Kind of hard to get into the details as an ELI5, but simply some plants ripen on the plant and some dont. It comes down to the production of ethylene gas in something called the climateric stage of ripening. Avocados will continue to produce this gas when plucked, which triggers avocado cells to continue respiration at a higher rate, which makes them continue ripening (also why closing them in a bag helps speed that up).
Grapes on the other hand, are preclimateric. Once you pluck them they will not produce ethylene, and not have this faster ripening effect (for the most part. Grapes and melons have exceptions to this rule)
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