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

1985 ◽  
Vol 48 (8) ◽  
pp. 663-667 ◽  
Author(s):  
J. R. BISHOP ◽  
C. H. WHITE
Keyword(s):  
Shelf Life ◽  
Skim Milk ◽  
Daily Basis ◽  
Prediction Equation ◽  
Plate Count ◽  
Regression Equations ◽  
Psychrotrophic Bacteria ◽  
Standard Plate Count ◽  
Standard Plate ◽  
Fluid Milk

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.

Estimation of Potential Shelf Life of Cottage Cheese Utilizing Bacterial Numbers and Metabolites

1985 ◽  
Vol 48 (12) ◽  
pp. 1054-1057 ◽  
Author(s):  
J. R. BISHOP ◽  
C. H. WHITE
Keyword(s):  
Shelf Life ◽  
Correlation Coefficients ◽  
Daily Basis ◽  
Prediction Equation ◽  
Detection Methods ◽  
Cottage Cheese ◽  
Plate Count ◽  
Psychrotrophic Bacteria ◽  
Cheese Curd ◽  
Aerobic Plate Count

A study was conducted to investigate the use of bacterial numbers and their metabolites as estimators of the potential shelf life of cottage cheese. Dry cottage cheese curd and cream dressing were obtained on the day of processing. Portions of the cream dressing were inoculated with Pseudomonas fluorescens P27 to result in approximate levels of 0, 1,000 and 100,000 bacteria per g in finished cottage cheese after combining the curd and cream. Samples, stored at 7°C, were sensorially evaluated on a daily basis and analyzed every 7 d for up to 35 d. On days of analysis each sample was subjected to preliminary incubation (PI) as follows: none, 21°C for 7 h, 21°C for 14 h, 13°C for 18 h and 18°C for 18 h. For each PI, samples were enumerated by aerobic plate count, modified psychrotrophic bacteria count and gram-negative (CVT) count. Samples were enumerated for the standard psychrotrophic bacteria count without PI. Samples were also exposed to 18°C for 18 h PI in plate count broth for impedance detection measurements. Endotoxin (lipopolysaccharide) concentration and proteolysis were determined by the Limulus amebocyte lysate assay and the o-phthaldialdehyde method, respectively. Bacterial enumerations proved to be of little estimative value as the highest correlation coefficient obtained was −0.61. Endotoxin, proteolysis and impedance detection methods resulted in high correlation coefficients as related to potential shelf life of cottage cheese, with values of −0.81, −0.87 and −0.90, respectively. A prediction equation was formulated from the data.

Identification and Control of Processing Variables That Affect the Quality and Safety of Fluid Milk

1999 ◽  
Vol 62 (6) ◽  
pp. 625-631 ◽  
Author(s):  
THOMAS J. GRUETZMACHER ◽  
ROBERT L. BRADLEY
Keyword(s):  
Shelf Life ◽  
Sequential Analysis ◽  
Microbiological Quality ◽  
Processing System ◽  
Plate Count ◽  
Quality And Safety ◽  
Standard Plate Count ◽  
Standard Plate ◽  
And Control ◽  
Fluid Milk

The objective of this study was to increase quality and safety of fluid milk by eliminating postpasteurization contamination as measured by extended shelf life. Milk shelf life was defined as the number of days for standard plate count to reach 20,000 CFU/ml in milk stored at 7°C. Sequential analysis of the fluid milk processing system indicated filling machine and pasteurizer were significant sources of postpasteurization contamination. Aseptically sampled milk from the pasteurizer outlet indicated a maximum shelf life of more than 30 days could be achieved. The pasteurizer can be a source of contamination when inadequately cleaned or maintained. The filling machine was a significant source of contamination. Shelf life of milk in 236-ml containers was reduced 20 days compared with milk sampled before the filling machine. Carton-forming mandrels, filling heads, and airborne microorganisms were sources of contamination within the filling machine. Eliminating sources of post-pasteurization contamination and proper cleaning followed by sanitizing with chlorine significantly increased milk shelf life in paperboard containers to 20.4 days from an initial shelf life of 9 days. Changing the sanitizing agent to peroxyacetic acid significantly increased milk shelf life to 33.9 days. Enclosing the filling chamber and adding sterile laminar flowing air significantly improved microbiological quality of air inside the chamber and reduced variance among milk shelf life samples.

Pyruvate as an Indicator of Quality in Grading Nonfat Dry Milk1

1982 ◽  
Vol 45 (6) ◽  
pp. 561-565 ◽  
Author(s):  
R. T. MARSHALL ◽  
Y. H. LEE ◽  
B. L. O'BRIEN ◽  
W. A. MOATS
Keyword(s):  
Geometric Mean ◽  
Regression Line ◽  
Skim Milk ◽  
Plate Count ◽  
Department Of Agriculture ◽  
Standard Plate Count ◽  
Geometric Average ◽  
Dry Milk ◽  
Standard Plate ◽  
Microscopic Count

Samples of skim milk and nonfat dry milk (NDM) made from it were collected, paired and tested for pyruvate concentration, [P], and Direct Microscopic count (DMC). The skim milk was tested for Standard Plate Count (SPC) and Psychrotrophic Plate Count (PPC). The geometric average DMC of skim milk was more than three times higher than that of the paired NDM samples. However, [P] of NDM was not significantly different from that of the skim milk. Although [P] of skim milk was poorly correlated with SPC and PPC, r = .31 and .26, respectively, it was relatively well correlated with DMC, r = .64. Data were widely dispersed around the regression line when [P] was ≤ 4.0 mg/L. However, [P] increased rapidly when DMCs were > 106/ml. A limit of 10 mg/L of [P] in NDM reconstituted 1:9 was chosen to represent the current U.S. Department of Agriculture Standard for DMC in NDM. This limit failed to classify about 10% of the samples correctly, assuming that each geometric mean DMC was correct. However, the probability that samples meeting the DMC standard would be rejected by the pyruvate test was quite low and the probability was moderate that samples which would be acceptable by the pyruvate test would be rejected by the DMC. For the latter, 28% of the samples having DMCs of ≥ 107/ml contained < 10 mg/L of pyruvate. No sample having ≥ 10 mg/L of pyruvate had a DMC of ≤ 107/ml. Pyruvate concentration in NDM did not change during storage at 5 or 32°C for 90 days.

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

1984 ◽  
Vol 47 (3) ◽  
pp. 206-208 ◽  
Author(s):  
J. J. RYAN ◽  
R. H. GOUGH ◽  
C. H. WHITE
Keyword(s):  
Microbiological Quality ◽  
Storage Period ◽  
Raw Milk ◽  
Plate Count ◽  
Bacterial Counts ◽  
Sample Group ◽  
Psychrotrophic Bacteria ◽  
Standard Plate Count ◽  
Milk Samples ◽  
Standard Plate

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.

Subpasteurization Heat Treatment to Inactivate Lipase and Control Bacterial Growth in Raw Milk

1982 ◽  
Vol 45 (6) ◽  
pp. 513-515 ◽  
Author(s):  
G. F. SENYK ◽  
R. R. ZALL ◽  
W. F. SHIPE
Keyword(s):  
Heat Treatment ◽  
Lipase Activity ◽  
Raw Milk ◽  
Heat Treatments ◽  
Plate Count ◽  
Psychrotrophic Bacteria ◽  
Standard Plate Count ◽  
Heat Treated ◽  
Standard Plate ◽  
And Control

Raw milk was heat-treated under subpasteurization and suprapasteurization conditions, cooled and stored for up to 72 h at 4.4 and 6.7°C. Milk lipase activity and bacteria counts were monitored in both unheated and heated milks. Inhibition of milk lipase activity ranged from 42 to 98% for treatments of 57.2°C for 10 sec to 73.9°C for 10 sec, respectively. The logs of Standard Plate Count after 72 h of storage at 6.7°C were 6.56, 4.86, 4.31, 4.00 and 2.82 for unheated and 10-sec heat treatments at 57.2, 65.6, 73.9 and 82.2°C, respectively. Psychrotrophic Bacteria Counts were also lower in the heated milks than in the unheated milk. The logs of Psychrotrophic Bacteria Counts after 72 h of storage at 6.7°C were 6.21, 2.45, 2.27, 1.33 and 1.00 for unheated and 10-sec heat treatments at 57.2, 65.6, 73.9 and 82.2°C, respectively. Heat treatment of raw milk supplies would result in limiting action of the milk lipase system and growth of bacteria.

Heat Resistant Psychrotrophic Bacteria in Raw Milk and Their Growth at 7 C

1978 ◽  
Vol 41 (2) ◽  
pp. 93-95 ◽  
Author(s):  
E. M. MIKOLAJCIK ◽  
N. T. SIMON
Keyword(s):  
Heat Treatment ◽  
Raw Milk ◽  
Plate Count ◽  
Psychrotrophic Bacteria ◽  
Standard Plate Count ◽  
Heat Resistant ◽  
Heated Milk ◽  
Standard Plate ◽  
Subsequent Storage ◽  
Microbiological Data

Microbiological data are presented for 109 raw milk samples, and for the same samples after heat treatment at 80 C for 12 min, and upon subsequent storage at 7 C for 7, 14, and 28 days. The Standard Plate Count of the raw milk averaged 110,000/ml with 65% of these organisms being penicillin-resistant. Immediately after heat treatment, 87% of the samples had psychrotrophic spore counts< 10/ml. After 14 and 28 days of storage, 50 and 83% of the samples had psychrotrophic counts ≥ 100,000/ml. It was concluded that growth of heat-resistant psychrotrophic organisms may cause spoilage of heated milk. No relationship could be demonstrated between gram-negative counts of raw milk, or initial mesophilic spore counts of heated milk, and bacterial numbers in heated, stored milk.

MANUFACTURING MILK QUALITY: A RE-EVALUATION

1971 ◽  
Vol 34 (5) ◽  
pp. 249-255 ◽  
Author(s):  
William S. LaGrange
Keyword(s):  
Milk Quality ◽  
Plate Count ◽  
Dairy Farmers ◽  
High Count ◽  
Psychrotrophic Bacteria ◽  
Standard Plate Count ◽  
Processing Plants ◽  
Standard Plate ◽  
Plate Counts ◽  
Microscopic Count

The bacteriological quality of manufacturing-grade milk is very similar to that marketed a decade earlier when bulk tanks first came into general use. Milk grading programs usually relied on reduction tests. These tests indicated that most milk supplies were good quality. Based on the Standard Plate Count, data is presented that show approximately one-third of the samples tested, in 1969–70 and in 1957–59, exhibit counts <200,000/ml. Considerable quantities of milk, received at processing plants have plate counts exceeding 1,000,000/ml. Dairy farmers learned they could substitute cooling for cleaning because psychrotrophic bacteria predominated the microflora of most high count bulk milk. These bacteria do not readily reduce resazurin and methylene blue. Psychrotrophs also tend to grow in clumps preventing an accurate evaluation of milk quality using the Direct Microscopic Count (DMC). USDA uses the DMC to test check manufacturing plant's milk supplies. Laboratories are recognizing the value of plating procedures, including the Plate Loop Count, to determine milk quality. Manufacturing-grade milk must be evaluated with a plating procedure before progress can be made in milk quality improvement. One grade of milk is far from being a reality if present levels of manufacturing-grade milk quality are considered.

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

1978 ◽  
Vol 41 (11) ◽  
pp. 902-906 ◽  
Author(s):  
DARRELL L. BIGALKE
Keyword(s):  
New York ◽  
Microbiological Quality ◽  
Raw Milk ◽  
Solution Temperature ◽  
Plate Count ◽  
Psychrotrophic Bacteria ◽  
Standard Plate Count ◽  
Wash Solution ◽  
Standard Plate

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.

Evaluation of a Rapid Impedimetric Method for Determining the Keeping Quality of Milk

1982 ◽  
Vol 45 (13) ◽  
pp. 1221-1226 ◽  
Author(s):  
S. B. MARTINS ◽  
S. HODAPP ◽  
S. W. DUFOUR ◽  
S. J. KRAEGER
Keyword(s):  
Shelf Life ◽  
Correlation Coefficients ◽  
Sample Volume ◽  
Plate Count ◽  
Impedance Method ◽  
Detection Time ◽  
Standard Plate Count ◽  
Milk Samples ◽  
Standard Plate ◽  
Quality Of Milk

Shelf-life of 151 pasteurized milk samples was recorded and correlation coefficients calculated using various microbiological factors: standard plate count (SPC), psychrotrophic plate count (PPC), coliform count (CC), and the impedance response detection time (DT) with incubation at both 21 and 32°C. These data were obtained for milk samples on the day of pasteurization as well as 4 and 8 d thereafter. Various treatments (media, dilution factors, temperature and sample volume) were compared. Of the SPC, PPC, CC and DT taken on the day of pasteurization, only the DT achieved a significant correlation with shelf-life. A correlation coefficient of 0.55 was obtained for one treatment applied to 61 samples and correlation coefficients of 0.28 to 0.32 were obtained for several other treatments applied to the entire 151 samples. Values as large as these could occur by chance in uncorrelated data with p<0.0005. Thus, of the total 61 samples, 80% were correctly classified by the impedance detection time test. It is concluded that for prediction of shelf-life on the day of pasteurization, the impedance method is superior to the SPC and the PPC. In addition, the impedance method is more rapid, i.e., 14 h vs.2 d for the SPC and 10 d for the PPC.

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

1984 ◽  
Vol 47 (6) ◽  
pp. 471-475 ◽  
Author(s):  
J. R. BISHOP ◽  
C. H. WHITE ◽  
R. FIRSTENBERG-EDEN
Keyword(s):  
Shelf Life ◽  
Correlation Coefficients ◽  
Plate Count ◽  
Impedance Method ◽  
Organoleptic Evaluation ◽  
Evaluation Standard ◽  
Psychrotrophic Bacteria ◽  
Standard Plate Count ◽  
Significant Relationships ◽  
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.

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