Color measurements were performed using the dissolution
method (Huang et al., 2009a). Chips (2 g) were soaked in
50 ml 1% HCl–ethanol solution for 20 min. A solution aliquot
(5 ml) was then transferred into a 100 ml volumetric flask and
brought to the mark with 1% HCl–H2O solution. Then the
absorbance at 530 nm was determined using 10 mm × 10 mm
cuvette and a UV/Vis-spectrophotometer (Techcomp Scientific
Instrument Ltd., Shanghai, China).