Storability of cultivars of Dioscorea rotundata poir

Abstract

Yams are very important staple food in West Africa. However, their storability is a problem as post-harvest losses of up to fifty percent had been observed. This study therefore investigated the storage physiology of undamaged tubers of three cultivars, Puna, Dorbari, and Chinchito of Dioscorea rotundata Poir., during two growing seasons. The study tested the hypothesis that tuber shelf life and related critical indices were the same for the cultivars and that D. rotundata yams had shelf lives shorter than three months. Experimentally, the tubers were stored in a naturally ventilated thatched shed at tropical ambient temperature and humidity. Cross sectional sampling was applied to the tubers, which were randomly selected every month for laboratory analyses. Randomized Complete Block Designs with four (4) replicates of tubers of the three (3) cultivars were applied. The following storability indices were determined: (i) visible sprouts as indicated by an observed bud; (ii) tuber weight; (iii) oxygen uptake and (iv) carbon dioxide evolution (by gas chromatograph analysis of air samples withdrawn from 12-litre closed systems at 24 ± 1 oC into which tubers were sealed); (v) dry matter content; (vi) starch and (vii) glucose contents (by coupled enzyme assay); (viii) soluble protein; (ix) ᾳ-amylase activity; (x) lysozyme activity (using a turbidimetric assay); and (xi) susceptibility to rot when mechanically damaged. The results showed that visible sprouts were observed five months after harvest for all three cultivars. Visible sprouts were used as an index of the end of natural shelf life. Statistical analyses showed that over the storage period, there were significant differences among the three cultivars in susceptibility to rot under mechanical damage (Sig. = 0.000 at p < 0.05), cultivar tuber weight (Sig. = 0.000 at p < 0.05) and oxygen uptake (Sig. = 0.000 at p < 0.05). In the closed system determinations, oxygen uptake and carbon dioxide evolution trends were second order polynomial reactions over one-week durations in the replicates. Intra-cultivar differences of dry matter were significant (Sig. = 0.022 at p < 0.05), as well as glucose (Sig. = 0.000 at p < 0.05), soluble protein (Sig. = 0.025 at p < 0.05), α-amylase activity (Sig. = 0.015 at p < 0.05) and in susceptibility to rot on mechanical damage (Sig. = 0.000 at p < 0.05). Storage time significantly affected susceptibility to rot on mechanical damage (Sig. = 0.001 at p < 0.05). The physiological changes however did not affect overall shelf life as defined. It was concluded that undamaged tubers of the three cultivars could be stored for use within 5 months after harvest if tubers suffered no mechanical damage. These findings are noteworthy for postharvest yam handling, storage, marketing and for domestic and national food security