Name (French louche = opaque, disreputable, suspicious) for the milkiness of a Aniseed spirit with the addition of water or very strong cooling. These are absinthe. Pastis and Pernod (France), aguardiente and pacharán (Spain), Aquavit (Scandinavia), arrack (East India), ouzo and Tsipouro (Greece), as well Raki (Turkey). However, the phenomenon is often referred to as "ouzo effect" after the Greek liquor.
With the help of the Louche effect, the anise content of these drinks can be compared. For the more turbid the liquid becomes at a certain mixing ratio, the more anise is in the spirit drink contain. The addition of water causes the clear liquid to opalize. The milky haze is formed by an oil-in-water emulsion which causes light scattering. The cause of the effect is the poor water solubility of the essential oil contained in absinthe anethole. It is therefore not a chemical but a physical phenomenon. The effect can also be achieved (without adding water) by cooling down the spirit.
The phenomenon is also known in nature as the Tyndall effect. It occurs especially on sunny afternoons in late summer. In the process, the sunlight is shattered by the smallest particles in the air, creating the impression that a golden mist is lying over the landscape.