Thermal pasteurization of shell eggs, at various time-temperature combinations, has been proposed previously and implemented industrially. This study was conducted to determine if shell egg heating rate, which varies with different pasteurization implementations, alters the Salmonella enterica serovar Enteritidis response to different stresses or expression of virulence. Shell eggs, containing Salmonella Enteritidis in yolk, were subjected to a low (2.4°C/min) or a high (3.5°C/min) heating rate during treatments that mimicked the pasteurization temperature come-up stage. The low heating rate protected Salmonella from the following processes: (i) lethal heat at the holding stage, (ii) loss of viability during 8-h cooling after heating, and (iii) sequential antimicrobial ozone treatment. Transcriptional analysis using Salmonella reporter strains revealed that the heat stress response gene grpE was transcribed at 3-fold-higher levels (P = 0.0009) at the low than at the high heating rate. Slow heating also significantly increased the transcription of the Salmonella virulence-related genes sopB (P = 0.0012) and sseA (P = 0.0006) in comparison to fast heating. Salmonella virulence was determined experimentally as 50% lethal dose (LD50) values in an in vivo model. The slow heat treatment mildly increased Salmonella Enteritidis virulence in mice (LD50 of 3.3 log CFU), compared to that in nontreated yolk (LD50 of 3.9 log CFU). However, when ozone application followed the slow heat treatment, Salmonella virulence decreased (LD50 of 4.2 log CFU) compared to that for heat-treated or nontreated yolk. In conclusion, heating shell eggs at a low rate can trigger hazardous responses that may compromise the safety of the final pasteurized products but following the thermal treatment with ozone application may help alleviate these concerns. IMPORTANCE Pasteurization of shell eggs is an important technology designed to protect consumers against Salmonella Enteritidis that contaminates this commodity. A low heating rate is preferred over a high rate during shell egg thermal pasteurization due to product quality concern. However, it is not known whether raising the temperature at different rates, during pasteurizing, would potentially affect product safety determinants. The current study demonstrated that slow heating during the pasteurization come-up stage increased the following risks: (i) resistance of Salmonella to pasteurization holding stage or to subsequent ozone treatment, (ii) recovery of Salmonella during the cooling that followed pasteurization, and (iii) Salmonella's ability to cause disease (i.e., virulence). Our findings inform food processors about potential safety risks to consumers resulting from improper use of processing parameters during shell egg pasteurization. Additionally, treating shell eggs with ozone after heat treatment could alleviate these hazards and protect consumers from natural Salmonella Enteritidis contaminants in shell eggs.
Keywords: D value; Salmonella Enteritidis; heating rate; ozone; pasteurization; shell egg; stress response; transcriptomic analysis.