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

1978 ◽  
Vol 41 (3) ◽  
pp. 168-177 ◽  
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.

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

1972 ◽  
Vol 35 (4) ◽  
pp. 203-206 ◽  
Author(s):  
G. B. Patel ◽  
G. Blankenagel
Keyword(s):  
Raw Milk ◽  
Bacterial Counts ◽  
Psychrotrophic Bacteria ◽  
Milk Samples ◽  
Standard Plate ◽  
Plate Counts ◽  
Subsequent Storage ◽  
And Storage ◽  
Common Defect

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.

Effect of Five Surface Area-to-Sample Volume Ratios During Preliminary Incubation on The Bacterial Count of Raw Milk1,2

1979 ◽  
Vol 42 (12) ◽  
pp. 968-970
Author(s):  
J. A. RITTER ◽  
B. E. LANGLOIS ◽  
J. O'LEARY
Keyword(s):  
Surface Area ◽  
Incubation Temperature ◽  
Bacterial Count ◽  
Raw Milk ◽  
Sample Volume ◽  
Bacterial Counts ◽  
Milk Samples ◽  
Standard Plate ◽  
Plate Counts ◽  
Incubation Temperatures

Effects of ratio of surface area to volume of sample during preliminary incubation (PI) and of different plate incubation temperatures on bacterial counts of raw milk samples were studied. One hundred and twenty Grade A raw milk samples collected during a 10-month period were divided into five 100-ml aliquots and allotted to one of five surface area to volume ratios. The ratios during PI ranged from 0 cm2/100 ml to 149.74 cm2/100 ml. Following PI, pour plates of each treatment were incubated at 26, 30 and 32 C for 72 h. The Standard Plate Counts (SPC) ranged from 89 × 101 to 20 × 108/ml, with the SPC of 73.6% of the samples being less than 1 × 105/ml. Counts after PI tended to be higher as the plate incubation temperature decreased from 32 to 26 C and as the ratio of surface area to volume of sample increased. None of the differences between the counts for the 15 treatment-incubation temperatures was significant. Counts of 61 samples increased less than one log count during PI, while counts of 33 and 16 samples increased one to two log counts and over two log counts, respectively. The greater the SPC, the smaller the increase in count during PI. Of the 81 samples with SPC less than 1 × 105/ml, 29 had counts after PI that exceeded 2 × 105/ml.

Rapid Enumeration of Psychrotrophic Bacteria in Raw and Pasteurized Milk

1976 ◽  
Vol 39 (4) ◽  
pp. 269-272 ◽  
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.

EFFECT OF TEMPERATURE AND TIME OF PLATE INCUBATION ON THE ENUMERATION OF PASTEURIZATION-RESISTANT BACTERIA IN MILK1

1963 ◽  
Vol 26 (11) ◽  
pp. 357-363 ◽  
Author(s):  
W. R. Thomas ◽  
G. W. Reinbold ◽  
F. E. Nelson
Keyword(s):  
Incubation Temperature ◽  
Bacterial Count ◽  
Raw Milk ◽  
Effect Of Temperature ◽  
Plate Count ◽  
Resistant Bacteria ◽  
Pasteurized Milk ◽  
Standard Plate Count ◽  
Standard Plate

This study was undertaken to determine the effect of temperature and time of plate incubation upon the count of thermoduric bacteria in milk. Specific types of thermoduric bacteria in pure culture, as well as those present in the mixed flora of commercial milk samples, were enumerated. Plate incubation at 28 C for 4 days was the temperature-time combination that produced the highest thermoduric bacterial count with laboratory-pasteurized milk. Incubation at 21, 32 or 35 C gave lower counts. Thermoduric bacteria subjected to pasteurization were more exacting in their growth temperature requirements than were unheated bacteria. Cultures of Arthrobacter sp., Micrococcus varians and Streptococcus sp. grew over a much wider temperature range before laboratory pasteurization than after the heat treatment. The incubation temperature and time currently recommended for the standard plate count, while presumably adequate for the enumeration of bacteria in raw milk, may not be equally satisfactory for the determination of the maximum viable bacterial population of pasteurized milk.

Effect of Agitation on Bacterial Aggregates in Pure Cultures and Raw Milk

1983 ◽  
Vol 46 (8) ◽  
pp. 681-685 ◽  
Author(s):  
ROBYN E. O'CONNOR ◽  
K. N. EWINGS ◽  
NEIL W. HOLLYWOOD
Keyword(s):  
Raw Milk ◽  
Mechanical Agitation ◽  
Filter Technique ◽  
Milk Samples ◽  
Pure Cultures ◽  
Plate Counts ◽  
Bacterial Aggregates ◽  
Direct Epifluorescent Filter Technique ◽  
Clump Size

A comparison of the effects of various mechanical agitation treatments on bacterial aggregates was performed on 8 pure cultures and 27 raw milk samples. Although both syringing and blending produced significant increases in total counts and psychrotroph counts, blending for 2 min gave the greatest increase in count. Use of the direct epifluorescent filter technique (DEFT) confirmed that syringing and blending reduced bacterial clump size to approximately 2 cells. These agitation treatments markedly improved the correlation between DEFT counts and plate counts.

329. The effect of the method of reconstitution and of the temperature of incubation on the plate count of spray-dried milk powder

1945 ◽  
Vol 14 (1-2) ◽  
pp. 175-183 ◽  
Author(s):  
Edith R. Hiscox
Keyword(s):  
Room Temperature ◽  
Incubation Temperature ◽  
Milk Powder ◽  
Raw Milk ◽  
Plate Count ◽  
Milk Supply ◽  
Plate Counts ◽  
Eighth Edition ◽  
Spray Dried

Bacteriological standards are used in the assessment of the quality of dried milk powder. From a knowledge of total numbers of bacteria and the relative numbers of the various types deductions are made as to the quality of the raw milk supply, the cleanliness of the plant and the over-all efficiency of the process. Figures have been published by several workers showing plate counts of roller- and spray-dried milk powder, but the technique of reconstituting and of plating varied. Sterile water appears to have been the general reconstituting fluid, but sometimes lithium hydroxide (N/10 solution) was used, especially when the powder was difficult to dissolve. The temperature of the reconstituting fluid is not always mentioned, but it appears to have varied from room temperature to 60–65° C. The usual incubation temperature for the plates was 37° C, though 21–22 and 55–56° C. were also used for special purposes. InStandard Methods for the Examination of Dairy Products, eighth edition (1), the technique laid down mentions water, carefully warmed to 43–49° C, as the reconstituting fluid, and either 37 or 32° C. (for 48 hr.) as the incubation temperature, the choice being left to theenforcement officials having jurisdiction. American workers have for some years strongly advocated the use of 32 rather than 37° C. as the incubation temperature for routine plate counts of samples of raw milk, the advantage being that slight inaccuracies in the temperature of the incubator have less influence on the plate count which is, moreover, closer to that obtained by direct counts. The data offered in this paper show that the advantage of a similar incubation temperature (30° C. was used in these experiments) is equally obvious in the plating of dried milk powder.

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

1990 ◽  
Vol 53 (7) ◽  
pp. 610-623 ◽  
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.

Bacterial Tracking in a Dairy Production System Using Phenotypic and Ribotyping Methods

1998 ◽  
Vol 61 (10) ◽  
pp. 1336-1340 ◽  
Author(s):  
ROBERT D. RALYEA ◽  
MARTIN WIEDMANN ◽  
KATHRYN J. BOOR
Keyword(s):  
Production System ◽  
Bacterial Contamination ◽  
Similarity Index ◽  
Raw Milk ◽  
Systematic Sampling ◽  
Dairy Production ◽  
Pasteurized Milk ◽  
Psychrotrophic Bacteria ◽  
Log Reduction ◽  
Automated Ribotyping

A systematic sampling plan was designed to collect raw and pasteurized milk samples throughout a single-raw milk source, dairy-processing operation experiencing reduced product shelf lives due to bacterial contamination. The objectives were to track bacterial contamination sources throughout a complete dairy production system and use this information to reduce bacterial spoilage losses in processed fluid products. Over a 5-week period, 233 bacterial isolates were collected, representative of different colony morphologies on psychrotrophic bacteria count (PBC) plates. Forty-five isolates (19%) were obtained from pasteurized milk and 188 (81%) were isolated from raw product. Thirty isolates were identified as Pseudomonas spp. by Gram stain and biochemical methods. Of these, 27 (90%) were postpasteurization isolates and 3 (10%) were raw milk isolates. Automated ribotyping revealed that raw and pasteurized Pseudomonas fluorescens isolates were indistinguishable (similarity Index >0.93), suggesting the possibility of postpasteurization contamination with bacteria from raw product. In the plant, filler nozzles were identified as the primary reservoirs of postpasteurization contamination. Nozzle replacement produced significantly lower finished-product PBCs at 7 days postprocessing (>4-log reduction) and extended fluid product shelf life.

THE INFLUENCE OF TIME AND TEMPERATURE OF INCUBATION ON THE PLATE COUNT OF MILK1

1974 ◽  
Vol 37 (4) ◽  
pp. 209-212 ◽  
Author(s):  
F. R. Roughley ◽  
C. K. Johns ◽  
K. L. Smith
Keyword(s):  
Incubation Temperature ◽  
Geometric Mean ◽  
Bacterial Count ◽  
Raw Milk ◽  
Plate Count ◽  
Geometric Means ◽  
Pasteurized Milk ◽  
Milk Samples ◽  
Significant Difference ◽  
The Mean

The geometric mean bacterial count of 131 raw milk samples, using plates incubated 48 h at 30 C, was 15% higher than the geometric mean when the bacterial counts were determined at 32 C incubation. When 72 h of incubation were used, the geometric mean of samples using the lower incubation temperature was 20% higher. The mean for all 72-h raw milk counts was 31% higher than that for 48 h counts. There was no significant difference between geometric means of 51 pasteurized milk samples using plates incubated at 30 or 32 C after 48 h incubation. After 72 h of incubation, the geometric mean obtained at 30 C was 10% higher. For all 72-h counts on pasteurized milk the mean was 55% higher than for 48 h. A number of both raw and pasteurized samples showed little or no increase in count following Preliminary Incubation at 12.8 C for 18 h while others “blew up” to high levels, suggesting undesirable contamination. The International Dairy Federation procedures tended to reflect the latter better than the SPC.

COMPARISON OF "DROP" AND "POUR" PLATE COUNTS OF BACTERIA IN RAW MILK

1948 ◽  
Vol 26e (6) ◽  
pp. 327-329 ◽  
Author(s):  
J. J. R. Campbell ◽  
J. Konowalchuk
Keyword(s):  
Preceding Paper ◽  
Raw Milk ◽  
Plate Method ◽  
Pure Cultures ◽  
Viable Bacteria ◽  
Plate Counts ◽  
Drop Plate

In the preceding paper it was shown that a "drop plate" method of determining the number of viable bacteria in pure cultures gives slightly higher counts than the usual "pour plate" method. In this paper it is shown that, in parallel counts made by the two methods on a series of samples of raw milk, the "drop plate" counts are some 27% higher than the "pour plate" counts. It is suggested that this discrepancy results from the more efficient breaking up of clumps and chains of bacteria by the dilution procedure used in preparing the "drop plates."

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