What causes chicken fat on the surface of soup, and does it add flavor to the dish?

Although the fat may add some flavor to the soup it also changes the mouthfeel. Instead of having a nice clear broth soup which is light and refreshing the fat in the soup with coat your mouth leaving a greasy feeling and taste. I cant be completely sure but if you were using the chicken thigh to just add flavor and not leave the meat in the soup you could use a chicken stock that your either make or buy from the store. As was stated above you could remove the fat from the soup. I would like to add another method to what Foodrules stated.

-Take a metal ladle and fill it with ice. Dip the ladle into your soup being care not to go deep enough to actually get any liquid in the ladle. They key here is that the cold ladle will solidify the fat and have it stick to it. You just remove the ladle wipe it clean with a paper towel and repeat until the fat layer is gone from your soup. Its very effective and easy.

The fat separates because boiling water is hot enough to render it from the meat, liquefying it and allowing the fat to float on top of the denser water.

In a professional setting, we skim the fat off the surface of stocks using a ladle, or by chilling it until the fat coagulates on top. Having tasted the fat, I would say it doesn't really add much in flavor, and it imparts a very unpleasant greasy texture. Stocks taste "cleaner" after the fat is skimmed.

If there's a solid layer of fat on top of your broth, it makes sense to remove it, but I wouldn't worry about a few spots of grease.

I have no proof, but I like the chicken fat and chicken fat usually enhances the flavour of soup better, so I would keep a bit of the fat there. Another way to reduce the fat is to use Chicken bones. I am not sure if they sell chicken bones in your area, but chicken bones still releases good chicken taste and it has a lot less fat.

A few ways below to take the fat out:

• One of the most effective way is what I call the cold method. It's not the best, but it's one that virtually requires no tool. Let the soup cold or even put in the fridge, then you will see a layer of fat on the soup. You simply take that layer of fat out

• The other method is to use coffee filter and something a coffee filter. You can easily get the fat out

• The less effective, but the easiest way is to use a spoon to try getting all the fat when the soup is boiling. You will notice the fat are usually on the top of the boiling bubble

To address the original question, the solid fat in the meat is melted by the hot water and since fat is less dense than water it floats to the top. Should you remove it? When I make chicken broth I do skim the fat, but I like to leave enough that there are some droplets on the surface. This gives a richer mouthfeel and flavor without making the soup greasy or heavy. The exception is if I am making broth to freeze. In that case I leave all the fat, or even add some if the broth is lean. The solid layer of fat on top protects the broth from freezer burn. When I take the broth out of the freezer I use a butter knife to pop the fat cap off before defrosting. I do save chicken fat from soup and use it instead of butter or olive oil when I cook onions.

To answer Walter's question, fat helps preserve heat in a bowl of soup by creating a barrier to evaporation. The soup in the bowl is cooling off in two ways: by radiating heat and by evaporative cooling at the surface. If you prevent the evaporative cooling by placing a barrier on the surface of the liquid (such as a layer of plastic wrap (not recommended) or a layer of fat or oil, the liquid will cool much more slowly. For the same reason a latte, with a layer of foam on top that prevents evaporative cooling, will stay hot in a glass much longer than a cafe leche made with coffee and milk but no foam.

One more comment

One of the reason japanese ramen soup usually has a bit of fat in it, it is to do with their cold weather. You will notice that their chicken and pork base soups always have a bit of fat in it. The fat in the soup help to preserve heat in the soup, so it is quite ideal in cold weather and the ramen will always arrive hot in front of their customers.

Not all the soups are fat. The soy base soup has not fat in it so as the miso base ramen