title: Microbiological quality of raw milk attributable to prolonged refrigeration conditions.
abstract: 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 × 104 cfu/ml and 1 × 104 cfu/ml, and were defined as storage life for quality (S LQ), and storage life for safety (S LS) aspects, respectively. The storage conditions also significantly affected the microbial diversity, where Pseudomonas fluorescens and Bacillus cereus were found to be the most predominant isolates. However, deep cooling (2 °C) and combination of heating and refrigeration (≤4 °C) significantly extended the S LQ and S Ls of raw milk.
authors.affiliation: College of Health and Biomedicine, Victoria University,Werribee, Victoria 3030,Australia.
authors.forename: Nuwan R
authors.initials: NR
authors.lastname: Vithanage
authors.affiliation: College of Health and Biomedicine, Victoria University,Werribee, Victoria 3030,Australia.
authors.forename: Muditha
authors.initials: M
authors.lastname: Dissanayake
authors.affiliation: Murray Goulburn Co-operative Co Ltd,Leongatha, Victoria 3953,Australia.
authors.forename: Greg
authors.initials: G
authors.lastname: Bolge
authors.affiliation: Faculty of Science, Engineering and Technology,Swinburne University of Technology,Hawthorn,Victoria 3122,Australia.
authors.forename: Enzo A
authors.initials: EA
authors.lastname: Palombo
authors.affiliation: Advanced Food Systems Research Unit,Victoria University,Werribee,Victoria 3030,Australia.
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Yeager
authors.affiliation: College of Health and Biomedicine, Victoria University,Werribee, Victoria 3030,Australia.
authors.forename: Nivedita
authors.initials: N
authors.lastname: Datta
mesh_terms: D000818:Animals; D001409:Bacillus cereus / Q000302:isolation & purification; D058491:Bacterial Load; D003612:Dairying / Q000379:methods; D005511:Food Handling / Q000379:methods; D005519:Food Preservation / Q000379:methods*; D063427:Food Quality; D006358:Hot Temperature; D008892:Milk / Q000382:microbiology*; D010447:Peptide Hydrolases / Q000378:metabolism; D059748:Proteolysis; D011551:Pseudomonas fluorescens / Q000302:isolation & purification; D012034:Refrigeration / Q000379:methods*; D013696:Temperature; D013997:Time Factors
chemical_list: D010447:Peptide Hydrolases
keywords: Raw milk; diversity; protease activity; proteolysis; psychrotrophic proteolytic bacteria; thermoduric psychrotrophs