Discussion 77 Determination of total nitrogen crude protein in plant material and feeding stuffs

If it is required to perform the determination by digestion and distillation, refer to Method 5.6b. 'Determination of organic plus ammonium-N by digestion and distillation', but use 2 g oven-dry plant sample ground to 1 mm. Also, for the calculation, multiply the sample titre minus blank titre by 0.35 to give the % N in the sample. Multiply the % N by 6.25 to get the % crude protein. This assumes there are 160 g N kg-1 plant protein. Traditional factors for other products are: almonds, 5.18; brazil nuts and peanuts, 5.46; coconuts and tree nuts, 5.30; dairy products, 6.38; wheat, 5.7. Note: about 20% of any nitrate present will be included.

The official EC method for crude protein in feeding stuffs may be downloaded from:

http://europa.eu.int/eur-lex/en/lif/dat/1993/en_393L0028.html This is document 393L0028 (which is presented in the Official Journal No. L1 79, 22.7.93, p.8), and is the Commission Directive 93/28/EEC of 4 June 1993. The determination of crude protein appears as the Annex and amends Point 2 of Annex I to Directive 72/199/EEC.

The AOAC semi-automated Kjeldahl method for crude protein in animal feed with determination by segmented flow autoanalysis is given by Padmore (1990, pp. 72-74). The method below is a modified version of that in Faithfull (1971a), which allowed several elements to be determined simultaneously. Two sample probes may be placed in the sampler dipper, and the capillary from each may be split to supply two different chemistries (auto-analyser modules) per pump. Although this is an efficient system, there are two drawbacks: a fault with the plumbing of one chemistry means the other one going through the same pump will have to be stopped in order to effect a repair; and if there are too many pump tubes attached to the pump, there is a danger that the fluid will stop flowing in one of the tubes because of the increased pressure required between the rollers and the pressure-plate to ensure that the bores of all the tubes are properly compressed. We will describe the single chemistry system, but point out that the addition of another chemistry utilizing the same digest solution is possible.

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