Most home-made ice-cream recipes you find contain eggs….why so? Eggs have multiple roles in cooking as leavening agent, thickening agent, a binder and as an emulsifier. In the ice-cream recipes egg yolk plays role of an emulsifying agent.
Before talking about emulsifiers in detail, we need to know that there are two types of liquids – water loving (hydrophilic) such as fruit juices, vinegar, alcohol etc. and water fearing (hydrophobic) dominated mostly by fat rich liquids such as oils and clarified butter. You must have tried mixing them in a salad dressing by shaking the bottle, but after sitting for a while they will be back to their own kind. Emulsifiers are agents that have capacity to bring these liquids together. Unlike water or oil, emulsifiers have a hydrophilic and a hydrophobic region in their structures. This allows them to identify with both the immiscible liquids, thereby creating a bridge to connect and retain them together.
Lecithin found in egg yolk is an emulsifier. During ice-cream making addition of lecithin prevents separation of water and fat particle found in milk resulting in a smoother ice-cream. Because of the same reason eggs are used in mayonnaise and hollandaise sauce.
Other emulsifiers used in cooking are mustard, soybean and honey. Mustard as an emulsifier is used in many salad dressing. Soybeans emulsifying properties come from lecithin, the same molecules that give this property to the egg yolk. Lecithin extracted from soybean is found in chocolates as an additive. It brings cocoa butter and other ingredients together and gives smoother texture to the chocolate.