Abstract
ABSTRACT In this study, we investigated the contamination of refrigerated raw milk produced in the western region of Paraná, southern Brazil, with psychrotrophic microorganisms, aiming to assay the proteolytic activity of the isolates and to identify Pseudomonas fluorescens, the main proteolytic species associated with the spoilage of milk products. Raw milk samples from 50 dairy farms were submitted to the counting of psychrotrophic microorganisms, being the microbiota characterized by its mesophilic behavior and proteolytic capacity, besides molecular identification of P. fluorescens. Of the samples evaluated, 94% had psychrotrophic counts ranging from 3 to 7.1 log CFU mL-1, and 48.5% of these showed mesophilic behavior. Of the isolates, 48.0% had proteolytic activity in at least one evaluated temperature (21 and 30°C), and 39.3% had proteolytic activity in both temperatures. Among the 61 isolates submitted to molecular identification by polymerase chain reaction (PCR), 86.8% contained the expression of the 16S gene characteristic for P. fluorescens. In this study, we demonstrated that P. fluorescens is the most prevalent psychrotrophic bacteria species in raw refrigerated milk and their proteolytic ability poses high risks to the dairy industry.
Highlights
MATERIALS AND METHODSRaw milk is an ideal medium for the growth of contaminating microorganism due to its pH close to neutral value, high water activity, and rich nutrient and mineral content
The prolonged refrigerated storage of raw milk may select some bacteria with psychrotolerant behavior, especially the genus Pseudomonas, that are responsible of the spoilage of milk and dairy products owing to their enzymatic activity (MUNSCH-ALATOSSAVA; ALATOSSAVA, 2006; NEUBECK et al, 2015; PORCELLATO et al, 2018)
It is widely reported that contamination of raw milk by P. fluorescens induces instability in ultra-high temperature (UHT) pasteurized milk during storage (GAUCHER et al, 2011; BAGLINIÈRE et al, 2012; STUKNYTĖ et al, 2016) and technological defects in cheese (MARTIN et al, 2011; CARRASCOSA et al, 2015), milk powders (CHEN et al, 2003) and others dairy products
Summary
MATERIALS AND METHODSRaw milk is an ideal medium for the growth of contaminating microorganism due to its pH close to neutral value, high water activity, and rich nutrient and mineral content. The prolonged refrigerated storage of raw milk may select some bacteria with psychrotolerant behavior, especially the genus Pseudomonas, that are responsible of the spoilage of milk and dairy products owing to their enzymatic activity (MUNSCH-ALATOSSAVA; ALATOSSAVA, 2006; NEUBECK et al, 2015; PORCELLATO et al, 2018). Some psychrotrophic bacteria produce and release proteases and lipases to the external environment, compromising the integrity of the milk constituents and the quality of dairy products (BAUR et al, 2015; NEUBECK et al, 2015; RIBEIRO JUNIOR et al, 2018).
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