Decomposition of food is due to the growth of bacteria, yeasts and moulds. If food is to be maintained in a wholesome condition the growth of these organisms must be prevented; this can be effected either by the application of heat sufficient to destroy them, by the reduction of the moisture content of the food, by the use of chemicals (which sterilize or prevent development of the organisms), or by subjecting them to low temperatures in the region of freezing point or below. These low temperatures do not destroy the organisms but by paralyzing activity inhibit growth.
The importance of refrigeration for the storage of perishable foodstuffs has been appreciated for many centuries. Long before the actual causes of deterioration were understood, it was realized that deterioration of certain foods was delayed by storage at low temperatures. It is known that the famous Elizabethan Sir Francis Bacon was very interested in the whole problem of refrigeration and was investigating it at the time of his death which may indeed be attributed to his experiments in this field. It was as a direct result of his attempts to preserve a fowl by filling it with snow that he developed the chill which caused his death.
Methods of refrigeration used in the past include the burying of food in the snow (still done in Arctic regions); storing it in underground buildings called Ice Houses lined with ice blocks cut out during winter months–a fairly common practice in this country a couple of centuries ago; and using caves which not being affected by outside variations in temperature maintain a constant temperature through the year. But it was not until a mechanical method was invented by which the temperature of the air in an enclosed space could be controlled automatically that the preservation of food by refrigeration became a really practical proposition. Modern refrigerators are designed to maintain a constant temperature throughout the year, the actual temperature varying according to circumstances. For short term storage to suit day to day requirements a temperature of 35°F. to 40°F. is adequate, while for long term work, such as the transport of meat from the other side of the world or the preservation of fruit and vegetables from summer to winter, a temperature well below freezing point is essential.
The food service manager who can rely on day to day delivery of all perishable goods uses the refrigerator mainly for short term storage, to preserve foods such as milk, fish and meat for a period of about 24 hours. It is also used for salads, ice cubes and in the preparation of foods which must be served really cold, i.e., at a temperature below that of the surrounding atmosphere. Some foods, as for example, butter, margarine and lard are kept in refrigerators at 35°F. to 40°F. over long periods, but lower temperatures are generally required for any long term storage.
In large kitchens it is usual to have at least three separate refrigerators one each for meat, fish and other provisions respectively, the temperatures of the first two being maintained at about 32°F. and of the third at 40°F. The use of a cool room sometimes called a larder for the storage of bread, cakes and pastry, etc., is still fairly general in kitchens in the UK. The temperature of this room should be below 50 F. and if such a temperature is not possible by natural methods refrigeration equipment should be installed.
The use of frozen foods requiring storage temperatures of 0°F. to – 57. is now generally established in the majority of large kitchens. Such storage can be in the form of factory-built cabinets or chests or of built-in cupboards or storerooms. A useful practice in planning for a deep free storeroom is to locate it within a refrigerated room, thus when the deep freeze is opened the cold air which is inevitably released flows into the surrounding refrigerators and conserves the surely of energy.
Refrigerators are also becoming more popular for the day to day storage of kitchen waste. There is inevitably a certain amount of waste matter discarded during the preparation of foods and particularly of vegetables and this, together with the dining room plate waste, frequently presents a problem if it cannot be collected immediately by the local authority or contractor for pig food. It is usually put into covered bins which are kept in an enclosure just outside the kitchen; in hot weather the contents of the bins quickly become insanitary, causing smells and attracting flies. If, however, these bins can be stored at temperatures of 35°F. to 40°F. the process of deterioration is delayed, and it is becoming common practice in building new kitchens to provide a cold room for this purpose.
Principle of Operation
The principle underlying the operation of a refrigerator is based on the fact that heat will always flow from a hot substance to a less hot one until both are at the same temperature. If therefore the temperature of one part of an enclosed cabinet can be maintained at, say, 40°F., in time the whole cabinet and any food put into it will be kept at this temperature also. In the manufacture of refrigerators two entirely different systems are used to produce the necessary cold section, known as the evaporator. One of these is called the Continuous Absorption type and the other the Compressor type. The former operates entirely by the application of heat, has no moving parts and is therefore quite silent, whereas in the latter an electric motor and fan are required and a certain amount of noise is inevitable. Although the latter is more commonly found because the absorption type is made only in sizes up to 8 cu. ft., both types are used by caterers and it is therefore worth while to consider briefly both methods of operation, since an understanding of the principles employed naturally contributes to efficient use.
Both systems are based on the fact that when a liquid changes into a gas it takes up heat from its surroundings. A gas chosen because it is easily liquefied is compressed and cooled; the pressure on the resulting liquid is then reduced, so that the full evaporates, with an automatic loss of temperature, and litt flows in from the surroundings.
The Compressor Type
The compressor type refrigerating unit consists of four main parts, the electric motor, the compressor, the condensent evaporator; the last three of these form a sealed unit filled with the refrigerant, which alternates between the gaseous and liquid state in circulating round the system. The electric motor is used to operate the compressor which is simply a pump used to compress the gas. This compression raises the temperature of the gas, a well-known thermodynamic principle, and the gas is then forced through a delivery valve into the condenser which consists of a series of coiled pipes with fins on the outside. A stream of cold air from a fan is blown on to these coils, cooling the gas and causing it to liquefy. In the next part of the circuit the liquid passes through an expansion device into the evaporator, a vessel maintained at a much lower pressure, and the effect of this is to cause the liquid to evaporate and become a gas once more. In this operation the temperature falls and the cold gas in passing through the evaporator which consists of a series of coils placed within the storage chamber of the refrigerator, absorbs heat from the surroundings including the stored food; it then returns to the pump for compression again.
The most common refrigerants used in this country for this type of refrigerator are methyl chloride and dichlorodifluoromethane, commonly called Freon 12.
The usual pressure from the compressor up to the expansion device at the entrance to the evaporator is 70-80 lb. per sq. in. and from the expansion device back to the inlet of the compressor a few lb. per sq. in. Moreover the refrigerant is a gas from the point where it leaves the expansion device through the evaporator to the compressor and again as far as that point in the condenser where its temperature has fallen sufficiently far to cause it to liquify. The mutual effect of temperature and pressure can be seen by reference to the above table: for instance, if the pressure is 70lb. per sq. in. the temperature of the gas must be dropped to 70°F. in order to convert it into a liquid.
The Continuous Absorption Type
In the absorption type of refrigerator ammonia gas and water are enclosed in a sealed unit in which sufficient pressure is maintained to condense ammonia at ordinary room temperatures. Ammonia is readily soluble in water but can be separated from it by heat. The refrigerating unit consists of four main parts, the boiler, the condenser, the evaporator and the absorber, all of which form one continuous sealed unit. A solution of ammonia in water is heated in the boiler by a gas jet, electric element or oil burner. The effect of this is to drive off ammonia gas at high pressure; the gas then passes through a series of coils constituting the condenser, where it is cooled and consequently liquified. The liquid ammonia so produced passes through to the evaporator where it vaporises in the presence of hydrogen, absorbing heat from the cabinei. The mixture of hydrogen and ammonia flows into the absorber where it meets a weak solution of ammonia connected to the boiler. The ammonia is dissolved and flows back to the boiler while the hydrogen is freed to return to the evaporator. In this way the cycle is continuous so long as heat is applied to the boiler.
Practical Refrigerators
The refrigeration facilities found in large kitchens may take the form either or a cabinet or of a built-in cold room. Cabinets are made in varying sizes up to 1000 cu. ft., those from 500 to 1000 cu. ft, usually being specially made to suit the space required. Dimensions rane from 4′ x 2′ x 5.0″ for a 25 cu. ft. cabinet to 7 “X 10″ ‘6″ for a 250 cu. ft. model. Cold rooms on a larger scale re made to fit in with the general design of the building. Cabinets unsist of an inner and an outer casing, the space herwoo buing filled with thermal insulating material; expanded peitsyrere of 3” to 4″ thickness is most commonly used for this purpose to vas. The inner casing is usually made of sheet steel finished with channel, and it must be so constructed that condensation and drippings cannot penetrate through to the insulation. The outer casing may also be of enamelled steel but wood, hard board or sheet asbestos suitably painted are more common for large refrigerators.
Since the cold room forms an integral part of the structure of a building it should preferably be planned and constructed at the same time as the main building although such rooms can if required be added at a later date. When several are required, as is common in large kitchens, it is advisable that they should be placed together so that the refrigerating machinery can be concentrated in one place. The final dimensions of a cold room will be less than those actually appearing on the plan owing to the special provision to be made for refrigeration and insulation. A thickness of 4″ insulation must be allowed on walls, floors and ceilings and 2″ granolithic on floors while a still further reduction of 12” width over the whole area of one of the sides must be made for the evaporator.
Refrigerators of over 100 cu. ft. are so made that it is possible to walk inside them; they are therefore sometimes referred to as the Walk In Type, while the smaller ones ranging from 50-100 cu. ft. are known as the Reach In Type. The main difference between the two is the size but the Walk In Type must have doors which can be opened both from inside as well as outside and the refrigerating machinery may be in a different position. In the Reach In Type this is usually fixed in a compartment either above or below the cold section so as to form a part of the whole cabinet. In the Walk In Type the position of the machinery varies. It may be fixed on the top provided the space in which the cabinet is installed exceeds the seven feet to eight feet of its height, or it may be installed at any convenient point within twenty feet of the cabinet.
In both types there is usually one main compartment for general use, fitted with shelves and hanging rails for meat, and a separate compartment for fish fitted with trays of sheet steel with perforated removable bottoms. It is essential that the interior of the cabinet and all the fittings should be so designed that they are easily cleaned. One way of achieving this is by the provision of light weight mobile racks which not only make the work of cleaning easier but also facilitate the loading and unloading of the shelves. It is not always appreciated that these cabinets are fitted with a floor drain for connection through a trapped gully to the main drainage system. This outlet is useful for both cleaning and defrosting operations, and if it is not available water will on occasions be found to leak over on to the adjoining floor.
Locating the refrigerator
Three points have to be considered before it is decided where to place the refrigerator. The use to which it will be put of course affects its position, but not to the same extent as in the placing of other pieces of equipment such as ovens and boilers. It is required for the storage of goods immediately they are delivered, it must be readily accessible for the issue of goods to the kitchen, and in addition it must be available for foods at certain stages of preparation.
The next point to consider is the temperature of the surroundings. Refrigerators are provided with thermal insulation to reduce the inflow of heat from the outside. The higher the temperature of the surroundings, the greater will be the intake of heat through this insulation and the higher the running costs. In fact if the refrigerator is located in too hot a situation the machinery may not be able to take the heavy load necessary to lower the temperature by the amount required. It is better therefore not to install the refrigerator in the kitchen but in an outside corridor. If, however, it must be put in the kitchen, it should be kept well away from boilers, ovens and hot plates. Moreover, it is advisable therefore not to install the refrigerator in the larder, since the refrigerating machine itself gives off heat in the condenser and thus will cause the temperature of the surroundings to rise unless free ventilation is available. The placing of cold rooms is a somewhat different matter, but here again it is important that the load on the refrigerating machinery due to inflow of heat from the surroundings should be minimized and they should therefore be located on N. or E walls to avoid sunshine.
The third point to consider is the noise of the machinery. It is important of course that this should not be excessive and when purchasing equipment one should make sure that the design and mounting are such that as little noise as possible is produced and transmitted. But even so with the compressor type system some noise is inevitable and the machinery should if possible be installed away from places where quiet is essential.
Performance
The duty of the plant is stated* as follows: “To maintain an inside temperature of 35°F. when the outside temperature is 80°F. and the running time of the compressor does not exceed 20 hours in a 24-hour day, and shall be capable of lowering the temperature of the cold room from 65°F. to 35°F. within 24 hours when stacked with food at 3lb. per cu. ft. (for this purpose the average specific heat of food is to be taken as 0.8).”
Thus a refrigerator should be able to stand up to all normal demands at all times, even in the hottest weather. But perhaps we should examine in more detail what is meant by normal demands. It is frequently not fully appreciated that food at room temperature which appears cold may be 20°F. to 40°F. hotter than the food kept in a refrigerator at 35°F., and that if the refrigerator is packed with such food the time taken to reduce the temperature of all of it to 35°F. will be 24 hours. This time lag may have important effects on preservation. If milk, for example is kept at a temperature of 65°F. to 70°F. the growth of bacteria which cause it to go sour is quite rapid, whereas if it can be cooled quickly to 35°F. and kept at this temperature the keeping qualities are considerably improved. If food at a still higher temperature is placed in the refrigerator, the load on the machinery is increased still more and cooling times are further extended. And if in addition the food is actually steaming when placed in the cabinet, the condensation on the evaporator is considerably increased causing it to become heavily frosted, and there. fore less efficient. It is an important rule therefore that all food should be cooled to room temperature before being placed in the refrigerator, although it is one which is sometimes ignored in practice. It sometimes happens in hot weather that large bowls of stewed meat in gravy are placed in the refrigerator while still hot and left overnight. This causes the temperature inside the refrigerator to rise and because of the heavy load placed on the machinery by the bulk of hot food, subsequent cooling only takes place slowly, so that the temperature inside the cabinet during the night may be actually higher than it is outside. The result is that when the meat is taken from the refrigerator on the following morning it is found to have ‘fermented’. What has actually happened is that the growth of spores of certain bacteria not destroyed during cooking has been encouraged by the conditions in the refrigerator, and the toxins and gas so produced have made the meat unfit for consumption.
Most refrigerators are provided with a thermometer and a glance at this from time to time is a useful check on the satisfactory running of the machinery. Temperatures will of course vary according to the use made of the refrigerator but the thermometer should usually read between 35°F. and 40°F. and should never exceed 50°F.
Provided that a refrigerator is used carefully, it should not present any maintenance problems and will operate satisfactorily for many years. It is usual to have the machinery examined and oiled at regular intervals by maintenance staff and this should ensure that it is always in good running order. In addition to this, however, the evaporator must be defrosted when necessary. A certain amount of drying takes place on the surface of all uncovered food placed in the refrigerator. This is because the air itself loses water vapor by condensation on to the other cold surfaces and in turn it takes up moisture from the uncovered food. Thus the cold surface of the evaporator gradually collects a coating of frost which when it reaches a thickness of 1″ begins to affect the efficiency of the machine. The evaporator must then be defrosted by cutting off the current and opening the door for a short time, so that the temperature rises above freezing point. The time a refrigerator will run before defrosting becomes necessary depends on the use to which it is put, but it is advisable to establish a fixed routine for this operation to ensure that it is not overlooked. It is generally found satisfactory to defrost once a week.
One of the major factors in achieving safe and efficient operation of cold rooms is the design of the door mechanism. To conserve energy it is vital that the opening of the door during use should have the minimum effect on the internal temperature. The question of safety to prevent an operator being trapped inside must also be covered. The door should be so designed that it can be opened and closed quickly and easily and in order to keep maintenance to a minimum the closure mechanism must be robust. For safety purposes the door must be capable of being opened easily and with certainty from the inside. The use of a transparent material such as l” thick acrylic plastic in the manufacture of the door or at least of a sizable window is yet another safety precaution now available.
Recent developments
In many cafeterias a cold counter for salads and cold sweets is now frequently installed in the servery. This may be either be a perforated shelf covered with broken pieces of ice on which the plates of food are put, or a metal shelf maintained at low temperature by refrigeration machinery. Such counters may be fixed or mobile, the latter type also being useful for service at other points than in the servery line. Plate XXXII shows a mobile cold counter used for the service of salads in the dining hall of a students’ residence in the U.S.A.
The use of refrigerators with mobile backs located in the wall between the kitchen and the servery has already been referred to in the previous chapter. Another aspect of this is shown in Plate XXXII where a refrigerated display unit for salads and cold sweets is accessible for replenishing through sliding doors from the salad section immediately behind in the kitchen.
The cold storage self-service unit for individual butter portions is another practical use of refrigeration. The butter supplied to the kitchen in & oz. pieces on cards of 1″ x ” with a paper cover is loaded into a container in piles so that when the bottom pat is withdrawn another falls into its place. The back of the container consists of a removable plate made of metal of a high specific heat which after being mechanically cooled is slid into place.