Among the commercially cultivated Brassicaceae (Cruciferae) plants, Brassica juncea, Brassica napus, Brassica rapa, and Sinapis alba store significant amounts of oil and protein in the seed. At present, Brassica seed proteins are primarily used for livestock feeding based on the nutritional value. The point of curiosity is whether the present knowledge on the protein structure, biochemical characteristics, nutritive value, and the recovery processes are inadequate to develop Brassica proteins into a usable plant protein source or these proteins are of substandard for uses beyond animal nutrition applications. Cruciferin (11S) and napin (2S) are the predominant storage proteins of Brassicaceae seeds that contribute to different properties and functions. A gamut of information is available on the chemistry, nutritional value, as well as the functionality in foods, and associated non-protein components of canola/rapeseed storage proteins. The intention of this article is to critically review what is known about the predominant storage proteins of commercially produced Brassicaceae seeds relative to the above aspects and identify the knowledge gaps.

OCL, Oilseeds and fats, Crops and Lipids

The two major storage proteins identified in Brassica napus (canola) were isolated and studied for their molecular composition, structural characteristics and the responses of structural features to the changes in pH and temperature. Cruciferin, a complex of six monomers, has a predominantly β-sheet-containing secondary structure. This protein showed low pH unstable tertiary structure, and distinctly different solubility behaviour with pH when intact in the seed cellular matrix. Cruciferin structure unfolds at pH 3 even at ambient temperature. Temperature-induced structure unfolding was observed above the maximum denaturation temperature of cruciferin. Napin was soluble in a wider pH range than cruciferin and has α-helices dominating secondary structure. Structural features of napin showed less sensitivity to the changes in medium pH and temperature. The surface hydrophobicity (S₀) and intrinsic fluorescence of tryptophan residue appear to be good indicators of cruciferin unfolding, however they were not the best to demonstrate structural changes of napin. These two storage proteins of B. napus have distinct molecular characteristics, therefore properties and functionalities they provide are contrasting rather than complementary.