Effect of barrel temperature on rehydration kinetics of direct-expanded tef flour breakfast cereal

The effect of barrel temperature (110, 130 and 150C) on the rehydration kinetics of tef flour extrudates was investigated. Rehydration was conducted in distilled water for 300-420 min using a constant-temperature water bath at 25C (±1C). Water absorption rate was high at the early stages (60-90 min) of hydration, decreased subsequently and finally approached an equilibrium. Three models, namely, Peleg's equation, first-order kinetic and normalized Weibull model, were fitted to the rehydration data. The three models described the rehydration characteristics adequately (R² = 0.95-0.99). The water absorption rate and equilibrium moisture content were found to be barrel temperature-dependent (P < 0.05). Extrudates processed at a barrel temperature of 150C exhibited a high water absorption rate, followed by those extruded at 130 and 110C, respectively. The equilibrium moisture content of extrudates processed at 150 and 130C was significantly (P < 0.05) higher than those extruded at 110C. In view of the values of the shape parameter βof the normalized Weibull model, the mechanism of water absorption appeared to be by diffusion for extrudates processed at 130C (β = 0.67), whereas in extrudates processed at 110 (β = 0.89) and 150C (β = 1.03), external resistance to mass transfer and relaxation phenomena, respectively, were found to be the water absorption mechanisms. In comparison, the normalized Weibull model described the rehydration characteristics relatively better, resulting in smaller RMSE value. Some extruded food products, like breakfast cereals, are consumed after being rehydrated (e.g., in milk). The rehydration characteristics, in turn, are influenced by the extrusion operating conditions. Fast rehydration without much change in the crispiness is a required attribute of breakfast cereals. Describing and modeling the kinetics of moisture uptake as a function of the operating conditions could lead to a better understanding and control over the mechanisms underlying the sorption process and the ability of the product to rehydrate. Such information is vital in process and quality optimization.