The Ouzo Effect
- Published: Mar 15, 2008
- Author: David Bradley
- Channels: NMR Knowledge Base
Fans of Ouzo will be all too familiar with the effect of adding water to their favourite tipple - the drink turns cloudy almost instantaneously. Now, new insights from NMR spectroscopy could explain why this happens and independent results on the same emulsification process might one day be exploited in novel drug formulations, food products, and cosmetics.
Although transparent when bottled, Ouzo, Pastis, Pernod, and other popular anise-flavoured alcoholic beverages form milky-white emulsions when diluted with water. The phenomenon is commonly known as the "Ouzo effect", a phrase coined by Vitale and Katz Researchers have demonstrated that these emulsions occur spontaneously and can remain stable for weeks and even months. It is a rare occurrence.
Indeed, there are few examples of two immiscible liquids spontaneously forming an emulsion in the absence of a suitable surfactant. Oil and water can be emulsified soap for instance, but form two separate liquid phases one floating above the other in the absence of the surfactant. This spontaneous behaviour could be attractive to the pharmaceutical and food and drinks industries perpetually aiming for stability of their products.
David Carteau, Dario Bassani, and Isabelle Pianet of the ISM-UMR at the University of Bordeaux in Talence, France, turned to NMR to help them follow the cloud-formation process in pastis. They used both DOSY (Diffusion Ordered Spectroscopy) and TOCSY (Total Correlation Spectroscopy) experiments to carry out their investigations. Stripped to its bare essentials, the ouzo effect occurs when an ethanolic solution of the essential oil trans-anethole is mixed with water to form a cloudy, metastable solution.
In 2007, Bassani and colleagues described a dynamic NMR study that led them to suggest a possible mechanism by which the aggregation of the cloud-forming colloidal particles might begin during the early stages of trans-anethole emulsification. "DOSY experiments indicate that the initially formed small aggregates undergo rapid coalescence to form larger droplets," explain the researchers, "Ostwald ripening of these droplets at the expense of the remaining small aggregates is responsible for the subsequent, slower time-evolution of the system." They also reported the effect of this aggregation process on the photochemical behaviour of the solution, having found the behaviour of the emulsion to be very different from trans-anethole in a non-colloidal state. This discovery could have implications for exending the shelf-life of various products.
The current research looks at the emulsification of trans-anethole in much more detail using powerful NMR technique. Analysis of the spectra showed that before reaching a molecular metastable state, trans-anethole molecules initially reorganize into small aggregates of a few tenths of a nanometre across. Because of their size, these aggregates are visible to standard liquid NMR experiments. However, once small aggregates coalesce in between these, small micrometre-size droplets form that are rendered invisible to NMR. The researchers describe this as a molecular "maturation" process.
In a related, but independent study Erik van der Linden of the Food Physics Group, in the Department of Agrotechnology and Food Sciences, at Wageningen University, The Netherlands and colleagues there and the INRA Group of Molecular Gastronomy, at UMR 214 INRA/AgroParisTech, in Paris, France, measured the stability of various emulsions prepared from commercial Pernod and compared the results to theoretical predictions of their formation.
van der Linden and his colleagues measured the interfacial tensions between growing droplets and used these data to predict droplet growth rates using an Ostwald ripening model and a model for "creaming" of the droplets. However, they found that their experimental observations were often the opposite of the predicted behaviour of the emulsions depending on the concentrations of the essential oil, water, and alcohol.
"More knowledge of the parameters that determine the stability of these emulsions, besides interfacial tension, solubility, and density difference, might lead to better control of the emulsification process," the researchers suggest. This could have applications in various industries. Regardless the enigmatic ouzo effect that has intrigued anis fans for decades remains a cloudy issue.
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
Ouzo offers a cloudy outlook