Impact of alkaline or acid digestion to antioxidant activity, phenolic content and composition of rice hull extracts

- Alkaline/acid digestion affect differently antioxidant activity of rice hull extracts.
- Heating (120 °C) during digestion positively influenced release of phenolics.
- Alkaline digestion provides extracts richer in phenolics and antioxidant efficiency.
- Only alkaline digestion assists recovery of p-coumaric and ferulic acids.
- Silicon level of hulls may be inversely related to p-coumaric/ferulic acid content.

The impact of alkaline or acid digestion to phenolic content (TPP), antioxidant activity (radical scavenging, copper (II) reducing and liposome oxidation) and phenolic composition of rice hull extracts was evaluated systematically for the first time. Alkaline digestion yielded extracts superior in TPP (13206-25498 mg gallic acid/kg dry hulls) and antioxidant activity. Chromatographic analysis showed that these were rich in hydroxycinnamic acids, whereas hydroxybenzoic acid/aldehyde derivatives, obtained as hydroxycinnamate degradation products, were the main phenolics in the extracts after acid treatment. Optimized alkaline digestion (120 °C, 120 kPa, 2 h) applied to hulls from selected rice varieties cultivated in Greece, showed a high p-coumaric (4879-6367 mg/kg dry hulls) and ferulic acid (1510-2037 mg/kg dry hulls) content, probably related to their low silicon levels (44,916-59,981 mg SiO2/kg dry hulls). Our study provides useful data for exploitation of rice hulls by the food industry.