Preparation of standard curves

(1) Standard curve preparation of xylose The standard xylose of 0.1 g was dissolved in 50 mL citric acid-sodium citrate buffer solution. After stirring and dissolving, it was transferred into 100 mL volumetric bottle. The standard xylose solution of 1 mg·mL^-1 was prepared by adding citric acid-sodium citrate buffer. Ten clean plugged test tubes were added with 0.1 mL, 0.2 mL, 0.3 mL, 0.4 mL, 0.5 mL, 0.6 mL, 0.7 mL and 0.8 mL standard solution respectively. Distilled water was added to make volume of 1 mL, followed by addition of 3 mL DNS reagent, and boiled for 10 minutes. Distilled water was added to make volume 25 mL after cooling to room temperature. 200 uL was added to 96-well enzyme plate to determine its absorbance at 540 nm by microplate reader (PerkinElmer Enspire). Standard curve was made with absorbance value as ordinate and xylose concentration as abscissa.

Standard curve of xylose linear regression was performed with absorbance (y) as ordinate and xylose content (x) as abscissa. The regression equation was obtained as follows: y = 0.2242x + 0.0135 (R² = 0.9988, n = 8). The linear relationship between xylose and absorbance was good in the range of 0.1 mg·mL^-1~0.8 mg·mL^-1.

(2) Standard curve preparation of glucose: The standard curve of glucose was prepared in the same way as that of the above steps. Xylose was replaced by glucose.

Standard curve of glucose linear regression was performed with absorbance (y) as ordinate and glucose content (x) as abscissa. The regression equation was obtained as follows: y = 0.305x-0.0064 (R² = 0.9991, n = 7). The linear relationship between glucose and absorbance was good in the range of 0.2 mg·mL^-1~1.2 mg·mL^-1