Apples/bananas release ethylene gas that hastens the maturing process of other fruits. These fruits, bananas especially, give off more ethylene than others, which will help ripen (or at least soften) any late bloomers.
If you don’t have any paper bags handy, use rice. Rice also traps ethylene effectively — in Indian households, mangoes are often submerged in a container of rice to speed up their ripening.
