MILK.
427. Composition of Milk.—The composition of milk not only varies with the genus and species of the mammal from which it is derived, but also depends in a marked degree on idiosyncrasy.[400]
Milk is a mixture containing water, proteids, fat, carbohydrates, organic and inorganic acids and mineral salts. There have also been observed in milk in minute quantities ammonia, urea, hypoxanthin, chyme, chyle, biliverdin, cholesterin, mucin, lecithin, kreatin, leucin and tyrosin. In the fermentation which milk undergoes in incipient decomposition there is sometimes developed from the proteid matter, as pointed out by Vaughn, a ptomaine, tyrotoxicon, which is a virulent poison.[401] The presence of these last named bodies is of interest chiefly to the physiologist and pathologist and can receive no further attention here.
From a nutritive point of view, the important components of milk are the fats, proteids and sugar, but especially in the nourishment of the young the value of lime and phosphoric acid must be remembered. The mean composition of the most important milks, as determined by recent analyses, is given below:
| Water. Per cent. | Sugar. Per cent. | Proteids. Per cent. | Fat. Per cent. | Ash. Per cent. | |
|---|---|---|---|---|---|
| Cow | 86.90 | 4.80 | 3.60 | 4.00 | 0.70 |
| Human | 88.75 | 6.00 | 1.50 | 3.45 | 0.30 |
| Goat | 85.70 | 4.45 | 4.30 | 4.75 | 0.80 |
| Ass | 89.50 | 6.25 | 2.00 | 1.75 | 0.50 |
| Mare | 90.75 | 5.70 | 2.00 | 1.20 | 0.35 |
| Sheep | 80.80 | 4.90 | 6.55 | 6.85 | 0.90 |
The mean composition of milk, as given by Watts and König, is given in the following tables:
| Watts. | ||||||
|---|---|---|---|---|---|---|
| Water. | Solids. | Proteids. | Fats. | Sugar. | Mineral Salts. | |
| Woman | 87.65 | 12.35 | 3.07 | 3.91 | 5.01 | 0.17 |
| Ass | 90.70 | 9.30 | 1.70 | 1.55 | 5.80 | 0.50 |
| Cow | 86.56 | 13.44 | 4.08 | 4.03 | 4.60 | 0.73 |
| Goat | 86.76 | 13.24 | 4.23 | 4.48 | 3.91 | 0.62 |
| Sheep | 83.31 | 16.69 | 5.73 | 6.05 | 3.96 | 0.68 |
| Mare | 82.84 | 17.16 | 1.64 | 6.87 | 8.65 | |
| König. | |||||
|---|---|---|---|---|---|
| Water. | Fat. | Casein and albumin. | Milk Sugar. | Ash. | |
| Woman | 87.41 | 3.78 | 2.29 | 6.21 | 0.31 |
| Mare | 90.78 | 1.21 | 1.99 | 5.67 | 0.35 |
| Ass | 89.64 | 1.63 | 2.22 | 5.99 | 0.51 |
| Cow | 87.17 | 3.69 | 3.55 | 4.88 | 0.71 |
The average composition of 120,540 samples of cow milk, as determined by analysis, extending over a period of eleven years, was found by Vieth to be as follows:[402]
| Per cent. | |
| Total solids | 12.9 |
| Solids not fat | 8.8 |
| Fat | 4.1 |
The quantity of solids and fat in milk is less after longer than after shorter periods between milkings.
The quantity of solids and fat in cow milk is less in the spring than in the autumn.
The chief organic acid naturally present in milk is citric, which exists probably in combination with lime.
The mean content of citric acid in milk is about one-tenth of one per cent.[403]
Citric acid is not found in human milk, and probably exists only in the mammary secretions of herbivores.
Among the mineral acids of milk, phosphoric is the most important, but a part of the phosphorus found as phosphoric acid in the ash of milk may come from pre-existing organic phosphorus (lecithin, nuclein).
The sulfuric acid, which is found in the ash of milk, is derived from the sulfur of the proteid matter during ignition.
Lactic acid is developed from lactose during the souring of milk as the result of bacterial activity.
Gases are also found in solutions of milk, notably carbon dioxid, which gives to freshly drawn milk its brothy appearance.
The ash of milk has the following composition expressed as grams per liter of the original milk:[404]
| Component. | Grams per liter. | Probable form of combination. | Grams per liter. |
|---|---|---|---|
| Chlorin | 0.90 | sodium chlorid | 0.962 |
| potassium chlorid | 0.830 | ||
| Phosphoric acid | 2.42 | KH₂PO₄ | 1.156 |
| K₂HPO₄ | 0.853 | ||
| MgHPO₄ | 0.336 | ||
| CaHPO₄ | 0.671 | ||
| Ca₃(PO₄)₂ | 0.806 | ||
| Potassium | 1.80 | (as shown above) | |
| and as potassium citrate | 0.495 | ||
| Sodium | 0.49 | sodium chlorid | 0.962 |
| Lime | 1.90 | (as shown above) | |
| and as calcium citrate | 2.133 | ||
| Magnesia | 0.20 | MgHPO₄ | 0.336 |
The percentage composition of the ash of milk, according to Fleischmann and Schrott, is expressed as follows:[405]
| Per cent. | ||
| Potassium oxid, | K₂O | 25.42 |
| Sodium oxid, | Na₂O | 10.94 |
| Calcium oxid, | CaO | 21.45 |
| Magnesium oxid, | MgO | 2.54 |
| Iron oxid, | Fe₂O₃ | 0.11 |
| Sulfuric acid, | SO₃ | 4.11 |
| Phosphoric acid, | P₂O₅ | 24.11 |
| Chlorin, | Cl | 14.60 |
| 103.28 | ||
| Less | Cl as O | 3.28 |
| 100.00 | ||
428. Alterability of Milk.—The natural souring and coagulation of milk is attributed by most authorities to bacterial action produced by infection from the air or containing vessels.[406] Pasteur, however, shows that fresh milk sterilized at a temperature of 110° may be exposed to the air without danger of souring.[407] After about three days, however, a fermentation is set up which is totally different from that produced by the microzymes naturally present in the milk. This point has been further investigated by Béchamp, who finds that the natural souring of milk is accomplished without the evolution of any gas, while the fermentation produced in sterilized milk by the microbes of the air, is uniformly attended by a gaseous development.[408] As a result of his investigations, he concludes that the souring of milk takes place spontaneously by reason of milk being an organic matter, in the physiological sense of the term, and that this alteration is produced solely by the natural microzymes of the milk.
According to Béchamp, the milk derived from healthy animals is capable of spontaneous alteration, which consists in the development of lactic acid and alcohol, and of curd in those milks which contain caseinates produced by the precipitating action of the acids formed. Oxygen and the germs which are present in the air, according to him, have nothing to do with this alteration in the properties of milk. Milk belongs to that class of organic bodies like blood, which are called organic from a physiological point of view, on account of containing automatic forces which produce rapid changes therein when they are withdrawn from the living organisms.
After milk has become sour by the spontaneous action of the microzymes which it contains, there are developed micro-organisms, such as vibriones and bacteria from a natural evolution from the microzymes.
Milk which is sterilized at a high temperature, viz., that of boiling water or above, is no longer milk in the true physiological sense of that term. The globules of the milk undergo changes and the microzymes a modification of their functions, so that in milk thus altered by heat, they are able to produce a coagulation without development of acidity. The microzymes thus modified, however, retain to a large extent their ability to become active. Human milk differs from cow milk in containing neither caseinates nor casein, but special proteid bodies, and also a galactozyme or galactozymase functionally very different from that which exists in cow milk. The extractive matter is also a special kind, consisting of milk globules and microzymes belonging particularly to it and containing three times less phosphate and mineral salts than cow milk. Boiling the milk of the cow or other animals does not render it similar to that of woman. There is no treatment, therefore, of any milk which renders it entirely suited to the nourishment of infants. The composition of the milk of the cow may be represented by three groups:
1. Organic elements in suspension; consisting chiefly of the globules of the milk, which are mostly composed of the fat, of an epidermoid membrane containing mineral matter of special soluble albumins and of microzymes containing also mineral matter.
2. Dissolved constituents; consisting of caseinates, lactalbuminates, galactozymase, holding phosphates in combination, lactose, extractive matter, organic phosphates of lime, acetates, urea and alcohol.
3. Mineral matters in solution; consisting of sodium and calcium chlorids, carbon dioxid and oxygen.[409]
It will be noticed from the above classification that Béchamp fails to mention citrate of lime. It is scarcely necessary to add to this brief résumé of the theories of Béchamp that they are entirely at variance with the opinions held by nearly all his contemporaries.
429. Effects of Boiling on Milk.—On boiling, the albumin in milk is coagulated and on separating the proteid bodies by saturation with magnesium sulfate no albumin is found in the filtrate. The total casein precipitated from boiled is therefore greater than from unboiled milk. Jager has shown that the casein can be precipitated from boiled milk by rennet, but with greater difficulty than from unboiled.[410] According to this author in 3.75 per cent of proteid in milk there are found 3.15 per cent of casein, 0.35 of albumin and 0.25 of globulin.
430. Appearance of the Milk.—The color, taste, odor and other sensible characters of the milk are to be observed and noted at the time the sample is secured. Any variation from the faint yellow color of the milk is due to some abnormal state. A reddish tint indicates the admixture of blood, while a blue color is characteristic of the presence of unusual micro-organisms. Odor and taste will reveal often the character of the food which the animals have eaten. Any marked departure of the sample from the properties of normal milk should at once lead to its condemnation for culinary or dietetic purposes.
431. Micro-Organisms of the Milk.—Milk is a natural culture solution for the growth of micro-organisms, and they multiply therein with almost incredible rapidity. Some of these are useful, as, for instance, those which are active in the ripening of cream, and others are of an injurious nature, producing fermentations which destroy the sugars or proteids of the milk and develop acid, alcohol, mucous or ptomaine products. It is not possible here to even enumerate the kinds of micro-organisms which abound in milk and the reader is referred to the standard works on that subject.[411]
For analytical purposes it is important that the sample be kept as free as possible of all micro-organisms, good or bad, which may be accomplished by some of the methods given below.
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.
Fig. 106.—Scovell’s Milk Sampling Tube.
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.[412]
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.[413]
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.[414] A simple apparatus, especially adapted to milk, is described by Beckmann.[415] 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.[416] 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.[417] 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.[418] 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.[419] Since the invention of so many quick and accurate methods of fat estimation these instruments have little more than a historical interest.
Fig. 107.—Lactoscope, Lactometer and Creamometer.
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.[420]
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.
Langlet has invented a lactoscope with a scale, showing the corrections to be applied for temperatures other than 15°. A detailed description of this instrument, as well as the one proposed by Pinchon, is unnecessary.[421]
445. Density of Sour Milk.—Coagulated milk cannot be used directly for the determination of the specific gravity, both because of its consistence and by reason of the fact that the fat is more or less completely separated. In such a case, the casein may be dissolved by the addition of a measured quantity of a solvent of a known specific gravity, the density of the resulting solution determined and that of the original milk calculated from the observed data. Ammonia is a suitable solvent for this purpose.[422]
446. Density of the Milk Serum.—The specific gravity of the milk serum, after the removal of the fat and casein by precipitation and filtration, may also be determined. For normal cow milk the number is about 1.027.
447. Total Solids.—The direct gravimetric determination of the total solids in milk is attended with many difficulties, and has been the theme of a very extended periodical literature. A mere examination of the many processes which have been proposed would require several pages.
The most direct method of procedure is to dry a small quantity of milk in a flat-bottom dish to constant weight on a steam-bath. The surface of the dish should be very large, even for one or two grams of milk; in fact the relation between the quantity of milk and the surface of the dish should be such that the fluid is just sufficient in amount to moisten the bottom of the dish with the thinnest possible film. The dish, during drying, is kept in a horizontal position at least until its contents will not flow. The water of the sample will be practically all evaporated in about two hours. The operation may be accelerated by drying in vacuo.
The drying may also be accomplished by using a flat-bottom dish containing some absorbent, such as sand, pumice stone, asbestos or crysolite. The milk may also be absorbed by a dried paper coil and dried thereon ([26]).
It is convenient to determine the water in the sample subsequently to be used for the gravimetric determination of the fat, and this is secured by the adoption of the paper coil method, as suggested by the author, or by the use of a perforated metal tube containing porous asbestos, as proposed by Babcock.[423]
The process is conveniently carried out as follows:
Provide a hollow cylinder of perforated sheet metal sixty millimeters long and twenty millimeters in diameter, closed five millimeters from one end by a disk of the same material. The perforations should be about 0.7 millimeter in diameter and as close together as possible. Fill loosely with from one and a half to two and a half grams of dry woolly asbestos and weigh. Introduce a weighed quantity of milk (about five grams). Dry at 100° for four hours. During the first part of the drying the door of the oven should be left partly open to allow escape of moisture. Cool in a desiccator and weigh. Repeat the drying until the weight remains constant. Place in an extractor and treat with anhydrous ether for two hours. Evaporate the ether and dry the fat at 100°. The extracted fat is weighed and the number thus obtained may be checked by drying and weighing the cylinder containing the residue.
The asbestos best suited for use in this process should be of a woolly nature, quite absorbent, and, previous to use, be ignited to free it of moisture and organic matter. A variety of serpentine, crysolite is sometimes used instead of asbestos. When the content of water alone is desired, it is accurately determined by drying in vacuo over pumice stone ([page 33]).
The methods above mentioned are typical and will prove a sufficient guide for conducting the desiccation, either as described or by any modification of the methods which may be preferred.
448. Calculation of Total Solids.—By reason of the ease and celerity with which the density of a milk and its content of fat can be obtained, analysts have found it convenient to calculate the percentage of total solids instead of determining it directly. This is accomplished by arbitrary formulas based on the data of numerous analyses. These formulas give satisfactory results when the samples do not vary widely from the normal and may be used with advantage in most cases.
Among the earliest formulas for the calculation may be mentioned those of Fleischmann and Morgen,[424] Behrend and Morgen,[425] Claus, Stutzer and Meyer,[426] Hehner,[427] and Hehner and Richmond.[428] Without doing more than citing these papers it will be sufficient here to give the formulas as corrected by the most recent experience.
In the formula worked out by Babcock the specific gravity of the sample is represented by S, the fat by F, and the solids not fat by t. The formula is written as follows:[429]
| t = ( | 100S - FS | ) (250 - 2.5 F). |
| 100 - 1.0753FS |
In this formula it is assumed that the difference between the specific gravity of the milk serum and that of water is directly proportional to the per cent of solids in the serum, but this assumption is not strictly correct. Even in extreme cases, however, the error does not amount to more than 0.05 per cent.
Since a given amount of milk sugar increases the density of a milk more than the same quantity of casein, it is evident that the formula would not apply to those instances in which the ratio between these two ingredients is greatly disturbed, as for instance, the whey.
The formula of Hehner and Richmond, in its latest form, is expressed as follows:
| T = 0.2625 | G | + 1.2F, |
| D |
in which T represents the total solids, G the reading of the quévenne lactometer, D the specific gravity, and F the fat.
Example.—Let the reading of the lactometer be 31, corresponding to D 1.031, and the percentage of fat be three and five-tenths, what is the percentage of the total solids?
Substituting these values in the formulas we have
| T = 0.2625 | 31 | + 1.2 × 3.5 = 12.09. |
| 1.031 |
To simplify the calculations, Richmond’s formula may be written
| T = | G | + | 6F | + 0.14. |
| 4 | 5 |
Calculated by this shortened formula from the above data T = 12.09, the same as given in the larger formula.
Calculating the solids not fat in the hypothetical case given above by Babcock’s formula, we get t = 8.46, and this plus 3.5 gives 11.96, which is slightly lower than the number obtained by the richmond process.
The babcock formula may be simplified by substituting the number expressing the reading of the quévenne lactometer for that donating the specific gravity, in other words, the specific gravity multiplied by 100 and the quotient diminished by 1000.
The formulas for solids not fat and total solids then become
| t = | L | + 0.2F, and T = | L | + 1.2F, |
| 4 | 4 |
in which L represents the reading of the lactometer. By the addition of the constant factor 0.14 the results calculated by the formula of Babcock are the same as those obtained by the method of Richmond.
In the following table are given the solids not fat in milks as calculated by Babcock’s formula. To obtain the total solids add the per cent of fat to solids not fat. To obtain total solids according to Richmond’s formula increase that number by 0.14.
Table Showing Per Cent of Solids not Fat in Milk
Corresponding to Quévenne’s Lactometer Readings
and Per Cent of Fat.
449. Determination of Ash.—In the determination of the solid residue obtained by drying milk, it is important to observe the directions already given ([28-32]).
In the direct ignition of the sample, a portion of the sulfur and phosphorus may escape oxidation and be lost as volatile compounds. This loss may be avoided by the use of proper oxidizing agents or by conducting the combustion as heretofore described.[430] In the official method, it is directed to add six cubic centimeters of nitric acid to twenty of milk, evaporate to dryness and ignite the residue at a low red heat until free of carbon.[431] It is doubtful if this precaution be entirely sufficient to save all the sulfur and phosphorus, but the method is evidently more reliable than the common one of direct ignition without any oxidizing reagent whatever.