What's Going On

• When cut or bitten, the cell walls of peaches and other fruits rupture and release enzymes and chemicals. One specific group of enzymes reacts with oxygen in the air and phenolic compounds in the fruit to promote browning. The darkened areas actually digest proteins, which serves to protect the fruit by disabling some insect larvae and irritating delicate membranes in small animal pests.

Slowing the Reaction

• Like any other chemical reaction, this defense -- called "enzymatic browning" by scientists -- can be affected by environmental conditions. It's slower at low temperatures, which is why fruit trays are refrigerated when they won't be served immediately. It also requires exposure to oxygen, so it is helpful to place large quantities of cut peaches and other quick-browning fruits into a bowl of water. The water restricts access to atmospheric oxygen, slowing the browning process. Finally, and most importantly from the home canner's perspective, acidity slows the browning reaction to a standstill.

Turning Sour

• Most home cooks know that dropping sliced fruit into water containing a splash of lemon juice will keep it from browning, though they might not understand why. Diluted lemon juice also works for canning purposes, but it adds a distinct flavor and its acidity can vary. Serious hobbyists typically rely on citric acid, readily available where canning supplies are sold, or ascorbic acid. To make an anti-browning soak for your peaches, use 1 teaspoon of citric acid for every cup of water. If you're using ascorbic acid, better known as vitamin C, crush 3,000 milligrams of tablets for every cup of water. Commercial anti-browning powders typically contain one or both of those crystalline acids.

A Few Other Tips

• Some canners prefer not to drop their peaches into a bowl of water, reasoning that it takes away from their flavor. As an alternative, you can collect them in a bowl, with only their own natural juices, and toss them with 1/4 cup of lemon juice or commercial anti-browning powder until they're well-coated. This doesn't give as consistent a result as the prepared water bath, but you can compensate by adding a sprinkle of citric or ascorbic acid to each jar before canning. You can also help prevent browning, and reduce your sugar consumption, by canning the peaches in apple juice or white grape juice rather than in a syrup.