EFFECT OF ASCORBIC-ACID IN COMPARISON TO CITRIC AND LACTIC-ACID ON LISTERIA-MONOCYTOGENES INHIBITION AT REFRIGERATION TEMPERATURES

This study analysed the effect of pH (3.0-6.5) as given by ascorbic ac id in comparison to citric and lactic acid at refrigeration temperatur e (4 and 10 degrees C) on the inhibition and survival of Listeria mono cytogenes in trypticase soya broth with yeast extract. Gompertz model was used to obtain microbial growth parameters and a linear model was fitted in the cases of bacteriostatic and bactericidal action of the a cids. for each acidifier, results were expressed as a function of the undissociated acid concentration (u.a.c.). Ascorbic and lactic acid pr oduced a decrease in the initial microbial counts. Two indices were de fined in order to analyse the influence of the acids: the inhibitory e ffect and the inhibition index. The inhibitory effect values showed th e effect on the slopes of growth and lethality curves and inhibition i ndex included the influence on lag phase duration. At pH 4 ascorbic ac id produced higher inhibitory actio than citric and lactic acid which can be attributed to the higher pKa value of ascorbic acid. However, i f the analysis is based on u.a.c, citric acid was more effective becau se with lower concentration higher indices were obtained.