CHAPTER IV
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 at Outset | In 12 hrs.at 50° F. | In 12 hrs.at 70° F. | In 50hrs. at 50° F. | In 50 hrs. or at time of curdling at 70° F. | No. hrs. to curdle at 50° F. | No. hrs. to curdle 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.
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.
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.