Say “processed food,” and most people think “cheese spread.” They’re right, of course. Cheese spread is, in fact, a processed food. But so are baked potatoes, canned tuna, frozen peas, skim milk, pasteurized orange juice, and scrambled eggs.

In broad terms, food processing is any technique that alters the natural state of food — everything from cooking to freezing to pickling to drying and more and more and more.

In this article, you can read all about how each form of processing changes food from a living thing (animal or vegetable) into an integral component of your healthful diet — and at the same time:

• Lengthens shelf life
• Reduces the risk of foodborne illnesses
• Maintains or improves a food’s texture and flavor
• Upgrades the nutritional value of foods

What a set of bonuses!

Where food is concerned, the term natural doesn’t necessarily translate as “safe” or “good to eat.” Food spoils (naturally) when microbes living (naturally) on the surface of meat, a carrot, a peach, or whatever reproduce (naturally) to a population level that overwhelms the food.

Sometimes you can see, feel, or smell when this is happening. You can see mold growing on cheese, feel how meat or chicken turns slippery, and smell when the milk turns sour. The mold on cheese, slippery slickness on the surface of the meat or chicken, and odor of the milk are caused by exploding populations of microorganisms.

Don’t even argue with them; just throw out the food. All food processing is designed to prevent what happens to the chicken (or the cheese or the milk).

It aims to preserve food and extend its shelf life (the period of time when it’s safe to consume and nutritious) by stemming the natural tide of biological destruction. (But wait! Not all microbes are bad guys. We use “good” ones to ferment milk to yogurt or cheese and to produce wines and beers.)

Reducing or limiting the growth of food’s natural microbe population not only lengthens its shelf life but also lowers the risk of foodborne illnesses. Increased food safety is a natural consequence of most processing that keep foods usable longer. This section discusses how food processing works.

• Temperature methods

• Cooking
• Canning
• Refrigeration
• Freezing

• Air control

• Canning
• Vacuum-packaging

• Moisture control

• Dehydration
• Freeze-drying (a method that combines methods of controlling the temperature, air, and moisture)

• Chemical methods

• Acidification
• Mold inhibition
• Salting (dry salt or brine)

• Irradiation

• High-pressure processing

Temperature control

Exposing food to high heat for a sufficiently long period of time reduces the natural population of bacterial spoilers and kills microbes that otherwise may make you sick.

For example, pasteurization (heating milk or other liquids such as fruit juice to 145 to 154.4 degrees Fahrenheit for 30 minutes) kills nearly all disease-causing and most other bacteria, as does high-temperature, short-time pasteurization (161 degrees Fahrenheit for 15 seconds).

Chilling also protects food. It works by slowing the rate of microbial reproduction. For example:

• Milk refrigerated at 50 degrees Fahrenheit or lower may stay fresh for almost a week because the cold prevents organisms that survived pasteurization from reproducing.

• Fresh chicken frozen to 0 degrees Fahrenheit or lower may remain safe for up to 12 months (whole) or 9 months (cut up).

Removing the water

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)

Controlling the air flow

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. Vacuumpackaging 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. 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.

Chemical warfare

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.) Here are the most common chemical preservatives:

• 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.

• 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.

• 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”).

Irradiation

Irradiation is a technique that exposes food to electron beams or to gamma radiation, a high-energy light stronger than the X-rays your doctor uses to make a picture of your insides. Gamma rays are ionizing radiation, the kind that kills living cells. As a result, irradiation prolongs the shelf life of food by:

• Killing microbes and insects on plants (wheat, wheat powder, spices, dry vegetable seasonings)

• Preventing potatoes and onions from producing new sprouts at the eyes

• Slowing the rate at which some fruits ripen

• Killing disease-causing organisms such as Trichinella, Salmonella, E. coli, and Listeria (the organism responsible for a recent outbreak of food poisoning from packaged meats and cold cuts) on meat and poultry

In 1998, the U.S. Food and Drug Administration (FDA — which had already approved irradiation for plant foods, pork, and poultry) put its stamp of approval on irradiating fresh red meat products as a way to enhance, but not replace, the safe handling and storage of meat in the processing plant, supermarket, and your kitchen.

As of 2005, there are also standards for irradiating fresh and frozen shellfish. And, no, irradiating food does not make the food radioactive. But you already knew that, right?