Sample preparation
Freeze-dried grape skins (20 g) were ground with a stainless-steel grinder (Omni-Mixer 17105, OCI Instruments, CT, USA) for 1 min, and then placed on a sieve (⩽0.25 mm) and the fine powder passing through the sieve collected (Xu, Zhang, Wang, & Lu, 2010). The powdered samples were stored at −20°C and used for subsequent analysis.