Avocado: Avocados turning brown
Avocados (and many other fruits) turn brown when exposed to air due to the action of an enzyme. To inhibit the browning, you must either minimize the exposure to air or slow the action of the enzyme. To do the former, cover the avocado with plastic wrap, making sure to press the plastic tightly against the surface of the avocado, or spray the avocado lightly with oil or cooking spray. Citrus affects the enzyme responsible for browning, which is why most guacamole recipes include lime juice, so rubbing extra avocado with lemon or lime juice will also work. Apparently, keeping the pit in guacamole only works because it keeps air away from the part of the guacamole touching the pit, and only keeps that part of it from browning.