Just this year, I discovered golden raisins. I like their light flavor in my morning oatmeal--not too sweet, and milder than their darker cousins. I didn't wonder what keeps the golden variety golden until recently, when I looked at the ingredients list: California seedless raisins, sulfur dioxide added as a preservative.
Chemical preservatives like sulfur dioxide help keep food fresh. Of the 32 ingredients in my favorite type of granola bar (oatmeal raisin), one is labeled as a preservative, although a few other ingredients also inhibit decay. Some grocery items have no preservatives at all--in particular those that are sufficiently preserved by freezing, drying, smoking, pickling, canning, or some other means. Chemical preservatives can't replace more stringent preservation methods, such as commercial sterilization, which destroys most enzymatic and bacterial activity. But chemicals can be used effectively to slow spoiling and keep microorganisms at bay.
Preservatives can be categorized into three general types: antimicrobials that inhibit growth of bacteria, yeasts, or molds; antioxidants that slow air oxidation of fats and lipids, which leads to rancidity; and a third type that blocks the natural ripening and enzymatic processes that continue to occur in foodstuffs after harvest.
Sulfur dioxide serves all three functions, which is one reason why it and related compounds called sulfites are found in so many household products. (A small percentage of the population is allergic to sulfites, though FDA states that, for the majority of the population, they are safe.) In my cupboards, I found sulfites in a package of sun-dried tomatoes, Turkish dried apricots, dried sweet potatoes, balsamic vinegar, red wine vinegar, lemon juice, and Hawaiian coconut syrup. Sulfites are also commonly used in wine preparation and to lengthen the life of fruit juices.
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