International Journal of Scientific & Technology Research

Home About Us Scope Editorial Board Blog/Latest News Contact Us
10th percentile
Powered by  Scopus
Scopus coverage:
Nov 2018 to May 2020


IJSTR >> Volume 3- Issue 4, April 2014 Edition

International Journal of Scientific & Technology Research  
International Journal of Scientific & Technology Research

Website: http://www.ijstr.org

ISSN 2277-8616

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

[Full Text]



A.M Magomya, D. Kubmarawa, J.A Ndahi, G.G Yebpella



Index Terms: Plants, protein, Total Nitrogen (TN), Amino acids, Kjeldahl, nitrogen-to-protein(N:P), conversion factors,



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 (N:P) 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 N:P conversion factors were calculated for each sample. The N:P 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.



[1]. Maynard, A. B. and Loosli, J. K. (1969). Animal Nutrition. McGraw-Hill, New York)

[2]. Herrera, C. M. (1982). Defense of ripe fruit from pests: Its significance in relation to plant-disperser interactions. Am. Nat. 120:218–241.)

[3]. Lourenço, S. O., Barbarino, E., Lanfer Marquez, U. M. and Aidar, E. (1998). Distribution of intracellular nitrogen in marine microalgae: Basis for the calculation of specific nitrogen-to-protein conversion factors. J. Phycol. 34:798–811

[4]. Conklin-Brittain, N. L., Dierenfeld, E. S., Wrangham, R. W., Norconk, M. and Silver, S. C. (1999). Chemical protein analysis: A comparison of Kjeldahl crude protein and total ninhydrin protein from wild, tropical vegetation. J. Chem. Ecol. 25:2601–2622.

[5]. Fujihara, S., Kasuga, A. and Aoyagi, Y. (2001). Nitrogen-to-Protein conversion factors for common vegetables in japan. J. Food Science. 66(3):412-415.

[6]. Milton, K. and F.R. Dintzis. (1981). Nitrogen-toprotein conversion factors for tropical plant samples. Biotropica 13(3):177–181.

[7]. Sedinger, J. S. (1984). Protein and amino acid composition of tundra vegetation in relation to nutritional requirements of geese. J. Wildl. Manage. 48:1128–1138

[8]. Izhaki, I. (1993). Influence of nonprotein nitrogen on estimation of protein from total nitrogen in fleshy fruits. J. Chem. Ecol. 19:2605–2615.

[9]. Levey, D. J., Bissell, H. A. and O’Keefe, S. F. (2000). Conversion of nitrogen to protein and amino acids. J. Chem. Ecol. 26: 1749–63

[10]. Sparkman D.H., Stein E.H and Moore S. (1958). Automatic Recording Apparatus for use in Chromatography of amino acids. Analytical Chemistry 30:119.

[11]. Association of Official Analytical Chemists – AOAC. (1990).Official Methods of Analysis, 15th Edn. AOAC, Washington, D.C, pp 556.

[12]. FAO Food Policy and Food Science Service, Nutrition Division: Amino-Acid Content of Foods and biological data on proteins. FAO Food and Nutrition Series No. 21. Food and Agriculture Organisation of the United Nations, Rome 1970.

[13]. Mattila, P., Salo-Väänänen, P., K. Könkö, H. Aro and T.Jalava. 2002. Basic composition and amino acidcontents of mushrooms cultivated in Finland. J. Agr.Food Chem., 50: 6419-6422

[14]. Lourenço, S.O., Barbarino, E., Joel C.D., Luis, O., Pereira and Lanfer Marquez, U. M (2002). Amino acid composition, protein content and calculation of nitrogen-to-protein conversion factors for 19 tropical seaweeds. Phycological Research.50: 233–241.

[15]. Diniz, G.S., Barbarino, E., Oiano-Neto, J., Pacheco, S. and Lourenço, S.O (2013). Gross chemical profile and calculation of nitrogen-to-protein conversion factors for nine species of fishes from coastal waters of Brazil. Lat. A m. J. Aquat. Res., 41(2): 254-264.

[16]. Jones, B.D. (1931). Factors for converting percentage of nitrogen in foods and feeds into percentages of proteins. U.S. Dept. of Agr. Circ. 183. Washington,D.C.

[17]. Jones, Jr., B.J. (1991). Kjeldahl method for nitrogen determination. Micro-Macro Publishing, Athens.

[18]. Khanizadeh, S., Buszard, D and Zarkadas, C.G (1989). Seasonal variation of proteins and amino acids in apple flower buds (Malus pumila Mill).J. Agr. Food Chem. 37:1246–1252

[19]. Sosulski, F.W. and Imafidon, G.I (1990). Amino acid composition and nitrogen-to-protein conversion factors for animal and plant foods. J. Agric. Food.Chem., 38: 1351-1356.

[20]. Yeoh, H. and Truong, V. D. (1996). Protein contents, amino acid compositions and nitrogen-to-protein conversion factors for cassava roots. J. Sci. Food Agric. 70: 51–54.

[21]. Tokoro, N., Sawada, M., Suganuma,Y., Mochizuki, M., Masuzawa, K., Aoyama, Y., and Ashida, K. (1987). Nitrogen composition of vegetables common to Japan. J Food Comp. Anal. 1(1):18-25.

[22]. Pirjo, P.S and Pekka E.K (1996). Determination of protein in foods: comparison of net protein and crude protein (N x 6.25) values. Food chemistry. 57(1): 27-31

[23]. Sriperm, N. Pesti, G.M and Tillman, P.B. (2011). Evaluation of the fixed nitrogen-to-protein (N:P) conversion factor (6.25) versus ingredient specific N:P conversion factors in feedstuffs. J. Sci Food Agric; 91(7):1182-6.

[24]. Yeoh, H.H. and Wee, Y.C (1994). Leaf protein contents and nitrogen-to-protein conversion factors for 90 plant species. Food Chem. 49: 245-250.