Filter paper assay procedure. Reagents: Whatman No. 1 filter paper cut into 1 × 6 cm strips (50 mg); buffer = 0.05 M Na citrate pH 4.8; glucose standards in buffer; dinitrosalicyclic acid (DNS). Reagent for reducing sugar . Filter or centrifuge culture sample to remove solids. Dissolve enzyme powders at 1.0 to 5.0 mg/ml in buffer. Dilute enzyme solutions in buffer. Place 0.5 ml enzyme solution and 1.0 ml buffer in 18 mm test tube. Add a filter paper strip and mix on Vortex mixer to coil the paper in the solution. Incubate 1 h at 50°C. Add 3 ml DNS reagent to stop the reaction. Place tubes in boiling water for 5 min and determine the amount reducing sugar as glucose. Include a blank tube (without filter paper) to correct for any reducing sugar present in enzyme preparation. The mg of glucose produced in this test is the filter paper (FP) activity. The DNS reagent  measures reducing sugar non-specifically. When glucose is used as standard, values for cellobiose will be about 15% low and values for xylose about 15% high on a weight basis.