To keep Granny Smith apples from turning brown, scientifically speaking, you need to either reduce the rate at which the enzymes on the apple's cut flesh react to oxygen in the air, or you need to reduce the surface area where the enzymes go to work. Luckily, there are readily available household ingredients that do both those things. And you can also buy easy to use commercial products designed specifically to stop apples and other fruits from turning brown.

Simply placing the apple slices or chunks in water keeps air from touching the surfaces and thereby reduces browning. Place a damp paper towel or saucer over the top of the slices to keep the top layer submerged. An even better method to keep the apples from turning brown is to mix 1/2 teaspoon of salt or two tablespoons of honey into each cup of water. Let the apple slices sit in the water for 10 minutes, and then drain and store them in the refrigerator until you are ready to use them. Rinse the slices, or eat them as they are if you don't mind a slightly salty or sweet additional taste to your apples.

Lemon, lime, orange, grapefruit and pineapple juice all contain enough citric acid to inhibit the enzymes on freshly cut apples to prevent browning for hours. Either squeeze the citrus fruit juice directly onto the apples, rub a pineapple slice over the apple's surface, or soak apple slices in citrus or pineapple juice for 10 minutes. It's not necessary to rinse the slices before you use them if you would like a citrus flavor in whatever you are making or if you like the flavor when you eat your raw apple slices.

Both citric acid and ascorbic acid, also called vitamin C, are available in powdered forms that you can add to the water you soak apple slices in to keep them from discoloring. The chemicals occur naturally in citrus fruits and other foods, but they are different substances that home canners use to help keep fruit from turning brown during the canning process. For your apples, use one teaspoon of ascorbic acid powder or six crushed 500 milligram tablets of vitamin C per gallon of water for a soaking liquid. If you have powdered citric acid, follow the directions on the package.