Tropical tuberous crops: getting to the root of the problem
Root and tuber crops contribute significantly to basic food requirements in urban and rural areas, especially for poorer communities. With rising urbanisation in many regions, there is an increasing need to store and transport fresh produce but perishability is often a problem due to the high moisture content of tubers (60-80% when fresh). The keeping quality of tubers decreases from yams, which can be stored for about four months, to sweet potato, and cassava, which deteriorate rapidly (within 1-7 days) after harvesting. However, during the last decade a greater understanding of the biochemical and physiological characteristics of these crops, together with an appreciation of the important quality attributes, has resulted in the development of strategies to reduce post-harvest losses. There is potential for further advances, and opportunities to reduce losses by genetic manipulation through breeding have been indicated. These will increase as biotechnological methods become more sophisticated.
Yams (Dioscorea spp.) are an important root crop in West Africa. White yams, in particular, are considered a prestigious food and are preferred to other foods by urban populations. High in nutritional value, they fetch a higher price than cassava in local markets and, despite the high labour costs of cultivation, it is a major cash crop. However, the harvested crop is easily bruised during storage and transportation, which reduces yields and income. Careful handling using appropriate tools for lifting the tubers is advocated and partially damaged tubers may be chipped and dried or used immediately. Traditional storage structures include pits, trench silos, and heaps in the field but these are difficult to manage i.e. to prevent pest attack and provide regular inspection of tubers. (Beetles have been revealed to be the predominant cause of storage damage with further spoilage occurring from fungal pathogens e.g. Fusarium). A raised hut with storage shelves made of locally available materials provides good ventilation and good access for inspecting tubers. During long storage, yam tubers loose moisture and shrivel but covering the tubers with yam vines, straw or other similar plant material may reduce this.
Sweet potato (Ipomoea batatas) is becoming an increasingly important crop in many regions, especially in areas of increasing population and where cassava yields are being severely affected by African cassava mosaic virus. Sprouting and spoilage are common with most forms of storage and tubers usually cannot be kept for more than 1-2 months although red varieties appear to store better than white. Like yams, the tuber is also highly susceptible to physical damage and subsequent deterioration (water loss and rotting). Significant differences exist between cultivars for rate of deterioration but trials conducted in Tanzania have revealed that selection for cultivars have to be assessed based on local preference for favoured characteristics (including less sweet varieties). Trials on ten cultivars in the Morogoro region have also demonstrated that all are susceptible to the main post-harvest pathogen in the region, Rhizopus. Traditional storage of sweet potato includes leaving the crop in the ground and harvesting only when needed. However, many late-harvested tubers are past their prime and damage from sweet potato weevil (Cyclas spp.), a universal pest, is more likely.
Cassava, or manioc, (Manihot esculenta) is one of the most reliable crops that can be grown under adverse growth conditions that are often unsuitable for other crop production and it has become an essential part of the diet of more than half a billion people. However, the production advantages of cassava are partly offset by the rapid deterioration of the roots, which can begin as quickly as 24 hours after harvest, and most varieties of cassava deteriorate within three to four days of harvest. Unlike other tuberous crops, cassava can be left in the ground from 8-24 months after planting as a safeguard against food shortages. Once harvested, simple techniques such as reburial, keeping under water, smearing with mud and stacking with daily watering help to preserve the tubers for a few days but most are eaten fresh or processed immediately after harvest. CIAT and NRI have developed a simple technique based on plastic-bag storage of fungicide-treated roots which increases storage time to about two weeks. Without application of the fungicide, storage is reduced to one week. The short shelf-life of cassava has played a major role in the evolution of cultural and post-harvest management practices (see next edition for Focus On Crop processing and marketing). To promote cassava further in rural development, FAO organized a forum of agricultural experts held in Rome from 25-28 April, 2000, which is to support the Global Development Strategy for Cassava initiated by IFAD in 1996.Further information: Market Oriented Yam Storage, GTZ publication http://www.gtz.de/post_harvest
Storage of fresh sweet potato, an extension guide, NRI, UK-NARO, Uganda publication