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U.S. Department of Health and Human Services


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List of Terms: P

Return to Comprehensive List of Terms


Did You Know?

In the early 1800s, the British Navy needed unbreakable canned goods for sea voyages, which spurred the development of tinned cans. By the 1860s, the production of tinned goods was a booming industry in the United States.


Did You Know?

Photo of Louis Pasteur
Louis Pasteur


Louis Pasteur, a chemist, developed pasteurization while researching the cause of beer and wine spoilage. The process was applied first in wine preservation. When milk producers adopted the process, pasteurization eliminated a substantial quantity of foodborne illness.


Did You Know?

In prehistoric times, people preserved food by drying it in the sun or storing it in cool caves. Today, science has developed other methods of preservation, such as canning, freeze-drying, and irradiation.

Salt is a good food preservative.
Salt is considered antibacterial because it restricts bacterial growth in many foods. It preserves foods by lowering the amount of "free" water molecules in foods. Bacteria need moisture in order to thrive, so without enough "free" water, they cannot grow well in foods that contain salt.


> Packaging 
Wrappers or containers used to protect food from dirt, germs, and damage. Packaging also keeps oxygen out and increases the shelf life of food.

Food Safety Implication: Packaging techniques use heat to kill pathogenic bacteria or other methods, such as placing food in sterile packaging, to protect food from micro-organisms that could cause spoilage or food-borne illness. Also, since bacteria thrive on oxygen, some types of packaging reduce the amount of oxygen available to bacteria to slow bacterial growth and extend the storage life of the food. Labels on product packages also provide information about how to safely use the product.

Food Safety Precautions:

  • Always read the label on retail packaging. Handling, storage, and cooking procedures can differ greatly for every product, and proper handling can ensure product safety.
  • Check the expiration, "sell by," and "use by" dates stamped on the package, and don't take risks by using old products.
  • Don't buy or use foods whose packaging has been broken, torn, or damaged.

Types of Packaging Commonly Used Today Include:

  1.  Shelf-Stable Packaging - Traditional shelf-stable packaging consisted of cans or glass jars. The newest shelf-stable packaging uses plastic containers. So, instead of opening a can of soup or stew, pouring it into a pan and heating it, you can pull a plastic soup package off the shelf and pop it into a microwave. Such products can be safely stored on the shelf without refrigeration. Assuming there are no breaks or tears in the package, these products should maintain top quality for more than a year.
  2. Packaging That Requires Refrigeration
    • Sous Vide (a French phrase for "under vacuum") - With this method, fresh raw ingredients or partially-cooked ingredients are vacuum-sealed in a plastic pouch. The pouch is heat-processed, then quickly chilled and transported under refrigeration. Sous vide products must be kept refrigerated. Like other vacuum-packed products, sous vide products will last 3 to 4 weeks refrigerated. To serve, you simply heat the bag in boiling water.
    • Vacuum Packaging (a.k.a. Modified Atmosphere Packaging) - Oxygen in the package is mixed with a gas (normally carbon dioxide and/or nitrogen) that slows spoilage, discoloration, and the growth of harmful bacteria. Vacuum packages now include raw pork tenderloin, fully-cooked roast chicken, tuna spreads, and tortellini. Some foods may require additional cooking or heating before serving, so always check the label.

Common Types of Packaging Materials Include:

  • Aluminum - Bottle caps and easy-to-open tops for cans.
  • Glass Containers - Durable, chemical-resistant, can be kept highly sanitary, and are ideal for the storage of solid and liquid foods.
  • Cardboard - Comes in a variety of shapes and sizes and serves as containers for food.
  • Plastic - Has been used extensively as a shipping material for liquids and perishable foods because of its high durability and insulation qualities.
  • Tin-Plated Steel Cans - Used predominantly for food storage.


> Parasite
A plant or animal that lives on or in another plant or animal, while making no beneficial contribution to that host.

Cryptosporidium, a parasite, can cause a
severe, life-threatening disease, particularly in
patients with AIDS.

Food Safety Implication: Food can become contaminated with parasites. Some parasites have an indirect life cycle. For example, they need an intermediate host (the food species) where they develop into a stage that is infectious to humans. Humans that consume the infested food, either raw or undercooked, become infected.

In parasites that are directly infectious, the parasite is physically transferred by the food through contamination with a human or other host waste.

Some Examples of Parasites That May Contaminate Food Are: Trichinella spiralis (trichinosis), which can be found in pork and Anisakis roundworm, which can be found in fish. For more about other common parasites, see Cyclospora, Cryptosporidium, and Toxoplasma gondii.


> Partnership for Food Safety Education (see Fight BAC!TM Campaign)

> Pasteurization 
The process of destroying microorganisms that could cause disease. This is usually done by applying heat to a food.

Food Safety Implication: Pasteurization is the heat processing of a liquid or food to kill pathogenic bacteria to make a food safe to eat. Using pasteurization to kill pathogenic bacteria has helped reduce the transmission of diseases, such as typhoid fever, tuberculosis, scarlet fever, polio, and dysentery. (Also see Typhoid Fever.)

It's important to note that foods can become contaminated even after they have been pasteurized. For example, all pasteurized foods must be refrigerated. If temperature is abused (e.g., if milk or eggs are not kept refrigerated), pasteurized foods can become contaminated. Therefore, it's important to always handle food properly by following the 4 Cs.

How It Works: Foods are heat-processed to kill pathogenic bacteria. Foods can also be pasteurized using gamma irradiation. Such treatments do not make the foods radioactive. The pasteurization process is based on the following time and temperature relationship.

  • High-Temperature-Short-Time Treatment (HTST) - Using higher heat for less time to kill pathogenic bacteria. For example, milk is pasteurized at 161° F (72° C) for 15 seconds.
  • Low-Temperature-Long-Time Treatment (LTLT) - Using lower heat for a longer time to kill pathogenic bacteria. For example, milk is pasteurized at 145° F (63° C) for 30 minutes.

Note: The times and temperatures depend on the type of food and the final result you want to achieve, such as retaining a food's nutrients, color, texture, and flavor and using a high enough temperature for a long enough time to kill pathogenic bacteria.

Processes Used to Pasteurize Foods Include:

  • Flash Pasteurization - Involves a high temperature, short-time treatment in which pourable products, such as juices, are heated for 3 to 15 seconds to a temperature that destroys harmful micro-organisms. After heating, the product is cooled and packaged. Most drink boxes and pouches use this pasteurization method as it allows extended unrefrigerated storage while providing a safe product.
  • Steam Pasteurization - This technology uses heat to control or reduce harmful microorganisms in beef. This system passes freshly-slaughtered beef carcasses that are already inspected, washed, and trimmed, through a chamber that exposes the beef to pressurized steam for approximately 6 to 8 seconds. The steam raises the surface temperature of the carcasses to 190° to 200° F (88° to 93° C). The carcasses are then cooled with a cold-water spray. This process has proven to be successful in reducing pathogenic bacteria, such as E. coli O157:H7, Salmonella, and Listeria, without the use of any chemicals. St pasteurization is used on nearly 50% of U.S. beef.
  • Irradiation Pasteurization - Foods, such as poultry, red meat, spices, and fruits and vegetables, are subjected to small amounts of gamma rays. This process effectively controls vegetative bacteria and parasitic foodborne pathogens and increases the storage time of foods.

Some Examples of Foods That Are Commonly Pasteurized:

  • Whole Eggs Removed from Shells and Sold As a Liquid - Large quantities of eggs are sold to restaurants and institutions out of the shell. The yolk and whole-egg products are pasteurized in their raw form. The egg white is pasteurized in its raw form if it is sold as a liquid or frozen product.
  • Dried Eggs - If eggs are sold dried, the egg white with the glucose removed is normally heat-treated in the container by holding it for 7 days in a hot room at a minimum temperature of 130° F (54° C).
  • Whole Eggs Pasteurized in the Shell - Traditionally, eggs sold to customers in the shell have not been pasteurized. However, new time/temperature pasteurization methods are making this possible. Egg whites coagulate at 140° F (60° C). Therefore, heating an egg above 140° F would cook the egg, so processors pasteurize the egg in the shell at a low temperature, 130° F (54° C), for a long time, 45 minutes. This new process is being used by some manufacturers, but it is not yet widely available.

Note: Pasteurizing eggs reduces the risk of contamination from pathogenic bacteria, such as Salmonella, which can cause severe illness and even death. Pasteurized eggs in the shell may be used in recipes calling for raw eggs, such as Caesar salad, hollandaise or béarnaise sauces, mayonnaise, egg nog, ice cream, and egg-fortified beverages that are not thoroughly cooked.

  • Milk - Pasteurization improves the quality of milk and milk products and gives them a longer shelf life by destroying undesirable enzymes and spoilage bacteria. For example, the liquid is heated to 145° F (63° C) for at least 30 minutes or at least 161° F (72° C) for 15 seconds.

Juice, Eggs, Milk
Today, many foods, such as eggs,
milk, juices, spices and ice cream,
are pasteurized.

Sometimes higher temperatures are applied for a shorter period of time. The temperatures and times are determined by what is necessary to destroy pathogenic bacteria and other more heat-resistant disease-causing microorganisms that may be found in milk. The liquid is then quickly cooled to 40° F (4° C).

Other liquids, such as juices, are heat-processed in a similar manner. Temperatures and times vary, depending on the product and the target organism. Other types of milk pasteurization include:

  • Ultrapasteurization - This involves the heating of milk and cream to at least 280° F (138° C) for at least 2 seconds, but because of less stringent packaging, they must be refrigerated. The shelf life of milk is extended 60 to 90 days. After opening, spoilage times for ultrapasteurized products are similar to those of conventionally pasteurized products.
  • Ultra-High-Temperature (UHT) Pasteurization - typically involves heating milk or cream to 280° to 302° F (138° to 150° C) for 1 or 2 seconds. The milk is then packaged in sterile, hermetically-sealed (airtight) containers and can be stored without refrigeration for up to 90 days. After opening, spoilage times for UHT products are similar to those of conventionally pasteurized products.


> Pathogen
Any microorganism that is infectious or toxigenic and causes disease. Pathogens include parasites, viruses, and some fungi/yeast and bacteria.

The 12 "Most Unwanted" Bacteria
(For in-depth information on each
bacterium, see the individual terms)
1. Campylobacter jejuni Campylobacter jejuni
2. Clostridium botulinum Clostridium botulinum
3. Clostridium perfringens Clostridium perfringens
4. Escherichia coli O157:H7
(a.k.a. E. coli O157:H7)
Escherichia coli O157:H7
5.Listeria monocytogenes Listeria monocytogenes
6. Salmonella Enteritidis Salmone lla Enteritidis
7. Salmonella Typhimurium Salmonella Typhimurium
8. Shigella Shigella
9. Staphylococcus aureus Staphylococcus aureus
10. Vibrio cholerae Vibrio cholerae
11. Vibrio vulnificus Vibrio vulnificus
12. Yersinia enterocolitica Yersinia enterocolitica

Food Safety Implication: If food is not properly handled, pathogens can contaminate food and cause foodborne illness when the food is eaten. (See The 12 "Most Unwanted" Bacteria at right.)


> Perishable
Food that is subject to decay, spoilage or growth of pathogenic bacteria unless it is properly refrigerated or frozen.

Food Safety Implication: Perishable foods can spoil quickly and become unsafe to eat if they are not properly stored.

Food Safety Precautions:

  • When shopping, buy perishable foods last and only when you can take them home right away.
  • Always store perishable foods in the refrigerator or freezer to prevent bacterial growth.
  • Avoid leaving perishables out at room temperature for more than 2 hours.

Perishable Foods Include:

  • Dairy Products
  • Produce
  • Meat
  • Seafood
  • Poultry 

> pH
The measure of the acidity (less than 7) or alkalinity (greater than 7) of a solution.

Acidity is measured on a pH scale of 0 to 14, with a neutral pH being 7. The pH is acidic if it is lower than 7. The more acidic, the lower the number on the scale.

pH scale 0 to 14
Most bacteria will not grow at pH levels
below 4.6. Microorganisms thrive in a pH range above 4.6.

Food Safety Implication: Most bacteria will not grow in acidic foods with a pH level below 4.6. Microorganisms thrive in a pH range above 4.6. That's why acidic foods, such as vinegar and citrus fruits, are not favorable foods for pathogenic bacteria to grow in. However, bacteria may survive in the food.

Some examples of high-acid foods include: all fruits, except figs, most tomatoes, pickles, sauerkraut, jams, jellies, marmalades, fruit butters, and fermented and pickled (treated with brine or vinegar solution to inhibit the growth of microorganisms) vegetables.


> Phage Typing
A classification procedure that uses bacteriophages (viruses that infect bacteria) to distinguish between bacterial isolates that belong to the same genus and species.

Each bacterial strain will exhibit resistance to some phages and be susceptible to infection by others. A battery of standard phages is used to test bacterial isolates. The profile of resistance and susceptibility is called the phage type.

Food Safety Implication: Phage typing helps food scientists determine if bacterial isolates obtained from groups of ill people are different or indistinguishable from each other.


> Plasmid Transfer with E. coli
Plasmids carry a number of genetic elements, which can be transferred between and among related bacteria. For example, E. coli cells can naturally transfer some plasmids from cell to cell by the process of conjugation.

Food Safety Implication: Plasmid transfer may cause new foodborne pathogens to emerge, and sometimes genetic elements that plasmids transfer from one bacterium to another can make the receiving bacterium pathogenic or resistant to antibiotics.

Plasmid Transfer
Plasmid Transfer


> Point of Service (POS)
The establishment where an implicated food is consumed or sold to the consumer. An establishment can include restaurants, grocery stores, caterers, banquets, or a private residence.

> Preservation 
A variety of methods used at the processing stage and at home to keep food safe from harmful bacteria and extend the storage life of food. (Also see Canning and Freezing.)

Food Safety Implication: Foods are preserved so that they can be safely eaten at a later time. Some preservation methods include: canning, drying, freezing, smoking, and pickling (treating foods with brine or vinegar solution to inhibit the growth of microorganisms).

Food Safety Precaution: Read and follow all instructions on food labels for storage, handling, and cooking of foods.


> Pulse-Field Gel Electrophoresis (PFGE)
The DNA "fingerprinting" method that scientists use to determine the source of bacteria in foods.

Pulse-Field Gel Electrophoresis
Pulse-Field Gel Electrophoresis

Food Safety Implication: Bacterial DNA "fingerprints" are generated by pulse-field gel electrophoresis (PFGE), which can distinguish between different strains of a bacterium, such as E. coli. Similar PFGE patterns suggest that 2 different E. coli isolates - such as one from a patient and one from food - come from a common source, and identifying these connections can help determine the cause of an outbreak. Finding similar patterns can also help scientists determine whether an outbreak is occurring, even if the affected persons are geographically far apart.

How It Works: The DNA is first digested into pieces by reacting the isolated DNA with enzymes that are able to specifically break the DNA molecule into individual pieces. The digested DNA is placed at one end of the gel. A pulsing electric field applied across the gel drives the DNA pieces into the gel over a period of hours. The smallest pieces slip through the pores of the gel more quickly, so the pieces are separated as distinct bands in the gel, based on size. The resulting pattern of 30 to 50 bands, which resembles a bar code is the "fingerprint."

A DNA "Fingerprint"


> PulseNet
A national network of public health laboratories that performs DNA "fingerprinting" on foodborne bacteria. The network permits rapid comparison of these "fingerprint" patterns through an electronic database at the Centers for Disease Control and Prevention (CDC). This network was set up in 1995.

PulseNet Logo
PulseNet Logo

Food Safety Implication: Today, the use of PulseNet helps to better detect the source of foodborne bacteria in foods, which cause foodborne illness. Any one of the laboratories (approximately 70 laboratories at this publication) in CDC's PulseNet network can fingerprint E. coli in less than 24 hours, whereas the process used to take days or weeks.

Using DNA "fingerprint" technology, PulseNet helps public health authorities recognize when cases of foodborne illness are occurring at the same time in geographically separate locales. These illnesses may be caused by the same strain of bacteria and may be due to a common exposure, such as a common food that was eaten. An epidemiologic investigation of those cases can then determine what they have in common. Control measures can be initiated and outbreaks stopped.

How It Works: Laboratories participating in PulseNet perform DNA "fingerprinting" by pulse-field gel electrophoresis (PFGE) on disease-causing bacteria isolated from humans and from suspected food using standardized equipment and methods. Once PFGE patterns are generated, they are entered into an electronic database of DNA "fingerprints" at the state or local health department and transmitted to the CDC where they are filed in a central computer.

When PulseNet is fully operational, all participating laboratories will have a direct link with the central computer at CDC. These laboratories will be able to submit new patterns to the national database online and obtain epidemiologic information associated with patterns in the database. If patterns submitted by laboratories in different locations during a defined time period are found to match, the CDC computer will alert PulseNet participants of a possible multi-state outbreak, so that a timely investigation can be done.

Foodborne Illness-Causing Bacteria Currently Being Tracked by PulseNet: Currently, PulseNet participants perform DNA "fingerprinting" by PFGE on E. coli O157:H7 isolates. In February 1998, CDC introduced a standardized method for PFGE analysis of Salmonella serotype Typhimurium that uses the same equipment. Over time, additional foodborne illness-causing bacteria will be tracked by PulseNet depending on their public health importance and the availability of specific DNA "fingerprinting" methods for that pathogen.

The Step-by-Step Process for Tracing a Food Implicated in a Foodborne Illness Outbreak:

Is PulseNet the only national network that tracks pathogens?

No. Currently, PulseNet tracks only Salmonella, E. coli O157:H7 (and other Shiga-toxin producing E. coli), Listeria monocytogenes, Shigella, Campylobacter jejuni, and Vibrio cholerae. The Public Health Laboratory Information System (PHLIS), a national network of public health laboratories, tracks all organisms. PulseNet is a part of PHLIS. Both networks help to better detect a foodborne outbreak in multiple states.

  1. A person experiencing the symptoms associated with foodborne illness (diarrhea, abdominal cramps, nausea, fever, vomiting, or body aches) goes to the doctor. The doctor makes an initial diagnosis and stool cultures from the patient are sent to a clinical laboratory.
  2. Medical lab tests are done on the stool culture. If the presence of foodborne bacteria is determined by the local clinical lab, an isolate of that bacterial culture is sent to the state health department lab for further testing, including PFGE (pulse-field gel electrophoresis). The state health department lab sends the PFGE results electronically to the Centers for Disease Control and Prevention (CDC).

    The DNA "fingerprint" (PFGE result) is compared at CDC with other "fingerprint" samples from the other states that do PFGE testing (approximately 70 laboratories in 51 states and territories at this publication). If there are other states reporting this same pattern, CDC notifies - via an automated e-mail - the state health departments, along with the Food and Drug Administration (FDA) and the U.S. Department of Agriculture (USDA), the 2 agencies that are responsible for the safety of food that travels interstate.

    In other instances, local and state health departments identify an outbreak or cluster of sporadic cases and the suspected food source without PFGE testing. When other similar outbreaks or clusters of sporadic cases are identified in a particular state or other states, PFGE testing is then used to link them along with the fact that a common food is involved.

  3. When there are several isolates with the same PFGE pattern, state health departments investigate to identify a common exposure, such as a common food that was eaten. Local health department representatives use standard questionnaires to interview both sick and well persons. First, they want to find out whether the foodborne illness was due to a point of service (POS) mistake. The point of service could be such places as your table at home, a picnic, or a restaurant. Health department representatives look for common answers - for example, a common meal that was eaten by all or a common event that was attended.
  4. If it is determined that the foodborne illness was due to food that was served at a banquet or restaurant, for example, local health department representatives will interview the food workers. They use standard questionnaires to find out what and how foods were prepared. In some cases, if there was a sick food worker on the job on the day in question, stool cultures from the food worker may be requested to determine if he or she was sick from the same organism and if his or her PFGE matches the persons who became sick. If the worker was the source of the contamination, a food source investigation would not be needed.   
  5. Once the common food is identified and the food source is suspected, CDC notifies FDA or USDA (the agency that is notified depends on which agency has jurisdiction over the food), who in turn uses trace-back techniques to determine the source of the food.
  6. Once the traceback investigation has determined the source of the outbreak, steps are taken to prevent further exposure or spread of the infection. For example, if the source is a specific food being distributed, a recall of that food will be implemented. If the source is an infected food worker, that person will be removed from any food-service duties until they have recovered and tested negative for that bacteria.


> Pure Food and Drug Act
The passage of this act in 1906 allowed the government to gain control over the economic adulteration (impurities) of food and use of chemicals. This act set up mechanisms for protecting the food supply. The Food and Drug Administration enforces this act.