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Have you ever wondered why it is so maddeningly difficult to get some powdered foods to dissolve?
For instance, take cocoa powder. Drop a load of cocoa powder into a cupful of milk1 and stir. What happens? You end up with a floating mass of infuriatingly dry powder, and no matter how much you bash away at it with a spoon, it steadfastly refuses to get wet.
The answer lies in the structure of the powder. Each grain contains some cocoa fat, and fat has a low affinity with water. All other things being equal, oils and fats would much rather float on top of water than mix with it. Drop a load of cocoa powder into water, and the water-hating fat molecules will tend to clump together away from the liquid, hiding inside bubbles of air and powder. As you stir, some bubbles rise to the surface and burst, depositing their still dry contents to float on top of the liquid, and other bubbles are carried down into the milk, bearing their own load of fresh dry powder from above. All you are doing is making ever-smaller bubbles, all containing infuriatingly dry powder.
In some of the smallest bubbles, the grains near the outside are not far enough from the water to avoid wetting, but they then form a sticky mass surrounding the rest of the bubble's contents, protecting them from further wetting. A mug of cocoa in this state will take a long, long time to break down by stirring with a spoon.
Of course, as every cook knows, the trick is to use only a little milk to start with, and then to add the rest when all of the cocoa powder has dissolved.
Why Does That Work?
Although the lumps still form, they occur in such a small amount of liquid that the resultant mess is quite thick. In a full glass of milk, it is easy for the cocoa-filled bubbles to dodge the spoon; in a thick paste they cannot move so fast and the sheer force applied by the stirring is much greater. The bubbles are soon broken up so that all the cocoa grains are wet, after which they will easily dissolve in the rest of the milk.
What About Flour?
Powders which have a high affinity with water, such as flour, are equally hard to dissolve, but for a different reason. This is because the individual grains in a blob of flour will quickly absorb water, swell up, and stick together, forming a protective ball around any dry grains that may have been in the middle of the lump. Heating the liquid does not help as this only increases the affinity of the grains for water and causes more clumping.
The best solution is to mix the flour with a little fat to separate all the grains, and then to slowly add the milk or water. This is why you first add flour to melted butter when making a roux for gravy or white sauce.