Special uses
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.

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:
  • the roots and tubers must be kept at an appropriate temperature, normally somewhat higher than ambient, in order to stimulate new skin growth;
  • the atmosphere must be kept moist but without free water on the surface of the roots or tubers; no new skin will be formed in dry air on injured surfaces;
  • some ventilation is needed for new skin growth, but an excessive air flow will dry the atmosphere and cause a drop in temperature;
  • the temperature must be kept steady; if it falls, water will condense on the surface of the roots and tubers and will encourage bacterial soft rot.
TABLE. Conditions suggested for the curing of roots and tubers
Temperature (°C)
Relative humidity (%)
Curing time (days*)
Irish potato
above 85
Sweet potato
above 90
above 90
Taro (dasheen)
30 35
above 95
30 35
above 80

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. 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.

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:
  • the necks of onions are very sensitive to decay if they remain wet, especially if the green tops are cut off before harvest;
  • drying the outer skins of the bulbs reduces decay and water loss;
  • roots damaged during harvesting are a common entry point for decay unless they are dried quickly.
Figure. 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.

Inhibition of sprouting
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:
  • the selection of varieties with long dormancy periods; suppliers of seed and planting material can be asked to provide information on storage characteristics of varieties produced under local conditions;
  • the use of chemical sprout suppressants for potatoes and onions to be stored. Some suppressants have to be applied to the growing crop before harvest (e.g. maleic hydrazide). Others such as tecnazene (which has both suppressant and fungicidal properties) are mixed as a dust or granules with potatoes as they are loaded into store. Suppressants are rarely used except in large-scale production and storage; they should be used only after consultation with extension workers. Little is known about the effectiveness of sprout suppressants when used on tropical root and tuber crops.
Fungicide application
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).

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.

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:
  • permissible (i.e. not prohibited) for use on the crop after harvest;
  • effective in controlling the post-harvest diseases of that crop;
  • used in accordance with the manufacturers' instructions and at their recommended concentrations (excessive residues on produce may lead to its rejection);
  • agitated continuously during use to prevent its settling out.
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.
Share on Google Plus


The publications and/or documents on this website are provided for general information purposes only. Your use of any of these sample documents is subjected to your own decision NB: Join our Social Media Network on Google Plus | Facebook | Twitter | Linkedin