Effects of Drying Methods and Pre-treatment Conditions on the Functional Component Contents and Antioxidant Activities in Egoma (Perilla frutescens (L.) Bitt. var. frutescens) Leaves

We investigated the effects of drying methods and pre-treatment conditions on the functional component contents and antioxidant activities in egoma (Perilla frutescens (L.) Bitt. var. frutescens) leaves. It was found that the contents of alpha-linolenic and rosmarinic acids in the leaves were higher after treatment by vacuum-microwave drying than after hot-air drying; alpha-linolenic acid in particular retained the same level of contents after vacuum-microwave drying and after freeze drying. Vacuum-microwave drying conditions were found to be superior at high power and short drying time for the retention of functional components. An evaluation of the blanching conditions used during pre-treatment found that there was no change in a-linolenic acid contents during boiling treatment, while the rosmarinic acid contents decreased with increasing boiling time. On the other hand, the contents of both components did not change during steam-treatment. Steaming as a pre-treatment therefore prevented the loss of rosmarinic acid before freezing and thawing. The DPPH radical scavenging activities of the samples decreased less under vacuum microwave drying. The activities of all drying treatments after steaming, freezing, and thawing were significantly higher than with no treatment other than freeze drying. For the retention of functional components and color, vacuum-microwave drying was found to be superior to the traditional method of hot-air drying, suggesting that drying the samples after pre-treatment by steaming may produce dried products with high antioxidant activities.