A rapid method for the estimation of the microbiological quality of refrigerated raw milk based on the aminopeptidase activity of Gram-negative bacteria

A rapid method for the estimation of the microbiological quality of refrigerated raw milk was developed based on the aminopeptidase activity of Gram-negative psychrotrophic bacteria, the dominant microbial population in refrigerated foods. Two versions of the test for measuring the aminopeptidase activity are reported (direct and indirect). A good correlation (r=0.93-0.95) between bacterial counts estimated by conventional methods and the aminopeptidase activity determined in milk was obtained. The sensitivity of the indirect assay was 2×104 cfu mL−1. This sensitivity limit complies with the level of detection required to satisfy regulations in many countries. The aminopeptidase test allows the assessment of raw milk quality in approximately 2.5 h, does not require either high-cost equipment or specialised operators, and results can be interpreted both spectrophotometrically and visually.