Beer Science Shenanigans
Alright boyos, I saw this article by Dylan Eiger [Dylan graduated with us and is getting his MD/PhD at Duke Med] a while back and I had to delve into some of the details.
First, take a look at this gif of a pint of guinness:
Now compare it to this:
Look at the bubble flow patterns…see the difference? The guinness bubbles seem to sink while the bottom beer bubbles float upwards.
Surprisingly, this phenomenon was of interest to scientists (there are even write-ups about it in prominent journals like Science). Unsurprisingly, those scientists were in Ireland [mostly].
There are a couple of facets that go into this, so let’s break it down step by step.
First, carbonation. Guinness is technically a nitro brew beer, which means that the brewmasters add nitrogen for carbonation during the brewing process. Nitro brews are typically 70% nitrogen and 30% CO2 (as opposed to 100% CO2 in typical beers) and are most often used for stouts or porters.
Before we get into the effects of nitrogen on the bubbles, you should know the effects of nitrogen on the taste: 1) The relative insolubility of N2 in beer is what creates the thick, creamy texture of the beer and 2) The inert chemical properties are what leave Guinness tasting smooth (CO2 likes to react with water, releasing hydrogen ions and making the beer acidic and sharp).
Now for the bubbly effects. Because of the way the nitrogen affects the viscosity of the fluid and because of the properties of nitrogen gas itself, nitrogen bubbles are far smaller than CO2 bubbles — on the order of 0.05–1mm vs. 1–1.5mm. So what are the implications? Well, close your eyes and imagine you are in a pool holding a tennis ball and a basketball (s/o to Dylan for this great visualization), which is easier to push down? The tennis ball, there’s a smaller buoyant force pushing it upward (note that the buoyant force still exists, it’s just smaller).
Alright, so the smaller bubbles are easier to pull down…but so what, they’re still bubbles and they are going to float upward regardless. Here’s where our Irish scientist, Dr. William Lee of Limerick (gah, what a wonderful title. Also St. Patrick’s Day appropriate) comes into play. He had some theories about Guinness bubbles that he put to the test. He took two glasses — one traditional, which narrows at the base, and one “anti-pint glass” which widens at base — and poured Guinness into them to see what happened.
I’ll give you a second to visualize it and take a guess what’s going on. If you’re struggling to do so, check out this video to see the results of Lee’s experiment:
Did you see it now? The bubbles in the pint glass were clearly falling, but the bubbles in the anti-pint glass were either stationary or moving up! Lee realized part two of this puzzle was that glass geometry altered the fluid dynamics because of the small bubbles and the thick viscosity. Specifically, Dr. Lee’s conclusion was that in the normal pint glasses, bubbles rising decrease “bubble density” along the edges of the glass, while in anti-pint glasses, bubbles rising cause increased bubble density on the glass edge. What does this mean? Let’s look at the illustration below from Lee’s paper.
Lee described bubble density roughly as the number of bubbles within a certain volume of space within the fluid. Part (A) illustrates a typical pint glass scenario, with a wide rim. Notice how as bubbles float up they separate from the glass and leave a region of low bubble density by the glass edge. Look at Part (B) now, which shows the anti-pint scenario. See how the bubbles approach the glass edge? That causes increased bubble density along the sides of the glass.
Again, so what? Here’s where it all comes together beautifully. The bubbles in the traditional pint are consistently moving away from the glass walls, so where do they go?-The center of the glass. This causes a column of high bubble density in the middle of the glass. The bubbles are still bubbles, so they want to float up. Remember, Guinness is quite viscous, and the bubbles move upward with some small amount of force. Because of this, the nitrogen bubbles induce a convection current in the fluid. As the bubbles are pulled from the side of the glass to the center, bubbles from the head of the beer are pulled back down along the edges to the areas of low bubble density to replace those that were just sucked into the middle. Here’s an illustration of the currents to make it easier to follow:
Eureka, there are your descending bubbles.
A few other notes on Guinness/nitro beers:
- Have you noticed the tap that bartenders use to pour Guinness in bars? It looks like this:
Remarkably, there’s about a million resources for where to get these taps and when you use them, but very little scientific explanation of exactly how they work. From what I understand, the insolubility of nitrogen in beer makes it very unstable. Small agitation should induce the expansion of the nitrogen into gas, causing more bubbles to form. That’s the purpose of this tap — it’s called a “restrictor faucet” by the way — it initiates the cascade of bubble cavitation when pouring, allowing for a nice full head to form . Without it, you would get a pour with far less head (hate to see that) and a texture that feels off.
In cans, Guinness was the first company to find a way to induce the same cavitation cascade with a device called a widget. You are probably familiar with it as the thing that rattles in guinness cans when you rock them. Widgets come in a variety of sizes and shapes depending on the use, but in cans they look like this:
This is essentially a ping pong sized plastic ball with a little hole in it. The can is under very high pressure and forces some of the nitrogen into the widget. Once the can is opened and depressurized, the nitrogen is forced back out through the hole, causing a chain reaction of bubble formation throughout the beer and creating that good good head.
2. Guinness is ~4.2% ABV, which means if you’re soft and pretending to be hard, you could take a couple Guinnesses to the face with fewer effects than a “regular beer”
3. If you’re trying to maintain dat figure, rest assured that guinness is fairly low-cal: only 198!
4.There is indeed a perfect amount of time it should take to pour a pint of Guinness: 119.5 seconds. (there are a couple of steps involved, look it up here)
Alright team, that’s all I’ve got for now. Hope y’all enjoyed
Sources:
http://giphy.com/search/guinness
http://www.hopculture.com/why-do-guinness-bubbles-sink-and-go-down/
http://aip.scitation.org/doi/10.1063/1.2929369
http://pubs.acs.org/doi/abs/10.1021/jf062973%2B
http://www.emba.uvm.edu/~tlakoba/07_fall/math_022/ExpDecayBeerFroth.pdf
http://aapt.scitation.org/doi/full/10.1119/1.3620416
http://aapt.scitation.org/doi/10.1119/1.4769377
http://www.irishcentral.com/roots/top-10-guinness-facts-93321039-237782481
