Dear Catherine

I actually do not know what happen with your sample. but I think you should first stop the reaction by adding immediately ten fold, your solution, cold water or ice. this is done before centrifuging and I think it will be enough to stop the hydrolysis. and I think this brown ppt is due to different constituent as lignin. did you analysis it in your MCC sample first?

Dear Mona

Thank you for your helping and answer. I did stop the reaction by the way you mentioned (ten fold cold water and putting in ice bath). I think if the material was lignin, it would't formed in the second centrifugation, would it?

I think perhaps it is the oxidized cellulose which has formed during acid hydrolysis and is more stable than cellulose in water due to polar functional groups, so I can't see it in the dispersion. but after centrifugation the heavier oxidized cellulose is deposited at the bottom of the falcon and can be seen, but I'm not sure.

Sorry Cathterine, I can not provide explanation  with your problem, here I send your paper maybe help what your need.

Thanks

Dear Catherine, 

You can collect the samples before and after the centrifugation, then freezed-dry them and check these powders by ATR-FTIR. If there is any change in the structure because of the oxidation process then the spectra should tell you. 

Hope you can find something interesting in your phenomenon! 

Le Quynh Hoa