Why Don’t Tomatoes and Fruits Rot When They’re Sun-Dried?

Why don’t tomatoes, apricots, raisins, and other fruits rot when they’re sun-dried?

The high concentration of sugars in dried fruit prevents bacterial growth, but the trick is getting the fruit dry before mold can grow. Even under optimal hot-and-dry conditions, like those of Central California or the Mediterranean, it takes four to six days to sun-dry tomatoes and two days or longer for stone fruits, says Paul Walker, director of sales for Traina Foods, growers and driers of apricots, peaches, nectarines, cherries, plums, pears, and tomatoes. The vast majority of sun-dried fruits are preserved with artificial chemicals. The most common of these are sulfites, which prevent bacterial growth and preserve color by absorbing oxygen, and also keep insects away from the drying fruit.

The only common sun-dried fruits not preserved with sulfites are dark raisins—they’re small enough to dry before rot takes hold, Walker says. Their color is a natural result of oxidation; golden raisins come from the same variety of grapes as dark raisins, but are sulfite-preserved to prevent browning. Without preservatives, all dried fruit becomes dark-colored.

About one in a hundred people, especially anyone with asthma, is dangerously allergic to sulfites. The FDA says they’re safe for most people. Sulfite concentrations in dried fruits generally range from about 1,000 to 3,500 parts per million—any higher and taste would suffer, says Stanley Gardner, director of sales for sun-dried tomato maker Valley Sun. Foods containing more than 10 parts per million must be labeled as such.

Organic and other sulfite-free sun-dried tomatoes use plain old salt to inhibit bacterial growth. It would be impossible to sun-dry tomatoes with no preservatives at all, Gardner says. But salt would ruin the flavor of sweeter fruits, so sulfite-free versions of those are usually oven-dried. If you’re really ambitious, you could try sun-drying your own fruit, but wait for good weather: The National Center for Home Food Preservation recommends a minimum of 85 degrees and a maximum of 60 percent humidity. To dry fruit without salt, Gardner suggests 12 to 18 hours in a 160-degree oven followed by a stint in the sun.