Specific sterilization of psychrotrophs in foods by inactivation of alanine racemase

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Psychrotrophic bacteria are widely distributed in nature, and often cause deterioration in foods stored at a low temperature. Accordingly, the bacteria have been studied to better understand food preservation, While psychrotrophs are easily sterilized by heat treatment, their extracellular hydrolytic enzymes survive and degrade the quality of foods. Therefore, sterilizing psychrotrophs before their growth in foods is important for food preservation at low temperature. However, there is no effective sterilization method of psychrotrophs in foods other than thermal sterilization, which often changes the original taste.Alanine racemase (EC 5.1.1.1) is a pyridoxal 5'-phosphate-dependent enzyme catalyzing racemization of L- and D-alanine. The enzyme occurs widely in bacteria and is essential in synthesis of peptidoglycan of bacterial cell walls by providing the D-enantiomer. The peptidoglycan biosynthesis is unique to bacteria, and therefore the enzyme has been a target of antibacterial dru … More gs. We examined the sensitivity of psychrotrophs to organic acids and spices in comparison with mesophiles, in order to develop an effective sterilization method of psychrotrophs based upon inactivation of alanine racemase.Fsychrotrophs such as P.fluorescens and B.psychrosaccharolyticus were found to be more sensitive to citrate, acetate, lactate, and succinate at 0.01% concentration than the mesophiles. The organic acid at the concentration also inactivated alanine racemase. Mesophiles were tolerant to the treatment and the intracellular alanine racemase in mesophiles was not inactivated.Also, psychrotrophs were more sensitive to various spices than the mesophiles, although the sensitivity of psychrotrophs to each spice was different depending on the species. When an equal volume of allspice, turmeric, nutmeg, and laurel leaves extracts was mixed and analyzed, the mixed extract completely suppressed the growth of all surveyed psychrotrophs at the concentration of 0.016% for 6 days. When eggplant, cucumber, and cabbage were soaked in the 0.016% mixed extract at 25oC for 10 min and stored at 4oC for 11 days, the growth of psychrotrophs was completely suppressed. The mixed spice extract may be useful for the suppression of psychrotrophs. Less