Chemistry, Physics | UNDERSTAND The peel of lemons contains essential oils, which are extracted in the production of limoncello. IgorZh/Shutterstock.com Limoncello and the science of emulsions How can you make oil and water stay mixed? A scientist’s curiosity about a lemon liqueur has revealed how to do this – with some promising industrial applications. By Leonardo Chiappisi Limoncello, the fragrant Italian liqueur made from lemons, is then mixed with a sugar syrup. Limoncello typically contains becoming increasingly popular around the world. This sweet approximately 30% alcohol and around 20% sucrose (sugar) and citrus digestif is an iconic item of Italian food culture – by volume, but as limoncello is often home made, the but it is also a complex colloidal system made of essential preparation method and final composition vary from family oils, ethanol, sucrose and water. to family. As an Italian chemist working at the Institut Laue-Langevin The essential oils so important to the flavour of limoncello (ILL)w1, I was curious to find out what ILL’s advanced are found in small pockets in the peel of citrus fruits, technology might reveal about this complex system. So, which burst and deliver the typical strong scent we notice earlier this year, my colleagues and I applied for beamline when peeling such fruit. These essential oils have a very time to conduct a small study, and it turns out that – as well complex composition: more than 60 different molecules as being delicious – limoncello has some rather special have been identified, with a class of organic molecules scientific characteristics. called monoterpenes being the main components. In lemons, the most abundant compound is limonene What is limoncello? (figure 1). In the traditional limoncello recipe, the citrus zest (obtained Limoncello is made by mixing two solutions: the ethanolic by scraping the outer part of the lemon peel) is macerated in extract containing the oils, and the aqueous sucrose solution. alcohol (ethanol) for several weeks. The zest contains most of Each of these starting solutions is completely transparent; the essential oils in lemons, producing the characteristic taste limoncello itself, however, is ‘turbid’, with a cloudy, opaque and colour of the liqueur. The ethanol and lemon extract is appearance. Turbid systems pervade everyday life: other 12 I Issue 48 : Autumn 2019 I Science in School I www.scienceinschool.org UNDERSTAND | Chemistry, Physics Surfactants Neutron scattering Phase diagrams Ages 16–19 Most students will recognise that oil and water do not Comprehension questions that could be used in class include: mix, and they may have heard the word ‘emulsion’ when · The limonene molecule has two possible enantiomers. helping a family member paint a room – but despite Identify the chiral carbon atom. these real-world examples, very few will have questioned · What volume of ethanol would you expect 5 litres of the chemistry behind such experiences. By encouraging limoncello to contain? students to question what is happening on a macro level · Turbid systems contain particles at a scale comparable and inspiring them with the chemistry occurring on a to the wavelength of visible light. What is the micro level, this article provides an accessible gateway to wavelength range of visible light? some key concepts. · Why could the science of limoncello prove useful for In addition, there is an opportunity to interpret a three- designing self-emulsifying systems? fold phase diagram, which allows students to use their · The phase diagram contains a ‘metastable ouzo mathematical skills to derive conclusions about physical region’. What is meant by the term ‘metastable’? characteristics, demonstrating that such skills are essential REVIEW Caroline Evans, Head of Chemistry, for scientific research. Wellington College, UK examples include ice crystals in a suspension of tiny droplets of one to form the emulsion. Lecithin and clouds, fat droplets in milk, and algae liquid in another. However, this only proteins contained in the egg yolk are in a pond. These different systems all happens at some specific ranges of also needed to stabilise the emulsion. contain particles or droplets at the scale composition (see phase diagram So why are ouzo systems important of hundreds of nanometres, which is text box). outside the kitchen? Some important comparable to the wavelength of visible This phenomenon of spontaneous industrial processes take place light. It is these ‘heterogeneities’ – tiny emulsion formation is called the ‘ouzo in emulsions – for example, amounts of solid or liquid suspended in effect’, after the famous Mediterranean polymerisation, where small molecules a fluid medium – that give these systems drink called ouzo that immediately (monomers) combine to form large their turbid appearance. becomes turbid when mixed with water, macromolecules, or polymers. Here, forming an emulsion. Indeed, ouzo emulsions are often created to bring The ‘ouzo effect’ is, from a scientific perspective, quite the reagents into close proximity so the So where does the turbidity of similar to limoncello, as it is made reaction can proceed quickly. If such limoncello come from? Water and from water, ethanol and the flavour ethanol are fully miscible (soluble component anethole, which – like Jorge Royan/Wikimedia Commons, CC BY-SA 3.0 BY-SA CC Commons, Royan/Wikimedia Jorge in each other), as are limonene and limonene – is highly soluble in ethanol ethanol – but limonene and water but only slightly soluble in water. are barely miscible. In limoncello, In contrast to these ouzo systems, this combination of three liquids spontaneously produces an ‘emulsion’: typical emulsions require a very high Nicola Graf Nicola energetic input – such as the shaking and stirring required to make the CH3 emulsion we call mayonnaise. Another very important difference between C ouzo systems and classical emulsions H C CH 2 is the absence of any stabilising agents. For example, mayonnaise is prepared H2C CH2 CH by emulsifying a vegetable oil with the water contained in egg yolk. The C process is long and tedious, and it CH H2C 3 requires a substantial amount of energy – provided by vigorous shaking and Figure 1: The structure of limonene stirring – to make the two liquids mix Limoncello bottles for sale in Capri, Italy www.scienceinschool.org I Science in School I Issue 48 : Autumn 2019 I 13 Chemistry, Physics | UNDERSTAND canbilgic/Shutterstock.com The ouzo effect: PhotoAZ/Shutterstock.com the Mediterranean drink ouzo (centre) immediately turns cloudy (right) when water (left) is added to it. Making mayonnaise requires not only the right ingredients to create the emulsion, but also quite a lot of energy. emulsions form spontaneously (as in under different conditions: when water 100 nanometres in diameter, regardless limoncello), requiring very little energy is added to the ethanolic extract; at of water content, sugar content input, if any, this obviously makes the different temperatures; and at different or temperature. These results are process more efficient and sustainable. sucrose concentrations (Chiappisi & surprising: the typical size of the oil-rich In addition, the polymer product needs Grillo, 2018). Neutrons are sensitive to domains in ouzo systems is normally to be recovered from the reaction the isotopic composition of the system, much larger, at several hundreds or medium at the end of the reaction, and they interact very differently with even thousands of nanometres in which is often the most challenging size (Grillo, 2003). In addition, their step of the entire process. However, if size is usually very sensitive to the composition or the temperature of the the system contains no stabilisers, the Limoncello can be kept extraction of the polymer and catalysts “ system – unlike in limoncello. is much simpler, as the components in the bottle for years: This makes limoncello a very interesting can easily separate once the emulsion- liquid, scientifically. The small size forming composition no longer exists. not bad for a metastable of the oil droplets seems to provide Another widely used application of system! its exceptional stability in relation emulsions is in pesticides, to enable ” to changes of temperature and these water-insoluble products to be composition, and also over time. In fact, diluted and spread onto fields. Using limoncello can be kept in the bottle for the two stable isotopes of hydrogen: an ouzo-type emulsion would avoid years: not bad for a metastable system! protium, 1H (normal hydrogen), and In contrast, drinks like pastis or ouzo also spreading unnecessary surfactants, the much rarer deuterium, 2H. In the tend to phase separate within a few which are often harmful to the research, the essential oil was extracted hours of preparation (which is why a environment. from a lemon bought in the local market pastis is always diluted with water in the (thus containing mostly protium nuclei), glass, just before consumption). Limoncello at the micro level while the ethanol and water were highly enriched with deuterium nuclei, as a So while we don’t yet fully understand As mentioned, the way limoncello contrast. why limoncello behaves so differently scatters light is quite revealing about to other ouzo-type drinks, we now have The analysis revealed that, in the liquid’s structure at the microscopic a better understanding of the science of limoncello, the size of the oil-rich level. Using radiation of shorter self-emulsifying systems – and how to domains is always around wavelength, X-rays or neutron beams design them for use in future products allow us to look in more detail at and processes. A Chezière/ILL structures and interactions within this References liquid, and at an even smaller scale. Chiappisi L, Grillo I (2018) Looking into We hoped to use the neutron scattering Limoncello: the structure of the Italian liquor facilities at ILL to see what they could revealed by small-angle neutron scattering. tell us about limoncello – and, luckily, ACS Omega 3: 15407-15415. doi: 10.1021/ we were allocated time on the small- acsomega.8b01858 angle neutron scattering (SANS) Grillo I (2003) Small-angle neutron scattering study of a world-wide known emulsion: beamline.