Henry R. (Henry Richard) Kenwood.

Public health laboratory work online

. (page 21 of 36)
Online LibraryHenry R. (Henry Richard) KenwoodPublic health laboratory work → online text (page 21 of 36)
Font size
QR-code for this ebook

would at times lead to the condemnation of milk derived from
healthy sources. These cellular elements have been usually
regarded as leucocytes or pus cells, and hence it is that a standard
has been suggested for the hygienic control of milk. There are
good grounds, however, for believing that most of the cells found
are young epithelial cells and not leucocytes. Savage has classi-
fied these cellular elements into polymorphonuclear cells, lym-
phocytes, large leucocytes, and doubtful cells.

Owing to the difficulty of recognizing pus cells from other
cells that may be present, it is not easy to say how frequently pus
resulting from inflammatory processes in the udder gets into milk ;
but in a large percentage of cases cells indistinguishable from pus
ceUs are present. Lymphocytes, which stain deeply and possess
nuclei which occupy nearly the whole of the cells, are not normal
constituents of milk except within three weeks of parturition;
but, like polymorphonuclear leucocytes, they are occasionally
present for a few days in the milk of an apparently healthy cow.
These bodies are found associated with staphylococci and strepto-
cocci generally in diseases of the udder, and they are to be found
associated with tubercle bacilli in tubercular mastitis. A high
cell count accompanied by streptococci apparently indicates
some udder trouble.




Cow's milk should be the normal, clean, and fresh secretion
obtained by completely milking the udder of the healthy cow,
properly fed and kept.

The Addition of Water. — Whether milk is naturally poor or has
been made so by the addition of water, the dairyman who sells
it defrauds the purchaser, for the latter demands and pays for
pure milk of average quality.

It is clear that the percentage amount of both fatty and non-
fatty solids will be reduced by any addition of water; but the
estimation of the amount of added water is always made from
the non-fatty solids, because these depart less from the average
than is the case with the fatty solids. The legal low limit for
non-fatty solids is one of 8-5 per cent.

Supposing, then, a sample yields 8 per cent, of non-fatty sohds.
Then if 8-5 per cent, of non-fatty solids denotes 100 per cent, of
pure milk, 8 per cent, denotes only about 94 per cent, of pure

Therefore there is about 94 per cent, of pitre milk in the
sample, and (100-94=) 6 per cent, of water has been

It is sometimes maintained by the milk-vendor that no water
has been added and that the milk is naturally poor milk. The
freezing-point test is said to be of value in settling this question,
for milk as drawn from the cow freezes at - o-550°C. whatever
its solids-non-fat content ; but the addition of water alters the
freezing-point, and a freezing-point above - o-530°C. is said to be
conclusive evidence of watering.

The ash should in every case be low when the solids-non-fat
arc low, or some mineral adulterant has been added.



Cream Abstraction.- — Though the milk from the same cow may
vary at times, the mixed product of many animals (" dairy
samples ") varies but little.

The legal low limit of 3 per cent, of fat is one which is reached
by all genuine dairy samples obtained from a fairly good herd
of cows, kept and fed under average conditions; though, as Bell
and others have shown, the milk of individual cows may some-
times fall below this limit.

The percentage reduction of fat (by the removal of cream) is an
easy calculation after an estimation of the fat has been made.
Suppose that the fat has been found to amount to just 2*5 per
cent. Then 3 per cent. -2-5 per cent. = 0-5 per cent, of fat has

2' ^ X TOO

been abstracted from the milk; or =83*3 per cent, of the

original fat remains and (100 -83-3= ) 167 per cent, of the total
fat originally in the milk has been removed. In cases where the
fat is low and the solids-non-fat are high there can be little
doubt that fat has been abstracted, and that the low fat is
not due to the dilution of. the milk with water.

The " toning down " of good milk by the addition of separated
milk is much practised, and large numbers of the samples analyzed
are found to barely reach the low legal limit of 3 per cent,
of fat.

As milk stands, a certain proportion of the fat quickly rises to
the upper layers, and a defence is sometimes set up by the
dairyman that a poor sample was due to the fact that such top
milk had all been sold, and the sample was some of the last of the
milk in the can. This defence is, in many cases, a wcU-recognized
subterfuge, for it is the duty of the vendor to mix the milk and
to supply fair samples to one and all alike. Failure to draw off
the " strippings " no doubt often accounts for the low figure of
fat in milk.

Samples collected on Sunday mornings are generally amongst
the poorest, for dishonest tradesmen adiilterate on these days
in order to meet the extra demand, due to the fact that more
people take their meals at home on that day.

In addition to water, there are other adulterants added to
milk. Chalk and starch were formerly used, but they are very
rarely, if ever, employed at the present day. Sodium carbonate
is rarely used to preserve the milk and to neutralize it when
sour. It may be tested for (E. Schmidt) by adding 10 c.c. of


alcohol to 10 c.c. of milk, followed b}' a few drops of i per cent,
solution of rosolic acid. Pure milk yields a brownish-yellow
colour, but if sodium carbonate or borax is present a more or less
marked rose-red colour appears.

Boric and salicylic acids, borax, " formalin," benzoates, fluor-
ides, peroxide of hydrogen, have been used as milk preservatives.
Either a mixture of boric acid and borax, or " formalin," has
been generally employed. Such chemical preservatives are now
prohibited in anj^ form of milk by the Public Health (Milk and
Cream) Regulations, igi2.

Boric acid with borax is largely added to milk during the
summer months, and the amount generall}' employed is
about 5 grains to the pint. Experiments go to show that
not less than 4 grains of a mixture of boric acid and borax
are necessar}- to preserve a pint of milk for twenty-four hours
in warm weather (Rideal, Foulerton). Salicylic acid is not so
frequently employed, because of its lesser solubility and
unpleasant taste.

" Formalin " is a commercial preparation containing about
38 per cent, of formaldehyde.

" Mystin " (a mixture of formic aldeh3?de and sodium nitrite)
has occasionally been emploj^ed as a preservative.

Annatto and turmeric, coal-tar d3'es and saffron, are j-ellow
colouring agents which arc added to give the milk a rich
yellow appearance. Annatto and coal-tar dyes are chiefly

The tests for the antiseptic and colouring agents in milk are
given in Chapter XIII., which treats of the subject of Anti-
septics and Colouring Agents in Food.

Cream. — Starch or gelatine is sometimes added to cream to
thicken it. Gelatine may be detected by adding to 10 c.c. of the
sample, 20 c.c. of cold water and 10 c.c. of a solution of acid
nitrate of mercury. The whole is then well shaken, allowed to
stand for live minutes, and then filtered. If much gelatine is
present, a clear filtrate cannot be obtained. A portion of the
filtrate is mixed with an equal quantity of a saturated aqueous
solution of picric acid, when a yellow precipitate forms if gelatine
is present (Stokes). Starch is detected by the bluing with
iodine solution.

Milk solids and other fats, and lime in cane-sugar sjTup, have


also been added to cream. Sucrate of lime may be detected by
estimating the lime in the ash of the cream (average percentage
of Ca0=22 per cent, of the ash). The same preservatives are
employed as in the case of milk.

Under the Pubhc Health (Milk and Cream) Regulations, 1912,
no preservative may be added to cream which contains less
than 35 per cent, by weight of milk fat, whereas in cream con-
taining 35 per cent, or more of milk fat the only chemical pre-
servatives permitted are boric acid, borax, or a mixture of these,
and hydrogen peroxide; but the addition of these preservatives
is subject to a system of declaration. Furthermore, no thicken-
ing substance may be added to cream. In these Regulations
" thickening substance " means sucrate of hme, gelatine, starch
paste, or any other substance which, when added to cream, is
capable of increasing its thickness. Neither cane nor beet
sugar are to be regarded as a preservative or as a thickening

Hand-skimmed milk is sometimes made to look like good rich
milk by the addition of condensed milk. An analysis of the ash
and non-fatty sohds will detect the fraud, since these will both
be in excess of their general proportions (more especially the
sugar), and the amount of soluble albumin will be diminished
(Faber) .

Hand-skimmed milk is generally slightly acid, and the specific
gravity is above 1032-5. The fat generally amounts to from
0-5 to 1-5 per cent.

The bulk of the samples of " separated milk " contain from
0"2 to 0*3 per cent, of fat.

Skimmed and separated milk must legally contain at least
8 7 per cent, of solids-non-fat.

Separated milk is sometimes " enriched " — that is, the butter-
fat taken out by the separator is replaced by an emulsion of
some other fat. In such case a separate analysis of the fat of
" the cream " must be made by the Reichert-Wollny process,
as described in the analysis of butter.

Condensed milk may be unsweetened or sweetened whole milk
(unskimmed or non-separated) concentrated to about one-third
of its original volume, and cane-sugar added; or it may be pre-
pared from sweetened skimmed or separated milk.

The following table, taken from Dr. Coutts's Report to the



Local Government Board {191 1), indicates the percentage com-
position of the chief classes of condensed milk upon the market :

Full Crbam.

Machine Skimmkd.











Total solids . .
Protein . .


Lactose . .
























In the analj'sis of condensed milk 20 grammes should be taken
and made up to 100 c.c. with water as a stock solution.

For Total Solids. — Evaporate 5 c.c. of this as in the case of

For Ash. — Incinerate the above. The ash averages 1-9 to
2 per cent.

For Fat. — Estimate by Adam's process. The fat averages
about 10 per cent.

The best method for obtaining a rapid and accurate estimate
of the fat in sweetened brands is the Gottheb process (the Werner-
Schmidt process being inapplicable) :

Into a graduated 100 c.c. tube put 10 c.c. of above solution;
add I c.c. of 40 per cent, ammonia; warm to 30° C; shake;
add 10 c.c. alcohol (95 per cent.), and shake. Add 25 c.c. ether,
and shake; add 25 c.c. petrol-ether, and shake; let settle; pipette
off 25 c.c. of the mixed ether solution, evaporate this and weigh.
Calculate as in the Schmidt process. The fat of sour milk, cream,
cheese, butter, etc., may aJl be reliably estimated by this process.

For Total Sugars (cane and milk). — To 10 c.c. of stock solution
add 40 c.c. of methylated spirit, add one drop of acetic acid,
shake (this will precipitate the curd and fat), filter. Evaporate
20 c.c. of the filtrate, and weigh the residue. Now incinerate this,
and subtract this ash from the total sugar weight. Multiply
the difference by 22 5, and then by 5, to give percentage of mixed
sugars. Apparently one should first multiply by 25, but allow-
ance has to be made for the volume occupied by the precipitated
* A partly skimmed milk.


curd and fat. Deduct milk-sugar (estimated Ijy Fehling's solu-
tion), and the difference is cane-sugar.

The milk-sugar (which averages 13 to 15 per cent.) is deter-
mined by titrating a 5 per cent, dilution of the milk with Fehling's
solution {vide pp. 340, 341). The cane-sugar may subsequently
be estimated by boiling the stock solution with citric acid, which
inverts the cane-sugar; the solution is cooled, neutralized with
potassium hydroxide solution, made up to a known volume, and
titrated with Fehling's solution.

Many brands of condensed milk contain a very laige amount of
sugar, the average being 38 to 40 per cent. ; while others are un-
sweetened. When opened, the latter have inferior keeping powers.
For Proteids. — Perform Kjeldahl's process on 10 c.c. of stock,
and multiply the N by 6-38. The proteid matter averages
9 per cent.

The " degree of condensation " may be approximately gauged
by dividing the percentage of solids by 12 -6, when the condensed
milk is unsweetened; or by dividing the percentage of fat by 3-6 in
other cases — 12-6 per cent, and 3-6 per cent, being, respectively,
the average amounts of total solids and fat in milk.

Brands of condensed whole milk (not " machine skimmed ")
ought to contain at least 10 per cent, of milk-fat and 25-5 per
cent, of non-fatty milk solids, of which the ash should constitute
about 2 per cent.

Heated Milk. — Sometimes it is required to know whether milk
has been steriHzed or boiled. In such a case 3 c.c. of milk may be
mixed with i c.c. of a freshly prepared 10 per cent, solution of
hydroquinone, and about 15 drops of hydrogen peroxide added.
If the milk has not been raised to a high temperature, an imme-
diate rose colour forms, but otherwise no colour is produced, as
the reaction is destroyed by exposure to a high temperature.

The following changes result when milk is boiled: Carbonic
acid gas is expelled and the calcium and magnesium salts are
therefore partially precipitated; the greater part of the phos-
phates are also precipitated. There is a slight diminution in
the organic phosphorus originally present ; a partial decomposi-
tion of the proteins. The skin which forms on the smTace (solely
when the heating is done in an open vessel) consists mainly of
lactalbumin. This pellicle has approximately the foUo\nng
composition: Fat, 45-5 per cent.; lactalbumin and casein, 51 per
cent. ; mineral ash, 3-5 per cent. The normal emulsion of the fat


globules is disturbed, so that the cream does not rise to form a
layer on the surface, the lactose is partially burnt (carameliza-
tion), and the milk therefore becomes slightly brownish in
colour. The boiling destroys the ferments in the milk and
probably also the antiscorbutic element of raw milk ; the natural
germicidal power of fresh raw milk is lost, and almost all the
bacteria are destroj^ed, those left consisting of sporing forms and
certain highlj' resistant varieties.

None of these changes takes place, appreciably, in "low tem-
perature pasteurization " — namely, the heating of milk to a
temperature of 60° C for thirty minutes — except the great
reduction in micro-organisms.

Milk Powders. — These are now very largely made, both from
whole milk and from skimmed milk. The powders are cream-
coloured, with a slight distinctive odour. The fat in whole-
milk powder should amount to at least 25 per cent. This can be
determined by the Gottlieb process or by the Soxhlet method.
Otherwise the analysis follows on the general lines described.

" Koumiss " consists of milk which has been skimmed of some
of its cream and sugar added; it is then partially fermented by
yeast or other ferments, wherebj^ much of the sugar is converted
into lactic and carbonic acids.

Bean or Synthetic Milk. — In the preparation of this milk soya
beans are washed and soaked in water, the outer integuments
being removed. The softened beans are then ground between
millstones and the powder boiled with water and filtered through
fine sieves. A cream-coloured liquid results, closely resembling
milk, but with a distinct beany odour and taste. For use, sugar
is added to suit the taste of the consumer. Bean milk on analysis
contains about 2-i per cent, of fat, 37 per cent, proteins, 1-4 per
cent, carbohydrates other than sugar, and 0-4 per cent, of
mineral ash.

Milk Standards.

In addition to the legal standards at present in force, certain
other standards are advocated. Only a small proportion (one-
sixth to one-eighth) by weight of the cow-dung which finds its
way into milk is recoverable (as dirt) from milk by centrifugaliza-
tion; but despite the difficulties involved, certain standards for
dirt in milk have been suggested. Most authorities agree that
milk ;ynelding more than i part of recoverable dirt per 100,000


is dirty. Houston suggests that, as a working standard, (i) the
deposit from a litre of milk obtained by sedimentation in a
special cylindrical separating funnel after twenty-four hours
should not exceed i part per 10,000 by volume; and (2) when
the deposit from (i) is centrifugalized, it should not exceed half
the above amount.

It has been suggested that, as a general rule, the recover-
able dirt should not be allowed to exceed 2 parts per 100,000 by
weight, and some advocate a standard as low as i part. As a
rough household standard, |- pint of milk placed in an ordinary
tumbler should not throw a visible sediment in two hours. But
dirt may be removed by trade filtration, which leaves behind the
harmful bacteria, and therefore the only satisfactory standards
are those based upon bacterial counts.

Seasonal standards of total bacterial counts are serviceable.
In Chicago, for instance, 1,000,000 bacteria per c.c. of milk from
May I to September 30, and half that amount for the remainder
of the year, is the standard of milk as it arrives in that city.
Savage prefers a standard of lactose fermenters of the coli type
of not more than 100 in winter and 1,000 in summer, and he
suggests that initial contamination may be best judged from the
number of Bacillus enteritidis sporogenes (the spores of which
abound in cow-dung), as that organism shows relatively little
tendency to multiply in milk.

Certainly leucocytes exceeding 1,000 per c.c. along with manj-
streptococci suggests the desirability of investigation.

In special (certificated) milk, which is sold at an enhanced
price, it is possible to impose such high standards aS' — freedom
from B. Uiberculosis, a total bacterial count below 10,000 per c.c.
(Class A) and 100,000 per c.c. (Class B), and delivery to the
consumer at a temperature of not above 10° C

A suggested standard for pasteurized milk is that the total
bacterial count should not exceed 1,000,000 per c.c. prior to
pasteurization, and 50,000 per c.c. when pasteurized and delivered
to the consumer.

Reductase test (Schmidt-MuUer) may serve as a standard for
freshness. The test reagent is made by adding 5 c.c. of a satur-
ated alcoholic solution of methylene blue (zinc chloride double
salt) to 195 c.c. of distilled water; it should be boiled every da^^
before using. One c.c. of the reagent is mixed with 20 c.c. of
milk, the surface is sealed with paraffin, and then the test-tube


and its contents are placed in a water-bath at 45" C. to 50° C.
Fresh milk should remain blue for twelve hours or more. The
reduction of methylene blue by raw milk (in the absence of
formalin) is due to bacterial contamination, and if the milk
decolorizes within one hour the organisms certainly exceed
500,000 per c.c.

Bacteriological Note.

Milk as secreted is free from organisms, but even in the milk
cistern of the udder and in the teat canals some bacterial infection
takes place ; while at every stage, from the udder to the consumer,
contamination with bacteria is possible, and under many of
the conditions which now prevail is invited. Organisms gaining
access to milk, unlike those in air and water, are usually in
an environment most favourable to multiplication, and as a
consequence milk as vended frequently contains one to five
millions or more organisms per c.c. ; the number, as is to be
expected, being considerably greater in hot weather.

Park* has shown that in New York, " with only moderate
cleanliness, such as can be employed by any farmer without
adding appreciably to his expense, namely, clean pails, straining-
cloths, cans or bottles, and hands ; a fairly clean place for milking,
and a decent condition of the cow's udder and the adjacent belly;
milk when first drawn will not average in hot weather over
30,000 and in cold weather not over 25,000 bacteria per c.c.
Such milk, if cooled to and kept at 50° F., will not contain at
the end of twenty-four hours over 100,000 bacteria per c.c. If
kept at 40° F. the number of bacteria will not be over 100,000
after forty-eight hoiirs."

The estimation of the number of bacteria in milk, or of some
special group of bacteria such as the B. coli group, is the natural
measure of the degree of contamination of milk, but the question
of the numbers to allow is beset with difficulties, largely owing
to the suitability of milk as a medium for the propagation of

A milk initially comparatively pure will frequently show after
the lapse of twelve to twenty hours many more bacteria than one
collected under much less cleanly conditions, and initially much
more heavily charged with bacteria, but examined after the lapse
of only three or four hours from milking.

* Journal of Hygiene, 1901, vol. i., p. 391.



These differences in the bacterial content are not only deter-
mined by the initial contamination and by the time since milking,
but also by the temperature at which the milk has been kept,
and the latter introduces a condition subject to great variation.

The milk should be collected in sterile glass-stoppered bottles.
Those used for the bacteriological examination of water may be
used, or the simple and efficient apparatus described by Delcpine
may be employed.

It consists of a metal case containing a 7 or 8 ounce bottle and
a milk-scoop. All the parts are thoroughly sterilized in the
laboratory before being sent out, and the sterilized case is opened
only at the time when the sample is taken. The sterilized scoop
is used to remove the milk from the cans or other vessels. When
obtained direct from a suspected cow, the milk may be milked

FIG. 38. — delepine's milk-collecting apparatus.

into the scoop. The metal cases are packed in refrigerating
boxes if necessary.

If the sample cannot be examined within an hour or so, it
must be transmitted packed in ice.

If milk from individual cows is being collected, the teats and
the milkers' hands should be washed and disinfected. In some
cases it is necessary to collect a separate sample from each
quarter, while for a complete examination fore, middle, and end
milk samples should each be collected.

As a rule condensed milks are free from preservatives. In
the sweetened milks the sugar is sufficient to inhibit the growth
of bacteria, and in the unsweetened the milk has been sterilized
at temperatures over 100° C. The processes carried out in con-
densing the milk are sufficient to destroy Bacillus coli, B. tuber-


culosis, and other pathogenic organisms, but spore-bearing
bacilh, streptococci, sarcinae, yeasts, and other saprophytes, are
often present, so that condensed milks must not be regarded as
necessarily sterile. It is probable that the bulk of the organisms
present have gained admission during the processes of cooling
and of filling the tins.

At the temperature of about lo^ C. the multiplication of lactic
acid forming bacteria is checked, and those organisms are de-
stroyed at a temperature of 70° C maintained for twenty minutes.
The ferments hitherto detected in cow's milk (peroxidase, reduc-
tase, catalase, etc), are mainly derived from bacteria; but certain
of such ferments which are found to be present in uncontamin-
ated milk, such as amylase, do not appear to be of value in diges-
tion and nutrition. The chief enzymes are destroyed at about
70° C. in thirty minutes. At 80° C all enzymes are destroyed;
but at 60° C. their activity is, if anything, slightly promoted.

The discovery by Ehrlich that passive immunity could be
produced by suckhng when the mother was immune, led to the
investigation of the presence in milk of precipitins, agglutinins,
opsonins, antitoxins, and other so-called " protective substances "

Online LibraryHenry R. (Henry Richard) KenwoodPublic health laboratory work → online text (page 21 of 36)