CHAPTER VII.

ON PRESERVED FOOD.

I PROPOSE in this chapter to allude shortly to the various methods employed for preserving food, and to show the relative value of these articles of diet.

The spontaneous decomposition of food, known as fermentation and putrefaction, is owing to the action of heat, air, and moisture; and in order to prevent this phenomenon, it will be necessary to remove one or more of the causes on which it depends. There are, moreover, substances called antiseptic agents, endowed with the property of preventing the decomposition of organic matters; which, when perfectly harmless to health, may be employed with advantage to preserve certain articles of food.

1. Preservation of food by cold.-It is probably a fact well known to the reader, that frozen organic matters do not undergo decomposition. Meat and fish are thus transmitted for sale from Archangel to St. Petersburg;1 provisions are also sent, packed in ice from remote parts of Britain to London, &c.; the application of a lesser degree

1 Lecture delivered at the Royal Institution, March the 30th, 1855, by the Rev. J. Barlow.

of cold is also extensively used to prevent too rapid a decomposition of food in hot weather, as in the case of fish, butter, &c. I have nothing to add on the state of food thus preserved; if it has not undergone the slightest decomposition, it is exactly in the same condition as when quite fresh, and consequently, as healthy and nutritive.

2. Preservation by exclusion of air.-There is no doubt that if an article of food be so prepared, the atmosphere can have no access whatever to it, this food will preserve for a very considerable length of time; the practical objection attending this method is the difficulty of preventing completely the contact of the atmosphere with organic substances. Articles of food intended for preservation, by exclusion of air, may be fluid or solid; if fluid, they are first to be thoroughly boiled, in order to expel the air they may contain dissolved within them, and next to be introduced through a small opening into metallic or earthenware vessels; when the vessel is overflowing, the opening must be soldered or stopped up; it is difficult, however, to prevent a very small quantity of air from being enclosed in the vessel. Such liquids, when kept in metal jars, may dissolve small quantities of the metal of which the vessel consists, and thereby acquire a very objectionable character.

Food is preserved on a very large scale by the exclusion of air and destruction of the small amount of oxygen contained in the jar; this is known as Appert's method. For this purpose, the jars, or boxes of tin plate, containing the article for preservation, are thoroughly corked or soldered, care being taken that the vessel be quite full, they are afterwards immersed in boiling water for a certain length of time; the air left in the jar is thereby de

stroyed, its oxygen being made to combine with some of the carbon of the substance, and, consequently, the contents of the vessel lose the power of decomposing. Ships of the English navy, on the coast of Africa, East Indies, West Indies, and China, where meat is not procurable, are thus provided with fresh provisions. Other methods are adopted for preserving articles of food by exclusion of air; thus an attempt has been made to preserve meat by surrounding it with a thick coating of gelatine; eggs are also prevented from decomposing by greasing and dipping them in milk of lime; Warrington has proposed a plan founded upon similar principles to preserve the flesh of buffaloes.1

There appears no objection to this method, beyond the difficulty of preventing the access of air to the article, and the necessity of inducing certain chemical changes by boiling the food; there is also a possibility that the metal of the cases might be dissolved to a slight extent by the preserved articles, the more so that juice of meat has an acid reaction. The method in question is objectionable especially from its expense. I am not aware that vegetable and animal food preserved by Appert's method is liable to adulteration.

3. Preservation by drying.-Articles of food are preserved by direct drying, with or without the application of artificial heat. Organic substances dried without being previously cooked are not likely to remain sound beyond a short period, and in moist and damp weather run the risk of decomposing; but if the articles have been cooked, or rather steamed at a high temperature, the albumen of the food is rapidly coagulated, enclosing in a hard covering, and thereby excluding from air, the other constituents of the substance. Vegetable and animal food

1 Potted meats are preserved on the same principle, air being carefully excluded by a tolerably thick layer of suet or lard.

thus preserved is found to keep very well for many years, and this method, both in a sanitary and practical point of view, has been found unobjectionable; we are indebted to Dr. F. Verdeil for this ingenious plan of preventing food from decomposing; and the English and French army, during the Crimean campaign, has been extensively supplied with vegetables preserved by his process.

My friend, Dr. Verdeil, having kindly showed me every stage of the operation in the extensive factory belonging to his company, at Paris, where he is the chief superintendent of the chemical and mechanical department, I shall enter into some details as to the various operations I

witnessed.

The vegetables intended for preservation in the Paris factory are first cut into small pieces, mostly by machinery, and then transferred to a number of shallow wirework trays. When a set of these trays is full, they are placed on a stand, which moves on a little railway, and can be pushed into an iron chest; a contrivance not unlike that employed in washhouses to convey wet linen into the drying chambers. These iron chests may be filled with steam, under a pressure of four or five atmospheres; and as soon as the stand is rolled in, the door is shut, hermetically fastened by means of a screw, and then steam freely admitted; by this means a temperature is rapidly obtained, sufficient to coagulate completely all the albumen of the vegetable, none of the constituents of the food being lost; five minutes suffice for this operation. The next step is the drying. The steamed vegetable is rapidly conveyed on the trays into a series of wooden chambers, where a strong draught is obtained by means of ventilators revolving very rapidly with steam power; the air

admitted into the drying chambers is made to pass first through iron pipes, maintained at a very high temperature, by a furnace, so that the desiccation is effected by hot air; the whole process takes so short a time, that from five to eight hours suffice to dry a large quantity of vegetables; they are afterwards set aside, ready for the market.

Food thus preserved, whether it be animal or vegetable, has the advantage

1. Of preserving in a fresh condition, though freely exposed to the atmosphere, for a great number of years.

2. Of being reduced to about one fifth of its original bulk, from its having lost all its water.

It is very remarkable how completely the prepared vegetables resume their shape and bulk when boiled in water; the soup possesses all the aroma of the food, to such an extent, that it is often difficult to notice the difference between these soups, and those prepared from recently gathered vegetables.

Food is, moreover, preserved by the peculiar action of certain substances, called antiseptic agents, which, when mixed with articles of diet, enable them to resist the decomposition induced by heat, air, and moisture. The principal of these methods are salting, smoking, and immersing in vinegar.

4. Preservation by salting.-This process is used more especially for preserving meat. The meat is cut into large pieces, and left in a tub for a certain number of days, covered with a thick layer of salt; the result of this operation is the extraction from the meat of a considerable amount of its juice, in which the salt becomes dissolved: this same brine is then removed from the tub and employed to salt another sample of meat, and so on until it has become so