No abstract

The functionality of the proteinaceous wheat xylanase inhibitor XIP-I in Aspergillus niger endoxylanaseaided breadmaking was investigated using small-scale breadmaking and in vitro experiments. At an optimal dosage for an increase in bread volume, the endoxylanase provoked a ∼15% increase of loaf volume in standard flour experiments, but the dough was significantly more sticky than in the control. No variations in bread volumes were observed in the presence of added XIP-I in endoxylanase dough but stickiness was much reduced. Arabinoxylan (AX) solubilisation from flour cell wall material was practically unaffected with XIP-I, and the viscosity of dough water extracts and AX intrinsic viscosity were slightly enhanced. In other breadmaking experiments using a flour from which water extractable material was inactivated, or a reconstituted flour (starch, gluten, water extractable arabinoxylans (WEAXs), water unextractable arabinoxylans (WUAXs)), no differences in loaf volumes were observed between control, endoxylanase and endoxylanase/XIP-I breads, but the high stickiness induced by endoxylanase was totally corrected by XIP-I. In these doughs, AX solubilisation was slightly affected by XIP-I whereas water extract viscosity and WEAX intrinsic viscosity were much higher than in endoxylanase doughs. In vitro experiments with purified AX and A. niger endoxylanase and XIP-I confirmed these findings, showing that the endoxylanase inhibition by XIP-I had more effect on the characteristics measured on WEAX (specific and intrinsic viscosities) than those measured on WUAX (AX solubilisation). Premixing substrate, endoxylanase and inhibitor in different combinationsshowed that XIP-I had a tendancy to bind WUAX, but not WEAX, which changed the balance between enzymic AX solubilisation and AX depolymerisation. As a result, a high viscosity of water extracts was preserved, which is beneficial for dough handling properties.

The effect of arabinoxylan structure on xylanase activity was investigated using a range of water-soluble and water-insoluble substrates isolated from wheat flour and several xylanases from families 10 and 11 of the glycoside hydrolases. The arabinose content of the substrates affected the activity in a linear manner related to the arabinose:xylose ratio and to different extents depending on the specificity of the xylanase. The soluble/insoluble feature of the substrates had a strong impact on the enzymatic activity and different selectivities (activity on insoluble arabinoxylan vs. activity on soluble arabinoxylan) were observed. There was no relationship between specificity and selectivity of a given xylanase.