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Cheese lovers all have a slightly different definition of melt, says John Brody, a technology principal and natural cheese specialist for Sargento Foods Inc., a Plymouth, Wis.-based cheese maker. Some people feel cheese is melted when it softens, for others "it has to bubble and flow," he says. The challenge of melting natural cheese smoothly has led many home and restaurant cooks to use processed cheese on cheeseburgers or for dishes like queso -- traditionally a mix of processed cheese, diced tomatoes and chilis. Processed cheese includes emulsifiers to stop protein separating from fat when heated, giving it a smooth melt. Other ingredients are often added to extend shelf life or achieve a particular color or flavor. Natural cheese is usually made of the same four primary ingredients: a lot of milk with a little bit of bacteria cultures, enzymes and salt. A cheese’s living bacteria can change the texture, flavor or how it melts.