الفهرس 
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Abstract
The main aim of this study was to isolate and identify some local xylanolytic fungal isolates and investigates the optimal conditions for xylanase production using some agro-residues i.e wheat bran, rice husk, corn cobs, rice straw, wheat straw and clover straw under solid state fermentation. Fifteen isolates belonging to three genera i.e Aspergillus sp., Penicillium sp. and Alternaria sp. were screened and tested for xylanase productivity using six agro-residues. The most active isolates were belonging to Aspergillus niger (2 isolates) and Penicillium citrinum (2 isolates) which were tested for xylanase production using 15 combinations between Agro-residues (1:1 w/w).Factors affecting xylanase activity produced by the most active isolates were also investigated. The best isolate for xylanase production were A.niger 15 followed by A.niger 1, P.citrinum 13 and P.citrinum 10. The combination of wheat bran+ clover straw proved to be the best for xylanase production and the maximum production was after 7 days of incubation at 30 ºc for P.citrinum isolates and 35 ºc for A.niger isolates. The optimum moisture content was 1:2 w/v substrate : distilled water (80%) for all selected isolates. Supplementation of fermentation media with xylan, cellulose or starch as additional carbon source increased xylanase production. As well as addition of ammonium sulphate and ammonium chloride as additional nitrogen source increased xylanase production. The results obtained in this study indicated A.niger 1 & 15 as a potential isolates for xylanase production using agro-residues under solid state fermentation specially by using the combination of wheat bran+ clover straw as a substrate. Interesting observation was that the produced enzymes showed and retained more than 90 % of their activities at 85 °C compared to the optimum at 70 °C. Furthermore, the produced enzymes showed optimum activity under acidic condition (pH 5.5) and retained about 90% of their activities at pH 4. In an attempt to use the produced xylanases in saccharification of different lignocellulosic wastes we obtained 197 mg sugar/g dry corn cobs after 8 h of incubation at 50 ºc with 500 IU/gds of A.niger 15 xylanase which constitute about 20% saccharification. This makes the produced xylanase is useful for various industrial applications such as biofuel production and pulp industries.