How can you keep it from turning brown?

When I make pesto, I put washed basil leaves, olive oil, asiago cheese or parmesan, pine nuts or walnuts, and sometimes I put salt and/or pepper into a small food processor and turn the processor on until the basil is all ground up. It always turns brown on the surface (top) of the bowl in a short time. I want it to stay green. I tried a little lemon and that didn't help.