Part 88

=432. Sampling Milk.=—It is not difficult to secure for examination representative samples of milk, if the proper precautions be taken. On the other hand, the ease and rapidity with which a milk undergoes profound changes render necessary a careful control of the methods of taking samples. The most rapid changes to which a mass of milk is obnoxious are due to the separation of the fat particles and to the action of bacteria. Even after standing for a few minutes, it will be found that the fat globules are not evenly distributed. Before securing the sample for analysis, it is necessary to well stir or mix the milk. A mean sample may also be secured from a can of milk by the sampling tube devised by Scovell, which will be described below.

In securing samples, a full detailed description of the cow or herd furnishing them is desirable, together with all other data which seem to illustrate in any way the general and particular conditions of the dairy. Samples are to be preserved in clean, well stoppered vessels, properly numbered and securely sealed.

=433. Scovell’s Milk Sampler.=—In sampling large quantities of milk in pails or shipping cans, it is exceedingly inconvenient to mix the milk by pouring from one vessel to another or by any easy process of stirring. In order to get representative samples in such conditions, Scovell has put in use a sampler, by means of which a typical portion of the milk may be withdrawn from a can without either pouring or stirring. The construction of the sampler is shown in Fig. 106, representing it in outline and longitudinal section. The tube a, made of brass, is open at both ends and of any convenient dimensions. Its lower end slides in a large tube b, closed at the bottom and having three elliptical, lateral openings c, which admit the milk as the tube is slowly depressed in the contents of the can. In getting the sample, a is raised as shown in profile. When the bottom of b reaches the bottom of the can a is pushed down as shown in the section. The milk contained in the sampler is then readily withdrawn.

=434. Preserving Milk for Analysis.=—Pasteurizing or boiling the sample is not advisable by reason of the changes produced in the milk by heat. The milk sample may be preserved by adding to it a little chloroform, one part in 100 being sufficient. Boric and salicylic acids may also be used, but not so advantageously as formaldehyd or mercuric chlorid. Rideal has observed that one part of formaldehyd will preserve 10,000 parts of milk in a fresh state for seven days. The formaldehyd sold in the trade contains about one part of formaldehyd in 320 of the mixture. One-half pint of this commercial article is sufficient for about twenty gallons of milk, corresponding to about one part of pure formaldehyd to 45,000 parts of milk. Rideal much prefers formalin (formaldehyd) to borax or boric acid as a milk preservative. No ill effects due to its toxic action have been observed, even when it is consumed in a one per cent solution.

Samples of milk can be kept in this way from four to six weeks by adding about one drop of the commercial formaldehyd to each ounce of sample. The analyst should remember in such cases that the formaldehyd may not all escape on evaporation, on account of forming some kind of a compound with the constituents of the milk, as is pointed out by Bevan.

Bevan suggests that the formaldehyd may not actually be retained in the sample, but that the increase in the apparent amount of total solids is due to the conversion of the lactose into galactose. This point, however, has not been determined.

Richmond and Boseley propose to detect formalin by means of diphenylamin. A solution of diphenylamin is made with water, with the help of just enough sulfuric acid to secure a proper solvent effect. The liquid to be tested, which is supposed to contain formaldehyd, or the distillate therefrom, is added to this solution and boiled. If formaldehyd be present, a white flocculent precipitate is deposited, which is colored green if the acid used contain nitrates. For other methods of detecting formalin and for a partial literature of the subject the paper mentioned above may be consulted.

One gram of fine-ground mercuric chlorid dissolved in 2,000 grams of milk will preserve it, practically unchanged, for several days. One gram of potassium bichromate dissolved in one liter of milk will also preserve it for some time. Thymol, hydrochloric acid, carbon disulfid, ether and other antiseptics may also be employed. No more of the preserving agent should be used than is required to keep the milk until the analysis is completed.

All methods of preservation are rendered more efficient by the maintenance of a low temperature, whereby the vitality of the bacteria is greatly reduced.

=435. Freezing Point of Milk.=—By reason of its content of sugar and other dissolved solids, the freezing point of milk is depressed below 0°. A good idea of the purity of whole milk is secured by subjecting it to a kryoscopic test. The apparatus employed for this purpose is that used in general analytical work in the determination of freezing points. Pure full milk freezes at about 0°.55 below zero, and any marked variation from this number shows adulteration or abnormal composition. A simple apparatus, especially adapted to milk, is described by Beckmann. The kryoscopic investigation may also be extended to butter fat dissolved in benzol.

=436. Electric Conductivity of Milk.=—The electric conductivity of milk may also be used as an index of its composition. The addition of water to milk diminishes its conductivity. This method of investigation has at present but little practical value.

=437. Viscosity Of Milk.=—The viscosity of milk may be determined by the methods already described. Any variation from the usual degree of fluidity is indicated either by the abstraction of some of the contents of the milk, the addition of some adulterant or the result of fermentation.

=438. Acidity and Alkalinity of Milk.=—Fresh milk of normal constitution has an amphoteric reaction. It will redden blue and blue red litmus paper. This arises from the presence in the milk of both neutral and acid phosphates of the alkalies. A saturated alkaline phosphate, i. e., one in which all the acid hydrogen of the acid has been replaced by the base has an alkaline reaction while the acid phosphates react acid. When fresh milk is boiled its reaction becomes strongly alkaline and this arises chiefly from the escape of the dissolved carbon dioxid. By the action of micro-organisms on the lactose of milk, the alkaline reaction soon becomes acid, and delicate test paper will show this decomposition long before it becomes perceptible to the taste. It is advisable to test the reactions of the milk as soon as possible after it is drawn from the udder, both before and after boiling.

=439. Determination of the Acidity of Milk.=—In the determination of the acidity of milk it is important that it first be freed of the carbon dioxid it contains. Van Slyke has found that too high results are obtained by the direct titration of milk for acidity, and when the milk is previously diluted the results are also somewhat too high. Good results are got by diluting the milk with hot water and boiling for a short time to expel the carbon dioxid. Twenty-five cubic centimeters of milk are diluted with water to about a quarter of a liter, as above, two cubic centimeters of a one per cent alcoholic phenolphthalien added and the titration accomplished by decinormal alkali. This variation of the methods of procedure, suggested by Hopkins and Powers, appears to be the best process at present known for the determination of acidity. The reader is referred to the paper cited above for references to other methods which have been proposed.

=440. Opacity Of Milk.=—The white color and opacity of milk are doubtless due to the presence of the suspended fat particles and to the colloid casein. On the latter it is probably principally dependent since the color of milk is not very sensibly changed after it has passed the extractor, which leaves not to exceed one-tenth of one per cent of fat in it. Some idea of the quality of the milk, however, may be obtained by determining its opacity. This is accomplished by the use of a lactoscope. The one generally employed was devised by Feser and is shown in Fig. 107.

The instrument consists of a cylindrical glass vessel of a little more than 100 cubic centimeters content, in the lower part of which is set a cone of white glass marked with black lines. Into this part are placed four cubic centimeters of milk. A small quantity of water is added and the contents of the vessel shaken. This operation is repeated until the black lines on the white glass just become visible. The graduations on the left side show the volume of water which is necessary to bring the dark lines into view, while those on the right indicate approximately the percentage of fat present.

Among the other lactoscopes which have been used may be mentioned those of Donné, Vogel, Hoppe-Seyler, Trommer, Seidlitz, Reischauer, Mittelstrass, Hénocque, and Heusner. Since the invention of so many quick and accurate methods of fat estimation these instruments have little more than a historical interest.

=441. Creamometry.=—The volume of cream which a sample of milk affords under arbitrary conditions of time and temperature is sometimes of value in judging the quality of milk. A convenient creamometer is a small cylinder graduated in such a way that the volume of cream separated in a given time can be easily noted. There are many kinds of apparatus used for this purpose, a typical one being shown in Fig. 107.

The usual time of setting is twenty-four hours. A quicker determination is secured by placing the milk in strong glass graduated tubes and subjecting these to centrifugal action. The process is not exact and is now rarely practiced as an analytical method, even for valuing the butter making properties of milk.

=442. Specific Gravity.=—The specific gravity of milk is uniformly referred to a temperature of 15°. Generally no attempt is made to free the milk of dissolved gases beforehand. This should not be done by boiling but by placing the sample in a vacuum for some time. Any of the methods described for determining specific gravity in sugar solutions may be used for milk (=48-59=). The specific gravity of milk varies in general from 1.028 to 1.034. Nearly all good cow milk from herds will show a specific gravity varying from 1.030 to 1.032. In extreme cases from single cows the limits may exceed those first given above, but such milk cannot be regarded as normal.

Increasing quantities of solids not fat in solution, tend to increase the specific gravity, while an excess of fat tends to diminish it. There is a general ratio existing between the solids not fat and the fat in cow milk, which may be expressed as 9: 4. The removal of cream and the addition of water in such a manner as not to affect the specific gravity of the sample disturbs this ratio.

The determination of the specific gravity alone, therefore, cannot be relied upon as an index of the purity of a milk.

=443.= =Lactometry.=—A hydrometer especially constructed for use in determining the density of milk is called a lactometer. In this country the one most commonly used is known as the lactometer of the New York Board of Health. It is a hydrometer, delicately constructed, with a large cylindrical air space and a small stem carrying the thermometric and lactometric scales. It is shown held in the creamometer in Fig. 107. The milk is brought to a temperature of 60° F. and the reading of the lactometer scale observed. This is converted into a number expressing the specific gravity by means of a table of corresponding values given below. Each mark on the scale of the instrument corresponds to two degrees and these marks extend from 0° to 120°. The numbers of this scale can be converted into those corresponding to the direct reading instrument, described in the next paragraph, by multiplying them by 0.29.

The minimum density for whole milk at 60° F. is fixed by this instrument at 100°, corresponding to a specific gravity of 1.029. The instrument is also constructed without the thermometric scale. The mean density of many thousand samples of pure milk, as observed by the New York authorities, is 1.0319.

The specific gravity is easily secured, and while not of itself decisive, should always be determined. The specific gravity of milk increases for some time after it is drawn and should be made both when fresh and after the lapse of several hours.

TABLE SHOWING SPECIFIC GRAVITIES CORRESPONDING TO DEGREES OF THE NEW YORK BOARD OF HEALTH LACTOMETER. TEMPERATURE 60° F.

Degree. Sp. gr. Degree. Sp. gr. 90 1.02619 106 1.03074 91 1.02639 107 1.03103 92 1.02668 108 1.03132 93 1.02697 109 1.03161 94 1.02726 110 1.03190 95 1.02755 111 1.03219 96 1.02784 112 1.03248 97 1.02813 113 1.03277 98 1.02842 114 1.03306 99 1.02871 115 1.03335 100 1.02900 116 1.03364 101 1.02929 117 1.03393 102 1.02958 118 1.03422 103 1.02987 119 1.03451 104 1.03016 120 1.03480 105 1.03045

=444.= =Direct Reading Lactometer.=—A more convenient form of lactometer is one which gives the specific gravity directly on the scale. The figures given represent those found in the second and third decimal places of the number expressing the specific gravity. Thus 31 on the scale indicates a specific gravity of 1.031. This instrument is also known as the lactometer of Quévenne. For use with milk, the scale of the instrument does not need to embrace a wider limit than from 25 to 35, and such an instrument is capable of giving more delicate readings than when the scale extends from 14 to 42, as is usually the case with the quévenne instrument.