When do we call a fruit ‘rotten’?

Answer

In itself this is not a simple question and there is no clear definition. Rot is a form of decay and it can be physiological or microbial. With a physiological cause, internal or external symptoms will always be visible. This is therefore a more clear form. Most likely you had the microbial spoilage in mind.

For a number of prepared foods there is a standard for how many micro-organisms may be contained, but this does not exist for unprocessed products, only that no external symptoms may be visible. But the spoilage obviously occurs earlier than when visible symptoms are apparent. As long as the structure, appearance and taste of a fruit are normal, we cannot speak of spoilage, although micro-organisms may already be present. If there is an influence on structure, texture or taste, we can speak of rot.