CONTROL OF SALMONELLA IN LOW-MOISTURE FOODS: THERMAL DEATH KINETICS AND MICROBIAL VALIDATION OF RADIO-FREQUENCY PROCESSES

Jie Xu

The safety of low-moisture foods (LMFs) is an emerging concern to the food industry arising from the increased number of outbreaks implicated in pathogenic contaminations. There is a need to understanding the mechanism of enhanced thermal resistance of bacteria in a low-moisture environment and validate innovative thermal processing technologies to improve the safety conditions of LMFs. The objectives of this study were to: 1) determine the influence of water activity (aw) at treatment temperatures on the thermal resistance of Salmonella in LMFs, 2) understand the impact of desiccation on the ultra-structure and thermal resistance of Salmonella, 3) develop a dry inoculation method for food powders, and 4) identify the lowest lethality zone of surrogate in wheat flour during radio-frequency (RF) pasteurization. Firstly, aw changes at elevated temperature were explored in three powder systems, wheat flour, almond flour, and whey protein. The thermal inactivation study was conducted by using two types of test cells, thermal death time test cell (TDT cell) and thermal aw cell (TAC). The results showed a linear relationship between logarithmic D-values of Salmonella and aw at treatment temperature, regardless of the food matrices and the test method. Secondly, the ultra-structure of desiccated Salmonella cells after thermal treatment was analyzed using transmission electron microscopy (TEM) and compared with that from tryptic soy broth (TSB). Salmonella cells could survive longer after desiccation with significantly increased thermal resistance. No significant difference in ultra-structure was observed in desiccated cells before/after thermal treatments. Thirdly, the usage of freeze-dried Enterococcus faecium NBBL-2354 (E. faecium) was explored and its potential to be used as an alternative to wet inoculum was evaluated in a RF pasteurization process. Results showed that freeze-dried E. faecium had strong stability of survival retain and thermal resistance in wheat flour when stored at 4℃ for up to 35 days. Freeze-dried E. faecium can be used as a valid surrogate for Salmonella in RF treatments. Fourthly, the lowest lethality zone of RF treated wheat flour was investigated by testing survivors of E. faecium at multiple locations. It was found that the lowest lethality zone was in the bottom layer when wheat flour was subjected to RF heating at 80℃ followed by a 20 min natural cooling. Bigelow model can predict the temperature-time dependent log loss of E. faecium at various locations and different RF heating rates. It can be concluded from the studies that aw of food matrices at treatment temperature is a determining factor influencing the thermal resistance of Salmonella. Salmonella cells showed enhanced thermal resistance to heat after desiccation. Freeze-dried E. faecium can be used as an alternative inoculum to inoculate LMFs. RF pasteurization was a promising and effective technology to process LMFs. Temperature history measured at cold zone can be used to develop an effective thermal process for control of Salmonella.

CONTROL OF SALMONELLA IN LOW-MOISTURE FOODS: THERMAL DEATH KINETICS AND MICROBIAL VALIDATION OF RADIO-FREQUENCY PROCESSES

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