Manufacturers show a growing tendency to remove the skin from all fruits and vegetables before drying. Because of custom or the type of skin, some kinds, notably prunes and apricots, however, are rarely peeled. Peeling may be done by hand or by specially designed machines. Many types of knives, with straight, curved or guarded blades, and handoperated cutting machines are obtainable for peeling, trimming, coring and otherwise preparing the material to be dried. Machines for peeling and coring both apples and pears in one operation are available. Friction or rotary mechanical peelers are particularly well suited for handling roots and tubers. All peelers of this type depend upon the rasping effect of rough surfaces of cement, corundum, etc., forming some part of the lining of the peeler, when the product is rotated rapidly within the cylinder by a moving bottom. The material is introduced at the top and discharged by a side door. These machines are usually equipped with water sprays, which wash off the dirt and the particles of skin removed by the peeler. Other types make use of an open flame which chars the skin so that it can be brushed or washed off. Several types of lye peelers are available. All depend upon immersion in or spraying with hot 190200 F. lye solution. The length of treatment must be determined for each batch of fruit. It should be long enough to permit the ready removal of skin by water sprays or by rubbing, but must not injure the flesh of the product being peeled. Following the peeling, the fruit or vegetable must be inspected and all remaining skin removed with trimming knives, especially adapted to different products. Lye is used to check the skins of prunes and grapes in order to facilitate drying. The fruit is in contact with the hot lye solution long enough to break the skin by many minute fissures or checks, but not long enough to loosen it. The concentration and temperature of the lye bath are similar to those for lye peeling. After the lye treatment the fruit is carefully washed to remove all traces of the lye bath before further processing. Fruits are sliced, cubed, shredded, or left whole. Vegetables are sliced, cubed, shredded, or chipped. The cutting is done by hand or by some one of the numerous machines on the market. Some of the machines consist essentially of rotating knives or cutting surfaces operated by hand or by power. In others the cutting surfaces are stationary and the product is forced against the blades. Special machines for cutting beans are made. In the most satisfactory type of cubing machine the slices are cut, carried to a die, and forced through by a plunger. Manufacturers find it desirable to pit or seed stone fruits, as pitted or seeded fruits dry more rapidly and sell more readily than those from which the pits or seeds are not removed. Machines for pitting, seeding and paring most fruits are for sale by foodmachinery manufacturers, but the greater part of dried fruits are pitted by hand. After the raw material is prepared in the desired form, no time should elapse before traying and subsequent treatments preliminary to its being placed in the drier. No definite rule can be given for determining the quantity of material to be placed on a unit area of tray surface. Experience will soon show the operators how much will insure even, rapid drying. Many of the larger fruits must be trayed only one layer deep. Overloading trays must be avoided. Trays should be light but capable of withstanding strain, and they should permit a maximum exposure of the materials. Trays of the type most often seen have a spreading surface formed either of wire screen or wooden slats held firmly in a narrow but rigid wooden or metal frame. Many of the trays now in use have been employed in sun or other drying. If new trays are made, the oneman tray, about 2 to 3 feet square, will be found to be the most convenient. Woodenslat trays can be more cheaply constructed than wirescreen trays, but they have a high rate of upkeep. These trays, however, are not affected by sulphur fumes or fruit acids, for which reason they are preferred for most fruits. The trucks for conveying loaded trays are of two general types. In one a skeleton frame is provided with cleats, upon which the trays rest. The cleats are from 2 to 4 inches from centre to centre above one another. These trucks are made of various combinations of wood or angle iron. The other, which may be called the stack type, has a low floor supported by wheels 3 to 5 inches in diameter. The trays are piled or stacked one above the other, and the desired space between them is maintained by the raised sides of the trays. Various alterations may be made for convenience in handling and loading. The original method of preparing fruits and vegetables for drying consisted in washing, peeling when desirable, cutting, and traying. By this method most darkcoloured materials made fairly presentable dried foods, but lightcoloured fruits and vegetables did not. The attempt to overcome this difficulty led to the introduction of blanching, or processing, and sulphuring. The blanching, or processing, agent is usually steam or hot water, which helps the product to retain its natural colour. In the steam treatment the material is subjected to live steam for the required period. In blanching by hot water, the temperature is maintained at 190 F. to the boiling point, depending on the material being treated. As a rule, steam blanching is preferred to blanching in liquid, because the loss of soluble constituents of the food is less, a better flavoured product results, and the use of steam is ordinarily more convenient. Lightcoloured fruits apricots, peaches, pears, and at times grapes and figs are sulphured in order to prevent discolouration during and after drying and to facilitate drying. Sulphuring plasmolyzes the cells and makes permeable the semipermeable cell membrane, thus facilitating the diffusion of water from the interior to the surface. When the general plan of operation makes it desirable, the fruit on trays is sulphured in an enclosed chamber, provided with an entrance for the sulphur gas and an exit for a draught. The chamber is usually large enough to hold one to two loaded trucks. Preferably the sulphur is burned in shallow pans stacked one above the other in zigzag formation. This method gives a large quantity of sulphur dioxide in a comparatively short time. Sometimes a sulphur stove is placed outside the chamber and the sulphur fumes are carried into the chamber by flues. Sulphuring should always be as light as possible to accomplish the desired results. The type of equipment best adapted to any particular use depends upon several factors. If the products are to be dried for home or farm use, then the equipment should be as simple as possible. If the dried material is to be prepared in large quantities for sale to the public, then the type of equipment will depend to a great extent upon the nature of the product desired. To meet these needs many devices have been originated and patented, so that all phases of drying are well covered. Materials can be dried more rapidly and at lower temperatures in vacuum than at atmospheric pressure. Such foods as are extremely susceptible to damage by heat are more safely dried in vacuum. However, vacuum driers are seldom used in largescale operations on the usual commercial grades of dehydrated fruits and vegetables. The apparatus is too expensive and usually the advantages gained are not sufficient to compensate for the added cost of equipment and operation. Enzymes are not inactivated by evaporation alone, and it will be found desirable in many cases to inactivate them by blanching or by other means if vacuum drying is used.
British Food Journal – Emerald Publishing
Published: Sep 1, 1942