Dry heating of egg white proteins: how a multiscale approach may help to predict foaming properties from 2D interface measurements

Dry heating is performed in egg product industries to pasteurise egg white powder. This treatment (55 to 80°Cduring a few days) is also used to improve egg white powder functional properties among other foamingproperties. Several studies have shown this foaming properties‟ improvement with dry heating length (Kato etal, 1989; Baron et al, 2003; Van der Plancken et al, 2007; Talansier et al, 2009) that Kato et al (1989) attributedto protein surface hydrophobicity increase. However, during these treatments, soluble and insoluble covalentaggregates were also generated (Kato, 1989; Van der Plancken et al, 2007; Talansier et al, 2009) that may beinvolved in foaming properties improveme nt. Conclusions from such a complex protein solution as egg whiteare difficult to draw; this is the reason why we choose ovalbumin, the major egg white protein (54% of totalprotein amount) and lysozyme (one of the most famous model protein) to identify the molecular speciesgenerated by dry heating that are responsible for foaming properties‟ improvement. Ovalbumin and lysozymefoaming properties are improved after dry heating (Kato, 1990a) and the protein undergoes some mildconformational changes close to the molten globule state as well as aggregation driven by hydrophobicinteractions and disulfide bonding (Kato, 1990b; Matsudomi, 2001). The present study has been performed toidentify the molecular species generated by dry heating responsible for foam ing properties improvement. Mostof the data of the literature were confirmed as we found that ovalbumin and lysozyme aggregated and thatdeamidation occurred. We also identified less negatively charged ovalbumin that we attributed todephosphorylation. We performed surface pressure and ellipsometric angle measurement on dry heatedovalbumin but also on dephosphorylated, desamidated and aggregated forms. Dry heated ovalbumin andlysozyme show faster adsorption kinetics to air water interface than non-heated one. However the equilibriumsurface pressure and surface concentration are quite close for ovalbumin whereas no equilibrium were reachedfor lysozyme. Shear elastic constant measurement showed higher values for dry heated ovalbumin during thefirst hour but no significant difference after 8 hours. Measurements of dilatational and shear modulus were alsoperformed to complete the data. Foaming properties of the different molecular species were measured whenpossible. This multidimensional approach helped in the understanding of the characteristic of the interfacial filmthat explain foaming properties. It seems that more than the values of surface pressure, ellipsometric angle orcomplex modulus at the equilibrium, it is the evolution of these values in the first few stage that is important topredict foaming properties