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?
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.