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Chateau Ksara Blanc de Blanc-A Wine from Lebanon
Grape 50% Sauvignon Blanc, 25% Chardonnay, 25% Semillon Facts That’s right, I said it! Lebanon!!! When you think of the top wine producing regions in the world, Lebanon most likely isn’t a part of that list. However; the fact that the country can even produce wine in such a place so torn apart by war, is nothing short of an absolute miracle! Believe it or not, Lebanon is actually one of the oldest…
George Vierra
Legs in a
Glass of Wine
“Legs” in a glass of wine or
spirits have been noted since millennia. As noted in Proverbs 23:31, “Look not
thou upon the wine when it is red, when it giveth his colour in the cup, when
it moveth aright.” Descriptions since Biblical times have changed. We now may
state, “Wow, just look at those legs!”
The language has changed, but the chemistry and physics have remained
the same.
Wine and spirits are basically
mixtures of water and ethanol (ethyl alcohol). As individual compounds, they
have two different physical distinctions. Alcohol has a lower boiling point
than water and hence, evaporates faster than water. Water has a higher surface
tension than ethanol.
The attractive forces between
molecules in a liquid are called surface tension. These forces hold the liquid
together. The same type a force acts between molecules of a liquid and those of
a solid surface. This force is called ‘interfacial tension’. If the interfacial
tension between a wine and a glass is a bit greater than the wine surface
tension, then this causes wine (or spirits) to climb the inside walls of a
glass. A point is reached at which the weight of the wine clinging to the glass
just balances the force trying to lift more. A pure liquid would arrive at a
steady state and a specific film height on the glass would be maintained. Wine is not a pure liquid. It’s a water
alcohol solution. On the wine film, alcohol evaporates faster than the water.
Once the inside of the glass is covered with a thin film, the wine film loses
some of its ethanol by evaporation. With the concentration of water increasing,
the film surface tension increases, as does the index of refraction. In the
areas where alcohol evaporates, the watery-wine left behind assumes a drop-like
form. The drops become heavier and the force of gravity becomes controlling and
the drops slides down the glass wall to the wine in the bowl. These legs can be
seen because the change in the refractive index makes the boundary between the
watery legs and the more alcoholic film visible. The channels of falling wine
appear as “legs”. Since this “surface tension engine” is driven by the ethanol
evaporation in the film, the higher the alcohol, then the greater the legs.
Glasses of pure water or alcohol show no legs. Different evaporation rates are
necessary. Place two glasses of the same wine side by side. Notice the legs in
both. Put a lid on one glass. In the lidded glass, evaporation ceases and legs
stop forming.
The elder brother of Lord Kelvin,
James Thompson, published a paper in 1855 in Philosophical Magazine,
titled “On certain curious Motions observable at the Surface of Wine and other
Alcoholic Liquors”. Thompson, therein, described the effect caused by
capillarity, or surface tension. Thompson described them as “tears of a strong
wine”.
“The phenomenon stems from the
dipole-dipole intermolecular forces in aqueous solutions. The combination of
the cohesive forces within the liquid and the adhesive forces between the
liquid and solid surface can explain, among other things, surface tension and
capillary action. In solution, the ethanol and water have cohesive forces
weaker than that of the molecules of pure water. The adhesive forces toward
glass surfaces are about the same as those of water. These adhesive forces are
stronger than these cohesive ones. This causes wine to adhere to and climb the
wall of the glass. As the ethanol evaporates, the cohesive force increases
until the wine falls in a thin stream. Upon reentering the surface of the wine,
the ethanol concentration is restored, the cohesive forces weaken and again the
wine climbs the walls of the glass.”
Many wine writers have attributed “legs”
to the glycerin in wine. There is no glycerin in wine. There is glycerol, an
alcohol. It is in minuscule percentages in wine and may add a touch of
sweetness. It has a boiling point of 554º F and does not evaporate like ethanol to
create this struggle of forces.
George Vierra 3
April 2005
Viticulture & Winery Technology
Napa Valley College
Napa, CA USA