Ultrasound and ethanol pre-treatments to improve convective drying: Drying, rehydration and carotenoid content of pumpkin

Among different effects, both ultrasound and ethanol technologies change the product structure and promote mass transfer when used as pre-treatments to improve the food drying. For the first time, their combined application was evaluated as pre-treatment to improve convective drying, as well as the properties of the dried food. Pumpkin cylinders were used as model food, being dried using air at 50 °C and 0.8 ± 0.1 m s⁻¹. Effects on drying and rehydration kinetics, as well as on the energy consumption and carotenoid preservation, were studied. Compared to the Control, all pre-treatments decreased the drying time in more than 48%. The combination of ethanol and ultrasound presented the greatest reduction in both drying time (59%) and energy consumption (44%). The pre-treatments also enhanced the rehydration properties in more than 28%: higher rehydration rate and an increase of water retention were achieved. Possible mechanisms, involving structure modification and mass transfer during pre-treatment and drying process were discussed. Additionally, the extraction of carotenoids due to pre-treatments was negligible when compared to the remarkable effect in avoiding this nutrient degradation during drying. Pre-treated samples preserved ∼100% of the carotenoid content, while the Control samples presented partial degradation (23%). This was explained by the negative effects evidenced by sample thermal history during drying. The results open new perspectives about an innovative method to improve the drying process and product quality by combining ethanol and ultrasound.