Picking out ripe fruit is an art form, and one which you can perfect with practice. Once you've tasted perfectly ripe fruit, you can understand why seeking it out is worth the effort. The rich flavor, superior texture, and intoxicating aroma when it is perfectly ripe can elevate a meal from ordinary to delicious, whether it is being eaten plain, used in a salad, or incorporated into a dessert. Depending on the fruit, there are a few general rules to help pick a ripe piece, but if you run into trouble, talk with your produce seller to get recommendations.
Fruit undergoes a series of changes as it ripens which have a profound impact on the flavor. If allowed to ripen all the way, fruit will grow sweet, losing acidity, and it will also get juicy, deepen in color, and start to emit a rich aroma: this is why it is important to examine it in room temperature conditions. Chilled produce prevents you from smelling the fruit for ripeness. In general, you should look for pieces that are even in color and texture and have a delicious odor. The color should be bright and full, with no hints of green or white unless the fruit naturally has these colors. With varieties like melons, a discolored area indicates where it rested on the ground, which also happens to be the sweetest and most delicious part of the fruit.
Fruit can be divided into two basic categories. Some types are able to ripen off the parent plant, especially when stored with bananas, which emit ripening agents. Other types cannot ripen off the parent plant, meaning that they must be vine or tree ripened. These fruits tend to be much more costly, as transporting delicate ripe pieces without damaging them is difficult.
Berries, citrus, cherries, dates, and grapes are all considered non-climacteric fruits. This means that once they are picked, they will not ripen any further, because the parent tree provides the ripening agents and sugars that sweeten them. When selecting these types in the market, look for plump, juicy, well scented specimens, because what you see is what you will get. Store them with care, and plan on eating them within a few days.
Climacteric fruits will ripen off the parent plant, and in the case of the avocado, they must be picked in order to ripen. Peaches, bananas, apples, melons, plums, persimmons, and tropical fruits will all ripen off the vine. Tropical species like mangoes, papayas, cherimoyas, and kiwis will also grow sweeter off the plant, but be aware that the aroma may never fully develop, because the scent compounds come from the plant it grows on. These fruits can be picked with less care than non-climacteric fruits, although you should select mature, evenly textured, firm specimens. They are ripe when the color is even, and the texture is yielding but not mushy. In the case of melons, look for those that are firm but not hard, with a rich aroma.
In the case of the avocado, buying green fruits is actually advised, because you can store them under refrigeration and ripen them as needed. Set an avocado out at room temperature to ripen, watching the color change from green to deep black. It is ripe when the knobbly skin is black and the flesh is slightly yielding to the touch.