Unfermented recalcitrant polysaccharide structures from rapeseed (Brassica napus) meal in pigs

Unprocessed and acid-extruded rapeseed meal (RSM) was fed to pigs as the only source of non-starch polysaccharides (NSP) and protein. Unfermented carbohydrate structures were analyzed. Acid-extrusion seemed to increase rigidness of the NSP-matrix in vivo, without affecting NSP-fermentability. Water-soluble NSP were almost completely fermented in the colon. From the water-insoluble unfermented carbohydrates 46-68% (w/w) was analyzed as the polysaccharides rhamnogalacturonan, (branched) arabinan, XXXG-type xyloglucan, linear xylan, galactomannan, and cellulose. A major fraction (35-54% w/w) of the unfermented carbohydrates was unexpectedly released as small uronyl-rich carbohydrates (<14 kDa) during alkaline extraction using 6 M NaOH. It follows that alkali-labile bonds hindered the complete fermentation of NSP in pigs and that fermentation may be improved by an alkaline pretreatment of RSM in the feed.