At one American establishment, apples are pared, cored, and sliced at once by hand machinery. The slices are then spread on galvanised screens and placed in the evaporator, a chamber running from the top of a large furnace in the basement upward, out through the roof of a 3-story building. The current of heated air is kept as near as possible to 240° F. (116° C). The screens of fruit rest on endless chains that move upwards at intervals of 3 to 5 minutes, when a fresh screen is put in below and one is taken off at the third story completed. The dried or evaporated produce is then packed in pasteboard boxes holding 1-5 lb., and these in turn are packed in cases of 200 lb. each.
A bushel of apples makes about 5 lb. of the dried fruit; and the process of evaporation is so rapid that the fruit loses none of its freshness and flavour. In some of the factories the cores and peelings are converted into vinegar; in others into apple jelly, out of which every variety of fruit jelly is made by the addition of flavouring extracts. Sweet corn, potatoes, and other vegetables have been successfully preserved by this process.
In properly evaporated fruit there is no loss of pleasant or valuable properties, but an actual increase of fruit sugar, from the fact that evaporation is essentially a ripening process, the development of sugar ranging from 10 to 25 per cent. in different fruits, as determined by chemical analysis. By the process of evaporation, properly conducted, in a few hours the juices are quickly matured, the maximum development of sugar is secured, and water pure and simple is evaporated, the change being analogous to the transition of the grape to the sweeter raisin, or the acid green apple to ripeness, with corresponding delicacy. The cell structure remains unbroken, and the articles, when placed in the rejuvenating bath of fresh water, return to their original form, colour, and consistency.
In evaporating cut fruits, such as apples, pears, and peaches, the correct method is to subject them to currents of dry heated air, so as to dry the cut surfaces quickly, preventing discoloration, forming an artificial skin or covering, and hermetically sealing the cells containing acid and starch, which yield glucose or fruit sugar. This principle is demonstrated in Nature's laboratory, in the curing of the raisin, fig, and date, which are dried in their natural skins - a process not applicable to cut fruits - in a tropical climate, during the rainless season, by natural, dry, hot air, in the sun; though a crude and slow process, the development of glucose or grape sugar is almost perfect.
A practical, economical, and inexpensive fruit drier is made by the American Manufacturing Company. In this evaporator separate currents of dry, heated air, automatically created, pass underneath and diagonally through the trays and then off and over them, carrying the moisture out of the evaporator, without coming in contact with the trays of fruit previously entered, and already in an advanced stage of completion. The greatest heat is concentrated upon each tray when it first enters the machine; and each tray subsequently entered pushes the previous one forward into a lower temperature. This operation is continued throughout, being rendered perfectly practicable by an inclined, divided evaporating trunk.
It is stated that about a third of the tea produced on Indian estates is cured in Davidson's so-called "sirocco" drying-closets. These are made in 2 forms, one having the fire-place adapted for wood, coal, bamboo, ekur grass, or similar fuel, the other suited only for a smokeless combustible, such as coke or charcoal. In the first-named from the furnace of the air-heater is lined with fireclay tiles, and is so arranged that when, from accidental breakage, any of the tiles require changing, this can be easily done through the fire-door, without having to take down any part of the apparatus. All the other parts of the air-heater are very durable, and not liable to breakage.
An improvement has been applied in the construction of the top of the air-chimney, which has completely obviated the necessity for any rain hood or cover (all of which had a harmful effect on the draught). The base of the air-chimney sits on an annular collar, so constructed that it collects and discharges, wherever required, any water which, on rainy days, may come down the chimney, either from a bad joint where it passes through the tea-house roof, or from condensation of vapour inside the chimney. This water collar is applicable to already existing "siroccos."
The trays are made from a special wood, which, after having carefully tested many qualities, has been selected as being the best suited to stand, without warping or twisting, the very high temperatures to which they are subjected; they are strongly put together, being brass-bound at the 4 corners, and the wooden battens screwed together with bolts and nuts, by which they can, if necessary, be tightened up from time to time.
With the exception of the trays, the closet is entirely constructed of iron, and can be readily put together and worked, even by an inexperienced person. It requires no motive power to create the draught, which is self-acting. When erected, it occupies a floor space of about 5 ft. by 4 ft., and is 10 ft. in height from base to where the air-chimney joins.
This apparatus is capable of drying 20-25 maunds (1 maund = 80 lb.) of green leaf per day of 10 hours, with 6-8 maunds of dry wood, or 2 1/2-3 1/2 maunds of coal fuel.
The second form differs only in having a brick-built fire-place.
With either form there should be a wooden platform for the man working it to stand upon, and a table for the trays to be rested on when the material is being spread upon them. One end of this platform should project underneath the apron tray upon the front of the drying-box, and should nearly touch the side of the air-heater casing. The dimensions of this table and platform should be as follows: -
Table, length, 8 ft.; width, 4 ft.; height, 6 ft. Platform, length, 8 ft.; width, 3 ft. 8 in.; height, 3 ft. 6 in. Stair to platform, with 2 steps, length, 8 ft.; width of each step, 1 ft.; height of each step, 1 ft. 2 in.
Of course the apparatus might he sank into the ground sufficiently to avoid having to use a platform at all; but in that case an open space of at least 1 ft. in width would have to be left on each side for admitting the cold air to the air-heater, and a stoke-hole opposite the fire would be necessary, as well as a similar place on the other side to get at the door for cleaning the chimney. A large excavation like this, however, is obstructive and generally objectionable in the floor of a tea house, and the makers prefer and recommend instead of it the use of the platform and high table, with the apparatus set on ground level, as hereinafter described. Should the apparatus be sunk into the ground, the sides of the excavation will require walls to prevent the earth from falling in; but if it be placed upon the ground level, no brickwork will be required.
It has occasionally been observed that the working of driers is more or less influenced by their situation in the tea house, and that it is desirable, when practicable, to have the apparatus placed on the opposite side of the house to the prevailing winds - i. e. as the winds prevail from S.W. it is well to have the driers situated on the N.E. side of the tea house, otherwise there may be a tendency to a down draught upon the chimney, which would materially affect the outturn of work; whereas, if placed on the opposite side, the tendency is just reversed, and the draught up through the apparatus is increased by the wind pressure.
Air-holes should always be kept open on the wind side (S.W.) of the house to admit the outer air freely, because each drier (when properly working) is continuously drawing away about 30,000 cub. ft. of air per hour from the house, and if the apparatus has to overcome any friction or resistance in obtaining this air supply, the outturn of work will be proportionally reduced.
It may be here stated as a general rale that (provided the proper working temperature of the drier is maintained) the better the draught of the apparatus, the larger will be the outturn of work; and, consequently, everything which tends to accelerate this draught should be observed and taken advantage of.
Further details respecting this apparatus may be obtained of the makers, Davidson & Co., Queensbridge Street, Belfast. It is obviously adapted to the drying of many other vegetable product.