Graduate Thesis Or Dissertation
 

A study of the accuracy in the methods of sampling, preserving, and testing milk for fat at Oregon milk plants

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  • A study was undertaken to determine, by methods and equipment, the variations between of sampling, preserving, and testing milk for fat during the various seasons of the year, and to try and solve the problem whereby the sampling, preserving, and testing of milk for fat at milk plants could be more uniformly accomplished by different operators. During the period from May 1939 to April 1940 a total of 2064 tests by the Babcock method and 1032 tests by the Mojonnier method were made according to a standardized procedure. A total of 900 daily samples of milk, 12 five day-, 60 seven day-, and 60 fifteen-day composite samples of milk were included in the tests that comprised 10 complete "Units" over the different seasons of the year. Small decreases occurred in the fat percentages of composite samples as compared to the average of daily tests on the same samples of milk. These decreases amounted to 0.016, 0.021, and 0.027 for the five-, seven-, and fifteen-day composites respectively by the Babcock method of testing and 0.023, 0.021, and 0.055 for the three storage periods by the Mojonnier method of testing. The above decreases wore not found to be significant when the data was subjected to a statistical analysis. The Babcock results averaged approximately 0.075 higher in the percentage than the Mojonnier results on the same fresh samples of milk and also on the 5, and 7 day composites. The Babcock results averaged 0.104- higher than the Mojonnier on the 15-day composites. The differences obtained by the Babcock and Mojonnier methods of testing on both daily and composite samples of milk were found to be very significant. Numerous variations from standardized procedure were conducted to determine the effect of such variations on the final fat test. This comparative study included more than 2000 individual tests by the Babcock method and 165 individual determinations by the Mojonnier analysis. Results of these tests showed that: Variations in handling 7, and 15 day composites, such as, storage temperatures of 70° F. or above, not mixing the daily portions properly, and using unsterilized bottles, caused decreases from the average daily tests which amounted to between 0.04 and 0.14. Formalin did not prove to be superior to mercuric chloride as a preservative of milk samples, Saponin did not prevent the decline in the test of composite milk samples. Glymol added to the fat column to remove the upper meniscus in the Babcock test lowered the test by 0.192. The following variations in the Babcock testing technique were found to have a significant effect on the final results obtained: Varying the temperature of the milk above er below normal when measuring the portion for the tests; Using acid of unstandardized strength and incorrect temperature; Varying the temperature of reading the fat column above and below normal; Varying the speed of the centrifuge above or below the specified speed; Using test bottles that varied more than 0.015cc. from the correct capacity of 1.600 cc. in the calibrated portion. The following variations in the Babcock testing technique were found to have slight but noticeable effects on the final fat tests: Not using a proper lighting system to determine the exact position of the upper meniscus in reading the test; Adding water to the tests of such a temperature that the fat column migrated downward at least one inch in tempering at 138° F.; Using speeds of the centrifuge above that which was specified. Results of miscellaneous studies showed that: Duplicate tests by the Mojonnier method checked within an average variation of 0.0064 and a maximum variation of 0.015; Pasteurization of milk prevented the increased difference in results between the Babcock and Mojonnier methods of testing on samples stored for 15- days; Smaller differences were obtained on the same tests by different readers when read by the reading light than were obtained when the light was not used; Errors of 0.05 in reading the Babcock test were attributed to inability to determine the exact position of the top meniscus on the fat column; Dumping the milk in weigh vats at milk plants did not cause enough agitation to incorporate the cream sufficiently to obtain an accurate sample; Closer agreements were obtained with the Babcock and Mojonnier methods on the same sample of milk from Holstein and Ayrshire cows than were obtained on the milk from Jersey cows.
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