Pigmented Rice Hull Extracts: Extraction of Phenolic Compounds and Their Antioxidant Activity in Oil-in-Water Emulsion

Abstract

Rice hull, an under-utilized agricultural waste, is always employed for low value purposes such as fuel. Hull of pigmented rice (Oryza sativa L., cultivars Sangyod) was used to produce rice hull extracts, and their antioxidant activities were observed. Initially, the effective conditions to prepare rice hull extracts were established by comparing enzyme- and ethanol-aided extraction methods. The extractor solvents—deionized water with Viscozyme-L (0.5 and 1% by weight of rice hull) at pH 6.25 and ethanol (concentrations of 50, 65, and 75%, volume per volume)—were mixed with rice hull (ratio of rice hull to extractor solvent was 1:10, weight per volume). The mixtures were heated at 50 °C for 0–5 hr. The extractor solvent and heating time influenced the total phenolic content (TPC) and antioxidant activities of the extracts. Generally, the TPC increased with increased extraction time. Using enzyme, a higher enzyme concentration effected an increase in the TPC and the antioxidant activities of the extracts. Ethanolic extraction was more efficient at recovering phenolic compounds than the enzyme-aided means. The extracts exhibited antioxidant activities through both primary and secondary mechanisms and their antioxidant abilities coincidentally increased with the TPC. Extraction using 65% ethanol for 2 hr could provide rice hull phenolic extracts (RHPE) with the highest TPC and antioxidant activities as indicated by radical inhibition effects, the reducing power and the ferrous chelating ability (P < 0.05). RHPE effected oxidative stability of soybean emulsions in different ways, depending on the applied concentration. At concentration levels of 0.5 and 1% (w/v), RHPE successfully improved the oxidative stability of the emulsions. However, a comparable oxidative stability with the control sample (emulsion without RHPE) was observed when RHPE was added at a level of 1.5% (w/v).