Storage and refrigeration
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| Standard Cyclopedia of Horticulture |
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Storage and refrigeration of fruits and vegetables. The storage house has become a very important adjunct to fruit-growing. In fact, fruit-growing would not be possible in present-day large-scale practice without storage, or some means to preserve the fruit from deterioration. Without some way to hold fruits in sound condition during the time required to transport them from the place of production, the development of the large fruit industries of the West and South would be impossible. The two most important factors underlying the success of modern fruit-growing are the discovery of methods of controlling insects and diseases and the application of refrigeration to the transportation and storage of the crops. The time required to transport fruit crops to the centers of consumption varies from a few hours to ten days, and in some cases two or three weeks are required, especially when the fruit is exported to foreign countries. Under these conditions, the trip to market really becomes a period of storage and the application of storage principles is as important as transportation principles. The object of refrigeration in the transportation of fruits involves only the holding of the fruit in sound condition long enough to reach the consumer. Storage, on the other hand, includes the holding of the product for a long or short period, thereby lengthening the season of marketing as well as increasing the territory over which it may be distributed. The lengthening of the season of marketing or period during which the product may be sold, is very essential when the production of fruit crops has reached a point greater than can be consumed during the normal season. This is aside from the general advantage of having a product out of season, for which many consumers are willing to pay fancy prices. The conservation of the surplus crops through storage enables the equalization of the selling or marketing season, giving the consumer the advantage of obtaining supplies during a longer period, and giving the producer a chance to market larger crops at profitable prices. The application of storage to the apple industry has resulted in making this fruit an all-the-year-round staple food, as supplies are easily held from one season to the next. The advantages of this to both consumer and producer are manifest. If all the apples produced had to go into consumption during the normal season of this fruit, there would be alternate periods of plenty and scarcity. The same is true, although to a less extent, of other fruit crops, such as pears, grapes, lemons, and oranges. Many of the vegetable crops are likewise held successfully in storage for longer marketing periods than their normal seasons. Potatoes, celery, cabbage, onions, and even lettuce and cauliflower are successfully stored for varying periods. Storage also finds application in other horticultural industries. The nurserymen are enabled to hold their stocks of trees, plants, cions, or cuttings until the proper season of planting or propagation arrives. Bulbs, lily-of-the-valley crowns, and cut-flowers are also successfully held in cold or cool storage much longer than would otherwise be the case. All of these examples are mentioned to indicate the wide application of storage to horticultural industries. There are several different methods of conserving or preserving food products. Among these methods may be mentioned curing, drying, salting, smoking, canning, use of chemical preservatives, and by refrigeration. Of these the last method is the only one by which the products are held in their original or "fresh" condition. All other methods involve a change in the character of the product. Cold-storage conservation aims to hold the product in such a condition that it may be used as fresh. If the storage does not accomplish this, it falls short of its principal object. Two kinds or methods of storage are recognized at present: These are (1) cold storage, and (2) common storage, sometimes known as "dry storage." The objects of these two methods of storage and their accomplishment are the same. "Cold" storage is designated as artificially cooled storage, or the holding of the products in rooms or buildings which are artificially refrigerated, i. e., the cooling is effected by means of machinery or ice. "Common" storage is the term applied to storage without ice or mechanical refrigeration, the cooling effect being obtained from the natural low temperature of the outdoor air. The aim of common storage is "to conserve the natural cold" in buildings which are specially constructed and equipped with proper ventilating devices or openings. It is difficult to determine the exact reason for designating this type of storage as "dry" storage. Both cold and common storage must be dry, excessive moisture in either case being detrimental. A possible explanation of the use of the term "dry" in this connection is the fact that in the first cold-storage houses ice was the refrigerant with necessarily more or less dampness about the plant; while in common storage, no ice is used ordinarily and consequently there is no dampness from this cause. Contrary to general belief, the use of cold storage is not confined to modern civilization. Frank N. Meyer, Agricultural Explorer for the United States Department of Agriculture, reports the finding of the use of cold-storage methods applied to fruits in remote parts of China, wholly out of touch with modern civilization. Meyer states that the Chinese have practised cold-storage methods for centuries. They are able to hold grapes from one year to the next by storing them in deep cellars which are kept cold with baskets of broken ice placed among the baskets of fruit. He found also that the Chinese fruit merchants keep perishable fruits in large thick-walled earthen jars, in the bottom of which a layer of broken ice is kept and over this, in wicker baskets, the fruit is held. The jar is covered by a wooden felt-covered lid. It is a long step from these ancient Chinese ice-cooled cellars and jars to the modern mechanically refrigerated storage house, but it is certainly of interest to find that the ancients understood the fundamental principles of the conservation of foods through the reduction of their temperature. There has been much discussion during the past few years regarding the application of storage (cold or refrigerated storage principally) to the conservation of foods. It seems unfortunate that the bulk of this discussion has taken a rather condemnatory stand. The dangers of the use of cold-stored food products have been over-emphasized. The present high cost of living has been at least partly ascribed to the supposed pernicious practice of "cornering" foodstuffs during their normal seasons at low prices, and holding them in storage to be sold at arbitrarily high prices. While there is no doubt that attempts have been made to corner foodstuffs in this way, experience shows that these attempts have been few and almost uniformly disastrous financial failures so far as the promoters were concerned. As will be shown later in this article, the holding of fruits or other foods in storage is rather expensive and not so simple as it seems to be at first sight. Not all products are uniformly suitable for storage and unless the greatest care is exercised both in selecting the product and preparing it for storage, serious losses from deterioration are certain. Another unfortunate fallacy, that cold-stored products are necessarily inferior as such, has become widely prevalent, due to campaigns in newspapers and magazines with a view mainly of casting odium upon stored products. There can be no doubt that much poor cold-stored food materials have been sold. Deteriorated foods have frequently been foisted upon consumers. The fallacy lies in attributing the deterioration alone to cold storage. Such deterioration can usually be traced to the poor condition of the product at the time it was placed in storage, to improper methods of preparing the product for storage, or to attempts to hold it too long. The application of the low temperature is not detrimental unless the temperature is low enough to injure the product by freezing. No product can be improved through cold storage. If it is in poor condition when it is placed in storage, it will be in much poorer condition when it is withdrawn. If it is in good condition at the time it is stored, it will remain in first-class condition throughout its normal life, provided always that the storage plants or rooms have been properly operated. Many fallacious arguments have been offered tending to prove that holding in cold storage is itself sufficient to render a given food product unwholesome. While unwholesome cold-stored products have at times been placed on the market, as admitted in the preceding paragraph, the condition of the goods was due to the three causes above enumerated and not alone to the application of refrigeration. The attempts to hold supplies which have deteriorated before they are placed in storage are frequently responsible for untimely deterioration, and the practice should be discouraged; but so far as fruits and vegetables are concerned, it is perfectly safe to state that no injurious effects have ever followed the use of these cold-stored foods. The evidences of deterioration are plainly visible in fruits and vegetables and there are no hidden germs or ptomaines. No one is likely to be deceived into eating a deteriorated fruit or vegetable. The agitation against cold-stored food products has resulted in a demand for legal regulation of the storage business. Several states have passed laws prescribing certain conditions which must be met. Attempts have been made to obtain federal regulation, and no doubt federal laws concerning the cold storage of foods will eventually be enacted. The state laws now in effect and the proposed federal legislation place arbitrary limits upon the time the products may be held; provision is also made regarding the marking of the products, and the inspection of the goods from time to time is provided. Legal regulation of this business is desirable and wise, but it is not wise to present arbitrary obstacles to the development of refrigeration. It is questionable whether the adoption of an arbitrary length of time for holding all products is wise or safe. Not only does the length of time vary for different classes of goods, but within the same class the condition of the product at the time it is placed in storage or its treatment previous to storage very materially influences the time the product may be held in wholesome condition. Thus, not all apples of a given variety may be held the same length of time. The length of the period of successful storage will depend upon the condition of the fruit, its stage of maturity, the care with which it has been handled, and also the promptness with which it has been cooled. The same principle holds true for all other fruit and vegetable products. It would seem, therefore, that some provision for the inspection of food products to determine their fitness for storage would be wise and would result in preventing many losses now sustained through the storage of unfit goods. So far, none of the laws passed or proposed makes this provision. This argument is not offered to defend the cause of cold-storage warehousemen. They have not been entirely free from blame in the past. The possibilities and the advantages of cold storage should be fully appreciated by consumers and producers alike. The necessity for refrigeration in horticultural industries will undoubtedly become more and more urgent, and the use of this important adjunct to modern fruit-growing is bound to extend its scope and receive wider application in the future. Cold storage is a modern economic necessity. Through this system of food conservation, the extension of markets and the territory over which different commodities may be distributed are very materially increased. In the use of cold storage or refrigeration, the first establishments depended on ice for the refrigerating medium. In the earliest plants natural ice was used. This was gathered or "harvested" during the winter and used during the spring and summer months. There is a very large quantity of natural ice used under modern conditions, but the manufacture of ice is increasing and the use of "artificial" ice is likewise extending, even in districts where natural ice may be obtained without difficulty. Systems of refrigeration. In modern cold-storage plants two systems of refrigeration are used: (1) ice refrigeration, and (2) mechanical refrigeration. There are several methods of applying each system. In the use of the first, ice alone may be the refrigerant, or a mixture of ice and salt may be used to obtain lower temperatures than are possible from ice alone. In the application of the ice-and-salt refrigerant, several systems may be used, as will be explained later. The ice used may be manufactured or natural, depending on the relative cost. In the mechanically refrigerated plant, the refrigeration is secured directly without first producing ice. It is apparent that the direct application of the refrigeration results in a very material saving in both time and energy. In this way, the cumbersome method of first manufacturing the ice and the consequent inconvenience in handling it are avoided. There are two kinds or styles of cold-storage plants— the general warehouse, provided with cold-storage equipment for handling all classes of commodities; and the storage plant constructed specially for the storage of a particular class of commodities—fruit or dairy products for example. It is evident that the large general warehouse may have a very great advantage in economy of operation. The general warehouse handles a mixed business which often results in a more economical distribution of overhead operating expenses than is possible when the entire burden of expense must be borne by a single commodity or single class of commodities. It is easy to see that the greatest efficiency and economy of operation occur where practically the entire capacity of the plant can be utilized during the year. This is hardly feasible with fruits, with the possible exception of apples which are now held largely during the greater part of the year. But even in the case of apples, there cannot be a full use of the equipment continuously, as the withdrawals will be gradual throughout the season. Nevertheless, the construction and operation of cold-storage plants for apples or other fruits is constantly increasing. These plants may be owned by special corporations or may be built and operated by growers' organizations or by growers who have sufficient acreage to warrant the extra provision of storage facilities. In some instances the ownership or control of cold-storage facilities has rendered fruit-growers independent of buyers' or speculators' manipulations of prices or marketing facilities. Mechanical refrigeration depends on the compression of a gas or vapor. The compression exerted heats and, in some instances, liquefies the gas. The heat is absorbed by means of cooling water, and when the gas is allowed to expand, an equal number of heat units is absorbed from the surrounding medium. This, briefly, is the general principle upon which depends the operation of refrigeration machines. The gases used may be air, ammonia, sulfur dioxide or carbon dioxide, commonly known as carbonic-acid gas. In the cases of air machines, the air is simply compressed under very heavy pressure and cooled by means of water. There is no liquefaction of the air attempted in the case of these machines. The advantages of the air-compressors are that they are comparatively easy to manipulate and there are no injurious effects in case of leakage from the compressed vapor. These machines are used to a great extent on shipboard, and in England to a much greater extent than in the United States. Improvements in the construction of air-compressing machines are resulting in their increased use. The disadvantage of the air-refrigerating machines is that they are relatively inefficient for low temperatures. There being no liquefaction of the gas, the advantage of the latent heat due to the change of state is absent, and consequently there is a loss of efficiency to the extent of the latent heat necessary, first, to change the gas to the liquid and then back from the liquid to the gaseous state. In the machines which utilize a liquefiable gas, that is, a gas which may be liquefied at comparatively high temperatures, the latent heat due to the change of state adds greatly to the efficiency of the machines, and consequently, much lower temperatures can be secured for a given amount of power. One of the essential qualities of the gas which may be used for this purpose is that it must be comparatively inexpensive and must be within the means of utilization by a comparatively simple apparatus. Ammonia, sulfur dioxide, and carbon dioxide are in general use in about the order named. Ammonia is the most common, and is in many respects the easiest to handle. It may be liquefied at a lower pressure and a higher temperature than either carbon dioxide or sulfur dioxide. Carbon dioxide is, therefore, somewhat less efficient to the extent that the high pressure requires more power, there is greater friction to overcome, and colder water is needed for condensation. These conditions are not always obtainable within ordinary means. Carbon dioxide has the advantage of being a non-irritating and non-poisonous gas. If leaks occur in the system, there is no great danger of serious injury either to the operators of the machinery or to the goods stored in the rooms. If leaks occur in the ammonia system or in the sulfur dioxide machines, there is very great danger of injury to the workmen about the plant and the food commodities exposed to these gases may be very seriously injured, even with a slight leakage. Nevertheless, the greater ease with which the ammonia machines can be manipulated is considered a sufficient advantage, and this type of machine is much the commonest now in use. The system of operation of the refrigerating plant is comparatively simple although rather complicated machinery is required. It is not essential that the fruit-grower who contemplates the erection of a refrigerating plant be conversant with all of the complicated details. It is well, however, that he understand the principles upon which the machines are designed. For the erection and planning of a complete storage plant depending on refrigerating machinery, the services of a competent refrigeration engineer are essential. While it may be possible for a mechanically inclined fruit-grower to design and have erected a complete refrigerating plant, it must not be forgotten that slight errors in the calculation of the power required and the capacity of the machinery necessary to yield given results will frequently render the operation of the plant very much more expensive than need be, or the efficiency of the plant may be very seriously impaired. Refrigerating machinery is, of necessity, expensive; it is likewise delicate in many respects. There are many styles and designs of refrigerating machines. All, however, are dependent upon the same general principles. There is, first of all, the motive power which may be either the gasoline or electric motor, or the steam engine, which furnishes the power to operate the compressor. The compressor exerts pressure on the gas, heating it to a rather high temperature. If the machine is operating with ammonia gas, the compression results in a dense hot vapor. From the compressor, this hot vapor passes to the condenser, which is a system of pipes arranged in such a way that streams of cooling water may be passed continuously around the pipes containing the hot gas. The absorption of the heat from the dense vapor condenses it into liquid ammonia. This liquid is ordinarily run into a receiver or reservoir, where it is kept for use as needed. From the receiver, the liquid ammonia is forced into what is known as expansion coils. These coils consist of series of pipes into which the liquid ammonia is allowed to enter. The liquid boils at a low temperature and changes from the liquid to the gaseous state, and in changing its state and expanding absorbs considerable heat from the surrounding medium; in this way the refrigeration is obtained. After expansion, the ammonia gas is passed back to the compressor where it is again compressed and afterwards liquefied, the same gas being used over and over again. Instead of the compressor, the same effect can be obtained from what is known as the absorption system. This is a combination of a chemical and mechanical process. No compressor is used. Dense aqua-ammonia, which is simply a strong solution of ammonia gas in water, is heated in a reservoir, and as the ammonia escapes from the solution, it is under heavy pressure and becomes a dense vapor. From this tank the gas is passed through condensing coils and liquefied, just as is done in the compression system. After liquefaction, it is allowed to expand in coils and the gas is then returned to a tank or a series of tanks known as absorbers. These absorbers contain cool water which readily absorbs or dissolves the ammonia gas. From the absorber, the solution is pumped into the heating tank, again heated, and the process repeated. When one contemplates the installation of refrigerating machinery, one of the most important factors to consider is the supply of water needed for condensing. This is ordinarily of very much greater importance than is appreciated. Large quantities of water are needed unless the temperature of the water is very low. The higher the temperature of the water-supply, the larger the quantity needed. The same water may be used continuously if there is some means at hand to cool it after it has served to absorb the heat from the condensing ammonia. In large refrigerating plants this is taken care of by means of large racks or towers located in some exposed place. In these towers, the water is run through screens which break it into many fine streams, thus increasing the evaporation and cooling the water by the absorption of its heat. Unless some means is at hand to cool the water, a constant supply must be provided; otherwise, the efficiency of the machinery will be very greatly reduced. This point is of the utmost importance when refrigerating plants are to be located in fruit-growing districts. In fact, the practicability of operating the refrigerating plant successfully depends primarily on the presence of a sufficient supply of water for condensation. Ice systems. Refrigeration may be obtained from the use of ice alone. In this instance, however, only cool-storage effects can be obtained, except in winter and in climates where the outdoor temperature is low enough to offset the lack of refrigerating effects from the ice. The minimum temperature obtainable from ice is its melting point, which is just above 32° F. or, under the very best conditions, about 33° or 34°. Generally, ice-cooled chambers cannot be maintained below a temperature of 38° or 40° and these temperatures are obtainable only under the most favorable conditions. To obtain a low temperature from ice, the addition of salt is necessary. The mixing of salt with the ice lowers the melting-point and, consequently, the temperature is lowered, although the rapidity with which the ice is consumed is very greatly increased. When the mixture of ice and salt is used, the quantity of ice necessary for storage is much greater. A lower temperature is necessary to hold the storage chambers at 32° F. because of the heat leakage into the chambers through the walls. There are two methods of obtaining refrigeration from ice and salt. One of these is the indirect method, known as the gravity-brine system, and the other is the direct circulation of air through the ice and salt mixture. The gravity-brine system, the invention of Madison Cooper, acts as the reverse of a hot-water heating system. It depends on the principle of the greater density of a cold liquid, and its consequent downward flow when confined in a system of pipes. The system consists, first of all, of coils of pipes filled with a strong solution of calcium chloride brine. One end of the pipe system is contained in a tank which holds ice and salt. The cooling effect of the ice and salt results in greatly reducing the temperature and increasing the density of the brine. From these "primary" coils the brine is conducted into what is known as secondary coils which are placed in the rooms or chambers to be cooled. The cold brine passes through the secondary coils, and, as it absorbs the heat from the rooms or chambers, its density is decreased and it flows upward and is returned to the top or primary coils, where it is cooled and the process repeated. Fig. 3700 is a diagram illustrating the principle upon which the Cooper gravity-brine system depends. At the top of the illustration the primary coils are shown and the methods of placing the pipe systems or coils is indicated. The cooling effects secured from the Cooper brine system are indirect in that the refrigeration obtained from the ice-and-salt mixture is first exerted upon the brine solution and this solution is the means of carrying the refrigeration to the place where it is needed. There is an inevitable loss in the indirect method because of the loss of refrigeration in the conducting pipes. In the second system of securing refrigeration from ice and salt, the air is passed directly through the mixture and, consequently, the refrigerating effect is obtained directly from the mixture. Experiments have shown that very low temperatures can be obtained by passing a current of air through an ice-and-salt mixture, the temperature depending on the proportion of salt used in the mixture. The higher the percentage of salt the lower the temperature obtainable. The device for obtaining refrigeration in this way is simple. It consists of a tank for holding the crushed ice and salt. At the bottom of this tank openings are provided through which a current of air can be forced. The particular working out of the design for obtaining refrigeration in this way has been accomplished by S. J. Dennis, of the United States Department of Agriculture. Dennis' apparatus has been used successfully in several plants on the Pacific coast, and owing to the fact that it was developed as a part of the Departmental investigations, its use by the citizens of the country is free. It is essentially an ice-and-salt tank of "magazine" type. As the ice is melted at the bottom the supply from above drops down. From 7 to 10 per cent salt has been found to yield satisfactory temperatures. The apparatus can be constructed by almost any ordinary mechanic who can follow detailed drawings or instructions. A fan is used to draw the air through the ice-and-salt mixture and to force the refrigerated air into the storage chambers. Ducts are provided for the return of the air, and in this way the same air is used continuously. There is another method of securing refrigeration from ice and salt which may be designated as the "tube system." In this system the mixture of ice and salt is contained in tubes located at the sides of the room. The tubes are filled with the mixture from the top and the refrigeration is obtained by the cooling of the air in contact with the tubes. The number of tubes necessary to cool a given quantity of goods depends upon the character, size, and insulation of the room. The tubes are constructed of galvanized iron and are about 8 or 10 inches in diameter. The tops of the tubes open above the storage room, so that the filling can be done without opening the room itself. A proper arrangement of pipes at the bottom to carry off the meltage is necessary, and in arranging for this meltage outlet, a proper trap must be provided in order to prevent the wastage of the cold air. This system has found considerable application in the Hudson River Valley of New York state, and is very effective for a short season of storage. Systems of applying refrigeration. Refrigeration may be defined as the cold obtained from a refrigerating medium or directly from mechanical appliances. The temperature-reducing properties of the ice-and-salt mixture and those of the liquid ammonia are the refrigerating qualities of these agencies. Refrigeration is ordinarily measured in terms of ice-melting capacity in a day of twenty-four hours. For example, a machine which is rated as yielding a capacity of ten tons a day is based upon the meltage of ten tons of ice in that time. Ordinarily, it is not the quantity of ice which can actually be produced by the machine, except when ice manufacture is the primary object. There are three systems of applying the refrigeration secured by mechanical means: (a) direct expansion; (b) brine circulation; (c) air circulation. In the direct-expansion system, the expansion pipes in which the gas is allowed to expand, and thereby produce the refrigeration, are located in the storage rooms. The claims for this system are that the work is direct; therefore there is no loss in conducting the refrigeration obtained from the pipe surface. The greatest objection to this system, especially with ammonia or sulfur dioxide plants, is the danger of leaks. Small leaks may allow a sufficient quantity of gas to escape to damage the goods stored in the rooms; there is also great danger to the workmen, which has already been referred to. The greatest care is necessary in constructing the direct-expansion system. With the perfection of mechanical devices for welding and fastening pipes, the danger from leaks has been reduced to a minimum, and there are many direct-expansion systems in operation in which no injuries have ever been recorded. One disadvantage is the fact that there is no reserve power except that which is contained in the liquid receivers. Should it be necessary to shut down the machinery for repairs for any considerable length of time, there would not be sufficient reserve to continue the refrigerating effects and the temperature of the storage rooms would be likely to rise to a considerable extent. In the brine-circulation system, the expansion coils are surrounded by a non-congealable brine such as a solution of calcium chloride, which has a very low freezing-point. The brine is cooled in the pipes and this cold brine is circulated through coils in the rooms by means of pumps. In applying this method, there is, first, what is known as a brine-cooler in which the actual cooling of the brine takes place; the cold brine is then collected in a brine tank of sufficient capacity to operate the plant for a considerable length of time. This really acts as a reservoir of cold, and from this reservoir the cold brine is circulated throughout the entire cold-storage plant, the temperature and quantity of the brine circulated being governed by the results desired. The coils of pipes in the rooms are commonly referred to as the "piping." These coils or racks of pipes are the room's equipment for refrigeration, and the number of coils or length of piping depends upon the size of the room and the temperature desired. Low-temperature rooms or freezers have a large number of coils and a great length of pipes. Pre-cooling rooms should also have heavy piping and many pipes. The advantages of the brine-circulation system are that there is no danger from leakage of ammonia or other irritating gas. The statement is also made that there is a more uniform temperature, the flow of the brine being under constant and easy control. The temperature of the brine is also under definite control. Perhaps the greatest advantage is that the supply of cold brine acts as a reservoir of refrigeration and a reserve supply can be drawn upon in case of necessary shutting down of the machinery. In placing the pipes in cold-storage chambers, it is important to have them located at the upper part of the room. Ordinarily, the dependence for the circulation of refrigeration throughout the room is on the natural circulation of the air within the room. The air in contact with the pipes is cooled and, being rendered more dense, flows to the floor, the warmer air from other parts of the room taking its place. In this way, a constant circulation throughout the room is maintained. It is easy to see, therefore, that the placing of the pipes in the upper part of the rooms is essential; otherwise, there is danger that the parts of the room above the pipes may be beyond the refrigerating effects. The air is, therefore, the medium of applying the refrigeration. It is due to convection currents that the transfer from the refrigerating pipes is effected, and it is very difficult to obtain a uniform cooling in all parts of the room unless the pipes are carefully placed. In the air-circulation system of applying refrigeration, there is a forced air circulation. The air is forced through conduits or ducts by means of fans. In this system the expansion coils of pipes are in groups or batteries in what are known as bunker rooms, or more correctly, coil rooms. The refrigerating capacity of the plant is, therefore, concentrated in one place. In arranging the coil pipes, baffles are placed in such a way that the air passing through the coil rooms must come in contact with all of the pipes. If all of the pipe surfaces are not reached by the air, the full refrigerating effect of the plant is not obtained. The coils may be direct-expansion coils or brine-circulating coils, that is, have the brine circulating through them. The Cooper gravity-brine system may also be used. For this purpose, the secondary coils are located in coil rooms where the air to be cooled can be forced through. In placing the fans for such a plant, the arrangement must be such that the air is drawn from the coil room and forced through ducts to the storage chambers. With this arrangement there is a constant pressure in the rooms which is preferable to the exhaustion of the air. Any leakage which occurs, therefore, is outward from the room instead of inward. Return ducts are provided which conduct the air back to the coil rooms, the same air being used continuously. Impurities from the storage rooms are absorbed by the air and deposited in the moisture which freezes on the cold pipes. The impurities, therefore, are very largely absorbed by the frost, and the circulation of the current of air through the coil rooms acts largely as a purifier of the air of the storage rooms. Some ventilation, however, is frequently desirable. The problem of ventilating a cold-storage chamber is a difficult one and special appliances must be provided for this purpose. The outer air cannot be admitted directly into the storage chamber unless it is at the same temperature as the air of the storage room. In warm weather, therefore, the admitted air must be cooled and in extremely cold weather it must be warmed. There are special mechanical devices for accomplishing both of these purposes. Shape of storage plants. Many storage plants are planned without consideration of the factor of the most economic shape; that is, the most efficient as well as the most economical size of plant to be constructed. The first consideration is that the plant should be planned to supply the required floor space and cubical capacity. In figuring the size of storage rooms to accommodate packages of fruits, the size of the fruit packages must be taken into consideration and enough space must be made to allow the air to circulate between the stacks or bales of packages. A barrel of apples, for example, requires 8 to 10 cubic feet. Another factor which must be considered is the economical handling of the packages in the storage rooms. Where the storage season is comparatively short, the extra expense of piling in high stacks must be considered. Where, however, the storage season is to be long, higher stacks may be made and, consequently, rooms of greater height will be most economical. After the size and the cubical contents of the chamber are determined, the next consideration is the shape of the plant or room. The most economical shape for a storage plant is the cube. This is due to the fact that the ratio between cubical contents and exposed outside surface is smaller for the cube than for any other shape. It is important to take this into consideration because of the fact that there is no perfect insulating material and, consequently, when the ratio of exposed outside surface is very high, the rate of heat leakage into the room is increased considerably when the shape of the room differs materially from the cube. Such a room must have either much heavier insulation or considerably more power must be supplied to offset the greater heat leakage. Sometimes limitations of space, as for example, utilizing parts of buildings, require that the rooms be of odd shapes. When this is necessary, it will require considerably more insulation or power, as suggested. The capacity of the plant must be determined by the nature of the commodity to be stored. Large rooms are easier to maintain at a desired temperature after the entire load of the room is reduced to the required temperature. In large rooms, however, it is more difficult to cool uniformly unless some special attention is given to the placing of the pipes, or the duct openings where forced air circulation is used. For periods of short storage, such, for example, as the more perishable fruits like berries, rooms of smaller capacity are more desirable than very large rooms. Insulation. There are three ways in which heat may be transferred: radiation, conduction, and convection. Radiation is the transference of heat from one body to another through a third medium without perceptibly affecting the medium. The heat which one feels when standing before a fire is radiant heat. The conduction of heat is accomplished by the passing of heat from one body to another by contact with the heated body. The heat that one feels when the hand is placed on a warm pipe is conducted heat. Heat is transferred by convection by means of a third medium, usually air. In attempting the construction of storage houses, all three methods of heat transference must be taken into consideration. The heat actually radiated is comparatively small in storage buildings. The quantity of heat transferred by conduction is greater, but the most important problem of heat transference is through convection currents. In order to offset this heat transference, specially constructed walls must be provided. A storage chamber is a room so constructed that the temperature may be maintained at or near a constant point. In order to offset changes of temperature, sufficient refrigerating capacity must be provided, or some means to prevent the actual transmission of the heat from the outside to the inside of the room. The latter provision is known as the insulation of storage rooms. Therefore, the rooms are constructed in such a way that the walls act as barriers against the transmission of outside heat into the room, or the loss of heat of the storage room to the outside in extremely cold weather. The best insulation against heat transmission is a vacuum. If it were possible to surround storage rooms with vacuum walls, the heat leakage into the room would be very slight, and after the rooms were once cooled to the desired point, it would not require machinery of great capacity to maintain a low temperature. It has been found difficult, however, to maintain a vacuum under ordinary circumstances. The outer air pressure is constant and leakage of air into the vacuum walls, although slight, gradually destroys the insulating effect. Attempts at vacuum construction on a large scale have not been successful. Air spaces, that is, walls made air-tight so that the air is closely confined, have been thought to be efficient insulation. Still air is a necessity where this method of insulating the wall is used. A slight leakage into the wall is sufficient to allow outer air to enter and, consequently, to destroy the insulating effects. In walls constructed of free air spaces, convection currents occur within the spaces, which act as effective transferers of heat either inward or outward, as the case may be. Fig. 3701 shows the action of convection currents within air spaces. When one air space is used, the transfer of the heat is very easily accomplished, as shown by the direction of the air currents—shown by the arrows in the diagram. Simply thickening the walls, therefore, does not act as a sufficient insulation. The insulating effect of air spaces is considerably improved by breaking up the walls into smaller air spaces. The diagram shows the convection currents occurring in walls of one, two, three, or four air spaces. As the number of spaces is increased, the effect of convection is very greatly reduced, so that a wall consisting of four air-tight spaces may be considered as fairly efficiently insulated. It is, however, extremely expensive to construct these air-tight divisions, and some other means of insulation is desirable. It is preferable to use some material to fill the walls. Such a filler breaks up the air spaces within the walls and confines the air in the small interstices between the particles. In this way, the air held within the wall approaches more nearly the desirable "dead-air" condition. Convection currents actually occur in filled walls, but they are very sluggish and the effect is very slight. Filled walls are effective barriers against heat conduction or radiation, provided, of course, that poor conductors of heat are used. The most effective insulating material is a substance of low-heat conductivity which has many pores or cells. These cells are filled with air (practically still air); consequently the efficiency of the heat barrier is increased. A number of substances are effective as insulation for storage walls. The requirements for an insulating material, besides non-conductivity of heat, are as follows: 1. Odorless; any strong odor would affect the goods stored in the rooms. 2. Moisture-proof or low capacity for moisture; dampness decreases the efficiency as an insulating material, and some substances ferment or rot when damp. 3. Vermin-proof; there should be no inducement for rats or mice to nest in the walls. 4. Non-liability to inherent disintegration or spontaneous combustion. 5. Lightness in weight: not only on account of the reduction of the actual weight of the walls, but because light materials are usually the best non-conductors of heat. 6. Elasticity; when packed firmly in the walls the material should not settle, as any settling within the walls results in open spaces in the insulation. After the walls are once constructed, these inequalities cannot be reached for repairs without completely rebuilding the walls. 7. Relative cheapness and economical handling; the material should not be so high in cost as to be prohibitive. In addition, the material must not be of such a nature that its economical handling is impracticable. 8. Must allow of practical application in general work; very specialized material which would not lend itself to general conditions could not be considered as efficient insulating material. The list of materials available for insulation may be divided into two classes: Those that can be considered as commercial insulation—that is, materials which are manufactured especially for insulating purposes; and common or waste materials. Among the most common of the first class are the following: granulated cork, cork sheets or boards or bricks, hairfelt, linofelt, mineral wool, and lith. Granulated cork is considered to be one of the best and most effective insulating materials. It is prepared from the trimmings of cork mills, and when used in the granulated state is simply filled into the walls and packed tightly. Cork sheets, bricks, or boards are manufactured of the cork particles which are compressed in molds at a high temperature. There is no cementing material used, the heat and pressure being sufficient to liquefy the natural gums and resins of the cork and these hold the particles together. Cork boards or sheets are also made by the addition of asphaltum pitch which renders the particles water-proof but may decrease the insulating efficiency. Hairfelt is manufactured of waste cattle hair which is washed and deodorized. It is pressed or felted together by special machinery into sheets from 1/4 to 1 inch in thickness. Linofelt is a patented material manufactured from flax fibers. It is prepared in sheets or quilts, from 1/4 to 1/2 inch thick, somewhat like cotton-batting. These sheets are ordinarily quilted between water-proof paper. This material is used largely for insulating household refrigerators and refrigerator cars. Mineral wool is also known as rock-wool, rock-cotton, rock-cork, or silicate cotton. This material is usually made from the slag of blast furnaces with the addition of limestone. Rock-wool is usually made from a mixture of granite particles and limestone. The crushed rock is mixed with coke and fused in furnaces at a temperature of about 3,000° F.; the molten slag or rock is run out through the bottom of the furnace by a high-pressure steam blast. This blows the slag into very fine shreds or fibers, much resembling fleece or wool. The result is a material which contains from 92 to 96 per cent air spaces and, although consisting primarily of a substance of high-heat conductivity, is fairly efficient as insulation. It is practically vermin-proof, fire-proof, and not liable to decay. It absorbs moisture very easily, and one of the greatest disadvantages is the difficulty of handling. The fibers are very penetrating and are glass-like, which result in considerable inconvenience in handling the material. Lith is a manufactured insulation composed of flax fibers, lime rock-wool, and water-proofing compound. It is prepared in boards of standard sizes and thicknesses and is accepted as a standard insulation by refrigerating engineers. It is a very efficient insulating material. Common forms of insulating material which are usually at hand or can be easily obtained for the construction of storage buildings are: straw, chaff, hay, dry grass, dry leaves, hulls of various grains, sawdust, and mill shavings. All except sawdust and mill shavings can be considered as suitable only for temporary structures. These materials are all fairly efficient as non-conductors of heat provided they are dry and means are used to keep them in a dry condition after being built into the walls. There is also some danger from the depredations of rats, and the greatest possible care must be used to prevent these rodents from gaining entrance. Sawdust from different woods has about the same insulating effect. The sawdust must be thoroughly dried, otherwise its efficiency as insulation is very greatly impaired and, in addition, there is danger of fermentation and heating, and even spontaneous combustion. It is more difficult to obtain dry sawdust than mill shavings, and whenever sawdust is used it should be very carefully dried before being placed in the walls. It has not as great elasticity as mill shavings and, consequently, is likely to settle after packing unless very carefully pressed into place. Mill shavings consist of small chips and shavings from planing mills. This material has largely replaced sawdust for insulating purposes and is much more effective. It is obtained easily in a dry condition, owing to the fact that the mills of this kind usually work dry lumber. It is much more elastic than sawdust and does not pack or settle down. If thoroughly dry, and means are taken to keep it so, it is a very efficient insulating material and will remain in good condition for many years. It should be packed in the walls at the rate of eight or nine pounds to a cubic foot. Whenever walls are filled with insulating material in loose condition, much will depend upon the method of constructing the walls. Not only is it necessary to use the lumber and insulating material in a dry condition, but unless the walls are properly built, the insulation will not remain dry any great length of time. Walls that are not practically air-tight allow the outside air to gain entrance and to mix with particles of insulating material; condensation of moisture takes place, and the insulating efficiency of the material is seriously impaired. The conditions for the condensation of moisture upon the insulation particles are ideal unless special means are used to prevent it. Contact with the inside walls lowers the temperature of the insulation to such an extent that when the warmer air from outside comes into contact with it, the moisture is deposited and absorbed. Therefore, it is necessary to build the walls in such a way that they will be practically air-tight. This is accomplished by having layers of elastic waterproof paper on the outside and inside of the walls. The proper method of constructing such walls is shown in Fig. 3702. The wall consists of two layers of matched boards on each side, between which the water-proof paper is placed. The figure also shows the proper method of overlapping the paper at the corners. It is very essential that these details be attended to; it is also necessary to prevent the tearing or breaking of the paper when placing it, and for this reason only elastic paper should be used. The more brittle forms of paper are so easily broken that it is almost impossible to place them without seriously injuring them. Any breaks at the corners or tears in the paper will allow considerable air leakage into the walls and very seriously impair their efficiency. This is probably one of the most important details in the construction of storage houses, both for cold-storage and for common-storage purposes. Too great stress, therefore, cannot be placed on this point. Storage temperatures; humidity. The general principles governing the application of low temperatures to the preservation of fruit products depend primarily on the fact that temperature is the most important factor governing the life activities of these products. A fruit or vegetable is a living organism in which the functions or life processes are continually proceeding as long as the body remains in a normal condition. The various processes of ripening depend upon the chemical and physiological changes within the organism. Contrary to common belief, the life processes do not cease when the fruit is removed from the parent plant. These processes continue until the life cycle of the organism is completed. The fruit organisms respire and transpire just as plants do and the measurement of the end products of these respiration and transpiration processes serves as an index of the rate at which the life activities are proceeding. Definite measurements on a large scale show that the temperature factor is the most important from the standpoint of the rate at which the life activities proceed. Each fruit organism has a definite life span or life cycle, and it is easy to see that if in any way these activities can be retarded, the life span can be lengthened. The reduction of the temperature of the organism materially reduces the life processes, and the rates at which these activities proceed is slackened to such an extent that the definite life functions of the organism may continue slowly during a long period. The retardation of the life activities through the reduction of the temperature thus induces a slowness of the rate and a consequent increase of the length of the life span. This is the essential physiological principle upon which the cold storage and common storage of fruits depend. The most satisfactory temperature for storage purposes is one which is low enough to reduce the life activities to a minimum but not sufficiently low to stop them entirely. It is important to remember that a complete stoppage of the life functions of a fruit organism means the death of it, and when this occurs, the fruit soon thereafter becomes unfit for food. Much careful investigation remains to determine the most satisfactory temperatures for various fruit products. Many factors are involved. One of the most important of these is the condition of the product when it is placed in storage. Fruits of the same kind and even of the same variety may have different storage qualities and require different storage treatments, depending upon the place where the fruit is grown or its previous handling. Until these factors are all known and controlled, it is unsafe to say that any particular temperature is exactly correct for all fruits. For this reason, it is difficult to limit storage periods by law, because no arbitrary limit can be satisfactory for all fruits. It would not be safe, for example, to state that apples should not be kept longer than a certain length of time. The same is true of pears, but with this fruit the conditions are even more extreme. Different varieties of apples and pears may be held in storage different lengths of time, and all warehousemen know by experience that the same varieties of fruits produced in different districts or in different seasons have different storage qualities. The proper storage temperature for a fruit should be the lowest possible—that is, the lowest temperature at which the fruit can be held without actual injury. This is due to the fact that when other conditions are satisfactory, the lower it is possible to hold a given product the longer it will remain in good condition. This means, then, that the freezing point of the fruit can be safely approached under ordinary conditions. There are, naturally, some important exceptions to this general rule. As has been indicated above, different fruits have different rates of life activities, and the more perishable fruits are those that have the most rapid rate. For example, perishable fruits like berries, peaches, and some varieties of grapes have very rapid life activities, while the less perishable fruits such as apples, pears, and the citrus fruits have a very low rate of life activities. This condition affects the storage period of a given fruit even under the most satisfactory condition. It is a well-known fact that the more perishable soft fruits cannot be held in storage for any great length of time. The naturally short life can be lengthened considerably but not to the same extent that the lifespan of the hardier fruits can be lengthened. Berries of various kinds, cherries and cranberries, may be hard frozen and held in such condition for several months when the product is intended for use in making sauce or pies. When the hard-frozen fruits are removed from storage they must be used immediately, as they soon become soft and break down physiologically. The most desirable cold-storage temperature for a fruit, according to present knowledge of the subject, is 32° F. for apples, pears, peaches, plums, strawberries, raspberries, loganberries, blackberries (short time), cherries, grapes, mangoes, celery, lettuce (short time). An apparent exception is in the case of apples from the middle coast section of California. The apples produced in this section require a somewhat higher storage temperature, due to the fact that a peculiar discoloration of the flesh develops when this fruit is held at the standard 32° temperature. This fruit is more safely held at about 35°. The varieties which are affected by this trouble are principally Yellow Newtown, Missouri Pippin, and, to a less extent, Yellow Bellflower. This apparent storage weakness seems to be confined to the mid-coastal district: apples from the mountains and other districts of the Pacific coast seem to possess normal storage qualities. For potatoes, 35° to 40° F., for citrous fruits, 45° to 50° F., are the most satisfactory temperatures. Citrous fruits seem to be an exception to the general rule that fruits of low life activities can be held at temperatures near their freezing-point. Investigations show that temperatures below 45° F. are injurious to citrous fruits, except for a very short period. The low temperature seems to affect the skin of the fruit, inducing the deterioration by scald or stains and the development of various fungous diseases. At a temperature of 45° to 50°, or a common-storage temperature of 50° to 60°, citrous fruits may be held for several months without serious deterioration, provided means are taken to prevent shriveling. Lemons are sometimes held from four to six months at common-storage temperatures without serious deterioration, when humidity conditions are carefully attended to. The proper humidity of the air of storage rooms is an important factor. Very little investigation of this important problem has been undertaken and, consequently, the fundamental factors governing the general principles of humidity conditions in storage rooms have not been definitely determined. Much shriveling of fruits in cold storage has been due largely to excessive evaporation on account of the free transpiration activities of the fruits. Transpiration, or the giving-off of moisture, occurs freely at high temperatures, less freely at low temperatures. The moisture, however, is being constantly given off even at low temperatures and when the air of the storage rooms becomes excessively dry, the fruit may become seriously wilted by excessive evaporation. The reduction of the temperature of the air reduces its water-holding capacity; consequently, as the air temperature is reduced to the freezing-point or below, all the excess moisture is removed and the air becomes saturated with water vapor for the temperature at which it is held. The total volume of water vapor is thereby greatly reduced. When the temperature of the air rises without the addition of moisture, its capacity for absorbing moisture from the fruit increases and, consequently, the drying effects due to refrigeration may be seriously overdone. The most satisfactory humidity condition in the storage room has never been correctly determined. Experience shows that the humidity condition should be as high as possible to prevent shrinkage from evaporation, but without danger from excessive moisture, which may induce the growth of mold. Excessive wilting of fruits in storage is not always due to evaporation. Fruits which are picked in an immature condition wilt and shrivel seriously under the most satisfactory storage conditions. From what has been said, it will be seen that humidity conditions in artificially refrigerated chambers very largely take care of themselves, due to the ameliorating effects of the refrigeration of the air. The control of the conditions becomes more important at high storage temperatures, e. g., it is very important in the storing of citrous fruits without artificial refrigeration. Under these conditions the humidity of the storage rooms or cellars must be held relatively high, because the higher temperature has a decided effect upon the life activities of the fruits, and a correspondingly high humidity is, therefore, essential. The fruit must be very carefully watched; otherwise, mold will occur when humidity conditions are too high. A relative humidity of about 80 to 85 per cent at a temperature of 50° F. has been found to be most satisfactory under the conditions which exist in California lemon-storage houses. It would not be safe to say that this humidity percentage is exactly correct, because the complexity of accurately measuring humidity conditions under different temperature conditions renders the problem very difficult. Common storage. The difference between common storage and cold storage has been explained (page 3246). The principal difference is that with cold storage, artificial refrigeration is used while in common storage there is no artificial refrigeration. Common storage is sometimes referred to as "dry storage," inferring that cold storage must necessarily be wet. This assumption is incorrect; cold storage is not in any way connected with moisture nor is it more likely to produce moisture in storage rooms, provided they are carefully conducted, than is common storage. Any excess of moisture in the cold-storage room means some defect in the construction of the plant or in its operation. It has been said that cold-stored fruits are more moist when withdrawn from storage than common-stored fruits. Here again, the difference is due to the difference in the temperature. The cold fruit from the artificially cooled storage chamber, coming in contact with the warm moist air, will condense moisture on its surface. Fruits from the ordinary storage rooms may not be cold enough to condense moisture; hence, the assumption that the cold-stored fruit is more moist than that from common storage. Common storage is not practicable for all fruits. The very active or highly perishable fruits cannot be held satisfactorily under common-storage conditions because there are no ready means at hand to cool them to the desired temperatures. Citrous fruits are eminently adapted for common storage. The curing of lemons is really a process of common storage. Winter varieties of apples and pears are also suitable for common storage. This method of storage is used to a considerable extent in New York and is coming into wide use in the Pacific Northwest. In the operation of common-storage rooms, dependence is placed on the ventilation for the cooling. There is a vast difference between ventilation due to the actual change of air by the opening of windows or flues into the room and the circulation of air. Ventilation means the admission of outer air, and circulation may refer merely to the movement of the air within the room or plant, the same air being used over and over again. This distinction is necessary because frequently the circulation of the air within the room is designated as ventilation. In the operation of common-storage rooms, the rooms are ventilated, or outside air is admitted, when its temperature is low enough to cool the fruit. The ventilators are closed during the day and during warm periods, thus conserving to a certain extent the low temperature obtained through the low-temperature outside air. It is essential, therefore, that there be cold nights or cold weather; otherwise, common-storage plants become mere cool-storage chambers, and the storage season is considerably shortened, due to the fact that the relatively high temperatures result in a high rate of life activities in the stored products. When the temperature of the common-storage room can be maintained somewhere near 32° early in the season there is no apparent reason why the storage period should not be extended to almost the same length of time that can be obtained under cold-storage conditions. In the early part of the season, especially when there are few cold nights, it is difficult to reduce the temperature of the fruit to the desired point. This is the critical period, as the rapidity with which the temperature of the fruit can be reduced determines the length of time the fruit may be held in good condition. It is easy to see, therefore, that under common-storage conditions, usually the fruit must remain at a comparatively high temperature for a considerable length of time. The ripening which occurs during this period of high temperature cannot be offset by low temperatures later on. The developments which take place in this period of high temperature shorten the life span under storage conditions, and when the temperature is high and the fruit held warm for a considerable length of time, the storage period may be very materially shortened. There are frequent warm spells during the fruit harvest, and the nights are not so cold. There is a widespread notion that common-stored fruits are better than cold-stored. It is difficult to understand how this opinion has become so fixed in the minds of many persons. It is probable that one reason is the fact that a comparison of fruit from common storage and from cold storage is really a comparison of fruits held under different conditions. The common-stored fruit is usually withdrawn after a shorter period and, therefore, may be in good condition. The cold- stored fruits are usually held for a long period and frequently the period is too long for the best condition of the product. Many carefully planned experiments show conclusively that cold-stored fruit remains in better condition during a longer period and, when carefully handled, remains in better condition after withdrawal than common-stored fruit. If the fruit is promptly and rapidly cooled at the beginning of the storage period, its life activities will be retarded to such an extent that the life span will be very materially increased. If this can be done under common storage, there is no reason why the fruit cannot be held in good condition. Frequently the fruit is placed in common storage during the fall and early winter; frequently also the common-storage room where the fruit is held is only a makeshift. After being held in this unsatisfactory condition for a time, the fruit is placed in cold storage later in the season when market conditions have not been favorable. This is the wrong way to store fruit. The time when cold storage is most urgently needed is at the beginning of the storage period, in order that the fruit may be promptly cooled. It would be more reasonable to remove the fruit from cold to common storage later in the season, because common- storage rooms may then be held in a satisfactory condition and the fruit would be in a much better condition for holding. It has been suggested that a combination of cold and common storage is really the solution of many of the problems of successfully holding the fruit in the district where it is produced. Where ice can be obtained at a reasonable price, it can be used at the early part of the storage season to cool the fruit promptly and quickly. After weather conditions are such that cool nights prevail, the place can be operated as an ordinary common-storage plant for the remainder of the season. Common-storage buildings. The earliest form of common-storage buildings for fruits was caves or pits. These were used for the storage of fruits under the impression that the earth is cool and also to protect the fruit from freezing in extreme winter weather. The earth is cooler than the outside air in summer; in winter it is warmer, under ordinary conditions. Ordinarily, the temperatures of the ground range from 50° to 60° and this temperature remains fairly uniform below the frost line which, of course, varies materially under different climatic conditions. The protection against freezing in winter, therefore, is ideal, but unless some artificial method of cooling the room is at hand, the temperature of the earth itself is too high for best storage conditions. In the later development of the cave storage, ice was used to cool the chambers; this was naturally not satisfactory, due to the dampness and to the difficulty of ventilating. Cellars have been a favorite place for common storage. They are open to the same objection as caves. Unless specially constructed and special means be provided for ventilation, the cellar is not an efficient fruit-storage chamber, except for short periods of time and for the protection of the products against freezing in winter. Cellars are difficult to ventilate unless special appliances are used. They may be ventilated by means of flues but the efficiency of such conduits is dependent upon differences in temperature, otherwise there will not be any appreciable movement of the air. Wind flues may be used; these are flues which have a funnel-like arrangement at the top, so designed that the mouth of the funnel is kept to the wind by means of a vane. The pressure of the wind entering the funnel creates a circulation of air through the cellar. In some instances these wind flues are found to work the reverse way during periods when the wind does not blow. In ventilating a cellar, there must be an outlet opening corresponding to the inlet opening. This is to allow the escape of the warm or foul air from the room. If it is possible to provide openings on all sides of the cellar, a current of air can be easily circulated through the room, especially if there is a breeze. The intake flues should open near the floor of the cellar. Fig. 3703 shows the proper placing of the inlets and outlets designed to ventilate cellar rooms. There should be a large number of openings to facilitate the ventilation of the cellar as rapidly as possible. Cellars are useful only for relatively high-temperature storage, and the necessity for insulating the walls of them is not sufficiently appreciated. The insulation must be sufficient to protect the cellar against the comparatively high temperature of the earth; otherwise, the temperature of the storage room cannot be held materially below the earth temperature. Common-storage rooms are frequently very cheaply constructed. The idea is prevalent that any old shed can be made to serve the purposes of a common-storage room for fruits or other products. The insulation is poor and, as a consequence, there is great fluctuation in the temperature. Proper insulation in the construction of a common-storage room is really more important than that for cold storage because there is no means of regulating the temperature except by ventilation or change of air, while the cold-storage room has artificial or mechanical means, the capacity of which can be increased to offset the heat leakage. To be effective, all common-storage plants must "conserve cold," and the necessity to provide efficiently against heat leakage through the walls, therefore, becomes doubly urgent. The insulation for a common-storage room or building may be of the commercial kinds, which have been described, or use can be made of some of the cheaper common materials, such as straw, chaff, dry leaves, sawdust, and mill shavings. The principles of constructing the walls and using the insulation are all applicable to common-storage buildings and should be followed carefully if one expects to secure the best results. In addition to the insulation of the walls, an outer ventilating space is effective, especially during warm weather. Fig. 3704 is a diagram which shows the proper method of constructing a common-storage wall with an outer ventilating space designed to carry off most of the heat absorbed by the outer wall. Windows are not satisfactory for ventilating common-storage plants. The openings should be at or near the floor and there should be corresponding openings at the top, as shown in the diagram illustrated by Fig. 3704. The taking-in of the outer air depends upon the difference in temperature between the bottom and top parts of the building; therefore the greater the number of openings, the more rapidly the air of the room can be changed. A false floor is a distinct advantage, and will add very materially to the efficiency of the plant. The construction and use of such a false floor is illustrated in Fig. 3705. When the false floor is used, the openings or ventilators should open directly under the floor, so that the outer air may have an opportunity to pass directly beneath the product stored in the room. A forced circulation is very much more satisfactory and will result in a more rapid change of air. In order to accomplish this, an exhaust fan should be placed at the top of the chamber, so that the air of the room can be drawn to the fan and exhausted into the outer air, thus creating a reduction of the air pressure within the rooms and the consequent drawing-in of the air to the room when the ventilators or traps of the room are open. Fig. 3705 shows a cross-section of such a chamber and the proper location of the fan. In the combination of the ice cooling and common storage, ice and salt or even ice alone can be used to cool the fruit at the early part of the season. The tube method may also be used with ice and salt or the gravity- brine system before described can be advantageously utilized. A diagrammatic cross-section of a combined ice-cooled and common-storage plant is shown in Fig. 3706. The design permits of the closing of ventilators and the opening of trap-doors, to utilize direct cooling from the ice stored above. A similar arrangement for the use of ice in small rooms can be made with the ice room or bunker placed at the end or side of the chamber. The Figs. 3707 to 3713 are diagrams showing the proper construction of walls and the method of insulating walls, ceilings, and floors both with commercial insulation and common materials. Figs. 3707, 3708, and 3709 show the proper method of applying insulation to stone, brick, and concrete walls; while Figs. 3710, 3711, 3712, and 3713 show the method of applying insulation to ceilings and floors. In the operation of common-storage plants, the fruit must be carefully watched at all times. The temperature should be taken frequently. It is very desirable that the actual temperature of the fruit itself be recorded from time to time. For this purpose, glass thermometers, the bulbs of which can be imbedded in the fruit, are desirable. Long-stem thermometers can be obtained which have the bulb at the end of a long tube and the recording scale at the upper end, thus allowing the temperature to be taken at the interior of the package. It is possible to note temperature conditions of the fruit by observing the influences of the temperature within the package upon the temperature of the room. When the fruit is thoroughly cooled throughout the mass, there will be little change in temperature after the closing of the ventilators. If the insulation of the room is effective, the change in the temperature of the air of the room will be very slight. If, however, there is any considerable heat left in the body of the fruit, there will be a marked and rather abrupt rise in the temperature after closing the ventilators. Careful attention to the condition of the fruit is necessary also to determine whether the humidity of the room is too high or too low. This will be shown by the appearance of the fruit. Excessive ventilation, i. e., the circulation of large volumes of air through the room, will cause shrinkage or shriveling, while insufficient circulation will favor mold. Special recording hygrographs which record the changes in humidity almost instantly upon a chart are very convenient adjuncts to storage rooms in order to observe the humidity conditions. Instruments which record both relative humidity and temperature on the same chart are obtainable. When one desires to operate properly, an investment in such an instrument is a distinct advantage. The length of time which different fruits may be held varies for the kind of fruits and even for different varieties of the same kinds of fruits. The importance of storage, then, is relative; it is most important for fruits which may be held longest. A short period of storage may be relatively as important for the short-season fruits, such as the perishable berries. The ability to hold these fruits even for a few days may result in a great profit due to changes in market conditions. The holding of short-period fruits for a brief time is important for canneries because the fruit may be held in its best condition and this may result in a great saving to the canneries or factories when sufficient help cannot be obtained. Storage is most important for the apple. This fruit has the longest storage period of all. There are cases in which apples have been held in fair condition for as long as two years. It is, of course, not profitable or desirable to hold apples as long as this. The most important season is during the winter and spring months and until the fresh fruits come into the markets. As indicated above, the cold storage of the apple has resulted in making it an all-the-year-round fruit. Many varieties are held from one season until the summer apples of the next season are available. The so-called winter varieties are held to the best advantage. There are three classes of apples: summer, fall, and winter. The summer varieties have the shortest storage season. The fall apples have a longer season but not so long as the winter varieties. It is upon the last class that dependence is placed for late-season supplies. The following fall varieties are the ones chiefly used for storage and, as a general rule, these may be held in first-class condition until the Christmas holidays or until the middle of January: McIntosh, Fameuse, Yellow Bellflower, Jonathan, Grimes. The following winter varieties are the ones of most importance for storage purposes: Baldwin, Ben Davis, Winesap, Yellow Newtown, Gano, Rome Beauty, Esopus, Northern Spy, Stayman Winesap, Banana, Ortley, Delicious, Lawver, Rhode Island Greening, Northwestern Greening, and York Imperial. The varieties of pears which may ordinarily be used for storage are: Bosc, Easter, Anjou, Clairgeau, Comice, Howell, Winter Nelis, Duchess, Sheldon, and Kieffer. Factors underlying successful storage. By means of investigations of the United States Department of Agriculture, the factors which govern the successful storage of fresh fruits have been carefully determined. The investigations have been extended through a number of years, since the work of Powell with apples in 1901-1902. There has been more work with apples than with other fruits but studies of the storage of grapes, peaches, pears, plums, cherries, and small-fruits have also been made. It is beyond the scope of this article to give in detail the results of researches with all of these fruits. In general, it has been found that there is a very definite relationship between the character of the fruit and the treatment given it in preparing it for storage, and its behavior in storage. The results from extensive experimental storage holdings have been consistent throughout; there have been no exceptions to the general principle of this definite relationship. It has been found, for example, that the influence of the place of production is frequently of great importance. The place and condition under which the fruit may be grown have a material influence on its behavior in storage. This is contrary to prevailing impressions but it is definitely certain. The character of the soil upon which the fruit is grown may have an important bearing on its storage quality. For example, apples from the lighter loam soils have better keeping quality than fruit grown on heavy or wet soils. In the study of the storage of grapes, it was found that the fruit grown in certain types of soils have better market and storage qualities. Some Tokay grapes, grown in California in light sandy soil, reach the limit of their market condition in November, while grapes of the same variety grown in heavy black soil may be kept in good condition until after Christmas. The Emperor grape, which has become an important storage fruit in California, is produced under best conditions in the red soils of the higher benches of the foothills of the Sierras. The same variety grown under valley conditions where the soil is of a different character, does not color so well and does not have as good storage qualities. The same is true of the Almeria grape, which is likely to become a very important storage fruit in California. At present the supplies of this grape come almost exclusively from Spain. The grapes are packed in granulated cork and the Spanish product is frequently held for several months in common storage. California-grown Emperor and Almeria grapes are packed in redwood sawdust and are successfully held in cold storage, the former until the middle of January, and the latter several months later. Differences of one to three months in the storage qualities of the same varieties of apples have been found to be due to the place of production. Mention has already been made of the storage weakness of the Yellow Newtown and other varieties grown in the Central Pacific coast district of California. The same varieties grown in the Pacific Northwest and in Virginia and other sections of the country are free from this particular weakness. The care of the orchard and method of culture given the trees have been found to be important factors. The character of tillage, pruning, age of trees or vines are also considerations, especially when taken in connection with different climatic conditions. The fruit from young trees or vines has weak storage qualities; it is usually large, coarse, sappy, and cannot be held in storage nearly so long as fruit of the same variety from older and more mature trees. Dense-headed trees produce fruit of poor color; the green, poorly colored apples produced under such conditions do not have high storage quality. Such fruit is very susceptible to the trouble known as storage-scald; the loss from this source may be avoided and the storage quality of the fruit may be much improved by better orchard methods. Pruning to open up the crowns of the trees will improve light conditions, especially where intense sunlight does not naturally prevail. Some growers actually cut away the leaves of the vines to allow light to color and mature the grapes to better advantage. In sections such as the arid regions of the Pacific coast, where intense light conditions prevail, the opening-up of the tree crowns must be done with greater care. It is not necessary to open up the trees to such an extent as is necessary where intense light is not naturally available. Late growth also affects the storage qualities, as it prevents the proper maturing of the fruit. In irrigated districts, the late application of water may stimulate the growth while the fruit is maturing and this may result in sappy poorly colored fruit of low storage quality. The question is often asked whether the fruit produced in irrigated districts has as good storage qualities as that from non-irrigated districts. The impression seems to prevail that it does not. This is erroneous, as has been shown by extensive investigations. It is manifestly impossible to compare directly fruit grown under irrigation in one district with fruit grown without it in another. The varieties are different and other factors may operate to change conditions in the one case or the other. There are thousands of boxes of irrigated fruits held in the best possible condition in storage, and this would seem to be a direct answer to the question of the keeping qualities of fruits grown under irrigation. It is necessary, of course, to have the irrigation properly applied; if overdone by applying large quantities of water late in the year in order to induce large sappy growth, the results are fruits of poor storage qualities. In sections which have dry summers, where tillage is depended on to conserve the moisture, the work must be thoroughly and properly done; otherwise, the moisture supply in the soil will be deficient, and the trees or vines will be under stress on account of the lack of sufficient moisture. Fruit produced under such conditions has very low storage quality. Any condition of soil, climate, and orchard treatment which results in the production of abnormal fruits may be important governing factors in their behavior in storage. Spraying for the control of insects and diseases is important from the storage standpoint. It is necessary that this work be thoroughly and properly done, as insect and disease injuries render the fruit liable to deterioration. The insect or disease may be of itself the cause of the decay or deterioration. There is also an indirect effect: when the trees are weakened by the effect of insects and diseases, the results may be weak fruit of poor storage quality. Seasons affect the quality as well as the quantity of the crop. In seasons of unusual drought, for example, the fruit may be so weakened that its storage qualities may be seriously impaired. On the other hand, unusually wet seasons result in the production of sappy fruits which deteriorate rapidly. In seasons of unusual drought, the orchards under irrigation have a distinct advantage, provided the application of the water is properly adjusted. The effect of frost may be beneficial or otherwise: A crop may be thinned to such an extent that its condition may be somewhat improved since over-production by the tree may result in weak fruit, while the thinning will improve this condition. On the other hand, where the frost is sufficient to destroy most of the crop, the remaining fruits may be sappy and overgrown and otherwise weak. The frost-injured fruits themselves have not as high storage qualities. During an unfavorable season, fruit which has been placed in storage must be carefully watched throughout the entire storage period. Its condition should determine the length of time it is held. The effect of storage is such that attempts to hold the fruit beyond its normal life period result in serious losses. Fruit of low vitality, when the limit of its life is reached, will deteriorate very rapidly after withdrawal from cold storage. It is important, therefore, not to wait until the fruit is ready to break down before withdrawal. Fully matured well-colored fruit keeps best and longest. Early notions that fruit for storage should be picked in an immature condition are erroneous. The fruits which are picked before full maturity have low storage qualities. There is serious deterioration from shriveling and, in case of apples, there is a definite relationship between the occurrence of scald and the state of maturity at which the fruit is picked. This disease is a peculiar browning or scalding of the skin of the fruit. It does not extend into the flesh except under very severe conditions. Immature fruit is seriously affected while fully mature fruit of the same variety may be held without deterioration from this cause. Full maturity means that the "ground color" is plainly developed, the flesh of the fruit firm, and the seeds fully grown and colored. This principle is correct for all fruits with the possible exception of most varieties of pears and lemons. Over-ripeness must be avoided. A designation of the proper stage of maturity for picking fruits is difficult; it must be learned by actual experience. Over-ripeness or over-maturity occurs when the fruit begins to soften. In some instances, growers are in the habit of allowing the crop to remain on the trees until all the fruits are fully colored. This is a wrong practice, as some fruits mature before others, and if allowed to remain until all are colored, may become over-ripe or over-mature. It is best to make more than one picking, especially with the earlier ripening varieties. Fruits on the outer branches exposed to full light ripen first and the best results in storage are obtained when these are held separately, unless the trees are well and properly pruned. Reference has been made to the importance of cooling the fruits promptly and rapidly, in connection with the ripening processes and life activities and the effects of temperature on these factors. Delay in storage, which means delay in cooling, during warm weather may shorten the storage period from one-third to one-half. Experiments with apples held at a comparatively high temperature for a period of ten days or two weeks before cooling, showed that fruit thus treated could be held only from one-third to one-half as long as the same varieties promptly stored and cooled after picking. There is also a direct influence on the occurrence of scald. Prompt cooling, as a rule, prevents the occurrence of this disease. This factor is of special importance for early-season fruits or early varieties of apples like Jonathan. The practice which prevails in many sections of allowing the fruit to accumulate for some time before placing it in storage is likely to result disastrously if the season happens to be warm. This is especially true where the fruit must be packed while warm. The ideal condition is the placing of the fruit under refrigeration immediately after picking from the tree and the nearer this can be approached in practice, the longer the fruit can be held in storage. The proper storage temperature for different kinds and varieties of fruits has already been referred to. The influence of a low temperature, especially for apples, is most important. Experiments show the occurrence of scald to be less severe at 32° than at a higher temperature. The use of a low temperature is also important because of the rapidity with which the fruit within the package can be cooled. In operating either a common- or cold-storage plant, the temperature of the fruit is an important factor to consider. When a fruit is first placed in the storage room, a considerable length of time may be required to reduce its temperature to the desired point, if means are not at hand to increase the cooling effects. In cold-storage rooms the cooling can be hastened by holding the air of the rooms at a temperature several degrees lower than the desired temperature. A temperature of 25° to 27° F. may be safely maintained until the fruits in the packages approach the storage temperature. In this way the operator can materially hasten the cooling effect, and this hastening is desirable. It is commonly assumed that the cooling should be gradual, but as yet there is no experimental evidence to indicate that rapid cooling is at all injurious. The investigations of the United States Department of Agriculture show conclusively that the character of the treatment given the fruit in preparing it for market or storage has a material influence upon its keeping quality. Fruits which are roughly handled and bruised or injured to any extent have their storage qualities seriously affected, and decay and deterioration follow the injuries. There are some forms of decay or deterioration which cannot develop unless there are injuries of some kind on the fruit. A break in the skin will allow blue mold to gain entrance, while a sound skin may prevent the development of this form of decay. Blue mold is one of the most common forms of loss both in common and cold storage and occurrence of this trouble is due almost exclusively to rough handling. The blue mold does not grow upon the sound skin of a healthy fruit. The importance, then, of handling the fruits with extreme care throughout all the processes of picking, grading, and packing cannot be too strongly emphasized. Bruises or breaks in the skin may mean a decayed fruit. This general principle has been established through a long series of careful investigations and thus far there has been no exception. There are other decays which affect apples and other fruits: the principal diseases affecting apples are brown- or ripe-rot, anthracnose, and bitter-rot. These diseases are not dependent upon the care in handling the fruit so far as the occurrence of bruises or injuries is concerned. The spores are present on the fruit when it is packed and the control of the disease in storage goes back to the orchard treatment and the control of the fungi on the trees. All three of these decays occur on the trees as canker spots and the spores which inhabit the fruits develop from these cankers. Control of the cankers by cutting out or spraying will materially reduce the occurrence of decay in storage. There are other forms of deterioration which, so far, have not been traced to any definite organism. These are obscure physiological diseases and result in the breaking down of the flesh of the fruit, or in burning of the tissues, or in a scalded appearance of the skin. These physiological troubles have been found to be due, at least to some extent, to rough handling or to pressing. A physiological breakdown also occurs in fruit which is over-ripe when it is stored. It also occurs seriously in fruit which has been delayed in cooling after picking. Physiological breakdown also occurs in fruit which is held beyond its normal life limit. Ordinary storage-scald has already been referred to. The nature of this disease is unknown but it is supposed to be due to the action of enzymes upon the skin of the fruit. There is another form of scald which, for want of a better term, has been designated as "soft scald." Ordinary storage-scald does not soften the skin except in the most advanced stages. The soft scald produces a softening of the skin and also of the flesh directly beneath the skin. It occurs also in more or less distinct areas or zones, sometimes extending completely around the fruit. The nature of this disease is obscure and, so far, storage treatment, orchard treatment, and temperature effects do not seem to have any bearing on it. It has been attributed to the freezing of the fruit in storage and, while definite results from the effects of freezing have not been obtained, it is possible that in storage rooms of uneven temperature conditions portions of the rooms may have temperature conditions sufficiently low actually to injure the fruit. For discussion of precooling, see the article Transportation. CH
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References
- Standard Cyclopedia of Horticulture, by L. H. Bailey, MacMillan Co., 1963
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