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To emulsify means to combine two liquids that normally do not combine easily, such as oil and vinegar.
Fat and liquid by nature are unmixable, and the goal when mixing a recipe is to form a water-in-fat emulsion. A well emulsified cake batter, for example, should not be curdled or weeping liquid. This is because the butter and liquids are in a stable emersion. If not stable, the batter will loose air cells. This results in a baked cake that is grainy or flat in texture, dry and flavorless, look uneven and may even sink.
Emulsifying is done by slowly adding one ingredient to another while whisking rapidly. The whisking disperses and suspends one liquid throughout the other. A third ingredient, called a liaison or emulsifier, is added because the two ingredients will separate. The emulsifier stabilizes the mixture. Mayonnaise is a classic example of emulsification; it is mixture of oil and vinegar or lemon juice that is emulsified by the addition of egg yolk, which contains the emulsifier lecithin. Emulsifiers are also found in egg white, gelatin, skim milk and mustard.
For example, after creaming fat and sugar together, the first step in making a Buttercake or Pound cake, is to beat in the room temperature eggs, ONE AT A TIME with the mixer on low. Each one should be fully incorporated before adding more. Room temperature eggs, worked in slowly, not only helps to incorporate more air in the batter and but also adds emulsifiers slowly from the egg yolks and will not break the fat (from the butter and egg yolks) and water emulsion (from water contained in the butter or fat and eggs). It results in a creamy batter that holds in the air bubbles in, previously created through creaming and beating.
SARAH SAYS: You can add eggs, cold right from the refrigerator when using a powerful stand mixer. They will quickly warm from the action or friction from the beaters. You will notice a slight curdling in the creamed butter and sugar or creamed butter/sugar and egg mixture, but it will come together when you start to add in the flour, during the next step. In some baking recipes, such as with cakes, the emulsion begins with the butter, sugar and eggs and continues while you add the dry ingredients, such as the flour, and/or the cream, milk or buttermilk.