Preparation of potato flour by freeze-thaw pretreatment: Effect of different thawing methods on hot-air drying process and physicochemical properties

Freeze-thaw (FT) pretreatment was applied to hot-air drying of cooked-potato flour preparation. Five thawing methods including air thawing (AT, 25 °C), running water thawing (RWT, 25 °C), refrigerator thawing (RT, 4 °C), ultrasonic-assisted thawing (UT) and microwave thawing (MT) were individually used. The rehydration rate, total energy consumption, starch composition, pasting characteristics, rheological and thermal properties of potato flour were evaluated. FT pretreatment improved the air-drying rate and shortened the drying time. RWT obtained the largest drying rate and reduced total energy consumption by up to 32.9%. The improvement of air-drying was attributed to lower initial water content and high moisture migration rate in freeze-thawed potato slices. Furthermore, FT pretreatment decreased the amylose to amylopectin ratio and increased the relative crystallinity and resulted in alteration of rheological and thermal properties of potato flour. The observed higher pasting temperature, final viscosity, setback value, gelatinization enthalpy (ΔH), elastic modulus, and viscosity modulus were associated with the decrease in amylose to amylopectin ratio and increase of starch crystallinity. Overall, FT pretreatment is effective to shorten the subsequent drying time of potato flour having higher relative crystallinity, with lower energy consumption.