Proteins contribute to food structure as three-dimensional networks in gels and solids but also as two-dimensional networks at interfaces of foams and emulsions. Interfacial properties of proteins have been a subject of considerable interest as very different films can be formed depending on the interactions and assembly of the protein molecules at the interface. Proteins with specific function on surfaces in nature provide unique properties, such as the extreme elasticity of hydrophobin films at air-water interface. In an attempt to improve the stability of food foams and emulsions, enzymatic means of tailoring interfacial properties of conventional food proteins have been sought for over a decade. While controlled hydrolysis can be used to improve foaming and emulsifying properties, increased interfacial elasticity and stability have been the aim using of cross-linking enzymes.
To gain more fundamental understanding of changes in the interfacial layer resulting from crosslinking, the mechanism of transglutaminase-induced cross-linking of interfacial beta-casein layer was investigated in tetradecane/buffer system.
The impact of enzymatic cross-linking on the properties of foams and emulsions suggest application potential in the past literature, even if the effect depends strongly on conditions of preparation, either prior to or after crosslinking as well as on the extent of reaction. Improved foam stability has been found in case of active enzyme present at the time of foaming. For emulsions, improved physical stability against Ostwald ripening and improved stability against oxidation have been reported. In addition, digestibility of protein has been shown to be altered by cross-linking.
In the current study, monolayer studies were carried out in a Langmuir trough, where incubation with the enzyme mostly affected the compression of the film through adsorption of transglutaminase to the interface. Interfacial shear rheology was used to follow the kinetics of formation of a visco-elastic film upon cross-linking. Substrate concentration affected the rate of the interfacial crosslinking, when enzyme was dosed per protein concentration. This was most likely due to the saturated substrate layer at the interface in all cases. Most of the beta-casein at the interface was not cross-linked by intermolecular links, but rather, intramolecular links were formed. Finally, studies of adsorbed beta-casein layers on polystyrene beads revealed that cross-linking reduced the thickness of the adsorption layer from 11-12 nm to 8-9 nm. These results suggest that it may be mainly intra-molecular cross-linking which modifies the physical interactions of beta-caseins at the interface resulting in a higher layer density and thus, formation of a visco-elastic network.
These observations may be important in consideration of mass transfer properties across the interface which affect not only physical but also oxidative stability of emulsions, and are crucial in development of emulsion-based encapsulation systems. The work was carried out with financial support from the Academy of Finland in Project in the postdoctoral researcher's project "Novel Protein-Based Emulsions by Engineering Interfacial Mass Transfer" by Riitta Partanen.
Partanen R, Forssell P, Mackie A, Blomberg E. Interfacial cross-linking of β-casein changes the structure of the adsorbed layer. Food Hydrocolloids 2013.
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