A rapid and quantitative method for assessing the whiteness of whitened lignin based on an in-depth analysis of reported methods.

The modification of lignin to a lighter color has been a concern in lignin valorization. In the previous reports on lignin color reduction, the methods of assessing the whiteness of lignin varied from each other, due to the absence of a universal method for characterizing the lightness/darkness of dark samples. In this paper, the reported methods for assessing the whiteness of lignin were analyzed, including "comparison of absorbance curve", "absorbance at specific wavelength", "ISO or TAPPI brightness" and "photograph observation". Our study revealed that various defects existed in these methods. In light of the Munsell color system, we herein proposed a quantitative method for evaluating the whiteness of lignin, based on the finding that the sum of the reflectance of samples with the same Munsell value (whiteness) are substantially equal. A model for converting the reflectance of lignin to Munsell value was established, and the effectiveness of the model was verified and discussed. The standard deviation of the model ranges from 0.23-1.10. The applicability of the method to liquid was also discussed in the paper. Moreover, because the model was derived from the Munsell color system, it can also be applied to characterize other medium to dark-colored objects besides lignin.