Determination Of Plant Proteins Via The Kjeldahl Method And Amino Acid Analysis A Comparative Study.

Abstract The amount of protein in most foods is usually determined by multiplying its Kjeldahl nitrogen content by a factor of 6.25. The reliability of this method in quantitating plant proteins was investigated. Ten lesser known plant leaf samples of nutritional significance among certain populations in Nigeria were used for this study. Protein contents of the plant samples were determined via the kjeldahl method using the conventional nitrogen to protein NP conversion factor 6.25 i.e. total nitrogen 6.25 and by summation of amino acid residues considered more accurate and taken here as the actual protein content. From data of total amino acid and total nitrogen specific NP conversion factors were calculated for each sample. The NP factors ranged from 3.24 to 5.39 with an overall average of 4.64. Protein contents were also calculated using this new factor. Comparison of the calculated protein contents showed that the traditional conversion factor of 6.25 overestimated the actual protein content of the samples. The degree of overestimation ranged from 16-93. Protein contents calculated with our adjusted factor 4.64 gave results that are in good agreement with the actual protein content. Our results indicate that calculation of protein content by N 6.25 is highly unsuitable for plant samples.