Cheese is a solid food made from the curdled milk of cows, goats, sheep, 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. 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 or not 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. Substitute "vegetable rennets" have been extracted from various species of the Cynara thistle family. In some societies, stored cheese is a hedge against famine and a good travel food. It is valuable for its portability, long life, and high content of fat, protein, calcium, and phosphorus. Cheese is lighter-weight, more compact, and has a longer shelf life than the milk from which it is made. Cheesemakers can place themselves near the center of a dairy region and benefit from fresher milk, lower milk prices, and lower shipping costs. Cheese's substantial storage life lets a cheesemaker sell when prices are high or money is needed. Some markets even pay more for "aged" cheeses, exactly the opposite case from conventional milk production. Cheeses are eaten raw or cooked, alone or with other ingredients. As they are heated, most cheeses melt and brown. Some cheeses melt smoothly, especially in the presence of acids or starch. Cheese 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 rarebit. 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.