On more than one occasion, I have heard or read pot shots taken at sparkling wine, most often by “red-whiners.” They often contend that champagne or sparkling wine would be simply a mediocre wine with out the bubbles to distract the drinker.   In this article the physical and chemical properties of bubbles will be described in their unique relationship to sparkling wine, demonstrating that bubbles are an integral part of the wine not a decoration or diversion.   Bubbles contribute to the visual, aural, sensational, olfactory, and physiological aspects of sparkling wine.

Visual:

           The immediately distinguishable characteristic of sparkling wine following the pop of the cork and pouring is the bubbles rising to the surface in rhythmic succession from points on the glass surface of the bowl or flute.   The points where the bubbles begin are known as nucleation sites.   Gérard Liger-Belair, an Associate Professor of Physical Sciences at the University of Reims Champagne-Ardenne, recently wrote a book titled Uncorked.   He is considered an expert in nucleation science and described in great detail how the bubbles appear in sparkling wine, their short lifespan and their ultimate fate.   The bubbles in sparkling wine do not appear from nothing but rather form on special substances.

           The bubbles in sparkling wine are principally but not entirely composed of carbon dioxide gas (CO2) that is dissolved in the wine.   The CO2 derives from fermentation and remains trapped in the tightly sealed bottle.   A sealed bottle of sparkling wine contains about 12 grams of dissolved CO2.   In the sealed bottle the dissolved CO2 is in equilibrium with CO2 in the vapor phase between the liquid and the bottom of the cork.   In other words, CO2 leaving the liquid phase at the same rate as CO2 is entering the wine. This is why you do not see any bubble trains in a sealed bottle of sparkling wine.   The pressure caused by all the dissolved CO2 is about 6 times greater than the atmospheric pressure we experience on earth.  

           Opening a bottle of sparkling wine drops the pressure in the bottle immediately from 6 atmospheres to 1 atmosphere putting the wine into disequilibrium.   At 1 atmosphere wine contains very little CO2.   Most of the 12 grams of CO2 must escape the wine for it to become in equilibrium with the atmosphere.   This establishment of equilibrium forces the CO2 out of the wine in the form of bubbles and vapor.   Liger-Belair calculated that an average glass of sparkling wine must liberate CO2 in the form of vapor and 2,000,000 bubbles to reach equilibrium and become flat.   Temperature influences CO2 liberation—cooler temperatures liberate CO2 more slowly than warmer temperatures.   This is why cool sparkling wine retains bubbles longer than warm wine.

           The bubbles in a glass of sparkling wine appear to emanate from little points in the glass, and it was believed to be scratches in the glass that provided the these nucleation points.   However, careful observation indicated that the bubbles were forming on pre-existing bubbles trapped in cellulose fibers probably left behind by cotton or paper used to dry the glass or from dust in the air.   Liger-Belair determined through careful observation that the minimum size starter bubble is about 0.2 micrometers in diameter; this is about 80,000 times smaller than the diameter of a dime. He observed with stop action photography bubbles growing from the seed bubble in the cellulose tube.   The bubble grows until it is large enough for buoyancy to detach the bubble and push it up through the wine.

           Without the pre-existing bubbles in impurities on the glass bowl of the flute, CO2 would not bubble up at all in sparkling wine.   For the wine to reach equilibrium the CO2 would just escape as a vapor through the surface of the wine.   To understand the energy limits to bubble formation and the minimum size of the seed bubble requires a brief look at the intermolecular forces called van der Waals forces holding neutral molecules together in wine.

           Van der Waals forces form intermolecular interactions that are not due to actual chemical bond formation but rather weaker transient forces.   Neutral molecule can become distorted leading one end of the molecule to become positively charged and the other end negatively charged.   The charges will attract opposite charges on other molecules.   Opposites do attract.   This attraction holds the molecules together.   For CO2 molecules to group together and form a bubble, they must push water molecules apart that are held together by van der Waals forces.   To try to imagine the amount of energy needed to push aside the molecules in wine for CO2 to group with other CO2s, envision a giant rally with thousands of people all from the same city with their arms interlocked, and interspersed randomly in this crowd are you and all your friends from a different city.   For you to get together at first, it takes an enormous amount of energy on your own to push through the crowd to meet just one of your townsmen and stay together, but with enough energy your group grows to a critical size that is able to push through the crowd. So it is the same for bubble formation.   The pre-existing bubble creates an open space for CO2 to move in. This works like an open space in a crowd for you and your townsmen to move into. Very tiny bubbles require great energy to form—more energy than is available in a cool glass of sparkling wine.   This is why there is minimum size needed for the pre-existing bubbles to start bubbles forming in a glass of sparkling wine.  

            To a pre-existing bubble with the minimum diameter of 0.2 micrometers, CO2 migrates from the solution into the bubble.   The bubble grows until it is large enough to detach from the fiber leaving a small bubble behind that will grow the next bubble like a tiny bubble pipe.   Bubble will continue to form until the CO2 is expended from the wine.

Aural:

            Try to notice the next time you pour yourself a glass sparkling wine that you can hear a popping sound as the bubbles explode on the surface of the wine.   These tiny explosions shoot little jets of wine in the air creating an atmosphere above the wine filled with tiny wine droplets as well as vaporized alcohols and aromatic molecules.

Sensational:

            The exploding droplets of sparkling wine combined with vaporizing constituents of the wine create a corona above the surface of the wine.   As one sips a freshly poured glass of sparkling wine, the little droplets of wine that land on the skin and in the nose and evaporate tickle the area around the mouth. Evaporating water and alcohol cool tiny patches of skin imparting a pleasant sensation.

            Beyond the pleasant spray from a freshly poured glass of bubbly, there is the complex mouth feel that comes from drinking sparkling wine.   The tingling sensation on the tongue comes from the wine foaming as it heats up in the mouth and rapidly releases more CO2. CO2 release from the wine is temperature dependent and going from 45° F in the glass to 98° F in the mouth liberates CO2 very rapidly for the wine.

            Another tingling sensation on the tongue comes not from bursting bubbles but from carbonic acid causing tiny acid burns.   (H2CO3 = CO2 + H20)   CO2 from the sparkling wine dissolves in the saliva in the mouth and comes in contact with the tongue tissue where it is converted to carbonic acid in a reversible reaction by the enzyme Carbonic Anhydrase.   The physiological role of carbonic anhydrase in the mouth apart from helping sparkling wine tingle is not known.

Olfactory:

            Bubbles enhance the aroma of sparkling wine in an interesting manner.   As noted before, the bubbles bursting at the surface of the wine release CO2 and send micro-jets of wine several inches into the air.   The bursting bubbles act as aroma launchers.   As a bubble rises through the wine, you can observe it getting larger.   The bubble increases in size rising through the wine by collecting more CO2 as well as many other compounds including flavor and aroma compounds.   The rising bubble picks up aldehydes—compounds that provide fruit smells like apple, mango, orange peel—and organic acids such as propionic and butyric acid.

Physiological:

            Sparkling wine often is accused of being a faster acting drink than still wine.   How often have you heard that champagne goes straight to my head?   It appears that the accusation has some credence.   Researchers at the University of Surrey in Guildford, England convinced some college students to participate in a study that compared the alcohol absorption rates between students drinking bubbly champagne or champagne flattened with a whisk.   The study found that after five minutes the flat champagne drinkers had only 0.39 mg/ml alcohol in their blood while the fizz drinkers consuming the same amount of alcohol had 0.54 mg/ml alcohol in their blood—a 28% higher level.   After 40 minutes the flat champagne drinkers had 0.58 mg/ml blood alcohol content and the bubbly drinkers were up to 0.7 mg/ml, still 17% higher than the controls.   Clearly the bubbles are enhancing alcohol absorption, but the researchers could not explain why.   They did speculate that the CO2 enhances movement of alcohol from the stomach to the intestines where 80% of alcohol is absorbed.   Perhaps this could be clarified by comparing scotch and soda drinkers with scotch and water drinkers to see if the bubbles are really the effective agents.

Conclusion:

            The elegant bubbles forming in a glass of sparkling wine are beautiful and hypnotic in their steady trains and contribute greatly to the character of the wine at many levels.   The bubbles help sparkling wine touch all the senses in ways unique among wines.