Release of ferulic acid from sugar‐beet pulp by using arabinanase, arabinofuranosidase and an esterase from Aspergillus niger

Aspergillus niger cinnamoyl esterase (CinnAE) is shown to be active towards a wide range of feruloylated oligosaccharides derived from sugar‐beet pulp (SBP). The esterase hydrolysed ferulic acid ester‐linked to either C‐2 of arabinose or C‐6 of galactose residues, and demonstrated the highest activity towards the feruloylated arabinose trisaccharide. However, CinnAE was able to release only 0.88% of total alkali‐extractable ferulic acid from SBP in 24 h when acting alone. To determine whether cell‐wall‐degrading enzymes could increase the release of ferulic acid by CinnAE, SBP was incubated with various carbohydrases [cellulase, polygalacturonase, endo‐arabinanase, alpha‐L‐arabinofuranosidase, endo‐(1,4‐beta‐D‐galactanase, beta‐D‐galactosidase]. These were added alone and in pairs, both in the presence and absence of CinnAE. We showed that all the carbohydrases tested were free of esterase activity. When individual carbohydrases were incubated with SBP, whether in the presence or absence of CinnAE, less than 1% of the feruloyl groups were released. When incubated with a mixture of endo‐arabinanase and alpha‐L‐arabinofuranosidase, the esterase was able to release 14 times more of the alkali‐extractable ferulic acid present in the whole pulp as free acid than CinnAE alone. Ferulic acid is linked either to L‐arabinose or D‐galactose in SBP, but no corresponding increase in ferulic acid release was detected when SBP was incubated with CinnAE plus endo‐(1,4)‐beta‐D‐galactanase and beta‐D‐galactosidase (both from A. niger). Hence feruloylated arabinans in SBP are readily available for hydrolysis by arabinan‐degrading enzymes, whereas feruloylated galactans are not available for hydrolysis by galactan‐degrading enzymes.