Analysis of the Variability in the Number of Viable Bacteria after Mild Heat Treatment of Food

Abstract

Variability in the numbers of bacteria remaining in saline solution and whole milk following mild heat treatment has been studied with Listeria innocua, Enterococcus faecalis, Salmonella enterica serovar Enteritidis, and Pseudomonas fluorescens. As expected, the most heat-resistant bacterium was E. faecalis, while P. fluorescens was the least heat resistant, and all bacteria showed greater thermal resistance in whole milk than in saline solution. Despite the differences in the inactivation kinetics of these bacteria in different media, the variability in the final number of bacteria was affected neither by the species nor by the heating substrate, but it did depend on the intensity of the heat treatment. The more severe the heat treatment was, the lower the average number of surviving bacteria but the greater the variability. Our results indicated that the inactivation times for the cells within a population are not identically distributed random variables and that, therefore, the population includes subpopulations of cells with different distributions for the heat resistance parameters. A linear relationship between the variability of the log of the final bacterial concentration and the logarithmic reduction in the size of the bacterial population was found.The safety and quality of many foods, such as minimally processed foods and some ready-to-eat products, are affected by a combination of multiple sublethal stresses due to processing and storage conditions (17, 32). The use of mild preservative processes instead of harsher ones affects the microbiological risk. It is therefore important to identify potential hazards and to perform accurate quantitative microbial risk assessments including probabilistic modeling approaches (17).It is unlikely that food products are contaminated by one unique bacterial strain or species. In fact, even cells from a pure culture derived from one cell are not necessarily identical (25). Nauta (16) differentiated between uncertainty, which is due to the lack of perfect knowledge about a parameter and which is reducible by further measurements, and variability, which reflects the true variability of a population and is therefore irreducible. He showed the importance of both parameters in quantitative microbial risk assessment models. The estimation of variability usually refers to the variation in the responses of the individual cells within the population, which may be homogeneous or heterogeneous. In a homogeneous population, the heat resistance parameters of the individual cells are identically distributed random variables. Heterogeneous populations include subpopulations of cells with different distributions for the heat resistance parameters. The heterogeneity of the individual cells or spores within a population is the most accepted explanation for the tails of inactivation curves (1, 30) and unexpected increases in the variability of the number of survivors after heat treatments (7,8,30).The microbial concentration in a food product at a given time is affected by the initial...

Effects of Modified Atmosphere Packaging, Food Life Extenders and Temperature on the Shelf Life of Ready-Made Dishes

“…E. faecalis is known for its strong heat resistance among lactic acid bacteria Aguirre et al, 2009;McAuley et al, 2012 and its detection in the current study suggests its potential use as a contamination indicator bacterium in precooked ready-made dishes. The test results for coliforms and S. aureus including the viable cell counts SD are shown in indicated that the gas displacement with inert gases was effective for improving the shelf life of deep-fried chicken, and that 80% nitrogen plus 20% carbon dioxide gas mixture was more effective than 100% nitrogen gas.…”

Accelerated storage testing of freeze-dried Pseudomonas fluorescens BTP1, BB2 and PI9 strains

“…AST can be also used to study the stability of freeze-dried bacteria. Supportive work was previously reported (An-Erl King et al, 1998;Achour et al, 2001;Ziadi et al, 2005;Hernández et al, 2006;Aguirre et al, 2009). The effect of different temperature on the survival of Gram positive bacteria was studied.…”

“…Thus, data obtained under accelerated tests are used for performing extrapolation studies and prediction of the real-time behaviour. One disadvantage of this approach is that it considers only the assessment of viable cells, while the residual water content is omitted (Hernandez et al, 2009 et al, 2008;Aguirre et al, 2009). AST can be also used to study the stability of freeze-dried bacteria.…”

Isolation of Highly Heat-Resistant Listeria monocytogenes Variants by Use of a Kinetic Modeling-Based Sampling Scheme

“…Further characterization of the LO28 HHP-resistant variants without mutations in ctsR also revealed increased resistance to heat for most of these variants (31). In addition, thermal inactivation of L. monocytogenes was previously fitted with a biphasic model, indicating the presence of a heat-resistant fraction (1,3,5). However, so far these variants could not be isolated after heat inactivation.…”