India has diverse climate to produce wide range of fruits and vegetables. However, these are highly perishable and require tender care after harvest. Both quantitative and qualitative losses occur in horticultural commodities between harvest and consumption. Qualitative losses, such as loss in edibility, nutritional quality, caloric value, and consumer acceptability of fresh produce, are much more difficult to assess than are quantitative losses but could be reduced with proper management practices. Quality standards, consumer preferences and purchasing power vary greatly across cultures or regions and these differences influence marketability and the magnitude of post-harvest losses. Overall, about one third of horticultural crops produced are never consumed by humans. Reduction of post-harvest losses can increase produce availability to the consumers at affordable price and bring more returns to the farmers. Proper handling and storage of fruits and vegetables is must to extend their shelf life to maintain the desired quality.
All fresh horticultural produce are living organisms, carrying on the many biological processes that are essential to the maintenance of life. They must remain alive and healthy until processed or consumed. The energy that is needed for these life processes comes from the food reserves that accumulated while the commodities were still attached to the plant. The processes by which these food reserves are converted into energy is called respiration (table1). Some of the energy that is produced through respiratory activity is utilised in maintaining life processes. Excess energy released during respiration increases the product temperature with consequent faster respiration rate and still higher production of heat with the consumption of food reserve. The amount of this vital heat varies with the type of product, variety, maturity or stage or ripeness, injuries, temperature, and other stress is related factors. This heat must be controlled in any temperature management program. It is normally desirable to achieve as low a respiratory rate as possible without danger of tissue injury or death. Each 10C (18F) temperature reduction reduces respiratory activity by a factor of 2 to 4. For example, the respiratory rate of a product at 5C (41F) would be only 1/4 to 1/16 of what it would be at 25C (77F). Good cooling and temperature management practices are, therefore, critical to lowering the rate of physiological deterioration.
Considerations in the cold store operation:
Temperature effect and its management: Since high temperature conditions are also injurious to perishable crops, cold storage is thus recommended to retad the undesirable metabolic changes, aging or ripening, softening , moisture loss and wilting etc. If best results are to be obtained, it is highly important to maintain constant temperature in cold storage room.Variations of 1C or 2C above or below the recommended storage temperature for prolonged period of time are quite stressful and there is danger of shorter storage life with increased decay and undue ripening. However 3 – 4 days at 1.5C usually would have little or no effect on apple (recommended storage temperature -1 to 0C), partly the temperature of the fruit will not rise as quickly as that of the air but 10 days at this temperature would probably shorten the life of fruit by at least a week. Simillarly, if cabbage is allowed to remain in storage, it will turn yellow and decay. Potatoes are likely to begin sprouting if temperature is too high and may become undesirably sweet if it is too low. In addition, fluctuations in temperature often cause condensation of moisture on stored products which is undesirable as it favours the growth of surface mold and development of decay.
Maintaining uniform temperature in all parts of a storage room is more important than avoiding small fluactions at a given point. Fruit stored in a part of the room where temperature is continuously higher than in another part ripens faster than stored in cooler part. This situation results in mixing of over ripe and prime fruit on removal or it may result undetected deterioration and decay in inaccessible locations. Temperature varaiations can usually be prevented if the storage rooms are well insulated throughout and and have adequate refrigeration capacity and the difference between the temperature of the refrigerant and that of the room is kept small. Proper stacking and adequate air circulation also helps to minimise temperature variations.
Storage room should be equipped with reliable, accurate thermostats that control the air temperature inside the cold room. Usually the thermostats are placed about 1.5 meter from the floor. Product temperatures should be taken in packages or within the bulk containers at various locations. A good quality thermometer is essential and to ensure accuracy it should be tested by immersing in an ice and water bath. Temperature in less accessible locations can be obtained by distant reading thermometers.
Relative humidity (RH): is defined as the moisture content (as water vapor) of the atmosphere, expressed as a percentage of the amount of moisture that can be retained by the atmosphere (moisture holding capacity) at a given temperature and pressure without condensation. The moisture holding capacity of air increases with temperature. Water loss is directly proportional to the vapour pressure difference (VPD) between the commodity and its environment. VPD is inversely related to the RH of the air surrounding the commodity.
The relative humidity of the air in the storage room directly affects the keeping quality of the products. If it is too low, wilting or physiological weight loss occurs in the stored horticultural produce. Higher humidity is beneficial for wound healing and periderm formation during curing of certain crops. To increase the relative humidity in the storage room, water may be sprinkled on the floor occasionally or misters may be installed.
As the difference in temperature of the refrigerating surface and the temperature of air in contact with the refrigerating surface increases, the humidity decreases. For example, if the air moving over the refrigerating surface is cooled to -1.1C and the surface temperature is -3.9C, the relative humidity of air will be 78 per cent because any water vapour in excess of the amount that can hold at this relative humidity will condense on the coil. On the other hand, if this air moves over the evaporating surface maintained at -2.8C, the RH will be 89 per cent. Since all the air in the chamber hardly comes in contact with the refrigerating surface, there will be variation of the RH in the store. To maintain a narrower temperature difference between air and refrigerating surface, a greater evaporator area is necessary. However, when it is not possible to maintain desired relative humidity by maintaining the narrow temperature difference, a pressure atomised or heat vaporised water can be added to the air.
Several different devices to measure the RH are available but most common and accurate one is psychrometer with two thermometers- one dry bulb and other wet bulb. From the difference in the temperature readings of these thermometers, relative humidity can be determined from psychrometer table or chart. Direct readings of relative humidity may be taken with hair hygrometer. Electrical hygrometers are being used increasingly for humidity measurements and to control humidifying equipments. Operation of these is based on the ability of hygroscopic film to change its electric resistance with small changes in the relative humidity. They must be calibrated periodically. Condensation of moisture on the commodity (sweating) over long periods of time is probably more important in enhancing decay than is the RH of ambient air. An appropriate RH range for storage of fruits is 85 to 95 per cent while that for most vegetables varies between 90 and 98 per cent. The optimal RH range for dry onions and pumpkins is 70 to 75 per cent. Some root vegetables, such as carrot, turnip, and radish, can best be held at 95 to 100 per cent RH.
Air circulation and package spacing
Air must be circulated to keep a cold storage room at an even temperature throughout. Product temperature in a storage room vary because the air temperature rises as the air passes through the room and absorb heat from the commodity. The need for rapid air circulation is greater during removal of field heat. Some times this heat is best removed in seperate precooling room that have more refrigerating and air circulation capacity. After field heat has been removed, a high air velocity is unnecessary and usually undesirable. Only enough air circulation be provided to remove respiratory heat and heat entering the room through exterior surfaces and doors. Air must be directed in such a way that it flows uniformily to all par ts of the room (figure 3). Air movement of 15 to 23 m/min through the stacks is usually sufficient to accomplish this. Usually the air circulation systems for apple are designed to provide 28 m3 of air/min/ ton of refrigeration capacity.Rapid circulation of air does not appreciably increase water loss if relative humidity is kept high.
However, if the humidity is low, commodities with no circulation show less shrivelling. Maintaining uniform temperature is easier in large room than in small ones if both are filled to capacity. This is due to large mass of commodity and the building which have higher thermal capacity thus produce flywheel effect that prevent rapid temperature change.
The nature of the container and manner of stacking are important factors that influence cooling rate. An elaborate system for air distribution is useless if poor stacking prevents air flow. Air follows the path of least resistance, so if spacing is irregular, the wider spaces get a great volume of air than the narrower ones. If some spaces are blocked , we get higher temperatures. Generalisation of stacking arrangement are difficuilt to make but the rows should be laid out such that direction of air movement is along the rows rather than across them. A 10 -20 cms spacing at side walls is desirable to deliver cold air to all levels. Most rapid cooling occurs when all sides of pallet boxes are exposed to air movement.
Water loss in storage
The loss of water from the harvested horticultural produce is a major cause of deterioration in storage. The fruits and vegetables contain 80 – 90 per cent water by weight, some of which may be lost by evaporation. This loss of water from living tissue (transpiration) must be minimised to avoid loss in salable weight and avoid wilting and shrivelling of produce. Some loss of weight also happens due to respiration of carbohydrate into CO2 but this is only a minor part of the total.Apples stored at 3C loose weight due to respiration @ 0.05 per cent/week where as due to transpiration @ 0.5 per cent/week. The rate of transpiration can be reduced by raising the relative humidity, by lowering the air temperature, by reducing air movement and by protective packaging. Water loss is generally higher during the first few hours or days in storagewhile the produce is still cooling. Water vapour moves from warm stored product into the air even though the relative humidity of the storage is 100 per cent. The vapour presure deficit in this case is much greater than that between relative humidities of 100 per cent and 50 per cent at a temperature of 0C. Loss of moisture can be minimised with protective packaging to supplement the benefit of refrigeration and high humidity. Polyethylene film can be used for consumer size packaging or for box liners.
Sanitation and air purification
Maintenence of sanitary conditions within storage room is essential for minimising development of contamination. During several months operation of a storage at -0.5C molds grow on the surface of packages and on the walls and ceiling of rooms under high relative humidity conditions. These surface molds may not rot the fruits and vegetables but they can have undesirable effectby producing substances that hasten scencence and give off flavour to the produce. Sanitary precausions may be taken in handling decayed produce. Rotting fruits and vegetables needs to be handled carefully and once removed from the container should be discarded promptly. Ozone haas been recommended to reduce growth of surface molds on walls and containers, to reduce mols spores in the air and to reduce storage odour, however it is not effective in controlling decay. A complete storage cleanup should be done when rooms are empty and well in advance of next loading date. Air pruification is recommended practice in storage rooms where odors or volatile may contribute to off flavour and hasten deterioration.
BVC Mahajan,
Punjab Horticultural Postharvest Technology
Center, Punjab Agricultural University,
Ludhiana
Mahesh Kumar,
Punjab Horticultural Postharvest Technology
Center, Punjab Agricultural University,
Ludhiana
