Canola proteins for human consumption: extraction, profile, and functional properties

Canola protein isolate has been suggested as an alternative to other proteins for human food use due to a balanced amino acid profile and potential functional properties such as emulsifying, foaming, and gelling abilities. This is, therefore, a review of the studies on the utilization of canola protein in human food, comprising the extraction processes for protein isolates and fractions, the molecular character of the extracted proteins, as well as their food functional properties. A majority of studies were based on proteins extracted from the meal using alkaline solution, presumably due to its high nitrogen yield, followed by those utilizing salt extraction combined with ultrafiltration. Characteristics of canola and its predecessor rapeseed protein fractions such as nitrogen yield, molecular weight profile, isoelectric point, solubility, and thermal properties have been reported and were found to be largely related to the extraction methods. However, very little research has been carried out on the hydrophobicity and structure profiles of the protein extracts that are highly relevant to a proper understanding of food functional properties. Alkaline extracts were generally not very suitable as functional ingredients and contradictory results about many of the measured properties of canola proteins, especially their emulsification tendencies, have also been documented. Further research into improved extraction methods is recommended, as is a more systematic approach to the measurement of desired food functional properties for valid comparison between studies.

Large scale purification of rapeseed proteins (Brassica napus L.)

Rapeseed (Brassica napus L.) cruciferin (12S globulin), napin (2S albumin) and lipid transfer proteins (LTP) were purified at a multi-g scale. The procedure developed was simple, rather fast and resolutive; it permitted the recovery of these proteins with a good yield, such as 40% for cruciferin and 18% for napin. Nanofiltration eliminated the major phenolic compounds. The remaining protein fraction was fractionated by cation exchange chromatography (CEC) on a streamline SP-XL column in alkaline conditions. The unbound neutral cruciferin was polished by size exclusion chromatography. The alkaline napin isoforms and LTP, adsorbed on the beads, were eluted as a whole fraction and further separated by an other CEC step at acidic pH. Napins were polished by hydrophobic interaction chromatography (HIC). The fractions were characterized by reverse phase HPLC, electrophoresis, N-terminal sequencing and mass spectrometry. All the fractions contained less than 5% of impurities.

Behaviour of a protein isolate from rapeseed (Brassica napus) and its main protein components — globulin and albumin — at air/solution and solid interfaces, and in emulsions

The behaviour of a rapeseed protein isolate (RI) and its main protein components - globulin (RG) and albumin (RA) - in adsorbed and spread monolayers, as well as in emulsions was investigated. Tensiometry, film-pressure area and Langmuir-Blodgett-techniques, and emulsion parameters were used to characterise the behaviour of the rapeseed proteins at various interfaces. The adsorption isotherms for albumin showed a plateau value for the surface pressure (Pi(e)) of 11.6 mN/m at the low critical association concentration (CAC) of 5.6x10(-8) g/ml. Both values were found to be distinctly higher for the globulin and the protein isolate. The isotherms of a mixture of RG and RA, which corresponds to the composition of RI, seems to be a superimposition of the isotherms of RA and RG. Contact angle measurements showed that all samples used were able to form LB-layers and to make hydrophilic glass surfaces more hydrophobic and vice versa. The changes in contact angle were more pronounced on hydrophobic glass surfaces. Monolayer and emulsion characteristics are dominated by the interfacial properties of albumin. The maximum film pressure reached by globulin was only about 8 mN/m. The globulin also possesses the lowest emulsifying activity. From the mean molecular area calculated for spread globulin, it is concluded that the globulin maintains its globular conformation at surfaces, which explains the low surface activity. Albumin and the protein isolate were highly surface active in monolayers and emulsion formation. The slightly different interfacial behaviour of the protein isolate compared with the corresponding mixture is probably due to additional effects of non-protein components and a partially denatured state of the protein.

Structural studies on native and chemically modified storage proteins from rapeseed (Brassica napus L.) and related plant proteins

Recent data on the structure and chemical modification of the two main storage proteins of rapeseed, the high-molecular mass 12 S globulin and the low-molecular mass 2 S protein (napin) are summarized and compared with those of related seed proteins. The 12 S globulin is built up of six subunits forming a quaternary structure which can be approximated by the model of a trigonal antiprism. The subunits, composed of a larger and a smaller polypeptide chain each, have a two-domain structure which is typical for all related plant proteins. These are characterized by a sedimentation coefficient of 11-13 S, a molecular mass of 300,000-360,000 g/mol and a high percentage of beta-sheet conformation. Increasing succinylation results in a step-by-step dissociation up to the subunits and to an unfolding of the latter at a critical level of modification amounting to 60-70%. These structural changes affect the functional properties remarkably. The napin fraction comprises a group of closely related and highly basic proteins with molecular masses of 12,000-14,000 g/mol, a high content of sulphur-containing amino acids and rich in helical conformation. They are built up of a larger and a smaller disulphide bridged polypeptide chain. Acylation does not abolish the secondary or tertiary structure which are stabilized by inter- and intrachain disulphide bonds. Acylation results, however, in a stabilization of the protein against heat-induced aggregation.

Binding of cellulose sulphates to the 12 S globulin and the low molecular mass basic protein fraction from rape seed in insoluble complexes

The binding of two differently substituted cellulose sulphates (CS) with DS 0.50 and 0.33 to two main rapeseed proteins, the high molecular mass neutral 12 S globulin and the low molecular mass basic protein fraction ("albumin") in insoluble complexes at pH less than Pi (protein) has been studied using turbidimetric titration and chemical analysis of the supernatant after coprecipitation. The binding of both types of CS to the globulin at pH 3.0-2.5 at the precipitation occurs at a substoichiometric CS-protein mass ratio. This result has been obtained both by turbidimetric titration and chemical analysis. Contrary to that, the CS binding to the albumin is substoichiometric according to the turbidimetric titration and stoichiometric according to the chemical analysis. The CS-protein mass ratio in the coprecipitates obtained of pH 2.5 and 3.0 is nearly independent on the CS concentration applied for the precipitation of the albumin. There is a typical dependence on the CS concentration, however, for the globulin at pH 3.0, which becomes less pronounced at pH 2.5. The CS-globulin complexes form sharp turbidimetric titration curves at pH less than Pi (pH 2.5-5.5), the maximum position of which shifts to lower pH with increasing percentage of CS. The analogous titration curves for the CS-albumin complexes are broader, owing to the heterogeneity of the albumin fraction. Both polyanions exert a solubilizing effect at a molar excess on both proteins. Regarding the weight part necessary for precipitation (W insol.), forming stoichiometric complexes (W stoich.) and solubilization (W crit.) of the proteins, the following range can be written: W insol. less than W stoich. less than W crit.