Food Freshness: How Food Quality Deteriorates

by Food and Drug Administration (FDA)

What could a roll of new carpeting possibly have in common with a slab of smelly, old salmon? They both emit chemically similar vapors — the carpet, as it's being installed; the fish, as it rots.

Chemists here at the Food and Drug Administration's National Center for Toxicological Research (NCTR) have identified "volatile amines" not only as the toxic connection between two seemingly unrelated products, but ultimately as a potential link to determining at what point a food product begins to deteriorate.

Food freshness is a key characteristic of overall food quality. And overall food quality is the result of all the desirable characteristics that make food acceptable to eat. Therefore, being able to tell when food is fresh is vitally important, at home, in a grocery store, or when dining out.

Seafood, for example, is one of the most difficult foods to keep fresh. Millions of bacteria are present on the surface, on the gills, and in the gut of virtually all seafood species. If you know when and where the fish was caught, you might be able to make an educated guess as to its freshness, provided that it's been properly stored. Inspecting the fish for color, resilience and sliminess would help. So might a check of its eyes to determine clarity and sheen. But in the end your nose might determine whether or not you buy the fish. If you're trying to determine whether to buy a piece of fish tightly wrapped in plastic and sitting on a Styrofoam tray, however, the decision might be more difficult.

The freshness and overall quality of food depend, in large part, on the distribution and marketing systems. Any mishandling of a food along the way can have a significant impact on its overall quality. To further ensure that food retains its high quality, consumers must practice careful food storage and handling habits at home, as well.

New technologies are emerging that aim to monitor temperature and other important variables that play critical roles in determining food freshness. It is hoped that such technologies will be useful in evaluating freshness during the movement of food from producer to supplier to consumers.

How Food Quality Deteriorates

A walk through any grocery store indicates the diversity of today's food supply — raw vegetables, baked goods, packaged meats, seafood, and more. The selection is a chef's delight. But that diversity also contributes to the challenges of ensuring that food is healthful and wholesome. Delight can become disappointment if undesirable changes in color, flavor, odor, or texture occur. For the most part, this deterioration is caused by enzymes — either contained within the food or produced by microorganisms, such as bacteria, yeasts and molds growing in the food.

Each food type and bacterium produce different chemicals or classes of chemicals. The chemicals produced vary and include volatile bases, volatile acids, volatile aldehydes, and volatile mercaptans, or sulfur compounds.

Spoilage bacteria (rather than disease-causing "pathogenic" bacteria) usually cause food to deteriorate most quickly because of their short reproduction times. They multiply very rapidly by a process called cell replication or binary fission — one cell divides and becomes two. If conditions such as moisture and temperature are right, for example, certain bacteria can reproduce in as little as 20 minutes. Within 20 minutes, one cell can become two; in 40 minutes, there would be four, and so on. The bacteria are slow to start, but the number increases quickly.

Some bacteria need only about four hours to adapt to a new environment before they begin rapid growth and threaten the window of time that a food item maintains its taste, texture, and nutritional value, known as shelf life. When it comes to food, this means consumers can have less than four hours to make a decision about whether to cool it, heat it, or eat it.

As the bacteria grow, the amount of enzymes produced by those bacteria increases. Enzymes are a normal component of food that help speed up or slow down chemical reactions. The enzymes in a banana, for example, cause it to change color from green to yellow, and then brown to black, as it matures. The ripening and softening of other fruits, such as peaches, tomatoes and apples, are other examples of enzyme action. Heat inactivates these enzymes, which explains why people might blanch vegetables. Cold temperatures also can inactivate enzymes, which is why you would refrigerate certain foods.

In addition, food may deteriorate as a result of chemical changes within the food itself or, more broadly, from temperature abuse.

"The odor that everyone associates with bad food is rancidity," says Dwight Miller, Ph.D., a chemist at NCTR. Rancidity, also called staleness, is caused by a chemical reaction that breaks down the molecular chains that make up fatty acids in fat to compounds called aldehydes, and may continue to smaller-sized fatty acids, resulting in the release of offensive or musty odors. So, as butter ages, it tastes stronger, just as peanuts become rancid with time.

In some cases, food deterioration may occur before flavor or odor changes are detectable. And because everyone doesn't have the same level of odor-detecting ability, Miller points out, "the untrained nose is not consistent."

A smoker, or even a woman wearing perfume, for example, can temporarily have a reduced ability to smell some odors. That's why, according to Miller, "We have to have a way that consumers can be taught to protect themselves from food that's gone — or is going — bad."

The numbers of microorganisms or enzymes present on a food product determine the degree of food spoilage. Since we cannot see them to count the number growing in our food, next to our untrained noses, we must rely on distributors, manufacturers, and grocers to provide us with some assurance that the products we purchase are fresh.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.