Storing avocados with one other food item will keep them ‘fresh for days’ and stop them from browning. Avocados turn brown when peeled or cut because of a process called oxidation. This is completely safe to eat but can affect their appearance.
In avocados and many other fruits, the enzyme polyphenol oxidase reacts with oxygen and phenolic compounds in the flesh, forming a brown pigment called melanin. But you can slow down this process by storing a cut avocado with one other food item – a cut onion. The reason the onion is able to keep the freshness of the avocado last is that the cut onion is releasing sulfenic acid as a volatile compound.
This compound, when in contact with moisture from the air or the avocado, will alter the avocado’s pH, removing the PPO from its effective pH range.
Plus, keeping the pair in a plastic bag or tupperware prevents oxygen from reaching the avocado, thereby reducing the enzymatic browning that may occur.
Sulfenic acid is the agent responsible for making you cry when cutting onions. It is a lachrymatory agent, a chemical that reacts upon contact with the eyes and triggers responses such as tearing and stinging.
Another, more well-known way to stop avocados from going brown when cut is simply to cover them a squeeze of lemon. The acidity stops the flesh from oxidising, which is what causes the color to change.
Lemon juice prevents oxidation because it contains citric acid, which acts as a natural antioxidant, reacting with oxygen in the air before the fruit’s natural enzymes can, thus preventing browning and discolouration.
Lime juice and cranberry juice are just as effective, as the acidic juice has to have a pH less than three.
