Effects of drying methods on antioxidant properties in Robinia pseudoacacia L. flowers

The effects of different drying methods (sun drying, hot-air drying, freeze drying and microwave-vacuum drying) on the antioxidant properties and phenolic content in Robinia pseudoacacia L. flowers (RPF) were investigated. Simultaneously, the correlation between phenolic content and antioxidant capacity was discussed. Results showed that antioxidant properties and phenolic content in dried RPF were affected by the drying methods to different extents. Freeze drying sample possessed the highest antioxidant activity including DPPH radical scavenging, reducing power and hydroxyl radical scavenging ability, and the total phenolic content was 47.30 mg gallic acid equivalents (GAE) /g DW. Microwave-vacuum drying samples had the best iron-chelating ability and its phenolic content (45.52 mg GAE/g DW) was comparable to that of the freeze drying samples (P> 0.05). Sun drying was the worst in antioxidant activities and phenolic content was only 29.15 mg GAE/g DW. Additionally, significantly positive correlation was found between the total phenolic content and antioxidant activity (P< 0.01). Microwave-vacuum drying was a potential drying technique for RPF.