This is used to make food products that are more solid and become less rancid at room temperature,
such as margarine.
Solubility in water
When liquid fats are added to water, the two liquids will not mix. This is because fats are neutral, so
are not attracted to the water molecules. If oil and water are mixed, the oil will rise to the surface and
float because it is less dense than the water. In order to make a stable fat/water mix, surfactant
molecules (molecules that contain both hydrophobic and hydrophilic parts) are needed. A common
example of a surfactant molecule is a detergent molecule, which is used to remove fat stains from
clothes – the water insoluble part of the detergent inserts into the grease stain, leaving the water
soluble part in the washing water, so the grease stain can be removed into the water.
Effect of heat
Fats, unlike water, are very susceptible to small changes in heat (water does not change significantly
on heating it between 0°C and 100°C, although at 0°C and 100°C exactly very significant changes
occur). Heating up to their boiling point will reduce the fluidity of a fat source, whereas cooling to its
freezing point will cause a gradual increase in viscosity. This is because different parts of the fat
molecule will melt at different temperatures, so the whole molecule is not as uniform as molecules of
water in a water solution, where each molecule will boil at exactly the same temperature.
The boiling temperatures of fats are much higher than the boiling temperature of water, and range
between 260°C and 400°C depending on the fat source. For example, the boiling point of olive oil is
about 300 °C.
However, fats will begin to decompose at temperatures well below their boiling points. This will begin
at a temperature called their smoking point. For example, the smoking point of olive oil occurs at
around 200 °C. A smoking point can be detected by the release of smoke (seen as visible fumes) and
discoloration of the oil. The fats will start to decompose.