1. Sterilisation. 2. Pasteurisation. 3. "Humanised milk." 4. Modified milk. Milk laboratories.

1. Sterilised Milk

The sterilisation of milk is accomplished by heating it up to the boiling point, 2120 F. In a vacuum this may be accomplished by a temperature a few degrees lower.

The construction of steam sterilisers for milk, beef tea, or other foods is based upon a very simple plan. It is a familiar fact that the steam which rises from water heated in a tin or copper can condenses in drops on the lid, and this condensed vapour drips back along the sides of the can. If the lid is raised a little and is made somewhat larger than the can, the condensed steam will drip down on the outside of the can instead of the inside. If a second larger can be inverted like a hood or jacket over the first, the steam condensed between the two cans drips back and may be made to rejoin the water from which it started. In this way a kind of perpetual motion is kept up so long as a flame is applied beneath the water to vapourise it. Bottles containing the food to be sterilised are suspended in racks in the upper part of the inner chamber in the atmosphere of steam, and their contents soon acquire the temperature of boiling water, 212° F.

Siebert removes the grosser impurities of milk by placing it on a thick layer of sterilised absorbent cotton in a clean glass funnel. A quart of milk will pass through in ten or fiteen minutes.

Mobrun preserves milk for six months by warming it in a tin receptacle, the only opening of which is through a lead tube. The milk is rendered sterile by the heat, arid all air is driven out through the tube, which is then compressed and soldered.

The taste of sterilised milk is peculiar and resembles that of boiled milk. If put in bottles which have been sterilised by boiling water or steam, by stopping them with pledgets of absorbent cotton which have been baked, the milk will keep fresh for a number of days - long enough to be carried upon a voyage to England or across the continent.

Milk drawn by clean hands fyssh from the cow into bottles which have been sterilised by boiling water is germ-free, and need not be further treated; but the milk usually served in cities from large cans which have been hawked about the streets has passed through several receptacles and been exposed to the air. (See Milk Adulteration and Impurities, p. 61.) It is better always to keep milk germ-free, but it is absolutely necessary to do so in summer if it is to be fed to infants. In 1899 sterilising apparatus was introduced at the Children's Hospital, Randall's Island, New York city, with the immediate effect of reducing the annual infant death rate from 44.36 to 19.86. It is often advisable to sterilise milk for feeding to typhoid-fever patients and others in whom asepsis of the alimentary canal is of paramount importance. If milk be too long sterilised it becomes of a brownish hue, owing to the conversion of its lactose into caramel.

It is suggested by Barlow that prolonged sterilisation of milk may lessen its antiscorbutic powers for young infants, but scurvy in infants is due also to other causes, such as feeding with artificial foods, and prolonged exclusive plain milk diet for adults reduces them to a condition resembling scurvy. (See Typhoid Fever, Milk Diet).

According to Leeds, the following alterations are produced in milk by sterilisation at 2120 F.:

1. The amylolytic ferment is destroyed.

2. The casein coagulates less readily by rennin.

3. The digestibility of casein by the gastric and pancreatic juices is somewhat retarded.

4. The fat is less promptly absorbed than normally from the intestine.

5. If the heating is continued for some time, the milk sugar is destroyed.

The mineral salts of milk are also dissociated in some degree from their organic compounds.

It is thus demonstrated that the chief change produced by sterilisation of milk, and even by Pasteurisation at 1670 F., is destruction of the enzymes or organic ferments of which this highly complex fluid contains a number. Babcock and Russell of Wisconsin have shown that when milk is rendered aseptic by salicylic acid, sodium fluoride or other germicides, a process of self-digestion goes on, which increases the longer the milk is kept. The result of this natural process is the formation of albumoses at the expense of casein, and the action is due to a ferment which these experimenters named "galactase," resembling trypsin.

Another ferment, an oxydase or anaeroxydase, has been demonstrated in cow's milk by a number of French experimenters. This ferment, as its name implies, causes a peculiar reaction in milk towards oxidising agents, such as turpentine or tincture of guars*

Spolverini identified a new ferment in cow's milk which resembles closely the glycolytic ferment of the blood.

A ferment, called by Bourquelot "lipase," has been identified in both human and cow's milk; its action is hydrolytic, resolving monobutyrin into glycerin and butyric acid.

In churning sterilised milk, butter forms more slowly than from raw milk, a period two or three times longer being required. Hirsch attributes this to toughening of the albuminoid envelopes (of lactalbumin) of the fat globules, which is produced by the heat.

For these several reasons an infant to be fully nourished on sterilised milk requires more of it than of raw milk.

Practically, in normal stomachs of either infants or adults these changes are not sufficiently pronounced to seriously interfere with the digestibility of the milk, but dyspeptic and catarrhal conditions of the stomach, especially in infants, make it highly sensitive to very slight modifications in the composition and reaction of milk.

In quite recent years it has been proved that the method of Pasteurisation is preferable to sterilisation (see 2. Pasteurised Milk, p. 88), but there is, at the present time, a decided reaction against the prolonged use of both these types of milk, in distinction from temporary emergency use, and von Behring in common with many American clinicians maintains that when mother's milk is unprocurable, the proper infant food is good, fresh, clean milk, not heated or otherwise modified than by necessary dilution or sweetening, von Behring states that fresh milk contains powerful antibodies which react against the bacillus acidi lactici and colon bacillus. As these are the two types of bacilli commonly present in the infant organism, and as the heat of sterilisation or Pasteurisation promptly destroys the antibodies, by the use of heated milk the infant is deprived of a protective agent against gastro-intestinal disorder.

T. M. Price in 1904 conducted a series of experiments at the Maryland Agricultural Experiment Station to determine the comparative digestibility in the calf of raw, Pasteurised, and sterilised milk. The experiments were decidedly in favour of the raw milk.

After all, in selecting a proper milk for infant feeding, the question of immediate digestibility is less important than that of permanent effect upon nutrition. The difficulty with infant feeding among the poor is to obtain really good, fresh, raw milk; hence in a given case the choice of feeding must ultimately depend upon the character of raw milk obtainable. When this is first-class the relative digestibility, as well as relative permanent nutritive value, stands as follows in order of preference: (1) raw, (2) Pasteurised, (3) sterilised.

The directions for predigestion combined with sterilisation of milk at the Philadelphia Hospital are thus given by Hirst:

"I. Have the nursing bottles prepared clean every morning.

"2. Take cream, five ounces; milk, two and a half ounces.

"3. Put in skillet; add pancreatin powder (pancreatih, two and a half grains; sodium bicarbonate, five grains); heat over alcohol flame for six minutes; stir and sip constantly; do not overheat.

" 4. Of this mixture put in each bottle six drachms (for a two-ounce bottle). Use funnel.

"5. Add to each bottle ten drachms of sugar solution. (Make sugar solution by dissolving an ounce of sugar of milk (one powder) in a pint of warm water).

" 6. Stopper the mouth of each bottle with dry baked cotton and sterilise for twenty minutes.

"7. Set aside to cool.

"8. Before use put bottle in warming cup; apply nipple immediately before giving it to infant".

The alimentary canal of the newborn infant is sterile - it contains no bacteria - but after the first few passages of meconium two species of bacilli and one of micrococci are found (Escherich, Bes-lau), which disappear with the meconium. There are but two species of bacteria found in the stools of healthy nursing infants, the Bacterium lactis aerogenes, obtained from the ileum below the duodenum, and Bacillus coli communis, from the colon.

In diarrhoeal conditions other bacteria, to the number of forty species, have been found (Booker). Their action in different diseases is not yet differentiated, but they are associated more or less directly with malfermentation and the formation within the intestines of gases and irritable or poisonous products. Many of these species thrive and are reproduced in hot weather outside the body in milk, in the atmosphere, and upon diapers and utensils. This explains the importance of absolute aseptic cleanliness in everything that pertains to the nursing and feeding of infants - cleanliness of the breasts and nipples, of the nurse's hands, of all receptacles for milk or other foods, of the child's body, and prompt removal and disinfection of soiled diapers or clothing.

Most of these bacteria grow best in milk, and the diarrhoeal diseases which are caused by and associated with them are absolutely preventable when the methods are understood of avoiding milk infection, and of sterilisation when it is infected. Vaughan says with proper emphasis: " If parents were willing to pay for wholesome uninfected milk half the fancy price which they readily give for some prepared baby food, their children would be better nourished and disease among them would be less frequent".