Thermal Resistance Of Microorganisms Research Articles

Impact of Heating Rates on Alicyclobacillus acidoterrestris Heat Resistance under Non-Isothermal Treatments and Use of Mathematical Modelling to Optimize Orange Juice Processing.

Alicyclobacillus acidoterrestris is a spoilage microorganism responsible for relevant product and economic losses in the beverage and juice industry. Spores of this microorganism can survive industrial heat treatments and cause spoilage during posterior storage. Therefore, an effective design of processing treatments requires an accurate understanding of the heat resistance of this microorganism. Considering that industrial treatments are dynamic; this understanding must include how the heat resistance of the microorganism is affected by the heating rate during the heating and cooling phases. The main objective of this study was to establish the effect of heating rates and complex thermal treatments on the inactivation kinetics of A. acidoterrestris. Isothermal experiments between 90 and 105 °C were carried out in a Mastia thermoresistometer, as well as four different dynamic treatments. Although most of the inactivation takes place during the holding phase, our results indicate the relevance of the heating phase for the effectiveness of the treatment. The thermal resistance of A. acidoterrestris is affected by the heating rate during the heating phase. Specifically, higher heating rates resulted in an increased microbial inactivation with respect to the one predicted based on isothermal experiments. These results provide novel information regarding the heat response of this microorganism, which can be valuable for the design of effective heat treatments to improve product safety and stability. Moreover, it highlights the need to incorporate experimental data based on dynamic treatments in process design, as heating rates can have a very significant effect on the thermal resistance of microorganisms.

Evaluation of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella enterica in milk powders at different storage times and temperatures

While the increase in thermal resistance of microorganisms at reduced water activity is demonstrated for low-moisture food products, the effect of storage time on the thermal resistance of microorganisms in low-moisture foods is not well established. As low-moisture foods are stored for long periods and are used as ingredients, cross-contamination can occur at any time period before the lethality step. Therefore, this study was designed to investigate the effect of storage time (30, 60, and 90 d) on the thermal resistance of Salmonella and Enterococcus faecium NRRL B-2354 in milk powders at a low water activity of 0.10 (conservative level). In this study, 2 milk powders, whole milk powder (WMP) and nonfat dry milk (NFDM), were inoculated with a 5-serotype Salmonella cocktail or E. faecium and equilibrated to a water activity of 0.10. The thermal resistance of Salmonella and E. faecium in WMP and NFDM were determined at different storage times (30, 60, and 90 d) at 85°C. The storage time had no effect on the thermal inactivation kinetics of Salmonella within 90 d of storage at 85°C. In the second part of this study, isothermal treatments were also conducted at higher temperatures (90 and 95°C) to evaluate the suitability of E. faecium as a surrogate for Salmonella in milk powders. The D-values of Salmonella in WMP with 30 d of storage at 85, 90, and 95°C were 7.98, 3.35, and 1.68 min. The corresponding values for E. faecium were 16.96, 7.90, and 4.16 min. Higher D-values of E. faecium indicates that it is a conservative surrogate. Similar results were found for NFDM. In general, D-values of both microorganisms are slightly higher in NFDM than WMP. Two primary models (log-linear and Weibull) were compared for their goodness-of-fit. The Weibull model was found to be more appropriate than the log-linear model. This study provides valuable information for establishing process validation for the pasteurization of milk powders.

Influence of Water Activity on Thermal Resistance of Microorganisms in Low-Moisture Foods: A Review.

A number of recent outbreaks related to pathogens in low-moisture foods have created urgency for studies to understand the possible causes and identify potential treatments to improve low-moisture food safety. Thermal processing holds the potential to eliminate pathogens such as Salmonella in low-moisture foods. Water activity (aw ) has been recognized as one of the primary factors influencing the thermal resistance of pathogens in low-moisture foods. But most of the reported studies relate thermal resistance of pathogens to aw of low-moisture foods at room temperature. Water activity is a thermodynamic property that varies significantly with temperature and the direction of variation is dependent on the product component. Accurate methods to determine aw at elevated temperatures are needed in related research activities and industrial operations. Adequate design of commercial thermal treatments to control target pathogens in low-moisture products requires knowledge on how aw values change in different foods at elevated temperatures. This paper presents an overview of the factors influencing the thermal resistance of pathogens in low-moisture foods. This review focuses on understanding the influence of water activity and its variation at thermal processing temperature on thermal resistance of pathogens in different low-moisture matrices. It also discusses the research needs to relate thermal resistance of foodborne pathogens to aw value in those foods at elevated temperatures.

Thermal Resistance of Microorganisms and Polyphenol Oxidase as Related to Solar Pasteurization of Concord Grape Juice

Pasteurization conditions for grape juice were examined and the concept of pasteurizing grape juice utilizing solar energy was explored in this study. The z-values of four selected micro-organisms in grape juice were approximately 5–8 C, whereas polyphenol oxidase in fresh grapes had a z-value of 8.0 C at pH 3 .4. Using a 0.5 m2 solar collector test module constructed for this study, 2.5 h were required to heat the grape juice from 10 to 85 C. The solar pasteurized juice and commercially pasteurized juice were different in flavor and color but not in pH, titratable acidity or soluble solids.

Combined Temperature-Concentration Effects on the Clotting Rate of Chicken Pepsin

Milk clotting rates with chicken pepsin were measured over a wide temperature range and at various enzyme concentrations, yielding isothermally linear relationship between the inverse of the clotting time and enzyme concentration. Two distinct approaches are shown with which the concentration-temperature-coagulation rate data set can be formulated into single overall mathematical equations. One approach uses an Arrhenius type format, whereas in the other, a direct temperature relationship is employed in a manner analogous to the way that thermal resistance of microorganisms is expressed. The overall rate equations enable accurate prediction of clotting times under given conditions of enzyme concentration and temperature and conceivably can be extended to include additional independent variables.

Determination of relationship between heating value and the thermal resistance of microorganisms in canned meat.

Meat was sterilized with gamma-radiation in an aluminum foil bag and then contaminated with a determined quantity of Bacillus subtilis spores. The bag was then placed in the centre of a can filled with meat. After autoclave sterilization, the whole contents of the aluminum foil bags were examined for the quantity of survived bacteria. This procedure enables the determination of thermal resistance of some bacteria on the condition that appears during typical heating processes. Moreover from all the critical areas all surviving microorganisms can be determined.