primarily of water.
However butter, which is solid, is also made from milk.
And why is milk white under white light?
Put some water in a glass. It is transparent, with no colour.
Then add some corn oil (the same amount as of water): oil forms a layer on top of the water; this layer
is transparent, and yellow.
Using a small whisk or a fork, mix vigorously the oil and water mix, and look at the “moving mixture”,
under white light: the mixture is white.
Wait for a while: oil droplets that were formed during whisking slowly rise up, forming again a layer of
oil on top of the water; the water layer and the oil layer appear transparent again.
Why the appearance of white during whisking?
You would get the same effect when whipping egg whites: naturally, egg whites are transparent and
yellowish, but on whipping, as the whisk incorporates air bubbles (remember that air is transparent),
the mixture turns white. In this second case, one can more easily see that the whitening is due to the
reflection of white light on the air bubbles: after whipping, there are millions of air bubbles, and so
millions of reflections.
The same is true for the mixture of oil and water, but, in this first experiment, the reflections are on oil
Mix the oil and water mix again and again, and try to see them! By the way, which should be the colour
when light were red? And blue?
Milk is an “emulsion”. This is a strange word, that needs some explanation. It appeared in the XVII th
century, when chemists observed that some liquids were like milk: white, and thicker than water. As
milk is obtained from milking cows, the product was called an emulsion, from the Latin word emulgere,
which means “to milk”.
More generally, emulsions are dispersions of droplets of one liquid, in another liquid, with which it will
II - 7 (of 9)
Cream and Butter
Imagine that you have some raw milk just taken from the cow. It’s a thick and white liquid, as we have
Leave it for one night and then look at it the following morning: using a ladle, recover part of the upper