Title: Applicability of pulsed field gel electrophoresis for the identification of lipolytic and/or proteolytic psychrotrophic Pseudomonas species in raw milk
Abstract: Many types of microorganisms are present in the milk collection environment and diversity in the raw milk microflora is typical, without dominance of a single species. The proportion of psychrotrophic bacteria in raw milk can vary widely and is associated with the level of farm hygiene. Studies in Europe have shown that typically, no more than 10% of the flora of good quality milk will be psychrotrophic with Pseudomonas species comprising a substantial proportion of these. Pseudomonas fluorescens, the most common species of the genus present in raw milk, has been involved in bacterial spikes (sudden elevations in total bacterial count) in farm bulk tank milk. Psychrotrophic Pseudomonas species play an important role in spoilage of UHT milk through the production of heat-stable lipases and proteases in raw milk that retain activity following UHT processing. Lipase and protease, produced by psychrotrophic Pseudomonas species are detected when the cell count exceeds ~10 cfu/mL. Prolonged refrigerated (4 °C) storage of raw milk increases the proportion of Pseudomonas species as do slightly higher temperatures (for example 6 °C) over a shorter period of time. This in turn increases the likelihood that they will produce heat-stable lipases and proteases. Furthermore, temperature fluctuations have been shown historically to occur in farm bulk milk, and the temperature of raw milk at the time of collection can vary widely. While less likely to occur today, both these scenarios could further compound the problem of Pseudomonas species proliferation in raw milk. The aim of the present study was to investigate the use of pulsed field gel electrophoresis (PFGE) for identifying sources of lipase and/or protease producing psychrotrophic Pseudomonas species at various pre-processing locations, and to track the types identified through the pre-processing environment. Incubation of raw milk was also carried out to simulate possible scenarios where the raw milk may be stored on the farm and in the silo prior to UHT processing. This enabled enrichment for spoilage bacteria and studies to identify sources of microorganisms that may contribute to lipolysis and proteolysis in raw and, subsequently, UHT milk or other long life dairy products. The impact of various storage conditions on the different Pulsed Field (PF) types of importance with regard to lipase and protease production was also assessed.
Publication Year: 2012
Publication Date: 2012-01-01
Language: en
Type: article
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