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boiling point, and yet above the thermal death point of lactic and pathogenic germs. The spores and germs not so killed are checked in their growth by the rapid cooling, which is an important part of the process.

Pasteurised milk cannot be distinguished from fresh milk, which it closely resembles in flavour and nutritive value. It can be preserved for long periods, and is not so liable to give rise to diarrhæal diseases.

Pasteurisation has also been resorted to in the case of wines, cream, etc., and with marked success.

Meat.-It has already been remarked that the putrefaction of dead animals is initiated by the saprophytes of the alimentary canal, which migrate from their normal habitat and invade the tissues in all directions. If, therefore, the bowels of the animal be completely excised as soon as it is slaughtered, it would be possible to delay the decomposition for some time. This is precisely what the butcher does every day; but without understanding the rationale of his method.

To prevent further putrefaction, the disembowelled carcass is placed in a refrigerating chamber, and in this frozen state can be transported to long distances. This method of “cold storage” is largely employed in Australia, and has proved eminently satisfactory to the public and capitalist alike.

The smoking of meat is also a reliable means of preserving it. The smoke of heated wood chips is rich in phenol and creasote, which are deposited on the flesh and prevent putrefaction.

But the best method of preserving meat consists in heating it to a temperature sufficient to destroy all bacteria and their spores, and then hermetically sealing the vessels. This is essentially the process used by Appert long ago, and has led to the growth of a gigantic industry.

Eggs.--The contents of fresh-laid eggs are not always germ-free, as bacteria are known to enter the oviduct and infect the albumin before the shell is deposited. Again, the shell is porous, and allows bacteria and oxygen to pass through it.

This gas is essential for the development of the embryo, and also for the organisms already contained in the egg.

The methods of preservation aim at making the shell impervious to the passage of bacteria and oxygen. This is accomplished by coating them with vaselin, or "water glass,” a material composed of sodium and potassium silicates.

Vegetables and Fruits.-Bacteria require a preponderance of water for their growth, and they cease to grow in substances in which this element is lacking. It is for this reason that fruits and vegetables dried by the sun's rays or artificial heat can be preserved for comparatively long periods.

In the case of substances like green peas, which are apt to lose their flavour by desiccation, recourse is had to Appert's process.

In the preparation of jams and marmalades, the fruits are boiled, sugar added, and the whole carefully packed in sterile glass jars. Sugar in itself is an excellent food, and not only exalts the nutritive value of these substances, but at the same time plasmolyses the bacteria, so that they are no longer capable of setting up decomposition. But in order to be effective it must be used in very strong solutions, otherwise it is apt to undergo alcoholic fermentation.

1 By“ plasmolysis” is meant a condition of the cell in which its protoplasm shrinks from the cell wall and aggregates into little

masses.

ᏢᎪᎡᎢ II.

CHAPTER IX.

BACTERIA IN DISEASE.

BEFORE considering the relations of bacteria to disease it would be as well to refer briefly to the organisms which are normally parasitic in man. They are present on the skin, in the respiratory passages, and in the digestive canal. Of these by far the largest number are met with in the intestines. Formerly it was supposed that bacteria were essential to normal digestion ; but it is now recognised that their presence is by no means of any advantage to the host. On the other hand, they may be distinctly harmful, for they are liable to multiply whenever the defensive forces of the organism are diminished. There can be no doubt that many forms of headache and anæmia are due to auto-intoxication from the poisonous secretions of these microbes. Cirrhosis of the liver has been produced in animals by the aid of acetic and butyric acids, which are the normal products of our intestinal bacteria. It may also be that these organisms, by devitalising the mucous membrane, prepare the

way

for the cholera germs which are thus enabled to gain a foothold in the intestines.

The microbic theory of disease, although formulated by Henle in 1840, has only been demonstrated within the last few decades. Pasteur's researches on butyric fermentation led him to note the marked resemblance between the processes of fermentation and disease, and

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