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forces which produce rapid changes therein when they are withdrawn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.
[Illustration: 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.
[Illustration: 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]
100_S_ - _FS_
_t_ = ( ----------------- - 1)(250 - 2.5 _F_).
100 - 1.0753_FS_
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:
_G_
_T_ = 0.2625 ---- + 1.2_F_,
_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?