Why is the beer foam so thick

purity command

In Germany the head of foam is a must. But does it also speak for special quality, does it improve the aroma? Conversation with Christoph Neugrodda, who is doing his doctorate on beer foam.



F.A.Z .: Can you tell the type of beer by its foam?

Christoph Neugrodda: It is possible to differentiate between light and dark beers based on the color of the beer foam. The foam texture or the bubble size distribution can also help. Another possibility is to draw conclusions about the type from the stability of the foam - a Pils beer should have a particularly stable foam due to its chemical composition.

While most of the light ones collapse more quickly.

That is not said. I have already analyzed a lot of beers and it is not generally the case. But it can be said that top-fermented beers achieve higher foam values ​​than bottom-fermented beers.

That surprises me. Wheat beers often have a bomb foam, but British ales, also top-fermented, sometimes have hardly any foam.

Yes, that's interesting - and as is so often the case in the brewery: “No rule, no exception”. The reason is that beer foam is not popular everywhere. There is a north-south divide in England. The closer you get to London, the fuller the beer glass has to be. But it can also be explained quite well why English beers naturally have little foam. Because English malt is relatively strongly dissolved, which means it contains fewer high-molecular proteins. Whereby proteins are essential for the beer foam and are also referred to as the "backbone of the beer foam". Finally: In terms of bunging, i.e. the setting of the CO2-Content in the beer, English beers have significantly lower values. This also creates less foam.

The CO2 push the foam?

Yes, it creates foam. The quality of the beer foam is made up of two components: stability and activity. If it has no foam activity, there is no foam at all. In general, you need surface-active substances that can form foam and you need an initiator. The latter is the CO2 or another dissolved gas in the beer. Now you still need nucleation sites - scratches, cracks, grains of dust or other bumps - where the gas can be released, and this is how the foam is created. When you think of Guinness, which is traditionally tapped with mixed gas - CO2 and N2 - the nitrogen creates a very special foam structure, a very creamy foam with high stability.

© dpaSo finely, it foams mainly with nitrogen in the beer

What exactly is beer foam?

From a highly scientific point of view, beer foam is a visco-elastic-plastic yield-stress fluid from the area of ​​soft matter. But in a more understandable and correct way, beer foam is a dispersant, i.e. a liquid phase - the beer - in which a disperse phase - a gas, for example CO2 - is finely divided. Simply put, the gas is surrounded by the liquid and this is how the foam bubble is created.

Why does this characteristic structure develop in foam?

This has to do with the formation of the foam and also with its decay. When you pour yourself a beer, you always have what is known as spherical foam at the bottom, with relatively round bubbles. This foam has a high proportion of liquid. As the foam breaks down, the liquid runs off as a result of the drainage and the foam dries out as a result. The dry foam is then more polyhedral. Everyone can easily try this out for themselves.

How does the color of the foam come about?

By refractions in the liquid film around the gas bubbles. There are surface-active substances in beer, mainly proteins, but also hops ingredients. They stabilize the gas bubble structure. The liquid film, which is called the lamella, separates the individual gas bubbles from one another, but the lamellae can also fuse with one another. The liquid film is nothing else than the beer. The foam color is created depending on how thick the lamellas are and what color the beer is. The wetter the foam, the more beer it contains and the more typical the foam becomes.

How do I get as much protein as possible in the beer?

Mainly about the malt. This is also the challenge when brewing beer; people once talked about the yin and yang of beer foam. Because, to put it quite simply, everything that is negative for the chemical-physical stability of the beer, i.e. the stability of preventing the formation of cloudiness, is very good for the beer foam. If you have a lot of protein in your beer, you will also be able to develop a better foam.

Does wheat malt produce better foam than barley malt?

When the brewer speaks of a good foam, he usually means the foam stability. Strictly speaking, one would have to speak of the beer foam quality, which is made up of the foam activity and stability. In wheat they definitely have a significantly higher protein content, and accordingly they have a higher foam stability than in barley malt.

What role does yeast play in the foam?

The yeast has a great influence on the stability. An extreme example: If yeast is left on the beer for too long and goes into autolysis, a certain enzyme is formed, proteinase A. It can only break down a single substrate, that is LTP 1, a very foam-positive protein. When brewers have real foam problems, it is mostly due to fermentation, to the quality of the yeast.

From the barrel or from the tank, the foam at Pilsner Urquell is completely different.

I recently drank a Pilsner Urquell tank beer, not pasteurized, without adding CO2 tapped. The foam was incredibly creamy, not to be compared with the one from the bottle. How can this be explained?

Yes, that is an interesting phenomenon. If you only have naturally bound carbon dioxide, the foam is somehow more stable. If, on the other hand, you make beers that are carbonized again before filling, you may have poorer foam stability than before with the tank. The binding of the CO2 in the liquid is just looser.

Can you say: super foam is the same as good beer?

For the German beer drinker, a stable, fine-pored foam is a sign of high quality. But that is a subjective assessment. There is research into how much the foam rating is country and even gender specific.

Does the foam say something about a special treatment of the beer, which in turn promotes a special taste? You are working in Weihenstephan on a research project on the topic: "Influence of texture and molecular composition of the foam on the aroma release from beer".

One mustn't forget that the beer foam also has a great sensory function. The beer foam acts as a so-called diffusion barrier. A beer with foam will stay fresh and tangy longer than one without. The release of gases and aromatic substances is inhibited by the foam head. Therefore, foam-free is always poured at professional tastings. In addition, our research project has shown that certain aromatic substances tend to accumulate in the beer foam. When you drink the foam with the liquid, you get a kind of aroma boost, as I would like to call it. In the retronasal perception, in the oropharynx, you have increased this release of bound substances, with the hop aromas playing a special role. A creamy foam also gives a completely different mouth feel. With a pure beer foam tasting, you will notice: It makes a difference whether you taste a foam that is only drawn with air, which is also possible, or with CO2 or nitrogen. The foam just bursts differently. The dry foam that was mentioned earlier also tastes significantly different from a wet foam.

There are beer shakers that swirl their glass from time to time while drinking so that new foam forms. The only disadvantage is that it also means recency, CO2, get lost.

You are right - in England there are so-called headkeeper glasses, in which a pattern has been scratched into the bottom of the glass, which results in an effect similar to the mousse point in a champagne glass. On a rough surface, CO is preferentially released2. You then have a permanent rising of bubbles. I always have a certain layer of foam on these glasses, but the beer becomes relatively varnished pretty quickly. That is the difference between foam activity and real foam stability.

I once read that the foam - this is particularly useful at large events like Oktoberfest - also dampens the spilling of the beer.

Yes, that was a great job done by physicists. This has to do with the cushioning properties of the foams. Something similar is the reason why espresso or cappuccino is easier to wear than coffee. It has already been considered whether this property should be used in the transport of dangerous goods.

The biggest enemy of the beer foam in the glass is probably the fat.

Yes, and detergent residue, rinse aid. Anything that lowers the surface tension ensures that foam is broken down more quickly and less foam is formed. Therefore, beer glasses should always be rinsed with clear water before pouring, also to smooth the surface inside so as not to create additional foam. The so-called bubble pelts in the beer glass are bad when a lot of bubbles form in one place. In this case, the glass has not been rinsed properly and there is still some dust in the glass.

Even if the beer is very cold, the foam often develops sluggishly, right?

Yes, for the same reason, hot beer is so popular in summer. The solubility of gases in liquids depends on the temperature. With a cold beer, the CO2 more stable in the liquid, there is less foam.

The German beer laws prohibit it, but which foam stabilizers are there?

In addition to certain hop products or metals (iron), PGA (propylene glycol alginate) is also used in England. This creates a very nice foam - very white, very fine-pored, especially in connection with nitrogen. But these products are banned in Germany.

Are there alternative tricks in Germany - the use of a highly concentrated hop product, for example?

That would then be hop extracts - and according to the German purity law, they may only be added in the hot area, but they would be more effective in the cold area. In addition, the German brewer doesn't really need anything like that. All of this can be achieved through technology, raw material selection and high yeast quality. What is also possible: If I notice that I have a foam problem, I can add pointed or short malt, for example. This is undissolved malt, so you bring in more of these viscosity-increasing or higher molecular substances and get a better foam.

Is adhesive foam in the glass a quality feature?

This is what is known as lacing, a property that beer drinkers like to see. For many, the drink rings in the glass, with which you can understand every single sip, are very popular.

Do you also speak for high quality?

Yes, there is research showing that foam adhesion correlates with foam stability.

In your dissertation you deal with the question of how beer foam can be improved. Who still needs tutoring? The established brewers will be satisfied with their foam, won't they?

Since the established brewers also use raw materials that are subject to natural fluctuations, knowledge of the beer foam is essential in order to maintain the unique selling point of the beer and, if necessary, to improve it. It would be a shame if my hoppy, highly aromatic beer had no foam and the aroma was quickly outgassed.

What else could the future bring? What possible foams have we not yet seen?

What could come in the future: foams glowing under black light maybe. Even if these were not in accordance with the purity law. And of course you can work with different gases and mixing ratios. We have just spoken of nitrogen. However, other gases and more suitable mixtures would also be conceivable. Perhaps there will also be the ultra-stable beer foam at some point, which would make the analysis much easier.

What would happen if you were to use nitrogen in the Pils?

That's not a problem at all, I've already done that myself. They create a particularly creamy foam and therefore have a different mouthfeel. However, nitrogen is not used alone, but as a mixed gas.


© privateChristoph Neugrodda

Christoph Neugrodda, Born in 1983, apprenticeship as a brewer and maltster at the Bitburger Brewery (2003 to 2006); Degree in brewing and beverage technology from the Technical University of Munich (2012); since 2012 research assistant at the chair for brewing and beverage technology in Weihenstephan.

Keywords: beer foam, gas mixture when tapping, headkeeper glasses, short malt, lacing, PGA, foam quality, foam stability, nitrogen
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What does the foam reveal about beer?

By Uwe Ebbinghaus

In Germany, the head of foam on beer is a must. But does it also speak for special quality, does it improve the aroma? Conversation with Christoph Neugrodda, who is doing his doctorate on beer foam.

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