Mitch offers some excellent advice:
A thought about juice oxidizing:
Lime and Lemon are natural antioxidants. Anything that you squeeze lime or lemon on oxidizes much more slowly. If your guacamole doesn’t have lime, it will turn brown quickly. With generous amounts of lime it will stay a vibrant green for days in the fridge.
The same goes for fruit salad. Chop up a banana or an apple and within minutes you’ll see them turning brown. Squeeze lime over it, and they stay fresh looking.
So by putting lime or lemon into your juice that you plan to drink later, you are definitely helping to protect against oxidation. The more the better, make sure it’s a quantity that you can really taste. Exactly how much this helps keep your juice unoxidized, a scientist would have to tell you.