Method 72 Determination of crude fibre

The EC official method is described in the Official Journal of the European Communities (EC, 1992), and uses specialized glassware. We will base the method on that of MAFF/ADAS (1986, pp. 90-92), (with Crown Copyright permission), but replace the alcohol and diethyl ether (used for washing the residue) with acetone, which is safer and also used in the EC method. The AOAC method is described by Padmore (1990, pp. 80-82). The traditional method uses 0.313 M NaOH, free from carbonate, but the EC method uses 0.23 M KOH. The method is suitable for plant material or animal feeds, but the following pre-treatments may be necessary, especially for feedstuffs.

Pre-treatments (if required). Samples containing >3% (>5%, EC method) calcium carbonate are pre-treated with excess 0.1 M HCl (3 x 30 ml 0.5 M HCl, EC method). Add the acid to the 3 g (1 g, EC method) weighed sample in a 500-ml short-neck round-bottom flask with ground glass socket size 34/35, and swirl for 1 min. Allow to settle, and then decant the supernatant into a 125-mm Whatman No. 541 filter paper, and wash the residue twice with water, decanting the washings into the filter paper. Allow both the residue and the filter to drain thoroughly. Bring 200 ml of 0.128 M sulphuric acid to boiling point, use a portion to wash any particles from the filter paper back into the flask, then add the remainder of the acid to the flask and proceed with refluxing as detailed below.

Samples containing >10% crude fat must be defatted before analysis with petroleum spirit (light petroleum, 40-60°C boiling range). This may be done either in a Soxhlet extractor, or in a beaker by stirring, settling and decanting three times with 30 ml petroleum spirit. Allow the fat-free sample to air-dry in a fume cupboard.


• Acetone, commercial 'drum' grade

• Hydrochloric acid, approximately 0.1 M - dilute 1 vol. of hydrochloric acid, approximately 36% m/m HCl to 100 vol.

• Petroleum spirit, (light petroleum), boiling range 40-60°C

Procedure. Weigh 3 g of either a feed sample (as received, ground to 1 mm; also take a separate sample at the same time for moisture analysis) or plant sample dried at ^65°C, into a 500-ml short neck round-bottom flask with ground glass socket size 34/35. Add 200 ml of 0.128 M sulphuric acid (measured at room temperature) which has been heated to boiling point, place the flask on a macro Kjeldahl heating unit, connect a coiled reflux condenser through which a steady stream of cold water is flowing, and bring to the boil within 1 min on full heat. Reduce the heat and continue to boil gently for 30 min. Reduce any excessive foaming by addition of 1-2 drops octan-2-ol, and swirl every 5 min to wash sample particles from the flask wall back into the acid. If any bumping has caused sample particles to enter the condenser, squirt a wash-bottle into the top of the condenser to wash them back into the acid using the minimum amount of water. Assemble a borosilicate glass Hartley type three-piece funnel with polypropylene support plate (e.g. Whatman, 530 ml, 125 mm) on which is a 125-mm Whatman No. 541 filter paper. Preheat the funnel by pouring boiling water into it. Turn the heat under the flask off and allow to stand for approximately 1 min before pouring into a shallow layer of hot water in the funnel. Adjust the suction so that the filtration is completed in less than 10 min. Wash the insoluble matter with boiling water until the washings are neutral to litmus paper. Wash the residue back into the flask using 200 ml (measured at room temperature) of boiling 0.313 M sodium hydroxide. Boil for 30 min, as described previously for the acid, then allow to stand for 1 min, and filter hot through a 60-ml, porosity No. 1 (P160, 100-160 pm) sintered glass crucible using gentle suction. Transfer the whole of the insoluble material from the flask to the crucible with hot water. Wash first with boiling water, once with approximately 0.1 M HCl, and then with water until the washings are neutral to litmus paper. Wash with three successive 25 ml amounts of acetone and suck dry, then air-dry in a fume cupboard until no smell of acetone can be detected. Dry the crucible and contents to constant weight in an oven at 130°C (as recommended by EC and AOAC methods), allowing to cool in a desiccator before weighing. Place the crucible in a cool muffle furnace, and increase the temperature to between 475°C and 500°C; keep at this temperature for at least 30 min and until ashing is complete. It is important not to overshoot the temperature, because the sintered crucibles will be damaged at ^ 515°C. Remove the crucible from the muffle furnace, cool in a desiccator and weigh.

Calculation. Divide the loss in weight on ignition by 0.003 to give the g kg-1 crude fibre in the sample, or divide by 0.03 to give the % crude fibre in the sample.

Note: The EC method also specifies a blank determination, where the weight loss on ashing should not exceed 4 mg. This should be subtracted from the loss in weight of the sample on ignition when calculating the % crude fibre.

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