TESTING MILK FOR TOTAL SOLIDS

The Importance of the Total Solids Test. Almost every state in the union, as well as the federal government, has standards defining law¬ful milk. These standards have been adopted to protect the public against milk of low butterfat and solids content brought about by watering or skimming the milk, and against milk from very low-test¬ing cows. When a producer's milk tests abnormally low in butterfat a milk company also will wish to run a test for adulteration. Bed the invention of the Babcock test and in the absence of any simpk test for determining the total solids content of milk, adulteration was very commonly practiced; today it is not common.

The milk standard is usually stated in terms of per cent butterfat. per cent S.N.F. and per cent T.S. The Federal requirements are 3.25 per cent butterfat, 8.5 per cent S.N.F., and 11.75 per cent T.S. The federal standard applies only to milk sold in the District of Columbia and milk shipped from one state to another. For example, 3 per cent milk is legal in New York State, but if a New York farmer ships milk into Massachusetts it must not only come up to the federal standard but also to the Massachusetts standard, which in this case happens to be higher than the federal standard.

The enforcement of these state standards usually rests with the state board of health, the state board of agriculture, or a dairy and food commission. City milk inspectors also enforce standards for the cities. In general, inspectors are hired to travel about the state, picking up samples from milk trucks, trains, stores, and so on. These samples are taken to laboratories and tested, and if they are
found to be below the standard, a warning is issued. If a sample taken after the warning shows no improvement, the offender is taken into court.

Methods of Determining the Total Solids in Milk. The only abso¬lutely reliable method, and the one that is likely to stand in court, is the gravimetric test. In brief, this test involves weighing out a small sample of the milk into a small dish on a chemical balance and then evaporating the moisture in an oven or over a hot-water bath, until trial weights show that the residue has reached constant weight. The percentage of total solids is then calculated. Incidentally, this is the only method that can be used for determining the total solids in milk products, such as cream, milk powder, ice cream, etc.

For making approximate tests on milk, there is an instrument on tte market known as the lactometer. From the lactometer reading and the percentage of butterfat, the percentage of S.N.F. or T.S. can be calculated with the aid of a formula.

Principle Upon Which fhe Lactometer Operates. The use of the lac¬tometer rests on the principle that a body floating in a liquid dis¬places an amount of the liquid equal to the weight of the floating body. In other words, if a glass cylinder is filled to the brim with milk and the lactometer is floated in it, the weight of the milk that runs over is equal to the weight of the lactometer.

Description of the Instrument. Two types of lactometers will be described. Until 1956, the Quevenne lactometer was the one in com¬mon use. At the bottom of the Quevenne is a large bulb filled with shot or mercury to keep the lactometer in an upright position when floating. Just above this is the bulb of a thermometer, the paper scale for which is located at the top of the instrument. Above this liulb is a large air chamber for floating the lactometer, and just above tin's air chamber is a paper lactometer scale, reading in lactometer degrees from 15 at the top down to 40.

In April 1956, Paul D. Watson of the Eastern Utilization Research Branch of the U.S. Department of Agriculture published his findings with a modified Quevenne lactometer, designed to give more accur¬ate results. This instrument works on the same principle and reads to 0.2 of a degree.

Common Factors Affecting the Weight of a Given Volume of Milk. TEMPERATURE. Heat causes milk to expand, and thus makes a given volume of it weigh less. Cold, on the other hand, causes the milk to contract and makes a given volume weigh more. This difference is plainly shown on the farm when milk is cooled. A can of warm night’s milk, put into the cooling tank at night, is no longer full in the morning and has to be “topped out” before shipping.

THE PERCENTAGE OF TOTAL SOLIDS NORMALLY PRESENT IN MILK. With the increase of the percentage of butterfat in milk the solids-not-fat also increase, so that a quart of 5 per cent milk, for example, will weigh slightly more than a quart of 3 per cent milk.

ADDITION OF WATER. Water is lighter than milk, and therefore a given volume of milk weighs less if it contains some extraneous water.
THE ADDITION OF SKIM MILK OH THE REMOVAL OF CREAM. Skim milk is heavier than milk; hence, the addition of skim milk to milk, or the removal of some of the cream, increases the weight of a given volume of milk.


Operation of the Test when Using the Quevenne Lactometer. The sample must be representative and must be thoroughly mixed as for the Babcock test. The temperature of the milk must be between 50° and 70°F. A glass cylinder is filled with the milk and set in some receptacle, such as a sink or a pan. The lactometer is then slowly lowered into it.

In a few seconds the lactometer becomes stationary, and the lac¬tometer scale is read at the point to which the milk rises on the lactometer scale (the meniscus formed by the milk on the lactometer scale is included). This reading is recorded, and the temperature noted and recorded. Because temperature affects the weight of a given volume of milk and hence the lactometer reading, obviously the readings must be taken at some definite temperature, or must be corrected to that temperature. The lactometer gives a correct read¬ing directly only when the milk is at 60°F. If the milk is some other temperature between 50° and 70°F, a correction factor can be used to give a corrected reading. This factor is 0.1 of a lactometer degree added to the lactometer reading for each degree of temper¬ature above 60° F, and subtracted for each degree below 60° F. Thus, if the lactometer reads 30 at 68° F, the correct reading is 30.8; and if it reads 29.5 at 53°F, the correct reading is 28.8. The lower the correction factor used, that is, the nearer the temperature is to 60°F, the more accurate the results will be.

The operation of the modified lactometer is the same except the milk must be at a temperature of 102°F ± 2° F. For each degree above 102° F add 0.2 to the reading, and for each degree below 102° F, subtract 0.2. The lactometer should be warmed to 102° F before use, and the milk temperature should be checked with an accurate thermometer.

Calculation of the Specific Gravity of Milk. The rule is to divide the Quevenne lactometer reading by 1,000, multiply by 100, and add 1 to result. Thus if the lactometer reading at 60°F is 32, 32  1,000 equal 0.0032  100 = 0.032 + 1 equal 1.032 specific gravity.

Calculation of the Per Cent S.N.F. and T.S. in Milk. To find the solids in milk, both lactometer and Babcock tests must be made and this formula must be used with the Quevenne lactometer:
¼ L + (0.2  per cent butterfat) = per cent S.N.F.
L is the corrected lactometer reading in degrees. For example, if the lactometer reading is 32.5 at 55°F, at butterfat test is 3.9 per cent, find the per cent S.N.F. and T.S.


The solids-not-fat may be found by using 0.33 F instead of 1.33 F, or by subtraction of the butterfat percentage from the total solids percentage. The total solids of skim milk samples may be calculated by dropping the constant ( 0.40) from the formula.

F. J. Henry and A. K. Harry: “Milk Production And Processing,” John Wiley & Sons Inc., New York, 1960