Antioxidant activity of fermented and nonfermented coffee pulp extracts

Coffee pulp contains natural antioxidants like hydroxycinnamic acids, most of which are covalently linked to the cell wall. These compounds can be released by fermentation or enzymatic processes. In this study, the antioxidant properties of fermented and nonfermented coffee pulp have been evaluated. Coffee pulp was fermented by solid-state fermentation using the fungus Aspergillus tamarii. Fermented and nonfermented samples of coffee pulp were extracted with aqueous methanol followed by alkaline hydrolysis. In both cases, the total polyphenol concentration was quantified by Folin-Ciocalteu method, then hydroxycinnamic acids were concentrated using ethyl acetate and quantified by HPLC. The antioxidant properties of samples were determined by radical monocation of 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulphonic acid) [[ABTS].sup..+]: the antioxidant activity was determined by kinetic parameters known as [ED.sub.50], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and antiradical efficiency (AE). Fermented extracts containing free hydroxycinnamic acids showed better antiradical activity against [[ABTS].sup..+] than the other nonfermented ones. There were no significant differences in the total content of polyphenols in fermented and nonfermented coffee pulp, but the content of total hydroxycinnamic acids was higher in the nonfermented coffee pulp extracts (47.1 g/kg) than in the fermented coffee pulp (30.9 g/kg). Nevertheless, the fermentation process increased the fraction of free hydroxycinnamic acids (47 %) and consequently decreased those covalently linked to the cell wall. The results of the antioxidant activity assays could be explained by the presence of free hydroxycinnamic acids. Fermented coffee pulp assays showed that free hydroxycinnamic acids were metabolised by A. tamarii. This study shows the potential of using coffee pulp as a natural source of antioxidants. Key words: antioxidants, hydroxycinnamic acids, [[ABTS].sup..+], solid-state fermentation