Lesson 2 - Beer Foam

Gushing is the effect of rapid CO₂ microbubble formation, expansion and release of the gas when discharging the bottle contents from overpressure down to ambient pressure. Foam is formed that then overflows the bottle. In extreme cases, it shoots from the bottle like a fountain, leaving little beer behind. It is a serious beer defect resulting in immediate consumer dissatisfaction and should be dealt with as the highest priority.

Gushing can be categorised as either:

• Primary – sporadic and related to malt defects

• Secondary – related to production faults or poor handling of the beer

Primary gushing

Epidemic occurrences of gushing in beer have been reported from different barley growing regions. These occur during periods of high rainfall before harvesting and are linked to mould activity on the barleycorns (typically Fusarium species, commonly F. graminearum and to a lesser extent, F. culmorum).

The following video shows beer gushing caused by Fusarium:

It is important for maltsters to measure the metabolites of Fusarium, notably deoxynivalenol or DON. This mycotoxin is otherwise known as vomitoxin.

As the name suggests, it causes nausea and vomiting in humans. DON is not the gushing factor, merely the most reliable marker for infection.

High levels of DON will cause a maltster to reject a batch of barley, and levels on a malt certificate of analysis (COA) will cause brewer rejection too. Maximum EU levels in processed cereals (malt) are set at 750 µg/kg (ppb).

The key to managing primary gushing is in the field. Maltsters and brewers have strict specifications in relation to Fusarium markers and will not buy infected grain. Breeding barley for disease resistance is a long-term goal; treating the crop with fungicides will provide a short-term solution.

Hydrophobins are a group of short-chain cysteine-rich polypeptides of 7-20 kDa that have been implicated as a cause of primary gushing. They are excreted by filamentous fungi and, like their name suggests, form impermeable hydrophobic coatings on their substrates (5-10 µm micro CO₂ bubbles formed during fermentation and subsequent processing).

The microbubbles are stabilised and prevented from further shrinkage and disappearance. When a packaged bottle is opened, these bubbles rapidly expand and explode because of high internal CO₂ pressure. This induces a CO₂ release from the beer into the gaseous phase. Fragments of the hydrophobic bound bubble wall act as further nucleation sites, resulting in bubbles rising en masse causing uncontrolled gushing.

To safeguard against gushing, the hydrophobin level in malt should not exceed 250 µg/g. Hydrophobin levels greater than 3 µg/L induced gushing in trials.

Secondary gushing

Secondary gushing is related to unintended nucleation sites in beer that give CO₂ an opportunity to break out of solution. These can be derived from several sources such as:

• Calcium oxalate – Malt contains oxalic acid and also calcium. Brewery water is often supplemented with calcium to a level of 50-100 mg/L. If there is enough calcium, it will precipitate oxalic acid as calcium oxalate, which are tiny crystals making up a portion of what is known as beerstone. This precipitation usually occurs in the mashing and wort boiling stages as well as in fermentation and maturation. Any remaining suspended crystals can be removed at filtration.

If calcium oxalate remains in the packaged beer, the crystals act as nucleation sites inducing the rapid release of CO₂ – resulting in gushing.

• Metal ions – metal ions, such as iron in its ferric form Fe³⁺, have been implicated in gushing. The ferric precipitates form to act as nucleation sites for CO₂ release. Nickel and cobalt are also mentioned as less important gushing agents.

• Surfactants – these are used in bottle washing and can cause gushing through lowering of surface tension. The gushing risk is higher with lower surface tension. Routine Q&A checks should eliminate this risk.

• Isomerised hop extracts – these products are often used to adjust beer bitterness in maturation and filtration; they include the reduced iso-α-acids. Oxidation of iso-α-acids can lead to gushing initiators. Dehydrated humulinic acid was shown to be a potent gushing initiator at 25 mg/L when added to beer. This can be prevented by both good storage and handling of the products.

• Diatomaceous earth – small diatomaceous earth (DE) particles, along with beer soluble ions extracted from this filter medium, can induce gushing. Both beer soluble iron (BSI) checks and good filter management, can assist with managing this infrequent causes of gushing.

• Beer packaging – bottles, crowns, cans, and liners have been connected to secondary gushing; however, these factors do not cause gushing by themselves. Perhaps the biggest risk in packaging is over carbonation on its own or in conjunction with N_2.

Gushing predictive tests

Currently, no reliable test exists for measuring gushing potential. The Carlsberg and modified Carlsberg tests have been used, but are generally considered not sufficiently reproducible, and are time-consuming to conduct. In the original test malt and water are mixed together in a blender, and 50 ml are added to a trial beer bottle, which is closed and shaken for 3 days. The quantity of beer lost on opening is determined. The modified test accepted by MEBAK excludes beer variables by using carbonated bottled water as the base. A third ‘doubly-modified’ Carlsberg test uses an EBC congress wort preparation added to carbonated water. This is shown to express gushing to a greater extent than the previous two tests.

It should be noted that testing of barley and malt before intake allows the user to accept or reject material that has the potential for causing gushing.