Avocados are semi-tropical, chilling-sensitive fruits that are harvested hard and ripen off the tree. Some varieties are soft and rich in oil, others firm and lean, better for slicing than mashing. Choose avocados that fill their skin and are either firm and unripe or give slightly to pressure, in the early stages of ripening. Avoid over-ripe soft avocados with loose areas of skin. Allow the avocados to ripen fully at room temperature before refrigerating. To speed up the ripening process, put an avocado in a paper bag with a ripe banana. To prevent avocado purées from turning brown press aluminium foil on the surface and cover well. Scrape any discoloured patches from the surface before using.