For aficionados simmering steel pots of aromatic broth, or those of us reaching for the easy-open can in the cupboard, one mystery of soup is the tendency of its ingredients to separate. The answer to what causes that filmy layer on the top of warming stock, or the thick layer that congeals on the leftovers in the refrigerator is a scientific one.
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Separation in Cooking
As the density of fat is lower than the density of water, fat floats on the surface of water. When making soup, recipes encourage cooks to begin with cold water that is brought to a long, slow simmer. This process allows fats to stay separate from the broth, rising to the top for skimming throughout the cooking. If the water comes to a boil, the fat will melt, becoming inseparable.
Add an egg white during cooking to further clarity stock. It will serve as a type of net to gather impurities, sending them to the surface for skimming.
Separation in Storage
When soups sit in storage, at room temperature, the ingredients will settle according to density, with the heaviest at the bottom, layering up through to the clear broth or oil at the top. This is a simple matter of density bringing the soup into a stratified organization requiring a good stirring to recombine all of the elements.
Separation in Refrigeration
Similar to the phenomena of separation in cupboard storage, soup will also separate by density in the refrigerator. Also, when chilled, the fats in a soup will gather and congeal. This is a convenient tool for removing fat from recipes— simply refrigerate the stock for a day before using and skim the collected fat from the top.