Italian prune plums are small and sweet and have a lower water content than most other plums. They have deep reddish-purple skin that darkens even further as they ripen. Their yellow flesh is denser and less juicy, which makes this type of plum perfect for drying into prunes, and also excellent for baking and preserving. Their flavor becomes more concentrated and they hold their shape better during baking.
When ripe, good-quality prune plums will be deep bluish-purple and have a natural, cloudy film on them, like blueberries. Choose fairly firm plums that yield to gentle pressure.
To accelerate the ripening process, store unripe plums in a paper bag with an apple or banana. Once ripe, keep the fruit in the refrigerator until you're ready to use it.