Natural cheeses can be classified a number of ways, the most common of which are: milk type (cow, goat, sheep, etc.); state, country, region, or place of origin; hardness; and whether / how the cheese was ripened.
Categorizing cheese by degree of hardness is the most universal and logical method used. It is also related to legal standards. Many cheese varieties have federal Standards of Identity that dictate allowable percentages of maximum moisture and minimum milk fat, and sometimes the age of the product. You will see examples of Federal Standards of Identity in the individual varieties we will cover. Since the amounts of moisture and milk fat in a cheese significantly affect its functional properties, the hardness category can be related to how a cheese performs in cold and hot cooking applications.
The following are basic guidelines for how moisture, milk fat level, and age can interact and affect cheese hardness.
Soft and fresh cheeses are more perishable because they have higher moisture content and, in many cases, bacterial cultures are not added during the cheese making process. Hard cheese types have lower moisture content and often have large numbers of bacterial starter or adjunct cultures present in them.
Cheese hardness can be affected by level of fat in the milk used as a starting ingredient. Some cheeses are made with whole milk, and some are made from partially skimmed milk. Cheeses that are made with whole milk have a creamier more buttery texture. Cheeses that are made from partly skimmed milk have a firmer more plastic texture.
As firmer cheeses age, their texture at first becomes somewhat softer. In the cases of firm cheese like Cheddar it has a dense, almost rubbery body as a young mild Cheddar, a creamier texture as a medium aged Cheddar, and a crumbly texture with more aging. Hard Italian-type cheeses such as Parmesan undergo a lengthy aging process during which moisture levels in the cheese decrease and complex flavors develop.