Hybrid Drying of Apples: A Comparison of Continuous and Intermittent Process Modes

In recent years, microwave and ultrasound technology has been under extensive development in drying technologies. Researchers are constantly searching for improved solutions or alternatives to hot air drying. The goal of this work was to determine the intermittent action of ultrasound and microwaves on convective drying. An examination of five specific cases of stationary and nonstationary drying processes was conducted. The evolution of moisture content and drying rate over process time was discussed, and the average drying rate and time, drying constant, effective diffusion coefficient, and specific energy consumption were also compared. To identify the differences between the dried products, the quality characteristics such as: water activity, color, shrinkage, rehydration, polyphenol content, odor, and flavor of apples were analyzed. The results indicate that intermittent drying provides a good alternative to convective drying, including when combined with microwave and ultrasound treatments. Applying microwaves or ultrasound intermittently resulted in an increase in the effective diffusion coefficient (by 68%) and drying rate (by 117%) and a reduction in drying time (by 53%), compared to convective drying. This processing method resulted in lower energy consumption by up to 13% and well-preserved quality attributes—this could be very promising for the production of healthy, ready-to-eat apple snacks.