobacillus plantarum > Listeria monocytogenes. Cells were adsorbed at all temperatures tested (5, 15, 25, and 35 degrees C) at levels that depended on incubation time, but the numbers of cells adsorbed were larger at higher incubation temperatures. Levels of adhesion of bacteria to dewaxed fruit were higher for L. monocytogenes and lower for Salmonella Typhimurium, L. plantarum, and S. aureus than were levels of adhesion to waxed fruit.
The prebiotic fructooligosaccharides (FOS) content of yacon makes this root an attractive alternative for the supplementation of a variety of food products. The preservation of yacon by fermentation has been proposed as an alternative to increase the probiotic content of the root concomitantly with its shelf life. Thus the fermented yacon could have significant functional content. The objective of this research was to characterize the biochemistry and microbiology of spontaneous yacon fermentation and define the viability of the proposed process. The biochemical analysis of spontaneous heterolactic fermentation of yacon showed a progressive drop in pH with increased lactic and acetic acids, and the production of mannitol during fermentation. The microbial ecology of yacon fermentation was investigated using culture-dependent and culture-independent methods. Bacterial cell counts revealed a dominance of lactic acid bacteria (LAB) over yeasts, which were also present during fermentation. Results showed that the heterofermentative LAB were primarily Leuconostoc species, which dominated the fermentation. The fermentation of yacon by Leuconostoc spp. is thus presented as a viable method to achieve long term preservation of this root.
: A nonacidified, deli‐type pickle product was used as a model system to study the potential use of biocontrol as a means to prevent the growth of pathogens in minimally processed fruits and vegetables (MPFV). Fresh pickling cucumbers were blanched and brined with sterile spices and garlic oil. The product was stored at 5 °C for 3 wk and then transferred to various abuse temperatures (16 °C, 25 °C, 30 °C). Lactic acid bacteria (LAB) were isolated and characterized as potential biocontrol agents, and the isolates were tested for bacteriocin‐like activity. A total of 118 LAB isolates were obtained. Among the LAB identified were species of Lactococcus, Leuconostoc, Lactobacillus, Weissella, and Enterococcus. Three isolates showed transient bacteriocin activity against—Listeria monocytogenes, and 7 isolates (Lactococcus) had bacteriocin‐like activity against other LAB. Although it did not produce a bacteriocin, a Lactobacillus curvatus isolate (LR55) was found to have desirable characteristics for use as a biocontrol (competitive exclusion) culture to enhance the safety of nonacidified deli‐type pickles.
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