Subject: RE: biochemistry: food test

*Given only ethanol, water, and blotting paper,
*how does one differentiate (the fat content)between full-cream
*milk and skimmmed milk?

The answer to this question is chromatography.

Chromatography is the technique used to separate complex mixtures that relies on the differential affinities of substances for a gas or liquid mobile medium and for a stationary adsorbing medium through which they pass, such as paper or gelatin. By way of explanation, let's look at the example given in the question:

Milk is an emulsion made up of three parts: an aqueous (water-soluble) liquid phase; a non-aqueous (fat) liquid phase; and a proteinaceous (casein - the curds used for cheese) solid phase. In this case, the phase we need to identify is the non-aqueous liquid phase, which contains the milk fat. Milk fat, like all fats, will not dissolve in water, but it will dissolve in an organic solvent, like ethanol. This is convenient, because the aqueous and proteinaceous phases are much less soluble in ethanol than in water.

Now comes the real answer. First, place a drop of each sample of milk near the bottom of the blotting paper, such that each drop is equidistant from the edge, and let them dry. Next, mix the ethanol and water, and pour the mixture into a shallow pan. Next, dip the bottom edge of the spotted blotting paper evenly into the liquid, being careful not to dunk the spots. Hold the paper in this position for a while, and you will notice that the blotting paper acts as a wick. As the liquid travels up the paper, it will pick up the milk on the spots, and carry the milk up the paper with it.

As the different parts of the milk move with the liquid, they will spend some time in solution and some time stuck to the paper. The more soluble each part is, the more time it will spend in solution, and vice versa. The more time each part spends in solution, the farther it will travel up the paper. Milk fat is soluble in ethanol, so it would always be in solution; and milk fat is insoluble in water, so it would always be on the paper. So if you mix the ethanol and water correctly (about 3:1 ethanol:water), the milk fat should travel farther up the paper than the other parts of the milk, thus separating them. After the paper is removed and dried, the milk fat should appear as a spot furthest above the original spot. The more milk fat, the bigger the spot.

There are many other forms of chromatography, that we use in the lab on a daily basis, but they all work on the same basic principles.