Ask Mr. Wizard

Over-carbonation issues


Greg Hutchinson — Essendon, Australia asks,

I seem to be having trouble with my beer when I bottle and leave the bottles for a while. They froth when I open them and keep slowly frothing after until half the bottle is finished. What can I do about this?


This general phenomenon is loosely referred to as gushing and has numerous causes in beer. Typically there is a primary culprit in a case of gushing, but it is not unusual for a single case of gushing to be caused by multiple factors. In order to keep this answer from becoming jumbled, I will address each cause separately.

Common causes of over-carbonation

Excessive carbon dioxide is the most common cause of gushing in homebrewed beer. This normally happens when too much priming sugar is added before bottling, but can also be caused by bottling beer with residual, fermentable extract. Most commercial beers that are bottle conditioned have been tested using the forced fermentation method to determine the final gravity while the batch is being fermented. This is an easy method where the test sample of wort is pitched with a very high concentration of yeast to accelerate fermentation; kind of a crystal ball approach that helps to see into the future. Although this method can be used at home (see page 82), it requires the brewer to sacrifice a bit of beer. Brewers can comfortably omit this method provided sufficient fermentation time and a stable hydrometer reading on 2–3 samples tested over 3–5 days at the end of fermentation.

Over-carbonation can easily be avoided by paying attention to the common causes. Here are a few tips on how to prevent this pesky problem:

  1. Always measure your beer volume before adding priming sugar. All too often, homebrew recipes prescribe the amount of priming sugar to be added to a batch. For example, add 2⁄3 cup priming sugar to your 5-gallon (19-L) batch. This sort of instruction is prone to error. A more precise and accurate instruction is to dose based on sugar weight and beer volume, for example adding 5.7 grams of priming sugar per liter of beer. The reason this is a better way of defining priming sugar additions is that granulated solids, like priming sugar, are compressible and should be measured by weight instead of volume, and a batch of beer does not magically yield a pre-determined beer volume to the bottling bucket.
  2. Use a carbonation calculator that takes into account the approximate amount of carbon dioxide in the beer you are bottling and the target carbonation level after conditioning has occurred. I am a bit old-school and like to grind through calculations using chemistry, paper, pencil, and a calculator. 1 mole of glucose (180 grams) yields 2 moles of carbon dioxide (88 grams), most beers after fermentation contain about 3 g of carbon dioxide per liter of beer, and most bottled beers contain between 5–6 g carbon dioxide/liter. Whatever the method, it should be robust.
  3. Unless you are an advanced brewer and really want to brew completely in accordance with the Reinheitsgebot, don’t use wort or “speise” as your priming sugar. This method requires the brewer to know how much fermentable sugar is present in the speise and requires a forced fermentation test and more math.
  4. Make sure your beer has stopped fermenting before cold crashing (if this is done) and bottling. Although this sounds like simple advice, there are many brewers who rely on airlock observations to determine when fermentation is complete. A stuck fermentation may appear to be complete, but add a bit of fresh bottling yeast, or simply rouse, and the fermentation may resume. This is why it is important to check your batch with a hydrometer; not only will this simple method indicate if the terminal gravity is stable, it also prompts the brewer to consider of the actual gravity is aligned with the expected gravity.

Over-carbonation associated with debranching enzymes

Most beers contain dextrins (branched carbohydrates) that cannot be hydrolyzed by alpha or beta amylase during the mashing process. Dextrins contribute body, some flavor, and calories to beer. Amyloglucosidase (Amylo for short) is a debranching enzyme that can be added to the mash and/or in fermentation to reduce or eliminate dextrins. Light beers and brut IPAs are produced using this type of enzyme. However, debranching enzymes have multiple exogenous sources; like the Amylo used for light beer, or found in hops, as well as diastatic yeast.

Diastatic yeast strains do not normally cause problems in beer when they are used as the primary fermenting strain . . .

So what about those funky yeast? Diastatic yeast strains do not normally cause problems in beer when they are used as the primary fermenting strain, for example in the production of saisons or funky Brett beers, they do contribute to increasing levels of carbon dioxide during bottle storage if added late in the game. Some beers, such as Orval, are dosed with Brettanomyces shortly before bottling and others are inadvertently contaminated with diastatic yeast strains. Over time, diastatic yeast strains will lead to increased levels of carbon dioxide in the package. Extreme gushing and even exploding bottles can result from the inadvertent inclusion of diastatic yeast in packaged beer. Diastatic yeast are almost always phenolic-off-flavor positive, or POF+; these yeast strains produce phenolic aromas during fermentation and are fairly easy to smell when present.

Beer gushing

Some beers gush, and the cause has nothing to do with over-carbonation. The term “gush” when applied to beer means that carbon dioxide uncontrollably escapes from a bottle or can when opened, oftentimes resulting in the entire contents of the package turning to foam. Gushing in beer is typically caused by one of two factors; oxalates and hydrophobin proteins. Malt contains oxalates that can survive into finished beer, where they precipitate as crystals. These oxalate crystals act as nucleation sites for carbon dioxide release, and can cause gushing. Oxalate is normally precipitated with calcium during wort boiling, and the most common method of oxalate removal is a vigorous boil in the presence of sufficient calcium.

Hydrophobin proteins are produced by certain fungi and, like oxalate crystals, act as nucleation sites for carbon dioxide release. When found in beer, hydrophobin proteins come from grain that has been infected with Fusarium in the field. Because Fusarium produces mycotoxins, unmalted adjuncts and malted cereals are tested for mycotoxins by suppliers. Even so, Fusarium-induced gushing is not uncommon. Grains can mold when stored in humid conditions, or when grain is accidentally wetted before or during storage. It is important for homebrewers to take note of any changes that occur during storage of grains at home, and to discard any suspect ingredients.

In summary, foaming bottles of beer can be caused by numerous factors. The good news is that good brewing practices will minimize this type of problem from occurring at home.

Response by Ashton Lewis.