Leave a slice of American cheese on a windowsill and after weeks it will dry, darken in color and curl. But rarely will it mold. Individually wrapped orange cheese slices melt smoothly on burgers and taste great straight from the package, but in most cases these soft slivers of heaven aren't technically "cheese" at all. More likely, they fall under the title of "pasteurized process cheese," "pasteurized process cheese product" or "pasteurized process cheese food."

Regular cheese, like Cheddar, for example, is made by heating milk, stirring in enzymes and cultures, separating the curds from the whey, salting and knitting the curds into a block or wheel to age.

Pasteurized process cheese, on the other hand, is a mixture of already-made cheeses that are reheated, blended together, pasteurized and mixed with an emulsifier to provide a uniform texture, mild taste, smooth mouthfeel and the consistent

melt that many people love.

Food technologists can lower the fat and then add flavor back in or create a variety of textures and tastes. Often, these "light" cheeses have less fat and fewer calories than regular cheese because they contain less actual cheese, more moisture and other additions. But with up to 20 ingredients, not to mention layers of extra processing, this "cheese" is far from its milky roots.

A small piece of true cheese offers much more flavor and satisfaction than a larger serving of processed cheese, which is why you don't need as much, says Max McCalman, author of Cheese, A Connoisseur's Guide to the World's Best. "If I'm hungry I'll eat the fake stuff if that's all I have, but even my daughter's dog knows the difference: he's thrilled to eat the rinds of real cheese, but if it's a processed cheese slice he often won't finish it."