Chemistry of Guinness: The Rise and Fall of Ireland's Most Prized Jewel

Chemistry of Guinness:

 The Rise and Fall of Ireland’s Most Prized Jewel

With the frothy head and the distinct color of the Guinness, there is seemingly no better beer to earn the name of the Nation’s Brew for Ireland, in which this beautiful creation originates from. But what makes this beer stand out from saturated field of brews that pop up everywhere nowadays? Guinness has stood up to the test of time, and arguably continues to remain at the top of the beer food chain across the world. In this blog, I hope to give a scientific backing to the rise and fall of the heavenly head of Guinness and offer a challenge to anyone who is willing to stare into the depths of the pint glass.

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  Background:

Sir Arthur Guinness of Ireland started his ale legacy at the St. James’s Gates Brewery in Dublin in 1759 where he put a 9,000 year lease on the brewery (1). During this time, he perfected every step of his beer making recipe, even formulating the proper way to pour the beer into a pint glass. It takes a total of nine in-depth steps to create Guinness. Which are as follows: ingredients, milling, mashing, separating, boiling, fermentation, maturation, packaging, and most important, enjoying (2).

Briefly going over the steps: during the ingredients step; malted barley, female hops, water and Guinness yeast are collected. However, not just any off the shelf ingredients are collected. The water for example, is prized for having high purity and softness, which in Ireland, is referred to as liquor because of the sheer importance of using this water for brewing. Also, the yeast that is used is highly specialized. The same yeast has been used from the time of Arthur Guinness himself, and a small amount is locked in a vault in order to replenish the supply if there were some unforeseen disaster that wiped out their reserves (2). The next six steps include the actual creation of the beer through distillation processes, but that would take up too much time to go through, so I urge you to click on the link at the end of this page to explore the brewing process in further depth.

The Lingering Head:

The idea of perfecting the pour (2) says that there is a very important aspect to Guinness that cannot be ignored. When pouring a pint of Guinness into a glass, the professionals know that the bubbles formed at the surface of beer must pass the “head height test” (2). During this test, the head must contain the proper amount of bubbles that separates Guinness from the rest of the herd. Interestingly, the amount of bubbles in the head equates to roughly 2 million bubbles (2) that contain a mixture of nitrogen and carbon dioxide gases. The gaseous mixture that gives Guinness its iconic head involves some very impressive chemistry, and even some physics. So the next time you go and enjoy a cold pint at your local pub, you will hopefully see the beer in a new light.   

  

So let’s dive into the frothy goodness and explore what makes the iconic Irish beer a true Irish gem. For starters, in the majority of most beers encountered, the carbonation during the bottling process is what forms the short lasting head because the gas used is strictly carbon dioxide. For Guinness however, the bottling process includes a combination of carbonation and nitrogenation. The addition of nitrogen gas reduces the bitterness of the hops to a certain extent, but you can read more about that in an article published by Chemical and Engineering News for a St. Patrick’s Day special (link on page 4: falling bubbles picture)². More importantly, with the addition of nitrogen, the head of the beer remains much longer than a traditional carbonated beer. This idea is directly related to how gases behave in accordance to Henry’s Law of gases. To understand how the head of the beer forms, we have to examine how this law pertains to packaged beer. But if you want a more general overview of Henry’s Law, please click on link at the end of page.  

Henry’s Law states that the solubility of a gas is related to the properties of the gas itself, the solvent type (usually water), the temperature of solvent, and most importantly for this blog, the surface chemistry taking place between the liquid and the surrounding environment. The chemistry that takes place at barrier between beer and the surrounding air is what causes the head of the beer. This surface interaction is related to Henry’s Law. This law says that as pressure increases, the amount of soluble gas increases within the solvent, and vice versa.

In a sealed container of beer, whether it be glass, can, or keg, the carbon dioxide gases continually escape and redissolve within the container forming a layer of CO₂ between the surface of liquid and cap of the container until the partial pressure of gaseous CO₂ is equal to the concentration of dissolved CO₂. Once you open the container, you are breaking the equilibrium formed by Henry’s Law. The atmospheric partial pressure of CO₂ is no longer in equilibrium with the dissolved concentration, therefore, the dissolved CO₂ will release into the atmosphere until equilibrium is achieved. In a perfect system, all the CO₂ into the beer will eventually be pushed into the atmosphere, but like I said before, at the surface of the beer there is some very interesting chemistry and physics taking place. For example, other factors in Henry’s Law are taken into account, specifically the properties of the solvent and gas.

It is obvious that water and air are two different states of matter that exhibit different chemical properties. The CO₂ cannot easily diffuse between water and air, and this is evident by the formation of the head. The head is a mixture of dissolved gas from the beer, and environmental gas trying to reach equilibrium. This however will never be accomplished with a carbonated beer because the atmosphere contains only 4% gaseous carbon dioxide. Since a carbonated beer contains 100% carbon dioxide, equilibrium will essentially push the CO₂ from in beer up and out into the atmosphere never reaching equilibrium since one beer cannot equilibrate the entire world’s concentration of atmospheric CO₂. This is why the head on a carbonated beer is very short-lived.

Fig. 1: Here is a diagram of Henry’s Law at work. You can assume that the piston represents the atmosphere. And the strength of the pressure applied, represented by the red arrows, corresponds to the amount of (a) CO2 and (b) N2 in the atmosphere. Since there is less gaseous CO2 in air in comparison to N2, there is less carbonic pressure applied to the beer, allowing the dissolved CO2 gases to easily diffuse into the air. The opposite is seen in (b) since the amount of atmospheric nitrogen is in a similar concentration of dissolved nitrogen within Guinness. Due to this reasoning, we see assume that atmospheric N2 is pushing down on the beer not allowing it to diffuse into the air, while the CO2 concentration in pushing up on the atmosphere, allowing diffusion to occur.

     It is at this point where the beauty of Guinness is truly revealed. Keeping what you just read about beer’s relation to Henry’s Law in mind, Guinness contains ~25% CO₂ and ~75% N₂ (3). Comparing carbon dioxide values, Guinness contains a quarter of the amount of CO₂ than a traditional carbonated beer. Since there is a small volume of CO₂ in Guinness, the dissolved gas will diffuse into the atmosphere at a slower rate than the fully carbonated beer, allowing the head to stay at the surface of the beer longer. What is fascinating about Guinness though is the nitrogen content. The nitrogen content in the beer is actually higher in the beer than in the atmosphere (~70%). Theoretically, this means that the atmosphere should push nitrogen gas into the beer to reach equilibrium (see Fig. 1). Which is almost true, to an extent at least. But if this were the case, the head of Guinness would only contain carbon dioxide gases that resemble a traditional carbonated beer. But this is not the case. Nitrogen gas does not like to be soluble, hence, the nitrogen will still try to release in the atmosphere while the atmosphere is trying to push nitrogen back into the beer. This push and pull of nitrogen to reach equilibrium causes the head of Guinness to not only remain on the beer for a longer amount of time, but have a higher amount of bubbles present. The large number of bubbles (remember that there is about 2 million bubbles there) causes the head to become frothy and creamy that we have all known to associate with the perfect pint of Guinness.

The Rise and Fall:

The thermodynamic equilibrium (Henry’s Law) between the nitrogen in the beer and the nitrogen in the air not only defines the texture of the head of Guinness, but also describes a weird phenomenon that the bubbles in the beer exhibit. If you are like me and get bored in the bar sometimes, you stare at the glass of beer in front of you and become mesmerized by the bubbles that form within the glass. If this has ever happened to you, you would have surely noticed that within a pint of Guinness there are bubbles that seem to fall downwards.

As we all know, bubbles are little packets of air. And since air is less dense than water, or beer for that matter, bubbles should rise up through the beer until it pops at the surface. This is how some of the dissolved gasses in the beer gets released from its soluble form into the atmosphere. The bubbles in the glass nucleate on little imperfections on the pint glass itself. It is at these points of nucleation where the dissolved gas molecules can aggregate until they become large enough to form gas bubbles. The formed gas bubbles rise up and interact with the atmosphere at the surface and readily diffuse into the atmosphere in accordance to Henry’s Law that I spoke about in depth at the beginning of this blog.

An interesting side note worth mentioning is the size difference between CO₂ and N₂ based bubbles. Carbon dioxide and nitrogen gases are similarly sized molecules, however, nitrogen based bubbles have a smaller volume than the carbon dioxide counterpart. The difference between the volumes is why the bubbles in a carbonated beer do not resemble the bubbles in nitrogen based beer like Guinness. The small volume of nitrogen based bubbles allows a higher quantity of bubbles to form within the head of the beer, which is also the reason why the head on Guinness tastes creamy and contains nearly 2 million bubbles.

Footnote aside, we have to remember that to reach equilibrium the atmosphere is pushing down on Guinness while the nitrogen content in beer is simultaneously pushing on the atmosphere, as shown in figure 1 on the previous page. And like I mentioned before, the nitrogen in the beer nucleates on the walls of glass resulting in the bubbles rising up towards the head. These are the nitrogen bubbles that are pushing on the atmosphere. There is no pushback from the atmosphere at this point because there is a high concentration of nitrogen bubbles coming from the nucleation sites on the glassware. However, in the middle of glass, there are no rising bubbles because there are very few nucleation sites at the bottom of the glass compared to the edges of the glass. Because of this, the atmosphere pushed down harder in the center of the glass than the edges. The push from the atmosphere causes the bubbles in the center of the glass to look like they are falling down. Once at the bottom of the glass, the bubbles are pushed to the edge of the glass where it can combine with the other nitrogen bubbles that are rising up. The path that the bubbles takes causes a small current within the beer (link at below of page) that keeps a continuous cycle of moving bubbles within the glass (3, 4, 5). This is the reasoning of why we can observe falling bubbles in of Guinness when we stare into the depths of the pint glass.   

Ireland’s Most Prized Jewel:

Guinness, the iconic Irish beer has much more than flavor that sets it apart from the rest of the beer world. From the unique combination of carbonation and nitrogenation to the phenomena of falling bubbles, it seems obvious why this beer remains to be an icon of a nation. However, the chemistry and physics involved in the final product of Guinness brewing is only one factor that leads the world to continually seek out this prized jewel. But you might be wondering why I keep referring to Guinness as a “jewel”, and I can tell you that it doesn’t merely mean that Guinness is a highly sought after precious stone. It also refers to another very important chemical aspect of the brew process that makes Guinness a literal jewel. So the next time you enjoy a pint at your local pub, I challenge you to uncover the ruby that Sir Arthur Guinness hid within every single ounce of liquid Guinness.

 References:

(1)   Yenne, Bill. Guinness: The 250 Year Quest for the Perfect Pint. Hoboken, New Jersey: John Wiley & Sons, Inc., 2007. Print.

(2)   The Beer Process.  Guinness & Co. 2008. Web. 23 Nov. 2015. <http:// www.guinness.com/en-us/thebeer-process-ingredients.html>.

(3)   Brunning, Andy. “The Chemistry of Guinness” Chemical and Engineering News 16 March 2015: 34. Print.

(4)   Shafer, N., et.al. “Through a Beer Glass Darkly.” Physics Today. Oct 1991: 48-52. Web.

(5)   Zhang, Y., et.al. “”Fizzics” of Bubble Growth in Beer and Champagne.” Elements. Feb 2008: 47-49. Web.

(6)   “Chemistry of Guinness: The Rise and Fall of Ireland’s Most Prized Jewel.” Unpublished, 2015. (*Note: This blog entry is adapted from a previously completed assignment for another class in which I am the sole author.)

(7)   All the pictures used in this blog are readily available from various  open-source web pages

Links for Additional Resources:

¹         Brewing Process

²         Henry’s Law

³         C&EN Guinness Article