Sunday, September 4, 2011

Legs for Days

Legs. Can't take it anymore. They don't mean anything. I find some inexperienced wine drinkers asking me about legs in wine, and proclaiming how nice the legs of a particular wine are . There is a general lack of understanding about what causes this phenomenon in wine. It has nothing to do with anything but the alcohol and water, and the interactions between the two of them.

Liquids have surface tension. Water has significant surface tension, if you don't believe me belly flop into the next pool you see. Did it hurt? Of course it did, and the fact that it did demonstrates one of the most significant facts about water; hydrogen bonding. There is a high amount of intermolecular forces at play within a solution of one or more chemicals. Have you ever paused to wonder why water, with its low molecular weight, boils at such a high temperature? Take methanol, which is water with a methyl (CH3-) group in place of one of the hydrogen atoms. It boils at ~64C, even though intuition (higher molecular weight) would seem to point towards the opposite. Again, we can attribute that to hydrogen bonding.

Hydrogen bonding comes about due to locally induced polarity on the water molecule. Oxygen is a very electronegative element, it strongly attracts clouds of negative charge towards it. When oxygen is bonded with two hydrogen atoms, as in water, it begins to draw electron density towards itself, creating a local negative charge centered on the oxygen atom. This causes a local positive charge to show on each of the hydrogen atoms. When the hydrogen local positive charge finds the oxygen local negative charge, a slight attraction occurs. The sum of all of these individual attractions between the molecules is what causes water to act the way it does. When ethanol is added to the mixture, we see a boiling point depression. Simply put, the water is not as attracted to itself because the large methyl groups block direct hydrogen bonding for a certain percentage of water molecules thus reducing the net sum of the intermolecular forces. This affects the surface tension of water. It also begins to give us a picture of what actually might be responsible for legs in wine.

There are several physical characteristics at play when a glass of wine is swirled. The capillary effect, just like a paper towel absorbing water, causes the wine at the utmost top of the glass to stick to the glass and sheet out until its surface area is maximized. All of the surface area of the water/ethanol mixture leads to evaporation. Because alcohol has significantly higher vapor pressure, it begins to evaporate much faster than the water. As the concentration of alcohol decreases, the surface tension of water increases. This draws more wine up along the side of the glass, but this wine has reduced surface tension and cannot easily mix with the reduced alcohol (higher surface tension) liquid it is heading towards. This causes the wine to bead up and drop back into the glass. This is a demonstration of the Gibbs-Marangoni effect, essentially describing liquid flow between two or more liquids of different surface tension.

Now for the try-at-home test....

Anyone who doesn't buy this explanation should try three simple experiments at home. First, swirl your usual wine glass and place your hand over the top to stop evaporation. You will see that legs do not form if the alcohol cannot rapidly evaporate. Second, buy yourself a bottle of grain alcohol. Put some water in the sink and then place a drop of everclear in the center of the water. Water will rapidly flow away from the everclear, demonstrating surface tension gradient flow. Third, mix the everclear with some water and swirl it in a wine glass. See the legs? This should debunk anyone still holding on to the notion that legs in wine are an indicator of sweetness or fruit quality.

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