When Does Milk Spoil? The Use of Rejection Threshold Methodology to Investigate the Influence of Total Microbial Numbers on the Acceptability of Fresh Chilled Pasteurised Milk (original) (raw)

Relationship between total microbial numbers, volatile organic compound composition, and the sensory characteristics of whole fresh chilled pasteurized milk

Food Packaging and Shelf Life, 2018

Total microbial numbers (TMN*) is a widely used, yet broad indicator of the quality of fresh chilled pasteurized milk (FCPM). Changes in the sensory characteristics (trained panel, n = 9), volatile organic compounds (VOC) composition (proton transfer reaction mass spectrometry, PTR-MS), and TMN were assessed for FCPM stored at 4.5°C ± 0.5 for up to 26 days (12 days past the stated shelf life). Significant differences (p ≤ 0.05) in its VOC composition (m/z 41, 43, 45, and 47) and the sensory characteristics (10 out of 17 attributes) occurred at various TMN levels over this time. However, owing to differences in the spoilage potential of the microorganisms present there was only a weak relationship between TMN, VOC composition, and the sensory characteristics, which indicates that both the nature of the microorganisms present as well as the numbers present play a critical role in determining the shelf-life of FCPM.

Microbiological quality of raw milk attributable to prolonged refrigeration conditions

Journal of Dairy Research, 2017

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 t...

Influence of time and storage temperature on raw milk deteriorating microbiota

Brazilian Journal of Veterinary Research and Animal Science, 2020

The quality of raw milk depends on initial microbial contamination and conditions of storage until industry processing. Considering the influence of time and storage temperature on raw milk microbiota, the objective of this work was to quantify and monitor the multiplication of these groups under different conditions. For this purpose, 41 samples of raw milk were collected immediately after milking, stored in the following storage conditions: 25 °C/2 h; 35 °C/2 h; 7 °C/24 h; 7 °C/48 h and 7 °C/60 h and analyses of aerobic mesophilic, psychrotrophic and proteolytic psychrotrophic microorganisms. The milk samples analyzed in the study had an initial mean count of mesophilic aerobes of 5.38 Log CFU/mL at Time Zero. The milk stored at 25 °C/2 h and 35 °C/2 h kept the mesophilic aerobic counts within the limits established by the legislation (5.48 Log CFU/mL), with an increase in counts of psychrotrophic and proteolytic microorganisms. When stored at 7 °C/24 h and 7 °C/48 h, the count of...

Selection of Tests for Monitoring the Bacteriological Quality of Refrigerated Raw Milk Supplies

Journal of Dairy Science, 1988

Raw milk samples were collected from 10 producer bulk tanks. Samples were then subdivided so that milks were subsequently stored at 1.7, 4.4, 7.2, and 10.0°C for 24 and 48 h. After storage, samples were analyzed by seven plating methods: standard plate count, psychrotrophic bacterial count, rapid psychrotrophic count, preliminary incubation count, mesophilic plate count, laboratory pasteurized count, and coliform count by violet red bile agar technique. Impedance protocols on a Bactometer® Model 123 for total count, psychrotrophic count, mesophilic count, and coliform count were also used to evaluate the bacteriological quality of the milks. Bacterial counts and impedance detection times were analyzed using nonparametric statistics. Impedance protocols for total count and psychrotrophic count were the best indicators of bacteriological quality. Preliminary incubation count was the best of the plating methods. The laboratory pasteurized count performed poorly. Impedance measurements provided information in the shortest time.

Evaluating the effect of storage conditions on milk microbiological quality and composition

Irish Journal of Agricultural and Food Research, 2018

In this study, the effect of storage temperature (2 or 4°C) on the composition of milk and microbiological load was investigated over 96 h. Milk samples were collected from farm bulk milk tanks after one complete milking and stored at 2 or 4°C over 96 h. Total bacterial count (TBC), psychrotrophic bacterial count (PBC) and proteolytic bacterial count (PROT) were affected by storage time and temperature and varied significantly between farms (P < 0.05). The levels of TBC, PBC and PROT bacterial count increased from 4.37 to 6.15 log cfu/mL, 4.34 to 6.44 log cfu/mL and 3.72 to 4.81 log cfu/mL, respectively, when the milk was stored for 96 h at 2°C. The milk samples stored at 4°C had higher increases in these bacterial counts after 72 h in comparison to milk samples stored at 2°C. The casein fraction content was lower in milk samples stored at 4°C, which could be due to high levels of PROT bacteria or enzyme activity in these samples. Milk stored for 96 h at 2°C has less impact on co...

Results from raw milk microbiological tests do not predict the shelf-life performance of commercially pasteurized fluid milk

Journal of dairy science, 2011

Analytical tools that accurately predict the performance of raw milk following its manufacture into commercial food products are of economic interest to the dairy industry. To evaluate the ability of currently applied raw milk microbiological tests to predict the quality of commercially pasteurized fluid milk products, samples of raw milk and 2% fat pasteurized milk were obtained from 4 New York State fluid milk processors for a 1-yr period. Raw milk samples were examined using a variety of tests commonly applied to raw milk, including somatic cell count, standard plate count, psychrotrophic bacteria count, ropy milk test, coliform count, preliminary incubation count, laboratory pasteurization count, and spore pasteurization count. Differential and selective media were used to identify groups of bacteria present in raw milk. Pasteurized milk samples were held at 6°C for 21 d and evaluated for standard plate count, coliform count, and sensory quality throughout shelf-life. Bacterial ...

Comparing PTR-MS profile of milk inoculated with pure or mixed cultures of spoilage bacteria

Food Microbiology, 2017

The volatile organic compounds (VOCs) associated with UHT milk (n¼8) inoculated with either pure inoculums of Pseudomonas fluorescens (two strains tested) or Chryseobacterium sp., or with mixed cultures of 2 or all 3 of the bacterial strains, and held at 4.5 C for up to 26 days was measured using proton transfer reaction e mass spectrometry (PTR-MS). The VOCs evolved included a range of carbonyl compounds, alcohols, esters, and acids and had significant qualitative and quantitative differences between the inoculums. Milks inoculated with paired (mixed) bacterial cultures attained patterns similar to the VOC composition of one of the pure inoculums, which could be attributed to the domination of these bacteria within the mixed inoculum. This study will help to characterize the spoilage of milk and provide important insights into understanding the factors that limit the shelf life of milk.

Characterization of Pasteurized Fluid Milk Shelf-life Attributes

Journal of Food Science, 2004

Pasteurized fluid milk samples were systematically collected from 3 commercial dairy plants. Samples were evaluated for microbial, chemical, and sensory attributes throughout shelf life. In general, product shelf lives were limited by multiplication of heat-resistant psychrotrophic organisms that caused undesirable flavors in milk. The predominant microorganisms identified were Gram-positive rods including Paenibacillus, Bacillus, and Microbacterium. Principal component analysis of sensory data collected using quantitative descriptive analysis showed that attributes related to milk flavor defects explained the largest amount of variance. These findings highlight the need to develop specific strategies for excluding bacterial contaminants from milk to further extend product shelf lives.