Heat-Resistant Psychrotrophic Bacteria Isolated from Pasteurized Milk

Psychrotrophic bacteria were isolated from 227 pasteurized milk samples which had a shelf life in excess of 20 days at 7.2 C. Of 700 cultures isolated, 135 were resistant to heating at 72 C for 16 sec and were able to re-establish growth at 7.2 C. Thirty-five cultures, representing 15 different types were subjected to detailed examination to determine their actions on refrigerated milk, growth temperatures, thermal resistance at various temperatures, and their identities. The spore-forming genus Bacillus occured most frequently. The non-sporing types were assigned to the genera Arthrobacter, Microbacterium, Streptococcus, and Corynebacterium.

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Comparison between the EC plate count test at 21 °C and an accelerated plate count method for assessing the keeping quality of pasteurized milk

Journal of Dairy Research ◽

10.1017/s0022029900030703 ◽

1992 ◽

Vol 59 (3) ◽

pp. 431-436 ◽

Cited By ~ 1

Author(s):

Sarah A. Langford ◽

Rohan G. Kroll

Keyword(s):

Plate Count ◽

Gram Negative ◽

Rapid Methods ◽

Pasteurized Milk ◽

Psychrotrophic Bacteria ◽

Commercial Interest ◽

Count Method ◽

Keeping Quality ◽

Plate Count Method

The keeping quality of properly refrigerated pasteurized milk and cream is primarily determined by post-pasteurization contamination by Gram-negative psychrotrophic bacteria (Phillips et al. 1981; Schröder et al. 1982). Reliable and rapid methods of assessing the levels of contamination by these organisms are therefore of commercial interest.

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Evaluation of an 18°C/45-Hour Plate Count Technique for the Enumeration of Psychrotrophic Bacteria in Raw and Pasteurized Milk

Journal of Food Protection ◽

10.4315/0362-028x-46.6.528 ◽

1983 ◽

Vol 46 (6) ◽

pp. 528-529 ◽

Cited By ~ 2

Author(s):

HARRY K. OEHLRICH ◽

ROBIN C. MCKELLAR

Keyword(s):

Plate Count ◽

Reliable Estimate ◽

Additional Test ◽

Bacterial Counts ◽

Pasteurized Milk ◽

Psychrotrophic Bacteria ◽

Milk Samples

Bacterial counts were done at 7°C for 10 d and at 18°C for 45 h on 93 samples of raw and 185 samples of pasteurized milk. As an additional test, catalase-positive microorganisms were enumerated at 18°C/45 h. Close correlations were obtained between the numbers of microorganisms following 18°C/45-h and 7°C/10-d incubations in raw (r2=0.866) and pasteurized (r2=0.936) milk samples. Similar correlations (r2=0.860 and 0.946) were noted for the 18°C/45-h-catalase and the 7°C/10-d methods for raw and pasteurized milk, respectively. Results suggest that incubation at 18°C for 45 h provides a reliable estimate of the numbers of psychrotrophs in raw and pasteurized milk and that the use of catalase does not improve sensitivity of the test significantly.

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Low Temperature Inactivation in Milk of Heat-Resistant Proteases from Psychrotrophic Bacteria

Journal of Dairy Science ◽

10.3168/jds.s0022-0302(76)84218-x ◽

1976 ◽

Vol 59 (3) ◽

pp. 391-395 ◽

Cited By ~ 51

Author(s):

J.T. Barach ◽

D.M. Adams ◽

M.L. Speck

Keyword(s):

Low Temperature ◽

Psychrotrophic Bacteria ◽

Heat Resistant ◽

Temperature Inactivation

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Rapid Enumeration of Psychrotrophic Bacteria in Raw and Pasteurized Milk

Journal of Milk and Food Technology ◽

10.4315/0022-2747-39.4.269 ◽

1976 ◽

Vol 39 (4) ◽

pp. 269-272 ◽

Cited By ~ 36

Author(s):

J. S. OLIVERIA ◽

C. E. PARMELEE

Keyword(s):

Dairy Products ◽

Correlation Coefficients ◽

Raw Milk ◽

Standard Methods ◽

Pasteurized Milk ◽

Psychrotrophic Bacteria ◽

Count Method ◽

Plating Method ◽

Rapid Enumeration ◽

Good Agreement

A plating method was developed to enumerate psychrotrophic bacteria in raw and pasteurized milk. Standard Methods agar plates were prepared according to Standard Methods for the Examination of Dairy Products, and incubated at 21 C for 25 h (21 C-25 h). Counts obtained by this method were in very good agreement with those obtained by the standard psychrotrophic count. The correlation coefficients between counts obtained by the 21 C-25 h method and by the standard psychrotrophic count method for 132 samples of raw milk and 190 samples of pasteurized milk were 0.992 and 0.996, respectively.

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Heat-Stable Proteases from Psychrotrophs in Milk

Journal of Food Protection ◽

10.4315/0362-028x-43.3.197 ◽

1980 ◽

Vol 43 (3) ◽

pp. 197-200 ◽

Cited By ~ 21

Author(s):

A. GEBRE-EGZIABHER ◽

E. S. HUMBERT ◽

G. BLANKENAGEL

Keyword(s):

Molecular Weight ◽

Gel Electrophoresis ◽

Dairy Products ◽

Raw Milk ◽

Gram Negative ◽

Psychrotrophic Bacteria ◽

Heat Resistant ◽

Heat Stable ◽

Single Enzyme

Twelve gram-negative psychrotrophic bacteria producing heat-resistant proteases that hydrolyzed casein were isolated from refrigerated raw milk. All were pseudomonads and the enzymes of the six most proteolytic cultures were examined further. The proteases were partially purified, and gel electrophoresis indicated that only a single enzyme was present in the preparation. The molecular weight of most of the proteases was approximately 45,000. All six enzymes retained some activity after being heated at 121 C for 10 min and casein was hydrolyzed at pH levels found in normal milk and many cultured dairy products. Although proteolysis was highest at about 40 C, considerable activity was evident at refrigeration temperatures.

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

Journal of Food Protection ◽

10.4315/0362-028x-41.2.93 ◽

1978 ◽

Vol 41 (2) ◽

pp. 93-95 ◽

Cited By ~ 19

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.

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The Automated Pyruvate Method as a Quality Test for Grade A Milk

Journal of Food Protection ◽

10.4315/0362-028x-41.3.168 ◽

1978 ◽

Vol 41 (3) ◽

pp. 168-177 ◽

Cited By ~ 10

Author(s):

R. T. MARSHALL ◽

C. C. HARMON

Keyword(s):

Incubation Temperature ◽

Micrococcus Luteus ◽

Raw Milk ◽

Salmonella Typhi ◽

Pseudomonas Fragi ◽

Pasteurized Milk ◽

Psychrotrophic Bacteria ◽

Natural Flora ◽

Pure Cultures ◽

Plate Counts

Concentrations of pyruvate in Grade A raw and pasteurized milks were determined by an automated procedure. The method was sensitive to 0.1 mg of pyruvate per liter and the coefficient of variation of the method was 1.6% for a sample of milk containing 3 mg/1. Since about twice as much pyruvate was produced in samples incubated 24 h at 20 C compared with 15 C, the higher temperature is suggested for the pyruvate difference test (ΔP); however, psychrotrophs would more likely be detected with an incubation temperature of 15 C. Pyruvate difference tests of pure cultures in steamed milk distinguished between concentrations approximating 102 and 103 Pseudomonas fragi, 103 and 104 Pseudomonas fluorescens, and between 104 and 105 Escherichia coli, Streptococcus faecalis, Lactobacillus acidophilus and Micrococcus luteus, but did not distinguish between 104 and 105 initial concentrations of six other gram-positive bacteria and Salmonella typhi. Both the initial pyruvate (IP) concentration and the ΔP in raw milk were correlated with the logarithm of the Wisconsin Mastitis Test score, suggesting that somatic cells contribute to pyruvate content. Correlations of IP and ΔP with Standard Plate Counts of raw and pasteurized milks were low or insignificant and only with raw milk from normal quarters was there a significant correlation of IP and ΔP with initial Psychrotrophic Plate Counts (PPC). Rates of pyruvate production by pure cultures in steamed milk and by the mixed natural flora of pasteurized milk were consistent with rates of growth. However, certain psychrotrophic bacteria reduced the concentration of pyruvate to undetectable levels after producing 10 or more mg/1. Suggestions are made regarding applicability of the test in controlling keeping quality of pasteurized milk and in receiving of bulk raw milk.

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Microbiological Safety of Cheese Made from Heat-Treated Milk, Part III. Technology, Discussion, Recommendations, Bibliography

Journal of Food Protection ◽

10.4315/0362-028x-53.7.610 ◽

1990 ◽

Vol 53 (7) ◽

pp. 610-623 ◽

Cited By ~ 35

Author(s):

ERIC A JOHNSON ◽

JOHN H. NELSON ◽

MARK JOHNSON

Keyword(s):

Heat Treatment ◽

Whey Protein ◽

Raw Milk ◽

Toxin Production ◽

Ph Control ◽

Heat Treatments ◽

Low Risk ◽

Pasteurized Milk ◽

Psychrotrophic Bacteria ◽

Heat Treated

Heat treatment or pasteurization does not adversely affect the cheesemaking process or the resulting physical properties of the cheese. Both types of heat-treatments can correct chemical changes that occur in cold stored raw milk. Thermization on the farm may help control psychrotrophic bacteria in cold stored milk. Some denaturation of whey protein does occur during pasteurization. Heat treatments slightly above current minimum pasteurization requirements can cause body/texture and moisture control problems in cheese. Loss of functionality can adversely affect the marketing of whey protein products. Cheeses made from pasteurized milk ripen more slowly and usually do not exhibit the flavor intensity of cheeses made from raw or heat-treated milk. Swiss and hard Italian type cheese, whose traditional flavor results in part from native milk enzymes and microflora, would also be adversely affected if milk pasteurization for cheesemaking were mandatory. The quality of cheese made from pasteurized milk is consistently better than cheese made from raw milk as evidenced by fewer body and flavor defects consequent to the growth of undesirable bacteria. Either pasteurization or heat-treatment enables improved uniform process control and quality during cheesemaking. Pathogens were prioritized as high, medium, or low risk in cheese. Three organisms, Salmonella, Listeria monocytogenes and enteropathogenic Escherichia coli were judged to be high risk threats to the cheese industry. Staphylococcus aureus was listed as low risk because growth and toxin production is readily suppressed by lactic cultures and acidity (pH) control in cheese. Three actions are recommended:Establish a guideline for minimum heat-treatment of milk for cheesemaking: 64.4°C (148°F) for 16 s or equivalent with adequate process control.Evaluate current safety technology and practices used for cheese manufacture. Support research with primary emphasis on the combined effect of heat-treatment and other current cheese technologies.Evaluate technologies not currently utilized in cheese manufacture for safety potential.

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