Assessing the antioxidant activity of melanoidins from coffee brews by different antioxidant methods

Antioxidant activity of instant coffees produced from the same green coffee beans roasted at three different degrees was analyzed. Coffee melanoidins were obtained by ultrafiltration (10 kDa cutoff) and subsequent diafiltration. Pure melanoidins were isolated from melanoidins after overnight incubation in 2 M NaCl and then ultrafiltered. Filtrates, corresponding to the low molecular weight (LMW) fraction noncovalently linked to the melanoidin skeleton, were also preserved. Antioxidant activity of coffee brews (CB), melanoidins (M), pure melanoidins (PM), and bounded melanoidin compounds (BMC) were tested using the 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2'-azobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and ferric reducing power (FRAP) methods. The correlation between the different methods was studied. The higher contribution of melanoidins to the total antioxidant activity of coffees was shown to be caused by the LMW compounds linked noncovalently to the melanoidin skeleton, as data from BMC confirmed. CB, M, and BMC fractions exert the highest antioxidant activity in aqueous media, whereas PM was not dependent on the reaction media. The highest correlation was found between DPPH and FRAP methods.