Thermal treatment of hen egg white lysozyme: structures, interactions, functionalities.
Among food ingredients, egg white is widely used for its surface activity and texturing properties. Egg white proteins are responsible for the foaming ability, but the relationship between protein structure and activity is not fully elucidated. The aim of this study was to obtain further information about this relationship by combining methods that allow to describe phenomenon occurring at a molecular to a macroscopic scale.
Model protein hen egg white lysozyme (lysozyme hydrochloride pH 3,5) was used to determine the structural consequences of heating in the dry state (80°C). The resulting proteins were employed to study the functional properties of lysozyme (interfacial behavior and microsphere formation propensity).
1. Heating in the dry state leads to the formation of succinimide residues. Five aspartyle residues were identified to be modified. The consequences of this chemical degradation are higher surface hydrophobicity, higher surface net charge, and decreased conformational stability. No secondary or tertiary structure changes could be shown. Another species resulting from dry-heating and differing from native lysozyme could be identified. However, no structural modification could be evidenced by the methods used in this study.
2. All treated samples had a high surface activity and were able to immediately adsorb to and associate at the air/water interface.
3. Moreover, foaming properties of dry-heated lysozyme were highly enhanced. Only two days of heating in the dry state were sufficient for lysozyme to have the better foaming properties (higher density and foam stability).
Another part of the work was aimed at determining the physico-chemical conditions needed for oppositely charged globular proteins to self-assemble into microspheres. This propensity for aggregation could be a newly functionality for egg white proteins. This study demonstrates that spherical particle formation is driven by charge compensation but also by “size compensation”. The volume ratio between the basic partner and the acidic one dictates the pH value for the optimal formation of microspheres. Dry-heated lysozyme was shown to have a higher propensity than native lysozyme to associate with ovalbumin, the major egg white protein. The resulting particles could also be more stable because intermolecular disulfide bridges take place in the microspheres.