Don't be alarmed.
You might be boiling chicken for chicken salad or a comforting casserole and you notice foam. It congregates around the edges of the pot and is somewhere between white and gray in color. That same foam tends to plague big pots of chicken stock made with chicken pieces or bones, and if you cook the stock long enough, you'll notice it breaks down and the broth becomes cloudy. Maybe the foam doesn’t bother you, or perhaps you’re the person who diligently skims it off. While it's safe to consume, maybe you’ve wondered why it's there in the first place. Let us explain that mysterious frothy layer that appears when you boil chicken.
What Is the Foam?
There is no appetizing way to say this, but that foam is made of coagulated chicken proteins.
"It is known as scum, not the most pleasant name. It is considered an impurity when in a stock and consists of coagulated proteins coming from any residual meat on the bones. The proteins transform from a liquid state to a solid state; they become firm and the process is irreversible," says Mark Traynor, Associate Professor and Director of Culinary Science at the Horst Schulze School of Hospitality Management at Auburn University.
As the cooking continues, the proteins tangle and bond to each other forming a network that traps air, which creates the foam and allows them to rise to the top of the pot.
Traynor notes that this also happens while cooking high-protein vegetables or legumes like lentils, chickpeas, or soy beans. You might also recognize this foam from poaching eggs, as the proteins from the egg whites are undergoing the same transformation.
Should You Skim It Off?
When it comes to boiling chicken, which will eventually be strained out of the water, there is no need to remove the foam. In stocks, where the cooking liquid will be retained, you want to consider removing it. Looks aren't everything, sure, but not only does the foam make the stock cloudy, it can impact texture and taste.
"It is in no way harmful at all. It is a food protein. However, it is undesirable due to its appearance, its texture, and its taste. The main issue is that when a stock boils, it becomes turbulent and can disperse the scum that has risen to the top of the stock back throughout the stock. This causes the stock to appear cloudy. Also, it can bind any other impurities such as fats, small bone fragments, or meat particles, and this can impact the texture, appearance, and taste," Traynor says.
Should You Boil Chicken and Other Meat for Stocks Separately?
Some people prefer to boil the meat and bones for stocks separately, then strain and rinse them before adding them back to the pot with aromatics. It's more work, but avoids having to deal with lots of foam and skimming.
"This is the classical way to make stocks and/or boil meats for soups or stews. Personally, I tend not to do this in order to save time. The consequence to this is that I have more scum to skim as the stock cooks," Traynor says.
If you want clear stocks for soups, you'll have to work for it either way. It's whether you want to plan ahead or would rather work as you go, skimming throughout the cooking process.
Tips for Removing the Foam
Sure, it's work to skim stocks, but Traynor says it can also be kind of therapeutic. To make things easier he has a few helpful tips.
Reduce the heat
Stick to a very gentle simmer, like the temperature you would use to poach an egg. Cooking the stock on a lower heat will help minimize the amount of scum that forms. "However, do not let the temperature drop too low as the stock can spoil or go sour," says Traynor.
Add an acid
Adding an acid (like vinegar or lemon juice) to the stock will speed up the protein coagulation. This will help the scum rise faster and in larger quantities, which will help reduce the amount of time needed to skim.
Use egg whites
A classic chef tip for clarifying stock (i.e. removing the foam) is to use egg whites. Traynor suggests to first strain and cool the stock. Then whip up two egg whites per gallon of stock in a pot and add the cooled stock. Heat and gently stir until the egg whites coagulate and rise to the top. This will form what chefs call a "raft," which collects the impurities in the stock and can be easily removed leaving clear stock behind.
Strain it well
The trick to clear stock is straining it through something with small holes; your colander just won't do. Pouring the stock through a fine strainer, coffee filter, or cheesecloth can better filter out that undesirable foam.
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