Olive pomace (OP) and olive stones (OS) are industrial by-products from the olive oil production. These residues have a high percentage of cellulose, hemicellulose, and lignin, which makes them a good source of fermentable sugars and xylan, as well as a potential alternative substrate for xylooligosaccharides (XOS) production. In this work, OP and OS were chemically characterized and used for the first time as a xylan source to produce XOS through direct fermentation by Bacillus subtilis 3610 containing the xylanase gene xyn2 from Trichoderma reesei. OS presented the highest potential for XOS production. The fermentation process was further optimized for this residue in terms of residue concentration (5, 10, 20, 40 and 60 g L-1), pH (5.0, 6.0, 7.0 and 8.0), and temperature (30, 37, 45 and 50°C). The highest total sugars yield (27 ± 2 mg g-1) was achieved after 12 h, using 20 g L-1 of OS at pH 7.0 and 45°C.CA and AC acknowledges her junior researcher contract (CEECIND/00293/2020) and her PhD scholarship (UI/BD/153689/2022), respectively, from the Portuguese Foundation of Science and Technology (FCT). This study was supported by FCT under the scope of the strategic funding of CIMO (UIDB/00690/2020 and UIDP/00690/2020), CEB (UIDB/04469/2020) and Associate Laboratory SusTEC (LA/P/0007/2020)info:eu-repo/semantics/publishedVersio
