Predictive modeling of inactivation, injury, and recovery of Listeria monocytogenes under combined effect of high pressure and temperature.

摘要

Milk and dairy products are a rich source of macronutrients, proteins, fats and carbohydrates; During processing of milk and dairy products, microbial contaminations occur. Among the several pathogenic microorganisms Listeria monocytogenes is one which is a potential threat to milk and dairy product because of its ability to grow at refrigeration temperatures.;High pressure processing (HPP) is gaining popularity among the nonthermal food processing technologies as it can maintain the freshness of the food products after the treatment. HPP can inactivate microorganisms in the pressure range of 500-700 MPa, extending the shelf-life of the treated food material.;Predictive modeling is an important tool for quantitative determination of the growth and inactivation of microorganisms under specific environmental conditions. The overall goal of the study was to evaluate the inactivation, injury, and recovery of L. monocytogenes under the combined effect of temperature and pressure.;In the current study, three pressure levels (400, 500, and 600 MPa) and three temperature levels (27, 43, and 60°C) were chosen. For each combination, six treatment times (pressure hold time is used to designate the treatment time) were chosen. Following each treatment, cell counts were obtained on nonselective (TSAYE) and selective (MOX) media to determine the healthy plus injured and only healthy cells, respectively. The inactivation of the microorganism was evaluated in terms of the log reduction values. Plating the milk samples on selective and nonselective media revealed that high treatment temperature promoted the inactivation of L. monocytogenes. At a constant treatment temperature the effect of increasing the pressure was also evident. As the treatment times for the seven combinations of pressure and temperature were different, the effectiveness of the treatments was evaluated by comparing the D-values of all the combinations by Tukey's pairwise comparison. Tukey's pairwise comparison on both media showed that the Dvalues of treatment condition 400 MPa, 27°C and 400 MPa, 43°C were significantly different from each other (p < 0.05) and from the rest of the five pressure and temperature combinations. Furthermore, the remaining five combinations were not significantly different from each other (p > 0.05). To determine the effect of plating media for a particular combination of pressure and temperature, the D-values obtained for both the media were also compared by Tukey's pairwise comparison. The comparisons showed that for 400 MPa, 27°C and 600 MPa, 27°C there was no significant difference (p > 0.05) in the D-values of both the media; whereas, for rest of the five combinations the D-values were significantly different (p < 0.05).;Mathematical representations for log reduction vs. time were obtained for both TSAYE and MOX media using the conventional two-parameter Weibull model. Although, the one-parameter Weibull model represented the correct interpretation of alpha, the R2 value of the predicted curve decreased (range was from 0.81 to 0.97 for TSAYE media and 0.82 to 0.95 for MOX media). So a four-parameter Weibull model with two characteristics times (alpha) and two curvatures (beta) were fitted to the log reduction values which improved the R2 values (range was from 0.95 to 0.99 for both TSAYE and MOX media) of the predicted curve.;The log reduction of injured cells was calculated at seven combinations of pressure and temperature by using the log reduction values obtained on the nonselective TSAYE) and selective (MOX) media.;Micro-differential scanning calorimetry (mDSC) study of UHT whole milk, inoculated UHT whole milk, and high pressure and temperature-treated inoculated UHT whole milk samples were done using a VP-DSC MicroCalorimeter. The purpose of the study was to determine if mDSC results can sufficiently differentiate between the untreated inoculated and treated inoculated whole milk samples with respect to the amount of cellular proteins content. Since sufficient differentiation between the inoculated untreated and inoculated treated UHT milk was not observed, mechanistic features could not be incorporated into the combined predictive model for injury and recovery.;With developing a combined model for injury and recovery following the high pressure and temperature treatment, milk samples were kept for 30 days at 4°C and plated at 2-day intervals on TSAYE and MOX media to evaluate injury and recovery. The predictive model enables the determination of the population of total, healthy, injured, and recovered cells at any time during the storage of milk product at 4°C following HPP treatment. In addition, this model can also help to estimate the product shelf-life and assess risk due to the recovery of the pathogenic microorganisms during the storage period. The procedure of this model is expected to be helpful in advancement of the field of predictive microbiology.