Why is alcohol fizzy?

By Peri Jones 

Beer, cider, sparkling wine…what do these have in common, except for being alcoholic? They also contain bubbles! It causes them to pop when opening a bottle, they have a “fizzy” taste on your tongue, and they might give you a little extra, ahem, gas.

All these are also alcoholic beverages, and follow a similar manufacturing process. The key process is fermentation. This is when the sugars and yeast react together to form alcohol and carbon dioxide.

Sugar + Yeast = Alcohol + Carbon Dioxide

The sugars can be extracted from fruits, like grapes for wine or apples for cider. Other ingredients are then added to give it flavour and colour, like hops in beer and leaving the skin on the grapes for differently coloured wines.

After the fermentation process is over, the left over liquid is bottled. But how does it become fizzy?

Often in pubs and bars, a keg is rolled into a cellar and attached to a pump which leads to the bar. This pump will force carbon dioxide into the keg, increasing the pressure and giving beer that well known fizz. It’s not always carbon dioxide though. Guinness is actually pumped with Nitrogen gas. Nitrogen is lighter than carbon dioxide and forms smaller bubbles. This gives the beer a much smoother finish and gives Guinness that well known frothy head.

Champagne bottles are rarely seen attached to a pump. After the fermentation process, additional yeast and sugar is added just before the bottling process. As the equation shows, this will create carbon dioxide, but this time the gas is sealed in the bottles. In turn, this increases the pressure in the bottle so when it opens, you have the well-known pop and fizz. This “second fermentation” is how sparkling wines are made.

The pressure within the bottles are five-and-a-half times higher than atmospheric pressure which is equivalent to twice the pressure within a car tyre. Carbon dioxide (CO2) and alcohol are formed when the yeast breaks down the sugar, but we don’t see bubbles within bottles. This is because the high pressure actually causes the carbon dioxide to dissolve in the water as carbonic acid (H2CO3).

Carbon Dioxide (CO2) + Water (H2O) = Carbonic Acid (H2CO3)

When you look in the bottle, there are no bubbles to be seen. So what happens when the bottle is opened? And how do the bubbles form?

After a gruelling examination of a variety of Champagnes, requiring many bottles to be opened and glasses to be poured (tough job), the bubbles were looked closer using microscopic photography. The bubbles created from a glass of champagne were compared to other liquids such as carbonated water. The carbonated water had much larger and irregular bubbles. This is because the carbonated water is made up of minerals which can cause the water to expel the gas randomly with a much greater force.

In Champagne however, the bubbles are smaller and follow a neat trail. “Blaseologists” are scientists that study bubbles. They have studied the formation of bubbles and how different things can vary the bubbles within champagne. They have concluded that the bubbles must be formed by either contaminants or dust particles from a cloth or in the atmosphere left within the glass before pouring or small microscopic ridges on the glass itself.

Both are key component in breaking down the carbonic acid and forming carbon dioxide bubbles. Unlike minerals in carbonated water, the dirt particles are much smaller which produces smaller bubbles. This essentially means that when you’re sipping on your glass of bubbly, the more bubbles you see in your glass, the dirtier your glass is! Although this might sound a bit horrible, wine experts believe that the bubbles play a key role in understanding the complexity of the ‘bouquet’ and helps with evaluating the taste.

So if you want to know if your best glassware is clean, crack open a bottle of Dom Pérignon and let the bubbles decide!

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Posted in Chemistry, Physics