Psychrotrophic Bacteria Counts Research Articles

Estimation of Potential Shelf-life of Pasteurized Fluid Milk Utilizing Bacterial Numbers and Metabolites

A study was conducted on use of bacterial numbers and their metabolites, and any possible interaction thereof, as estimators of the potential shelf-life of pasteurized fluid milk. Whole and skim milk samples were obtained on the day of processing. Samples of each milk were inoculated in duplicate with 0, 1,000, or 100,000 bacteria/ml with a pure strain of Pseudomonas fluorescens P27. Samples, stored at 7°C, were analyzed for microbiological and bioichemical parameters every 5 d for up to 20 d, with organoleptic evaluations conducted on a daily basis. On days of analysis, each sample was subjected to various preliminary incubations. Bacterial enumerations conducted were psychrotrophic bacteria count, standard plate count, gram-negative bacteria count, and modified psychrotrophic bacteria count. Lipopolysaccharide (endotoxin) concentrations, degree of proteolysis and impedance detection were also determined. All bacterial enumerations and proteolysis were significantly related to potential shelf-life of pasteurized fluid milk (whole, skim, and combined) but were of little predictive value. Endotoxin concentration and impedance detection were highly significantly related to shelf-life, and provided predictive regression equations. Using combined data from whole and skim milk, impedance detection resulted in the preferred prediction equation suitable for pasteurized fluid milks.

Rapid Impedimetric Method for Determining the Potential Shelf-Life of Pasteurized Whole Milk

Potential shelf-life of 100 pasteurized whole milk samples, obtained from retail outlets and from dairy processors, was investigated. Parameters studied were: organoleptic evaluation, Standard Plate Count (SPC), psychrotrophic bacteria count (PBC), modified psychrotrophic bacteria count (mPBC), Moseley test (MSPC), and impedance detection time (IDT) at 18 and 21°C. Correlation coefficients were obtained for all possible data combinations in an attempt to detect significant relationships between the parameters studied and the true shelf-life of the product. None of the direct counts (SPC, PBC, and mPBC) correlated well enough with shelf-life to allow shelf-life prediction. Moseley test (MSPC) appeared to possess an adequate relationship to shelf-life with a correlation coefficient at −0.84. IDT 21°C and IDT 18°C proved to have the most significant relationships to shelf-life with correlation coefficients of 0.88 and 0.87, respectively. Therefore, the impedance method had three advantages over the Moseley test: (a) it was a better predictor of shelf-life, (b) it was less labor intensive, and (c) it required only 1–2 d, as opposed to 7–9 d to complete.

Preliminary Incubation Count as an Index of Raw Milk Microbiological Quality During Storage

During a 5-month period, 200 raw milk samples were collected from two Louisiana milk plants. Standard Plate Count (SPC), Psychrotrophic Bacteria Count (PBC), and Proteolytic Count (PC) of each sample were initially determined, then monitored daily during a 5-d storage period at 2.2°C. As hypothesized, all bacterial counts increased during the storage period. The magnitude of the increase in bacterial numbers during storage was further investigated by dividing the milk samples into bacteriologically acceptable and unacceptable groups based on SPC or Preliminary Incubation (PI) count. An SPC of 1.0 × 105/ml and PI counts of 1.0 × 105/ml, 1.5 × 105/ml, 2.3 × 105/ml, and 3.0 × 105/ml were used to repeatedly dichotomize the 200 raw milk samples into two groups. Median SPC, PBC, and PC for each acceptable and unacceptable group were then calculated. Dichotomization based on PI counts yielded acceptable sample groups having consistently lower bacterial counts during storage than did the acceptable sample group, which resulted from the dichotomization based on a SPC of 1.0 × 105/ml. The results of this study indicated that the PI count is of considerable value for raw milk quality control.

Microwave Treatment of Pasteurized Milk

AbstractThe average shelf life of pasteurized fluid milk is 10.7 days. The purpose of this study was to examine the possibility of extending this with the application of microwave heat. This would be convenient for the growing number of small households. For these individuals, it is difficult to use a quart of milk in 10.7 days.Pasteurized homogenized milk in 200 ml lots was exposed to 2450 MHz microwave treatment. After 85, 97, 100 and 120 second treatments, the temperatures increased to 45°, 50°, 55° and 60°C respectively. Both Standard Plate Counts and Psychrotrophic Bacteria Counts were dramatically reduced with increasing temperatures. Psychrotrophs were reduced from a level of 1.8 × 106/ml to a nondelegable level when samples were brought to a temperature of 60°C. informal taste tests indicated that microwave-treated 8 day old pasteurized milk heated to a temperature of 60°C had a longer shelf life than the control and all other samples tested.

Influence of Microwave Radiation on Psychrotrophic Bacteria

Pure cultures of bacteria in nutrient broth at 107 or 108 organisms/ml and various raw meat products were exposed to either 915 or 2450 MHz microwaves. After various timed exposures, temperature increases were noted and samples were removed for plate-count determination of survivors. In nutrient broth, all psychrotrophic bacteria counts were dramatically reduced by short exposure (5 to 20 sec) to microwave radiation. In some instances, initial counts of 107 to 108/ml were reduced to zero after 15 sec of microwave exposure and a temperature of 60 C. Moraxella-acinetobacter (MA-3), Serratia marcescens, Pseudomonas synthaxa, and Alcaligenes viscolactis were extremely susceptible to the microwave treatments. When raw poultry parts, with initial total counts of 104/cm2 of surface area, were exposed to microwave radiation, organisms were reduced by 1 log cycle in 20 sec and by 2 log cycles in 40 sec. Total counts on microwave-treated chicken skin were reduced from 105/cm2 to 103/cm2 in 30 sec. Refrigerated storage of microwave-treated chicken parts indicated that such treatment could substantially increase shelf life.

Effect of Low-Temperature Cleaning on Microbiological Quality of Raw Milk and Cleanliness of Milking Equipment on the Farm

Milking equipment on four farms each in the St. Paul. Minnesota area and the Ithaca, New York area was cleaned at wash solution starting temperatures of 120, 130, 140, 150, and 160 F. The effect of wash solution temperature on raw milk microbiological quality was determined by the Standard Plate Count (SPC), Psychrotrophic Bacteria Count (PBC). Coliform Count (CC) and Laboratory Pasteurized Count (LPC). Cleanliness of the farm milking equipment was determined by microbiological counts (SPC, PBC, CC) of circulated sterile water, visual examination and determination of residual calcium on the equipment. Statistical analysis of the data indicated that Klenzade farm detergents and Agway farm detergents can be used at temperatures such that the final wash solution temperature does not drop below 105 F and have no significant effect on cleaning performance, microbiological quality of raw milk or microbiological status of the milking equipment.

Influence of Yield Grade of Fresh Lamb Carcasses on Shrinkage and Psychrotrophic Bacteria Counts during Transport

Influence of Yield Grade of Fresh Lamb Carcasses on Shrinkage and Psychrotrophic Bacteria Counts during Transport

BACTERIAL COUNTS OF RAW MILK AND FLAVOR OF THE MILK AFTER PASTEURIZATION AND STORAGE

A total of 216 raw milk samples with a variety of Standard Plate Counts and psychrotrophic bacteria counts were laboratory-pasteurized, stored at 7 C, and then evaluated for flavor after 1 and 2 weeks. Results showed that milk with counts of >1,000,000/ml before heating frequently developed objectionable flavors after pasteurization and subsequent storage. The most common defect was a bitter flavor which appeared within 2 weeks after pasteurization in nearly all samples which as raw milk had counts exceeding 10,000,000/ml. This off-flavor developed in spite of small numbers of organisms in the pasteurized product and in the absence of post-pasteurization contamination.