I have been making Pesto for years. It is one of the delights of summer for me, my family and friends. It browns a bit on top of the container, and I usually put a thin layer of olive oil on top and it is fine.
This batch is turning brown more quickly, even with the olive oil, and what is worse, when it is mixed with hot pasta, it turns a muddy brown from the heat. It tastes good, but I have never had it be so brown before. I haven't changed my recipe- except that I used fresh garlic from the farmer's market instead of the usual garlic bulb. It made me think of when i have pesto in a restaurant- it isn't brown - is some kind of antioxidant added - Vitamin C powder or something like that? I would love to know how food service people keep it green - although I won't use BHA and such preservatives.