Protein analysis of the common mushroom Agaricus bisporus

The crude protein content of the common mushroom (Agaricus bisporus) has been reported to be 19-38% on a dry weight (DW) basis, but measurements are complicated by the presence of non-protein nitrogen-containing compounds. Reliability of protein measurement methods were compared by extracting mushrooms in defined ways and the resulting fractions were analysed in terms of protein content (amino acid analysis and protein-dye binding), total nitrogen (Kjeldahl), free amino acids (as ninhydrin positive compounds) and ammonia/urea content. A general accepted protein determination method based on protein-dye binding was compared with amino acid analyses on the same samples and proved to be reliable. In the soluble fraction, most nitrogen of non-protein nitrogen is due to urea and ninhydrin positive compounds. In the fraction obtained by heating in dilute alkali the same substances were found, but not in the same (relative) quantities. In this fraction about 21% of the nitrogen was of unexplained origin and could be N-acetyl glucosamine solubilised from the cell wall. The protein content of the mushrooms examined was at most 0.5% on fresh weight (FW) basis and 7% on DW basis, which is four times lower than usually reported.