Use Of Low Temperature (part1)

The activities of food spoilage agents are very much dependent temperature. Enzymes require

particular optimum temperature for their catalytic reactions; so do the microorganisms

for their activities. Even the rate of pure chemical reactions is influenced by temperature, but these are not terminated as easily as enzyme-xcatalyzed reactions when temperature beyond the optimum range is encountered.

              Temperature manipulation is a very useful tool for extending storage life of foods. When enzyme and microbial activities are

undesirable in foods, temperature control may become necessary. Keeping food above the maximum temperature required for enzyme and

microbial activity may mean encouraging chemical reactions as temperature is increased. Moreover, the nutritional quality of food is

damaged if it is stored at a high temperature for a long time. Thexalternative procedure for checking the problems posed by enzymes and

microorganisms is to hold food at temperature below the minimum forxtheir activities. Low temperature also retards simple non-enzymatic

chemical reactions in foods.

               Normally enzyme activity and growth of food spoilage and pathogenic organisms best proceed at moderate temperature, i.e., in the

mesophilic range. Progressive reduction in temperature below this initiates gradual decrease in the activity of food spoilage agents. Below a certain temperature, all life activities cease and so food is saved from deterioration and spoilage. The choice of temperature usually depends upon the objective of storage. If short-term storage is the aim, then the temperature couid be decreased to near or slightly below the minimumxrequired for the enzyme and microbial activities. In case food is to be stored for a long period, then the temperature has to be reduced far below the minimum at which any life activity can occur.

                The terms cold storage and freezer storage, respectively, describe storage under the two situations. Cold storage refers to the storage condition where food is held at temperature above its freezing point. Freezer storage, on the other hand, describes the situation where food is held in frozen state at temperatures lower than the freezing point,

which incidentally corresponds to temperatures far below the minimum conducive for the activities of enzymes and microorganisms.

1. Equipment :

The equipment required in low temperature storage installalions is basically a refrigeration system whose power may he through non- mechanical. or mechanical means. In the non-mechanical or natural systems, ice or a suitable freezing mixture is employed. In mechanical refrigeration systems, the liquid refrigerant that boils and vaporizes at very low temperature, circulates in a closed system. It absorbs heat from its environment and is transformed to the gaseous state. The gaseous

refrigerant is reconverted into liquid state through a suitable mechanism that may involve either a vapour absorption cycle' or a vapour

cormpression cycle'. In systems using the vapour absorption cycle, the réfrigerant moves from the liquid phase to the gaseous state: the gas is absorbed in a suitable fluid and the liquid refrigerant is regenerated. This is employed in the manufacture of domestic refrigerators that work with gas, kerosene or other similar source of heat.

              Refrigerators using the vapour compression cycle employ a device, the compressor' to bring about the compression of the gaseous rerigerant. The electrical household refrigerators, freezers and common commercial equipment are of this type. In its simplest form, a vapour compression mechanical refrigeration system consists of four basic Components-compressor, condenser, expansion valve and evaporator.

              The compressor is the heart of the system and provides energy for its operation. It compresses the gas circulating in the hermetically. Sealed refrigeration system and passes it to the condenser. Here the gas IS cooled and condensed to liquid form. The liquid refrigerant is passed to the evaporator at a high pressure through an expansion valve that results in changing the fluid refrigerant to an atomized

vapour-liquid mixture at low pressure. In the evaporator the refrigerant obtains heat from the

surrounding atmosphere and vaporizes. The gaseous vapours again through the pass compressor and the cycle is repeated.

2. Refrigeration System :

Refrigeration systems are generallyxthree groups based on the operating attainable:-

1. High Temperature System :

Used for air conditioning and cold storage equipment where temperatures between -3.9°C

(25°F) to 7°C (45°F) or higher are needed.

2. Medium Temperature System :

These are used for food storage and other applications requiring temperatures between

3.9°C (25 F) and -17.8°C (0°F).

3. Low Temperature System :

Employed where temperatures of 17.8°C (0°F) or lower are needed.

3. Use Of Above Freezing Temperature :

The simplest form of equipment available for storage of foods at above freezing temperature is the domestic refrigerator and food displaying cabinets installed in supermarkets. temperature is lowered by the use of vapour compression mechanical refrigeration system. The shelf life of food commodities than when the same commodities are stored in commercial cold stores equipped with other sets of controls.

               The commercial cold stores operating in Pakistan are large insulated rooms equipped with'a mechanical cooling system that lowers

the temperature of the chamber. The controlled atmosphere storage facilities (CA storage) are equipped with other mechanisms whereby Humidity, ventilation and composition of gases inside the chamber may a regulated. These provide optimum conditions to the commodities, ensuring lono shelf life.

1. Objectives Of Cooling Foods :

Low temperature reduces the rate of chemical and biochemical reactions and retards the activities of microorganisms. A Tan temperature by 10°C reduces the rate of these reactions by one-half. Thus, life of a food could be doubled by merely lowering its temperature by 10°C or quadrupled by reducing 20°C from the ambient temperature. However, this assumption does not hold good as each food material responds to temperature changes differently. A major object of cooling foods is, therefore, to increase their storage life. Holding foods at low temperature also protects their nutritive value and prevents moisture losscthrough normal metabolic activities and evaporation.

            In the industry, cooling of foods may be done for purposes other than preservation, Bread is cooled after baking to facilitate slicing, while beef is cooled for ageing to improve its sensory characteristics. In the production of carbonated beverages, water is cooled before carbonation to increase the solubility of carbon dioxide. Wort is cooled in the brewing industry to precipitate some undesirable components and again after fermentation, the young beer is held at chilling temperature for impregnation of carbon dioxide and precipitation of other insoluble substances.

2. Pre-treatment Of Food For Low Temperature Storage :

Food raw materials get contaminated from different sources when they are gathered, harvested or slaughtered. Some contaminants

such as microorganisms can be troublesome even under very ideal handling conditions. Food meant for cold storage is prepared according

to the requirements for each particular commodity. Beef carcasses are washed, dewatered and then stacked in the chambers that are usually equipped with ultraviolet lamps. Eggs may be dipped in suitable mineral oil and then brought inside the cold store. Fruits are sorted for over-ripe or under-ripe ones, while vegetables are washed, drained and then stored.

            Quite often the microbial load of fresh food destined for cold storage is reduced by washing, heat treatment, use of chemicals or

irradiation. Lemons, papaya or nectarines are immersed in hot water at a temperature of 46 to 54°C for one to four minutes to pasteurize. Some fruits and vegetables such as cucumbers and root crops are waxed to Improve their appearance and keeping qualities. Shell eggs are usually dipped in light mineral ol 12 to 24 hours after laying-the treatment retards dehydration as well as loss of carbon dioxide and maintains.

              Chemicals are very frequently employed in the treatment of food materials prior to cold storage. Chlorine, acetates, ozone, sulphur dioxide and methyl bromide are commonly used to treat fruits and occasionally vegetables to prevent the growth of microorganisms. Ripening of plantains is retarded by application of purafil, while mangoes are treated with 2, 4, 5 trichlorophenoxy acetic acid for the same purpose. Sprouting in such commodities as onions, potatoes and carrots is prevented by the

application of phenyl carbamates, maleic hydrazide or vapours of nonyl alcohol. Ethylene gas is often used as a colour modifier to degreen citrus fruits.

             Irradiation has also been very helpful in cold storage of many commodities. Quite often chilled meat is irradiated to destroy the surface

microflora and parasites. Primarily, irradiation is used to sterilize the chambers that are now a days equipped with ultraviolet lamps, especially

in rooms for storage of meat and cheese. Irradiation of the atmosphere also helps when higher relative humidity and increased storage

temperatures are preferred.

3. Cold Storage Procedure :

At home fresh cómmodities-like onions, garlic, ginger, potatoes, sweet potatoes and others are stored in a cool corner of the house for quite some time. Meat, fish, eggs and other perishable commodities are kept in the refrigerator, if not utilized immediately. With storage at lower than ambient temperature, the shelf life of the food materials is considerably extended. Table 5.1 shows the useful storage life of some foods of plant and animal origin at different temperatures.

              At home fresh cómmodities-like onions, garlic, ginger, potatoes, sweet potatoes and others are stored in a cool corner of the house for quite some time. Meat, fish, eggs and other perishable commodities are kept in the refrigerator, if not utilized immediately. With storage at lower than ambient temperature, the shelf life of the food materials is considerably extended. Table 5.1 shows the useful storage life of some foods of plant and animal origin at different temperatures.

Generalized average useful storage life of animal and plant foods at different temperatures.

In practice, the feod processor dees net take any chances Wilth his commodities. The food is stered under controlled conditions, which guarantee extension in the shelf life, Large capacity cold storages are employed for preserving fresh food raw materials such as fruits, vegetables and tubers.

4. Factors Affecting Cold Storage Of Foods :

The physiological phenomenon in plant and animal tesues is the shelf life. All plant materials respire even after harvesting. The rate varies from one material to anether. During storage of fruits and vegetables, oxygen is taken up and carben diexide and water are evolved. Respiration in plant materials also results in heat generation and loss in the quality of the product. Animal tissues under anaerobie respiration, converting glyeogen into glucose and finaly to lactie acid. The energy produced during this precess eventually gets dissipated as heat, while the lactie acid causes a fall in the pit of meat from above 7.0 to as low as 5.1. All respiration in meat ceases after a postmortem period of one to 36 hours due to non-replenishment of glyeogen oF glucose. The refrigeration requirements of plant and animal foods are dependent on several factors, These are temperature relative humidity, composition of storage atmesphere and ventilation.

5. Temperature :

The choice of temperature for refrigeration storage of foods depends primarily on the nature of food, estimated desired period of storage, composition of the storage atmosphere and pre-treatment of the raw material. The metabolic activities in some plant materials are very high that result in the production of heat during storage. For example, one ton of green beans, sweet corn, okra or green peas stored for 24 hours at 4.5 °C, generate over 252 kilocalories (1,000 British Thermal Units, BTU) of heat. Under similar conditions, over 504 to 1260 kilocalories (2,000 to 5,000 BTU) of heat are generated when the same quantity of carrots or potatoes are stored. Most fruits are slow in respiration and thus release less heat during storage. Grapefruits, lemons oranges, cabbage, onions and tomatoes yield below 504 kilocalories (2,000 BTU) of heat under the same conditions and time.

             Since the rate of metabolic activities of each food varies, so does the storage-life expectancy under any specific situation. Depending upon other conditions, some animal tissues, firm, ripe fruits and vegetables may be stored at optimum parameters of chilling temperature- and relative humidity for a period of less than two weeks. In an atmosphere containing normal amounts of oxygen and carbon dioxide, beef, mutton, poultry, fish, lemons, nectarines, cabbage, carrots, green peas and spinach may be kept at just above freezing temperature for maximum storage life. Oranges, pineapples and potatoes will best be stored at 2 to 7°C. Bananas, grapefruits, lemons, limes, mangoes, tomatoes, green beans cucumbers, and sweet potatoes are kept at a temperature- between 7 to 13°C for maximum life. Green lemons, oranges, cabbage. carrots, potatoes, sweet potatoes and eggs may be stored for over three months at optimum conditions of temperature and relative humidity.

             Good insulation is essential to have adequate and uniform temperature inside the chamber throughout the storage period. The insulating material should normally be non-toxic, strong and with low heat conducting properties. Another factor that affects the temperature of cold storage chambers is the temperature difference between refrigeration coils and the storage atmosphere-smaller temperature difference is preferable over a large one, since the latter promotes vapour condensation on cooling coils. Proper air circulation equipment

maintains a uniform temperature in cold stores. The specifications of the equipment have to coincide with cooling requirements of the food

material.

6. Relative Humidity :

Control over relative humidity in the storage chamber is vitai for extended storage. Too high relative humidity, above the optimum level. Encourages microbial growth. Moulds grow in a relative humidity of 85 to 90 percent, yeasts require 90 to 92% and bacterial growith occurs on the food surface at near saturation. Relative humidity below the optimum results in moisture loss, causing wilting in fruits and vegetables or

damage to the appearance of animal tissues, thereby incurring económic losses. In many vegetables, a decrease of 3 to 6% moisture will result in a marked loss in quality.

           The optimum relative humidity for a particular raw material depends upon the storage temperature. In recommended relative humidity at 0°C is 92%, at 2.2°C 88% and 75% at

4.4°C. Thus, whenever temperature for storage is specified for a particular material, the relative humidity has also to be stated.

7. Composition Of The Storage Atmosphere :

In cold storage chambers, an atmosphere containing higher percentage of carbon dioxide and lower oxygen content than are found in air, is maintained to suppress the normal physiological processes in plant materials. Carbon dioxide content of above 10% significantly retards microbial growth. on the food surface. Similarly, reducing oxygen concentration from the normal 21% to 10% or lower decreases the rate of respiration. The problem in manipulating the gas atmosphere lies in the difficulty of control. The storage process known as Controlled Atmosphere Storage' or CA storage' has technically solved this. In CA storage, machines including scrubbers control the amount of different gases in sealed and insulated storage atmosphere, Özone may also be used where higher relative humidity is employed, since it helps in the control of microorganisms. Eggs, for example, keep as well in a relative humidity of 90% in the presence of 1.5 ppm of ozone as in 85% in its absence.

8. Ventilation :

Ventilation in cold storage chambers is important to prevent the development of stale odours and flavours and remove them from the

atmosphere. This is also helpful in maintaining a uniform temperature and relative humidity. In case adequate ventilation or air circulation is not provided, then food in local areas of high humidity may undergo microbial decomposition. This would also prevent uniform product composition in the storage atmosphere.