Dairy products are derived from milk, the secretion of the mammary glands of mammals, usually cows (bovine), sheep, goats, buffalo, mare, camel, or yak. Most dairy products originate from bovine milk and, to a lesser extent, sheep and goat milk. As milk contains approximately 80 to 90 percent water, it is prone to undesirable microbial growth with concomitant product deterioration. To prevent this problem from occurring, and to ensure a longer shelf life, milk is processed to form different products such as ice cream, cheese, milk powders, yogurt, butter, lactose, and anhydrous milk fat (also known as butter oil). Milk can be separated into a cream fraction and a skim milk fraction by a centrifugation technique called separation. This process concentrates the fat present in the milk into the cream phase, leaving a skim or partially skimmed phase with much lower fat content. The speed of centrifugation can be adjusted to yield different fat content in cream. Milk processing applies different preservation techniques to allow for longer storage of dairy products. Milk powders are produced by concentration to remove some of the water, followed by atomization into a fine mist and drying at high temperatures. Heat and dehydration (water removal) are employed to give a long shelf life for milk powders. Ice cream is a dairy product preserved by the action of freezing. Yogurt and cheese are both fermented products. A bacterial culture is used to inoculate milk, for which the primary function is to lower the pH from 6.7 (typical for fresh bovine milk) to 4.2 for yogurt and in the range 4.6 to 6.0 for most cheese varieties. The bacterial cultures also assist in breaking down proteins and fats in the milk product to develop some of the flavor. The preserving function of added bacterial culture is to compete with unwanted pathogens for nutrients. Thus cheese and yogurt are preserved by dehydration, acidification, and competition with pathogens for survival in the product.