Comparison of different nitrogen detection methods after Kjeldahl digestion of dietary fiber preparations
F. Urbat (1), P. Mueller (1), A. Hildebrand (1), D. Wefers (1), M. Bunzel (1) (1) Karlsruhe Institute of Technology, Karlsruhe, Germany.
Just as non-starch polysaccharides, lignin, and other polymers, proteins are part of the plant cell wall. However, they are not included in the definition of dietary fiber. Thus, it is necessary to adjust the dietary fiber fraction for its protein content in addition to its ash content. The official AOAC-Method (2009.01) for the determination of total dietary fiber by an enzymatic-gravimetric method and liquid chromatography recommends using either Kjeldahl analysis or a combustion method for protein determination. Approaches for nitrogen detection after Kjeldahl digestion are not further specified. Therefore, three different nitrogen detection methods were compared with regard to accuracy, precision, and practicability: the conventionally used titration with hydrochloric acid following steam distillation, a spectrophotometric determination based on a method of Willis et al. (1996), and the determination with an ammonia sensitive electrode. The spectrophotometric assay was tested using disposable cuvettes or 24 well plates for high throughput. It was demonstrated that all methods give very similar results, although the standard deviation of the high throughput spectrophotometric assay was slightly higher, probably due to smaller pipetting volumes. In terms of efficiency nitrogen detection by using the ammonia sensitive electrode was found to be a fast and easy to use approach. Finally, the methods were applied side-by-side on soluble and insoluble fiber fractions of several food samples (wheat, buckwheat, wild rice, pear, and asparagus) to demonstrate their applicability on different fiber types.
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