Effect of freeze-thaw pretreatment combined with variable temperature on infrared and convection drying of lotus root

The effects of freezing temperature, times of freeze-thaw (FT) pretreatment combined with variable temperature on infrared and convection drying of lotus root were studied. The FT pretreatment improved the drying rate (from 0.0188 to 0.0261 min−1), reduced the total energy consumption (from 4.77 to 3.73 kW h/kg), and resulted in a more porous structure of the final product with low shrinkage rate (from 83.39 to 73.04%), reduced hardness (from 41.67 to 24.49 N) and with a crisp texture (from 0.36 to 0.17 s). The inflection point of the change in state of water was determined using low field NMR during drying. Effect of the drying conditions on the quality of final products was also monitored. Variable temperature drying not only effectively solved the problem of color deterioration, but also promoted faster drying rate with reduced energy consumption and a better quality product. •Freeze-thaw improved drying rate, formed crisp texture and saved energy.•Cell membrane was destroyed and micro channels formed after pretreatment.•Variable temperature drying solved the color change problem.