Ask Mr. Wizard

Over-Carbonating My Bottles


Talley Pollard - Little Switzerland, North Carolina asks,

I have been having problems with over-carbonation in my bottles. I give my batches plenty of time to finish the secondary and check the hydrometer reading to be sure the fermentation is done. Then i add 5 oz. (0.14 kg) of corn sugar and wait about two weeks before sampling the first bottle. It is almost always over-carbonated and nearly all foam. What am I doing wrong? My brewery is at 4,000 feet, could my altitude be the problem?
Talley Pollard
Little Switzerland, North Carolina


I think your problem is too much sugar added for bottle conditioning. But before I jump into this topic, I want to focus on the state of beer when it is opened. All carbonated beverages are super-saturated with carbon dioxide, meaning that there is more CO2 in solution when the container is opened than permitted by the atmospheric pressure outside of the container. This is why carbonated beverages are fizzy when the pressure of the container is released.

In the case of beer, carbonation levels up to about 3.5–4.0 volumes or about 7–8 grams of carbon dioxide per liter cause little problem when a bottle or can is opened. Most beer in the world contains somewhere between 2.5–2.8 volumes of carbon dioxide (~5–5.6 g/L) and bottle conditioned styles from Belgium and German weizen beers are often in the 3.5–4 volume range. These beers do not typically gush, even when opened at higher elevations. I have enjoyed many a fine beer on the tops of mountain peaks without major incident.

The thing about super-saturated liquids is that anything that is a nucleation site can cause rapid and seemingly explosive gas release. A fun parlor trick in the kitchen is to heat water in a very clean stainless steel pot with fairly pure water. If you control things just right, which typically happens by mistake, you can cause water that is hotter than the boiling point, but not yet rolling, to gush into steam by tossing in a packet of powder or something as innocuous as a tea bag. The same sort of thing occurs when soda is poured over coarse ice cubes or beer is poured into a glass containing a few salt crystals. But under normal conditions, a bottle of beer can be opened and poured without too much fanfare.

So now it’s time to take a huge turn in the flow of this question. And that is onto the topic of why the metric system makes problem solving easy. Bet you didn’t see that one coming! Above I slipped in the metric equivalent to the volume, which is a unit that both makes sense and no sense at the same time. A liter of beer containing 3 volumes of carbon dioxide would fill a balloon with 3 liters of carbon dioxide if all of the carbon dioxide were driven from solution. And this cannot happen under atmospheric pressure. And doing any simple math with this weird term is simply not feasible. The metric system solves all of these problems.

Hold onto your bottle opener! When one gram of glucose is fermented by yeast (assuming 100% efficiency), 0.49 grams of carbon dioxide is produced. When you add 5 ounces of priming sugar (corn sugar, aka glucose) to your bottling bucket (I am assuming weight here, not 5 ounces of volume) you are adding 142 grams of glucose. And when that glucose is fermented by yeast during bottle conditioning it yields 70 grams of carbon dioxide. Add in the assumption that your nominal batch size is 5 gallons or 18.9 liters, this equates to 3.7 grams of carbon dioxide per liter of beer attributed by the priming sugar. But beer after fermentation and cold conditioning, even at atmospheric pressure contains at least 3 grams of carbon dioxide per liter of beer, bringing the total up to about 6.7 grams per liter, or 3.4 volumes in US terms. This is a wee bit on the high side of things, but nothing to give huge concern.

The assumption above about your hypothetical carbonation level assumes a beer volume of 18.9 liters (5 gallons). If you fiddle around with the numbers in my logic above with your actual bottling volume, say 15 liters, you will discover that you may have about 8 g/liter or 4 volumes of carbonation in your beer. This level of carbon dioxide coupled with your elevation very well could lead to gushing bottles, especially when dealing with beer that is likely to contain more yeast solids (nucleation sites) than commercial beer.

The basic problem is likely a result of using too much priming sugar. But the underlying problem, with this and others, may be that weights and measures cited in recipes are all based on wort and beer volume. If you follow a recipe for a 5-gallon (19 L) batch of beer and end up with only 4 gallons (15 L) the ingredient additions that are pegged to beer volume need to be adjusted. Likewise, if you are adding hops based on 10 gallons (38 L) of wort after boiling and you predict only ending up with 8 gallons (30 L), you should reduce hop additions by 20%.

Whenever I encounter a problem that simply does not add up, the first thing that comes to my mind is the accuracy of measurements. Many homebrewers don’t measure a lot of things because of the seemingly precise instructions of recipes. My bet on the cause of your problem is in part, if not entirely, due to assumptions made about beer volume at packaging, the weight of sugar required for the job and/or the relationship between sugar volume and sugar weight.

Response by Ashton Lewis.