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Science

Cooking food and additives

Emulsions and emulsifiers

Immiscible liquids do not mix together. For example, if you add oil to water, the oil floats on the surface of the water. Then if you shake the two together, tiny droplets of one liquid become spread through the other liquid, forming a mixture called an emulsion.

Emulsions

Mayonnaise and emulsion paints are emulsions. The table describes two other emulsions.

Two types of emulsion

type of emulsionexampleminor componentmajor component
water in oilbutterwaterfat
oil in watermilkfatwater

In an emulsion, the oil and water gradually separate out again. Tiny droplets join together until eventually the oil is floating on the water again. To stop the two liquids separating, we need a substance called an emulsifier.

Emulsifiers

Emulsifiers are molecules that have two different ends:

  • a hydrophilic end - 'water-loving' - that forms chemical bonds with water but not with oils
  • a hydrophobic end - 'water-hating' - that forms chemical bonds with oils but not with water.

Lecithin is an emulsifier commonly used in foods. It is obtained from oil seeds and is a mixture of different substances. A molecular model of one of these substances is seen in the diagram.

Emulsifier molecules

The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil. In this way, the water and oil droplets become unable to separate out. The emulsion is stabilised.

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