+313. Testing whey for fat.+--Because of the small amount of fat in whey, it is difficult to obtain a representative sample. The best way, if the entire amount cannot be placed in a vat and stirred, is to catch a little of the whey at intervals as it is being drawn from the vat. The sample to be tested is measured with the pipette the same as the milk and placed in the skimmed-milk test bottle. The same acid is used to test whey as to test milk but because there are not so many solids to destroy, not so much is used. If as much acid is used with whey as with milk, it will burn the fat and so interfere with the reading of the test. Just enough acid is added to destroy the milk substances except the fat, or enough to turn the contents of the test bottle dark brown. This usually requires filling the acid measure one-quarter of an inch under the graduation. The remainder of the test is the same as for whole milk. +314. Testing cheese for fat.+--The sample of cheese to test for fat is obtained by removing the sample with a cheese-trier. This sample is called a "plug." Different plugs from the same cheese will test various percentages of fat so that it is difficult to secure a representative sample. The usual practice is to take three plugs, one near the center, another near the outside and the third between the first two. The plugs should be put into glass-stoppered bottles to prevent the evaporation of moisture. These plugs are then chopped up very fine. It is of course impossible to measure the cheese as with milk and whey, but it is weighed (Fig. 70). If the cheese is soft it can be stirred with a spatula until well mixed. A soft cheese usually sticks to the neck of the test bottle. After being weighed, it can be dissolved in a little sodium hydroxide and poured into the bottle. Different amounts may be used, commonly 4½ or 6 grams, but 6 grams is to be preferred. This is placed in the Babcock cream bottle since there will usually be more fat than can be read in a milk bottle. After the material has been placed in the test bottle, about two-thirds of an acid-measure of warm water is added to assist in dissolving the cheese. [Illustration: FIG. 70.--Apparatus necessary to test cheese for fat.] The acid is added the same as with the milk. If all the cheese particles are not destroyed, and therefore do not disappear, a little more acid will complete the solution. Centrifuging is performed as with the milk.