Skip to main content

Among the many sub-disciplines of physics are quantum mechanics, cosmology and the physics of fruit.

It’s not a field that universities or learned societies recognize, but it fits the investigation of how reservoirs in the skin of citrus fruit burst and shoot out micro-jets of aromatic oil at more than 30 feet per second.

Andrew Dickerson and his colleagues at the University of Central Florida in Orlando investigated the phenomenon purely out of curiosity. Anyone who has handled a lemon or a navel orange may have noticed that when the skin is bent, a little bit of oil comes out in a tiny spritz. These are to be distinguished from the wayward squirts of juice that can hit a dinner companion when you are trying to add a dash of lemon to your sole.

The scientists used high-speed videography to track how the process works and found sacs of oil in the relatively soft part of the skin just below the more rigid outer layer. They used pliers to bend skin of several citrus fruits and found that at a certain point, the stress on the skin causes a break and the oil reservoir empties in a burst.

Micro-jets are found in other plants and in animals as well, such as spitting termites and spiders. Why citrus plants show this action when the skin is bent in an extreme way isn’t known, although the oils are toxic to some insects, plants, and microbes. The micro-jet phenomenon is the reason that bartenders twist an orange or lemon peel to release the flavor, but that is not likely to have played a part in their evolution.

The experiments, reported in the Proceedings of the National Academy of Sciences, may be helpful in fields like medicine or commercial printing or even in devising monitors for stress in bridges. For instance, a micro-jet sensor might indicate when too much stress is put on part of a bridge, bursting and spraying a dye.

That’s speculation. Dickerson said that he had undertaken the investigation because of conversations eight years ago with David Hu of Georgia Tech when Dickerson was an undergraduate working in Hu’s lab.

And one of the things he takes away from the findings, after viewing the high-speed videos, is that everyday life is filled with hidden marvels, “ubiquitous examples of natural beauty.”

“How many times have you peeled an orange?” he asked. “How many other things in life pass us by and we don’t appreciate them?”