Beer Science: Chemistry of the Beer Head - The Foam Factor

I sought to determine and understand the chemistry of one of the most aesthetic pleasing aspects of beer, the luscious foamy frothy head of foam.  Below are pictures of two different beers I have tasted in the past.  Notice the difference in the beer head, or amount of foam, at the top of each.  Each has a different amount, color, and consistency of beer foam.  

There were three questions I wanted to answer:

(1) Chemistry of the Beer Head: What is the chemistry of beer foam?
(2) Beer Head Size: Why do some beers form a larger head than others?
(3) Foam vs. Quality: Is the size of beer head indicative of beer quality?

(1) Chemistry of the Beer Head

What is the chemistry of the beer head?  I had expected this question would be an easy one to answer.  Well, I was wrong.  The chemistry involved in beer foam formation and retention is incredibly complex.  Although there seems to be some disagreement on the exact chemistry responsible for beer foam, there is agreement on the principles of the chemistry involved in creation of the beer head.

Let's start with what you know about beer.  Most beers usually some level of carbonation in the form of carbon dioxide (CO2) as a result of yeast fermentation.  These CO2 bubbles rise to the surface and make beer foam, right?  Well, that's part of the story.

Now, let's compare with two other drinks you may be familiar with.  Let's compare a soda, a root beer float, and a beer.  What happens when you pour a can of soda?  The trapped carbonation travels to the surface in the form of bubbles, the bubbles reach the liquid surface, and the bubbles immediately explode.  When you sniff a can of recently poured soda, you can feel the bubbles tickle your nose. 

Soda Fizz 

Now, think about a root beer float.  When you put ice cream in a glass and then pour root beer on top, you get an enormous foamy and long-lasting head.  And the bubbles that form are larger than normal soda bubbles.  Why is that?  The fat molecules from the ice cream actually coat and "protect" the CO2-filled bubbles.  As the CO2 bubbles rise to the surface they become stabilized by ice cream fats.  Therefore, root beer float bubbles do not burst nearly as fast as soda bubbles. 

 

Root Beer Float

So what's happening with beer foam is somewhere in between that of soda and root beer float bubbles.  But because there is so much chemistry happening in beer, the foam phenomenon is a lot more complicated and open to debate.  The reasoning behind beer foam is the same as that of the root beer float.  Upon pouring a beer, carbonation bubbles rise to the surface, and become associated (or coated with) with molecules found in beer.  But more akin to soda fizz, beer bubbles typically dissipate over time and are not as long-lasting as bubbles from a root beer float.

Researchers have isolated numerous proteins that they believe contribute to formation and longevity of beer foam.  Some of the proteins believed to contribute to the formation and longevity of beer foam are derived from albumin and hordein found in barley.  One particular albumin protein found in barley is called Lipid Transport Protein 1, or LTP1.  LTP1 was shown to be necessary in formation of a large foam head.  Yet another protein, called protein Z, affects the longevity of beer foam, and helps stabilize a foam head. 

Beer Foam

  
But whichever proteins may be responsible, the same principles appear to be in action as with other drinks.  Much like other carbonated beverages, opening a beer bottle or can causes carbonation bubbles to form as CO2 attempts to escape a now-depressurized container.  As these CO2 bubbles ascend to the surface of your glass, they get coated with proteins of various types.  These proteins form a protective coating around the CO2 bubbles.  And thus a beer head is created with relative strength somewhere between that of soda fizz and root beer float foam.


(2) Beer Head Size

Why do some beers form a larger head than others?  Research has shown that malt proteins contribute to head formation in beer.  So do beers that contain more barley produce larger beer heads?  More malt would mean more malt proteins.  The answer is yes.  If you have beer which is higher in malt proteins, these beers tend to form and maintain larger beer heads.  Wheat malt contains more protein than barley.  Brewers therefore may add wheat malt to beer (0.5 lbs. per 5 gallons) to increase the size of their beer head.

Barley

There are also lots of other factors responsible for beer head formation and stability.  Here is an list of some of these factors:

• Malt choice.  As mentioned, malt contains proteins that affect the size (LTP1) and stability (protein Z) of a beer head.
• Yeast happiness.  These organisms are very particular to being stressed out.  If you stress yeast out (low pitch rate, under-aeration, high gravity beers), they start making chemicals that could result in destruction of beer foam proteins.
• Glassware cleanliness.  Grease destroys bubbles.  You know this is you've ever put your finger in a recently-poured glass of soda.  The dirtier your finger the more rapidly the soda foam will dissipate.  Keep glassware clean if you are looking for good beer foam.
• Alcohol content.  Higher alcohol beers dissipate beer foam more rapidly by destroying the relationship between the CO2 bubble and protein coat.
• Physical factors.  Correct boiling time on the wort, temperature, nitrogen, carbonation level, and aeration levels can affect formation of beer head.

(3) Foam vs. Quality

Is the size of beer head indicative beer quality?  Many years ago, I used to be annoyed when my beer had a large layer of foam.  Now, I almost get disappointed when my beer does not produce at least a moderate level of foam.  I think that many beer drinkers would agree that beer foam causes a psychological and anatomical reaction in the brain.  Beer foam can deliver perceived and actual smell.  The presentation and visual aspects of a beer will certainly influence your perceived quality of a beer.  And everyone knows that how a beer smells affects how we perceive the taste of a beer.

If you drank two beers: one from a clean glass and one from a dirty glass (i.e. one had a large head of foam and one did not), would there be a difference in flavor, perceived or otherwise?  This sounds like an interesting experiment to conduct.  Anyone want to try it?  Let me know what you find.

In summary, the presence of numerous proteins in malt have a significance influence on the formation and retention of beer foam.  These proteins coat and protect carbonation bubbles as they reach the surface of your beer.  But beer foam formation and retention is also affected by many other variables.  Typically, beers higher in malt (and thus beers that possess a higher concentration of malt proteins; especially what malt) produce larger beer heads.



References:
Root Beer Foam
Barley Protein Foam Factors
Dressed in Lace
Beer Foam Techniques
Foam Science