Previous studies on plum fruit skins and industrial plum pomaces (Sójka et al., 2015) have shown that the predominant flavonol in the plum extracts was rutin, whereas neochlorogenic and chlorogenic acids were the main hydroxycinnamic acids. Although the present study also found high amounts of rutin in the samples, with maximum concentrations during the later stages of fermentations (on the 9th or 14th day) (Table 1), however the isoquercitrin was the major flavonol in both processed plum residues. This result could be explained by the ability of the filamentous fungi (especially from the genus Aspergillus) to produce a-rhamnosidase (Lin et al., 2014), an enzyme known for its capability of removing one terminal a-rhamnose present in many natural glycosides. It is possible that part of the rutin accumulated during SSF processes was