Graduate Thesis Or Dissertation
 

The microbial stability of refrigerated intermediate moisture foods

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z316q4376

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  • The lag phase, the specific growth rate, the minimum a [subscript w] required for growth and the temperature characteristic of Fsendomonas fluorescens, Brochothrix thermosphacta, Salmonella typhlmurium, Streptococcus faecalis and, Staphylococcus aureus have been studied in liquid model media under controlled temperature and a [subscript w] conditions. The results show that the lag phase increases and the specific growth rates decreases when the temperature is lowered or the a [subscript w] is reduced. The minimum a [subscript w] required for growth increases when the incubation temperature is lowered. At refrigeration temperature the minimum a [subscript w] for growth of mesophiles is higher than that of psychrotrophs. The temperature characteristic (representing the activation energy for growth) of psychrotrophs is lower than that of mesophiles, and therefore mesophiles are more sensitive to temperature changes. All of these growth parameters depend on the a [subscript w] controlling solute. The solute effect can be traced to the ability of the solute to penetrate into the cell and can be explained by the osmoregulatory mechanism. In general, it was found that the effect of the solute on these parameters had the order glycerol<NaCl<sucrose. A comparison of three models for the effect of temperature on growth rate (linear, square-root, and Arrhenius) showed that the linear model had both the advantage of simplicity and accuracy. In general, the Arrhenius model showed the poorest fit to experimental data. A predictive method was developed to assess the microbial stability of liquid model systems exposed to fluctuating temperatures. The strategy was to predict lag time and growth rate by different regression equations, and to estimate microbial growth using an integral function of the accumulated time. A lag time predictive model was developed using a linear relationship between lag time and reciprocal growth rate. A linear equation was used to predict growth rate as a function of temperature. The model was used to predict the growth of B. thermosphacta in liquid model media with low a [subscript w] and exposed to fluctuating temperatures. The predictions were acceptable at a 95% confidence level. This method will be used in future studies to predict the microbial stability of refrigerated foods exposed to temperature abuse.
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