For the preservation of grain no further precautions are necessary beyond gathering it when ripe, and keeping it dry.
The simplest form of desiccation is by ordinary sun- and wind-drying, as conducted in hay-making. The next step is by radiated sun-heat, as in coffee-drying; a farther advance is made by the application of artificial heat, as in hop-drying and tea-drying. The primary object in all these cases is the removal of the water mechanically present, and without whose presence fungoid growths and decay cannot exist. As a curative agent simply, the application of heat is, however, unnecessary and injurious, causing a partial destruction of the flavour, and more or less fermentative change. Research has proved that between the limits of 32° and 60° F. (0° and 15° C.) vegetable substances retain their flavour and all other qualities, while giving up their moisture, no fermentative action being engendered. This has led to the adoption of the Cold-Blast System
The fruit or vegetables are deprived of moisture by subjection to dried air at a low temperature. The air is compressed in a chamber containing chloride of calcium, or any other compound possessing strong dehydrating qualities. Chloride of calcium is in practice probably the best, as it so readily gives up the absorbed water on being heated. The compressed and dried air is then admitted into a chamber containing the substances to be treated. The expansion lowers its temperature somewhat, which should be maintained between 32° and 60° F. (0° and 15° C). The substances are distributed throughout this chamber on perforated trays, so as to be fully exposed to the current of cold dry air passing through. All the moisture is thus removed, without the least detriment to the flavour, colour, and other virtues of the substance acted upon. The process has a great advantage over hot-drying, both in the cost entailed and the result achieved. Fruit and vegetables thus prepared, and packed with ordinary care, remain good for an indefinite period, and resume their natural shape and dimensions when placed in water.
(1) Great quantities of vegetables continue to be prepared by this process, which has been in use for some time by Whitehead and other well-known firms. A common method of conducting the operation is as follows: - The fruit or vegetable is pared and cored, if necessary, and then finely shredded. The shreds are spread on galvanized-iron wire screens in the evaporator, a 3-storeyed chamber, through which passes a current of air heated to 240° F. (116° C). The screens rest on endless chains, that move upwards at intervals of 3 to 5 minutes, when a fresh screen is put on below, and a finished one is taken off at the top. The evaporation is very rapid. The cores and peelings of apples, etc, are made into vinegar. (2) Another plan is by means of a vacuum-pan, heated to 120° to 170° F. (49° to 77° C). The air is dried by passage over chloride of calcium. The operation occupies 20 minutes.
Vegetables are submitted for a few minutes to steam at 70 lb. a sq. in., then dried by air at 212° F. (100° C), subjected to hydraulic pressure so as to form tablets, and, when required for use, are soaked in cold water for 5 hours.
The potatoes are peeled and cut into discs, and are scalded by immersion in nearly boiling water. They are then dried hard in an oven. To preserve the white colour, they are treated with water acidulated with 1 per cent, of sulphuric acid. They are then washed in cold water, and dried.
For preserving potatoes in store, the floor is sprinkled with fine quick-lime; this is covered with a layer (4 to 5 in. thick) of potatoes; this by a sprinkling of quick-lime again, and so on, using the lime in the proportion of about 1 measure to 40 measures of potatoes. This method checks disease when it is present, and improves the potatoes if they are watery or waxy. Layers of straw and powdered plaster of Paris may be substituted for the lime.
Sacc's process for preserving vegetables is as follows: - The vegetables are warmed to destroy their rigidity, and are then packed in barrels, and surrounded with 1/4 their weight of acetate of soda in powder, by which their moisture is absorbed. In summer the action is immediate; but in winter it may be necessary to put the barrels into a room heated to 68° F. (20° C). After 24 hours, the vegetables are removed, and kept in a dry atmosphere. For use, they are soaked in cold water for 12 hours.
The preservation of vegetables by cooking them in sealed cases is dependent upon the destruction of all organic germs by the heat of the boiling and the perfect exclusion of air. An example of the simplest form is the canning of tomatoes. The fruits are scalded to loosen the skin, and then dipped in sieves into wat«r, heated by injection of steam, for 1/2 minute. They are then skinned, and picked over, and passed into the steamer. Thence they fall into the hopper, and are fed by the "stuffer," a cylinder worked by a treadle, into the cans. The filling of these is adjusted by boys, and they arc sealed up. The cans are then boiled for 2 hours, then partially cooled, the air is let out by a pin-hole, and they are immediately soldered up, and the cooling is completed.
Many other vegetables are canned in a similar manner. Those which have a green colour lose it during the operation, by the destruction of the chloro-phyl. The same remark applies to those dried by heat. The green colour may be replaced by adding a solution of chlorophyl, exhausted from other plants; or the natural colour may be retained by treatment with alkaline earths, according to Possoz, Biardot, and Lecuyer.
In pickling vegetable substances, advantage is taken of the curative properties of acids, alcohol, sugar, saltpetre, salt, etc.
Curing by means of acids, as acetic acid, vinegar, etc, is the process commonly known as " pickling." In the ordinary way, the vegetables are kept soaking for a long time in brine, and are then pickled by acetic acid. An improved method, by which months of time are saved, is to exhaust them under an air-pump, and then to force in spiced vinegar under a pressure of 45 lb. per sq. in.
This is too expensive for commercial purposes. An example is the preservation of cherries in brandy.