Biology Online, an informational website about biology, included this definition in its "Dictionary" section (accessed Feb. 20, 2007):
"Milk - A white fluid secreted by the mammary glands of female mammals for the nourishment of their young, consisting of minute globules of fat suspended in a solution of casein, albumin, milk sugar, and inorganic salts."
Douglas Goff, PhD, Professor of Food Science at the University of Guelph in Ontario, Canada, stated on his webpage "Dairy Chemistry and Physics" (accessed Feb. 20, 2007):
"The role of milk in nature is to nourish and provide immunological protection for the mammalian young... Milk is also a very complex food with over 100,000 different molecular species found. There are many factors that can affect milk composition such as breed variations, cow to cow variations, herd to herd variations - including management and feed considerations, seasonal variations, and geographic variations. With all this in mind, only an approximate composition of milk can be given: 87.3% water (range of 85.5% - 88.7%); 3.9% milkfat (range of 2.4% - 5.5%); 8.8% solids-non-fat (range of 7.9% - 10.0%) including protein (3.25%), lactose (4.6%), minerals (0.65%), acids (0.18%), enzymes, gases, and vitamins...
Due to its role in nature, milk is in a liquid form. This may seem curious if one takes into consideration the fact that milk has less water than most fruits and vegetables. Milk can be described as an oil-in-water emulsion with the fat globules dispersed in the continuous serum phase, a colloid suspension of casein micelles, globular proteins and lipoprotein particles, and a solution of lactose, soluble proteins, minerals, vitamins and other components."
Walter L. Hurley, PhD, Professor of Animal Science at the University of Illinois, provided the following definition on his website "Lactation Biology" (accessed Feb. 20, 2007):
"The newborn mammal is still very much dependent on the mother for provision of a nutrient supply that matches its digestive capabilities. This nutrient supply is milk...
Milk is of paramount importance to survival, proper development, and vigorous growth of the neonate [newborn]. Milk is the only supply of the water, organic nutrients, and minerals to which the neonate has access. Milk supplies everything to the neonate except air! It has a high caloric value and generally is balanced for the nutrient needs of the rapidly developing young of that species. Colostrum (the first milk taken from the mammary gland after parturition) and mature milk also contain nonnutrient substances (such as antibodies and bioactive factors) that may be important for growth, development, and survival of the neonate."
Britannica Concise Encyclopedia offered the following description (accessed Feb. 21, 2007):
"Liquid secreted by the mammary glands of female mammals to nourish their young. The milk of domesticated animals is also an important food source for humans. Most milk consumed in Western countries is from cows; other important sources include sheep, goats, water buffalo, and camels. Milk is essentially an emulsion of fat and protein in water, along with dissolved sugar, minerals (including calcium and phosphorous), and vitamins, particularly vitamin B complex. Commercially processed cow's milk is commonly enriched with vitamins A and D."