If a glass funnel is not available, a very small jelly bag can be made of fine gauze and used in the same way. The washing water should be used in small quantities and directed to concentrating the dirt in the apex of the bag. After washing, the latter can be turned outside in, to permit of readier examination of the dirt. The bag should be well washed in cold water, then boiled and dried, and is then ready for future use.

The acidity of milk is a very useful guide to its age. Milk has the curious property of being "amphoteric," i.e., it is both slightly acid and slightly alkaline when fresh. As its age increases, however, so does its acidity, and at a rate varying with the temperature and moisture contents of the atmosphere in which it is placed. Old and acid milk is heavily contaminated with bacteria, a proportion of which are likely to be injurious to health.

The simplest method of testing the acidity is to procure a few little books of blue and red litmus Lactometer and Test Tube Fig. 7. Lactometer and Test Tube test papers, and these can be had from any philosophical instrument maker or laboratory furnisher. The strips of test paper are torn out and dipped in the milk. When the milk is quite fresh it will, owing to its amphoteric condition, change the red litmus paper slightly blue, and the blue litmus paper slightly red. Old milk changes blue litmus paper to a bright red because of its decided acidity.

The above tests do not indicate if the milk is poor or rich, but this can be determined by the lactometer, an instrument for ascertaining in a simple way the specific gravity. The lactometer is shown in Fig. 7.

It is graduated usually from 25° to 36°, corresponding to specific gravities 1.029 to 1.038. It is graduated to degrees and half degrees. Sometimes a thermometer is combined with the instrument. The specific gravity rises as the temperature is lowered and decreases with increase of temperature, so that it is important to make the test at the figure at which the lactometer was graduated, which is usually 60° F. Failing this, an allowance has to be made for higher or lower temperatures. The milk to be tested is well mixed, and placed in a deep vessel, and the lactometer placed in it, holding it at first at an angle. It stands upright and remains deeper or higher according to the specific gravity. The reading is taken on the stem at the level of the milk. As the latter is drawn up a little round the stem, about a half degree should be added on to get the true figure. Thus, if the apparent reading is 31, the true reading may be taken as 31.5. This is the average figure for good milk, corresponding to a specific gravity of 1.0315; anything above this is all to the good. Lower readings mean inferior quality, the latter being proportionate to the lowness of the readings. The tests are most conveniently made in a glass cylinder (Fig. 7), which may be purchased with the lactometer. As there are many inaccurate instruments in the market, it is necessary to go to a reputable maker, because an unreliable lactometer is worse than useless.

The following table gives, in a condensed form, the allowances to be made when the temperature is above or below the standard (60° F.):

Thus if the thermometer indicates 40° F., and the lactometer 29.1°, the true reading at the standard temperature of 60° F. is 31°, corresponding to a specific gravity of 1.031. Intermediate figures can readily be averaged. Care should be taken to wash the lactometer with cold water under the tap, as otherwise the milk will dry on it and render it inaccurate.

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 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:

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.

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.

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.

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.

CHAPTER V

THE BACTERIOLOGY OF FERMENTED OR SOURED MILK

A CHAPTER FOR STUDENTS

During the last few years much work has been done in investigating the action of various classes of organisms—bacteria, yeasts, and moulds—upon milk and its products. While, however, the attention of the dairyman has been chiefly directed to the propagation of acid-producing organisms and the use of pure cultures of lactic acid bacteria in their relation to butter and cheese making, a new sphere in micro-biology has been disclosed by the study of the effects caused by the combined growth of two or more different classes of organisms in milk and the consequent production of lactic, alcoholic, and gaseous fermentations. The simultaneous occurrence of these fermentative changes is responsible for the formation of such beverages as keffir, koumiss, milk-wine, etc. It has therefore become essential, in connection with the study of new developments in the milk industry, that we should make a more intimate acquaintance with the bacteriology of the ferments involved.

Keffir (kephir, kifyr, kiafyr, kephor, kyppe) is the name given to an acid, slightly alcoholic drink, which for many centuries has been prepared by the nomadic tribes in the Caucasus. The characteristic fermentation is induced by the addition of so-called keffir grains. These are yellow or golden-yellow, warty, and furrowed flakes or nodules, the former varying in size from that of a rice grain to that of a bean, while the latter are often about an inch across and one eighth of an inch thick. Bearing in mind the fact that the preparation of keffir has been carried on for many centuries, it is not surprising that the origin of these grains should be surrounded by myths.

The belief is prevalent among the Mohammedan tribes of the Caucasus that keffir grains were, in the first instance, presented by Allah, as a sign of immortality, to one preferred tribe. Others hold that, in past ages, they were found by shepherds growing on a shrub in the Caucasian highlands; while, according to Skolotowski,47 they were originally found adhering to the walls of an oaken vessel used for the preparation of airam. This is a soured milk beverage similar to keffir, but possessing a weaker alcoholic fermentation, and prepared from goats' milk by the addition of pieces of calf's stomach. This would undoubtedly serve to introduce various species of lactic acid bacteria, and will be referred to in the portion dealing with soured milks. Keffir is prepared by the Caucasians from cows', sheep's, or goats' milk, and the operation is carried on in large leathern tubes or bottles. After the addition of the grains or seeds to the milk the vessel is placed in a cool chamber, and the fermentation is allowed to proceed for one or two days, by the end of which time the normal fermentation is at an end. During this period the keffir grains have increased enormously in size, assume a bright yellow colour, and lose their sour buttery smell.

Previous to the removal of the fermented liquid, a portion of the bottle is firmly bound from the rest by a stout cord, and the greater portion of the remaining keffir is quickly removed for use, thus avoiding, as far as practicable, any outside infection. After the addition of fresh warm milk the cord round the end of the bottle is removed, and the old and new milk thoroughly mixed for a time in order to ensure uniform inoculation of the new milk for the next fermentation. During the winter months the leathern vessels are often placed in the sunshine, so that the temperature remains at 61° to 65° F.

The necessary agitation of the vessel is said to be supplied in the form of kicks by passers-by or by the children during their play.

The beverage prepared in this way is so gaseous in character that it is often blown forcibly from the vessel during removal, and possesses, according to Podowyssozki,48 a very acid taste.

During any interruption in the preparation of keffir in the above manner, the grains are taken out, and after having been well washed in clean water, are spread out on a clean cloth to dry in the sunshine. They thereby assume a characteristic cheesy or buttery odour and become rather darker in colour. Thorough desiccation is essential in order to prevent subsequent mouldiness or disease of the grain.

In European countries the grains are subjected to a preliminary soaking in water for five to six hours and then placed in four to five changes of milk, each change having a duration of two to three hours. As soon as the grains commence to rise to the surface of the milk, they may be used for the actual preparation. To this end, a small quantity of the grain is added to freshly boiled milk and allowed to stand for eight to twelve hours at a temperature of 55°-62° F. with agitation of the flask every two hours. By this time the milk, now known as Sakwaska, has become abundantly inoculated with the organisms essential to the fermentation, and after the removal of the grains, may be poured into well-corked flasks for the secondary brew. The flasks should be kept at a lower temperature for twenty-four to forty-eight hours, by which time the product is ready for consumption.

According to the temperature and length of period to which this subsequent fermentation is allowed to proceed, the resultant keffir is more or less acid and gaseous. The grains may again be used for starting a fresh portion of milk, and a regular supply obtained in this manner. Well-fermented forty-eight-hours-old keffir should be an effervescent beverage with prickling and acid taste and a consistency and smell similar to sour cream. Large, persistent bubbles should form on the surface of the liquid and the casein be present as an extremely fine flocculent precipitate which remains suspended for a considerable time.

From the third day there ensues a gradual peptonisation of the casein. If the temperature at which the secondary fermentation has occurred should be higher than 72° F., or if the milk has not been sufficiently agitated, then the casein will be present in the form of porous small flakes, which on shaking form a fine emulsion.

The chemical changes undergone by the milk during the preparation of keffir are confined almost exclusively to the milk sugar. As already stated, a slight peptonisation occurs in old samples, but this depends very largely upon the method of preparation and purity of the culture. Hammersten49 and Essaulow50 show, however, that this is not a concomitant of normal fermentation. According to Hammersten, normal keffir contains—

In no case should the acid be higher than 1.0 per cent., and the alcohol more than 0.75 per cent.

Biology of the Keffir Grain.—The first communication on the biology of the keffir grain seems to have been made by Kern.51 He regarded the grain as a zoöglœa composed of bacilli and yeasts, the latter being regarded as the ordinary beer yeast (Saccharomyces cerevisseæ), while to the former he gave the name of Dispora caucasica. As the name indicates, this bacillus possesses two polar spores, and germination of these proceeded in the same manner as with Bac. subtilis. As, however, pure cultures of the organisms were not made, and the descriptions and illustrations made by Kern fail to show any distinctive characteristics, it seems probable that accidental confusion with other organisms must have occurred.

Krannhals52 succeeded in isolating ten different keffir bacteria among which were several sporulating bacteria. Here too it is impossible to attach any importance to the results, as the artificial preparation of keffir, by means of these bacteria, was not attempted. Beijerinck53 studied the organisms constituting keffir grains and attached prime importance to the occurrence of two organisms, viz., (a) a yeast, Saccharomyces kefir, which was capable of inverting milk sugar by means of an enzyme (lactase) and afterwards fermented the products with the formation of alcohol and carbon dioxide, and also (b) a non-motile non-sporulating bacterium, afterwards Lactobac. caucasicus. The latter, when cultivated on gelatine, gave rise to tough warty colonies about 1/40 in. diameter, and was regarded as one of the lactic acid bacteria found in milk which has been incubated at 77° to 90° F. and afterwards incubated at a higher temperature, 100° to 104° F. Scholl54 isolated three different organisms, of which a yeast inverted milk sugar for the lactic acid bacteria, while Dispora peptonised the albuminoid matters.

Adametz55 failed to isolate Dispora, and came to the conclusion that ordinary lactic bacteria and yeasts played the most important part in the fermentation.

Essaulow found in keffir grains six different organisms—yeast cells, cocci, short thick bacilli, bent bacilli, long threads, and motile bacteria. The two latter would seem to be Bacillus subtilis, while the others may be regarded as Bacterium acidi lactici (Hueppe), Bacterium aërogenes, and Streptococcus lacticus (Grotenfeldt). Pure cultures were insufficient to produce keffir, while mixed cultures of Bacterium acidi lactici and yeasts were effective.

Freudenreich,56 to whom we owe a record of very carefully executed experiments, could not arrive at a satisfactory explanation of the rôle of Bacillus caucasicus. This organism is described as being 5-6 µ long and 1 µ thick, slightly motile, and possessing bright refractive spots at the poles of the bacilli. It is extremely difficult to cultivate, and forms flat, small greyish colonies of irregular outline. The bright refractive spots above referred to are, however, granules taking the usual stains quite readily, and not spores as supposed by Kern.

Freudenreich also found three other organisms—a yeast and two streptococci. The yeast, to which he gave the name Saccharomyces (Torula) keffir, forms small oval or roundish cells 2-3 µ wide and 3-5 µ long. The optimum temperature would seem to be about 72° F.; the maximum 82° F. This organism is unable to ferment milk directly, but is able to decompose maltose and glucose with gas production. It does not coagulate milk, but imparts to it a characteristic taste and is unable to withstand desiccation for more than a few days.

Of the two streptococci isolated, Streptococcus a resembles organisms of the group Streptococcus lacticus in appearance, but is able to ferment milk, with weak acid and gas production, and is capable of inducing coagulation.