Characterization of heat-moisture treated Dioscorea alata purpurea flour: impact of moisture level

Dioscorea alata purpurea (purple yam) is a seasonal yam that is not commercially exploited due to its undesirable characteristics; having high pasting temperature and forming a soft, sticky and uncohesive texture. Hydrothermal modification could improve the physicochemical properties of the yam flour. The effect of moisture level (20, 25, and 30%, w/w) during heat-moisture treatment (HMT) of D. alata flour at 105 °C for 24 h on amylose content, thermal properties, swelling power and carbohydrate leaching were investigated. Increasing moisture level during HMT slightly decreased amylose content and gelatinization enthalpy, while significantly increased the gelatinization temperatures of the flour. Heat-moisture treated flour exhibited reduced carbohydrate leaching and limited starch granule swelling when heated at 55 to 95 °C, compared to the native flour. The enhancement of starch granule rigidity, rearrangement of amylose and amylopectin and interactions between starch and non-starch components are proposed as the factors contributing to the changes observed in the HMT D. alata flours. Moisture content during HMT play a significant role by acting as plasticizer in assisting mobility of starch components. In conclusion, the results highlight the feasibility of utilizing heat-moisture treated D. alata flour in products requiring high thermal stability and low cooking loss.