Effects of ultrasound and blanching pretreatments on mass transfer and qualities of hot‐air drying of perilla ( Perilla frutescens L.) leaves

The work investigated the influences of blanching (B) and ultrasonic-assisted blanching (UB) as pretreatment on mass transfer, drying kinetics, color, contents of nutrient, and aroma compounds of hot-air-dried perilla leaves. The samples were pretreated at two temperatures (70℃ and 80℃) and two durations of pretreatment (1 min and 3 min). Principal component analysis and hierarchical cluster analysis were used to distinguish sample characteristics of different pretreatments. The results showed that the condition of UB pretreatment had a higher drying rate compared to B pretreatment. The effective moisture diffusivity (Deff) of pretreatment for UB is higher than B. The UB pretreatment method yielded the lower ΔE value and higher preserved content of chlorophyll, flavonoid, and rosemary acid at the pretreatment temperature of 70℃. Moreover, GC-MS results showed a lower perilla ketone relative content under the same conditions. According to the results, the quality of the dried perilla leaves after performing low-temperature UB pretreatment (70℃) was improved relatively. Practical applications Perilla leaves are rich in antioxidant substances and have extensive culinary usage given their characteristic aroma and flavor. Convective drying is known to have some adverse effects on agricultural products. However, Ultrasonic pretreatment, as a modern food processing technology, can shorten the drying time and preserve the color, aroma, and nutrient content of food. The purpose of this study was to investigate the quality of dried perilla leaves with B and UB pretreatments, and to compare the differences in drying characteristics, color, nutrient content, and aroma compounds by different pretreatments. The pretreatment and dehydration parameters should be considered with respect to further utilization of dried perilla leaves. Nonetheless, sonication is worth to be taken into account due to the significant effects of energy expenditures, inhibiting browning of perilla leaves without damaging the quality of final product.