Understanding the photochemistry of anise-flavoured drinks could help to extend their shelf life, say researchers in France. 

Dario Bassani and colleagues at the University of Bordeaux 1, in collaboration with researchers at the drinks company Pernod Ricard, have studied the photochemistry of trans-anethole - the compound responsible for the distinctive flavour of drinks such as sambuca, ouzo and pastis. 

Photochemical reactions can dramatically alter the taste and smell of anise-flavoured drinks. 'Upon exposure to light...trans-anethole undergoes isomerisation to the cis isomer which, instead of the pleasant aroma of anise, possesses an aroma similar to that of hay,' said Bassani. 

When a concentrated ethanol solution of trans-anethole is rapidly diluted with water it spontaneously forms a cloudy microemulsion - a phenomenon known to many as the ouzo effect. Bassani's team wanted to find out how this emulsification would affect the photochemical reactions of trans-anethole. 

The researchers studied the effect of light on trans-anethole dissolved in ethanol-water mixtures of varying proportions. At higher water concentrations, emulsification occurs because anethole is soluble in ethanol, but not in water. The anethole molecules aggregate together within the emulsion droplets. 

The researchers found that when the anethole molecules were aggregated, they were much less photosensitive - the isomerisation from trans to cis, which is responsible for the change in aroma, was inhibited. 

Christian Bochet, an organic photochemist at the University of Fribourg, Switzerland, praised the research. 'Chemical reactions in confined media...have been increasingly studied in the last few years. Increasingly sophisticated nanocontainers have been designed, synthesized and studied. The beauty of this work is that such confined media can be found in very simple systems, such as commonly used alcoholic beverages,' he said. 

'While some research groups study the photochemical behaviour of styrene derivatives in expensive nano-beakers, Bassani shines light on a glass of pastis and gets similarly interesting results!' he added. 

Rachel Warfield