HOW COME? Why milk is white
What gives milk its white color? asks a reader. Also, why does skim milk look more bluish than regular milk?
Got milk? If so, it's white.
Mammals make white milk in their mammary glands, which evolved from sweat glands. But instead of producing salty water, the mammary glands make food for newborn animals.
All mammal milk contains protein, fat, milk sugar (lactose), minerals, vitamins and other nutrients.
Cow's milk is about 87 percent water, with about 4 percent fat by weight. We all know milk contains calcium, and that calcium tablets (such as Tums) are naturally white. But it turns out that the reason why mammal milk is white is both more complicated and more interesting.
While we think of milk as a uniform liquid, it's actually an emulsion -- a combination of substances that can't be perfectly mixed. Salad dressing is an emulsion, which is why we have to shake the bottle to mix up the oil, vinegar and water before we pour it on.
Look at milk under a powerful microscope, and you'll see particles of protein and fat suspended in water. (In fact, the word "emulsion" comes from a Latin word meaning "to milk.")
About 80 percent of the protein in cow's milk is in the form of casein. Casein's long-chain molecules link up to form bumpy spheres about 90 to 150 nanometers across. (A nanometer is a tiny billionth of a meter.) These protein spheres also contain most of the milk's calcium and half of its phosphate, minerals essential for bone growth in baby mammals.
Scientists call these tiny protein balls "micelles." Each teeny micelle has a diameter less than the wavelength of visible light. So the micelles are actually too small to reflect light like a mirror. However, they're the perfect size to scatter white light every which way when it streams through a glass of milk. Meanwhile, the tiny globules of fat scatter light, too. The combined effect makes us see white.
However, there's more to the white milk story. White light contains a hidden rainbow of colors. Its shortest wavelengths -- the blues and violets -- are scattered more strongly than the longer-wavelength reds. But the particles in a glass of milk are so concentrated that the blue tint is drowned out by the scattering of the rest of the colors. However, skim milk lacks the fat of regular milk. So the light-scattering is done almost solely by the protein particles -- giving skim milk a bit of a bluish tint. Dilute milk with water and shine a flashlight through, and you'll see even more blue.
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