Fruit and vegetables cover a wide range of produce that vary considerably in their postharvest behaviour, but are generally more perishable than the roots and tuber crops and invariably much more perishable than grains. Watch the video below for an overview of fruit and vegetable postharvest losses by NRI's Dr Debbie Rees.
Source of losses
The most common causes of loss are: mechanical injury, injury from temperature effects and pests and diseases. Rotting by fungal and bacterial pathogens is often indicative of physical damage or physiological deterioration. The soft texture and high moisture content of fruit and vegetables render them susceptible to mechanical injury, which can occur at any stage from the field to market and not during storage alone. Injury may arise because of poor harvesting practices, the use of unsuitable containers to transport the crop from field or to market, improper packing (over- or under-packing) of containers, careless handling of the produce or the containers in which it is packed.
The injuries, which may result in immediate loss or lead to further deterioration, can take many forms. Produce that is dropped may split on impact, or may suffer internal bruising (which may not be visible externally). Superficial grazing of the skin of the produce may result from poor handling and soft produce, especially leafy vegetables will be susceptible to crushing. When the outer skin of produce is damaged fungi and bacteria can enter, leading to rapid decay and increased water loss from the damaged area can occur. The rate of respiration may also increase resulting in a rise in temperature within the consignment.
Perishable crops show a wide range of temperature tolerance but are often particularly vulnerable to injury when exposed to extremes of temperature. It is generally accepted that perishable commodities should be kept cool to delay the onset of deterioration as long as possible. Fresh produce exposed to high temperatures caused by solar radiation will deteriorate rapidly. Long exposure to tropical sun will cause severe water loss especially from thin-skinned produce and leafy vegetables. It is not unusual for produce left in the sun after harvesting in the tropics to reach temperatures as high as 50°C. Respiration increases with increased temperature and produce that is packed or transported without cooling or adequate ventilation will quickly become unusable.
Tolerance of low temperatures is important in relation to cool storage. Injury from freezing is likely at temperatures between 0° and minus 2°C. Although some commodities may be tolerant of slight freezing their storage life will be reduced since produce recovering from freezing will be very susceptible to decay. Some commodities, particularly those of tropical or subtropical origin are susceptible to chilling injury, i.e. exposure to low but non-freezing temperatures (although these can be as high as 12-14°C). The effects of chilling injury (which may not become apparent until the produce is removed from the chilled environment) include discoloration, skin pitting, abnormal or uneven ripening and susceptibility to rapid decay. Fruit and vegetables for domestic consumption in Africa are not generally stored under temperature control. Chilling injury becomes particularly important where produce is exported, in which case it can limit the export range.
Insect pests are rarely a cause for major concern in perishable crops. However, when perishable crops are attacked, damage can be serious. Insect infestation usually occurs in the field before harvest and damage is caused by the larvae burrowing through the produce (e.g. fruit fly). Further development of the infestation can be a problem if produce is stored for long periods. As in the case of stored cereals and pulses, rodents and birds sometimes cause damage and loss during storage.
More serious loss and deterioration arise from diseases caused by fungi and bacteria, often the result of infection of the crop in the field. Loss in quantity occurs where deep penetration of decay makes the infected produce unusable and loss of quality occurs when the disease affects only the surface of the produce. It is sometimes possible to remove the affected areas and the undamaged portion consumed.
With the absence of temperature control facilities during storage/ transport and with poor roads, often the only option for trading fruits and vegetables any significant distance is through processing, such as drying. In this case losses may occur where the final moisture content is not well regulated. Mycotoxin contamination can be an issue for some products, for example chilli peppers.
There are few accurate figures available for losses of fruit and vegetables measured by a described methodology since there are no generally accepted methods for assessing PHLs of fresh produce comparable to those for grains. It is possible to find individual cases with losses ranging from 0% to 100%. Even when figures have been obtained by direct measurement, they may be of limited value because they refer to loss for one specific commodity, in one location and for one specific set of conditions. Moreover, the extent of loss can vary tremendously within a short time. The figures generally indicate a total weight loss and do not normally distinguish between loss of food per se and loss of moisture during storage or due to the metabolic processes that continue after harvest.
Loss reduction opportunities
PHLs of fruit and vegetables commodities can arise for a variety of reasons, not all of which are directly related to storage and deterioration is usually due to the physiology of the produce itself rather than to external factors such as insects. Because of these differences it is usually necessary to design a different set of intervention programmes to reduce PHLs in fruit and vegetables. Emphasis must be placed on good management of the supply chain (which may or may not involve storage), with the objective of delivering a high quality product from field to market.