Cream is a product dairy farmers produce from whole milk. In non-homogenised milk, the butterfat present rises to the top. Cream producers skim this fat layer off to make cream and make different grades of the product. The main difference between grades is the percentage of butterfat present. In the United Kingdom, two of the most common grades are double cream and single cream.
In Britain, single cream refers to a milk product that has between 10 per cent and 12 per cent butterfat. This relatively low level of butterfat creates a thin liquid similar to the U.S. product called half and half. In contrast, double cream in Britain refers to a cream product containing 48 per cent butterfat. This is richer than any cream product commonly found in the United States; our heavy whipping cream has about 40 per cent butterfat.
Because of its high milk content, single cream is too thin to be whipped into soft or hard peaks. Double cream, however, is an excellent cream for whipping. It whips quickly into soft and then hard peaks. Some cooks find that double cream is so thick it strains their mixer. They add about 1 tbsp of milk to every 1 cup of double cream to thin it for a better consistency for whipping. Because it whips so well, it is easy to overwhip double cream. This will produce butter and a thin liquid called whey. Cooks whipping double cream should stop the mixer every few seconds and lift the beater from the bowl to check if peaks are hard enough yet. Once overwhipped, it is not possible to turn butter back into cream.
Because double cream has more butterfat, it is more caloric. Double cream has 140 calories per 30ml which includes 15.2 gm of fat, 1 gm of carbohydrates and 1.9 gmof protein. Single cream is much lighter; the same serving size has 40 calories with 5.4 gm of fat, 1.2 gm of carbohydrate and 1 gram of protein.
It may not be convenient to keep several kinds of cream in the house. When purchasing only one kind of cream from U.K. grades, some chefs recommend double cream. This is because double cream can be thinned down with milk to make single cream if desired, but single cream cannot be converted into double cream at home. This makes double cream the more versatile product to keep on hand.