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The risk of deterioration of produce during transport can be reduced in several ways.
8.3.1 Trucks used to transport fresh produce. Most fresh produce is now moved in road vehicles, with lesser amounts by sea, air or inland waterways. The vehicles in most common use are open pick-ups or bigger trucks, either open or enclosed. The use of road vehicles is likely to increase, so users should give attention to the following:
8.3.2 Handling and stowage practices. Although the shape and condition of trucks are important factors in fresh produce transportation, the loading and stowing methods in vehicles are pertinent to damage and loss:
Although every care may be taken to observe all the above precautions, the standards of driving remain a difficult problem to overcome. In many cases, drivers are induced to speed in order to make more money for themselves or their employers. Whenever possible, only experienced and responsible drivers should be employed.
8.3.3 Other modes of transport. Fresh produce is transported by many other means, from head-loads to air-freighting. In all cases, the same conditions should be observed. Produce must be:
8.3.4 Rail transport. In some countries a large amount of produce is carried by rail.
The advantages are:
Rail transport, however, requires extra handling since road transport is needed to and from the rail journey; transport by road alone usually is a door-to-door service.
8.3.5 Water transport
Inland. Waterway transport is used in some countries to move produce to markets. Much of the produce carried in this way is packed in locally made crates or sacks. The vessels employed are often mixed passenger-cargo craft, and no special handling is provided for fresh produce.
Sea. Short-distance transport of fresh produce in small ships without refrigeration is common in countries of island communities (e.g. the Philippines). Ships often accommodate passengers and general cargo, and no special provision is made for fresh produce, which may be stowed in unventilated holds. Losses are high, owing to rough handling by porters, inadequate packaging and overheating in unventilated holds or near engine rooms.
There is much room for improvement in this mode of transport. A model for organized and efficient sea transport is the refrigerated shipment of commercial crops such as bananas, although a modest investment by the small-scale shipper could greatly improve performance.
8.3.6 Air freight. As with shipping, the international trade in the air-freighting of high-value exotic crops is generally well organized. In some countries where road links are poor (e.g. Papua New Guinea), produce is carried by air from production areas to urban markets. Costs are high and losses often heavy because of:
Even though changes are made in packaging and handling, it is unlikely that the overall situation will improve much until road links are established between producers and consumers.
The routine packing operations, such as cleaning, selection, grading and packing of produce are discussed in Section 7. Apart from these, some crops which are seasonal and subject to long-term storage, or are highly perishable and transported over long distances to market, require special treatments in order to slow deterioration and minimize losses.
These treatments may be applied before, during or after packing and are supplementary to the routine measures, such as temperature and moisture control, which aim to reduce losses in all fresh produce.
The term "curing" is applied to the measures used to prepare starchy staple root crops and onions for long-term storage. The method of curing root crops is, however, quite different from that used on onions.
9.2.1 Root crop curing. The curing of root and tuber crops replaces and strengthens damaged areas of corky skin, restoring protection against water loss and infection by decay organisms. The principal crop subjected to curing is the Irish potato, but curing is also effective in some tropical root crops.
Although details vary from crop to crop (Table 3), the following conditions must always be observed:
TABLE 3. Conditions suggested for the curing of roots and tubers
|Crop||Temperature (°C)||Relative humidity (%)||Curing time (days*)|
|Irish potato||13-17||above 85||7-15|
|Sweet potato||27-33||above 90||5-7|
|Taro (dasheen)||30 35||above 95||4-7|
|Cassava||30 35||above 80||4-7|
* In practice, at least sewn days should be allowed for wring.
** Dioscorea alata and D. rotundata
Because all root and tuber crops are damaged to some extent during harvest and handling, curing must be carried out as soon as possible. This can be done by limiting ventilation, thus allowing the temperature to rise enough to promote curing. At the same time the air will become moist owing to the normal production of water by the roots and high rate of evaporation from injuries (Figure 9.1).
The conditions for Irish potato storage are we!! established; but those for tropical root crops are mostly based on experimental data. The storage life of sweet potatoes and of aroids like taro and cocoyam is usually rather short owing to their susceptibility to post-harvest decay. Cassava is subject to rapid internal discoloration and decay.
9.2.2 Curing dry bulb onions. The curing of dry bulb onions, carried out immediately after harvest, is a drying-out process. Under dry, warm conditions harvested onions are left in the field for a few days until the green tops, outer skins and roots are fully dried. Under wet conditions, it may be necessary to dry onions on racks or trays under cover.
The curing of onions is necessary because:
Figure. 9.1 This simple method of curing yams damaged in harvesting or handling has been used in West Africa
If properly carried out, this technique will provide the necessary warm and moist atmosphere to aid in healing skin damage. It can be adapted for other root crops (Reproduced from Careful storage of yams: some basic principles to reduce losses. Commonwealth Secretariat, London)
Cutting off the green tops of bulb onions is not recommended for small-scale producers because it greatly increases the risk of losses from decay if the bulbs cannot be dried quickly under controlled conditions.
In large-scale commercial production, where the green tops are cut off mechanically before harvest, drying is often carried out using artificial heat with forced ventilation. This technique is not economical for small-scale production.
Field-dried onions can be stored up to two months under ambient conditions in well-ventilated trays on pallets or in a field windbreak. Dried onions should never be allowed to come into contact with damp soil.
Sprouting of both potatoes and onions is a problem in temperate countries, where they are stored for up to eight months. Long-term storage may not be necessary in warmer climates where growers may produce more than one crop a year.
Two methods are employed to reduce sprouting:
Post-harvest application of fungicides to control decay is used on several major crops which are either stored or undergo long periods of transport to distant markets (citrus, bananas, apples, etc.). Fungicides are normally used only on produce which is washed and drained dry before packing (see Chapter 7).
9.4.1. Application method
Spray or mist. For small-scale operations application is by hand-held knapsack sprayer (Figure 7.3) or for large-scale commercial operations by a mechanized spray rig in conjunction with a moving belt or roller conveyor. Produce is sprayed to runoff to ensure complete coverage.
Drenching. A simple mechanized recirculating system pumps fungicide in a cascade over produce passing beneath it on a belt or roller conveyor (Figure 7.4). This system has no spray nozzles to wear out or become blocked, and the high flow rate through the pump keeps the mixture agitated. It may be necessary to add a non-foaming wetting agent in the suspension to counteract possible drag-out of the fungicide if foaming occurs.
Dipping. Where small quantities of produce are to be treated, the fungicide mixture is made up in a small container and produce is dipped by hand. Excess fungicide is allowed to drain back into the bath (Figure 7.2). The fungicide suspension must be agitated constantly. Workers dipping by hand may develop skin reaction to some fungicides, and they should be supplied with rubber gloves for their protection.
Smoke or fumigant. Fungicide can be applied in the form of dust or vapour in closed containers (e.g. diphenyl wraps or pads in citrus boxes), or in sealed bulk stores (e.g. tecnazene in potato stores). Such treatments are relatively rare. Bulk-store fumigation requires skilled operation and is normally carried out by contractors.
Hot water (fungicide treatment). Although hot-water dips have known to be effective for the control of post-harvest decay of some tropical fruits, the treatment has not been widely adopted because of the difficulty of applying it on a commercial scale. A heated fungicide dip has been shown to control anthracnose and has been used commercially in Australia. The operation requires close technical management and allows very little margin for error. It is not generally applicable to small-scale production.
9.4.2 Controls on fungicide treatment. The use of fungicides after harvest is normally subject to more stringent regulation than would be applied to their use on growing crops. The range of chemicals available for post-harvest treatment of fresh produce is small, with strict limitations on both the concentrations used and the permitted levels of residues on treated produce at the retail or processing stage.
Users of post-harvest fungicides must observe that the fungicide for any crop is:
Those in charge of operations must make sure that employees using fungicides observe all the precautions applicable to their use and that they wear the necessary protective clothing.
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.
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.
10.2.1 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.
10.2.2 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.
10.2.3 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.
The natural limits to the post-harvest life of all types of fresh produce are severely affected by other biological and environmental conditions:
10.3.1 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.
10.3.2 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 temperature tolerable by the crop.
10.3.3 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.
10.3.4 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.
10.4.1 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:
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 (Figure 10. 1).
10.4.2 Clamps. These are simple, inexpensive structures used to store root crops, particularly potatoes in Europe and Latin America (Figure 10.2).
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 10.2 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)
10.4.3 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).
10.4.4 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.
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.
Chapter 4 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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