A thick head of beer foam is one of those qualities of beer that divides people – you either love or loath it. For me, I have to admit that, I love to see a good beer capped with a crown of beautiful white foam. But I would say that, I have spent a good deal of my research career, within the brewing industry, looking at beer foam stability. However, in my eyes the quality of the beer, and my enjoyment of it, is reduced if there is no foam.
In Belgium as much as half of the content of the glass can be foam and that is perfectly acceptable. But, for some, foam is just wasted beer. In the UK, historically, there has been a geographical North/South split in the argument over whether foam is good or bad. For example, in the South, pub landlords are very careful to give you a full pint of liquid. However, our friends in the North, so it is claimed, will always insist on a good finger or two of foam on their beer and landlords will attach little plastic nozzles, called sparklers, to the dispense taps to improve foam formation. The argument on beer foam is not only reserved for the pub, it has even been subject of High court judgements which stated that foam should be regarded as an integral feature of beer but that the customer can, if they wish, ask for a full measure of liquid beer.
Love it or hate it beer foam is unique!
Interestingly beer is the only alcoholic beverage that produces and maintains a stable head of foam. Other beverages such as Cider and Champagne will only produce light foams when they are first poured but which will rapidly collapse. So why is beer the only alcoholic beverage capable of producing a stable head of foam? The science behind beer foam is far from completely understood and is astonishingly complex. But if you like foam on your beer and want to know how to retain it in beer then read on…
Beer contains Protein and Carbon Dioxide.
Beer contains a large volume of dissolved carbon dioxide and it is thanks to this gas that it is able to produce foam. Typically a packaged beer contains between 2.2 and 2.8 volumes of carbon dioxide. That is for every millilitre of beer there is between 2.2 and 2.8 millilitres’ of carbon dioxide. As such beer is described as a supersaturated beverage which means that the brewer has managed to dissolve more gas in the beer than is theoretically possible under normal living conditions (25ºC and 1 atmosphere of pressure). However, when you open a bottle of beer, apart from the initial pop of the bottle, the carbon dioxide usually remains dissolved in the beer. To generate foam we need to be able to get the dissolved carbon dioxide to come out of solution and for this we must have a process known as nucleation. Nucleation is a phenomenon which creates the bubbles you see rising to the surface of the beer whilst in the glass. Nucleation sites allow carbon dioxide gas bubbles to form, grow and eventually burst free and float to the surface of the beer. Typical nucleation sites include cracks and scratches in the surface of the glass the beer is served in, insoluble particles in beer or gas pockets introduced during dispense. In fact beer dispense is generally when the most foam is created as the turbulent flow introduced into the beer through pouring into a glass introduces lots of gas pockets which aid bubble nucleation.
The presence of carbon dioxide alone is not sufficient to generate and maintain a stable head of foam. It is the presence of protein in beer that allows it to produce a stable head of foam. Essentially the protein in beer stabilises the carbon dioxide gas bubbles. But the way that this occurs is quite complex and is worth covering in greater depth.
Why does beer foam collapse?
Once the beer has been poured into the glass and there is hopefully a nice covering of foam a number of physical processes occur which will cause the foam to collapse. They are foam drainage, bubble coalescence and disproportionation and it is worth discussing each in a little more detail.
From the moment that the foam is formed gravity causes beer trapped between the gas bubble walls to drain from it. Anything that reduces the rate that beer drains from the foam will help to increase the stability of the foam. So for example a highly viscous liquid, which is more resistant to pouring, will drain slower from foam, under the force of gravity, than a liquid of low viscosity. Beer has a viscosity only slightly higher than that of water so very little improvement in foam stability is as a result of viscosity. The problem with foam drainage is that as beer drains from foam the regions between the bubbles narrow until their walls touch leading to merging or coalescence of the bubbles to produce bigger bubbles. This coarsens the foam making it less attractive and crucially less stable.
A coarse foam, where the bubbles are all different sizes, leads to the final physical process involved in foam collapse, disproportionation. Beer foam bubbles contain carbon dioxide which as a gas exerts a pressure on the bubble walls. In small bubbles carbon dioxide will exert a greater pressure than is found in big bubbles. If a small bubble is sited next to a big bubble the gas contained within the bubbles will try to reach equilibrium. The result is the carbon dioxide dissolves through the bubble wall of the small bubble into the larger bubble. The small bubbles therefore disappear and the big bubbles get bigger and ultimately less stable. This phenomenon happens more quickly at higher temperatures, but is reduced if the gas pressure above the foam is increased. Next time you have a beer you can test this out for yourself by covering your beer glass. You should find that the foam lasts longer. This is often claimed to be the reason why German beer steins have lids.
Surface tension and beer foam.
Before considering what it is in beer that helps to stabilise foam this is perhaps a good point to consider the property of surface tension, an essential force in the process of foam formation and stabilisation. We have, I am certain, all experienced the primary school experiment where we first learn about the property of surface tension. A small pin is placed on the surface of a beaker of water and the expectant children wait for it to sink. However, this does not happen as miraculously the pin somehow manages to float. The reason the pin does not sink is due to molecular attraction within the liquid resulting in an elastic film at the surface of the liquid.
The best way to understand surface tension is to consider a single molecule within the main body of the liquid. The molecule is pulled equally in all directions by surrounding molecules and therefore has a net force of zero. However, a molecule at the surface of the liquid is only pulled downwards by molecules beneath it as there is no counteracting attractive force between the molecules at the surface of the liquid and the air above. Therefore the molecules at the surface of the liquid are subject to an inward force which is only balanced by how much the liquid can be compressed. In effect the liquid is squeezing itself together which drives the tendency to make the surface as small as possible, hence water droplets are round. Anything that stresses a surface to become bigger, such as gas bubbles, is in opposition to the surface tension which will try to restore the liquid to its original condition.
What is it about beer that helps to stabilise foam?
When gas bubbles are introduced into a liquid the surface area is increased which as we have already seen is in opposition to the force of surface tension and unless stabilised the bubbles collapse. In beer there are a number of materials that act to stabilise the bubbles of carbon dioxide and counteract the collapsing force of surface tension. The main stabilising material in beer is protein, which comes from malt. Proteins are wonderfully complex substances which have, as part of their structure, areas that are water loving (hydrophilic) and water hating (hydrophobic). It is those proteins which are mainly hydrophobic in character that migrate into and help stabilise the carbon dioxide bubbles. There they encounter other hydrophobic materials such as molecules from hops which give beer its characteristic bitter flavour. As an interesting aside if you taste beer foam it is quite bitter relative to the liquid beer, this is due to the concentration of the hydrophobic bitter substances in the foam. It is the interaction between the proteins and the hop bitter molecules that holds the bubbles together and produces the stable foam that we associate with beer.
Foam is a very delicate structure and just as there are materials in beer that promote foam there are other materials that will interfere with it. The main culprits are substances known as lipids, which include fats. Like proteins they come from malt and have areas which are hydrophobic and hydrophilic. Because lipids have regions of their structure which are hydrophobic they will behave in a similar way to foam stabilising proteins and migrate into the walls of the carbon dioxide bubbles. There, rather than stabilising foam, the lipids interfere with the interactions between proteins and hop bitter acids with the result that the foam collapses. Good brewing practice should ensure that very low levels of lipids survive into the final beer. It is much more likely that these substances are introduced into beer whilst it is in the glass from sources such as waxy materials in lipstick or any grease associated with typical bar snacks like potato crisps. In fact in those countries where foam is not seen as a positive feature of beer consumers have been known to dip a chip into their beer to assist its collapse.
Over the years brewers have tried to devise methods by which a stable head of foam will remain on the beer until the bottom of the glass. Two common methods involve introducing a little bit of nitrogen into the gases used for beer dispense and scratching the bottom of beer glasses to encourage carbon dioxide release from the beer. Nitrogen comprises around 78% of our atmosphere and it is not very soluble which makes it a very good gas for producing stable foam. Carbon dioxide is reasonably soluble in water and as such can dissolve through the bubble wall which is predominantly liquid. Nitrogen, because of its poor solubility does not dissolve so well through the bubble wall so the bubbles that are formed are far more stable. Thus nitrogen produces very creamy foams which are made up of very small bubbles which are highly stable. However, the drawback of using nitrogen gas is that it interacts with the taste buds on your tongue “softening” the bitterness of the beer and making the beer taste more creamy. Some consumers like this property where as other do not.
As home brewers how can we influence beer foam?
Well that is a good question and initially you could say that there is not much we can do. However, there are a number of stages within the brewing process where we can influence the final foam in beer.
If you consider that the presence of carbon dioxide and protein are essential for forming beer foam then anything that we do to increase the carbon dioxide in beer or preserve the protein content of beer should be positive to foam. For example you could consider the following:
- Mashing temperatures i.e. mashing in at 45-52°C to facilitate further protein breakdown does not help beer foam.
- Duration of wort boil – a longer wort boil will facilitate greater protein removal and can therefore be negative for foam.
- Fermentation temperature – a warmer fermentation temperature can give rise to a more vigorous fermentation and therefore generation of more foam which reduces the foam potential of the finished beer.
- Yeast health – Yeast that is stressed can breakdown (autolyse) and release proteolytic enzymes which can survive into the finished beer causing protein breakdown and therefore reducing the foam potential.
- Addition of hops – the hoppier the beer the greater the foam potential.
- Beer conditioning – The more carbon dioxide you have in your beer the more foam potential there is.
Love it or loathe it beer foam is one of the characteristics of beer that make this drink that we love so much so unique.