The Microscopic Jungle

• A variety of microscopic organisms can cause illness, including bacteria, viruses, molds and parasites. Collectively, they're referred to as pathogens. Like any other living creatures, they require favorable conditions to grow and flourish. Foods that are too acidic, or too high in salt or sugar, discourage the growth of pathogens. They can be killed by irradiation, high temperatures or, in some cases, by freezing. Most pathogens require oxygen to survive, and they all need a certain amount of moisture. Pathogens reproduce most freely at temperatures between 40 and 140 degrees Fahrenheit, referred to as the food safety "danger zone." That's why refrigeration keeps foods safe, even when every other factor favors the pathogens.

Letting the Atmosphere In

• Most shelf-stable foods are processed to eliminate any bacteria that might have been present, through heat, irradiation or other processes. Then they're sealed in cans, jars or airtight pouches, which prevent air from contacting the food. That means pathogens from the outside can't enter the food and infect it. It also means that any bacteria or spores surviving in the food are deprived of oxygen by the packaging, and won't be able to grow. Once you open the packaging, these foods become perishable and must be refrigerated.

Letting the Atmosphere Out

• Sometimes manufacturers take an additional step toward ensuring an oxygen-free atmosphere inside their packaging. Instead of drawing the air out of their packaging and creating a vacuum, they force the air out by filling the package instead with an inert gas, such as nitrogen. Any pathogens in the food suffocate in the oxygen-free environment, just as humans would. When the packaging is opened, the inert gas escapes and the outside air -- with its supply of fresh oxygen -- enters. At that point, the food becomes perishable again and must be refrigerated. Convenience foods such as sandwiches and baked goods are often packaged this way.

Refrigeration for Quality

• To make the whole issue of refrigeration and food safety just a bit more confusing, many manufacturers recommend refrigerating their products for quality reasons rather than food safety. These foods are safe from pathogens because of their high levels of salt, sugar or acidity, but are labeled "refrigerate after opening" to extend their storage life or because they simply taste better when cold. For example, pickles can safely be held at room temperature, but they often lose their crisp texture as a result. Most condiments are also safe at room temperature, but taste better or resist separating when they're kept cold.