The Book of Cheese

by Charles Thom; W. W. Fisk

CHAPTER XIX

TESTING

In connection with marketing, a certain amount of testing of the products should be practiced, to determine exactly the results and grades of products. This includes the testing of the whole milk, whey and cheese for fat, the milk for casein, and the cheese for moisture. In factories in which the milk is bought on the fat basis, it is necessary to test each patron's milk for fat. If there is a cheese-moisture law in the state, it is necessary to test for moisture. The whey should be tested to learn the loss of fat in the manufacturing process and to ascertain whether the losses have been reduced to the minimum.

308. The fat test.—The test commonly used to determine the fat in milk is known as the Babcock. The principle of this test is as follows: Fat exists in the form of very small globules. Because the fat globules are lighter than the other milk constituents, under the influence of the force of gravity most of them rise to the surface. There, mixed with the other milk substances, these globules form a layer of cream. Babcock found that by adding to the milk sulfuric acid of proper strength and temperature, the casein, the milk-sugar and the albumin are decomposed and the sticky quality of the milk is destroyed. The acid does not decompose the fat but leaves it free to come to the surface of the mixture. Under centrifugal force, this fat is quickly brought to the surface. By using a known quantity of milk and having a scale graduated in percentage of the amount of milk, the percentage of fat can be determined. Fig. 69 shows the necessary equipment.

Fig. 69.—Apparatus necessary to test milk and whey for fat and total solids.

There are three kinds of bottles employed in making the test, one with a very large neck which is used when testing materials high in fat-content such as cream, butter and cheese. This is generally called a cream-test bottle. It is graduated from 0 to 50 per cent. When testing materials with a small amount of fat such as whey, skim-milk and buttermilk, a test bottle with two necks is used, one with a small bore for the fat and the other neck with a larger bore to add the milk, acid, water. It is graduated from 0 to 0.5 of 1 per cent. There is a third bottle between the other two to test whole milk. This is known as a whole-milk bottle. It is graduated from 0 to 8 per cent. All of the glassware should comply with the laws.

309. Sampling the milk.—One of the most important parts of testing is to obtain a fair sample of the milk. The milk to be tested may be in a vat or in a farmer's can or a composite sample jar. If the milk is bought on the fat basis, that of each patron is not tested daily, but a small quantity, about half an ounce, is taken each day and placed in a jar; this is known as a composite sample. It is the usual practice to number the patrons and have a sample bottle for each patron with his number on it. Some substance must be added to preserve the milk and to keep it from souring or coagulating. It is difficult to secure a fair sample of sour milk. A wide-mouthed jar is preferred for keeping milk samples. This must be kept closed to prevent evaporation. Each day when milk is added to the composite sample, the bottles should be shaken to prevent the cream drying. Composite samples are tested at least twice a month. The milk may be mixed to obtain a fair sample, by stirring in the vat or by pouring from one bottle to another. Vigorous shaking should be avoided as this is likely to cause churning. One should see that all the cream is removed from the sides of the sample bottle and that it is evenly distributed through the milk. The sample of milk is now measured out with the pipette. This is graduated to deliver 18 grams of milk, and holds 17.6 c.c. Hold the pipette between the thumb and second finger of the right hand with the tip below the surface of the milk, draw the milk by suction with the lips until it is filled well above the graduation. Quickly place the forefinger over the opening and at right angles to the pipette. By gently and carefully raising the forefinger, allow the milk to run down until the surface is exactly level with the graduation. To obtain an accurate reading, the pipette should be on a level with the eye. Then with the left hand, hold the milk test bottle in a slanting position and place the tip of the pipette into it about one-third of an inch and at a slight angle. Now let the milk slowly flow down the side of the neck of the bottle, making certain that none is blown out by the escaping air. When all has run out of the pipette, blow out the drop which remains in the tip. Then measure out another sample in the same way, as the test should be made in duplicate.

310. Adding the acid.—The sulfuric acid should have a specific gravity between 1.82 and 1.83. It should be kept in glass-stoppered bottles or carboys to prevent the absorption of moisture from the air, which will reduce its strength. Acid that is too strong might burn the fat. The acid is a strong poison and will burn if it comes in contact with the flesh or the clothing. In such case, it should be removed by washing with plenty of water. An alkaline substance such as ammonia or bicarbonate of soda should be applied to remove any acid not washed away.