When I was little, my mom went through what she calls her “Earth Mother” phase (it was the ’70s). At one point, she decided to make her own root beer. She sent away for a book with the recipe and some bottle caps. We followed the recipe, had a great time using my great-grandfather’s bottle capper, and put the bottles in a cool corner of the basement to finish. Some time later, my brother and I went down to the basement to play and found sticky root beer on the walls and ceiling. The bottles hadn’t exploded (we had those heavy-duty, glass pop bottles back then; the ones that you drank in the store so you got your nickel back), but the cap had blown off one of them. After that, I remember taking the bottles outside, carefully – “Point the bottles away from your face. … No, Jenny, don’t point it toward your brother” – and dumping the contents, which smelled pretty strongly of bread dough. What went wrong, and how does carbonation science work?
It was all about our carbonation methods. We Americans love our fizzy drinks – soda pop, beer, champagne. Studies show that carbonation has a “taste.” It activates the portions of our tongues that taste sour things (which is why flat drinks taste, well, flat).
Carbonation is created when carbon dioxide (CO2) dissolves in liquid. The amount of CO2 that will dissolve is dependent on temperature and pressure. CO2 will dissolve in a liquid until “equilibrium” is reached – when the amount of carbon dioxide in the liquid and the amount in the air above the liquid are the same. In a sealed container, an excess of the gas can be dissolved if it is used to pressurize the airspace above the liquid. When you open a carbonated drink to the outside air, the pressure is released, and the CO2 leaves the liquid to reestablish equilibrium. If you recap the bottle, enough escapes to return to equilibrium, leaving the liquid just a little bit flatter. If you leave it open to the air, most of the CO2 will leave the liquid, and the drink will go completely flat.
Dissolved CO2 can be added to beverages in two ways: forced and natural. In forced carbonation, CO2 is added to the drink under pressure. Most bottles and cans of soda and beer use this kind of carbonation. When using natural carbonation, yeast and a little bit of sugar are added right before bottling to encourage a last bit of fermentation (see our American Brown Ale on Page 61). The yeast metabolizes the sugars and creates carbon dioxide and alcohol. The pressure and CO2 build up inside the bottle and carbonate the drink. The amount of sugar and yeast added must be carefully controlled to keep the beer from becoming over-carbonated. The carbonation process stops when the yeast either runs out of food or is encouraged to go dormant.
Usually, in naturally carbonated sodas like Mom’s root beer, because you want sweetness to remain in the final drink, the yeast reaction is controlled by refrigerating the bottles – most yeast becomes dormant at temperatures below 40 degrees. We had attempted natural carbonation and succeeded a bit too well. The yeast had done its work and created enough pressure to pop the tops off the bottles. I’m not sure if we forgot to put them in the fridge at the scheduled time or if the basement wasn’t cool enough for the right yeast action, but our little experiment was certainly memorable. Mom says the rhubarb wine she made turned out much better.
Web Editor Jenn Nemec’s mom also made her own yogurt, which Jenn does not remember with fondness.