Canned food is a modern convenience most of us take for granted, but metal food cans are a surprisingly recent invention. Early versions contained lead, but today's canning materials of steel, tin and aluminum have evolved into products that are lightweight and versatile, and give canned goods a shelf life ranging from one to five years. Metal cans ensure that food is unspoiled, tamper-free and maintains its nutritional value.

Canning was first invented because Napoleon needed his armies supplied with unspoiled food. The first versions weren't metal but rather glass containers that kept food from spoiling when the jar was heated with an airtight seal. The next step was to use tin cans, which contained lead. Once that was deemed unsafe, lead-free steel was pressed into service. Today, to kill harmful bacterial, cans go through a variety of processes such as pasteurization, boiling, even irradiation.

The majority of cans used for canned foods around the world today are made from steel. Although there is a thin layer of tin on the surface, there is only 5 to 6 pounds of tin per one ton of steel used. Some people still use the term "tin can" to refer to this type of packaging. The Can Manufacturers Institute reports that more than 1,500 types of food items are packed into steel cans of various shapes and sizes. Newer innovations include cans with twist-top, resealable lids, as well as the easy-open type with pull-tabs on the lids. Steel cans are also environmentally-friendly, 100 percent recyclable and made from a minimum of 25 percent recycled content.

Aluminum is preferred for beverages because the metal is easily molded and resistant to corrosion and rust. Aluminum also withstands pressures of up to 90 lbs. per square inch, important for carbonated drinks. Although it's usually easy to tell aluminum cans from steel by sight, one way to differentiate the two is to use a magnet; steel will stick to it, but aluminum won't.

Almost all metal food and beverage cans contain an industrial chemical called bisphenol A, or BPA. The chemical is used in epoxy resins that coat the inside of food cans. There is a growing concern from scientists and health organizations like MayoClinic.com that BPA can seep into food or beverages. This is a problem because BPA has been linked to prostate and breast cancer, heart disease and damage to the prostate gland of fetuses, infants and children. To avoid this danger, look for cans labeled BPA-free.