PWDec11sinha-4 17/11/11 10:37 Page 25 physicsworld.com Feature: Cocktail physics iStockphoto.com/philipdyer Cocktail physics Many top chefs are well known for taking a more scientific approach to cooking in recent years, and some “mixologists” are now extending the same philosophy to cocktails. Naveen N Sinha and David A Weitz explain the theory and techniques behind these increasingly exotic mixed drinks We are in the midst of a culinary revolution, as high- techniques borrowed from research laboratories in end chefs around the world exploit scientific knowledge physics and chemistry, such as rotary evaporators, ther- and technological advances to create spectacular mocouples and centrifuges, are helping bartenders to dishes. Ferran Adrià, known for his world-acclaimed put their innovative drinks ideas into practice. Concepts restaurant El Bulli in Catalonia, has pioneered the use from thermodynamics as well as the physics of colloids, of hydrocolloids to create yogurt spheres, carrot foam gels and other forms of “soft matter” can help explain and other novel foods. Other chefs, such as Heston the flavour, appearance and “mouthfeel” of these bev- Naveen N Sinha is a Blumenthal at the Fat Duck in Bray, UK, Grant Achatz erages. So get your cocktail shakers and bar spoons PhD student at the at Alinea, Chicago, and Wylie Dufresne at wd~50, ready as we take you through what you need to know to Harvard School of New York, are exploring science-based techniques, create cocktails that look, taste and feel fantastic. Engineering and including the use of liquid nitrogen, enzymes and con- Applied Sciences and David A Weitz is the trolled temperature baths, to create remarkable juxta- Full of flavour Mallinckrodt positions of new flavours and unexpected textures. Be it in beer, wine or spirits, the physical properties of Professor of Physics The same trend is happening, in parallel, with cock- ethanol, especially its solubility and volatility, help to and Applied Physics tails. For years bartenders have relied on trial and error deliver flavours that are impossible to achieve using at Harvard University, to refine recipes, but now the same techniques that water alone. What we think of as “flavour” actually has e-mail nsinha@ fuelled the culinary revolution are allowing a more sys- two main components: taste and aroma. As food-sci- seas.harvard.edu tematic approach to developing new drinks. Tools and ence author Harold McGee puts it, “Tastes provide the Physics World December 2011 25 PWDec11sinha-4 17/11/11 10:37 Page 26 Feature: Cocktail physics physicsworld.com At a Glance: Cocktail physics mented alcoholic beverages, such as beer and wine, rarely exceed about 20% ethanol by volume, since G Having perfected yoghurt spheres, carrot foam and other novel foods using new higher levels are toxic to most of the strains of yeast that technological tools, some chefs are now turning their attention to cocktails produce them. Higher concentrations must therefore G Cocktails have traditionally been developed by trial and error but can now be be reached through distillation, in which the fermented understood in terms of thermodynamics and soft-matter physics beverage is heated to preferentially extract the ethanol, G The physical properties of ethanol, the basis of every spirit, enable the delivery of which has a lower boiling point than water. The plant flavours impossible to achieve using water alone material used during fermentation, such as molasses G Equipment such as rotary evaporators and whipped-cream dispensers are now for rum or agave for tequila, gives an intense flavour to used to extract flavours, along with traditional distillation and soaking methods the final distilled beverage. Additional plant materials G The appearance and texture of drinks can be controlled by clarification, the supplied during or after distillation, such as the juniper decision to stir or shake, or the production of foam, for example using egg whites to berries used for making gin, also contribute to the stabilize air bubbles flavour. Because of the high concentration of aromatic molecules extracted from the plants during the pro- duction process, spirits are some of the most intensely Get mixing! flavoured foods. Indeed, only a few drops of Chartreuse, a French liqueur made using nearly 130 Here is a cocktail recipe you can try for yourself, from leader in the field Ferran Adrià. herbal extracts, can entirely change a cocktail’s flavour. Distillation has been used for thousands of years to Hot and cold gin fizz create spirits, going as far back as Mesopotamia and ancient China, but continues to be improved through Ingredients Method applications of scientific knowledge. For example, For the base syrup: For the base syrup: some bartenders, such as the award-winning Tony 150 g sugar Mix ingredients and bring to a boil. Conigliaro of London bar 69 Colebrooke Row, are 150 g water Remove from heat, cool, then refrigerate. experimenting with a device commonly found in the science lab – the rotary evaporator (figure 1). This For the frozen lemon mix: For the frozen lemon mix: device extracts aroma molecules that would otherwise 250 g lemon juice Mix all ingredients cold, then freeze. be destroyed by the higher temperatures in traditional 150 g base syrup (see above) Once frozen, blend in a blender until distillation techniques. The rotary evaporator lowers 150 g gin fluid. Keep in the freezer. the pressure inside a rotating container holding the li- quid to be distilled, causing the more volatile compo- For the hot lemon foam: For the hot lemon foam: nents to evaporate and leaving behind the undesirable 150 g egg whites Break egg whites with a whisk. Add the water, sugar, pigments and other large molecules. A 130 g lemon juice remaining ingredients. Strain and pour condensing coil uses a coolant to turn the vapours back 70 g gin into the iSi Whip using a funnel. Load the into a liquid – the final intensely flavoured product – 145 g base syrup (see above) iSi Whip and keep in a water bath at which is collected in a separate flask. A habañero 80 °C, shaking occasionally. liqueur is one illustrative example: the capsaicin that Equipment makes the chilli taste so hot is non-volatile, so only the 0.5 litre iSi Whip To serve, 3/4 fill a cocktail glass with fruity and floral compounds end up in the distillate, 1 cartridge of N2O frozen lemon mix. Top up with hot foam. yielding a liqueur that retains all the flavour of the chil- lies but without any nasty burn. Another way to intensely flavour spirits is to soak foundation of flavour, and aromas provide the tremen- ingredients in high concentrations of ethanol, thereby dous variety.” Although we can perceive just five basic infusing the aromatics into the alcohol. This process tastes on the tongue (sweet, sour, salty, bitter and traditionally requires many days for the ethanol to fully savoury), there are thousands of aromas that we can penetrate the ingredients and extract the desired com- sense through olfactory receptors in the nose – be they pounds. Now, however, flavour infusion can be the caramel notes of rum or the oaky smell of bourbon. achieved in just a few minutes, using a technique pio- Alcohol is far more effective than water at deliver- neered by Dave Arnold, author of the blog Cooking ing these aromatic components, since typically they are Issues and director of culinary technology at the French not especially water-soluble. Water molecules are polar Culinary Institute in the US. Coffee-flavoured vodka, and so prefer to be near other polar molecules to min- for example, can be made by combining ground coffee imize their interaction energy. This encourages non- beans and vodka in a whipped-cream dispenser – a polar molecules, such as the aromatics, to leave the pressurized device typically used to create foams such liquid phase and vaporize into the surrounding air, as whipped cream at the touch of a button, now well where they contribute to the aroma of the drink. The known by the commercial name “iSi Whip”. What hap- presence of ethanol mediates this polar/non-polar pens is that nitrous oxide, which is also in the canister interaction and allows high concentrations of aromat- and under high pressure, dissolves in the vodka. The ics to remain in an aqueous solution. For this reason, high pressure of the liquid displaces any air bubbles in ethanol is used to extract and deliver flavours from a the coffee grounds. When the pressure is released, the range of sources, including flowers, spices, nuts, fruits nitrous oxide rapidly bubbles out of the solution, just and herbs. as when a can of carbonated drink is opened. Releasing Distilled alcoholic liquids, called spirits, are the these bubbles draws flavour molecules from the coffee essential component of any cocktail. Naturally fer- grounds into the vodka, flavouring the alcohol and 26 Physics World December 2011 PWDec11sinha-4 17/11/11 10:37 Page 27 physicsworld.com Feature: Cocktail physics 1 From lab to kitchen 2 Shaken or stirred? Cooking Issues Dave Arnold, Mike Betancourt and Leo Stein A rotary evaporator, typically found in a science lab, can be used to extract delicate aroma molecules that would be destroyed by the higher temperatures of traditional distillation. The liquid in the rotating chamber (red) is gently heated while under a vacuum, causing it to These manhattan cocktails have identical ingredients, but the shaken version (left) has a froth vaporize at a lower temperature. The condenser coils (blue) liquefy the on the surface and a cloudy appearance, caused by vigorous shaking that incorporates air vapour, which is collected in the flask on the left (green). bubbles, while the stirred version (right) is clear.