Nov. 13, 2002 What keeps golden raisins golden? Sulfur dioxide. What is sulfur dioxide? A preservative. What do preservatives do? They keep foods fresh and inhibit the growth of bacteria, yeasts or molds.
Everyone has heard about food preservatives, but how do they work? Chemical & Engineering News, in the Nov. 11 issue, explains what these useful chemicals can do to keep food safe and palatable. Chemical preservatives can't take the place of stronger preservation methods, like commercial sterilization, which kills most bacteria and enzymes, but they can be used effectively to retard spoiling and to stop the growth of harmful microorganisms.
Chemical & Engineering News is a weekly newsmagazine published by the American Chemical Society, the world's largest scientific society.
Preservatives can be grouped into three general types: antimicrobials that block growth of bacteria, molds or yeasts; antioxidants that slow oxidation of fats and lipids that leads to rancidity, and a third type that fights enzymes that promote the natural ripening that occurs after fruits or vegetables are picked.
Sulfur dioxide works in all three areas and is part of a larger group called sulfites, compounds found in numerous foods. A small percentage of people are allergic to sulfites, but the FDA says the preservative is safe for the majority. Some vinegars, fruit juices and dried fruits contain sulfites.
Sulfites block the growth of microbes by interrupting the normal functioning of their cells, according to Hassan Gourama, Ph.D., associate professor of food science at Pennsylvania State University.
Propionates are antimicrobials that help keep bakery products fresh. Propionic acid occurs naturally in apples, strawberries, grains and cheese. It works against bread molds and spores of a bacterium that leads to a condition called "rope" that makes bread inedible. Benzoates, another weak antimicrobial, fight fungi, working best in areas at a low acid level, where there are few bacteria. Benzoates are found naturally in cranberries.
Nitrites and nitrates most commonly are used to preserve meats and block deadly botulism bacteria. These preservatives also give cured meat its fresh pink color. Without nitrates or nitrites meat turns brown. The only potential problem with nitrites, Gourama says, is that they react with amino acids to form nitrosamines, cancer-causing agents. The levels used in cured meat are low and are not of concern, however, he adds.
Antioxidant preservatives stop the chemical breakdown of food when products are exposed to the air. Unsaturated fatty acids in oils and lipids are especially susceptible to oxidation and will take on a rancid flavor and odor as a result.
A third group of preservatives attacks enzymes in food that cause fruits and vegetables to over-ripen after harvest. One enzyme, for example, causes apples and potatoes to turn brown soon after they are cut or peeled. Acids such as ascorbic acid (vitamin C) and citric acid retard this process by making the pH level uncomfortably low for the enzyme.
Gourama says that at present many food scientists are searching for more preservatives in natural products. Some new antimicrobials have even been found in the microorganisms themselves, he says.
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