Microbiological quality of raw milk attributable to prolonged refrigeration conditions

Refrigerated storage of raw milk is a prerequisite in dairy industry. However, temperature abused conditions in the farming and processing environments can significantly affect the microbiological quality of raw milk. Thus, the present study investigated the effect of different refrigeration conditions such as 2, 4, 6, 8, 10 and 12 °C on microbiological quality of raw milk from three different dairy farms with significantly different initial microbial counts. The bacterial counts (BC), protease activity (PA), proteolysis (PL) and microbial diversity in raw milk were determined during storage. The effect of combined heating (75 ± 0·5 °C for 15 s) and refrigeration on controlling those contaminating microorganisms was also investigated. Results of the present study indicated that all of the samples showed increasing BC, PA and PL as a function of temperature, time and initial BC with a significant increase in those criteria ≥6 °C. Similar trends in BC, PA and PL were observed during the extended storage of raw milk at 4 °C. Both PA and PL showed strong correlation with the psychrotrophic proteolytic count (PPrBC: at ≥4 °C) and thermoduric psychrotrophic count (TDPC: at ≥8 °C) compared to total plate count (TPC) and psychrotrophic bacterial count (PBC), that are often used as the industry standard. Significant increases in PA and PL were observed when PPrBC and TDPC reached 5 × 104cfu/ml and 1 × 104cfu/ml, and were defined as storage life for quality (SLQ), and storage life for safety (SLS) aspects, respectively. The storage conditions also significantly affected the microbial diversity, wherePseudomonas fluorescensandBacillus cereuswere found to be the most predominant isolates. However, deep cooling (2 °C) and combination of heating and refrigeration (≤4 °C) significantly extended theSLQandSLsof raw milk.</jats:p

Mapping the thermo-tolerant proteases in ultra high temperature (UHT) treated milk using molecular approaches

Refrigerated storage of raw milk selects for psychrotrophic bacterial genera such as Pseudomonas (mainly Pseudomonas fluorescens), Bacillus, Acinetobacter, Hafnia, Klebsiella, Rahnella, Stenotrophomonas and Aeromonas, Serratia, which are disseminated by means of feed, faeces, bedding material, soil, air, water and milking and processing equipment at the farming and processing environments. These bacteria produce a wide range of extracellular proteases that are resistant to the current heating regimes used in Ultra High Temperature (UHT) processing, typically 135-150 °C for 2-10 s. The residual bacterial proteases can lead to the development of quality defects in UHT milk, including bitterness, increased viscosity, sedimentation and age gelation, all of which, cause spoilage and shelf-life deterioration during ambient storage and transportation of UHT milk and dairy products

Biodiversity of culturable psychrotrophic microbiota in raw milk attributable to refrigeration conditions, seasonality and their spoilage potential

Refrigerated storage of raw milk promotes the growth of psychrotrophic bacteria, some of which produce heat-stable exoenzymes causing dairy product spoilage. The effects of storage conditions and season on the biodiversity of psychrotrophs in raw milk were examined using matrix-assisted laser desorption time of flight mass spectrometry and 16S rRNA analysis. The ability of psychrotrophs to produce protease, lipase and phospholipase C was determined. The predominant genera found were Pseudomonas (19.9%), Bacillus (13.3%), Microbacterium (5.3%), Lactococcus (8.6%), Acinetobacter (4.9%) and Hafnia (2.8%); a considerable number of isolates were hitherto unknown species and genera. Diversity varied significantly (P &lt; 0.05), depending on the storage temperature, time, initial microbiota and season. The predominant isolates showed significantly higher heat-stable exoenzyme activities after heating at 142 degrees C for 4 s. Improving the quality of milk products may require differential processing of raw milk depending on the type of microbiota present, storage temperature and seasonality. (C) 2016 Elsevier Ltd. All rights reserved