GIFSQ Tools · Module 1 — Food Composition & Proximate Analysis · Tool 9 of 500
Acid then alkali digestion leaves the fibre behind. Weigh the dried residue and what is left after ashing, and the difference is the crude fibre. Add the dry matter % if you want it on a dry basis.
Try sample values — fills the form with the worked example below.
–
Crude fibre (%)
–
Residue (g)
–
Fibre %, dry basis
Equation: Crude fibre % = (mresidue − mash) ÷ msample × 100
From crucible weights: mresidue = (crucible + dried residue) − empty crucible; mash = (crucible + ash) − empty crucible.
Dry basis: Fibre % (dry) = Fibre % ÷ (dry matter % ÷ 100).
Method: Weende crude fibre, AOAC 978.10 (animal feed) / AOAC 962.09 — digest with 1.25 % sulfuric acid for 30 minutes, then 1.25 % sodium hydroxide for 30 minutes, dry the residue at 105 °C, weigh, then ash at 550 °C and weigh again.
Assumptions: the residue left after acid and alkali digestion is fibre; ashing removes all organic matter, so what is left is mineral ash.
Limitations: crude fibre underestimates true dietary fibre — much of the hemicellulose and some cellulose dissolves in the alkali step, and soluble fibre is lost entirely; results shift with digestion time and reagent strength, so run the method exactly as written; for total dietary fibre use the enzymatic-gravimetric method instead.
Rounding: results to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
From crucible weights: mresidue = (crucible + dried residue) − empty crucible; mash = (crucible + ash) − empty crucible.
Dry basis: Fibre % (dry) = Fibre % ÷ (dry matter % ÷ 100).
Method: Weende crude fibre, AOAC 978.10 (animal feed) / AOAC 962.09 — digest with 1.25 % sulfuric acid for 30 minutes, then 1.25 % sodium hydroxide for 30 minutes, dry the residue at 105 °C, weigh, then ash at 550 °C and weigh again.
Assumptions: the residue left after acid and alkali digestion is fibre; ashing removes all organic matter, so what is left is mineral ash.
Limitations: crude fibre underestimates true dietary fibre — much of the hemicellulose and some cellulose dissolves in the alkali step, and soluble fibre is lost entirely; results shift with digestion time and reagent strength, so run the method exactly as written; for total dietary fibre use the enzymatic-gravimetric method instead.
Rounding: results to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: sample 2.000 g, empty crucible 25.000 g, crucible + dried residue 25.120 g, crucible + ash 25.020 g. Residue = 25.120 − 25.000 = 0.120 g; ash = 25.020 − 25.000 = 0.020 g; crude fibre = (0.120 − 0.020) ÷ 2.000 × 100 = 5.00 %. With dry matter 87.00 %, fibre on a dry basis = 5.00 ÷ 0.87 = 5.75 %.
Enter values above to see results.
Typical crude fibre: white wheat flour 0.3–0.5 %, whole wheat flour 2–3 %, oats 10–11 %, bran 40–45 %. Crude fibre is always lower than dietary fibre for the same food — report which one you measured. Tool 9 of 500 — GIFSQ
