My wine breathes, has a nose, body, and legs, but no pulse!
(Part 4, by Raymond R. Wells, Ed.D.)
No, I don’t need a defibrillator. And you don’t need a doctorate to understand these terms as applied to wine.
Continuing our anatomy and physiology of wine, legs:
You may have seen someone swirl their wine, hold it up, and comment on its nice legs (the drops that form near the top of the glass and flow back down the inside of the glass), as if it were some kind of indicator of quality. Well, it is NOT! Legs (also called tears of wine) are only indicative of the alcohol content; the higher the alcohol level, the more legs that will form. The 2 main chemistry/physics forces involved are: Surface tension – the attractive forces between the molecules of the liquid. This is what allows water strider insects to walk around on the surface of a puddle. And, interfacial tension – the attractive forces between the molecules of the liquid and the molecules of a solid surface. This causes the “capillary action” seen when a nurse pricks your finger and the blood climbs up a narrow tube of glass.
Wine is mostly a mixture of water and alcohol. The wine climbs the sides of the glass due to interfacial tension (capillary action). Because the alcohol evaporates faster (it has a lower surface tension, higher vapor pressure, and therefore a lower boiling point than water), the wine continues to climb the glass until gravity overcomes the interfacial forces. The water at the top, with reduced alcohol concentration, has a higher surface tension which causes it to form beads and run back down at the same time as liquid with a higher alcohol concentration climbs upward. The water rich drops have a slightly different refractive index, allowing increased visibility of the “legs”. A simple test to verify that this phenomenon is due to evaporation is to cover the top of the glass of wine to prevent the evaporation; and you will see that legs fail to form. Unfortunately, visual examination of a wine’s legs is not an accurate indicator of the alcohol content because there needs to be around a 5% difference in alcohol level to see a significant difference in the legs. Also, the cleanliness of the glass and/or the presence of any soap residue can have an effect on the formation of legs.
So the next time someone comments on their wine’s great legs, you can be nice and say, “How interesting!” Or if they are a pompous horse’s patoot, you can tell them that they are full of baloney (or some more odiferous material).
In wine, truth and health!