Plate Loop Count Research Articles

Trends in Somatic Cell Counts, Bacterial Counts, and Antibiotic Residue Violations in New York State During 1999–2000

Milk quality data on a month-by-month basis from March 1999 to December 2000 were studied from five of the largest milk plants operating in New York State. The analyses focused on bulk tank somatic cell count (SCC), bacterial counts in the form of plate loop count (PLC), and antibiotic residue violations in the pool of milk of New York State, their mutual relation, and the influence of farm size. The average SCC was 363,000 cells/ml, the average PLC was 24,400 bacteria/ml, and the average number of antibiotic residue violations in the pool of milk was 3.9 per 1000 producers. Each month between 72 and 88% of the milk pool had SCC levels in compliance with the European Union (EU) requirements (SCC<400,000 cells/ml).The findings in this study suggest that larger farms had lower SCC and PLC but more antibiotic violations. However, the larger farms contribute most to the SCC and PLC of the total pool of milk. Farms with high SCC also had higher PLC and more antibiotic violations. Measurable improvements in overall quality of the pool of milk in New York state would most likely occur by targeting incentives, education, and training programs for any farms with very high SCC and for larger farms with SCC between 400,000 and 750,000 cells/ml.

Ontario Bulk Milk Somatic Cell Count Reduction Program: Progress and Outlook

The objective of this study was to describe the results of the ongoing somatic cell count (SCC) reduction program in Ontario, Canada and to provide an outlook regarding the possible scenarios beyond the final stages of the current reduction program. The data were from all of the approximately 9500 farms in the province of Ontario during the last 10 yr and included monthly data for kilograms of milk sold, mean milk component measurements, and milk quality measurements (bulk milk SCC, plate loop count, and inhibitor presence).Four of five steps in the control program had a significant impact on the mean monthly bulk milk SCC. The total decrease in mean SCC that was attributable to the program was approximately 80 × 103 cells/ml. The monthly occurrence of inhibitor violations increased significantly. Cross-classification of the annual mean bulk milk SCC and the incidence of inhibitor violations indicated that the incidence specifically increased for farms with a relatively high bulk milk SCC. In 1994, bulk milk SCC increased, primarily because of farms that had a mean bulk milk SCC between 150 × 103 and 450 × 103/ml. The small relative contribution of farms with higher bulk milk SCC was due to their relatively low production and the small number of farms in those classes.The Ontario SCC reduction program was initially successful in the reduction of mean bulk milk SCC. Further reduction will require the active participation of farms with a relatively low bulk milk SCC. Not only is it necessary to penalize farms that exceed thresholds, but also an incentive to prevent farms with good udder health management from increasing bulk milk SCC is of great importance. The increase in inhibitor violations is of concern and requires further attention.

A comparison of the Bactoscan 8000S to three cultural methods for enumeration of bacteria in raw milk

The bacteriological load of bulk tank raw milk samples was determined using the Bactoscan (BSN) 8000S and three cultural methods: standard plate count (SPC), spiral plate count (SPC) and plate loop count (PLC). A total of 3640 milk samples (2918 natural and 722 spiked) were analyzed. The repeatability of each system was assessed at critical cut-off counts of log 10 4.0 and Iog 10 5.0. In descending order, kappa values of within-system-agreement (repeatability) at Iog 10 4.0 were: SPC = 0.87, SPC = 0.70, BSN = 0.67 and PLC = 0.42. Descending kappa values at log 10 5.0 were: BSN = 0.96, SPC = 0.95, PLC = 0.84 and SPC = 0.78. BSN results demonstrated good correlation with the three methods: SPC ( r = 0.79), PLC ( r = 0.72) and SPC ( r = 0.73). These results suggest that the rapid and automated Bactoscan 8000S could provide a reasonable alternative to the three cultural methods for enumeration of bacteria in bulk tank raw milk when counts fall within the instruments specified applicable range of log 10 4.0-log 10 5.477.

Ontario Bulk Milk Somatic Cell Count Reduction Program. 1. Impact on Somatic Cell Counts and Milk Quality

The objective of this study was to evaluate the bulk milk SCC control program in Ontario and to observe the effect of the program on milk components and milk quality. The data were from the approximately 9500 farms in Ontario, Canada. Data on milk quality, milk components, and kilograms of milk produced were collected monthly from January 1985 through September 1991.The SCC data had a significant seasonal pattern. The expected lowest mean SCC occurred in April, and the expected highest mean SCC occurred in October. The SCC control program had a significant impact on the mean monthly SCC. A decrease of approximately 58×103 cells can be attributed to the program.Percentage of fat and lactose increased significantly with decreasing bulk milk SCC. Bulk milk SCC had very little effect on protein percentage. In herds that produced milk with lower SCC, we observed significantly lower plate loop count, lower incidence of inhibitor violations, and lower freezing point of the milk.

Variation in Food Microbiological Tests Used to Evaluate Analyst Performance

Procedures for evaluating analyst performance in state and federal milk and food laboratory quality assurance programs depend on the estimates of variation for the methods used. The total components of variance for a method consist of replicate error, among-analysts variance, and analyst-sample interaction components. Eleven methods were evaluated in this study. Total variance estimates of milk analytical procedures for the Standard Plate Count, plate loop count, coliform plate count, direct microscopic somatic cell count, electronic somatic cell count and Wisconsin mastitis test were 0.01045, 0.01371, 0.01590, 0.01265, 0.00504 and 4.06, respectively. Estimates of the logarithmic total variance of food analytical procedures for the aerobic plate count, coliform most probable number (MPN), Escherichia coli MPN, fecal coliform MPN and Staphylococcus aureus MPN were 0.00853, 0.10205, 0.14705, 0.14780 and 0.24245, respectively. The study showed that current analyst performance levels used by state and federal laboratory quality assurance programs are satisfactory.

A Flooded Plate Loop Count Procedure

A modification of the plate loop count procedure for milk samples is described which incorporates use of a 0.5-ml diluent loop-rinse onto pre-dried agar medium in a 100-mm petri plate. The sample is distributed using a glass spreader, the plate is dried and incubated using standard procedures. Colony formation was more uniform than with the conventional method. The method did not appear to significantly change the statistical characteristics of the plate loop count on raw milk samples. A correlation coefficient of .91 was obtained between the flooded and the conventional plate loop count.

Collaborative Evaluation of the Plate Loop Technique for Determining Viable Bacterial Counts in Raw Milk

A collaborative evaluation of the plate loop count technique was undertaken involving 13 laboratories with 29 technologists and 27 plate loops, The log variance of the Plate Loop Count (PLC) was 0.007, compared with a log variance of 0.001 for the reference Standard Plate Count (SPC), both of which fell within the tolerance limit of 0.012 considered acceptable for reproducibility by Standard Methods. The overall log10 mean PLC (1.975) compared favourably with the log10 mean SPC (1.960). Despite this overall image of precision and accuracy of the PLC, comparative data between individual technicians and/or loops was significantly different. Factors which contributed to this inaccuracy and imprecision of the PLC included improper loop calibration and significant variation in technique not only between analysts but also by individual analysts between loops.

Opportunities to Improve Milk Quality

Many opportunities exist to improve the bacterial quality of milk, particularly of manufacturing-grade milk. Test results, using plate counting procedures by 10 Iowa dairy laboratories, reflect a gradual improvement in recent years in manufacturing-grade milk quality. In recent months, 70 to 84% of 3,000 to 5,000 individual Iowa farm's manufacturing-grade milk samples tested each month were placed in Class I (plate-loop count of less than 500,000/ml). Problems still exist in farm milking equipment sanitation, undersized bulk tanks with inadequate cooling capacities, and delays in processing raw milk. USDA DMCC results on milk samples obtained from milk-storage tanks in Iowa dairy manufacturing plants reveal that in 1977–78 only 5.8 to 13.1% of the commingled raw milk would be classified as Class I. These results reflected a deterioration in bacterial quality of raw milk from the time milk leaves the farm until held in the plant storage system. Part of the quality deterioration can be blamed on failure to empty and clean plant storage tanks often, allowing too much time before milk is processed, and on milk not at 40 F or less. Cheese and butter exhibits reveal that poor milk quality has a negative influence on product flavor quality. Quality assurance programs for dairy farms, milk transportation, and plant storage must stress sanitation, cold temperatures and minimum times before processing to obtain good bacteriological quality milk and pleasing-flavored dairy foods.

The 3M Automatic Colony Counter in the Bacterial Evaluation of Manufacturing-Grade Milk

Twenty-seven raw milk samples were analyzed for their bacterial quality by the Standard Plate Count (SPC) and the plate loop count (PLC). All resultant plates were counted by three different technicians either manually or with a 3M Brand Automatic Colony Counter, Model 630. Results emphasize the superiority of the SPC over the PLC for samples with high bacterial numbers. The automatic colony counter tended to underestimate the number of colonies on a plate. Correction factors for conversion of PLC counts to SPC and automatic to manual counts were calculated. It is recommended that the PLC and the automatic colony counter be used with appropriate correction factors for routine analysis of large numbers of samples.

Plate Loop Method for Determining Total Viable Count of Orange Juice

The plate loop count as described in Standard Methods for the Examination of Dairy Products. 13th Edition, was evaluated as a simplified technique for determining total viable counts in single strength orange juice. This procedure requires use of standardized loops (0.01 and 0.001 ml) for making serial dilutions instead of pipets and dilution bottles. Data show there is a statistically significant difference in results between the plate count and plate loop count procedures for determining total viable population of orange juice. However, since the difference is so small for normal operating conditions, the plate loop method is a reliable alternate to routine plating procedure. It can also be done without flaming the loop between samples. The plate loop count saves time and equipment, but requires analysts to be specially trained to maintain this degree of accuracy.

MANUFACTURING MILK QUALITY: A RE-EVALUATION

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.

PREDICTION OF STANDARD PLATE COUNT OF MANUFACTURING-GRADE RAW MILK FROM THE PLATE LOOP COUNT

Plate Loop Counts and Standard Plate Counts of manufacturing grade raw milk samples were compared. With bacterial counts of 200,000/ml and less, the Plate Loop Count approximated the Standard Plate Count very closely. With counts greater than 200,000, the agreement was not as close. Differences between counts from the two methods, however, were not statistically significant (P<0.05) within either count range.An adjustment factor was determined to enable the Plate Loop Count to more accurately predict the Standard Plate Count of milk with a Standard Plate Count >200,000/ml. The Standard Plate Count can be reasonably predicted from the Plate Loop Count by the formula: [Display omitted]

COMPARISON OF PLATE LOOP AND AGAR PLATE METHODS FOR BACTERIOLOGICAL EXAMINATION OF MANUFACTURING GRADE RAW MILK

Summary Plate loop counts and standard plate counts on each of several manufacturing grade raw milk samples (handled in cans or in farm bulk tanks) have been compared. On the average, the plate loop count (PLC) was lower than the standard plate count (SPC) regardless of the type of handling of milk on the farm, can or bulk tank. Agreement between the SPC and PLC seemed to depend upon the bacterial-count levels present in milk. Statistical analyses indicated significant differences, at 1% level of probability, between the average bacterial count by SPC and PLC methods regardless of count level (≤100,000/ml or &amp;gt;100,000/ml) in case of can milk samples. On the other hand, in case of farm bulk tank milk samples, no significant differences, at the 1% level of probability, between the average bacterial count by SPC and PLC methods were obtained, when the counts were equal to or less than 100,000 per ml; when the counts exceeded 100,000 per ml, significant differences were present. Since the bacterial counts of manufacturing grade raw milk samples are likely to exceed 100,000/ml, the data presented in this investigation indicate that, until the bacteriological quality of manufacturing grade milk supplies undergoes substantial improvement, the PLC method does not appear to be a suitable substitute for the SPC method for routine bacteriological examination of such milk supplies.

A PLATE LOOP METHOD FOR DETERMINING VIABLE COUNTS OF RAW MILK

A plate loop count method for determining viable counts of raw milk has been described. The technique uses a 0.001 ml calibrated loop attached to a continuous volume syringe for rinsing the sample into a standard petridish prior to pouring with agar. Results with this method compare closely with the standard plate count performed simultaneously on the same sample.