Date of Award:
Master of Science (MS)
Nutrition, Dietetics, and Food Sciences
Department name when degree awarded
Department of Dairy Manufacturing
A. J. Morris
A. J. Morris
The importance of an accurate measurement of fat in milk deliveries is fully appreciated by most producers and plant operators. Since milk is purchased on a fat basis, the accuracy of the procedure used in sampling and testing the milk has often been questioned by both the buyer and seller.
A composite sample to be theoretically correct, should consist of aliquot portions of the milk shipments represented. Where aliquot portions are to be taken, a milk thief is commonly suggested. This device takes a vertical column of milk from the weigh tank. The idea is that the height of the column will be proportional to the amount of milk. This will be true only if the weigh tank has a flat bottom and vertical sides. As a result, this device is not so well suited for this purpose, and besides, it is slow and cumbersome. For this reason, the principle of aliquot portions is usually ignored in sampling.*
The milk dipper is commonly used for sampling because of ease and speed with which samples are procured and the relatively small cost of the dipper. Since the dipper takes equal portions of milk regardless of the amount of milk delivered, there has been and still is considerable doubt as to the efficiency of the dipper as a method of sampling milk.
Accurate sampling is important not only from the standpoint of the producer who is interested in receiving payment for all of the butter fat he delivers to the creamery, but also from the standpoint of the creamery operator, who experiences considerable difficulty accounting for all the butter fat he purchases. Butter fat losses encountered in the operation of the creamery are far too high, according to dairy engineers. These and other similar problems connected with the sampling and testing of milk have led various institutions and individuals to devise methods of sampling milk that would help overcome these difficulties. Some of the sampling devices that have been developed for use with milk are shown in figure 1.
The procedure of sampling milk according to the Official and Tentative Methods of Analysis of the Association of Official Agricultural Chemists is:
In sampling bulk milk thoroly (sic) mix by pouring from one clean vessel into another 3 or 4 times. If this procedure is impracticable, thoroly (sic) stir the milk for at least 30 seconds with a suitable appliance long enough to reach to the bottom of the container. If cream has formed on the milk, continue the mixing until all cream is detached from the sides of the vessel and evenly emulsified throughout the liquid.*
The New Zealand proportional sampler is the outgrowth of a producer's attempt to take advantage of the sampling by equal portions.
To illustrate this let us assume that a producer has a daily production of 500 pounds of 4.0 per cent milk. In two days he would deliver 2 x 500 x 0.04 or 40 pounds of fat. However by removing some skim milk on the first day and adding it to the second day's milk, this same production might result in deliveries of --
400 pounds of 4.9% milk containing 19.6 pounds of fat. 500/1000 pounds of 3.4% milk containing 20.4/40 pounds of fat.
When equal portions are taken in sampling, the composite sample of these two deliveries would test 4.15 per cent fat. Thus the producer would be credited with 1000 x 4.15 or 41.5 pounds of fat instead of the 40 pounds actually produced and delivered. Where producers' shipments vary greatly from day to day this possibility should not be ignored.
The work undertaken in this study has been confined to the comparison of various types of sampling devices and to determine if there is a significant difference between these samplers. Five different samplers were selected for this comparison (figure 1). These samplers are: (1) Dahlstrom proportional samplers; (2) Fassett proportional sampler; (3) the dipper; (4) the milk thief; and (5) the New Zealand proportional sampler.
Hoskisson, William A., "Comparison of Various Types of Milk Sampling Devices" (1940). All Graduate Theses and Dissertations, Spring 1920 to Summer 2023. 5106.
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