Abstract
Improper food processing is one of the major causes of foodborne illness. Accurate prediction of the thermal destruction rate of foodborne pathogens is therefore vital to ensure proper processing and food safety. When bacteria are subjected to pH and thermal stresses during growth, sublethal stresses can occur that may lead to differences in their subsequent tolerance to thermal treatment. As a preliminary study to test this concept, the current study evaluated the effect of prior pH and thermal stresses on thermal tolerance of Salmonella and Staphylococcus using a tryptic soy broth supplemented with yeast extract. Bacteria incubated at three pH values (6.0, 7.4, and 9.0) and four temperatures (15, 25, 35, and 45°C) for 24 hr were subjected to thermal treatments at 55, 60, and 65°C. At the end of each treatment time, bacterial suspensions were surface‐plated on standard method agar for quantification of bacterial survival and further calculation of the thermal death decimal reduction time (D‐value) and thermal destruction temperature (z‐value). The effect of pH stress alone during the incubation on the thermal tolerance of both bacteria was generally insignificant. An increasing pattern of D‐value was observed with the increment of thermal stress (incubation temperature). The bacteria incubated at 35°C required the highest z‐value to reduce the 90% in D‐values. Staphylococcus mostly displayed higher tolerance to thermal treatment than Salmonella. Although further research is needed to validate the current findings on food matrices, findings in this study clearly affirm that adaptation of bacteria to certain stresses may reduce the effectiveness of preservation procedures applied during later stage of food processing and storage.
Highlights
Knowledge of bacterial responsiveness over a range of conditions enables predictions of bacterial growth and destruction
The levels of Salmonella tended to increase with the increment of pH at incubation temperatures of 35 and 45°C, while similar observations were made with the decrement of pH at 15 and 25°C, indicating that slightly alkaline and acidic environment favored bacterial growth at 35 and 45°C, and 15 and 25°C, respectively
The net effect (p ≤ 0.03) of thermal and pH stresses on the decrease of z‐value was observed in the increment of both thermal stress temperature from 35 to 45°C and pH from 6.0 to 9.0. These results indicate that the bacteria subjected to thermal stress at 35°C required the highest increase in temperature to obtain the thermal death decimal reduction time throughout the pH stresses
Summary
Several scientists indicated tolerance of foodborne pathogens for non‐optimal pH and temperature (Gandhi & Chikindas, 2007; Glass, Loeffelholz, Ford, & Doyle, 1992; Shachar & Yaron, 2006), survival of foodborne pathogens in non‐optimal pH and temperature (Rocourt & Cossart, 1997; Zhao & Doyle, 1994), and resistance to the lethal effects of very low pH (Leyer, Wang, & Johnson, 1995) Aligning with those reports, when bacteria are subjected to pH and thermal stresses, sublethal stresses can occur that may lead to differences in terms of their tolerance to thermal process that follows. The terms “cool,” “ambient,” “warm,” and “excessive heat” for 15, 25, 35, and 45°C, respectively, defined as in U.S Pharmacopeia 659(USP 2017) are used for description purpose of thermal stresses in this article
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