The FTIR can be used to detect the cellulose destruction (the finger print region between 800-1400), but I recommended you to calculate the average degree of polymerization of carbohydrates (DP).
FTIR spectra can be used. It is recommended that additional technical studies, to corroborate the results obtained and thus can then be used to quantify only the FTIR
you can learn a lot about the interpretation of cellulose FTIR spectra and the changes you can follow with FTIR from Schwanninger et al. in Vibrational Spectroscopy 36 (2004) 23–40 Effects of short-time vibratory ball milling on the shape of