Cheese is a solid food made from the curdled milk of cows, goats,
sheep, water buffalo or other mammals. The milk is curdled using
some combination of rennet (or rennet substitutes) and acidification.
Bacteria acidify the milk and play a role in defining the texture
and flavor of most cheeses. Some cheeses also feature molds, either
on the outer rind or throughout.
There are hundreds of types of cheese produced all over the world.
Different styles and flavors of cheese are the result of using different
species of bacteria and molds, different levels of milk fat, variations
in length of aging, differing processing treatments (cheddaring,
pulling, brining, mold wash) and different breeds of cows, sheep,
or other mammals. Other factors include animal diet and the addition
of flavoring agents such as herbs, spices, or wood smoke. Whether
the milk is pasteurized may also affect the flavor.
For a few cheeses, the milk is curdled by adding acids such as vinegar
or lemon juice. Most cheeses, however, are acidified to a lesser
degree by bacteria, which turn milk sugars into lactic acid, followed
by the addition of rennet to complete the curdling. Rennet is an
enzyme traditionally obtained from the stomach lining of young cattle,
but now also laboratory produced. It acts by breaking down the major
milk protein casein into small fragments, leading to coagulation.
Substitute "vegetable rennets" have been extracted from
various species of the Cynara thistle family.
Cheeses can be eaten either raw or cooked, alone or with other ingredients.
As they are heated, most cheeses melt and brown. Some cheeses, like
raclette, melt smoothly; many others can be coaxed into doing so
in the presence of acids or starch. Fondue, with wine providing
the acidity, is a good example of a smoothly-melted cheese dish.
Other cheeses turn elastic and stringy when they melt, a quality
that can be enjoyed in dishes like pizza and Welsh rabbit. Some
cheeses melt unevenly, their fats separating as they heat, while
a few acid-curdled cheeses, including halloumi, paneer and ricotta,
do not melt at all and can become firmer when cooked..