Why fruits don’t get mouldy during the blooming and growing stages, but when ripening instead?

Question

By some reason Youtube decided to recommend me some mouldy videos, and one of those I saw a fruit getting ripened whilst fully exposed to the outside environment (e.g. left outside a fridge.)

After a while, the fruit rotted and mould started to form, and that made me wonder;

Why is mould visible only/mostly after rotting and why mould isn’t as predominant or even none existent when the fruit is still developing, even though the fruit is fully exposed to the outside world. Aren’t the conditions already optimal for mould to grow regardless?

In: Chemistry

Answer 1

The same reason your arm doesn’t get moldy and rot while it’s attached to your body, but it will if you sever it and leave it laying outside.

Fruit that has fallen or been picked from the plant no longer gets nutrients from the plant, so it essentially is dying or dead tissue (aka senescence).

Now, that said, fruits and vegetables fall into two categories: climacteric and non-climacteric. Climacteric fruit/veg (e.g. bananas, apples, tomatoes, etc.) will continue to ripen after being picked, and non-climacteric fruit/veg (oranges, strawberries, peppers) will not.

The main difference is ethylene production. Non-climacteric fruit ripen without ethylene, and thus need to reach maturity while still connected to the plant. On the other hand, climacteric fruit ripen due to ethylene which continues to be produced by climacteric fruit as part of senescence (post-picking) in sufficient amounts.

Fun fact: this is where the phrase “one bad apple spoils the barrel” comes from. If you put a ripe apple (which is producing ethylene) in a barrel with less ripe apples, the ethylene the ripening apple produces will speed the ripening of nearby apples and their ethylene from ripening will speed the ripenign of other nearby apples and so on. A bunch of ripening plants next to each other will create a kind of chain reaction of ripening that leads to the fruit reaching full ripeness (and eventually going bad) more quickly.

That’s also the reason people tell you to put green bananas in a bag to ripen them faster — makes it harder for the ethylene to escape into the air. More ethylene = faster ripening.