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U.S. Department of Health and Human Services


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List of Terms: U-V-W

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Did You Know?


Human viruses cannot grow in foods. Since viruses are very host-specific, a human virus will rarely multiply even in foods that are still alive (like oysters). However, they can persist for a long time.


> Ultra High Pressure (UHP) Treatment 
A new process used at the processing stage that utilizes very high pressure to kill bacteria in foods.

Typhoid MaryUltra High Pressure (UHP) Treatment Machinea2z-t3.jpg
Photo: Flow International Corporation
Today, some food producers are using a new method called Ultra
High Pressure (UHP) Treatment to kill harmful bacteria in foods.

Food Safety Implication: Ultra High Pressure destroys bacteria, but does so using pressure, rather than high temperatures or chemical additives. Thus, foods, such as juices, salsas, cold cuts, and other moist foods, are made safer without affecting the vitamins and flavor.

How It Works: Using specially-designed equipment, packaged food is exposed to 50,000 to 100,000 psi (pounds per square inch) of pressure for a short time. The ultra high pressure interferes with the metabolism and structure of bacteria and destroys these living cells, but does not crush the food.

As long as the food is mostly air-free and contains water, hydrostatic pressure doesn't crush food because the water in the food protects it from physical damage. However, living bacteria are destroyed by the effects of high pressure on their cellular functions.


> U.S. Department of Agriculture (USDA)
A department of the United States government that has many varied responsibilities, including food safety. The primary agency in USDA responsible for food safety is the Food Safety and Inspection Service. (See Food Safety and Inspection Service.)


> "Use By" Date
This is the last date a consumer is recommended to use a product while it is at peak quality. This date is recommended for best flavor or quality. It is not a "sell by" or food safety date. (Also see "Best If Used By," Expiration Date, and "Sell By" Dates.)


> Vacuum Packaging (see Packaging)


> Vibrio cholerae
This bacterium occurs naturally in the aquatic and marine environment. It causes cholera, a severe disease that, if untreated, could cause death.

Vibrio cholerae
Vibrio cholerae

Sources: Raw and undercooked seafood or other contaminated food and water. The contamination is the result of the food or drinking water mixing with water from sources that receive the untreated feces of cholera victims.


Incubation: 6 hours to 5 days after eating contaminated food.

Symptoms: They are often absent or mild. Some people develop a severe illness with profuse diarrhea, vomiting, and leg cramps. Loss of body fluids can lead to dehydration and shock. Without treatment, death can occur within hours.

Duration: 7 days.


> Vibrio vulnificus
A bacterium that is in the same family as those that cause cholera. It normally lives in warm seawater and is part of a group of vibrios that are called "halophilic" because they require salt.

Vibrio vulnificus can cause infection in people who eat raw, contaminated seafood, often shellfish, or have an open wound that is exposed to seawater. It is a rare cause of disease, but it is also underreported.

Vibrio vulnificus
Vibrio vulnificus

Sources: Raw fish and shellfish, especially raw oysters.


Incubation: Usually within 16 hours of ingestion or exposure to organism.

Symptoms: Diarrhea, abdominal pain, nausea, vomiting, fever, and sudden chills. Some victims develop sores on their legs that resemble blisters.

Duration: 2 to 3 days.

Is it true that some viruses are resistant to heat and cold? If so, what food safety precautions should consumers take?

Foodborne viruses are not especially resistant to heat. Most virus outbreaks are the result of foods that are not cooked, or are contaminated after cooking. Many viruses are very stable in the environment, but cooking is good at denaturing the proteins that protect the virus. That's why it's important to cook food, including seafood, thoroughly before eating it.

On the other hand, it's true that viruses are resistant to cold. Chilling or freezing does not eliminate viruses. Good agricultural and manufacturing practices along the farm-to-table continuum are needed to keep food from being contaminated by pathogens in the first place.

> Virus 
A non-cellular particle that consists minimally of protein or nucleic acid (DNA or RNA). In order to survive, it must replicate inside another cell, such as a bacterium or a plant and animal cell. (Also see Noroviruses.)

Norwalk Virus
Photo: F.P. Walker, U.S. EPA
The Norwalk Virus is a potent
norovirus that is rapidly gaining
groud as a public health threat.

Food Safety Implication: Food serves as a transportation device to get viruses from one host to another. Once the contaminated food is eaten, a virus can multiply in living cells and cause foodborne illness in humans. Food can become contaminated with viruses in a number of ways, such as:

  • A Food Handler - who picks, processes, prepares, or serves food and is shedding (excreting the virus in their stool). If the person practices poor hygiene, he or she may transfer the virus to food.
  • Contaminated Water - used to irrigate or wash foods.
  • Seafood - grown or collected from contaminated water and in which viruses have colonized. Foods such as oysters, which pump a lot of water each day through their bodies and filter out microorganisms, are very likely to collect viruses from the water.
  • Cross-Contamination - of safe food by contaminated food (or liquids dripping from such foods). For example, raw seafood juices that come in contact with fresh fruits or vegetables. Sometimes, this phenomenon makes it more difficult to pinpoint the actual food involved in a foodborne outbreak.

What's the difference between viruses and bacteria?

The differences between viruses and bacteria are numerous. Viruses are the smallest and simplest life form known. They are 10 to 100 times smaller than bacteria. The biggest difference between viruses and bacteria is that viruses must have a living host - like a plant or animal - to multiply, while most bacteria can grow on non-living surfaces.

Also, unlike bacteria, which attack the body like soldiers mounting a pitched battle, viruses are guerilla fighters. They don't attack so much as infiltrate. They literally invade human cells and turn the cell's genetic material from its normal function to producing the virus itself.

In addition, bacteria carry all the machinery needed for their growth and multiplication, while viruses carry mainly information - for example, DNA or RNA, packaged in a protein and/or membranous coat. Viruses harness the host cell's machinery to reproduce. In a sense, viruses are not truly living," but are essentially information (DNA or RNA) that float around until they encounter a suitable living host.

Why are shellfish and salads especially friendly to viruses?

Shellfish are especially friendly to viruses because shellfish pump water through their bodies and thus, concentrate food and contaminants, such as viruses, from the water. Once the virus is on or in the shellfish - a live host - it may persist for a longer time than if it were suspended in the water. Some shellfish are eaten raw or lightly cooked, which increases the risk of foodborne illness.

Produce used for salads, lettuce, spinach, etc., grow low to the ground where they are more likely to come in contact with contaminated, organic fertilizers, such as manure. Also, sometimes produce is irrigated with contaminated waters or picked by farm workers with poor hygiene practices. The complex, multi-layered surfaces of salad produce are more difficult to clean after picking than, for example, the surface of an apple or potato. Finally, because salads are usually eaten raw, there is no heating step that would inactivate the viruses

How It Causes Disease: Viruses cause disease in humans by tricking healthy cells into duplicating the virus's nucleic acid instead of its own, which lets the virus multiply. Once the virus is duplicated, the healthy cell usually dies.

Some Examples of Foodborne Viruses:

  • Norovirus, previously Norwalk virus and other Norwalk-like viruses
  • Hepatitis A
  • Rotavirus (mainly affects young children)

Some Examples of Human Viruses:

  • Influenza (causes the flu)
  • HIV (causes AIDS)
  • Polio (causes poliomyelitis)
  • Rhinovirus (causes colds)
  • Rubella (causes German measles)


> Water Activity (aw
The amount of water that's available to support bacterial growth in different foods.

Food Safety Implication: Bacteria require a certain amount of "free" water in order to grow. The more available water, the faster the bacteria will grow. Bacteria, yeast, and mold multiply rapidly at a high water activity - above 0.86. Meat, produce, and soft cheeses have water activity between 0.86 and 1.0, and thus support rapid bacterial growth.

How It Works: Water activity is measured on a scale of 0 to 1.0 and is derived from a measurement of the amount of moisture in a food product and the amount of solutes (a dissolved substance). The greater the solutes in a specific amount of moisture, the lower the water activity.

Water activity is lowered by the addition of solutes, such as salt or sugar. These food constituents bind water molecules together, making it unavailable for use by micro-organisms. Preservation methods that use large amounts of salts or sugars work by reducing the water activity.


> Water Quality
The nature or state of water for consumption or use on foods along the farm-to-table continuum.

The nature or state of water for consumption or use on foods along the farm-to-table continuum.

Food Safety Implication: Along the farm-to-table continuum, water quality dictates the potential for pathogen contamination. For example, at the farm, growers use good agricultural practices (GAPs) to minimize the risk of contaminated water being used on the produce. At processing facilities, good manufacturing practices (GMPs) are followed to minimize microbial contamination from water used during processing. (Also see Good Agricultural Practices and Good Manufacturing Practices.)