A recombinant strain of the protease-deficient, non-acidifying pH mutant Aspergillus niger D15 (A. niger D15 [abfB]) strain was developed to secrete alpha-l-arabinofuranosidase (AbfB) free of endo-1,4-beta-xylanases for selective hydrolysis of xylan into hydrogels. The A. niger D15 [abfB] strain expressed the alpha-l-arabinofuranosidase abfB gene under the transcriptional control of the glyceraldehyde-3-phosphate dehydrogenase promoter (gpd (P) ) and glucoamylase terminator (glaA (T) ) in fermentation cultures containing 10 % glucose. The yield, activity, purity, kinetics and ability of the recombinant AbfB to selectively hydrolyse xylans into hydrogels were assessed. The recombinant AbfB secreted in 125-mL shake flasks and 10-L bioreactor fermentation cultures had specific activities against rho-nitrophenyl-alpha-arabinofuranoside of up to 4.4 and 2.7 U g(-1) (dry weight), respectively. In addition, the recombinant AbfB was present as a single protein species on silver-stained 10 % sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The recombinant AbfB had optimal activity at 40-55 A degrees C and pH 3.0 to pH 5.0 and was stable at temperature and pH of up to 60 A degrees C and pH 6.0, respectively. About 20 % of the available arabinose in the xylan was released by the recombinant AbfB from the hydrolysis of low viscosity wheat and oat spelt arabinoxylans and about 9 and 5 % from bagasse and bamboo arabinoglucuronoxylans, respectively, that led to the formation of the hydrogels. Therefore, the constructed A. niger D15 [abfB] strain presented a microbial system for the production of recombinant AbfB with the required purity for the modification of xylans into hydrogels.