The secret of Limoncello

Why is the Italian lemon liqueur so milky?

The Italian lemon liqueur Limoncello is a mystery to chemists for its persistent haze. © NelliSyr / iStock
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Mysterious turbidity: The Italian lemon liqueur Limoncello is not only popular for its fine taste, it is also chemically a special feature. For in him water, alcohol and lemon oil form an emulsion that remains surprisingly long stable - without any binder. Now, for the first time, chemists have defined the microstructure of this mixture more closely - and found surprising results. Because the oil droplets in the liqueur are unusually small and stable.

After the Christmas goose, the French foie gras or the Italian festive menu, it tastes particularly good: an ice-cold limoncello as a liquid dessert. The sweet, milky-yellowish lemon liqueur has a long tradition in southern Italy but is also popular elsewhere in Europe. "Many Italians make the lemon liqueur themselves by putting lemon peel in alcohol for several weeks, " explains Leonardo Chiappisi of the TU Berlin.

Permanent "ouzo effect"

However, limoncello is also exciting scientifically: "Above all, we are interested in the so-called 'ouzo effect' of limoncello, " explains Chiappisi. This effect is named after the ouzo, an aniseed liquor that is usually a clear liquid. By adding water, however, it suddenly becomes milky and opaque. "We were interested in the physical basis for this stable mixture of alcohol, oil and water, " says Chiappisi.

Because normally oil and water repel and must first be forced to a stable mixture by adding an emulsifier or a surfactant. With limoncello, however, this emulsion of citrus oils, water and alcohol remains stable even without a binder. The turbidity is retained and it does not settle even after prolonged standing.

Liqueur in the neutron beam

Chiappisi and his team used state-of-the-art technology to keep track of the permanent cloudiness of this liqueur: they analyzed the composition and microstructure of limoncellos with the aid of neutron scattering. "The neutron beam responds to the number of neutrons in a molecule. Accordingly, he leaves the sample at a different angle than he enters and allows us to reconstruct what the structure looks like, "explains Chiappisi. display

For their analyzes, the researchers enriched the water and the alcohol of the liqueur with deuterium, a hydrogen variant with an additional neutron in the atomic nucleus. The hydrogen in lemon oil, however, they left unchanged. This allowed them to differentiate between the different substances.

The secret of the droplets

The result: The unusually durable turbidity of Limoncello is due to the tiny size of its ltr pfchen. The lemon oil forms in this triple mixture of water, alcohol and Tropfenl drops with a radius of only 100 nanometers. They are so small that they can remain in suspension for a long time and are significantly smaller than other well-known triads of water, alcohol and oil, as the researchers report.

And the investigations revealed something else: to the surprise of the scientists neither the sugar content nor the temperature influence the microstructure of the limoncello. Although the liqueur is usually drunk ice-cold, even at room temperature this does not detract from its typical turbidity. Apparently the alcohol stabilizes the drops even under such conditions, as Chiappisi and his team discovered.

The researchers were able to solve a taste secret of the lemon juice in their experiments. With a water content of around 50 percent, which corresponds to a high-quality limoncello, only about two-thirds of the oil is trapped in droplets. As a result, however, the rest of the etheric oil is free and can be released as a lemon fragrance. That could give limoncello its particularly intense aroma and taste.

It remains puzzling

However, why Limoncello has so unusually small oil droplets remains a mystery for the time being. "The topic is complex, and it will take a lot of research until we understand the phenomenon, " says Chiappisi. When the time comes, the results will not only be important for manufacturers and connoisseurs the chemical industry could benefit as well.

"This very property of holding water-repellent chemicals in an emulsion with water is of interest to the specialty chemicals industry, " says Chiappisi. "Because with such a mixture one would not have to add surfactants or emulsifiers, which later have to be laboriously removed and disposed of. Thus, this property of citrus fruits could help to develop environmentally friendly solvents, plastics or insect repellents. "(ACS Omega, 2018; doi: 10.1021 / acsomega.8b01858)

Source: Technische Universität Berlin

- Nadja Podbregar