Abstract
Milk spoilage is caused by the presence of proteolytic enzymes produced by Pseudomonas spp. during storage at low temperatures. The aim of this study was to identify Pseudomonas spp. in raw milk and investigate their associated proteolytic properties at low temperatures. Raw milk samples (n = 87) were collected from 87 bulk tanks in Shaanxi Province in China. Pseudomonas spp. were identified using Pseudomonas specific 16S, universal 16S rRNA sequencing, and rpoB gene sequencing. The proteolytic properties of Pseudomonas spp. were examined using milk agar, quantitative trinitrobenzenesulfonic acid assay, and by the presence of alkaline metallopeptidase gene (aprX). A total 143 isolates from all 87 samples were confirmed as Pseudomonas, and were identified as belonging to 14 Pseudomonas species. Of these, 40 (28.0%) isolates revealed proteolysis on milk agar at 2°C, 74 (51.8%) at 4°C, 104 (72.7%) at 7°C, and 102 (71.3%) at 10°C. However, proteolytic activity of 45 (31.5%) isolates exceeded 2 μmol of glycine equivalents per mL at 7°C, followed by 43 (30.1%) at 10°C, 18 (12.6%) at 4°C, and 7 (4.9%) at 2°C. The results reveal proteolytic activity of Pseudomonas spp. present in milk and their spoilage potential at different temperatures.
Highlights
Spoilage of milk resulting from the contamination of dairy products with psychrotrophic microorganisms results in significant losses for the food industry and is a particular concern of the dairy industry (Dogan and Boor, 2003)
The 14 Pseudomonas species are divided into 4 different groups, according to Anzai et al (2000), namely, P. fluorescens group, P. chlororaphis group, P. putida group, and unknown group
Our results are in agreement with those of Chambers (2003), who reported that P. fluorescens was the dominant Pseudomonas spp. in milk and those of Dogan and Boor (2003), who identified 51% of Pseudomonas isolates as P. fluorescens in fluid milk products and dairy processing plants using API20 NE
Summary
Spoilage of milk resulting from the contamination of dairy products with psychrotrophic microorganisms results in significant losses for the food industry and is a particular concern of the dairy industry (Dogan and Boor, 2003). Pseudomonas spp. can grow over a temperature range of 4–42◦C, with an optimal growth temperature above 20◦C (Chakravarty and Gregory, 2015). They are present in different environments and are frequently linked to food spoilage, especially, that of raw milk (Quigley et al, 2013; Chakravarty and Gregory, 2015). The growth of the Pseudomonas is often associated with the production of extracellular enzymes (e.g., peptidases and lipases)
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