In the Weendes method when the sample is subjected to acid and alkali digestion, we obtain a residue comprising ash (mineral matter) of the feed and the resistant fraction of carbohydrate. When the residue is ignited the organic matter gets oxidized leaving the inorganic residue or ash. Thus the difference in weight of the residue before and after ashing gives the weight of crude fiber.
Weight of the sample - W gm
Residue (crude fiber + mineral matter) - WI gm
Ashing at 600 c Organic matter Oxidised
Ash remains - W2 gm
Crude fiber = W1-W2.
% of crude fibre = W1-W2/ W X 100