Assay of proteinases of psychrotrophic bacteria in milk using Hide Powder Azure and a simple procedure for clarification of milk

Abstract

SummaryThe clarification of milk by addition of solutions of Triton X-100 and EDTA after digestion of added Hide Powder Azure (HPA) was found to provide a simple method of determining the extracellular proteinase activity of Gram-negative psychrotrophic bacteria in pasteurized whole milk. The light absorbance of the clarified HPA digestion product was measured directly, after a brief incubation period, and was stable to storage of samples in diffuse daylight for at least 2 d. Proteinase produced by growth in refrigerated whole milk of as few as 2·5 × 106cfu ml−1ofPseudomonas fluorescensAR11 was detected.