In days, sometimes hours, bread goes moldy, apple slices turn brown, and bacteria multiply in mayonnaise.

But you can find all of these foods out on the shelf at the grocery store, hopefully unspoiled, thanks to preservatives.

But what exactly are preservatives?

How do they help keep food edible and are they safe?

There are two major factors that cause food to go bad: microbes and oxidation.

Microbes like bacteria and fungi invade food and feed off its nutrients.

Some of these can cause diseases, like listeria and botulism.

Others just turn edibles into a smelly, slimy, moldy mess.

Meanwhile, oxidation is a chemical change in the food's molecules caused by enzymes or free radicals which turn fats rancid and brown produce, like apples and potatoes.

Preservatives can prevent both types of deterioration.

Before the invention of artificial refrigeration, fungi and bacteria could run rampant in food.

So we found ways to create an inhospitable environment for microbes.

For example, making the food more acidic unravels enzymes that microbes need to survive.

And some types of bacteria can actually help.

For thousands of years, people preserved food using bacteria that produce lactic acid.

The acid turns perishable vegetables and milk into longer lasting foods, like sauerkraut in Europe, kimchi in Korea, and yogurt in the Middle East.

These cultured foods also populate your digestive track with beneficial microbes.

Many synthetic preservatives are also acids.

Benzoic acid in salad dressing, sorbic acid in cheese, and propionic acid in baked goods.

Are they safe?

Some studies suggest that benzoates, related to benzoic acid, contribute to hyperactive behavior.

But the results aren't conclusive.

Otherwise, these acids seem to be perfectly safe.

Another antimicrobial strategy is to add a lot of sugar, like in jam, or salt, like in salted meats.

Sugar and salt hold on to water that microbes need to grow and actually suck moisture out of any cells that may be hanging around, thus destroying them.

Of course, too much sugar and salt can increase your risk of heart disease, diabetes, and high blood pressure, so these preservatives are best in moderation.

Antimicrobial nitrates and nitrites, often found in cured meats, ward off the bacteria that cause botulism, but they may cause other health problems.

Some studies linking cured meats to cancer have suggested that these preservatives may be the culprit.

Meanwhile, antioxidant preservatives prevent the chemical changes that can give food an off-flavor or color.

Smoke has been used to preserve food for millennia because some of the aromatic compounds in wood smoke are antioxidants.

Combining smoking with salting was an effective way of preserving meat before refrigeration.

For antioxidant activity without a smoky flavor, there are compounds like BHT and tocopherol, better known as vitamin E.

Like the compounds in smoke, these sop up free radicals and stave off rancid flavors that can develop in foods like oils, cheese, and cereal.

Other antioxidants like citric acid and ascorbic acid help cut produce keep its color by thwarting the enzyme that causes browning.

Some compounds like sulfites can multitask.

They're both antimicrobials and antioxidants.

Sulfites may cause allergy symptoms in some people, but most antioxidant preservatives are generally recognized as safe.

So should you be worried about preservatives?

Well, they're usually near the end of the ingredients list because they're used in very small amounts determined by the FDA to be safe.

Nevertheless, some consumers and companies are trying to find alternatives.

Packaging tricks, like reducing the oxygen around the food can help, but without some kind of chemical assistance, there are very few foods that can stay shelf stable for long.