Molecular cooking: physical transformations in Chinese 'century' eggs

Over two thousand years ago the Chinese developed a method to preserve eggs such that they remain edible for many months. The room temperature, physico-chemical preservation process that is used to prepare 'century' eggs transforms the egg white into a yellow, transparent gel with optical and mechanical properties that are very different to those of the familiar white protein aggregate that forms upon boiling a raw egg. Here we show that boiled egg white gels can be further transformed into an elastic and transparent gel using the Chinese preservation method. We demonstrate that the resulting protein gel is made of fine-stranded globular assemblies of partially denatured protein, and resembles the aggregates formed by colloidal particles interacting through long-range electrostatic repulsion combined with short-range attraction. These gels are not only highly deformable but are also very stable, maintaining their structure even when boiled. We suggest that the mechanism responsible for gelation in century eggs illustrates a non-specific aggregation pathway available to globular proteins.