Heat Resistance Of Microorganisms Research Articles

High Heating Rates Affect Greatly the Inactivation Rate of Escherichia coli.

Heat resistance of microorganisms can be affected by different influencing factors. Although, the effect of heating rates has been scarcely explored by the scientific community, recent researches have unraveled its important effect on the thermal resistance of different species of vegetative bacteria. Typically heating rates described in the literature ranged from 1 to 20°C/min but the impact of much higher heating rates is unclear. The aim of this research was to explore the effect of different heating rates, such as those currently achieved in the heat exchangers used in the food industry, on the heat resistance of Escherichia coli. A pilot plant tubular heat exchanger and a thermoresistometer Mastia were used for this purpose. Results showed that fast heating rates had a deep impact on the thermal resistance of E. coli. Heating rates between 20 and 50°C/min were achieved in the heat exchanger, which were much slower than those around 20°C/s achieved in the thermoresistometer. In all cases, these high heating rates led to higher inactivation than expected: in the heat exchanger, for all the experiments performed, when the observed inactivation had reached about seven log cycles, the predictions estimated about 1 log cycle of inactivation; in the thermoresistometer these differences between observed and predicted values were even more than 10 times higher, from 4.07 log cycles observed to 0.34 predicted at a flow rate of 70 mL/min and a maximum heating rate of 14.7°C/s. A quantification of the impact of the heating rates on the level of inactivation achieved was established. These results point out the important effect that the heating rate has on the thermal resistance of E. coli, with high heating rates resulting in an additional sensitization to heat and therefore an effective food safety strategy in terms of food processing.

Computational fluid dynamics approaches in quality and hygienic production of semisolid low-moisture foods: a review of critical factors.

Low-moisture foods have been responsible for a number of salmonellosis outbreaks worldwide over the last few decades, with cross contamination from contaminated equipment being the most predominant source. To date, actions have been focused on stringent hygienic practices prior to production, namely periodical sanitization of the processing equipment and lines. Not only does optimum sanitization require in-depth knowledge on the type and source of contaminants, but also the heat resistance of microorganisms is unique and often dependent on the heat transfer characteristics of the low-moisture foods. Rheological properties, including viscosity, degree of turbulence, and flow characteristics (for example, Newtonian or non-Newtonian) of both liquid and semisolid foods are critical factors impacting the flow behavior that consequently interferes heat transfer and related control elements. The demand for progressively more accurate prediction of complex fluid phenomena has called for the employment of computational fluid dynamics (CFD) to model mass and heat transfer during processing of various food products, ranging from drying to baking. With the aim of improving the quality and safety of low-moisture foods, this article critically reviewed the published literature concerning microbial survival in semisolid low-moisture foods, including chocolate, honey, and peanut butter. Critical rheological properties and state-of-the-art CFD application relevant to quality production of those products were also addressed. It is anticipated that adequate prediction of specific transport properties during optimum sanitization through CFD could be used to solve current and future food safety challenges.

Microbiological aspects of thermally processed foods

The knowledge requirements of the food microbiologist working with thermally processed products are very challenging; particularly in relation to minimally processed foods. In the past, the minimum criteria for sterilized canned foods was very well-defined, however, the diversity of food products, packages and related heat processes are now very difficult to summarize in terms of minimum requirements. The overall heating characteristics of these products can be complex, particularly for mixed particulate products or multicompartment packages. Similarly, the death kinetics of the contaminating micro-organisms can be markedly different dependent on the composition of the food product. These factors combined with the consumer demands for 'fresher' foods has resulted in product, process and package development and innovation progressing at a much faster rate than in the past. While food safety is the most important factor, there are also other issues such as shelf-life and visual quality that have strong market influences and must therefore also be considered by the microbiologist.

Microbiological safety and sensory characteristics of salmon slices processed by the sous vide method

Abstract The aim of this work is to evaluated the microbiological quality and the sensory characteristics of salmon slices processed by the sous vide method under different conditions of time–temperature when stored at 2 and 10 °C. Three different combinations of time–temperature were studied (5 min/65 °C, 10 min/90 °C and 15 min/90 °C). The product stored at 2 or 10 °C was evaluated periodically to study its sensory quality, microbiological condition and proximate composition. Batches stored at 2 °C had lower growth rates of mesophiles and psychrotrophs. Staphylococcus aureus , Bacillus cereus , Clostridium perfringens and Listeria monocytogenes were not found in any of the samples. Neither aerobic nor anaerobic sporeforming bacteria were detected in the salmon processed at 90 °C for 15 min and stored at 2 °C after 45 days. The heat treatment of 90 °C for 15 min was the most effective for extending the shelf life of fish (>45 days). However, under these conditions the sensory characteristics were not optimal. It can be conclude that the treatment at 90 °C for 15 min was the most effective one to ensure the safety and to extend the shelf life of sous vide salmon. However, under these conditions the sensory characteristics were not optimal immediately after processing. This study emphasizes the importance of heat treatment and storage temperature in order to ensure the safety particularly in a fat fish, since fat can acts as a protecting agent in the heat resistance of microorganisms.

Microbicidal efficacy of superheated steam. II. Studies involving E. faecium and spores of B. xerothermodurans and B. coagulans

AbstractThe authors' studies on the heat resistance of microorganisms and the superheating of steam (Spicher etal., Zbl. Hyg. Umweltmed. 201, 541 – 553, 1999) have been continued, using the vegetative bacterium, Enterococcus faecium, and spores of B. xerothermodurans and B. coagulans. The temperature of the saturated steam was 68 °C(E. faecium), 105 °C (B. xerothermodurans) and 110 °C (B. coagulans), respectively. The steam was superheated by 30 – 40K, as a maximum. The test organisms had been fixed to fibre glass fleece. The time of exposure to saturated or superheated steam after which 50% of the bioindicators yielded no viable germs was used as a measure of resistance of the organisms. In the discussion, reference has also been made to the findings for B. subtilis and B. stearothermophilus spores obtained in the preceding study.E. faecium exhibited a maximum resistance when superheating the steam by 5K. The superheated steam required a 74-fold exposure time compared to saturated steam of 68 °C. Resistance became gradually reduced as superheating was further increased. Even superheating by 30K required a 11-fold exposure time.The bacterial spores exhibited maxima of resistance on superheating by 23 – 30K. In these cases necessary exposure times were 119 (B. subtilis), 30 (B. xerothermodurans), and 4 times (B. coagulans, B. stearothermophilus) longer than those required in saturated steam. In the superheating range below 10K, behaviour patterns varied. Thus, heat resistance may initially become reduced with increasing superheating (B. coagulans), remain on almost the same level as under exposure to saturated steam (B. stearothermophilus), reach a stage of weakly enhanced resistance (B. xerothermodurans), or approach a maximum of resistance in an almost linear mode (B. subtilis). It appears that there are differences between strains of one and the same bacterial species.

Heat resistance of microorganisms: an improved method for survival counting

A modification of the plate count method is described which brings important advantages upon the normal plate-counting system as used in heat resistance determinations ( D t). It saves cost and time, and makes less liable the appearance of handling errors. Precision of counting in the range of 70–20000 colonies per plate was studied and D t values obtained by survival counting at three ranges of number of colonies per plate were compared. Precision of this system was shown to be as high as that of the pour plate method as normally used, and no significant difference was found among thermal death rates ( D 105 values) obtained at the three ranges of number of colonies per plate tested.

Effect of pH and Sodium Chloride on Heat Resistance of Microorganisms in Eel Bouillion and Eel Fillet

The purpose of this investigation was to modify the conventional method of processing frozen eel (sirayaki). The thermal resistance of E. coli Type I in eel bouillion with varied pH values and NaCl concentration was studied first. The effect of NaCl concentration, baking and freezing on the weight loss and survival of microorganisms in eel filets was then studied in a frozen baked ell processing factory. The resistance of E. coli decreased at pH values far from neutral and also decreased with increasing NaCl concentration at pH 7.0 in all tested eel bouillions. However, the decrease was gradual when the concentration was higher than 10%. The combined effect pH and NaCl lowered the D value for at least 0.96 log cycle when compared with the control. Frozen baked eel pretreated in 10% saline solution for 3 seconds showed reduced weight loss of at least 0.35%, but the reduction in microbial count was not significant. However after freezing there was a significant decrease in the number of microorganisms (p<0.01) in saline-soaked fillets.