Browning is a natural process that occurs when the chopped basil is exposed to the air (oxidation). There are three ways to help prevent browning:   1. Limit the exposure to air by pressing plastic wrap directly on the surface of the pesto before sealing to store. 2. Pour a thin layer of olive oil on top of the pesto to cover its surface before sealing to store. 3. Blanch the basil leaves briefly before processing.  It locks in the bright green color of the leaves. To blanch the basil, dunk the basil leaves into boiling water for 5 to 10 seconds then submerge in ice water. Pat the leaves completely dry before making the pesto.   Note that you only need to use one of these tactics if you make the pesto in advance and store it separately before preparing the entire pasta dish!