The two basic varieties of avocados available in the United States are Hass and Fuerte--although many other varieties are available worldwide. Regardless of the type of avocado you purchase (or pick), the ripening process is the same. Avocados do not ripen on the tree. All avocados are picked hard and allowed to ripen after harvest. Most stores sell hard avocados, expecting them to ripen at the customer's home, although a few stores sell ripe avocados off the store shelves. Avocados are ripe when the skin turns a deeper shade of green and the fruit is slightly soft to the touch.
Buy or pick large, unblemished avocados with no dark or soft spot. Shake the avocado near your ear and listen for a rattle, which would indicate that the pit (seed) has come loose and is rattling around inside of the fruit. Never buy an avocado with a loose seed, as this indicates an overripe fruit.
Place your unripe avocados (only as many as you wish to currently ripen) in a brown paper bag with a yellow banana. Close the top of the bag and set it on top of a refrigerator or in a cupboard. The bag will hold in the ethylene gas that avocados release as they ripen. The yellow banana will release additional ethylene gas, which will shorten the ripening period for your avocados.
Open the bag and check to see if your avocados have ripened after three days inside the paper bag. Look for a darkening of the skin and gently squeeze the fruit to see if it has softened. If not ripe, return the avocados and the banana to the paper bag, seal the top and allow the avocados to ripen for two more days.
Open the bag and check the avocados two days after the last inspection (five days total after being placed in the bag). If still not ripe, return to the bag with a ripening banana and seal the bag. Allow the avocados to ripen for another two days. Continue this process until your avocados are ripe. Your avocados should ripen within three to seven days.