1 Controlled conditions
The term "storage", as now applied to fresh produce, is almost automatically assumed to mean the holding of fresh fruit and vegetables under controlled conditions. Although this includes the large-scale storage of some major crops, such as potatoes, to meet a regular continuous demand and provide a degree of price stabilization, it also meets the demands of populations of developed countries and of the richer inhabitants of developing countries, providing year-round availability of various local and exotic fruits and vegetables. 

In many developing countries, however, where seasonally produced plant foods are held back from sale and released gradually, storage in a controlled environment is not possible because of the cost and the lack of infrastructural development and of maintenance and managerial skills. Even in developed countries, however, there are still many people who, for their own consumption, preserve and store fresh produce by traditional methods.

2 Storage potential
Much fresh produce (i.e. that which is most perishable) cannot be stored without refrigeration, but the possibilities for extending the storage life of even the most durable fresh produce under ambient conditions are limited.

Organs of survival. The organs of survival which form the edible parts of many crops such as Irish potatoes, yams, beets, carrots and onions have a definite period of dormancy after harvest and before they resume growth, at which time their food value declines. This period of dormancy can usually be extended to give the longest possible storage if appropriate conditions are provided. This factor is called the storage potential.
It is important to recognize the variation in the storage potential of different cultivars of the same crop. Experienced local growers and seed suppliers can usually provide information on this subject.

Edible reproductive parts. These are largely confined to the fruits or seeds of leguminous plants (peas and beans). In their fresh condition these products have a brief storage life which can be only slightly extended by refrigeration. They can also be dried, and then are called pulses. Pulses have a long storage life, provided they are kept dry, and do not present a storage problem of the sort affecting fresh produce.

Fresh fruit and vegetables. These include the leafy green vegetables, fleshy fruits and modified flower parts (e.g. cauliflower, pineapple). The storage potential of these is very limited under ambient conditions. They quickly deteriorate because of their fast respiration rates, which cause rapid heat buildup and the depletion of their high moisture content.
Traditional methods of preservation are sun-drying or simple domestic processing into conserves (with sugar) and pickles (with brine or vinegar). Most fresh fruit and vegetables have a storage life of only a few days under even the best environmental conditions.

3. Factors affecting storage life
The natural limits to the post-harvest life of all types of fresh produce are severely affected by other biological and environmental conditions:

Temperature. An increase in temperature causes an increase in the rate of natural breakdown of all produce as food reserves and water content become depleted. The cooling of produce will extend its life by slowing the rate of breakdown.

Water loss. High temperature and injuries to produce can greatly increase the loss of water from stored produce beyond that unavoidably lost from natural causes. Maximum storage life can be achieved by storing only undamaged produce at the lowest

Mechanical damage. Damage caused during harvesting and subsequent handling increases the rate of deterioration of produce and renders it liable to attacks by decay organisms. Mechanical damage to root crops will cause heavy losses owing to bacterial decay and must be remedied by curing the roots or tubers before storage. Curing is a wound-healing process discussed in Chapter 9.

Decay in storage. Decay of fresh produce during storage is mostly caused by the infection of mechanical injuries. Furthermore, many fruits and vegetables are attacked by decay organisms which penetrate through natural openings or even through the intact skin. These infections may be established during the growth of the plant in the field but lie dormant until after harvest, often becoming visible only during storage or ripening.

4. Storage structures

Ventilated stores. Naturally ventilated structures can be used for the storage of produce with a long storage potential, such as roots and tubers, pumpkins, onions and hard white cabbage. Such stores must be designed and built specifically for each intended location. Any type of building can be used provided that it allows the free circulation of air through the structure and its contents.

The following essentials must be observed:
  • the building should be located at a site where low night temperatures occur over the required storage period;
  • it must be oriented to take maximum use of the prevailing wind for ventilation;
  • the material covering the roof and walls should provide insulation from the heat of the sun; grass thatch on a bush-pole frame can be very effective, particularly if it is wetted to provide evaporative cooling;
  • double-skinned walls will provide better insulation, if cost allows;
  • white paint applied to surfaces of man-made materials will help to reflect the heat of the sun;
  • the structure should be built in the shade of trees if they do not interfere with the prevailing air flow; beware of bush fires and of trees falling during storms;
  • provide ventilation spaces below the floor and between walls and roof to give good air flow;
  • if the store is subject to cold night temperatures, fit movable louvres and adjust them to limit the flow of warm air into the store during the day.

These are the basic requirements of a ventilated store. Such stores may be constructed to various levels of sophistication, using, where it is economically acceptable, fan-assisted ventilation controlled by differential thermostats. This type of store is in common use in Europe for the bulk storage of Irish potatoes and onions in locations where external winter conditions make possible the accurate control of the storage temperature.
Simple open-sided, naturally ventilated structures may be used to store seed potatoes at high altitudes in warm climates. They cannot be used for table potatoes, which will turn green, develop a bitter taste, or even become toxic if exposed to light for more than a few hours

Clamps. These are simple, inexpensive structures used to store root crops, particularly potatoes in Europe and Latin America. The potatoes are placed on a bed of straw I to 3 m wide, but not more than 1.5 m wide in warm climates. A ventilating duct should be placed along the bottom. The piled potatoes are covered with about 20 cm of compacted straw which can subsequently be encased in soil, applied without compaction up to 30 cm deep. The clamp system can be modified for different climatic conditions. In warm climates extra straw casing may be used instead of soil in order to give added ventilation.

Figure: This structure. called a clamp, is commonly used for storage of potatoes and other root crops. It is used mostly in temperate regions but is also effective at high elevations m warmer climates

(Reproduced from Principles of potato storage, International Potato Centre, Lima, 1981)

Other simple storage methods. Windbreaks are narrow, wire-mesh, basket-like structures about I m wide and 2 m high, of any convenient length, on a raised wooden base, and are used for short-term storage of dried onions in the field. The onions are covered on top with a 30 cm layer of straw, which is in turn held down by a polythene sheet fastened to the wire mesh. The windbreak is built at right angles to the prevailing wind to obtain maximum drying and ventilation.
Onions can also be woven into plaits on twine and hung in a cool dry place, where they will keep for several months (Figure 10.3).

Refrigerated and controlled-atmosphere storage. For large-scale commercial operations, refrigerated storage may be used in a cold-chain operation to carry regular consignments from production areas to urban markets and retailers. This can be a highly complex operation requiring expert organization and management. Cold storage can also be used for long-term storage of seasonal crops such as potatoes and onions. The storage life of some fruits, such as apples, can be extended by combining refrigeration with a controlled environment consisting of a mixture of oxygen and carbon dioxide. These last are expensive operations with high maintenance and running costs, and demand skilled and experienced management. They have relatively little application to small-scale production in developing countries.

Preservation methods for fruit, vegetables and root crops
1 Processing avoids waste
In most countries, the production of many perishable food crops is seasonal, making them available only during short periods of the year. During this short time, they are produced in greater quantity than the market can absorb, so the surplus of many of these crops must be processed and preserved to avoid wastage of the food and loss of income to the grower. Modern methods of food storage and preservation, such as refrigeration and freezing, are now widely used in developed countries. These methods are, however, rare in many of the developing countries, but surpluses of many seasonal local crops can be preserved for later use by various processing methods requiring only simple and inexpensive equipment.

2 Principles of fresh-produce processing
This describes how fresh produce deteriorates and decays after harvest owing to the activities of:

Enzymes. These complex chemicals are present in very small amounts in all living material. All living activities are under their control; they continue to function after harvest, causing a natural breakdown of fresh produce. Enzymes in fresh produce must be destroyed if the processed product is to be stored.

Micro-organisms. These are the moulds, yeasts and bacteria which can attack and decompose both living and dead plants and animals. They are active spoilage agents of preserved produce; if they are not destroyed or inactivated, they can even render it poisonous by their activities.
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