Discussion 71 Determination of acid detergent fibre cellulose and lignin

These are a trio of methods, and are described together. The acid detergent fibre (ADF) can be carried out as a separate analysis, but the ADF residue is required to determine the lignin from the weight loss on oxidation by potassium permanganate. Any tannins would also be removed by KMnO4. The residue from the oxidation step contains mainly cellulose (plus any cutin, which appears as dark flecks in the white cellulose) and mineral ash. The cellulose content is determined as the weight loss on ashing. The method is adapted from that of Van Soest and Wine (1968). The AOAC method for lignin uses highly corrosive 72% v/v sulphuric acid instead of permanganate-buffer reagent, and also requires asbestos filter aid, which precludes use of the residue for other determinations. The efficiency of the lignin oxidation depends on the particle size not significantly exceeding the specified mesh. We use 1 mm, but Van Soest and Wine (1968) use 20-30 mesh, approximately 0.5-0.9 mm. It would be worth setting up a spreadsheet for calculations involving large numbers of samples. For further details see Chapter 4, where other aspects of the various fibre determinations are presented more fully.

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