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.