The preservation of meat, fruit, dairy produce and other perishable commodities which are shipped from overseas is of great importance to health. Different types of foodstuff require widely different refrigerating conditions
CARBON DIOXIDE REFRIGERATING MACHINES in the motorship Tuscan Star, a twin-screw vessel of 11,499 tons gross. She has a total of 640,000 cubic feet of refrigerated cargo space. The refrigerating machines are of the vertical-enclosed single-acting CO₂ compression type, coupled directly to electric motors. The Tuscan Star, of the Blue Star Line, was built in 1930.
ICE is particularly suitable for food preservation. Its suitability is due, not so much to its low temperature as to the large amount of heat that it absorbs during the process of melting. It is therefore not surprising that the earliest refrigerating machines were built simply for the purpose of making ice.
Heat tends to flow from a substance at a high temperature to one at a lower temperature. The purpose of a refrigerating machine is to reverse this natural process and cause heat to be transferred from substances at low temperatures to other substances at higher temperatures. Most modern refrigerating machines operate on what is known as the “vapour compression” system. They depend on the fact that when any substance undergoes a change of physical state from solid to liquid, from liquid to vapour, or vice versa, a large amount of heat is either absorbed or given out without any accompanying change in temperature.
For example, while water is boiling, heat is continually being absorbed to convert it into steam, yet the temperature of the water remains unaltered and identical with that of the steam formed. When the steam is condensed and has again become water, due to contact with some cooler substance, the heat which was required to convert it into steam is given out and absorbed by the cool substance. Similarly, a lump of melting ice remains at a temperature of exactly 32° Fahrenheit, in spite of the fact that it is continually absorbing heat. Again to freeze the water formed by its melting the same amount of heat must be extracted from it, yet the whole process of melting and refreezing, with the large transferences of heat involved, can be effected without any change of temperature. This heat which is evolved or absorbed without producing alteration of temperature, is termed “latent heat”.
In a refrigerating plant operating on the “vapour compression” system an easily liquefiable gas, called the refrigerant, is liquefied by compression in the refrigerating machine — which is simply a gas compressor of special design — and by the removal of the latent heat of liquefaction in a condenser. The refrigerant then passes through a regulator valve and is evaporated in suitable pipe coils, or some equivalent appliance under a lower pressure produced by the suction of the compressor. In the evaporator the heat required for vaporizing the refrigerant is absorbed directly from the air in a cold room, from some liquid which is to be cooled or from a non-freezing calcium chloride “brine” which is circulated through cooling appliances. This cycle of operation, involving a change of physical state from vapour to liquid and back again to vapour, is accompanied by the liberation and absorption of a quantity of heat many times greater than that due to the mere changes of temperature of the refrigerant. Furthermore, since the circuit is closed, the same refrigerant is used over and over again.
For marine purposes, carbon dioxide is by far the most widely used refrigerant in modern machines of the vapour compression type. On land, for large and medium-sized plants, anhydrous ammonia holds the premier position, but is beginning to feel the competition of “Freon” (dichlorodifluoromethane). Its higher price is probably the chief hindrance to its wider adoption. Small machines use methyl chloride, sulphur dioxide or ethyl chloride.
The first large-scale commercial application of mechanical refrigeration was to the transportation of fresh meat to Great Britain from overseas, a problem which became a matter of urgent national importance about seventy-five years ago. The arrival of the Strathleven in the London Docks in 1880, with a cargo of frozen Australian beef and mutton, marked the beginning of the era of long-distance food transportation. Large numbers of refrigerated vessels were built in later years and, with the cold storage capacity provided on land at the ports and in the principal inland towns, the problem of providing a satisfactory food supply for the population of the British Isles was, to a considerable extent, solved.
Chain of Preservation
The solution was, however, incomplete, as there was still a break in the chain between producer and consumer. Food, kept in an excellent state of preservation during transport and its stay in the large cold stores, had to undergo a further period of storage on the premises of the retailer, where something was needed to continue the process of preservation. Until recently this was done by the use of chemical preservatives. The harmful effect of the widespread use of these substances on the health of the population came to be realized and their use, with a few exceptions, was forbidden by law.
This prohibition could scarcely have been issued if it had not been known that other means of preservation were available in the form of small refrigerating plants of a size suited to the retailer’s needs, comparatively inexpensive and automatic in operation, so that the difficulty of providing skilled supervision was overcome. The retail meat trade was the first industry to adopt extensively the small, automatically-refrigerated, cold storage chamber. Other retailers of perishable foodstuffs are now following the example of the butcher, and the final link has been added to the chain of preservation by the introduction of the domestic refrigerator into the nation’s homes.
EARLY REFRIGERATING MACHINE working on the “cold air” system now obsolete. This machine was built about 1885 for marine use. Drive is by a compound steam engine having two cylinders in tandem (on the left). Air, drawn from the cooled space, is compressed in the middle cylinder, then cooled, and then expanded while doing work in the heavily lagged cylinder on the right. The air is finally returned to the cooled space.
In speaking of cold storage it is important that the difference between chilling and freezing should be appreciated. Meat to be tender and palatable requires to be hung in a cool atmosphere for a fairly long time to “condition”. Cold storage, at a temperature of about 34° Fahrenheit or slightly lower, is suitable for this process and the butcher’s cold chamber used for “home killed” meat is as necessary for conditioning purposes as for storage. Because of the length of the voyage, the time which must necessarily elapse between the killing and consumption of meat shipped from Australia and New Zealand is too long for chilled storage. So this meat is frozen hard before shipment and kept frozen during the voyage and in the large cold stores in Great Britain. On the other hand, meat imported from Argentina is not frozen, but is kept in chilled storage in the ships which carry it.
Broadly speaking, the freezing of foodstuffs is resorted to only when chilled storage, for one reason or another, proves inadequate. Freezing, although it enables storage to be prolonged almost indefinitely, tends to impair quality and damages some commodities severely, the outstanding example being fruit, which is completely spoilt if frozen. Other foodstuffs, such as mutton and lamb, are affected only to a negligible extent. Scientific investigation has, however, led to the discovery of special methods of freezing by means of which it is hoped that all objections to the process may ultimately be overcome. Some of these methods have already achieved considerable success in commercial use.
Although mechanical refrigeration finds its widest application in the storage and transport of foodstuffs, it has come to play an important part in the manufacture and preparation of food. It is used not only for storage purposes but also for the control of temperature in the making of margarine, in the curing of bacon and ham, in the manufacture of chocolates, sausages and meat pies, in the brewing of beer and in the large dairies where milk is subjected to treatment such as pasteurization or sterilization. Ice cream, now consumed in vast quantities, would be a rare and expensive luxury in the absence of refrigerating machinery to produce the low temperatures required for freezing, hardening and storing it. Refrigeration is used also in air conditioning, in the refining of oils, in the testing of scientific instruments, in bacteriological research, in excavation work, and the sinking of mine shafts through wet ground or quicksand and so on. The fishing industry, which still requires' enormous quantities of ice for the trawlers operating on distant fishing grounds off the coast of Iceland and in the White Sea area, depends on refrigeration for its supplies of the ice in which the catch is packed for the return voyage.
Gas Storage
The largest plant of its kind in the world is situated at Grimsby, Lincs, and is capable of making 1,100 tons of ice every twenty-four hours. There are four ammonia compression refrigerating machines of the vertical enclosed single-acting type. Each machine has four cylinders and is coupled directly to an electric motor of 600 horse-power, running at 250 revolutions a minute. There are six ice-making tanks through which a rapid circulation of cold “brine” is maintained. Immersed in this brine are nearly 10,000 rectangular cans containing the water which is to be frozen. The cans in some of the tanks are of 2 cwt., and in others of 24 cwt. Capacity.
As the trawlers require the ice to be delivered on board crushed into small fragments, ice-crushing machines are provided. Some of these are at a considerable distance from the factory, and the heavy blocks of ice travel to them along an overhead sloping runway down which they slide at a velocity which reaches about 40 miles an hour towards the end of the run.
The blocks are white and almost opaque, because costly agitating gear must be provided in each can if clear ice is required.
An application of refrigeration of great importance is the transport of fruit from overseas and the storage of home-grown fruit, especially apples. For this purpose the ingenious system known as “gas storage” is used. The outstanding difference between fruit and most of the other commodities for which cold storage provides a method of preservation, is that a fruit is not a dead substance but a living organism, which must be kept alive because decay begins when death takes place.
FOUR VERTICAL ENCLOSED AMMONIA COMPRESSORS in the engine room of the Grimsby Ice Co., Ltd., Lincolnshire. This is the largest ice-making plant of its kind in the world, as it can manufacture opaque ice at the rate of 1,100 tons a day. The four-cylinder single-acting compressors are coupled directly to motors of 600 horse-power.
Fruit keeps alive by breathing, absorbing oxygen from the atmosphere and giving off an equal volume of carbon dioxide. This process is accompanied by the evolution of heat. The amount of heat generated by a single apple, or other fruit, is exceedingly small, but the amount generated by a mass of fruit with an aggregate weight of many tons is far from small. Moreover, the process is cumulative. The warmer the fruit, up to a point, the more quickly it respires and the more rapid is the evolution of heat. Thus the temperature of a large mass of fruit, in a store or in a ship, tends to rise rapidly. This means that the quantity of heat to be removed by a refrigerating machine from a space containing fruit is larger than from one containing the same weight of meat or any other dead substance, so that a more powerful refrigerating machine is required for the fruit, assuming that the same temperature is to be maintained in either instance. For the initial cooling of freshly picked warm fruit a still more powerful refrigerating machine is needed.
All fruits are, unfortunately, easily damaged or killed by too high a temperature, too low a temperature, or by suffocation, which occurs if the atmosphere in the store, due to the respiration of the fruit and the lack of ventilation, becomes deficient in oxygen and too heavily charged with carbon dioxide. Thus adequate ventilation with fresh air must be provided and a proper circulation of air throughout every part of the store must be maintained by power-driven fans. The object of cold storage applied to the keeping of fruit is simply to delay ripening and prolong the term of its natural life by slowing down its respiration or, in other words, the rate at which it lives.
Generally the maintenance of a suitable temperature with adequate ventilation proves satisfactory, but there are exceptions. Apples grown in Great Britain are placed in cold storage at the end of the summer for sale chiefly during the following March and April.
Apples that Breathe
With certain varieties it was found that a temperature low enough to preserve them for this period caused internal damage to the tissues of the fruit, so that although it appeared to be in excellent condition when taken out of the store, it went bad soon afterwards. Its life, after removal from store, was too short. If, however, the temperature in the store were raised a few degrees, such damage did not occur, but the fruit went bad in the store.
Research showed that at a temperature well above the danger limit, a certain proportion of carbon dioxide in the atmosphere of the store, with a corresponding reduction in the amount of oxygen, caused a retardation in the life processes of the apple much greater than that obtainable with temperature control alone, ft a store be made gas tight the apples contained in it, by their respiration, gradually reduce the volume of oxygen present and replace it by an equal volume of carbon dioxide. It still remains essential, however, to provide refrigerating machinery to maintain the correct temperature and to arrange for a small amount of accurately controllable ventilation with fresh air to prevent the attainment of such an excess of carbon dioxide and, simultaneously, such a deficiency of oxygen that the apples would be suffocated. That is the process known as gas storage.
Enormous quantities of fresh fruit are now imported into Great Britain from South Africa. Nearly all this fruit is shipped from Capetown, Port Elizabeth, East London, Durban or Lourengo Marques, where it arrives by rail from the fruit-growing districts inland. On reaching the port it is warm, and often has to await the arrival of a vessel with suitable refrigerating equipment. At the ports large fruit pre-cooling stations have been erected which serve the double purpose of providing suitable storage for the fruit during the waiting period and of cooling the fruit so that, when placed in the refrigerated spaces in the ship, it is already at the correct temperature.
ICE-MAKING TANK for an ouput of 50 tons of clear ice a day. When freezing is complete the ice cans are lifted in rows, immersed in warm water to free the ice blocks and then tipped. The cans are then refilled by means of the filling gear shown, and returned to their position in the ice tank.
