Selectivity for water-unextractable arabinoxylan and inhibition sensitivity govern the strong bread improving potential of an acidophilic GH11 Aureobasidium pullulans xylanase

An acidophilic xylanase of Aureobasidium pullulans (XAPI) was recombinantly produced, characterised and its functionality in bread making compared with that of Bacillus subtilis Xyn A xylanase (XBS), an enzyme frequently used in bread making. Prominent characteristics of XAPI were its high specific activity towards arabinoxylan (AX), its relative preference for hydrolysis of water-unextractable AX (WU-AX), its optimal activity under acidic conditions and its sensitivity towards TAXI (Triticum aestivum xylanase inhibitor) and XIP (xylanase inhibiting protein). Optimally developed dough containing XAPI had a considerably drier feel after mixing and was less sticky after fermentation than dough treated with XBS. Both xylanases improved bread loaf volume by 24% under the conditions of the process. In spite of the higher dosage of XAPI necessary to obtain this result, the similar loaf volume increase coincided with less solubilisation of WU-AX and even lesser degradation of solubilised AX (S-AX) and water-extractable AX (WE-AX) by XAPI than by XBS during the bread making process. This is probably due to different dynamics and extent of xylanase inhibition in the dough matrix for both xylanases. AX solubilisation by the acidophilic XAPI was not boosted by the drop in pH in the dough during fermentation. Results show that significant bread loaf volume increase goes hand in hand with excellent dough properties when a limited solubilisation of WU-AX during mixing is coupled to retention of a high molecular mass of S-AX and WE-AX during the rest of the process.