Gushing, also known as fobbing, is the occurrence of more or less vigorous overflowing of beer upon opening the bottle or can. Generally anything that promotes the rapid release of the gas in beer will cause gushing. Any particulates that form sites for carbon dioxide to gather (called nucleation sites) essentially cause the gas to be released very rapidly.

Gushing has been attributed to several factors. “Primary gushing” is attributed to beers brewed from defective malt (derived from barley harvested under wet conditions) and is apparently the major cause of the issue that shows up periodically in commercial beers. The substances responsible for gushing are hydrophobic polypeptides derived from several mold strains—most commonly those denoted as Fusarium. The components have been given the name hydrophobins.

Other causes of gushing fall under the term “secondary gushing” and include but are not limited to the presence of metal ions such as iron, nickel, and tin; calcium oxalate; some isomerized hop extracts; a rough surface (forming the nucleation sites) on the inside of the bottle; and inadequate washing and rinsing of bottles prior to filling. Calcium oxalate can also be an issue. Oxalate anions, derived from the husks of the grains used in brewing, can precipitate out with calcium and form distinct crystals, which form the seeds or nucleation sites for CO₂ breakout. This can be an especially difficult problem for brewers producing bottle-conditioned beers with high levels of carbonation. Brewers encountering gushing caused by oxalate crystals will attempt to ensure that enough calcium is present at the start of the brewing process to allow the precipitation of the oxalate at various stages in the process prior to filtration and bottling. Unfortunately this is a delicate system of chemical equilibrium and calcium addition is not always permanently successful. If enough calcium is used to end up with 80 mg/l of calcium in the finished wort, the oxalate cause of potential gushing will be minimized. In 2001 German researchers argued that 99% of gushing is caused by calcium oxalate in beer.

Prevention of malt-based primary gushing depends on the control of malt quality, whereas prevention of secondary gushing requires the removal of other potential nucleation materials. The treatment of beer with adsorbents and tight filtration can help. Brewers experiencing gushing problems will minimize iron content from brewing water and/or DE filtration medium (leaching caused by reverse flow) and analyze variety-dependent hop alpha acid/polyphenol ratios. In the latter case, polyphenols have been cited as tending to act as gushing promoters, as do very tight naturally derived beer protein clusters.

Of course, at a very basic level, any factor that causes beer to be severely overcarbonated will cause gushing, whether it is improper application of carbon dioxide or overattenuation or overpriming of bottle-conditioned beer. Overattenuation problems are common in bottle-conditioned beers using wild yeast strains such as Brettanomyces, which can continue to ferment out sugars that Saccharomyces strains will not.