Storability of sweet potatoes (Ipomoea batatas (L.)) under tropical conditions: physiological and sensory aspects.

Abstract

The shelf-life of the sweet potato storage root under tropical marketing conditions limits its potential for marketing. This research aimed to identify the physiological characteristics that affect the shelf-life of sweet potato cultivars when they are exposed to tropical marketing conditions. Weight loss was the key limiting factor in storability under the conditions studied. The range in weight loss was large amongst the 39 cultivars tested, and varied between 5 to 15% per week. Weight loss related to the marketable appearance. It consisted mainly of water loss and only 10% was due to respiratory metabolism. Sweet potatoes with high rates of water loss were more susceptible to rotting. The role of periderm characteristics (thickness and permeability), root-size, root surface area/mass ratio and shape were investigated. Although significant differences were observed among cultivars, these characteristics could not account for the variation in storability. The level of damage severely affected the rates of weight loss, with transpiration rate through damaged areas many times higher than through undamaged periderm. Breakage was found to be the most severe form of damage, having a great impact on weight loss for 14 days. Cultivars differed in susceptibility to damage after standardised damage treatments. Susceptibility to breaks was greater for long thin roots. Skinning injury was negatively related to the periderm thickness. Wound healing ability was a major factor for the shelf-life of sweet potato cultivars. It was demonstrated that lignification of wounds as measured by phloroglucinol staining, correlates with reduced susceptibility to weight loss, water loss and microbial attack. A lignin index was used to express the probability that lignification occurs. Cultivars differed significantly in their lignin indices under tropical marketing conditions. A high dry matter content generally coincided with a low lignin index. This relationship was consistent for 19 cultivars tested. Sensory evaluation of five sweet potato cultivars resulted in five distinct sensory profiles. During storage some of the cultivars lost some of their flavour but little changes were observed for textural properties. It was concluded that changes in sensory aspects are not a limiting factor for storage of sweet potato.