High pressure induced pH change and its effect on the inactivation of Lactobacillus plantarum and Escherichia coli Public Deposited

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  • Microbial lethality is strongly pH dependent in processing regimes including high hydrostatic pressure processing (HPP). HPP induces the dissociation of weak acids that potentially causes a shift in pH in some systems thereby increasing acidity while pressure insensitive buffers do not undergo dissociation. The degree of high pressure induced pH shift depends on the nature of the acid. Furthermore, in the presence of weak organic acids with antimicrobial properties a pH reduction will increase the concentration of the more active protonated form of these acids. The first objective of this study was to determine the differences in lethality between pressure sensitive and pressure insensitive buffers. The second objective was to determine the synergistic effects of antimicrobial organic acids and pressure sensitive buffers in the reduction of microbial population by HPP. Escherichia coli and Lacto bacillus plantarum were utilized to study differences between buffers of low susceptibility (HEPES, ACES, MES) and high susceptibility (citrate, phosphate) to pH shifts under pressure. In general dissimilar levels of inactivation were observed between pressure sensitive and insensitive buffers. At low pH conditions pressure sensitive buffers achieved higher inactivation levels as was hypothesized. At neutral pH the differences were smaller for L. plantarum. However certain combinations (pH 5 and 6) appear to contradict the general trend observed. In the course of this study it was found that L. plantar'am was less resistant than E. coli to HPP processing under most conditions. The effect of antimicrobials (acetic, propionic, sorbic, and benzoic acids) in combination with HPP on a strain of Lactobacill'us plantarum was studied. The cultures were resuspended in citrate buffers at pH 3 and pH =pKα of the acids, and HPP treated for one minute over a range of pressures. Survivor curves were fitted using a Weibull equation. Despite conditions in which the organic acid was fully protonated, no microbial effect was observed at pH 3, possibly because of the superior lethal effect of high acidity over the presence of antimicrobials. At pH = pKa, three of eight conditions showed statistically significant differences from control even though all conditions showed an improvement in lethality. Concentration of antimicrobial had a minimal impact. In all cases, the incorporation of antimicrobial organic acids even at levels below published MIC resulted in a consistent trend of increased lethality under pressure.
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