Comparison of different peptidase substrates for evaluation of microbial quality of aerobically stored meats

Different aminopeptidase and endopeptidase substrates were assessed for the detection of enzymatic activity of microorganisms collected from the surfa ce of aerobically cold-stored pork and beef. The most sensitive substrates were fluorogenic Ala-7-amino-4-methylcoumarin (Ala-AMC) or Leu-AMC and colo rogenic Ala-p-nitroanilide (Ala-pNA). Activity on natural oligopeptides, e. g., bradykinin or alpha(s1) casein fragment 1 to 23, was very low. The corr elation coefficient (r) between log surface counts of 66 meat samples and l og fluorescence or absorbance after incubation of surface microbial cells f or 2 h with Ala-AMC, Leu-AMC, and Ala-pNA was 0.89, 0.83, and 0.82, respect ively. A distinct yellow color was obtained with Ala-pNA when the surface c ount was similar to 10(6) CEU/cm(2). Although correlation and sensitivity w as better, no clear advantage is obtained with the use of the fluorogenic A la-AMC or Leu-AMC instead of Ala-pNA, a substrate proposed by Alvarado et a l. (J. Food Sci. 57:1330, 1992) for rapidly assessing the microbial quality of refrigerated meat. The correlation coefficient (r) between time of cold storage and surface count was 0.69.