A test of two methods for plant protein determination using duckweed

In a series of experiments, the Bicinchoninic Acid (BCA) method for protein content analysis was operationalised and compared to the common practice of crude protein calculations from Kjeldahl Total-Nitrogen determinations (crude protein = 6.25 x Total N). The BCA method involves bovine serum albumin as an external standard and a colour complexation with bicinchoninic acid and Cu. Procedure optimisation results were: extractant to sample volume (20: 1), extractant concentration (0.1M HCl), extractant procedure (24h, room T) and, if necessary, a second extraction round. For solutions of the albumin standard, both methods approached the known protein content closely (within 2%, no significant differences). In a paired analysis of duckweed (Lemna gibba L.) samples, the BCA and crude protein method gave 20.5 +/- 0.3 and 21.3 +/- 0.1% protein of duckweed dry weight, respectively. Although the difference was small (4%), it was significant (paired t-test: p = 0.04). The BCA method had a lower precision: its coefficient of variation was 5% versus 1% for crude protein. The slight over-estimation and greater simplicity of the Kjeldahl method justifies the common use of the multiplier 6.25 to estimate protein content in duckweed. (C) 2000 Elsevier Science B.V. All rights reserved.