Nitrogen-to-protein conversion factors revisited for applications of microalgal biomass conversion to food, feed and fuel

Accurately determining protein content is important in the valorization of algal biomass in food, feed and fuel markets. Conversion of elemental nitrogen to protein is a well-accepted and widely practiced method, but depends on developing an applicable nitrogen-to-protein conversion factor. The most complete method to determine this factor takes six different hydrolyses of the subject material and these are not always carried out in reported literature studies. We report new data for conservative conversion factors determined from 21 algae samples along with over 50 amino acid profiles from the literature, representing distinct cultivation conditions for fresh and marine algae. We find that the amino acid profile among different algae samples is consistent, however the large variability between strains in non-protein nitrogen (up to 54% in microalgae) causes variability in the calculated conversion factor. We include our calculated novel nitrogen-to-protein conversion factors for model and commercially relevant biofuel algal strains and compare these with the literature.