Writer Alex La Guma once called pubs “a fountain of wisdom and a cesspool of nonsense.” So, you can understand why Charlton’s Matthew Daley is a little skeptical of something he heard one night at his local establishment, while enjoying a few draft beers with a friend.
“Every time we took a sip, the head of the beer would leave a ring on the inside of the glass,” he explained.
That’s when the nugget of wisdom (or nonsense) was dispensed. Matt can’t quite remember if it was the bartender or his friend, but one of them said, “When the head of the beer does that in the glass after each sip, it’s an indication that the glass is clean.”
And like beer foam to the side of a glass, the exchange has stuck with him.
“I’ve been pondering it ever since. I’ve been curious about it and been wondering how true it can be?”
Daley reached out to the Curiosity Desk in the hopes that we could get to the bottom of what’s going on at the top of his beer glass. And I was more than happy to take up the case.
I began my quest to find an answer by visiting a few bars myself to get a sense of how widespread this so-called fact is among area bartenders.
Some, like Russell Trainor at Avenue in Somerville’s Ball Square, looked at me like I had two heads when I brought it up.
“Never,” he said. “Never have I ever heard of this.”
Others, like John Tringale at Carroll’s in Medford had, but were a little fuzzy on the details.
“I forget what it is, but the glass is either clean or dirty if it has, like, the suds on it,” he said.
Still others, like Daren Denis at Davis Square’s Elm Street Tap Room were in the same boat as our Daley.
“One of my managers was telling me that story so I just believed him,” said Denis. “Seemed like a cool thing to say. So yeah, [I] don’t know the exact science behind it all, but hey!”
And so, I turned to someone who might know the exact science behind it: A scientist who’s an authority on what he calls the “FIZZics” of beer.
“There are a lot of interesting physics and chemistry going on in a glass of beer,” said Stanford University chemistry professor Richard Zare.
The bubbles that make up a beer’s head are due to carbon dioxide that is dissolved in the beer. Zare said that, in general, for bubbles to form in a liquid — a process called nucleation — they need a place to do it. Aptly, it’s called a nucleation site.
“Where to the bubbles form when you start to boil water?” Zare asked. “Do they form in the middle of the water? Never. They actually form on the wall — the sides.”
It’s the same with beer that’s being poured in a glass. The bottom and sides of the glass provide nucleation sites for thousands of CO2 bubbles. From there, Zare said one of two things typically occurs: The bubbles stay small and get crushed, or they grow large enough to escape.
“And then they rise and they go to the top of the surface,” he explained. “And so you form the head on things like a beer.”
But each bubble has a little business to take care of during its journey to the head of the glass.
“On its way up, it is doing some other chemistry: it’s grabbing onto other compounds that are in the beer,” said UMass Amherst food science professor John Gibbons.
Those other compounds include yeast, some residue from the hops, and a few proteins — including one that is crucial to answering Daley’s question: Lipid transfer protein one, aka LTP1.
“This LPT1 protean is found in most plants,” Gibbons said. “You find it in barley, wheat, corn, rice, you know? All these fermentation grains.”
And as a lipid, LPT1 is extremely hydrophobic, which means it really does not want to be in or around water. Unfortunately for LPT1, a glass of beer is mostly water. But all those CO2 bubbles that are produced when the beer is poured offer the LPT1 a chance to make a break for it.
“This is like its opportunity to get out of the water,” Gibbons said. “So, it clings onto the bubble and takes it for a ride to the surface.
Gibbons says that all of the bubbles that comprise a beer’s head are laden with LPT1, just looking for a way out.
“But the ones that are on the side of the glass can grab onto something,” Gibbons explained. “They can anchor onto something. And that’s why you see — every time you take a sip — you get this really nice line. It’s that protein that really doesn’t want to interact with water and it’s just sticking to the side of the glass.”
There’s a name for this phenomenon. It’s called lacing. And while the amount of lacing will be affected by everything from the type of beer you’re drinking to the speed at which you drink, Gibbons says one thing is universally true: The glass really does have to be clean for it to happen.
Gibbons said that substances on the inside of the glass can discourage or entirely prevent lacing from occurring. That includes a number of materials most commonly responsible for a dirty glass in a bar or restaurant — like lipstick, grease, and soap residue.
“All of that stuff can interact with the foam and kill the foam and the head,” he said. “And end up killing the lacing effect, too.”
Now, there is one notable caveat. Stanford’s Zare says if you see bubbles clinging to the side of the glass below the surface of the beer, there’s a good chance the opposite is true. Scratches in a glass — but also things like lint, food particles or other specks of grime — can give those bubbles that are too small to escape a place to hide and not get crushed. So, if you see bubbles dancing around the sides of your glass, it might not be as clean as it should be.
If there is something you've been itching to know more about, email The Curiosity Desk and Edgar might just look into it for you. For more from The Curiosity Desk, follow Edgar B. Herwick III on Twitter and subscribe to the GBH News YouTube Channel.
