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The Science of Apples

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Apples rank among the world’s most popular fruits. Not only are they delicious eaten out of hand, but they’re also a crisp addition to salads, a star in autumn pies, and a sweet complement to savory meat dishes. Here’s how to choose the right apple for what you’re making and how to keep it tasting its best.

What makes an apple crisp and juicy?
The cells of apples are filled with flavorful juices composed of water, sugar, acids, and aromatic esters (compounds made of one acid and one alcohol molecule). The spaces between the cells are filled with air, which accounts for as much as 25% of the volume of a ripe apple (that’s why apples float). In a ripe apple, the cells bulge with juice, which stretches the cell walls and compresses the air between the cells. When you take a bite, the cell walls break (hence the crispness), and the juice bursts out. At the same time, the trapped air is released, transporting the apple’s aromatic esters up the back of your throat to the olfactory membranes in your nose, and you taste the distinctive flavor of that particular apple. Apples that contain less air aren’t as flavorful, crisp, or juicy, but they’re better for cooking.

What makes an apple sweet or tart?
All apples have a balance of sugars and acids, but the balance changes by variety. Granny Smith apples are high in acid and lean in sugar, for example, and Fujis are decidedly sweet with a subtle acidity. Regardless of the varietal differences, the sweet-sour balance of any apple evolves during storage. Apples are climacteric, which means they contain starch that converts to sugar after harvest. In addition, malic acid, the primary acid in apples, is consumed by the fruit once picked and is used for energy over time in storage. This means that all apples are at peak tartness right after picking and gradually become sweeter the longer they’re stored.

How do you a tell a good apple from a bad one?
As an apple ripens, its cells fill with water, and green chlorophyll in the skin breaks down, revealing deeper green, yellow, red, and pink colors underneath. When picking apples from the tree or the market bin, look for brightly colored fruit that feels plump, firm, and somewhat heavy. This heaviness indicates maximum water retention and juiciness.

After harvest, apples continue to ripen due to ethylene, a vaporous hormone produced by the fruit. With continued exposure to ethylene, apples become increasingly soft, shrunken, and lighter in weight. Hemicelluloses and pectic substances that hold water in the apples’ cells (and keep the cells separate) eventually break down, causing moisture to escape and the apple skin to wrinkle. Overripe apples will appear shrunken and wrinkled, and feel soft when pressed. They may also taste dry and mealy due to lack of water and desegregation of the plant’s tissues as the hemicelluloses and pectic substances weaken.

Can one bad apple spoil the whole bunch?
Yes. Jostling and impact injuries can bruise an apple, leaving behind soft brown spots that are easily attacked by the fungus Penicillium expansum. Known as blue mold or soft rot, P. expansum rapidly spreads from one bad apple to those nearby, particularly in warm, humid conditions, which encourage mold growth.

What’s the best way to store apples?
Properly stored apples will taste crisp and juicy months aft er harvest. To minimize bruising and untimely rotting, apples should be handled gently and kept separate during and aft er harvest; that’s why produce distributors ship apples in soft trays with individual cradles.

For the longest storage, keep apples in the refrigerator produce drawer to slow their natural ripening processes and discourage mold growth. If your fridge has drawers with adjustable humidity, set the apple drawer to about 85% humidity, which helps keep apples from drying out prematurely.

How do I keep apples from browning?
Browning in fruit is caused by exposure to oxygen. Bruised or cut apples release enzymes from the damaged cells that mix with phenols, the fruit’s aromatic compounds, and react with oxygen to form new molecules that appear brown in color.

So how do you keep cut apples nice and white in, say, a fresh apple salad? There are a few options. The easiest way is to slow the enzymes’ activity with acid and cold temperatures. To do this, put cut apples in a bowl of cold acidulated water (add 1?4 cup lemon juice, 2 tsp. vinegar, or 500mg crushed vitamin C tablets to every 4 cups cold water). To avoid the mild acidic flavor imparted by this method, you can simply keep cut apples submerged in plain cold water to shield them from oxygen; however, the lack of added acid will cause enzyme activity to increase more rapidly when the apples are removed from the water. You can also toss cut apples in sugar or syrup to shield the cut surfaces from oxygen-this method works well for sweet fruit salads. Finally, you can deactivate enzymes by dipping cut apples in boiling water for at least 1 minute. This blanching method soft ens the fruit slightly, but is a good choice if you want the apple to stay white without an acid or an oxygen-shielding solution.

What makes an apple good for cooking?
In general, apples that are high in acid, such as Granny Smith and Braeburn, hold up best during cooking. Acids enhance our perception of other fl avors, and because heat tends to dissipate aromatic molecules, cooked dishes made with high-acid apples retain more fl avor. In addition, acids are necessary to strengthen pectin (the “glue” that holds fruit cells together), which helps apple slices keep their shape in pies and crisps. Apple varieties with less air, like Rome Beauty and Braeburn, are best for baking whole because theywon’t collapse as their water evaporates, their juices concentrate, and their cells contract. Although any apple can be used for making applesauce, those with less air, like McIntosh, make creamy smooth applesauce, while crisp, tart apples, like Granny Smith, make chunkier, looser sauce.

For more information about apples, see our ingredient profile.

David Joachim and Andrew Schloss are the authors of the award-winning reference book The Science of Good Food.

Illustration: Janet Stein


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