CC Image courtesy of Kalense Kid.
Posted by Lauren Thomas on March 4, 2013.
If you enjoy a glass of wine while reading this blog, this is a perfect opportunity to perform a quick chemistry experiment. (And if you don’t have a glass of wine at hand, here’s a great reason to go pour one!) Either take a sip of wine or swirl your glass, then check out the surface on the inside of the glass. You should notice a thin film of wine, which will drip down the sides of the glass back to the surface of the wine. The descending film is referred to as wine “tears” or “legs.” This phenomenon is easy to create, but how does it happen, and can it tell us anything about the wine?
How it happens has a beautifully scientific explanation. Wine is made up of minerals, polyphenols, sugars, acids, glycerol, ethanol (alcohol), and, mostly, water.1 These last two components are especially important in the formation of wine tears due to their different chemical properties. Wine tears are formed because ethanol evaporates more quickly and is more non-polar than water. Ethanol is also surface-active in wine; this results in a higher concentration of ethanol molecules than water molecules at the surface of wine. Ethanol’s ability to evaporate more quickly than water, particularly at the thinnest part of the film (on the sides of the glass), will leave behind more watery wine in that area. Then, surface tension starts to come into play.
Water, in general, has a high surface tension; basically water likes to cling tightly to other water molecules. Ethanol has a lower surface tension than water, as it is more non-polar. A mixture of water and ethanol (i.e. wine) has a lower surface tension than a solution of pure water.2 Thus, after ethanol evaporates from the surface and the ethanol concentration decreases, the remaining water molecules increase the surface tension of the wine, creating a surface tension gradient. The wine left in the bulk liquid responds to this gradient by climbing toward the top of the film to reduce the surface tension. This forms a ring around the wine glass above the main surface of the wine, and the ring grows as this process repeats. Eventually, gravity pulls the ring back into the wine, creating what we refer to as tears or legs. This phenomenon is known as the Marangoni Effect.
So what does this effect tell us about the wine? Many people mistakenly correlate more wine tears with higher quality. There’s also the idea that wine tears are caused by glycerol and predict sweetness. Both of these ideas are incorrect (and now you know why). What wine tears can actually tell us about a wine is its alcohol content. Because ethanol is a main contributor to wine tears, alcohol content is positively correlated to the volume of the tears and the length of the time that the tears linger on the glass. That is, high-alcohol wine will have larger tears that remain on the glass longer.
Because wine tears are prompted by ethanol evaporation, conditions external to your wine glass will impact its tears. Warm weather will accelerate ethanol evaporation, making tears form more easily. Other weather conditions, like humidity, affect tear response as well. To stop tears, try covering your glass completely (if you’re using your hand, try to eliminate any gaps). When the glass is capped, you won’t see any tears because ethanol will not be lost to evaporation and the system will remain at equilibrium. Experiment with different wines in different environments to see the variety of tear formations wine can display. Now, go pour yourself a second glass and share your new scientific knowledge with a friend. Cheers!
Lauren Thomas is a M.S. Candidate in the Department of Food Science and Technology at Cornell University, where she studies enology and researches phenolic extraction from red wine grapes. Lauren received a B.S. in Biology with Honors from Roanoke College. She highly recommends checking out what Finger Lakes wines have to offer.
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- Gavin Sacks, Ethanol and Fermentation, lecture notes distributed in FDSC 4400 “Wine and Grape Flavor” at Cornell University, Ithaca, NY, on 23 January 2013. ↩
- Marcos Gugliotti, Tears of Wine, 21 J. Chemical Educ. 67–68 (2004). ↩
- Howard A. Stone, The Fluid World: Flows, Films, and Foams, Harvard U., http://athome.harvard.edu/programs/stone/imageframe/37.html (last accessed Mar. 3, 2013). ↩