Limes preserve the green color because they contain vitamin C (ascorbic acid), a mild antioxidant. Most fruits and vegetables contain one or more major antioxidants. They use them as natural pesticides and preservatives for their little veggie bodies.
An antioxidant is exactly what it sounds like -- it is "anti-oxygen." Oxygen is dangerous stuff. It degrades anything it touches by "oxidizing" it. Rust is metal that has been touched by oxygen. In the same way, oxygen is harmful to the surface of the avocado and it turns brown when oxygen degrades it (also when enzymes work on it). So to stop the degradation and browning, we have two choices:
1. Keep the avocado away from oxygen. Put it under water or dip it in oil. Or very tight contact with plastic wrap.
2. Cover the avocado with an effective antioxidant. This has been my method of choice, and there are many cheap and effective antioxidants available. By the way, "antioxidant" is another word for "preservative." Food preservatives are simply antioxidants. They can be purified from plants (we already know they contain natural antioxidants, that's why they are so healthy), or they can be manufactured from other raw materials. The most effective antioxidants we know of today to protect fruit (avocados) from oxygen is sulfur, which is both an antioxidant and an antibacterial agent.
health supplements" and also feared as "artificial preservatives."
The best known preservative is sulfur. It is both antibacterial and an antioxidant. To preserve our avocado and keep the oxygen away from its precious green meats, we need sulfur! I have been using potassium metabisulfite for some time now and it works wonderfully well. It is potassium bound to sulfur, and when it touches water it releases sulfur into the water. Just put a little bit of the sulfur powder in water and wash the avocado with it. When you want to eat the avocado you can remove the sulfur by washing or you can simply eat it, as I do. If you are not allergic the sulfur is harmless. After the wash the avocado will stay green and fresh and protected from rotting and bacteria for over a week.