The invention relates to a process for preparing a composition comprising hydrolysing arabinoxylan to arabinoxylan-oligosaccharides in spent grain. Further the invention relates to an arabinoxylan-oligosaccharides rich composition. Further, the invention relates to the use of the arabinoxylan-oligosaccharide rich composition to for promoting the flora of the colon and/or for promoting the growth of lactic acid bacteria.