Foods that can be safely stored at room temperature, or "on the shelf," are called "shelf stable." These non-perishable products include jerky, country hams, canned and bottled foods, rice, pasta, flour, sugar, spices, oils, and foods processed in aseptic or retort packages and other products that do not require refrigeration until after opening. Not all canned goods are shelf stable. Some canned food, such as some canned ham and seafood, are not safe at room temperature. These will be labeled "Keep Refrigerated."

In order to be shelf stable, perishable food must be treated by heat and/or dried to destroy foodborne microorganisms that can cause illness or spoil food. Food can be packaged in sterile, airtight containers. All foods eventually spoil if not preserved.

Napoleon is considered "the father" of canning. He offered 12,000 French francs to anyone who could find a way to prevent military food supplies from spoiling. Napoleon himself presented the prize in 1795 to Chef Nicholas Appert, who invented the process of packing meat and poultry in glass bottles, corking them, and submerging them in boiling water. Without realizing it, he sterilized them, stopping bacterial spoilage and growth.

This military secret soon reached England where, in 1810, Peter Durance patented the use of metal containers for canning. Englishman William Underwood migrated to Boston and established a canning plant in 1821. This was the beginning of canning in the United States. Underwood (even today, a brand of "deviled ham") is America's oldest canning company.

Canning is a way to store food for long periods of time. It is a method of preserving where food is placed in airtight, vacuum-sealed containers and heat processed at 250 °F (121 °C). This destroys microorganisms and inactivates enzymes. As the food cools, a vacuum seal is formed that prevents any new bacteria from getting in. Since the food in the container is commercially sterile, it does not spoil. Once the container is opened, however, bacteria can enter and begin growing in the food. Any unused portions must then be refrigerated in clean containers.

The canned food industry in the United States stopped using lead-soldered cans in 1991. In 1995, the Food and Drug Administration issued a final rule prohibiting the use of lead solder in all food cans, including imported products. Metal cans, which are made of sheet steel — sometimes with a coating of tin — are now welded closed at the seams. The inside of the can may also have an enamel or vinyl protective coating.

Commercial canning is done under tightly controlled conditions — careful sanitation and the necessary time and temperature under pressure, but there are still limits to how long it will preserve food. There are several factors that limit the shelf life of canned foods. First, cans can rust over time. Shipping accidents, where cans fall and dent or are crushed, also cause container problems.

Then there's can corrosion. In all foods, but especially in high-acid foods like canned tomatoes, natural chemicals in the food continually react with the container. Over several years, this can cause taste and texture changes, and eventually lower the nutritional value of the food.

High temperatures (over 100 °F) are harmful to canned goods too. The risk of spoilage jumps sharply as storage temperatures rise. In fact, canned goods designed for use in the tropics are specially manufactured.

Store canned foods and other shelf stable products in a cool, dry place. Never put them above or beside the stove, under the sink, in a damp garage or basement, or any place exposed to high or low temperature extremes. Temperatures below 85 °F are best. Check your pantry every few weeks and use canned goods you have had on hand for awhile. Don't purchase bulging, rusted, leaking, or deeply dented cans.

Yes. Unused portions of canned food may be refrigerated in the can, but to preserve optimum quality and flavor, place the unused portion in a glass or plastic storage container.

Some canned hams are shelf stable and can be stored in the pantry up to 2 to 5 years at room temperature. These hams are generally not over 3 pounds in size. They have been processed to kill all spoilage bacteria and pathogenic organisms such as Clostridium botulinum, Salmonella, and Trichinella spiralis. The product is free of microorganisms capable of growing at ordinary room temperatures. However, unusually high temperature storage — above 122 °F (50 °C) — may result in harmless thermophylic bacteria multiplying and swelling or souring the product. If this happens, it should not be eaten.

Canned hams purchased refrigerated and bearing the "Keep Refrigerated" designation on the label are not safe to store at room temperature. This type of ham has been processed at a time/temperature sufficient to kill Trichinella spiralis. However, the normal cooking process for this product will not destroy the spores of Clostridium botulinum and Clostridium perfringens should they be present on the raw hams before canning. These two bacterial pathogens can grow if the perishable canned hams are not kept refrigerated. Consequently, the consumption of these canned hams may result in foodborne illnesses. "Keep Refrigerated" canned hams are also not free of spoilage bacteria that may eventually grow. Such hams may be stored in the refrigerator for up to 6 to 9 months.

Discard heavily rusted cans. Cans that are heavily rusted can have tiny holes in them, allowing bacteria to enter. Surface rust that you can remove by rubbing with your finger or a paper towel is not serious. You can keep these canned foods. If you open the cans and there is any rust inside, do not eat the food. Rust (oxidized iron) is not safe to eat.

If a can containing food has a small dent, but is otherwise in good shape, the food should be safe to eat. Discard deeply dented cans. A deep dent is one that you can lay your finger into. Deep dents often have sharp points. A sharp dent on either the top or side seam can damage the seam and allow bacteria to enter the can. Discard any can with a deep dent on any seam.

Cans of food that freeze accidentally, such as those left in a car or basement in sub-zero temperatures, can present health problems. Frozen cans could swell because the food inside expanded when frozen. However, cans can be swollen because of contamination with Clostridium botulinum or spoilage-causing organisms. Do not use any swollen cans; discard them.

Also, discard frozen cans that are not swollen but have been allowed to thaw at 40 °F or higher. Cans that have thawed and refrozen are not safe.

A frozen can that has not thawed can be safely defrosted in the refrigerator and used. If the canned food is still frozen, let the intact can thaw in the refrigerator before opening. If the product doesn't look and/or smell normal, throw it out. Do not taste it!

If the product does look and/or smell normal, thoroughly cook the contents right away by boiling for 10 to 20 minutes. At altitudes below 1,000 feet, boil foods for 10 minutes. Add an additional minute of boiling time for each additional 1,000 feet elevation (for example, at 3,000 feet, boil for 12 minutes). Spinach and corn should be boiled for 20 minutes at all altitudes. This is due to the high density of these vegetables. Products can then be refrigerated or frozen for later use.

Some cans make a hissing sound when opened because they are vacuum-packed and the noise is a result of air pressure. This is perfectly normal. However, if a can hisses loudly or the contents spurt forcefully out of the can when opened, it may be an indication that the food is unsafe. Do not taste or use such food. Place the can and its contents in a heavy garbage bag. Close and place the bag in a regular trash container or bury it in a nearby landfill.

Canned seafood occasionally contains small fragments of a glass-like substance. According to the U.S. Food and Drug Administration (FDA), these crystals pose no danger to consumers. Known chemically as magnesium ammonium phosphate, commonly called "struvite," the crystals can form from certain natural constituents of the fish or shellfish after they are commercially canned.

While extremely rare, a toxin or poison produced by the bacteria Clostridium botulinum ( C. botulinum) is a very serious danger in canned goods. Botulism is a deadly food poisoning. The botulism bacteria — rod shaped under the microscope — grow best in anaerobic (absence of oxygen) conditions. Since the canning process forces air out of food, the C. botulinum bacteria may find incorrectly or minimally processed canned foods a good place to grow and produce the toxin. Low-acid vegetables such as green beans, corn, beets, and peas, which may have picked up C. botulinum spores from the soil, are at risk.

The botulism spores are heat-resistant, can survive in foods that are incorrectly or minimally processed, and are difficult to destroy. While high cooking temperatures will kill the normal C. botulinum organism, it takes even higher temperatures to kill the spore. That's why the canning of low-acid foods is done with a pressure canner. If the spores are not killed in the canning process, they can become normal cells again and produce the deadly toxin.

If you eat C. botulinum-contaminated food, symptoms will develop in 12 to 48 hours. The poison attacks the nervous system, causing double vision, droopy eyelids, trouble swallowing, and difficulty breathing. Without treatment, a patient can die of suffocation — the nerves no longer stimulate breathing. There is an antitoxin, which has reduced the number of deaths from botulism, but patients may still suffer nerve damage, and recovery is often slow.

To avoid botulism, carefully examine any canned food that looks suspicious. The risk is greater if containers have been canned at home without following safe canning procedures. Never use food from containers showing possible botulism warnings — leaking, bulging, or badly dented cans; cracked jars or jars with loose or bulging lids; canned food with a foul odor; milky liquids surrounding the vegetables that should be clear; or any container that spurts liquid when you open it. Don't even taste the food!

Throw canned goods that are suspect away carefully. You don't want animals, children, or anyone else who might rummage through the trash to get ill. Double bag the cans in plastic bags that are tightly closed. Then place them in a trash receptacle (non-recyclable trash) outside of the home.

The botulism risk is greater for home-canned foods that have not been processed using safe canning guidelines. If it is possible that any deviation from the USDA-endorsed methods occurred, to prevent the risk of botulism, low-acid and tomato foods should be boiled in a saucepan before consuming even if you detect no signs of spoilage.At altitudes below 1,000 feet, boil foods for 10 minutes. Add an additional minute of boiling time for each additional 1,000 feet of elevation. However, this is not intended to serve as a recommendation for consuming foods known to be significantly underprocessed according to current standards and recommended methods. It is not a guarantee that all possible defects and hazards with non-recommended methods can be overcome by this boiling process. The recommendation is to only can low-acid and tomato foods. Properly home-canned fruits have never needed the 10-minute boil.

Except for infant formula and some baby food, product dating — having a "use-by," "sell-by," or "best-if-used-by" date — is not required by Federal regulations. Dating is for quality, not safety. However, if a calendar date is used, it must express both the month and day of the month (and the year, in the case of shelf-stable and frozen products). If a calendar date is shown, immediately adjacent to the date must be a phrase explaining the meaning of that date, such as "sell by" or "use before." While there is no uniform or universally accepted system used for food dating in the United States, dating of some foods is required by more than 20 states. A shelf-stable product can be safely used after the "sell-by" date. Products displaying a "use-by" date, although still safe, may not be of acceptable quality after the "use-by" date.

Cans must exhibit a packing code to enable tracking of the product in interstate commerce. This enables manufacturers to rotate their stock as well as locate their products in the event of a recall. These codes, which appear as a series of letters and/or numbers, might refer to the date or time of manufacture. They aren't meant for the consumer to interpret as "use-by" dates. There is no book or Web site that tells how to translate the codes into dates; however, you can contact the manufacturer for further information. Cans may also display "open" or calendar dates. Usually these are "best-if-used-by" dates for peak quality.