Like all living things, the microbes on food need water to survive. Dehydrate the food, and the bugs won't reproduce, which means the food stays edible longer. That's the rationale behind raisins, prunes, and pemmican, a dried mix of meat, fat, and berries adapted from East Coast Native Americans and served to 18th- and 19th-century sailors of every national stripe. Dehydration (loss of water) occurs when food is
^ Exposed to air and sunlight
^ Heated for several hours in a very low (250 degrees Fahrenheit) oven or is smoked (the smokehouse acts as a very low oven)
Just as microbes need water, most also need air. Reducing the air supply almost always reduces the bacterial population. The exception is anaerobes (microorganisms that can live without air), such as botulinum organisms, which thrive in the absence of air. Go figure!
Foods are protected from air by vacuum-packaging. A vacuum — from vacuus, the Latin word for "empty" — is a space with virtually no air. Vacuum-packaging is done on a container (generally a plastic bag or a glass jar) from which the air is removed before it's sealed. When you open a vacuum-packed container, you hear a sudden little pop as the vacuum is broken.
jfe. If there's no popping sound, the seal has already been broken, allowing air inside, and that means the food inside may be spoiled or may have been tampered with. Do not taste-test: Throw out the entire package, food and all.
About two dozen chemicals are used as food additives or food preservatives to prevent spoilage. (If the mere mention of chemicals or food additives makes the hair on the back of your neck rise, chill out with Chapter 22.) Here are the most common chemical preservatives:
1 Acidifiers: Most microbes don't thrive in highly acidic settings, so a chemical that makes a food more acidic prevents spoilage. Wine and vinegar are acidifying chemicals, and so are citric acid, the natural preservative in citrus fruits, and lactic acid, the natural acid in yogurt.
1 Mold inhibitors: Sodium benzoate, sodium propionate, and calcium propionate slow (but do not entirely stop) the growth of mold on bread. Sodium benzoate also is used to prevent the growth of molds in cheese, margarine, and syrups.
1 Bacteria-busters: Salt is hydrophilic (hydro = water; phil = loving). When you cover fresh meat with salt, the salt draws water up and out of the meat — and up and out of the cells of bacteria living on the meat. Presto: The bacteria die; the meat dries. And you get to eat corned beef (which gets its name from the fact that large grains of salt were once called "corns").
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