At the ACS conference in New Orleans Shirley O. Corriher and Sally Mitchell talked about using food and cooking as ways to teach students about chemistry in a more engaging and compelling way. Some examples that were given was making Kool Aid as an example of the importance of molarity, and to use the concept of baking bread as an introduction to kinetics and gas laws. However, one of the little things that caught my ear was when Shirley O. Corriher said you should never cook asparagus in lemon juice.
The lovely thing about science is you can perform an experiment to test if this advice is true, and so I did. Below is a picture of two groups of asparagus both of equal deliciousness before I broiled them in the oven. The asparagus was seasoned with olive oil, salt, and a pinch of sugar. I added fresh squeezed lemon juice to the group on the right.
After broiling for 10 minutes, I observed the following.
The asparagus dipped in lemon juice (right) was more yellow-brown, and it did not have that nice green healthy sheen to it anymore. Both groups still tasted good, but I would say the asparagus without lemon juice did taste slightly better. The reason for this is shown below. The citric acid in the lemon juice will presumably chelate the magnesium from chlorophyll and protonate those ringed amines. The product that results from this chemistry is phaeophytin, which is a yellow-brown compound.
This is likely to be seen with cooking any chlorophyll rich food with a citric acid fruit. If you still want to impart onto your veggies that lemon flavor it would be best to use lemon zest.