How to prepare fresh basil?

Spending my summers in the Hamptons, I grow lots of fresh basil in my yard. I love making my own pesto sauce but it usually turns brown eventually. What am I doing wrong?

Traditionally pesto is a specialty of Genoa in Italy, it is crushed in stone mortars. When using a food blender the metal blades and the oxygen in the air help oxidize the pesto which naturally turns it brown. The trick for great pesto is to combine 3 cloves of garlic, 1/4 cup of toasted pine nuts 7 packed cups of basil leaves and 2-2 1/2 cups of olive oil. Blend the ingredients in a food blender for no longer then 30 seconds then add 1/2 cup of grated Parmesan cheese and a half unflavored crushed pill of vitamin c (an antioxidant). Season with fresh ground pepper. I would suggest storing the pesto in small airtight containers in your freezer for the winter. Then you may take out small batches as needed.