Before passing on to other matters, reference must be made to poisonous products other than bacteria which occur in milk and set up ill-health. Vaughan, of Michigan, pointed out at the London Congress of Hygiene in 1891 that he had separated a poisonous alkaloid, which he called tyrotoxicon. This, as its name denotes, was a toxic or poisonous substance, probably produced by some form of microbe. It may be taken as a type of the organic chemical substances frequently occurring in milk.
METHODS OF PRESERVING MILK
From the somewhat extensive category of diseases which may be milk-borne, it will be suitable now to speak of some of the means at our disposal for obtaining and preserving good, pure milk.
We considered at the commencement of this chapter the most frequent channels of contamination. If these be avoided or prevented, and if the milk be derived from cows in good health and well kept, the risk of infection is reduced to a minimum. But we have seen that much, if not most, of the pollution of milk arises after the milking process and during transit and storage preparatory to use. Bacteria are so ubiquitous that to prevent the entrance of any at all is almost beyond hope. Can anything be done to prevent their multiplication or to kill them in the milk? Fortunately the answer is in the affirmative.
There are two means at hand to secure these results. First, we may add to the milk various chemical or physical preservatives. Borax or boric acid, formaldehyde, salicylic acid, and other chemical bodies are used for this purpose. The commonest of these is that named first. The Food and Drugs Act (Section VI., 1875) permits the addition of an ingredient not injurious to health if the same is required for protection or preparation of the article in question. It is, however, a difficult matter to determine what amount of boric acid is injurious to health, for this differs widely in different persons. It has been laid down by one authority that even so small an amount as one-tenth per cent. might have inconvenient results, owing to its cumulative effect. Formaldehyde is without doubt an excellent antiseptic, and the more its efficacy becomes known so much the more probably will it be used. The salicylates, which are mild antiseptics, have long been used as preservatives. These substances, then, can be added to milk in quantities not recognisable to the taste (salicylic acid about .75 grain, and boracic acid .4 grain, to the litre of milk). They will materially increase the time that milk will remain sweet, they will prevent a number of micro-organisms living in the milk, and will inhibit multiplication of others.[60] Secondly, it is possible very perceptibly to remove the infectivity of milk by filtration and temperature variations.
Filtration has been practised for some time by the Copenhagen Dairy Company and by Bolle, of Berlin. The filters used consist of large cylindrical vessels divided by horizontal perforated diaphragms into five superposed compartments, of which the middle three are filled with fine sand of three sizes. At the bottom is the coarsest sand, and at the top the finest. The milk enters the lowest compartment by a pipe under gravitation pressure, and is forced upwards, and finally is run off into an iced cooler, and from that into the distribution cans. By this means the number of bacteria is reduced to one-third. The difficulty of drying and sterilising enough sand to admit a large turnover of milk is a serious one. This, in conjunction with the belief that filtration removes some of the essential nutritive elements of milk, has caused the process to be but little adopted. Dr. Seibert states that if milk be filtered through half an inch of compressed absorbent cotton, seven-eighths of the contained bacteria will be removed, and a second filtration will further reduce the number to one-twentieth. One quart of milk may thus be filtered in fifteen minutes.
The common methods now in vogue for the protection of milk are based upon germicidal temperatures. Low temperatures, it is true, do not easily destroy life, but they have a most beneficial effect upon the keeping quality of milk. At the outset of the process of cooling, strong currents of air are started in the milk-can, which act mechanically as deodorisers. But if the temperature be lowered sufficiently, the contained bacteria become inactive and torpid, and eventually are unable to multiply or produce their characteristic fermentations. At about 50° F. (10° C.) the activity ceases, and at temperatures of 45° F. (7° C.) and 39° F. (4° C.) organisms are deprived of their injurious powers. If it happens that the milk is to be conveyed long distances, then even a lower temperature is desirable. The most important point with regard to the cooling of milk is that it should take place quickly. Various kinds of apparatus are effective in accomplishing this. Perhaps those best known are Lawrence's cooler and Pfeiffer's cooler, the advantage of the latter being that during the process the milk is not exposed to the air. It must not be forgotten that cooling processes are not sterilising processes. They do not necessarily kill bacteria; they only inhibit activity, and under favourable circumstances the torpid bacteria may again acquire their injurious faculties. Hence during the cooling of milk greater care must be taken to prevent aërial contamination than is necessary during the process of sterilising milk. No cooling whatever should be attempted in the stable; but, on the other hand, there should be no delay. Climate makes little or no difference to the practical desirability of cooling milk, yet it is obvious that less cooling will be required in the cold season.
We now come to the protective processes known as sterilisation and pasteurisation. As we have already seen, sterilisation indicates a complete and final destruction of bacteria and their spores. As applied to methods of preserving milk, sterilisation means the use of heat at, or above, boiling-point, or boiling under pressure. This may be applied in one application of one to two hours at 250° F., or it may be applied at stated intervals at a lower temperature. The milk is sterilised—that is to say, contains no living germs—is altered in chemical composition, and is also boiled or "cooked," and hence possesses a flavour which to many people is unpalatable.
Now, such a radical alteration is not necessary in order to secure non-infectious milk. The bacteria causing the diseases conveyable by milk succumb at much lower temperatures than the boiling-point. Advantage is taken of this in the process known as "pasteurisation." By this method the milk is heated to 167–185° F. (75–85° C.). Such a temperature kills harmful microbes, because 75° C. is decidedly above their average thermal death-point, and yet the physical changes in the milk are practically nil, because 85° C. does not relatively approach the boiling-point. There is no fixed standard for pasteurisation, except that it must be above the thermal death-point of pathogenic bacteria, and yet below the boiling-point. As a matter of fact, 158° F. (70° C.) will kill all souring bacteria as well as disease-producing organisms found in milk. If the milk is kept at that temperature for ten or fifteen minutes, we say it has been "pasteurised." If it has been boiled, with or without pressure, for half an hour, we say it has been "sterilised." The only practical difference in the result is that sterilised milks have a better keeping quality than pasteurised, for the simple reason that in the latter some living germs have been unaffected.
Sterilisation may of course be carried out in a variety of modifications of the two chief ways above named. When the process is to be completed in one event an autoclave is used, in order to obtain increased pressure and a higher temperature. Milk so treated is physically changed in greater degree than in the slower process. The slow or intermittent method is, of course, based on Tyndall's discovery that actively growing bacteria are more easily killed than their spores. The first sterilisation kills the bacteria, but leaves their spores. By the time of the second application the spores have developed into bacteria, which in turn are killed before they can sporulate.