Lesson 2 - Beer Foam
In this lesson we will discuss the physics and mechanics of foam formation and collapse, along with the chemistry of the compounds involved with beer foam.
Foam is affected by the brewing of raw materials and by wort and beer processing within and outside the brewery. The measuring of foam will be discussed and we will end the lesson by looking at gushing.
Beer Processing
Most of the beer processing effects on foam happen in fermentation where yeast is active. The plant design and use of process aids influence the outcomes as well.
Fermentation
Fobbing results in the reduction of foam potential. This is potentially the area of greatest foam loss. Vessel dimensions, fermentation temperature, cooling effectiveness, surface finish, and internal pressure all affect foam loss or fobbing.
Once foam formation has taken place in the fermenter the foam potential is lost, and subsequent beer foam will be poorer as a result.
Other important fermentation effects are:
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Beer movement needs to be gentle. Low shear variable speed pumps along with a design for a slow initial fill and gentle chilling, are all foam-positive.
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Yeast strain can affect foam. S cerevisiae yeast (typically ales) bind with foam stabilising surfactants. Beers brewed with S pastorianus (typically lagers) are more foam-positive as they do not possess the function to remove these proteins.
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Yeast health plays a significant role too. Unhealthy yeast secretes proteinase A, which digests foam proteins. Lipids can also be excreted from unhealthy yeast, adversely affecting foam. Conditions which induce yeast stress are important considerations in modern brewing practices. Ultra-high gravity brewing (> 18 °P), along with increased percentages of adjuncts and tall fermenters exerting pressure on yeast, all potentially contribute to stressed yeast. Poor yeast management practices such as the use of low viability yeast, late removal, excessive shear on yeast during removal, long storage, high storage temperatures, and excessive re-use of yeast (high generations) also contribute.
- Dried yeast is higher in proteinase A than wet yeast.
Filtration
Filters, especially sterile membrane filters are associated with reduced foam stability. This is likely to be due to the removal of foam positive material.
Process aids
These can be both foam-positive and foam-negative.
- Isinglass finings are foam-positive as they remove lipid.
- Historical haze reducing proteases are very damaging to beer foam as they have side activity on foam protein.
- Silicone antifoam preparations can improve foam by reducing the fobbing in the fermenter, thus preserving foam-positive material that would be lost on the tank wall. Overuse will be foam-negative if the antifoam is not all removed during filtration and it acts on beer foam in the glass.
- Process aids that encourage yeast growth such as yeast food and exogenous enzymes (to increase fermentability) can have a foam-negative effect through fobbing.
Foam Stabilisers
These are used by some brewers at filtration. The most common stabiliser is propylene glycol alginate (PGA), which is derived from a polysaccharide extract of seaweed. It is used as a foam protectant at a dosage of ~50 mg/L.
Foam is improved by up to 10% by PGA binding and destroying lipids, as well as acting as a foam protein stabiliser. The negatively charged PGA binds with the positively charged amino groups of the protein in the bubble gas liquid interface. This results in increased viscosity and stable foam.
PGA is expensive and can cause haze problems in beer if the preparation carries too much negative charge and overreacts with protein. PGA is an additive that remains in the final product and is considered by some consumers as a chemical addition to beer.