Effect of Superheated Steam and Hot Water Spray Treatment on Polyphenol Oxidase Activity in Sweet Potato Leaves

In this study, a newly developed heating system, which consisted of superheated steam and hot water spray (Aqua-gas) treatment, was applied to sweet potato leaves. Upon initiation of Aqua-gas treatment, the internal temperature of sweet potato leaves was immediately increased, and reached 95 degrees C in 30 seconds. Notably, polyphenol oxidase (PPO) activity was not deactivated by short-term Aqua-gas treatment, while 5-minute treatment almost completely inactivated PPO activity. Moreover, the amounts of caffeic acid and 7 species of caffeoylquinic acids (CQAs) in sweet potato leaves after Aqua-gas treatment were significantly higher compared to leaves subjected to boiling treatment. Therefore, we propose that Aqua-gas treatment is a useful heating system to maintain the nutritional components such as CQAs.