The complex cellular structure and chemical nature of fruit and vegetable tissues retard evaporation so that under no conditions of temperature and humidity does the rate of evaporation from them equal that from a free water surface. When conditions are such that surface evaporation from the tissues exceeds the rate of moisture diffusion to the surface, the surface becomes dry and hard and seals in the moisture. This condition, known as case‐hardening, is overcome by reducing the temperature of the air or by increasing the humidity. The maximum rate of drying, then, is attained by using the highest temperature which will not injure the product, the humidity being sufficient to prevent case‐hardening. In general practice the temperature of the air entering the drying chamber should not exceed 160° to 170° F. The humidity at the air‐outlet end of the drier should not greatly exceed 65 per cent. In driers employing recirculation the conditions of temperature and humidity may be largely controlled by varying the recirculation. The velocities of air flow which produce the most efficient results in the drying chamber depend upon several conditions. In general the rate of drying increases with the velocity of air movement. Low air velocities tend to bring about slow and uneven drying. Exceedingly high velocities may not be used profitably because a point is app ched at which the materials will be blown from the trays or at which the increased speed of drying will not offset the cost of operating a larger fan. Velocities of 600 to 800 feet per minute through the drying chamber are satisfactory in tunnel driers; lower velocities are permissible in compartment driers. The most practical means of removing moisture from the air, and at the same time conserving heat, is through the steady discharge of a portion of the air leaving the drying chamber. The rest dries efficiently when mixed with fresh air from the outside and reheated. All forced‐draught driers, therefore, should be provided with recirculation ducts connecting the air‐outlet end of the drying chamber with the heaters and with dampers controlling the air discharged, recirculated, and drawn from the outside. Dehydrated fruits and vegetables should have a uniform moisture content low enough to inhibit undesirable microbic and chemical changes within the food, and they should be free from any part of the life cycle of moths or other insects. The moisture content of dehydrated foods directly controls deterioration within the food, and the protection afforded by sulphuring or blanching will not prevent insufficiently dried products from soon becoming unfit for use. Dehydrated products having a low moisture content are not readily attacked by insects. In the long run the additional protection afforded by a low moisture content will more than make up to the producer the loss resulting from the longer drying time and greater weight shrinkage involved. To assure best keeping qualities the moisture content of fruits containing much sugar should not exceed 15 to 20 per cent., while that of other fruits and vegetables should not exceed 5 to 10 per cent., the preference in both cases being for the lower percentage. The texture, or feel, of products is a guide in determining when the proper stage of dryness has been reached. At a given moisture content products usually feel softer when hot than after they have been cooled, and often they feel softer after standing until the moisture has become evenly distributed throughout the pieces than when first cooled. A rough test for moisture in dried fruits is to take up a double handful, squeeze it tight into a ball, and release the pressure. If the fruit seems soft, mushy, or wet, and sticks together when the pressure is released, the moisture content is probably 25 per cent. or more. If the fruit is springy, and, when the pressure is released, separates in a few seconds to form pieces of approximately the original size and shape, the moisture content is usually about 20 to 25 per cent. If the fruit feels hard or horny and does not press together, falling apart promptly when the pressure is released, the moisture content is probably below 20 per cent. At the proper stage of dryness vegetables look thoroughly dry and are often hard or crisp. The Association of Official Agricultural Chemists has published a method for the determination of moisture in dried fruits. In using methods of this type, care must be taken to select a composite sample from different parts of the lot, so that it will be representative of the lot as a whole, and directions for preparing the sample must be carefully followed in order to obtain dependable results. Products are never uniformly dry when removed from the drier. Large pieces and pieces not as directly exposed to the currents of heated air as most of the material contain more moisture than the rest. Products should be stored in large bins until the moisture becomes evenly distributed. This period of curing will usually take several weeks. An alternative method is to place the dried product in large friction‐top cans for curing, thus insuring complete protection from contamination and insect infestation. Leafy vegetables, like spinach, must remain in the drier until the moisture content of the stems is very low. At this point the product is bulky and the leaves are brittle. For economy in packing and handling it is desirable to reduce the bulk by compression. For this purpose the leaves are exposed to currents of cool damp air until they have reabsorbed just enough moisture to make them slightly flexible. For convenience in handling and to facilitate the application of heat or fumigation, products should be packed in the room where they were cured and stored. Such a room should be strictly clean, dry, cool and well ventilated. The doors should fit tightly, and the windows should be covered with fine‐mesh screen to exclude dust and insects. An abundance of light assists in detecting the presence of insects and in keeping the room clean. The types of containers chosen for packing will depend largely upon the severity of the storage conditions, with particular reference to the humidity and to chances of insect infestation. An ideal container would be one which, while moderate in cost, would keep the product from absorbing moisture when exposed to the most severe conditions of storage and shipment, and would be impervious to insects. Sealed tin cans and glass jars give absolute protection against moisture absorption and insect infestation. Friction‐top cans are nearly as good. Tin containers, necessary for export shipments of dehydrated foods, are more expensive than paper containers. Wooden boxes are generally used for bulk goods. Liners of heavy paper or cardboard, and sometimes additional liners of waxed paper, are used. The use of moisture‐proof cellophane packages is increasing. All types of paper containers with which experiments have been made allow the absorption of moisture when the products are stored in damp places. Also paper containers do not give perfect protection against all insects, some of which can bore holes in paper, while the larval forms of others are so small that they can crawl through the slightest imperfections at the joints where the cartons are sealed. Most products, however, keep satisfactorily in double or triple moisture‐proof cellophane or waxed‐paper bags packed in waxed, moisture‐proof cartons, provided the initial moisture content is low and no live insects in any form enter the package. Packing in insect‐proof and moisture‐proof packages cannot be too greatly stressed.
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