PROTEIN AND NONPROTEIN NITROGEN CONTENTS OF SOME OILSEEDS AND PEAS

Abstract

Nonprotein nitrogen (NPN) was extracted from seven species of oilseeds and three cultivars of peas (Pisum sativum L.) by three methods. Method 1 was extraction of meal nitrogen with dilute sodium hydroxide and removal of alkali-soluble proteins by trichloroacetic acid (TCA) precipitation. Methods 2 and 3 were extractions of meal nitrogen with 70% ethanol and 1% TCA, respectively. The three solvents extracted vastly different quantities of nitrogen from the meals. Method 3 gave the highest values for NPN followed by methods 1 and 2 in that order. The nitrogen extracted by ethanol was probably the true NPN content of the meals because of the lack of solubility of oilseed and pea proteins in this solvent. The oilseed meals contained more amide nitrogen than the peas. None of the meals contained any significant quantities of nitrate nitrogen. Amino acid analysis of NPN fractions of meals obtained by method 1 showed the oilseed meals and peas to contain, in free state, all the protein amino acids except cystine or an appreciable amount of methionine. The NPN fractions of the meals contained, except in safflower (Carthamus tinctorius L.), high quantities of ammonia, glutamic, and aspartic acids. Safflower NPN fraction contained, in addition to ammonia, more proline and alanine than glutamic and aspartic acids. Mustard (Brassica juncea Coss.) and pea NPN fractions also contained high concentrations of arginine. The other protein amino acids were present in trace or relatively small concentrations. The major conclusion drawn from the data was that the NPN of the seed species used in the study was highly variable and depended on the method and solvent of extraction.