Tara C. McIntosh

Polypeptide profile and functional properties of defatted meals and protein isolates of canola seeds

Laboratory-defatted meals from four types of canola seeds were analysed for protein profile by reducing and non-reducing forms of sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). In the absence of a reducing agent (2-mercaptoethanol), four major polypeptide bands (16, 18, 30 and 53 kDa) were prominent in similar ratios in all varieties. In the presence of mercaptoethanol, significant reductions in intensity of the major bands occurred, suggesting that the major polypeptides contained smaller units which were held together by disulphide bonds. Meals from Brassica napus seeds had higher protein solubilities than meals from Brassica rapa seeds. Meals with higher protein solubility values also had higher foaming capacity (FC) values. Generally, the acid-precipitated (pH 4.0) protein isolates (APPIs) had higher FC values than the calcium-precipitated isolates (CPPIs). On the other hand, the CPPIs formed emulsions with higher values of emulsifying activity index (EAI) when compared to the APPIs. The results indicate that variations in functional properties of protein isolates and meals between the four seed types were probably due to differences in protein conformation in aqueous solutions rather than differences in polypeptide composition. © 2001 Society of Chemical Industry

Comparative study of the emulsifying and foaming properties of defatted coriander (Coriandrum sativum) seed flour and protein concentrate

Abstract Emulsifying and foaming properties were determined for coriander protein products (defatted flour and protein concentrate) at pH 4.0, 7.0 and 9.0 and the results compared with those obtained for defatted soybean flour. Mean oil droplet size and interfacial protein concentration was smallest for emulsions (∼17% oil, v/v) stabilized by the coriander protein concentrate, when compared to the coriander and soybean flours. Polypeptide composition of the interfacial protein membrane of the emulsions was different from the polypeptide composition present in the respective coriander flour and protein concentrate. In contrast, soybean flour-stabilized emulsions contained similar polypeptide composition to that of the flour. Soybean flour formed the greatest amount of foams at pH 4.0, 7.0 and 9.0 followed by the coriander flour, which had greater amounts of foam at pH 4.0 and 5.0. The foam stability of both the coriander flour and protein concentrate were significantly (P⩽0.05) less than those of the soybean flour. It was concluded that the reduced level of non-protein components in the coriander protein concentrate favoured increased surface activity at the oil–water interface but not at the air–water interface.

Structural Properties of Cruciferin and Napin of Brassica napus (Canola) Show Distinct Responses to Changes in pH and Temperature

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.

Structural and Physicochemical Property Relationships of Cruciferin Homohexamers

Heteromeric cruciferin from wild type (WT) Arabidopsis thaliana and homomeric cruciferin CRUA, CRUB, and CRUC composed of identical subunits obtained from double-knockout mutant lines were investigated for their structural and physicochemical properties. A three-step chromatographic procedure allowed isolation of intact cruciferin hexamers with high purity (>95%). FT-IR and CD analysis of protein secondary structure composition revealed that all cruciferins were folded into higher order structures consisting of 44-50% β-sheets and 7-9% α-helices. The structural and physicochemical properties of homohexameric CRUC deviated from that of CRUA and CRUB and exhibited a compact, thermostable, and less hydrophobic structure, confirming the predictions made using 3D homology structure models.