Amino Acid Composition of Oilseed Meals and Protein Isolates

Abstract

Amino acid analyses of oilseed meals indicated that varietal differences in amino acid composition were much greater in soybean, turnip rapeseed, rapeseed and sunflower than in safflower and flax. In general, soybean and rapeseed proteins contained high proportions of essential amino acids required for human nutrition such as leucine, lysine and threonine while flax, sunflower and safflower proteins contained more of the nonessential arginine, aspartic acid and glutamic acid. Rapeseed proteins were also rich in methionine + cystine and proline while soybean had a higher level of phenylalanine. Because of serious deficiencies in lysine, the oilseed meals and protein isolates of flax, sunflower and safflower rated poorly in essential amino acid indices and protein scores. Higher ratings were obtained for soybean and rapeseed meals and isolates but soybean was deficient in sulfur-containing amino acids while the rapeseed species were low in isoleucine. Since the latter amino acid is rarely deficient in mixed diets, the rapeseed products appeared to have the best protein for the supplementation of human diets.The nitrogen-to-protein conversion factors were calculated from the amino acid compositions of the oilseed meals and protein isolates. The values for the present oilseed proteins varied between 5.33 and 5.74, indicating that the standard factor of 6.25 caused a substantial overestimation in crude protein content. A common factor of 5.50 is proposed for oilseed meals and, based on literature values, for all seed proteins.