Effect of microwave drying and oven drying on the water activity, color, phenolic compounds content and antioxidant activity of coconut husk (Cocos nucifera L.)

The coconut (Cocos nucifera L.) husk is basically composed by fiber and pith material and remained under-utilized. This is an important source of phenolic compounds that could be used as functional ingredients. The aim of this study was to determine the effect of: oven-drying (OD) and microwave drying (MD), on the water activity, color, phenolic compound content and antioxidant activity of coconut husk. The OD was performed at 60 °C for 12 h and MD was performed at 900 W for 10 min. The total phenolic content (TPC) in fresh coconut husk was 64.2 mg GAE/g dry wt and significant higher than observed after OD and MD of 35.8 and 45.5 mg GAE/g dry wt, respectively. Ten phenols were identified in fresh and dehydrated coconut husks. The husk MD showed an increase in the content of gallic, 4-hydroxybenzoic, ferulic and syringic acids and epicatechin compared with the fresh; while coconut husk OD and MD, showed a decrease in the content of vanillic acid, vanillin, catequin and kaempferol. The antioxidant activity decreased after both OD and MD. However, MD resulted in a better antioxidant activity in husk than OD. MD of husk resulted into better retention of preserved color, TPC and TFC than OD.