New optical technique uses light to reveal internal contents of chicken eggs

Scientists have demonstrated a non-invasive technique that uses light to reveal the hidden contents of chicken eggs, potentially helping to curb the meat industry's practice of killing billions of male chicks at birth. The study, publishing June 19 in the Cell Press journal Newton, found that when light enters an intact bird eggshell, it bounces back and forth many times, with photons traveling as far as two meters within a chicken egg's tiny, four-centimeter interior. By illuminating the optics of chicken eggs, researchers may be able to determine the sex of a developing embryo within, assess the quality of an egg's interior, and determine whether intact eggs have been fertilized. 

"This previously unobserved phenomenon might help to investigate the content of the egg non-invasively even during the embryo development in fertilized and incubated eggs, addressing the ethical dilemma of culling male chicks" says Lennard van den Tweel, a researcher with the hatchery technology company HatchTech B.V. in the Netherlands and an author of the study. 

Since male chicks cannot lay eggs and don't grow fast enough to be profitable, over 300 million are slaughtered shortly after birth each year in European hatcheries alone. The practice is widely opposed by animal rights advocates, with many countries pushing for laws to end the practice. 

Optical spectroscopy techniques that non-invasively reveal a specimen's physical, chemical, or structural properties using light have traditionally been used in medicine, including for mammography and to monitor the function of the lungs and brain, as well as to address agricultural issues, including measuring fruit and wood, and environmental applications such as snow monitoring. 

While such light-driven techniques have also been used to study egg properties, "the complex way light migrates through the egg is largely ignored" says van den Tweel. 

To investigate chicken eggs' undiscovered optical properties, the researchers measured the time it takes for light particles, or photons, in the visible and near-infrared wavelengths to travel specific distances as they migrate through whole eggs, studying the role of the eggshell as an "integrating sphere" that traps photons within the enclosed space. The team looked at the way photons behave as light passes through unbroken eggshells both before incubation and during eight days of embryonic development. 

We were impressed by how efficiently the eggshell traps photons. Two meters is a very long pathlength, not easily matched by other natural materials. The finding of the integrating sphere effect immediately gave sense to several studies showing temporal emission tails that were initially attributed to fluorescence." 

Vamshi Damagatla, study author and post-doctoral researcher at Politecnico di Milano

However, the "complex and unusual" optical properties of chicken eggs will require further research using other approaches in order for the researchers to fully "disentangle" their data and improve the sensitivity of the technique developed in this study, says van den Tweel. 

The researchers also plan to study how the photon effects they observed change throughout a chicken embryo's developmental process and the optical properties of each individual component of the egg. 

"The highly scattering nature of the avian eggshell might have evolved to protect the embryo from ultraviolet light or reduce heat dissipation when parents are out foraging," says Damagatla.