Radio Frequency Drying Characteristics of Three Scallion Parts and Their Impacts on Qualities

Abstract

The scallion white, scallion core, and scallion leaf, with the same mass and same layer height, respectively, were subjected to RF drying at an electrode gap of 80 mm and a target temperature of 75°C, and their drying characteristics and qualities were measured and analyzed. The results showed that the scallion core required the longest drying period of 420 min and obtained the slowest temperature rise rate under the same layer height condition, while the scallion leaf of the same layer height was RF dried fastest, with an overall drying rate of over 6.0%·min⁻¹. Under the same mass condition, scallion white obtained the fastest overall drying rate of around 5.5%·min⁻¹, but scallion leaf took the least time to reach the temperature of 75°C due to its largest amount of RF power absorbed and presented the largest effective diffusion coefficient of 1.42 × 10⁻⁸ m²/s. Therefore, scallion leaf can be RF dried the easiest, which consumes the least drying time under either condition. The dried scallion leaf obtained the minimum color change and shrinkage percentage of 65% and the maximum allicin content of 30.17 mg/100 g, while the scallion core had the largest ascorbic acid (VC) content of 30.39 mg/100 g under the same mass condition, and the scallion white possessed the best rehydration ratio of 5.56 under either condition. It was found that the increase in layer height and/or mass not only caused the decrease in drying rate for the scallion parts, but also led to a slightly increased allicin content (p > 0.05), decreased shrinkage percentage, rehydration ratio, and VC content. Particularly, VC content of the scallion core significantly decreased by 28% (p < 0.05) as its layer height increased by 2 mm. Since the scallion core needed about two times the drying period than that for scallion white, it was recommended that they should be RF dried separately to save time and energy and guarantee product qualities. These results and findings not only revealed the different drying characteristics of the three scallion parts and their impacts on product qualities, but also could provide the drying industry some useful information about the practical processing methods in scallion drying.