Sparkling wine is serious business, but it’s also serious science. The process of making methode champenoise sparklers sparkle, from secondary fermentation in the bottle to the time you take a sip, is governed by the physical laws of how gases behave. Scientists who care about fluid dynamics have written a lot about the physics of sparkling wine, but their work isn’t just about what happens in the laboratory. Physics has a lot to say about the way we serve and enjoy our bubbly, too.
A standard 750 mL bottle of sparkling typically contains about 9 grams of dissolved carbon dioxide (that’s 11-12 g/L), equivalent to just less than 5 liters of CO2 at typical atmospheric conditions (i.e., outside the bottle.) Some of that CO2 escapes when the bottle is opened, some escapes as the wine bubbles in your glass, and some escapes in your mouth when you take a sip. (Some probably also escapes in other ways later on, but we’re not going to discuss that here.) Here are four considerations that influence how that CO2 escapes and why you should care.
1. Glass shape
That flutes are better for bubble preservation than coupes is common knowledge. Even at the initial phase of pouring, coupes result in three times more wine-to-air contact than do flutes. The real difference between the two glasses occurs after the wine has been poured, however, when the much larger surface area of the coupe means that CO2 can diffuse across the wine-air barrier faster. Translation: sparkling wine in a coupe goes flat faster than sparkling wine in a flute because of “invisible” CO2 loss at the surface of the wine, not because one shape necessarily produces more or faster bubbles than another.
Neither flutes nor coupes do much to emphasize a sparkling wine’s aroma. That’s ironic, given that one of the hidden functions of bubbles is as aroma enhancers. As bubbles rise, they mix the wine, dragging wine from the bottom of the glass to the top. As wine at the surface gives up its volatile aromatic molecules into the air, it’s replaced with fresh wine from the bottom such that, on the whole, more aromatic molecules make it out of the wine and into the air.
So, with all of this aroma-enhancing mixing going on, why don’t we tend to notice the nose of sparklers much? While flutes are great for preserving bubbles, they don’t give your nose much space to appreciate the smelly molecules suspended in the headspace above the wine. Compare a glass of sparkling – especially a relatively full glass, as is often poured – with a standard glass of white or red wine and you’ll see that there’s a lot less room in the glass for those molecules to concentrate. Coupes are worse; there’s plenty of room for your nose above the wine, but the open shape allows aromatic molecules to move freely into the air rather than remaining concentrated in the glass. It doesn’t help that sparkling wine is customarily served very cold. Cold temperatures dampen our ability to perceive smell and slow down the movement of aromatic molecules.
2. How to clean your glassware
How bubbles form in the glass depends a lot on the type of glass and how it was cleaned. The basic bubbling process is called heterogenous nucleation, which is physics short-hand for what happens when an irregularity – a crystal, a rough surface, another bubble; aka, a nucleation site – gives a microscopic gas bubble a place to hold on. The nucleation site then collects progressively more microscopic gas bubbles and grows larger until the bubble is big enough to push harder against the wine above it than the wine pushes down on it. At that point the bubble rises and bursts at the surface. Tiny imperfections on the inner surface of a sparkling wine glass make great nucleation sites, as do invisible fibers from a towel used to dry or polish the glass or trace tartaric acid crystals left over from a previous pour. Bits of dust collected while sitting in a cabinet between New Year’s Eves work well, too.
So, in theory, a cleaner glass will mean slower bubble formation because there are fewer nucleation sites. And, in practice, this is true. When sparkling is poured into a glass “washed” with formic acid – super-strong, nasty stuff that will strip anything and everything off of the glass – it looks like still wine; bubbles just don’t happen because there isn’t anything to start the nucleation process. The down-side to a very clean glass, then, is that the sparkling doesn’t look very sparkling in the glass. The up-side is that all of that CO2 is still dissolved in the wine and ready to form bubbles when you take a sip – your mouth is full of rough bits that make excellent nucleation sites – which produces an extra-creamy feeling in your mouth. Conversely, not washing your glass thoroughly, using a towel to dry the inside of the bowl, or not carefully rinsing a glass that’s been sitting awhile means that your wine will go flat fast after a short period of over-aggressive bubbling.
Some sparkling wine glasses are deliberately etched with minuscule scratches at the base of the glass where the bowl meets the stem. It’s easy to tell when you’re sipping from such a glass; the steady, thick stream of bubbles from the center of the bowl is a sure clue. Pretty, but your sparkling will go flat a bit faster.
The surface of crystal stemware is rougher—at the microscopic level – compared with glass, so crystal glasses will also yield more bubbles faster (and flatness faster) than glass ones.
3. Pouring strategies
It’s pretty and dramatic to pour in a long stream from the bottle into a glass sitting flat on the table – the “tongue” of wine extending from bottle to glass makes for a nice show, and a tall head of foam forms in the top of the glass. But pouring this way does two things. Firstly, that long tongue exposes a lot of wine to a lot of air during the pouring step. Because CO2 moves across that wine-air boundary by diffusion, lots of wine-air contact means lots of CO2 lost, which means less bubbling in the glass and your mouth. Secondly, pouring this way creates a lot of turbulence in the bottom of the glass. All of that wine swirling around allows more CO2 to escape from the wine in the glass (the reasons why are more complicated than I have space to explain here) and, moreover, creates air bubbles in the wine that effectively increase the area of wine exposed to air even more. The result? An impressive head to begin, but a pour that falls flat faster. Warmer temperatures make all of this worse because CO2 diffuses out into the air faster as temperature increases.
A better alternative is to pour sparkling wine the same way we tend to pour bottled beer. By keeping the bottle close to the glass, the length of that wine “tongue” shortens and the surface area of wine in contact with air decreases. By tilting the glass so that the wine slides gently down the side, less turbulence happens in the bottom of the glass and fewer air bubbles are incorporated. The result is very little foamy head, but a longer lasting fizz. And, as an added bonus, you’re almost guaranteed not to encounter the unpleasant surprise of foam spilling over the edge of your glass.
Henry’s law and the ideal gas law is part of why opening a warm bottle of sparkling wine is so ill-advised. (Okay, only part of the reason – cold temperatures dull our perception of both acid and sugar, so warm sparkling wine is also likely to taste unpleasantly tart or unpleasantly sweet – but Henry’s law is why opening warm sparkling risks poking your eye out. Henry’s law tells us (among other things) that the higher the temperature, the less CO2 can dissolve into the wine. Less CO2 dissolved into the wine means more CO2 hanging out in the space between the wine and the cork, which means that the gas inside the bottle is under higher pressure. The result? A cork flying at very high velocity, potentially. This is also part of why sparkling wines are disgorged at near-freezing temperatures, since colder wine means less CO2 lost in the interval between the lees being removed and the bottles being topped up.
Warmer temperatures mean faster-moving CO2 molecules after pouring, too, which means faster CO2 loss, so warmer sparkling goes flat faster in your glass, too.
I would love to be able to tell you how fast you have to drink your glass of bubbly to enjoy the last sip before it goes flat, but I can’t. As you can surmise from the rest of this article, there are just too many factors involved in how fast sparkling falls: temperature, glass shape, how the glass was washed, how the wine was poured, how much you move your glass around… Moreover, the age of the sparkling matters, too. We all know that cork isn’t completely air-tight, and if outside air can (and does) get into a bottle of still wine, then CO2 can (and does) get out of a bottle of sparkling. We don’t have a lot of data on exactly how bubbles change with bottle age, perhaps partially because not a lot of people with the right equipment have looked, but also because so much depends on the porosity of any given cork. Since each cork is different and allows for a different amount of air movement in and out of the bottle, the way each bottle of sparkling will age is different. The advent of technology like the Zork, screwcap, and Nomacorc that either form a virtually air-tight seal or that have a consistent rate of gas transmission for every closure will start to change that but, for now, the use of those closures just isn’t significant for age-worthy bottles.
The bottom line: physics is great fun, but probably less fun (for most people) than a good glass of sparkling. Don’t fret too much about the fluid dynamics, and enjoy your bubbles.
[author] [author_image timthumb=’on’]http://palatepress.com/wp-content/uploads/2010/07/yoga-headshot-2010-thumb.jpg[/author_image] [author_info]Erika Szymanski was blessed with parents who taught her that wine was part of a good meal, who believed that well-behaved children belonged in tasting rooms with their parents, and who had way too many books. Averting a mid-life crisis in advance, she recently returned to her native Pacific Northwest to study for a PhD in microbial enology at Washington State University. Her goal, apart from someday having goats, is melding a winery job to research on how to improve the success rate of spontaneous ferments. When tending her Brettanomyces leaves enough time, her blog Wine-o-scope keeps notes on why being a wine geek is fun.[/author_info] [/author]