Milk is nature's buffet for babies, providing all the nutrition a mammalian infant needs for early development outside the womb. Cow's milk provides a range of nutrients for humans, although intolerance to lactose — a difficult-to-digest sugar — is a common affliction.
Milk consists of about 5 percent lactose, 3.7 percent fats and 3.5 percent proteins. Calcium-rich casein is by far the most common protein, and it is the combination of casein and certain fats that give bovine juice its color.
The color white is nature's light buffet, in that it results from all the wavelengths of visible light being reflected into the eye. Casein and certain fats reflect wide ranges of wavelengths, causing milk to appear white.