Modelling the effect of pH, sodium chloride and sodium pyrophosphate on the thermal resistance of Escherichia coli O157:H7 in ground beef

Abstract

The objective of this study was to assess the combined effects of temperature, pH, sodium chloride (NaCl), and sodium pyrophosphate (SPP) on the heat resistance of Escherichia coli O157:H7 in minced beef meat. A fractional factorial design consisted of four internal temperatures (55.0, 57.5, 60.0 and 62.5°C), five concentrations of NaCl (0.0, 1.5, 3.0, 4.5 and 6.0 wt/wt.%) and SPP (0.0, 0.1, 0.15, 0.2 and 0.3 wt/wt.%), and five levels of pH (4.0, 5.0, 6.0, 7.0 and 8.0). The 38 variable combinations were replicated twice to provide a total of 76 survivor curves, which were modelled by a modified three-parameter Weibull function as primary model. The polynomial secondary models, developed to estimate the time to achieve a 3-log and a 5-log reduction, enabled the estimation of critical pH, NaCl and SPP concentrations, which are values at which the thermo-tolerance of E. coli O157:H7 reaches it maximum. The addition up to a certain critical concentration of NaCl (~2.7-4.7%) or SPP (~0.16%) acts independently to increase the heat resistance of E. coli O157:H7. Beyond such critical concentrations, the thermo-resistance of E. coli O157:H7 will progressively diminish. A similar pattern was found for pH with a critical value between 6.0 and 6.7, depending upon temperature and NaCl concentration. A mixed-effects omnibus regression model further revealed that the acidity of the matrix and NaCl concentration had a greater impact on the inactivation kinetics of E. coli O157:H7 in minced beef than SPP, and both are responsible for the concavity/convexity of the curves. When pH, SPP or NaCl concentration is far above or below from its critical value, the temperatures needed to reduce E. coli O157:H7 up to a certain log level are much lower than those required when any other environmental condition is at its critical value. Meat processors can use the model to design lethality treatments in order to achieve specific log reductions of E. coli O157:H7 in ready-to-eat beef products.

Some Like It Hot: Heat Resistance of Escherichia coli in Food

“…The effect of NaCl addition on solute accumulation and heat resistance of E. coli is observed at concentrations that are typical for food systems. A critical concentration of NaCl in ground beef, about 2.7-4.7%, substantially increased heat resistance of E. coli O157:H7 at 55-62.5°C (Juneja et al, 2015). In addition, pre-exposure to 5% NaCl at room temperature for 24 h increased the heat resistance of E. coli O157:H7 at 55°C (Bae and Lee, 2010).…”

Modelling the effect of pH, sodium chloride and sodium pyrophosphate on the thermal resistance of Escherichia coli 0157:H7 in ground beef

“…Values used were initial sachet temperature = 4°C, heat-up target temperature = 58.165°C (i.e., 99% of 58.75), sachet thickness = 0.0005 m (=½ × 1 mm thickness), and for the ground beef, (Pan & Singh, 2001) thermal conductivity = 0.38 W m −1°C−1 , density 5 = 1019.5 kg m −3 , specific heat (Anon., 1972) = 3520 J kg −1°C−1 , and water-bath heat transfer coefficient (Bedecarrats, Strub & Peuvrel, 2009;Anon., 2015) = 500 W m −2°C−1 . (Large changes in the value of the heat transfer coefficient for heating and cooling over the range of water temperatures reported by Juneja et al (2015) can be demonstrated to not meaningfully affect predicted heat-up times (Davey, 2015)). 4.…”

“…We acknowledge that the work of Juneja et al (2015) possibly represents very valuable data for a major pathogen. However we wish to draw attention to the following:…”

Inactivation kinetics of food-borne pathogens subjected to thermal treatments: a review

“…Ohmic heating has been used to inactivate Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes in orange and tomato juice [19]. Populations of foodborne pathogens during heat treatment change with heating temperature, water activity (a w ), heating rate, pH, heat shock, recovery medium and composition/physical characteristics of the foods [20][21][22][23][24][25][26][27]. For example, when the ground beef inoculated with Escherichia coli O157:H7 was cooked in a water bath for 1 h at temperature of 55-62.5 C, the D-value (the time required at a certain temperature to reduce a specific microbial population by 90% or by a factor of 10) was significantly lower in ground beef adjusted with pH 4.5 than pH 5.5 [22].…”

“…under conventional and novel thermal technologies are reviewed but only the primary model (the food-borne pathogen evolution as a function of heating time) is concerned [28]. However, since the inactivation kinetics are actually influenced by several factors, such as different bacterial strains, age of the culture, food composition (fat, NaCl, pH and a w ), processing parameters, and physiological state of the organisms, some researchers have established secondary models for predicting survival curves under different conditions [20,24,[29][30][31][32]. Also omnibus models incorporating the primary and the secondary models are further considered for predicting survival curves of pathogens [23,33,34].…”