Effects of Aqua-gas Drying Conditions on Functional Components and Antioxidant Activities in Egoma ( Perilla frutescens (L.) Bitt. var. frutescens ) Leaves

Aqua-gas (AQG; superheated steam containing micro-droplets of hot water) was developed as a pretreatment technology before primary processing, such as sterilization, without sacrificing the material yield and maintaining qualities such as functionality. Its application to drying treatment, however, had yet to be studied. In this study, we examined its applicability to drying treatment of egoma (Perilla frutescens (L.) Bitt. var. frutescens) leaves. Results revealed that when egoma leaves were treated with AQG, they underwent a drying process that progressed with time, as was the case for treatment with superheated steam (SHS). AQG treatment produced dried leaves with a final wet weight-based water content of <10 %. We then investigated the effects of different drying methods on functional components in egoma leaves, and found that such components (e.g., alpha-linolenic and rosmarinic acids) were preserved during treatment with AQG, or AQG followed by SHS, at levels comparable to those by freeze drying (FD). In contrast, rosmarinic acid content was reduced after drying with SHS alone or hot air drying (HAD). In addition, egoma leaves were successfully sterilized by treatment with AQG, SHS, or AQG followed by SHS, or by steaming followed by HAD. Common bacteria remained viable after HAD treatment. Moreover, egoma leaves dried with AQG, AQG followed by SHS, or steaming followed by HAD had significantly higher DPPH radical-scavenging activity than the FD-treated product. In contrast, such activity was reduced after HAD. In addition, radical-scavenging activity correlated well with total soluble polyphenol content and polyphenol oxidase activity. These results suggest that AQG treatment shows promise as an efficient approach to completing the multiple steps generally required for drying leafy vegetables (blanching, drying, and sterilization) in one step, while preserving functional components.