Dear Kitchen,
What makes pesto turn brown? This weekend I bought a ton of basil from the greenmarket and made a nice green pesto, but within a minute of being exposed to the air it turned a deep unappetizing shade of brown. What did I do wrong? I used basil, olive oil, walnuts & pine nuts, and garlic. (I left out the parma). This happens when I make guacamole as well. Is something wrong with my water?
Thanks,
M
Dear M,
There is nothing wrong with your water. The reason pesto (and guacamole) turns brown is because of a chemical reaction the cut basil has from being exposed to air. The reaction is known as oxidation.
To prevent oxidation, I blanch the basil leaves for a few seconds in boiling water until they turn bright green. Remove the basil from the boiling water, shock it in ice water and pat completely dry on a tea towel, then proceed with your pesto recipe. Acid of some sort can also help. Try squeezing in a bit of lemon juice.
To prevent the pesto from changing color during storage, place the pesto in a container and pour a thin layer of oil over the top of the pesto. This prevents the air from getting to the pesto. Although I've never tried it, another way keep the pesto bright green is to add one-eighth teaspoon of vitamin C powder.
© 2009 Apartment Therapy
You can also add flat-leaf parsley to your pesto for a nice color boost.
I agree with Grant: I ALWAYS put fresh lemon juice in my pesto, and the color keeps. Tastes good too.
same thing with guac - lots of lime tastes good - how can you be making guac without it?
also, for guac, put the avacodo pit in the same bowl as the guac, cover it, and store before serving in the fridge - has never turned brown on me!
Of course, the lime tastes better.
Even better than the old pit-in-the-guac trick is lightly pressing a piece of plastic wrap onto the guac itself. Less air = less oxygen = less oxidation. Plus no slippery pit to fish out.
I also agree about lashings of lime juice!
Is this an issue just of aesthetics? Does the brown do any harm?
I recently read on cooksillustrated.com (or maybe their email newsletter) that if you blanche the basil before you make the pesto, it keeps the green color...
To repeat Grant's advice: This sort of browning is a caused by enzyme in the food that reacts when the tissues are damaged (cut or bruised). It's the same enzyme that causes apples, peaches, mushrooms, bananas, potatoes, etc. to discolor.
Chilling the food below 40 degrees slows the process, as does acid (vitamin C powder is ascorbic acid), salt, sulfur (often used on dried fruit) and limiting the enzyme's contact with oxygen (by covering w/ plastic or immersing in water). Blanching/cooking will destroy the enzyme and prevent the problem. Pick your solution(s) based on what's appropriate for your dish.
Eliminating the parmesan cheese probably cut down on the salt, which may have made your pesto turn brown quicker.