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.

Sulfites inhibit microbial growth through a number of actions, says Hassan Gourama, associate professor of food science at Pennsylvania State University. They react with the energy currency of the cell, adenosine triphosphate; inhibit some metabolic pathways; and block cellular transport systems. Other antimicrobials alter microbial membrane or cell wall permeability or destroy the genetic material.

In addition to its antimicrobial action, sulfur dioxide inhibits degradation reactions in fruits, Gourama says. It keeps raisins and other dried fruits from losing their light color by blocking both enzymatic browning and a nonenzymatic browning reaction between reducing sugars and amino acids called the Maillard reaction. The reaction darkens raisins, alters their flavor, and reduces essential amino acid levels.

Antimicrobials are found throughout the grocery store. Propionates hold sway in the bakery aisle. Propionic acid occurs naturally in strawberries, apples, violet leaves, grains, and cheese. The acid is effective against bread molds and the spores of the bacterium Bacillus mesentericus, which cause an inedible condition in baked goods called rope.

Other weak organic acid antimicrobials include benzoates, found naturally in cranberries, and sorbates. Because these compounds work best at a low pH--in the range that excludes much bacterial growth--they are used primarily as antifungals. Esters of p-hydroxybenzoic acid, also known as parabens, are similar to benzoic acid but effective at a higher pH. Many beverages, jams, pickled products, salads, cheeses, meats, and margarines contain benzoates or sorbates.

Nitrites and nitrates roost mainly among the packaged meats. They are the food industry's primary chemical defense against the bacterium Clostridium botulinum. They also impart a pink, fresh hue to cured meat. Nitrates readily convert to nitrites, which then react with the protein myoglobin to form nitric oxide myoglobin. During cooking, this is converted to nitrosohemochrome, a stable, pink pigment. In the absence of nitrates or nitrites, meat turns brown. However, "the only problem with nitrites is that they react with amino acids to form nitrosamines"--cancer-causing agents, Gourama says, adding that "the levels that are used in cured meat are not of concern right now."

ANTIOXIDANT preservatives, such as butylated hydroxytoluene, butylated hydroxyanisole, tert-butylhydroquinone, and propyl gallate, stop the chemical breakdown of food that happens in the presence of oxygen. Unsaturated fatty acids in oils and lipids are particularly susceptible to autooxidation. In this process, a free radical initiates peroxide formation at fatty acid double bonds. The chain reaction propagates to other double bonds, and aldehyde, ketone, and acid-termination products eventually build up to create the rancid off-flavors characteristic of oils and fats gone bad. Antioxidant preservatives sop up the free radicals that help initiate and propagate these reactions.

A third group of preservatives targets enzymes in the food itself that continue to metabolize after harvest. The enzyme phenolase, for example, goes to work as soon as an apple or potato is cut. It browns the exposed surface. Acids such as citric acid and ascorbic acid (vitamin C) inhibit phenolase by making the pH uncomfortably low for the enzyme.

And metal-chelating agents such as EDTA (ethylenediamine tetraacetic acid) can remove the metal cofactors that many enzymes need. Chelators also make it difficult for bacterial and fungal enzymes to carry on.

Currently, Gourama says, many food scientists are searching for preservatives in natural products. Some of the newest antimicrobials have been found in microorganisms themselves as they form their own chemical defenses when competing with each other for space and nutrients. For example, nisin and natamycin--cheese preservatives called bacteriocins--are harvested from microorganisms. Other potential natural preservative sources include honey, milk, and even dried plums as scientists seek new sources and combinations of safe, effective preservatives.

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