Keeping from browning can feel like a losing game, but one simple trick can keep them fresh. Avocados turn brown when peeled or cut because of a process called oxidation. This is completely safe to eat, but it can affect their appearance.

In 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 using a simple method. Cover them with a squeeze of , wrap it in cling film and store in the fridge in an airtight container. 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.

Storing a cut avocado with a cut onion will also prevent browning. The reason the onion is able to keep the avocado fresh is that it releases sulfenic acid, 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.