Effects of Storage on Physical and Viscoelastic Properties of Yam Tubers

Abstract

The aim of the study was to evaluate the effects of storage period on the physical and viscoelastic properties of yams (Gbangu, Ogoja and Agboyian) tubers. The yam tubers were stored in a ventilated hut at a temperature of 28˚C ± 5˚C and 80± 15% relative humidity for 225 day. Using the universal testing machine, the viscoelastic properties of the yam tubers were measured two times; 0 and 225 days after storage. While the yam tubers physical characteristics were measured four times 0, 75, 150, and 225 days after storage using electronic weighing scale and digital caliper. The research results showed that storage duration had significant effect (p ≤ 0.05) on the physical characteristics, (mass, length, diameter, size, surface area, sphericity, and shape) of the yam tubers. Their values declined monotonically during storage, and the seven physical characteristics investigated in the study fit quadratic functions with high coefficient of determination (R² ≥0.94). The slight variation in sphericity with storage is an indication that shrinkage is not isometric. Gbangu tuber sphericity decreased from 39.02 to 36.69%, Ogoja tuber decreased from 38.23 to 36.14%, while Agboyian tuber decreased from 49.75 to 46.10%. During storage, Agboyian tuber shrinkage was about 42 %, Gbangu tuber was about 32%, Ogoja tuber was about 30%. In addition, storage duration had significant influence on the viscoelastic properties of the yam tubers. The constants K₁ and K₂ decreased significantly during the storage period. The lower the value of K₂ is, the higher the resistance of the yam tuber to elastic deformation. Higher value of the k₁ depicts lower force relaxation rate for yam tuber under loading. Force relaxation test is useful for estimating susceptibility of yam tubers to mechanical damage, and the results indicate that the D alata yam cultivar exhibited lower elasticity and viscosity than the D rotundata; therefore the Agboyian yam tubers are more prone to mechanical damage than the Gbangu and Ogoja tubers.