CALIFORNIA AGRICULTURAL EXPERIMENT STATION CIRCULAR 340 Revised, DECEMBER, 1949 r? TESTING MILK AND CREAM = ■ D. H. NELSON Revised by E. L. JACK UNIVER / . CALIFORNIA • BER ANYONE can test milk or cream successfully — even if he's never done it before or isn't trained in laboratory work. You may not hit the mark the first time, but with practice you can perfect your technique and increase your speed and accuracy. Your results will be dependable . . . You have clean equipment. You get a sample which is typical. You follow directions in detail . . . Precision is important — remember, your results will be no more pre- cise than your operation. ■ HIS CIRCULAR tells you how to make simple, easy tests for quality in milk and cream. But you will find it helpful if you can first watch an experienced milk tester at work. These are the tests which are described: Milk Fat in Milk 6 Milk Fat in Cream 10 Milk Fat in Skim Milk 12 Acidity in Milk and Skim Milk 12 Acidity in Cream 13 Specific Gravity and Solids in Milk and Skim Milk 14 Sediment in Milk 15 Sediment in Cream 16 THE AUTHORS: D. H. Nelson was Associate in Dairy Industry in the Experiment Station; resigned June, 1937. E. L. Jack is Professor of Dairy Industry and Dairy Technologist in the Experiment Station. CLEAN EQUIPMENT is a must . . . The success of your operation and the accuracy of your results depend upon using equipment which is absolutely clean. Be sure to follow these directions with utmost care. The method for cleaning testing equipment is the same for all tests. Follow these directions in washing all equipment, such as sample bottles, pipettes, and test bottles. Wash the equipment immediately after use, before it dries, as follows o © e Rinse thoroughly with clean water having a temperature between 90° and 100° F. Brush thoroughly in a solution of some alkali, such as sal soda or some suitable commercial cleaning preparation made for that purpose. About a tablespoon of powder in 6 or 8 quarts of water is usually enough. Temperature of the solution should be between 110° and 120 F. Rinse the equipment at least twice with clean water at about the same temperature as that used for washing. For glassware difficult to clean, use a cleaning solution composed of potassium dichromate in concentrated sulfuric acid. To make the clean- ing solution: into a 2-liter pyrex beaker or flask dissolve 60 grams of potassium dichromate (IGCnO-) in 50 milliliters of water. Cool and add slowly 500 ml of concentrated sulfuric acid (H2SO4). Heat if necessary, until all potassium dichromate is dissolved. Add an additional 500 ml of concentrated sulfuric acid. [3] 3 Top: several implements used in taking milk samples; bottom left: a sample bottle; bottom right: testing the specific gravity of sulphuric acid (see page 6). [4] THE SAMPLE is important . . . Since you can't test an entire lot of milk, you must rely on a small portion taken in such a way that it is typical of the whole lot. The sample must be truly representative. The first and most important step in any analysis is to obtain a correct and truly representative sample, without which you cannot expect accuracy in results regard- less of the care and precision of the analysis. Milk is frequently sampled on the farm, at shipping stations, on receiving plat- forms, and in the laboratory. Wherever taken, the sample must be correct and truly representative. The quantity of sample required will depend upon the number of tests to be made. A sufficient quantity should be taken to permit at least one re-test in case the first test is in error. Usually about 2 fluid ounces are sufficient, except when specific gravity and sediment tests are made. (For exact amounts, see detailed directions for tests.) Procedure for Sampling Milk Mixing If milk is to be sampled from a milk- weighing vat, a mechanical stirrer is re- quired for adequate mixing. If milk is to be sampled from cans, stir thoroughly with a hand stirring rod. If milk is to be sampled from a smaller container, such as a pail or sample bottle, mix thoroughly by pouring from one clean vessel into another three or four times. Continue mixing until all cream is detached from the sides of the vessel and evenly emulsified throughout the liquid. Obtaining the sample A sampling dipper, milk sampling tube, or a graduated pipette may be used. See upper picture opposite page. If propor- tionate samples are not necessary a dipper to obtain the required amount is satis- factory. If the weights of the different quantities of milk are known, as in cow-testing work, a graduated pipette to take proportionate quantities from each milking is satisfac- tory. Proportionate samples may best be taken from a milk-weighing vat with a sampling tube, provided the sides of the vessel are vertical and the bottom is flat. Fill containers completely to prevent churning, if samples are to be hauled. Stopper tightly and mark for identifica- tion. See lower left picture opposite page. Composite samples may be accumu- lated from each day's delivery for an eight-day period. Proportionate amounts are added each time to a sample bottle containing a preservative. Keep tightly stoppered. Mix thoroughly, using a gentle rotary motion to avoid churning after each addition. The prescribed amount of preservative (mercuric chloride or formaldehyde) may be used in sample bottles thus held. The preservative will interfere with some analyses; for this reason, preservatives are generally used to preserve samples for fat tests only. One mercuric chloride tablet (no. 1 size) or 4 drops of formal- dehyde will keep an eight-ounce sample of high quality milk while 8 days' sam- ples are collected; more may be necessary with poor milk. Store composite samples at 35-40° F. Never freeze. Keep samples where they cannot be tampered with until they are tested. [5] MILK FAT in Milk . . . is easily determined by the Babcock Test, in which sulfuric acid and a centrifuge are used to separate the fat. The Babcock Test for determining fat in milk, invented by Dr. S. M. Babcock in 1890 at the Wisconsin Agricultural Ex- periment Station, has since been adopted throughout this country. It is based (1) on the use of concentrated sulfuric acid to dissolve the solids-not-fat and produce heat that aids in coalescing the fat glob- ules, and (2) on the use of a centrifuge to separate the fat completely from the acid mixture, which has a higher specific gravity. The acid The acid used is commercial sulfuric acid, water-clear, and strong enough to have a specific gravity of between 1.82 and 1.83. It is extremely dangerous to handle. It attacks glass, lead, and oil or fat very slowly, but most other substances very rapidly. If spilled the acid should be immediately washed away with large quantities of cold water, and its corrosive action checked with a dilute alkali such as ammonia or alkali washing solution. Sulfuric acid should be kept tightly stop- pered at all times; otherwise it may ab- sorb moisture from the air and become too weak for use. The acidimeter illustrated on page 4 is the instrument commonly used for de- termining the strength of commercial sulfuric acid. It is standardized to read directly the specific gravity at 60° F. The centrifuge The centrifuge or tester may be turned by hand, by electric motor, or by steam. These three types of centrifuges are shown in the illustrations. The steam or electric centrifuges with heating elements are preferable because they can be easily operated at a uniform speed and a uni- form temperature. In every case the speed of the centrifuge is very important and depends upon the diameter of the socket carrier. The diam- eter is measured from the bottom of the sockets on one side to the bottom of those directly opposite, when the sockets are in a horizontal position. The correct speeds for testers of vari- ous diameters are given below : Revolutions per Diameter of minute of socket socket carrier carrier 14 inches 16 inches 18 inches 20 inches 21 inches 22 inches 24 inches 875-925 825-875 775-825 725-775 710-760 700-750 675-725 The centrifuge must be securely fas- tened to a level and solid foundation. Milk test bottles The only milk test bottle legal in Cali- fornia is graduated on the neck to read from to 8 per cent when 18 grams of milk (delivered by the 17.6-ml pipette) The centrifuge may be turned by hand is used for the test. Each of the smaller divisions represents 0.1 per cent. Such a test bottle is shown on the next page. The milk pipette The pipette is graduated to contain 17.6 ml of water at 20° C, and has a delivery tube small enough to be easily slipped into the neck of the test bottle and long enough to extend through the neck. Procedure in testing milk for fat Preparing the sample 1. Adjust the temperature of the sample if cold to near 95° F in a 95° F tem- pering bath. 2. Mix thoroughly by pouring from one container to another at least 3 or 4 times. The milk should be poured down the side of the container to avoid partial churning of the fat. 3. If lumps of cream do not entirely dis- appear, warm sample to a temperature not exceeding 100° F and mix again. 4. Immediately after preparing the sam- ple, suck milk into pipette a little above the mark and hold it there by placing a finger on the top of the pi- pette. See center picture next page. 5. Release pressure of finger and allow milk to run slowly out of the pipette until the top of the milk coincides with the mark on the pipette. 6. Insert delivery tube of the pipette through the neck of the test bottle and remove finger to allow the milk to run into the bottle. 7. Blow out drop remaining in pipette and remove pipette. 8. Mark bottle with pencil to identify. Test each sample in duplicate, where practical 1 . Add 17.5 ml of sulfuric acid at 70° F by tipping the test bottle slightly so that the acid will run down the side of the neck. See right hand picture next page. 2. Revolve the test bottle while adding the acid in order to wash down any traces of milk adhering to the neck. 3. Mix acid and milk by a smooth, ro- tary motion until a uniform color is obtained throughout. Hold the neck of the bottle between the thumb and . by electric motor ... or by steam. til forefinger, and rotate the contents with a wrist motion such that the con- tents will swirl about the interior of the bottle and become mixed together rapidly. Be careful to use the proper motion for mixing and to continue for a sufficient time to be sure all the solids-not-fat are dissolved. The con- tents will develop a dark chocolate color. 4. Place bottles in the centrifuge and arrange around the carrier at equal intervals to keep it balanced. If neces- sary to keep the socket-carrier bal- anced, test bottles containing water should be used. 5. Centrifuge bottles for 5 minutes after the machine has attained the proper or "normal" speed, then fill each bottle with hot soft water (140° F or above) to the base of the neck. 6. Centrifuge test bottles again for 2 minutes after proper speed has been attained. Bring the fat column within the graduated portion of the neck of the bottle by adding more hot soft water. 7. Centrifuge finally for 1 minute at the proper speed. 8. Transfer test bottles to a water bath of such depth that the top of the fat column does not come above the level of the water. See left hand picture opposite page. 9. Maintain temperature of the bath between 130° and 140° F as long as any bottles are in it. 10. Hold test bottles in the water bath at least 10 minutes. 1 1 . Read each test immediately upon its removal from the water bath. The fat column at the time of reading should be translucent, golden yellow or amber, and free from visible sus- pended particles. Reject all other tests, especially in highly accurate work. See right hand picture opposite page. 12. Record immediately. 13. Empty the bottles. Use a special jar or lead sink because this waste acid is still very corrosive. It must not be poured into ordinary drains or where animals can contact soil saturated with it. If the bottles are vigorously shaken while being emptied, the sedi- ment in the bottom is easily removed. Clean bottles by the procedure de- scribed on page 3. Left: a Babcock test milk bottle; middle: drawing milk sample into pipette; right: adding the sulphuric acid. Ml ■ ■*# v * k*:!*?^ / m;m ..;<■;;:■ sw „« ■rm ~> 4 • '.A 1 Left: removing test bottles from water bath; right: making a careful reading. IMPERFECT TESTS . . . Off-color fat columns, suspended particles, and bubbles cause imperfect tests. Column should be translucent, golden-yellow or amber, and entirely free from visible specks. Dark-colored fat columns containing black specks may be caused by: 1. Too strong acid. 2. Too much acid. 3. Too high a temperature of either milk or acid before they are mixed together. 4. Adding the acid directly into the milk, instead of allow- ing it to run down the side of the neck of the bottle. 5. Allowing the acid and milk to stand in the bottie loo long before mixing. 6. Not using a smooth rotary motion when mixing. Pale-colored fat columns containing white specks may be caused by: 1. Weak acid. 2. Too little acid. 3. Low temperatures of either acid or milk before they are mixed. 4. Insufficient mixing. Bubbles appearing in the fat column are caused by: 1. Using hard water to fill the bottles between the centrifuging periods. [9] MILK FAT in Cream . . . is also determined by the Babcock Test, but certain important changes in procedure and equipment are necessary. Cream varies so greatly in its specific gravity that a pipette does not always deliver the same weight. Also, the proper amount of cream cannot be delivered from the pipette. Air bubbles are fre- quently trapped in the cream, making it impossible to fill the pipette properly. All these difficulties can be overcome by weighing the cream into the test bottle instead of measuring it as with milk. Cream balances These balances range in capacity from 1 to 12 bottles, but the 4-bottle torsion balance is very satisfactory. Set the bal- ance on a level, solid foundation, where it is protected from even the slightest draft. Check for accuracy at regular in- tervals. The balance is very sensitive, must be handled carefully, and preferably should not be carried or moved. The oper- ator should stand directly in front of it and should keep the beam locked at all times, except during the brief moment necessary to observe whether it is in balance. When properly balanced, the pointer will swing one or two divisions equally on each side of center without coming to rest. Cream test bottles Two cream test bottles are accepted for official work in California. They are 9 inches high and are graduated to read from to 50 per cent, the smaller divi- sions representing 0.5 per cent each. See upper picture opposite page. One bottle is designed to read directly the percent- age of fat when 18 grams of cream are used; the other when 9 grams of cream are used. Procedure in testing 1 . Take cream samples in the manner described for sampling on page 5, although greater care must be used because the viscosity of cream makes a uniform mixture more difficult to secure. 2. Prepare sample by warming to be- tween 90° and 110° F, to make mixing and pipetting easier. 3. Avoid heating the sample too fast or too long or allowing the warm cream to stand too long. These errors will permit melted fat to separate and will make it impossible to mix the sample properly. 4. Mix the warm sample thoroughly by pouring from one container to another 3 or 4 times. 5. Place test bottles on balance and bal- ance by the tare weight. 6. Place weight on one pan (9 or 18 grams according to the test bottle used) and introduce cream to balance the weight exactly by means of a pi- pette into the test bottle on the oppo- site pan. See illustration. If too much cream is added, a small portion may be poured from the test bottle, and the latter returned to the balance. 7. Remove weight from pan and add cream to bottle on that pan to counter- balance weight of cream in opposite pan. Experience will enable you to check the flow of cream from the pipette when the correct charge has been delivered. Be careful to get all the cream into the test bottle and not on the outside or on the pan of the balance. The amount of acid to be added varies with the amount of cream in the test bot- tle and the richness of the cream. Usually from 12 to 14 ml of acid are required for the 18-gram charge. The amount of acid necessary will decrease as the richness of the cream increases, because there are [10] " 7 less solids-not-f at to be dissolved and less moisture to weaken the acid. Since the richness of the sample cannot be foretold, you must learn to add sufficient acid to give a coffee-brown color after mixing the acid and cream. The color of the cream mixture is considerably paler than the dark chocolate color in milk. A darker color indicates that too much acid has been added. When adding the acid, re- volve the bottle to wash down the cream adhering to the sides of the neck. The manner of mixing the acid and cream and of centrifuging the cream tests is exactly the same as described for milk on pages 7 and 8. Reading of cream tests 1 . After the tests have been in the water bath at 130° to 140° F for at least 10 minutes, and just before the test is read, allow a few drops of glymol (1 quart of a white mineral oil colored red with 1 ounce of alkanet root or with oil-soluble aniline dye) to run down the side of the neck of the test bottle. This glymol straightens out the upper meniscus or crescent-shaped curve of the fat column, making pos- sible a more accurate reading. 2. As shown in the photograph on this page, place one point of the dividers at the extreme bottom of the fat col- umn, and then adjust the other point to the straight line at the bottom of the glymol. 3. Without changing the spread of the dividers, place one point on the zero mark on the neck of the bottle. 4. Read percentage of fat where the other point meets the scale. 5. Shake test bottles while emptying into the waste-acid jar as directed on page 8, and then clean thoroughly as de- scribed on page 3. Imperfect tests and their causes are discussed on page 9. Top: official milk and cream test bot- tles; middle: balances for weighing the cream; bottom: reading the fat percent- age in cream test. MILK FAT in Skim Milk . . . Use Babcock Test here, also. The Babcock method of testing skim milk requires a special double-neck test bottle (see picture) having one large neck for introducing the charge and one neck with a fine bore and graduations suffi- ciently small to record 0.01 per cent fat. The directions outlined for testing milk should be followed with a few exceptions. Use slightly more acid and add it in 2 portions, mixing thoroughly after each. When skim-milk test bottles are being placed in the centrifuge, the larger neck should be toward the center. The tests are centrifuged for 10, 3, and 2 minutes, water being added between each period as in testing milk. The glassware should be carefully cleaned as described on page 3. A Babcock test skim milk bottle. «*0® ACIDITY in Milk and Skim Milk . . . Mann's acidity test, used frequently to determine the acidity of milk and its prod- ucts, is based upon the use of a dilute alkali to neutralize the acidity and of an indicator to show when the necessary amount of alkali has been added. Alkali solution This solution must be of the proper strength, designated as 1/10 normal. It may be prepared by accurately diluting the concentrated solution sold by dairy supply companies, following the direc- tions furnished by the manufacturer and taking care to rinse all the concentrated solution thoroughly from the bottle. If the necessary equipment for precise meas- urement of materials is available, the solu- tion may also be prepared by dissolving is determined by Mann's acidity test. 4 grams of pure sodium hydroxide in dis- tilled water and adding sufficient distilled water to make 1,000 ml of solution. The burette The amount of alkali used in the acidity test is conveniently measured from a bur- ette graduated so that each small division represents 0.1 ml. The burette shown on the opposite page is attached to the bottle of alkali for convenience in filling. Its tip below the pinchcock must first be filled with alkali free from bubbles. Procedure 1 . Prepare sample in the manner de- scribed on page 5, using 17.6-ml pipette to transfer 18 grams of milk into a white porcelain cup. [12] 2. Add to the cup 2 to 4 drops of the indicator solution (SYs per cent solu- tion of phenolphthalein in 90 per cent ethyl alcohol). 3. Make sure that no bubbles of air are trapped in the tip of the burette con- taining the alkali. 4. Add the alkali solution drop by drop, noting the reading at start. 5. Stir sample in the white cup with a glass rod to mix each drop of alkali thoroughly as it is added. 6. Continue process until a faint, but definite, pink color remains after mix- ing for a /2 minute without further additions of alkali. Extreme care is necessary in detecting this point. 7. Take reading from the burette and subtract this figure from original bur- ette reading taken before starting the test, to obtain the number of milliliters of alkali solution added to the cup. Divide the resultant figure (the num- ber of milliliters of alkali thus added) by 20 to obtain the percentage of acid- ity in the sample. ACIDITY in Cream . . . is also measured by Mann's test, with a few minor changes. Use Mann's acidity test, described in the preceding section, with these changes : 1 . Use a 9-ml pipette to transfer 9 grams of the cream to the white cup. 2. Fill pipette with warm distilled water to rinse out the cream. 3. Add this 9 ml of rinse water to the cream in the white cup. 4. The indicator and alkali are added in the same manner as for milk, and the number of milliliters of alkali used is determined. 5. Calculate the percentage of acidity in the cream by dividing by 10 the number of milliliters of alkali used. Mann's acidity test equipment. T' I I SPECIFIC GRAVITY and SOLIDS . . . in Milk and Skim Milk . . . are determined by lactometer, but variation from average in milk samples makes it necessary for the operator to be extra skillful and to exercise extreme care in conducting test. A special TYPE of hydrometer, known as a lactometer, is used to determine the specific gravity and the percentage of solids-not-fat in milk. The solids-not-fat, when determined by this method, may be in error by as much as 0.3 per cent, unless the operator is skilled and observes ex- treme care in the procedure. The most important reason for this variation in accuracy is that the milk sample may differ widely (both in composition and behavior) from the "average" used in determining the formula for calculating the results. The method is based upon the princi- ples that a freely floating body displaces a quantity of liquid weighing the same as the floating body and that the total- solids content determines the density of a solution. In the case of milk, the fat and the part called solids-not-fat constitute the total solids. The lactometer Of the several types of lactometers on the market, the most acceptable is gradu- ated in "Quevenne degrees" with the small divisions representing 1/10 degree. Many technicians prefer the large lactometer recommended by the Bureau of Dairy Industry, United States Department of Agriculture, but about a pint of milk is required to float it properly. Lactometers are graduated on the stem, reading from the top down. A scale read- ing from 27 to 40 is a convenient range. Since milk having less solids will allow the lactometer to sink deeper, the lower readings on the scale must be at the top of the stem. The lactometer. Procedure for determining specific gravity and solids in milk 1 . Take sample as directed on page 5. 2. Keep cold (40° to 50° F) for 1 hour, or preferably longer, and then place in cool water, which will slowly raise the temperature to between 55° and 65° F, preferably to 60°. An hour or more should be required to reach 60°. 3. Mix milk thoroughly by pouring from one container to another until a ho- mogeneous mixture is obtained. Care should be taken that these containers do not change its temperature. 4. Pour milk into a cylinder of the same temperature. 5. This cylinder should have a diameter at least 1 inch greater than the largest diameter of the lactometer and be deep enough to float the lactometer. 6. Fill cylinder to such a point that it will overflow when the lactometer is placed in the milk. 7. Place lactometer immediately in the milk and push down to wet its stem. Care should be used to have it float freely, not near the sides of the cyl- inder. 8. Allow lactometer to stand at least % minute and not more than 2 minutes before reading. 9. Blow all bubbles from the surface, particularly around the stem. 10. Read lactometer {see picture) to the nearest small division on the scale at the top of the meniscus (crescent- shaped curve) on the stem, keeping the eye at the same level as the menis- cus. If the temperature of the milk is not exactly 60° F, the lactometer read- ing must be "corrected" for temper- ature. This correction is made by adding 0.1 for each degree above 60° F, or by subtracting 0.1 for each de- gree below 60° F, provided the grad- uation is in Quevenne degrees. 1 1 . Calculate the specific gravity and the percentage solids-not-fat by sub- stituting in and solving the following formulas. Where L is the "corrected" lactometer reading (that is, the lac- tometer reading corrected for tem- perature) and F is the fat test obtained by the Babcock method and expressed as per cent (that is, if the milk tests 3.8 per cent) , then F equals 3.8. i^ooo + l = s P ecific g ravit y L F — + — = per cent solids-not-fat 4 5 r For example, if the corrected lactom- 32 eter reading is 32, then 1,000 + 1 = 09 o o 1.032. Then if F=3.8, then ~ + -£-= 4 5 8.74 per cent of solids-not-fat. SEDIMENT in Milk . . . Follow the directions which come with commercial sediment testers, but be sure to observe these precautions. The sediment test is a simple method of determining the amount of extraneous matter in milk. It is based upon the capac- ity of a cotton pad to remove very small particles so that they may be seen. The several sediment testers available on the market give satisfactory results when properly used. Instructions accom- pany each tester when purchased and therefore will not be repeated here. However, you should observe certain precautions not usually mentioned by the manufacturers. The sample should be taken as directed on page 5. (Some Dairy Inspection Serv- ices prefer to take sediment samples from the bottom of milk cans.) Since exactly 1 pint of milk is used in the test, a quart should be taken for a sample. The sample Making a sediment test. is then thoroughly mixed by pouring from one container to another 3 or 4 times until a homogeneous mixture is obtained. If necessary, it should be warmed to 100° F in order to soften lumps of cream. When the cotton disk is fitted into the tester, exactly 1 pint of the prepared sam- ple is forced through the disk. Be careful to get all the milk to pass through the disk and not around it. Especial care should be used to have the last teaspoonful of milk pass through, since this milk will otherwise rinse away much of the sedi- ment when you try to remove the disk from the tester. The sample of milk is given a numeri- cal rating for sediment by comparing the appearance of the cotton disk with a standard series of prepared disks, shown in illustration. SEDIMENT in Cream . . . A sediment test of cream is made in the same manner as the sediment test in milk. Only 4 ounces of the cream are used, instead of 1 pint as in milk. Also, the cream is diluted with about % pint of boiling water— clean water, free from even small visible particles of dirt. If the cream is sour, the boiling water should contain a small amount of alkali, such as baking soda. An alkali solution for sour cream is made by dissolving 2 tablespoonfuls of baking soda in 3 quarts of water and passing it through a cotton disk to remove any sediment before heat- ing it for use in diluting the cream. Standard series of prepared sediment-rating disks. 7Im-12,'49(B5143)