Many of Polish soups use cream as one of the ingredients. In general, any kind of the cream is suitable, however, for soups and sauces, Poles usually use cream with 18% fat.
Surprisingly, adding it to your dish is not as easy as it may sound, because proteins in the cream are prone to so-called denaturation. Denaturation is a process in which native structure of proteins is destroyed, leading to precipitation of little white tufts, which do not look appealing on the plate.
If you are pondering on how to add the cream to the soup so that it doesn't get denaturated, remember: never add the cream directly into the pot with a hot soup. It is high temperature what destroys proteins.
And here come traditional ways, which I have been taught by my grandmother and mother. You should not add the cream to the hot soup – quite contrary. Add a little portion of the soup to cold cream and mix it thoroughly. Repeat the cream becomes warm and only then pour it into the pot. While pouring, you should constantly vigorously stir the contents of the pot. Never pour the cream into the boiling soup – proteins become denaturated instantaneously.
There is another way, which meaning remains the same. It can, however, be more reasonable, as it is less time consuming. Simply pour a small portion of your soup into some vessel. Let the soup in the vessel (and in the pot) cool down. Pour the cream into the vessel and mix. Then, add this diluted cream into your soup, while stirring it vigorously.