Pernod's super-stable emulsion defies chemists' predictions

Love it or hate it, Pernod is well known for its distinctive flavour. But now it appears that it also has some distinctive - and baffling - chemical properties. 

According to Elke Scholten, now based at the Massachusetts Institute of Technology, US, and her colleagues in France and the Netherlands, the white emulsion that forms when the aniseed-flavoured alcoholic drink is mixed with water, defies the rules of theoretical chemistry. 

Pernod - the branded beverage akin to other anise-based spirits such as pastis, ouzo or sambuca - consists of a solution of ethanol and essential oils from star anise and other herbs and spices. The oil is primarily trans-anethol - an ester that gives the drink its characteristic smell. 

"The usual theories don’t apply here" - Elke Scholten

Because trans-anethol is insoluble in water, it forms a cloudy-white emulsion when diluted - a phenomenon familiar to any Pernod drinker, and known as the ouzo or pastis effect. 

Scholten’s new work shows that this emulsion exhibits very unusual properties. Her team used a ’drop tensiometer’ to measure the energy between the two liquids - this gives an indication of how stable the emulsion is and if the oil droplets will remain suspended in the solution. 

’The usual theories don’t apply here,’ Scholten told Chemistry World. Her experiments showed that, contrary to the usual behaviour of oil-water emulsions, the stability of the droplets increased as more ethanol was added to the mixture. Since the oil is soluble in ethanol, this would be expected to make the emulsion unstable and decrease the size of the oil droplets. 

We’re not sure what is missing exactly,’ said Scholten. ’It might be due to the fact that the interface [between oil and water] is not as sharply defined as for usual emulsion droplets.’ 

Stable emulsions can usually only be produced by shaking, disrupting the solution with ultrasound or adding a chemical that affects the surface tension of the solution. But Pernod and water spontaneuosly forms an emulsion - a process that could be valuable to industry if it can be harnessed. 

’This procedure could be used in low-energy emulsification,’ said Scholten. ’The droplets are fairly homogeneous in size and stable for a very long time, which is useful for the fabrication of nanocapsules.’ 

Victoria Gill