UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION CIRCULAR No. 230 TESTING MILK, CREAM, AND SKIM MILK FOR BUTTERFAT By J. C. MARQUAKDT Division of Dairy Industry This circular describes an approved and practical method of using the Babcock Test that is suitable for the dairyman, the manufacturer of dairy products, and the official test supervisor. The directions given below are expressed in such a manner that the tests can be made by persons not familiar with chemical analysis or laboratory practices. However, persons not familiar with such procedure would find it helpful to observe the work being done at a local creamery or other place where tests are being made before attempting to operate the test independently. TAKING THE SAMPLE A proper sample is of first importance in any analysis. If the sample is improperly taken, the results will be of no value, regardless of the operator's knowledge or skill. If the milk is mixed by pouring from one container to another a proper representative sample can be obtained. This method is used in connection with cow-testing work. The milk from a complete milking is poured into another pail and sampled at once. It is essential that the sample be taken immediately after mixing, since the fat soon begins to rise again. In large vats the mixing may be successfully accomplished by the aid of mechanical agitators. When samples of milk are taken directly from the can, a milk stirrer (Fig. 1) may be used to thoroughly mix the top and bottom milk immediately before the sample is secured. Composite sampling. — A composite sample is composed of pro- portional quantities of different lots of milk. In the official test work, composites are taken with a graduate pipette and a certain quantity of milk is taken for each pound produced by the animal at each milking. The common practice is to take 3 cubic centimeters per Z UNIVERSITY OF CALIFORNIA EXPERIMENT STATION pound. This enables the superintendent of dairy tests to check the work of the test supervisor by comparing the test of the composite sample with the computed average test. Composite samples should be kept in a clean, well-sealed container to prevent the evaporation of moisture. Approved types of containers are shown in Fig. 1, and bottles with a capacity of four to sixteen ounces are used for preserv- ing the samples. Those most commonly used in milk plants have a capacity of six to eight ounces. In order to reduce the labor and expense of testing, the composite method of sampling is practiced by most large whole-milk plants. The type of samplers commonly used in sampling large quantities of milk are also shown in Fig. 1. THE BABCOCK TEST The Babcock Test is based upon the fact that strong sulphuric acid will dissolve the milk solids other than fat, and produce heat which causes the fat globules to come together more easily. By cen- trif uging in specially constructed bottles, the milk fat is completely separated from the serum which has a higher specific gravity. Test bottles. — The bottles used for testing milk by the Babcock method are marked with divisions on the neck which read from zero to 8 per cent when 17.5 cubic centimeters of milk (18 grams) is used for the test. Each of the smaller divisions on the neck of the bottle represents one-tenth per cent. (.1%). Milk test bottles marked with divisions on the neck reading from zero to 10 per cent are sometimes used. Each of the smaller divisions on the neck of these bottles represents two-tenths per cent (.2%). The use of the 10 per cent bottle is not legal in California, and con- sequently, the 8 per cent bottle must be used for official testing. Milk pipettes. — The glass milk pipette shown in Fig. 2 is used for measuring the quantity of milk required for the test. A mark sur- rounding the neck of the pipette indicates the measure of milk to be used. When the pipette is filled with milk to this mark it contains 17.6 cubic centimeters, and 17.5 cubic centimeters or 18 grams of milk is delivered into the test bottle, .1 cubic centimeter adhering to the inner surface of the glass. The tester. — The centrifugal machines used for the Babcock test vary in size from 2-bottle to 48-bottle machines. These machines may be operated either by hand or by steam, or electricity. (Fig. 3.) In all cases, it is essential to have the centrifuge securely fastened to its base. Circular 230] TESTING MILK AND CREAM FOR BUTTERFAT 5 Fig. 1. — (1) Type of rod commonly used for stirring milk. (2) Dipper type of sampler now in general use for taking milk samples. (3) The McKay sampler occasionally used for milk sampling but designed for cream sampling. (4) Thief sampler which is efficiently used for milk sampling. (5) Two types of bottles satisfactory for preserving composite samples. 4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The steam machines are preferred to the other types owing to the uniformity of speed and also to the maintenance of an even tempera- ture which they make possible. Acid. — Commercial sulphuric acid with a specific gravity of 1.82- 1.83 is used for the test. It is essential to have acid of this strength in order to obtain clear butter fat readings. The acid should always be kept in a tightly closed container as the sulphuric acid weakens when exposed to the air, owing to its property of absorbing moisture 3 I <& «2J cfel 3 / \ CZZIS 17.5 c c c9-= ^-4 Fig. 2. — (1) Milk test bottles, 8 per cent. (2) Milk pipettes, 17.6 cubic centimeters capacity. (3) Acid measure, 17.5 cubic centimeters capacity. (4) Acidometer. r s from the atmosphere. Sulphuric acid is extremely dangerous owing to its corrosive action. When acid is spilled, it is essential to remove it immediately with water, and then check the corrosive action of the acid with an alkali. Lead sheeting is used to protect testing tables as it is not attacked by sulphuric acid. The acidometer illustrated in Fig. 2 is the instrument commonly used for determining the strength of commercial sulphuric acid. It is standardized to read directly the specific gravity of sulphuric acid at 60° F. Convenient apparatus for use in disposing of waste acid is shown in Fig. 4. Circular 230] TESTING MILK AND CREAM FOR BUTTERFAT DIRECTIONS FOR MEASURING MILK INTO THE TEST BOTTLE Pour the milk sample from one container into another several times to insure thorough mixing. Mixing should be done by pouring down the side of the container and not into the center of it. This is neces- sary so as to avoid the introduction of air into the milk. Then immedi- ately draw the milk into the pipette some distance above the mark. By a slight release of the forefinger on the mouth of the pipette the milk is lowered directly to the mark. The stem of the pipette is then inserted into the neck of the test bottle and the milk allowed to run in. By blowing into the pipette the last drop of milk is forced into the bottle. In cases in which the results of testing are of great im- portance, as in the testing of cows for advanced registry, all tests should be made in duplicate. The milk bottles should be marked so that the samples can be identified. H Fig. 3. — (1) Steam turbine tester. (2) Electric centrifuge. (3) Hand centri- fuge. Adding acid. — The milk and acid should be at a temperature of from 55°-70° P., at the time the acid is added to the sample. In adding the 17.5 cubic centimeters of acid to the milk, it is essential to slope the test bottle so the acid will pass down the side of the neck of the bottle. Adding acid in the central region of the bottle would cause a charring of the sample and might cause the spilling of acid. The acid and milk should be mixed by a rotary motion of the bottle. Centrifugmg the samples. — The samples should be placed in the tester and centrifuged for a period of &\e minutes, If all the pockets in the tester are not filled with test bottles, the bottles should be evenly distributed around the tester so that the centrifuge will be evenly balanced. The speed of the centrifuge is determined by the diameter of the wheel. (See Table 1.) The bottles are filled to the base of the neck with water after the five minute period. To insure clear tests, the water used should be either soft or distilled, and when adding it to the tests it should be b UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION at a temperature of from 140° to 180° F. Centrifuge the bottles for two minutes after this addition of water. Then stop the tester and add water up to the 7 per cent mark. Centrifuge one minute longer, and remove the bottles from the tester for reading. Reading the tests. — The samples are placed in a water bath (Fig. 5) at a temperature of 130° to 140° F. for a period of five minutes or more. Care should be taken that the fat in the neck of the bottle is completely below the level of the water in the bath. Fig. 4. — Convenient jar for handling waste acid. In reading the fat column is conveniently measured with a pair of dividers. The measurement should include the lower and the upper curve of the fat column. (See Fig. 7.) Then transfer one point of the divider to the zero mark on the neck of the bottle. The other divider point will indicate directly the per cent of fat in the sample. vm TABLE I Showing Speed of Centrifuge for Testers of Different Diameters Diameter Revolutions Diameter Revolutions of wheel of wheel of wheel of Wheel in inches per minute in inches per minute 10 1000-1100 18 775- 825 12 900-1000 20 725- 775 14 825- 975 22 700- 750 16 825- 675 24 575- 625 CIRCULAR 230] TESTING MILK AND CREAM FOR BUTTERFAT 7 Abnormal tests. — Charred samples may result from using too much acid, using warm acid or milk, adding acid directly to the milk instead of permitting it to run down the side of the neck by sloping the bottle, allowing samples to remain too long before centrifuging, or from using acid of a strength greater than that specified, or allowing milk and acid to stand before mixing. The formation of curd in the tests is due either to using weak acid, insufficient mixing, or to using milk or acid that is too cold. Using hard water causes the formation of a white foam. Fig. 5. — Type of water bath used for milk and cream samples. TESTING CREAM FOR BUTTER FAT The range in specific gravity of cream of various fat contents is very great. Hence, an 18 gram sample of cream cannot be obtained by means of a 17.6 cubic centimeter measurement. Furthermore, as cream of high test has a thick consistency, much of it would adhere to the walls of the pipette in case it were measured into the test bottle. Since separator cream also contains more or less air it is essential to weigh it in order to obtain a proper sample. Cream test bottles. — The bottles used for testing cream by the Bab- cock method are marked with divisions on the neck of the bottles to read from zero to 50 per cent. Eighteen gram or 9 gram bottles are used. They are designed to read directly the percentage of butter s UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION fat. The smaller divisions on the necks of both types of bottles repre- sent five-tenths per cent (.5%) each. The types of cream bottles commonly used are shown in Fig. 6. Cream test balances. — The balances used in weighing cream are specially designed and range in capacity form one to twelve bottles. The four-bottle Torsion Balance illustrated in Fig. 6 is very satis- Fig'. 6. — Cream balance and test bottles. factory. Cream balances should be carefully checked for accuracy by a person familiar with their construction. Careful handling of the cream balance is imperative for good results. Preparation of the cream sample for testing. — A sample of cream can be prepared by pouring it from one container to another in a manner similar to that described for milk. Most cream samples, how- ever, are clotted when they are presented to the operator for analysis. These samples are placed in a bath of water at 90° to 110° F. This treatment restores the liquid condition of the cream and makes it possible to follow the procedure described for preparing milk samples. Circular 230] TESTING MILK AND CREAM FOR BUTTERFAT 9 Weighing the cream for analysis. — The bottles are placed in the sockets of the balance pan and balanced. By means of a pipette, a charge of cream is passed into the cream test bottle. Considerable skill is required by the operator in order to check the flow of cream when the pointer of the balance indicates that the correct charge has been delivered. If too much cream is added, a small portion can be poured from the test bottle and the bottle again placed upon the scales and cream added from the pipette until it is accurately balanced. Adding the acid. — Cream contains less solids-not-fat than milk and for this reason less acid is required in making the cream test. The amount of acid necessary to be used will decrease as the richness of the cream increases. The development of a coffee brown color after mixing the acid and cream is an indication that sufficient acid has been added. The development of a darker color is a sign of too much acid. Under normal conditions, with the cream and acid at a tem- perature of 70° F., approximately 4 to 8 cubic centimeters of acid are required for a 9 gram sample, and 8 to 12 cubic centimeters would be required for an 18 gram sample of average cream. While adding the acid, it is necessary to revolve the cream bottle so as to allow the acid to wash the cream into the bulb of the bottle. In mixing, the bottle should be rotated evenly so that none of the mixture is expelled from the bottle. After mixing the cream and acid, water at a temperature of about 180° F. is added to the sample. The addition of water should be sufficient to bring the contents of the bottle up to the base of the neck. The purpose of adding the water before whirling is to check the action of the acid and prevent charring. Centrifuging the samples. — The initial whirling should cover a period of five minutes as for milk. The fat is then raised into the neck of the bottle by the addition of water at a temperature of about 180° F. After this, the bottles receive a final whirling for a period of three minutes. Beading the tests. — At the completion of the second whirling, the bottles are placed in a bath of water at 135° to 140° F., for a period of at least ten minutes. Just before reading the tests, a few drops of glymol (one quart of a white mineral oil called Hydrol colored with 1 oz. alkanet root) are added to the tests. The addition of the glymol straightens the upper meniscus or crescent-shaped curve of the fat, making possible an easier and more accurate reading. The tests are 10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION then read by the aid of a pair of dividers. The reading is taken from the lower part of the lower meniscus to the straight line formed by the upper meniscus and the glymol. (Fig. 7.) THE PROPER. METHOD CREfiM 3 MILK TEST5 / 50 145 35 50 Z5 20 15 * 5