A Thermal Resistance Study of STEC in Low-moisture Foods with the Use of Differential Scanning Calorimeter (DSC)

Introduction: According to CDC's Foodborne Disease Outbreak Surveillance System database, from year 2007~2011, there have been eight outbreaks associated with low-moisture foods including nuts, cheese, cookie dough, and wheat snack foods involving Shiga toxin-producing Escherichia coli O157:H7 (STEC), which led to over 318 cases of foodborne illness. Yet, insufficient data on STEC thermal inactivation has been obtained in low-moisture foods.   In this study, a differential scanning calorimeter is used to measure STEC inactivation kinetic parameters in low moisture environments. 

Purpose: The objective of this study was to use a differential scanning calorimeter to measure D- and z-values, and therefore determine the microbial thermal resistance, of STEC.

Methods: Six strains of outbreak related E. coli were individually grown on tryptic soy agar with yeast extract (TSAYE).  The cells were harvested and inoculated into a moist buffer solution, simple model low-moisture matrix, and low-moisture foods (i.e., flour and peanut butter). Samples were individually heated using a differential scanning calorimeter (DSC).  The DSC was able to achieve a reproducible and accurate thermal environment. Following heat treatment, microbial survivors were enumerated via plate count.

Results: The six strains showed greater thermal resistance levels in corn syrup and peanut butter compared with buffer solution (P < 0.05). At the same processing duration, approximately 95°C was needed to reduce the outbreak strains of STEC in peanut butter by 5-log CFU/ml, whereas 85°C and 75°C were needed for corn syrup and buffer solution, respectively.  D₆₅°_C ranged from 0.26-0.92 min for the strains tested in buffer.  Those same strains exhibited a 10-100 times increase in resistance in corn syrup (D₆₅°_C= 2.6-108.6 min).

Significance: The measurement of the increased heat resistance of STEC in low moisture foods will improve science-based risk prevention by ensuring process lethality in these types of foods.