Pakistan grows substantial quantity of fruits and vegetables. The production of these horticultural products has been persistently on increase. The annual average production of fruits was 3.701 million tons and of vegetables 3.442 million tons for the five years ending 1989-90. In 1990-91, the production increased to 43.955 million and 3.829 tons respectively. Now, the output is up by 20 and 15 per cent, respectively.

However, the marketed volumes are much lower than the quantum produced. About 30-35 per cent output is lost due to defective marketing techniques, including the storage.

Proper and scientific arrangements for storing highly perishable products like fruits and vegetables minimize the production losses, extend the availability period, regulate supplies and prevent the spoilage of seasonal carryover stocks.

This aspect of marketing is grossly ignored at all levels. The existing storage units, besides being inadequate in number, are not scientifically managed. Products requiring varying degrees of temperature are generally kept on a flat level. No proper research in this regard has been undertaken. Nor have the research results obtained in developed countries been adopted with necessary modifications.

For instance, we in Pakistan still believe that onion is a commodity which is difficult to be kept in cold storage when scientific research elsewhere has proved that it can be stored at 32F with relative humidity level of 60 to 70 per cent for dry or cured bulbs of this vegetable.

Desirable environment: The function of fruits and vegetable storage is to provide an environment that will permit produce to be stored as long as possible without depreciating its quality. The quality is composed of flavour, texture, moisture content and other factors related to edibility. The desired environment can be secured by controlling the temperature composition and circulation of the storage atmosphere.

The fruits and vegetables need special treatment as these commodities have living tissues. As a result of tissue respiration they continually generate heat. It is generally believed that the lower the temperature the less is the amount of heat given off. But reduction of temperature can be harmful as is indicated by the development of low temperature disorders.

In case the produce is severely frozen the tissues die and when it is later on thawed and stored, decay sets in making the produce worthless. Besides temperature, there are also other factors that affect the storage environment. Fruits and vegetables being living organisms consume oxygen and give off carbon-dioxide and other volatiles into the storage atmosphere.

Further they also give off moisture and the rate of moisture loss is generally decreased by adding to the moisture contents of a storage atmosphere. But excessive moisture may generate growth of micro-organisms and thus hasten deterioration.

Temperature control is the most important single factor in operating in storage. What is the most suitable temperature depends on the nature of produce and frequently on its particular variety or its ultimate use.

Heat extraction is the method adopted to achieve a desired temperature reduction. Quantities of heat are measured in the British thermal units (BTU) equipment capable of extracting 12.000 Btu per hour is rated as having one ton refrigeration capacity. A frequently used rule of thumb is that one ton of refrigeration is required for each thousand bushels of storage.

The heat that needs to be removed comes from three main sources: heat leakage, field heat and metabolic heat or heat of respiration. Heat leakage means heat leaking into a storage room from the outside.

The higher the temperature differential between the inside and the outside of a storage room, the greater will be the amount of heat leaking inwards. Wheat, leakage is controlled mainly by insulating materials used in the structure of a building.

Field heat is generally the greatest portion of the total refrigeration load. It represents the heat that has to be removed from produce and containers in order to reduce their temperature to the desired holding temperature.

Heat of respiration is an important part of the total heat load in storages. It may also be the cause of local heating and rapid produce deterioration if air movement is inadequate. This can occur whenever a fruit or vegetable itself or a package or a method of storage prevents transfer of heat of respiration.

Humidity of storage means the amount (or percentage) of moisture in the atmosphere at a given time. Humidity values are affected by variations in temperature. For instance, if air has a relative humidity 80pc at 40F, it will fall below 80pc if the temperature is raised and will rise above 80pc if the temperature is lowered.

Maintaining the proper relative humidity is of great significance. If it is lower than optimum, produce will lose moisture by evaporation, and will wilt or shrive. On the contrary very high relative humidity may result in the deposition of moisture on produce and on other surfaces.

Such wet spots promote the growth of rot organisms staining (particularly on onion) softening and even splitting of skin have been cooled below the freezing point, which ranges between 27F and 30F. To avoid it no fruit or vegetable should be allowed to freeze. Even when no apparent damage is done produce may depreciate faster in storage.

Chilling injury on the other hand, is due to metabolic disturbance resulting from low temperature above the freezing point of the tissues control of chilling injury is one of the primary factors on which, storage, temperature recommendations are based.

Symptoms of chilling injury are extremely variable. They include deep scars or breakdown in some verities of apples, pigmentation complication on tomatoes, browning disorder or excessive sugar accumulation in potatoes, pitting of skin of bean and susceptibility to fungal disorders in peppers.

The commodities having different temperature requirements should not be stored together. Thus apples should not be stored with carrots, celery, cabbage, potatoes or onions.

Similarly, celery and onions should not be stored together. Potatoes sometimes impart an earthy flavour to fruits, particularly at high temperature. As a rule dairy products should not be stored with fruits and vegetables.