The influence of moisture content migration and sugar concentrations in fungal biodeterioration of stored sweet potato (Ipomoea batata (L) lam) root tubers of Jos and environs

Abstract

Harvested sweet potato (Ipomoea batatas (L) Lam) root tuber loss on the Jos Plateau is attributed to fungal biodeteriogens. Five improved cultivars: CIP4400168, Ex-Igbariam, Tanzania, TIS 8164 and TIS 87/0087 were collected, propagated, harvested and stored in a barn. The fungal colonisers, sugar concentration, moisture content migration within the cultivars circumference were investigated. Standard methods were used to identify the biodeteriogens and those that were amylolytic in activity. Physico-chemical analyses of the peels and five layers (depths) from it within each cultivars circumference were determined. Depths of penetration of the circumferences by the colonisers were monitored and each was plated out on standard media, divided into 3 batches and incubated at 250C, 370C and 450C. A total of 30 species of fungi: 3 Ascomycetes, 22 Hyphomycetes and 2 Phycomycetes, composed of 2 species of thermophiles (Mucor pusillus lindt and Scytalidium thermophilum Austwick), 4 species of thermotolerants and 24 mesophiles were identified. The genus Aspergillus had the highest representation; 3 yeasts: Candida albicans Berkhout, Rhodotorula species and Saccharomyces cerevisiae Hansen were also isolated. A. niger, A. oryzae, A. terreus, E. nidulans and Mucor pusillus were amylolytic in activity. Prior to storage Tanzania had peel MC of 49.60% and in-depth (5th layer) MC of 52.55%, but had 31.52% peel MC and 45.36% in-depth MC after 8 weeks of storage indicating that MC migration was directed towards its peels. Similar MC was recorded for the other cultivars after 2, 4, 6 and 8 weeks of storage. Ex-Igbariam peel prior to storage had the highest glucose content of 11.98% and an in-depth content of 0.21% as compared to CIP 4400168 which had 7.51% peel glucose content and in-depth content of 6.41% but after 8 weeks of storage, the former had 14.71% peel glucose content while the letter had increased to 9.67% glucose content. Similar tread in terms of greater peel glucose content were recorded for the other cultivars after 2, 4 and 6 weeks of storage. In terms of penetration of the decayed cultivars, 33 fungal colonisers were isolated on the peel with the highest concentration of glucose; 19, 14 and 2 fungal species were isolated from the second, third and the central layers respectively. This study has shown that the deteriorative and degradative activities of fungi during the cultivars storage were influenced by moisture content migration that mobilised glucose from the inner layers and concentrated it on the peels, encouraging proliferation and growth of the decaying biodeteriogens thereby reducing their shelf-life. To increase the shelf-life of the harvested cultivars plant extracts are being experimented with to control their enzymatic browning and to formulate sweet potato starch that could be utilised in food, pharmaceutical and textile industries and flour in bread, biscuit and cake baking.