Also known as the 'sugar-shortening' method, the sugar and shortening fat are blended together first and then creamed by added mixing.
During creaming, small air cells are formed and then incorporated into the mix. This mix becomes larger in volume and softer in consistency. The exact time for proper creaming is controlled by several factors: the temperature of the shortening or fat (21°C is best). Cold shortening (eg butter, margarine etc) is not plastic enough to incorporate quickly and hold air cells. By the same token, fats that are too warm (24°C or more) will not be able to hold as much air, nor give as much volume because they are soft and cannot tolerate the friction of the machine and constant mixing.
Sugar and shortening are creamed at a medium speed until soft and light. High speed mixing tends to destroy or reduce the number of air cells that are formed and incorporated during the early stages of mixing.
During the second stage, eggs are added in several portions. Very often, an inexperienced baker will add the eggs too quickly before they are absorbed in the mix and curdling will result. Egg contains albumin, or egg white, and the yolk. The yolk of the egg contains a fat that coats the surface of the cells formed in creaming and allows the cells to expand and hold the liquid added (egg whites, milk etc) without curdling.
Curdling is the result of having more liquid than the fat-coated cells have a capacity to retain.
A creamed mix that has been carefully mixed and does not curdle has a water-in-fat solution. Cake mixes that curdle are those in which the water or liquid has been released by the cells and created a fat-in-water emulsion.
Adding eggs too quickly or adding all the other liquid (milk) at once will cause curdling. Addition of a small portion of the flour at the start of the mix will help to eliminate curdling in mixes with high liquid content. Adding flour alternately with the liquid after the mix is creamed will also eliminate the curdling tendency. The batter is then mixed slowly until smooth and the flour completely mixed in and wet.