What you see: Yellowing arugula leaves
What it is: The leaves are breaking down chlorophyll as they age
Eat or toss? Eat! Nothing unsafe happening here, but once some of your arugula flashes the yellow card, it’s time to hurry up and eat the rest. As it loses green, it loses nutritional value too.
Your arugula breathes. Technically, respires, but, in any event, its cells take in oxygen and release carbon dioxide,* even as the leaves look to be lying perfectly still in their plastic clamshell container.
All plants respire, even after we harvest them, but leaves like arugula tend to do so at particularly fast rates. This is because the mission of a leaf is typically to grow fast and not concern itself with slowing down and storing things for later (unlike, say, a potato).
A fast-breathing arugula leaf ages much faster than the plodding potato. This is because with every “breath” the arugula is putting more of its internal plant machinery into motion and using more of its limited nutrient stash. And, since it’s now cut off from the rest of the plant, its stash can’t be replenished.
But, the arugula cells can look to some of their own internal components for additional nutrients. Nutrients like those contained in chlorophyll, the pigment that's central to photosynthesis and gives them their green color. So, the cells start to harvest what they can by breaking down the chlorophyll and when the chlorophyll is broken down, the leaf’s signature hue fades to yellow.
Thus we’re left with yellowing leaves. The process, called senescence, is what you see on fall trees—except that the leaf deliberately sends nutrients back into the rest of the tree before it naturally falls off.
“Senescence is self-programmed cell death,” said Jim Monaghan, Director of the Fresh Produce Research Centre at Harper Adams University. “It’s meant to break down and recirculate to the rest of the plant,” he said.
Yellowing indicates nutrient breakdown, which also means that there are fewer nutrients available to us leaf-eaters. But in terms of our human diets, the change indicated by a few yellowing leaves is pretty marginal and shouldn’t cause you to abandon your arugula. Instead, consider it a hint to hurry up and make some salad. And if you want to get all official about it, consider that agricultural standards set by the United Nations Economic Commission for Europe consider “slight defects in colouring” to be a nonissue.
To stave off yellowing, make sure to keep your arugula cool. Cooler temps slow down respiration and slower respiration means slower aging (sorry, however, to report that sleeping in a fridge does not slow the human aging process).
*Yes, takes in oxygen and releases carbon dioxide. That's called respiration. Photosynthesis, a different process, takes in carbon dioxide and releases oxygen. The Royal Society of Chemistry has more.
Jim Monaghan. Director of the Fresh Produce Research Centre. Harper Adams University. Newport, Shropshire. United Kingdom.
UNECE Standard FFV-58 concerning the marketing and commercial quality control of leafy vegetables. 2017 Edition. Working Party on Agricultural Quality Standards of the United Nations Economic Commission for Europe.
Quality and Post Harvest Physiology of Rocket Leaves. Fresh Produce. Global Science Books. Anastasios S. Siomos and Athanasios Koukounaras. University of Thessaloniki.
Quality Changes during Storage of Spinach and Lettuce Baby Leaf. Acta horticulturae. November 2010. A. Spinardi, G. Cocetta, V. Baldassarre, A. Ferrante and I. Mignani. Dipartimento di Produzione Vegetale, University of Milano
Colour changes of fresh-cut leafy vegetables during storage. Journal of Food, Agriculture & Environment Vol.2 (3&4) : 40-44. 2004. A. Ferrante1, L. Incrocci, R. Maggini, G. Serra and F. Tognoni. Dipartimento Produzione Vegetale, University of Milan.
Chemistry for Biologists. Gas exchange. Royal Society of Chemistry.