The influence of postharvest conditioning and storage protocols on the incidence of rots in White Yams (Dioscorea rotundata Poir.) in Ghana

Abstract

The post-harvest conditioning of root and tuber crops (such as potatoes and cassava) by exposure to elevated temperature and humidity ('curing') has long been used to extend their storage life. The use of this strategy on yams is not, however, well documented and appears not to be practised with any consistent conviction in West Africa. During the period 2000 to 2003, a series of on-farm trials was conducted on White Yams (Dioscorea rotundata Poir.) in Ghana to assess whether curing and other related protocols could be exploited. The factors investigated included the impact of yam variety, tuber maturity (immature 'milk' yams and physiologically mature 'ware' yams), different curing environments (plastic bags, clamps or modified storage rooms) and subsequent storage in different structures (pits or barns). Interactions were observed between the conditioning treatments and varieties, and the different storage structures had a significant impact on yam weight loss, sprouting and the incidence and prevalence of rots. Overall 'milk' yams were more prone to deterioration than 'ware' yams and, irrespective of the possible benefits of curing, were much better conserved in traditional pits than in barns. Irrespective of treatment, rots were more often associated with the upper sections of the tubers, suggesting a link with pre-harvest insect damage and harvest cuts at the site of the vine. It was determined that the levels of relative humidity in pits, clamps and modified storage rooms could engender curing, whereas the humidity in plastic bags was excessive and simply brought about rapid rotting. Achieving the correct temperature for curing was problematic. Ambient temperatures (high 20°C) proved sub-optimal whereas temperatures above 37°C were too high. These field trials suggest, however, that the conditions necessary to cure yams can be established in rural communities with limited resources.