A group of researchers has used DESY’s brilliant X-ray source PETRA III to analyse the structural changes that take place in egg yolk when you cook it. The work shows how different constituents in the yolk of a chicken egg contribute to the development of the soft-grainy microstructure of egg yolk which one receives when it is heated. This may not only be of interest to the food industry but may also serve as a model for the physics behind biologically relevant non-equilibrium processes such as protein aggregation which are connected to a number of severe diseases. The team of researchers from University of Siegen, Tübingen University, DESY and TU Dortmund now report new insights into these processes in the journal Nature Communications.

Heated egg yolk is widely used in our kitchens, in food industry and in biotechnology. However, only little is known about the out-of-equilibrium processes occurring during thermal treatment of egg yolk’s contents leading to the well-known grainy gel microstructure. By using coherent synchrotron radiation at PETRA III, a group of researchers headed by Christian Gutt from University of Siegen and Frank Schreiber from Tübingen University investigated the functional contribution of egg yolk constituents’ proteins, fat molecules – so called LDLs which stands for low-density lipoproteins –, and yolk-granules to the development of the grainy-gel microstructure and microscopic dynamics during cooking.

Apart from its food, biological, and also therapeutic value the diversity of proteins and high concentration of LDLs make egg yolk also an ideal candidate for studying the physics behind biologically relevant non-equilibrium processes. Most often denaturation and aggregation of proteins and LDLs are undesirable in biological systems, especially in relation to the pathogenesis of several human diseases such as Alzheimer’s disease, Parkinson’s disease, atherosclerosis, and others. Furthermore, the instability and aggregation of protein drugs, caused by deviations from optimal conditions of temperature, pH, ionic strength, etc. are one of the main challenges the pharmaceutical industry faces.

“To get a better understanding of what happens inside biological systems, it is important to understand the nanoscopic non-equilibrium processes involved in protein aggregation on the relevant time and length scales,” explains Nimmi Das Anthuparambil, lead author of the study working in Gutt’s group and at PETRA III beamline P10 at DESY. The team employed a technique called X-ray photon correlation spectroscopy (XPCS) which makes use of X-ray image series of the protein denaturation and LDL aggregation processes. The scientists used the yolk of conventional chicken eggs which was filled into 1.5-millimetre capillaries for the experiments. During heating the egg yolk, they observed the samples at temperatures from 63 to 100 degrees Celsius over time intervals of up to 2 hours at a time resolution of down to 10 milliseconds. “We observe that the temporal evolution of the structure of the protein gel network, the aggregation of LDLs and the microscopic relaxation rates each follow particular scaling laws, which nicely fit onto master curves when scaling the waiting times during cooking with their respective characteristic time scales. This means that the mechanism which leads to the formation of the grainy gel-microstructure is identical irrespective of the cooking temperature, but the speed of these processes depends on the temperature,“ says Anthuparambil.

From their large data sets the team could construct even a generic time-temperature phase diagram for the out-of-equilibrium processes during egg yolk gelation. “This diagram is impressive as it nicely illustrates the coupling of the nanoscale processes that give rise to the gel structure in a wide range of time-temperature combinations,“ says Nafisa Begam (Tübingen University) from the researcher´s team.

“These experiments make ideal use of the coherent fraction of the X-ray radiation produced at PETRA III,” notes Fabian Westermeier, member of the P10 team at PETRA III. “However, this fraction is limited. The coherent part of the beam at DESY’s future project PETRA IV will be a hundred times larger, enabling up to 10000-times better time resolution for measuring and understanding these processes.”

Christian Gutt points out that the results of the studies financed by the federal ministry BMBF are based on a close collaboration between DESY and the University teams and explains: “We expect our results to be relevant beyond egg yolk and that similar scaling laws will accompany a large range of denaturation phenomena and nanoscale structure formation in biologically significant dense protein systems.”

Future research from this collaboration will explore the aggregation of egg yolk proteins caused by deviations from optimal conditions of salt and pH. This could provide insights on the suitable conditions for stabilizing the proteins, which has implications for storage conditions of future protein-based drugs.

Reference
Exploring non-equilibrium processes and spatio-temporal scaling laws in heated egg yolk using coherent X-rays: Nimmi Das Anthuparambil et al.; Nature Communications, 2023, DOI: 10.1038/s41467-023-41202-z