HANDLING OF MILK
MODERN DAIRY PRACTICE
As we have seen, the dairy industry is a very ancient one, and has been
intimately associated with the development of civilisation.
Within historical times dairying has always formed a prominent feature
in connection with agriculture, and the use of milk in one form or
another has been common to every civilised nation.[42]
The greatest progress, however, in the study of milk has taken place
since about the year 1890, at which time the dairy industry seems to
have attracted the general attention of food specialists and scientific
investigators throughout the world. Since then it has been considered
worth while to enact laws in different countries with regard to the
regulation and control of the milk supply.
Since 1903 there has been an International Dairy Federation formed, and
it has held conferences at Brussels, Paris, The Hague, and Buda-Pest,
and in 1911 it will hold a conference in Stockholm. The Federation was
started in a very humble way in Brussels, and owes its origin, to a
large extent, to a distinguished Belgian agriculturist, Baron Peers of
Oostcamp, Bruges; but at the present day a general committee composed of
representatives of nearly every civilised nation has been formed, and
delegates from such countries attend the Congresses, which are held
every two years. The literature which has arisen out of these
International Congresses has been disseminated in different countries,
and has been instrumental in placing the dairy industry on a thoroughly
scientific basis.
_Milk Supply of the United Kingdom._--The milk supply of the United
Kingdom has steadily grown from year to year, and in relation to the
population works out at fifteen gallons per head. The manner in which
these figures are arrived at is shown in the following estimate:
The population of the United Kingdom is now about
45,500,000. The number of cows or heifers in calf or in
milk in June, 1909, was 3,360,600; the number in 1910 was
probably about 4,400,000.
Of these about 300,000 were heifers that had not yet
produced any milk. The actual milking class, therefore,
comprised about 4,100,000 cows and heifers; of these,
about 600,000 were heifers that calved in the winter and
spring of 1909-10, and 300,000 were heifers that calved in
the summer and autumn of 1910. The number of cows that
produced two or more calves may be taken to be about
3,200,000; of these about 600,000 should have produced
their second calf in the winter and spring 1909-10, and
would be milked as heifers in the summer and autumn of
1910; the number of mature cows from which a full season's
supply of milk was obtained during the twelve months from
June 5, 1909, to June 4, 1910, was apparently about
2,600,000. A large quantity of milk is yielded during the
year by cows sold or lost during the twelve months before
the census. Possibly ten per cent. of the milk produced in
the twelve months from June, 1909, to June, 1910, was
yielded by cows that were sold or lost before the census
of June, 1910.
It is estimated that the 3,200,000 cows (including the
600,000 that up to the winter of 1909-10 were heifers)
produced, on the average, 44 cwts. (480 gallons) of milk
per head in the twelve months from June 5, 1909, to June
4, 1910; the 300,000 heifers that calved in the summer and
autumn, 30 cwts. (330 gallons) per head; the 600,000
heifers that calved in the winter and spring of 1909-10,
15 cwts. (165 gallons), making the total quantity of milk
produced in the twelve months by cows and heifers on the
farms, and that produced calves during the twelve months
(June, 1909-1910), 158,800,000 cwts. (1,746,800,000
gallons), or about 426 gallons per head, and about 400
gallons per head for all the cows and heifers in milk or
in calf in 1910. There remains to add the milk yielded by
the cows that were sold during the twelve months, and of
cows and heifers in feeding pastures that were milked
during the twelve months, June to June, 1909-10, and which
probably formed one tenth of the whole supply, making the
total supply for the twelve months 176,444,000 cwts., or
1,940,884,000 gallons. This equals 2 tons, or 440 gallons
per head, crediting the whole supply to the 4,400,000 cows
and heifers in milk or in calf in June, 1910. At 7-1/4d.
per gallon the value of milk produced in the United
Kingdom in the twelve months was £58,600,000. Including
the value at birth of the calves, the total value of the
produce of the milk-giving class would be about
£62,000,000. The value of the milk, butter, cheese, and
cream sold or consumed in farmhouses would be about
£48,000,000, or equal to about 24 per cent. of the gross
annual income of farmers.
The average consumption of new milk is about 15 gallons
per head of the population. During the twelve months of
1911, the quantity required for this purpose will be about
682,500,000 gallons, or about 35 per cent. of the total
supply; calves will require about 10 per cent. of the
supply; the quantity available for butter and cheese will
equal about 55 per cent. of the supply.[43]
_The Milk Industry in the United States._--In the United States of
America, where the habits of the people are somewhat analogous to those
in the United Kingdom, it is estimated that the milk from five million
cows is annually consumed, which averages twenty-five and one half
gallons per year for each person, or equal to an ordinary sized
tumblerful each day.[44]
Such a vast industry, so intimately associated with the food of the bulk
of the people, naturally invites the closest study, and, as a
consequence, the literature on the subject, which has arisen during the
last twenty years, has been of a voluminous character, not only from the
point of view of practice, but from that of bacteriology, chemistry, and
hygiene.
A pure milk supply is essential to health, and it seems unfortunate that
the ordinary milk producer should, in a great many cases, take up an
antagonistic attitude to the scientific methods of handling milk. There
is a body of opinion being created, however, which is likely to alter
this attitude in the next generation, and this is attributable to the
fact that so much excellent work has been done at numerous dairy
colleges and institutes in all civilised countries that the dairy
industry is emerging from a period of rule-of-thumb procedure to its
proper place as one of the technical arts.
_Transmission of Disease in Milk._--It is not to be wondered at that the
handling of milk should now be regarded as a technical business, seeing
that milk-borne disease is one of the commonest with which we have to
deal.
The commoner diseases which have been transmitted by milk are scarlet
fever, typhoid, diphtheria, tuberculosis, sore throat epidemics. Others
of a more complex character have been traced to the same source of
infection, and the clearest possible evidence has been furnished of the
transmission of diseases by means of micro-organisms, which have
contaminated the milk supply.
It is therefore necessary to watch over the milk from the source of
supply to its consumption. It is primarily on the farm and in the
cow-house that methods of handling in a hygienic way should be insisted
on, as microbial contamination increases at a prodigious rate, and it is
the early microbe therefore which does the most damage.
The milk in the udder, for all practical purposes, may be assumed to be
sterile, and the contamination which takes place originates, therefore,
from external sources.
One of the principal means of infection is from hairs which fall from
the cow into the milk, and many of which are carriers of dangerous
micro-organisms.
There is also a certain amount of offensive dirty matter which may fall
into the milk-pail, and carry with it undesirable germs.
These impurities may, to a certain extent, be eliminated by good
straining, but a surer prevention is to have the cow-house perfectly
clean and free from dust, as dust specks are in many cases the vehicles
of disease germs. Cleanliness is, in fact, the essential feature in
modern dairying, not only in the cow-house, but in the milking utensils,
the drainage, etc., and, above all, the milker should be of cleanly
habits.
The flavours of milk sometimes arise from the absorption of
evil-smelling gases in the cow-house, or from a peculiar taint from
certain roots and feeding stuffs, and in such a case it is desirable
that aëration should take place in a fresh clear atmosphere, so that
oxygenation may have the effect of eliminating and destroying the
foreign odours and flavours which may be present. If this process of
aëration is carried out at blood heat, the result is generally highly
satisfactory.
_Milk Management._--There have been many excellent tables of rules
published for the management of dairies in different countries, but they
are necessarily framed within certain limitations which apply to all.
The following is an excellent set, which put concisely the conditions
necessary to be observed in the modern cow-house:
1. The cow should be sound--no disease should exist in
the animal.
2. The feed should be good and free from aromatic
substances. If these aromatic foods are used, they should
be employed according to those methods which will not
cause odours or flavours to appear in the milk.
3. The cow should be groomed, and hair about the udder
preferably clipped.
4. The udder should be moistened during milking.
5. The milker should be a neat, tidy person.
6. The milker should be free from disease, and should not
come in contact with any communicable disease.
7. The milker's clothes and hands should be clean while
milking.
8. The pail should be sterilised.
9. The stall should be such as to reduce the amount of
disturbance of dust and dirt.
10. There should be good light, good ventilation, and
good drainage in the cow-house.
11. The cow-house should always be kept clean.
12. Feeding and bedding, unless moist, should be done
after milking.
13. A dustless milking-room is desirable.
14. Milk should not stand in the cow-house.
15. If milk is aërated, it should be done before cooling
and in pure air.
16. The sooner the milk is cooled after milking the
better.
17. Keep the milk as cold as possible when once
cooled.[45]
The supply of milk is conducted, to a large extent, by towns' dairies,
which depend for their supplies upon the dairy farm in the country, and
it is obvious that a certain period of time must elapse, in the
generality of cases, before a town's dairy receives its supply in the
ordinary course, and this constitutes the greatest difficulty in modern
dairy practice, owing to the liability of the milk to absorb bacteria,
which during transit may multiply enormously.
The multiplying of bacteria in milk at different temperatures is easily
demonstrated, and the result of this has been stated in various forms
many times over. As a graphic means, however, of showing the increase
that takes place in the numbers of germs present, and the consequent
product of acidity, the table below by Conn may be given.
The consequent result of the increase in bacteria is the production of
lactic acid, which produces the souring so familiar in milk which has
been kept in the household at a high temperature.
_Numbers of Bacteria per c.cm. in Milk kept at
Different Temperatures._
--------------------------------------------------------------------------
Number In 12 In 12 In 50 In 50 hrs. or at No. hrs. No. hrs.
at hrs. at hrs. at hrs. at time of curdling to curdle to curdle
Outset 50° F. 70° F. 50° F. at 70° F. at 50° F. at 70° F.
--------------------------------------------------------------------------
46,000 39,000 249,500 1,500,000 542,000,000 190 56
47,000 44,800 360,000 127,500 792,000,000.36 hrs. 289 36
50,000 35,000 800,000 160,000 2,560,000,000.42 hrs. 172 42
--------------------------------------------------------------------------
What actually happens is that the lactic acid is produced by the
breaking up of the milk sugar, and the appearance of this sourness is an
indication that a period has been reached in the age of the milk which
may be described as being--unwholesome.[46]
It is necessary, therefore, for the town's milk dairy to be equipped in
such a way as to deal promptly with the milk supply.
We have seen that the milk should first of all be aërated at blood heat,
so as to liberate objectionable odours, after which it should be cooled
to as low a temperature as possible, by means of well water. When these
operations have been performed on the farm, milk should be sent as
rapidly as possible to the distributing towns' dairies, and should be
transported in refrigerated waggons, cooled preferably with ice, during
the journey. On arrival at the town dairy, it will be necessary to
pasteurise the milk--that is to say, the milk should be heated to such a
temperature as will destroy any pathogenic organisms which may be
present, and the pasteurising temperature should therefore be in excess
of the thermal death-point of all such organisms.
Pasteurisation owes its origin to Pasteur, and has become an adopted
method throughout the dairy industry, and there are many mechanical
devices termed "pasteurisers" (see Fig. 8) which are used for the
carrying out of this particular operation. The form of one of these is
that of a vertical jacketed cylinder with paraboloidal surface, around
which steam is made to pass, so as to maintain the temperature at about
176° F. Milk is allowed to flow in at the bottom of the paraboloidal
surface, and is caught by mechanical agitating arms, which revolve at a
given speed, and by this action milk is distributed centrifugally over
the paraboloidal surface, and is forced out by the same action, at the
top of the apparatus, after being heated.
[Illustration: PASTEURISER
FIG. 8.--The milk enters from the bottom and circulates to
the top of the inside cylinder, which is paraboloidal in
construction. It is heated as it passes through the
apparatus, and is discharged at the top at a temperature
of 176° F.]
The centrifugal action is sufficient to raise the milk some three to
four feet, through a tube, and this is taken advantage of so as to cause
the milk to flow over a conical cooler, described as a primary cooler,
and in which water is made to circulate. As the hot milk descends over
the conical cooler it gives up most of its acquired heat to the water,
and, in practice, is reduced in temperature to within 4° of the
temperature of the water.
Below this primary cooler is fixed a cooler of the same size and shape,
which is termed a secondary cooler. In it, brine at a temperature of
about 35° F. is circulated from a refrigerating machine, and, as the
milk falls over the secondary cooler, it is cooled to a temperature of
about 40° F., when it may be looked upon as being pasteurised and free
from all pathogenic organisms, in which state it will keep for a
considerable length of time.
It is desirable that the milk should, as soon as possible after the
cooling takes place, be delivered to the consumers, and be kept under
cool conditions, either in bottles or in a closed vessel covered over
with muslin, so as to keep out specks of germ-laden dust.
Briefly speaking, the foregoing is an outline of what is carried on in
the ordinary dairy practice.
There are many modifications of this practice, such as the introduction
of regenerative heaters, so as to utilise a portion of the heat of
pasteurisation, which would otherwise be wasted.
In some cases, again, it is considered necessary to conduct the primary
and secondary cooling over coolers furnished with mantles, so that the
atmospheric bacteria which are everywhere present should be shut off
from the falling milk.
Ordinarily, however, the equipment for a town's dairy consists of:
1. Steam-boiler to generate steam for pasteurising, scalding, etc.
2. Motive power, which may be either a steam-engine, gas-engine, or
electric motor.
3. Refrigerating machine, which is used for supplying cold brine to the
secondary cooler. In many cases it is also used for cooling a room in
which the milk and cream are stored.
4. Milk-receiving tank.
5. Milk-strainer.
6. Pasteurising apparatus, and primary and secondary coolers.
Such a plant is necessary in order to conduct an ordinary town dairy
business in anything like a hygienic way, and is designed only for the
handling of milk intended for domestic consumption.
There are times when another plant might be necessary, such as a plant
for the separation of milk, or for utilising it for the production of
butter or cheese, such operations being subject to the fluctuations in
the milk supply.
It is sometimes desirable also to use up an excess of milk for cheese or
butter-making; hence it is necessary to provide such apparatus as has
been indicated.
_Preparation of Soured Milk._--The foregoing description has been given
in some detail, as showing the ordinary practice, and we now come to
consider how it can be modified so as to provide for the production of
soured milk. It may first of all be premised that within the next few
years the preparation of soured milk as an ordinary production of the
dairy will be universal, and will form a part of the ordinary dairy
practice. The apparatus, therefore, which is necessary is one of
considerable interest to all who are engaged in the dairy industry.
As will be seen from the chapter describing the preparation of soured
milk in the dairy, this process can be conveniently carried on, so as to
utilise the plant which is at present in general use. The milk can be
received in the same way, pasteurised and cooled to about blood-heat,
after which its preparation as soured milk is a very simple matter, and
only requires a certain amount of careful attention.
For the keeping of soured milk, a cold room cooled by a refrigerating
machine would be desirable, so as to maintain the fermented milk at a
low temperature and prevent over-fermentation.
Apparatus has been designed so as to handle soured milk on a large
scale, and one of the machines is shown on the illustration (see Fig.
9). It is simply a jacketed cylinder with a cover and an agitating gear.
The inside of the machine is nickel-plated, and there is an arrangement
whereby the cooling may be done rapidly, through a coil inside the
jacket, this coil being connected to the brine circulation of the
refrigerating machine.
[Illustration: CONTINUOUS APPARATUS FOR THE PRODUCTION OF
LARGE QUANTITIES OF SOURED MILK
FIG. 9--This apparatus is made by the Dairy Machinery and
Construction Company of Shelton, Conn., U S A. The milk is
agitated inside a jacketed cylinder, where it is allowed
to incubate at about blood heat. The milk can be rapidly
heated and also rapidly cooled by means of this
apparatus.]
The machine is filled with milk containing three per cent. of fat, which
has been previously pasteurised to about 190° F., and cooled down to
about 90° F.; at this point the pure culture of _Bacillus bulgaricus_ is
introduced, and the agitator is kept working, so as to mingle it
thoroughly with the milk. The agitator is then stopped until the acidity
shows a test of 0.9 to 1.0 per cent., when the agitator is again
started, and cold brine from the refrigerating machine is turned on to
the cooling pipes, so that the product is thoroughly broken up, and
cooled down to 40° F.
The milk is then transferred to a bottle-filling machine (Fig. 10),
poured into bottles and hermetically sealed, after which it is ready for
consumption. When it has to be kept for any time it should be placed
in a cold room where there is a temperature not higher than 40° F.
The process, therefore, is a simple one, and lends itself to the
ordinary dairy business, without involving any great expenditure on
account of a new plant.