Viscoelastic properties of sweet potato complementary porridges as influenced by endogenous amylases

Abstract

Sweet potato (Ipomoea batatas L.) roots contain amylolytic enzymes, which hydrolyze starch thus having the potential to affect the viscosity of sweet potato porridges provided the appropriate working conditions for the enzymes are attained. In this study, the effect of sweet potato variety, postharvest handling conditions, freshly harvested and room/ambient stored roots (3 weeks), and slurry solids content on the viscoelastic properties of complementary porridges prepared using amylase enzyme activation technique were investigated. Five temperatures (55°C, 65°C, 70°C, 75°C, and 80°C) were used to activate sweet potato amylases and the optimum temperature was found to be 75°C. Stored sweet potato roots had higher soluble solids (⁰Brix) content in the pastes compared to fresh roots. In all samples, activation of amylases at 75°C caused changes in the viscoelastic parameters: phase angle (tan δ) and complex viscosity (η*). Postharvest handling conditions and slurry solids content significantly affected the viscoelastic properties of the porridges with flours from stored roots yielding viscous (liquid-like) porridges and fresh roots producing elastic (solid-like) porridges. Increase in slurry solids content caused reduction in the phase angle values and increase in the viscosity of the sweet potato porridges. The viscosity of the porridges decreased with storage of sweet potato roots. These results provide a possibility for exploiting sweet potato endogenous amylases in the preparation of complementary porridges with both drinkable viscosities and appropriate energy and nutrient densities for children with varying energy needs.

Keywords: complementary porridge; endogenous amylases; phase angle; sweet potato; viscosity.

Figure 1

Saccharification of sweet potato slurries of Kakamega variety at different temperatures. (a) Changes in soluble solids (⁰Brix), (b) peak ⁰Brix

Figure 2

Effect of variety and postharvest handling conditions on sweet potato paste soluble solids during heating at 75°C

Figure 3

Effect of variety, postharvest handling conditions, and slurry solids content on complex viscosity (η*) of sweet potato porridges. (a) Fresh NASPOT 10, (b) stored NASPOT 10, (c) fresh Kakamega, and (d) stored Kakamega

Figure 4

Effect of variety, postharvest handling conditions, and slurry solids content on the final shear viscosity of sweet potato porridges. Values are mean ± standard deviation (n = 3); different lower case letters indicate significant differences at p <.05

Figure 5

Effect of variety, postharvest handling conditions, and slurry solids content on the phase angle (tan δ) of sweet potato porridges. (a) Fresh NASPOT 10, (b) stored NASPOT 10, (c) fresh Kakamega, and (d) stored Kakamega