The mix for a stirred cream is made much as baked-custard mixes are. An especially rich cream may call for yolks only, as many as 4 or 5 per cup/250 ml milk. The eggs and sugar are mixed with scalded milk or cream, and the mixture is then stirred constantly on the stovetop until it thickens enough to cling to the spoon, at around 180°F/80°C. The gentle heat of a double boiler minimizes the possibility of curdling, but it takes longer than direct heat. The thickened cream is then strained of any coagulated egg or other solid particles, and cooled, with occasional stirring to prevent the proteins from setting into a solid gel. An ice bath will cool the cream quickly, but demands more frequent stirring to maintain an even texture. Fruit purees are generally added after the cooling, because their acidity and fibrous particles can cause curdling during the cooking.