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.

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.

The acid measure holds 17.5 c.c. and it should be filled to the graduation. Then this acid should be added to the test bottle. The bottle should be held at an angle and slowly rotated so that the acid will rinse down any milk remaining in the neck of the bottle. Immediately mix the acid and milk by whirling the body of the bottle in a circle five or six inches in diameter. The mixture should not be allowed to go into the neck of the bottle while mixing. Continue shaking for about a minute after all the curd has disappeared. One should avoid pointing the neck of the bottle toward any person in the mixing operation. The acid unites with all the milk substances except the fat and generates much heat.

311. Centrifuging.—There are two machines in common use for centrifuging, one that runs by mechanical power and the other smaller and runs by hand. If the machine and atmosphere are very cold, the apparatus can be warmed by placing hot water in it. This is not necessary in a steam machine. In a factory where there are a number of samples to test, a power machine is usually employed. In this machine there are pockets or cups in which to set the test bottles. The machine or disk must be balanced by placing bottles in opposite pockets. These pockets are hinged so that when standing still the bottle is in an upright position and when the centrifuge is running, it is in a horizontal position. The machine should then be covered and started running. It should be run at the speed indicated. After five minutes, stop the machine and fill the bottles with boiling water up to the neck. This can be done without taking the bottles out of the machine. A pipette or slender-spouted vessel may be used to add the water. Whirl the bottles two minutes, then add more boiling water to bring the fat column into the graduated part of the neck of the bottle. Then whirl one minute. The test should be read at once or the bottles kept at a temperature of 130° to 140° F. until ready to read.

312. Reading the test.—To read the test, subtract the reading at the bottom of the fat column from that at the highest point. The curved meniscus which always forms at the top of the fat column should be included in the reading. Duplicate samples should not vary more than O.2 of 1 per cent. Standard Babcock test bottles and pipettes should always be used. In some states the agricultural experiment stations examine all glassware and mark it to make certain that it conforms to the requirements of the state law. In New York, glassware found to be correct is branded "S. B.," which means State Brand. In some states a person must have a license to test milk or cream, when it is paid for on the fat test. Such a person must pass an examination to show that he understands the test before a license, will be granted. The license may be revoked if the work is not honestly performed.

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.

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.

315. Reading the test.—In a cream-test bottle the neck is so much wider that there is a much larger meniscus. In order to obtain an accurate result, the meniscus should be removed. This is done by carefully adding a substance called glymol, which is a mineral oil colored red. Usually about one-quarter of an inch of glymol is added to the fat column. This should not mix with the fat. The bottles should be placed in a hot water bath 135° to 140° F. for four minutes before reading. The temperature at reading should be 135° to 140° F. The reading is then taken from the bottom of the fat column to the line between this and the glymol. The bottle is graduated for 18 grams of material, but as only a part of 18 grams of cheese was used for the test, the reading should be multiplied by the part of 18 grams used. For example, suppose 6 grams of cheese were used and the test read 12 per cent fat. Since 6 is one third of 18, the actual percentage of fat is 3 times 12, or 36 per cent.

316. The Hart134 casein test was devised to determine the percentage of casein in milk. A special test bottle and centrifuge are necessary. The method of making the test is as follows: Place 2 c.c. of chloroform in the casein test tube, add 20 c.c. of a 0.25 of 1 per cent solution of acetic acid at a temperature of 65° to 75° F. This solution of acetic acid is made by diluting 10 c.c of glacial acetic acid with 100 c.c. of water, then dilute 25 c.c. of this solution to 1000 c.c. with water; 5 c.c. of milk at a temperature of 65° to 75° F. is then run into the bottle. The bottle is then covered with the thumb and inverted and the mixture shaken vigorously for exactly twenty seconds. It is then centrifuged within twenty minutes at a speed of A Quevenne lactometer. Fig. 71.—A Quevenne lactometer. 2000 revolutions a minute. The bottle should stand ten minutes before reading the percentage of casein. There are other tests for casein but they are very complicated.

317. Solids in the milk.—Because not only the fat but all the solids are utilized in cheese-making, it is important to know the amount of the solids in the milk. This is ascertained by determining the specific gravity of the milk and knowing the fat-content; the solids not fat can then be calculated.

318. The lactometer.—The specific gravity of liquids is measured by an instrument called a hydrometer. Its use is based on the fact that when a solid body floats in a liquid, it displaces a volume of liquid equal in weight to its own. Hydrometers are in many cases so made that the specific gravity can be read at the point where the scale is even with the upper surface of the liquid. A hydrometer that is especially adapted to milk is called a lactometer. There are two lactometers in common use, the Quevenne and the Board of Health.

The Quevenne lactometer.—This is a long slender hollow piece of glass weighted at the bottom to make it float in the milk in an upright position (Fig. 71). The upper end is slender and contains the scale. This scale is graduated from 15 at the top to 40 at the bottom. Each reading on the scale corresponds to the point marked specific gravity on a hydrometer, except that the figures are not complete. For example, 15 on the Quevenne scale means a specific gravity A Board of Health lactometer. Fig. 72.—A Board of Health lactometer. of 1.015; a reading of 30 on the Quevenne scale means a specific gravity of 1.030, and so on. The Quevenne lactometer is graduated to give correct results at a temperature of 60° F. The milk should be at this temperature. If the temperature is below or above this, a correction must be made to the reading. The temperature should not be more than 10 degrees above or below 60° F. The correction for each degree in variation of temperature can be made by adding 0.1 or subtracting 0.1 from the lactometer reading, as the case may be. If the temperature is above 60° F., the correction is added to the lactometer and if it is below 60° F., the correction is subtracted from the lactometer reading. The reading should be taken when the lactometer is floating free in the milk. The scale is read exactly at the surface of the milk. The better lactometers have a thermometer with the scale just above or opposite the lactometer scale.

The Board of Health lactometer.—This is very similar to the Quevenne lactometer except that the scale is graduated from 0 to 120 (Fig. 72). The point on the scale of the lactometer that floats at the surface in water is represented by 0, and 100 represents the specific gravity of 1.029. On the Board of Health lactometer, the 100 degrees or divisions from 0 to 100 equal 29 divisions on the Quevenne. Therefore, one division on the Board of Health equals 0.29 of a division on the Quevenne. To convert Board of Health reading to Quevenne, multiply by 0.29 and to convert Quevenne to Board of Health, divide by 0.29. The correction for temperatures above or below 60° F. is made the same as with the Quevenne, except 0.3 is added or subtracted from the reading instead of 0.1 as with the Quevenne.

319. Calculating the solids not fat in the milk.—When the lactometer reading and fat-content of the milk are known, there are several formulas for calculating the solids not fat. In the following formulas, L equals Quevenne lactometer reading at 60° F., and F equals the percentage of fat in the milk:

L + 0.7 F
———— = S.N.F
5

L + F
———— = S.N.F
4

L
— + 0.2 F + 0.14 = S.N.F
4

320. Testing cheese for moisture.135—There are two methods of testing cheese for moisture. The following is a simple test devised by H. C. Troy:

The ordinary butter moisture test, in which a metal cup is heated over a flame, cannot be used for determining the percentage of water in cheese because the high temperature developed in operating that test drives from the cheese other substances with the water. Also, particles are lost by spattering when the cheese is heated with any degree of rapidity in the shallow butter-moisture cups. To overcome these difficulties, the new method here described has been developed for the purpose of determining the percentage of moisture in cheese. The apparatus consists of:

1 double-walled copper drying cup

1 centigrade thermometer registering to 200°

1 alcohol lamp

1 tripod

1 special flask

1 scales sensitive to 0.01 gram

1 set of weights, 0.01 to 100 grams

The body of the copper drying cup may be made in two parts. One of the parts is a jacket that forms the outer wall of the apparatus. It has a flat bottom 4½ inches in diameter, and the perpendicular wall is 4½ inches in height. The inner part of the cup must have a flat bottom 2¾ inches in diameter and a side wall 3¾ inches high. A flange attached to the upper rim of the inner part extends out at right angles to the cup wall and forms a cover for the space between the walls when the two parts are put together. The flange is bent down around its outer edge to make it fit snugly over the upper rim of the outer jacket. It thus holds the inner cup securely in place, leaving a space about ¾ inch wide for oil between the walls and bottoms, and permits the apparatus to be taken apart readily. A circular opening about ½ inch in diameter is made through the flange to permit the insertion of a thermometer for taking the temperature of the oil or the melted fat which is used in the space between the walls. Lard or tallow serves best for use in this space; a readily inflammable oil should not be employed. The thermometer may be permanently held in place by passing it snugly through a hole bored in a cork, the cork being then fitted into the hole through the flange. A flat metal cover is placed on the cup when making a test. This cover has a hole through the center just large enough to permit the neck of the drying flask to extend up through it. The cover assists in keeping the body of the flask at a constant temperature by preventing the entrance of cold air currents. The thermometer should register changes in temperature between zero and 200° C. The alcohol lamp should yield a flame about ¼ inch in diameter and ¾ inch high. The tripod should be about 6 inches high and of proper diameter at the top to support the oil bath.

An ordinary flat-bottom glass Erlenmeyer flask, of such a diameter as to fit neatly into the oil-bath cup, may be used to hold the cheese during the drying operation; but a special glass flask serves better. It is made with a flat bottom 2½ inches in diameter, which will fit into the cup of the drying apparatus. The side walls of this flask should be perpendicular for about 1 inch, when they should begin to slope in toward the base of the neck, which should be located about 2 inches above the bottom. The neck of the flask should be 1 inch in diameter, with perpendicular walls, and its length should give the flask a total height of 4¾ inches. When the apparatus (Fig. 73) is put together for the first time, the melted fat or oil may be placed in the outer jacket and the inner cup may then be fitted into position, or the parts may be put together first and the oil then poured into the space between the cup walls through the opening where the thermometer is to be placed. The oil should fill the space to within an inch of the top. The cork through which the thermometer has been passed is then fitted into the opening. The thermometer bulb should be placed in the oil about half an inch above the bottom of the outer jacket. The apparatus is then placed on the tripod over the alcohol lamp. A flame ½ inch in diameter and ¾ inch high will give sufficient heat to hold the bath at the proper temperature. The temperature may be regulated by raising or lowering the lamp or by changing the size of the flame by adjusting the wick. Hundreds of tests may be run without taking the apparatus apart or changing the oil. The copper drying cup can be made by any tinsmith. The other parts may be ordered through any dairy or chemical supply company.

Fig. 73.—Apparatus necessary to test cheese for moisture.