Anyone who’s ever purchased an avocado will testify that, after taking several days to reach the point of perfect ripeness, they remain at that point for an incredibly short amount of time before morphing into a brown, sludgy mess. As if to confound this problem, if you do catch them at the optimum ripeness, they turn brown incredibly quickly after being cut open if not eaten straight away. As always, there are chemical processes at work that are to blame for this occurrence.
The flesh of avocados is made up of mainly fatty acids, such as oleic acid and linoleic acid. They contain very little sugar or starch. Avocados don’t start to ripen until they are picked from the tree, and if you put them into the fridge whilst still unripe, it can prevent them from ripening at all. Putting into the fridge once they have reached the point of ripeness, however, can prolong the time at which they stay at this point for several days.
The rapid browning of avocado flesh is a consequence of its exposure to oxygen in the air, as well as the presence of phenolic compounds in the avocado itself. In the presence of oxygen, an enzyme avocados contain, called polyphenol oxidase, aids the conversion of phenolic compounds to another class of compounds, quinones. Quinones are capable of polymerising, taking the smaller molecules and joining them together to form a long chain, to produce polymers called polyphenols. This polymerisation manifests itself as a brown colouration to the flesh. The browning doesn’t happen in the intact avocado, not only because the flesh isn’t exposed to oxygen, but because the phenolic compounds are stored in the vacuole of the plant cells, whilst the enzymes are found in the surrounding cytoplasm. So, both damage to these cell structures and exposure to oxygen is required for browning to occur.
This browning isn’t unique to avocados – the browning of many other fruits, such as apples, is also a consequence of this reaction. For the fruit, it’s not a purely aesthetic process. Quinones are compounds that are toxic to bacteria, so their creation from phenolic compounds serves a practical purpose for the fruit by enabling it to last a little longer after exposure to oxygen before beginning to rot.
Browning of avocados can be prevented in several ways. One of the most effective is to rub lemon juice on the exposed flesh of the fruit. The enzymes which enable the enzymatic browning reactions to occur are sensitive to acidic conditions, and work much slower in them. Another option is covering the avocado flesh tightly in cling-film. This prevents oxygen from reaching the flesh, and thus browning cannot take place. Chilling the avocado in the fridge can also slow down the enzymes to an extent, as their activity is lower at lower temperatures. The commonly touted method of leaving the seed pit in the avocado to prevent browning does work – but only on the part of the avocado that it’s shielding from oxygen. Exposed areas of the flesh will still turn brown in time.
One final fact about avocados that I feel compelled to include here doesn’t actually have anything to do with chemistry, but with the fruit’s name. The Aztecs named the trees it grew on ‘āhuacacuahuitl’. The Aztec word for avocado was also used euphemistically for ‘testicle’ – so one reading of the tree’s name is ‘testicle tree’. Arguably, not the absolutely linguistically correct reading, but it’s a great fact to unsettle your fellow diners with.
References & Further Reading