Biuret and Bradford Methods Suitability for Protein Quantification in Rapeseed Meal

Being attractive for insects, non-genetically modified rapeseed is valuable for maintaining environmental biodiversity. Primarily, the rapeseed is an important industrial crop which is used for production of vegetable oil. Oil extraction from rapeseeds results in the generation of substantial amounts of rapeseed meal which is used either as a protein rich feed additive or as a source for preparation of protein containing ingredients for food industry. Both applications require frequent evaluation of protein content. Although Kjeldahl method is considered standard, it is not appropriate for routine evaluation of protein content in protein extracts. The aim of the study was to evaluate suitability of biuret and Bradford methods for protein quantification in rapeseed meal extracts. After consecutive triple extraction of proteins with water, 5% NaCl, 70% ethyl alcohol and 0.1 N NaOH, protein evaluation of each albumin, globulin, prolamin and glutelin extraction aliquot demonstrated overall lower protein content by Bradford method compared to biuret method. The most pronounced differences in protein content were observed with prolamin fraction where three fold higher protein concentrations in each extraction aliquot was observed when biuret method was applied for the evaluation. Comparative quantification of the total protein of each of the four fractions followed a similar trend of lower protein content evaluation by Bradford method. Overall results indicated biuret method as more suitable for protein quantification in rapeseed meal extracts which was confirmed by comparison with data obtained by Kjeldahl method.