THE CEEAMING ABILITY OF MARKET MILK 3 
METHODS USED 
The studies at the plants included a history of the raw milk 
received and the collection of detailed data on the equipment and 
methods of operation. Samples were collected during each step of 
the process, and the cream volume and butterfat percentage were 
determined. The cream-value determinations were made by placing 
100 cubic centimeters of the sample in a graduated cylinder with 1 
cubic centimeter graduations. Each cubic centimeter of visible 
cream in this cylinder represented 1 per cent of the total volume. 
The cream-volume readings are not referred to in this bulletin as a 
rate per cent, however, but as units, in order to avoid confusion with 
other percentage rates. The cream-volume readings were made 
after the samples had remained in storage for from 20 to 21 hours. 
All samples on which cream-volume determinations were made 
were cooled quickly to approximately 15° F., and then placed at 
once in the refrigerator, unless otherwise stated. The methods used 
eliminated the factors of slow cooling and of agitation during cool- 
ing in so far as possible. 
The percentage of fat in the under layer was determined by plac- 
ing 100 cubic centimeters of the sample in a glass tube plugged at 
the bottom with a cork. An outlet tube extended through the cork, 
by means of which a definite quantity of the under milk could be 
slowly drawn off. These tubes were placed in the refrigerator with 
the other samples and the milk was drawn from them at the time the 
cream-volume readings were made ; the milk was then tested for 
butterfat. These tests are referred to in the tables as " the butterfat 
in the under layer." 1 These under-layer butterfat tests were made 
to check the accuracy of the cream-volume readings. Butterfat tests 
were run on all samples on which cream-volume determinations 
were made. 
In order to eliminate the factor of a high-temperature heating 
medium in vat pasteurization, a direct-acting piston pump was con- 
nected with the pasteurizing vat at the dairy hall, University of 
Minnesota, so that a large quantity of the heating medium could be 
pumped through the coil. 
EFFECT OF PASTEURIZING MARKET MILK 
Table 1 is a summary of the results of pasteurizing market milk 
at temperatures varying from 113° to 118° F. Most of these tests 
were made at commercial plants, though a few of them were con- 
ducted in the board of health laboratory and at the dairy hall. Uni- 
versity of Minnesota. Unless otherwise stated, the market milk 
used was that received at commercial pasteurizing plants, varying 
in age from 1 to 28 hours, with an acidity less than 0.2 per cent. 
A study of the table shows that in the majority of cases there was 
a reduction of approximately two units in cream volume for milk 
pasteurized at 115° F. for 30 minutes, and a still greater reduction 
at higher temperatures. In a few cases there was no change, and in 
some cases the pasteurized sample showed an increase over the raw 
control sample. In many cases where this occurred, however, the 
cream volume of the control sample was abnormally low owin<r to 
