MATHEMATICALLY MODELING THE REPAIR OF HEAT-INJURED LISTERIA-MONOCYTOGENES AS AFFECTED BY TEMPERATURE, PH, AND SALT CONCENTRATION

Heat-injured cells of Listeria monocytogenes were inoculated into List eria repair broth (LRB) adjusted to various pH levels (4.2, 5.0, 6.6, 8.0 and 9.6) and salt concentrations (0.5%, 2.5%, 5.0%, 7.5% and 10.0% w/v) at controlled temperatures (4, 10, 22, 37 and 43 degrees C) in a complete factorial manner (5(3)). Repair of the injured microorganism s was evaluated using selective and non-selective plating media. The G ompertz parameters, which were generated by fitting the equation with the bacterial counts, were used to calculate the repair percentage as a function of time from which the repair time was estimated. Ail growt h curves fit the Gompertz equation well (R(2) greater than or equal to 0.972). A first-order model described the repair trend closely (R(2) = 0.989 +/- 0.011). Heat-injured Listeria could fully repair in LRB on ly under 63 of 125 conditions tested during 21 days of incubation. Ref rigeration temperature was the most effective means to prevent the rep air of heat-injured Listeria. The minimum temperature required for rep air increased with an increase in NaCl concentration. The pH ranges at which the repair could occur were narrower at 4 and 10 degrees C than those at higher temperature. The repair was observed in media contain ing 10% NaCl between temperatures of 22 and 43 degrees C at pH 6.6.