Evidence for the Presence of a Pentosanase Inhibitor in Wheat Flours

The solubilisation, by a pentosanase preparation fromAspergillus niger, of arabinoxylans from water-unextractable pentosans (WUP) isolated from wheat flour was much reduced when carried out in flour aqueous extracts as medium, instead of pure buffer. When flour extracts were previously heated at 100°C, the extent of arabinoxylan solubilisation was almost restored. The heating at 100°C and centrifugation of the flour extracts removed approximately one-third of the soluble protein but very low amounts of arabinoxylan. Increasing the concentration of extracts decreased the extent of WUP arabinoxylan solubilisation. There was slight variability between wheat cultivars Apollo, Soissons and Thésée in the extent of the inhibitory effect. Compounds responsible for this effect were mainly present in wheat grain endosperm but also in bran. Different microbial xylanases fromA. niger(Grindamyl S 100 and E1, an endoxylanase purified from this commercial preparation) andTrichodermastrains (C1, a partially purified cellulase/hemicellulase complex, and the commercial preparations Veron HE and Multifect XL) were strongly inhibited. Also the arabinofuranosidase activity present in Grindamyl S 100 was inhibited but to a lower extent than xylanases. Pronase treatment and protein addition in the extracts had no effect on the level of inhibition.