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The Science of Ripening

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By David Joachim and Andrew Schloss
From Fine Cooking #135, pp. 28-29

Any good cook will tell you that great food starts with great ingredients, and when it comes to fruit, this means choosing what’s perfectly ripe. Here’s a glimpse into how the ripening process works so that you can feel, smell, and taste your way to fruit that’s bursting with flavor.

Why is fruit so enticing?
Ideally, to get eaten! In fact, that’s its entire reason for being. To humans and other animals, a fruit is something delicious to eat, but to a plant, it’s a means of survival. Fruit protects and nourishes the plant’s seed; as the seed matures, the fruit surrounding it ripens, getting bigger, juicier, sweeter, and more colorful. These changes entice us (and other animals) to eat the fruit and, with any luck, drop the seed elsewhere, where it can grow into a new plant. Ripening their fruit is how plants have trained animals to help them reproduce and multiply.

Why does underripe fruit taste bad?
Early in the ripening process, the fruit synthesizes compounds such as alkaloids and tannins. These fight infections and cause the underripe fruit to taste bitter and astringent, warding off eager eaters who might take the fruit (and seed) before it’s ready.

What happens as fruit ripens?
As fruit continues to grow, its storage cells expand, engorging it with water, sugars, starches, organic acids, vitamins, and minerals, and its skin turns from green to other appealing colors, such as red, orange, or yellow. Starch and acid contents decrease while sugar content increases, and alkaloids and tannins disappear. Aromas develop as the acid and protein composition changes, and the fruit’s texture softens as the substances that hold up its cell walls begin to break down. All of these changes make the fruit ripe and ready to eat.

What causes these changes?
Enzymes–the catalysts of the microbiology world–are behind many of these changes. For example, color develops when membranes containing chlorophyll weaken, allowing enzymes to destroy the green pigment. This, in turn, allows other secondary pigments (the ripe fruit color) to shine through. Enzymes also convert starches into sugars.

Most of these changes are accelerated by ethylene, a hormone that develops in the fruit during ripening. How ethylene works isn’t fully understood, but it probably makes cell membranes more permeable, allowing the enzymes to do their jobs. One thing we do know is that once ethylene reaches a certain concentration in a fruit, the cells switch from expelling oxygen (a byproduct of photosynthesis) to expelling carbon dioxide. This switch in “breathing” means that a fruit is quickly approaching peak ripeness. (Of course, fruits don’t “breathe” with lungs like humans, but they do get the energy needed to ripen by absorbing oxygen and giving off carbon dioxide via a process known as cellular respiration.)

Does fruit continue to ripen after harvest?
It depends on the fruit. Some fruits do continue to ripen and develop flavor after harvest, including apricots, avocados, bananas, kiwis, mangos, pears, peaches, nectarines, persimmons, and tomatoes. These fruits are called climacteric because there’s a critical period in their development when they ripen through a burst of increased cellular respiration. During their final stages of maturity, these fruits also retain more ethylene. Climacteric fruit can both retain the ethylene it produces and absorb ethylene from its environment. Commercial produce companies use this to their advantage by picking many types of these fruits when they’re underripe to minimize damage during shipping, then gassing them with ethylene to help them ripen as they’re brought to market.

Non-climacteric fruits can ripen fully–and develop all of their sweetness–only on the plant. These fruits include berries, grapes, cherries, melons, pineapples, and citrus. They don’t become sweeter once picked because they don’t store their sugars as starches, so there are no starches to convert to sugars. Ideally, these fruits should be picked at the height of ripeness and enjoyed soon thereafter.

Can I speed ripening?
Yes, if it’s a climacteric fruit. To hasten ripening, store it in a warm place (70°F to 90°F) to increase its cellular respiration. You can also increase its intake of ethylene by loosely enclosing the fruit in a paper bag or perforated plastic bag along with an ethylene-rich fruit such as a ripe apple or banana. Together, these methods work well for ripening rock-hard avocados or peaches overnight.

How should I store fruit that’s already ripe?
Refrigerate it to slow down the fruit’s cellular respiration. You can also reduce the available oxygen and increase the available carbon dioxide by enclosing your fruit in a plastic bag in the fridge. In the produce business, this process is called controlled atmosphere storage, and it’s what keeps ripe apples fresh before they get to market. Before eating, let your chilled fruit sit at room temperature for a few hours to improve its aroma.

Some important exceptions: Uncut tomatoes, bananas, and other tropical fruits should never be chilled. Temperatures below 50°F can cause “chill injuries” to these warm-weather fruits, including discoloration, the development of a mealy texture, and loss of flavor and aroma.

When is it ripe?

Apricot: Deep orange, soft, aromatic
Banana: Dull yellow skin with many brown spots, shrunken neck, yields to light pressure
Berry: Vivid color, plump, strong aroma
Bing Cherry: Deep maroon-black, firm, juicy
Grape: Full, plump clusters tightly attached to pliable green stems, brown seeds
Hass Avocado: Deep green-black skin, yields to light pressure
Lime: Light green, heavy, plump, tields to light pressure, smooth and blemish-free skin, thin and flexible peel
Melon: Firm, heavy, ends yield to light pressure, rich aroma
Passion Fruit: Dark purple, wrinkled skin, yields to light pressure, heavy, tropical aroma
Peach or Nectarine: Deep gold-yellow background color, blemish-free skin with no green, slightly wrinkled near stem, heavy, yields easily to light pressure, highly aromatic
Pineapple: Plump, heavy, gold or light green skin that yields to light pressure, highly aromatic, no mold on bottom
Tomato: Vivid color, plump, heavy, glossy and blemish-free skin, fragrant

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