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Many bars rinse a beer glass immediately before serving. I've consulted with bartenders and Google, and there's some confusion about it. Some sites claim it reduces foaming.

Another perk is that beer pours better onto a wet surface than a dry one as the friction of a dry glass can cause CO2 to come out of solution and create foam. source

Some seem to say it increases foaming.

When you rinse a beer glass, it becomes more slippery, and there’s less friction when beer fills it. This allows a more even, clean pour and a substantial, fragrant head. source

I'm skeptical of both claims. If water on a glass affects foaming, one might reasonably expect that the first drops of beer on the glass would have the same effect.

This smells like an urban legend to me. Is there any science that a wet beer glass has any effect on foaming?

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    Where does the second source say that there is more foam? – FooTheBar Sep 19 at 13:41
  • It claims that with a wet glass you get "a substantial, fragrant head", which implies that without a wet glass, you don't--or to a lesser extent. If it's not clear, head means foam. – Flimzy Sep 19 at 13:47
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    I think you misinterpreted the second quote. It argues that wet glass = less friction = finer bubbles, which would create a head with a firmer consistency and make it more "substantial". It doesn't claim anything about the amount of foam, only it's consistency. The first source claims that dry glass = more friction = more foam, which in turn would give the foam a fragile consistency due to bigger air pockets (bubbles). – Elmy Sep 19 at 14:02
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    "more slippery" - that's layman's terms for the ability of the surface tension of water to hold itself to the nucleation points of glassware which prevents the beer from coming into contact with them, reducing further off-gassing. You should have asked this on Physics ;) – Mazura Sep 20 at 3:20
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    I agree with @HeadhunterKev's conclusion (by the same proof method ;) ). erhaps the folks at Beer, Wine & Spirits.SE could produce some eveidence as well ... – Hagen von Eitzen Sep 20 at 17:46
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In 1953, a paper looking at some of the factors involved in beer foam answered this question almost as an incidental aside.

The main experiments described monitoring the CO2 emissions of beers and carbonated water over time, and the effect of bubbling CO2 through the liquid.

However, in the discussion section, the author slips in a description of another experiment.

Practical considerations in the serving of beer—effect of drying the glass.—The presence of bubble nuclei in glass of beer depends, as has been described, on the glass being imperfectly wetted. The wettability can be reduced by coating the glass surface with hydrophobic substances, such as silica or paraffin wax, but this is obviously not practical possibility in serving beer. An important practical consideration is, however, that the glass should be dried, because it is then less completely wetted than glass still damp from rinsing, and the evolution of carbon dioxide after pouring is correspondingly more brisk. In an experiment carried out to demonstrate this, two half-pint glasses, one wet, the other dry, were filled from the same bottle of pale ale. The concentration of carbon dioxide in both glasses immediately after pouring was 0.30%. After 30 min. there was no detectable loss of carbon dioxide from the wet glass, but the concentration in the dried glass had fallen to 0.24%. This was sufficient to maintain slight head, whereas there was no head on the beer in the wet glass.

Summary

[...] 3. The rate at which carbon dioxide is lost depends on the extent of bubble formation; this depends on imperfect wetting of the glass surface and it is therefore important that glasses in which beer is to be served should be dried.

Whether you agree with the tastes of the author in how much head is desirable on a beer, their experiment confirms that wetting a beer glass does change the foam characteristics.

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    Interestingly, it seems that neither the dry glass or wet glass have an effect on the CO2 immediately after pouring, but the wet glass does not lose as much CO2 over time, which counterintuitively means it loses its head over that time span, whereas the dry glass maintains it. Unfortunately, they don't seem to describe what the head is like immediately after pouring, at least in the quote you provided. – called2voyage Sep 19 at 19:33
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    But anyone who left the beer in the glass for 30 minutes wasn't interested in the head, anyway. – Ring Sep 19 at 19:56
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    This is interesting, thanks for posting! However, I don't know if I can fully trust this source - will have to test it myself. ...and of course, being a good scientist it may require multiple trials and various beer types for controls. – BruceWayne Sep 19 at 22:08
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    @Mazura: The trouble with such appeals to intuition is that they can be countered with equally plausible (or even more plausible) counter-arguments: I could argue that the beer itself is very wet. In a dry glass, the nucleation sites will rapidly get just as wet as they would have with water, making any effect negligible. Meanwhile, in a wet glass, the water will quickly mix with the rest of the beer, until it is merely slightly more diluted beer, making any effect negligible. It is a fine conjecture, only defeated by empirical evidence. – Oddthinking Sep 20 at 4:31
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    This answer would be improved by either stating the conclusion of the cited study (a dry glass leads to longer head) or at least highlighting the relevant portion in the citation. As it is, the one, single important part of the answer gets lost in irrelevant quibbling. – GreySage Sep 20 at 20:58
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Does wetting a beer glass change the foam characteristics?

Yes, it reduces nucleation on the surface of the glass; reducing foaming below the head. Bubbles under the head rise and enter the head altering the size of the bubbles there, larger bubbles in the head pop and drain further reducing the foam on top.

See: "Beer - A Quality Perspective", (287 pages) by Charles W. Bamforth, Department of Food Science and Technology, University of California, Davis, CA. Sections: Drainage, Creaming, Coalescence, and Disproportionation (pages 4-8).

If you want your head to last you want it to be composed of a thinner layer of fine bubbles rather than be any thickness with larger bubbles.

Page 2:

Foam and Cleanliness Too much foam with larger bubbles leads more quickly to too little foam. The image on the right is proof that the glass was clean when filled.

Page 3 & 4:

"Bubble formation and size
Despite beer being supersaturated with carbon dioxide, bubbles will not form spontaneously unless nucleation occurs (Figure 1.2), promoted by a particle, fiber or scratch in the glass (Prins and Marle, 1999) or the dispense mode, be that tap (Carroll, 1979) or bottle (Skands et al., 1999). These nucleation sites should ideally be small to create smaller bubbles that create foam that is most appealing to the drinker (Bamforth, 2004a). A desirable attribute of nitrogenated beers, due to the lower partial pressure of nitrogen gas compared to CO₂ is the production of much smaller bubbles (Carroll, 1979 ; Fisher et al., 99). Such principles are applied in the use of nucleated glassware such as the “headkeeper” style (Parish, 1997) or as a partial function of widgets (Brown, 1997 ; Browne, 1996) which will be discussed later in the chapter. Finally, the control of dispense angle and low dynamic surface tension leads to smaller bubbles with a homeodisperse size distribution, which results in the desirable “creamy” foam characteristic (Ronteltap et al., 1991).

Figure 1.2 Figure 1.2 Schematic representation of the bubble formation sequence (after Ronteltap et al., 1991).

Page 43:

Nitrogen vs. CO₂ Figure 1.17 Comparison of the foam bubble size of nitrogen vs. CO₂.

See also our sites:

How to clean a glass: YouTube video: "5 Beer Clean Glass" (Channel: Studiobar Boutique) (Presented by: Peter Coors) or view a written description: "Beer Clean Glass – Oxygen Out, Aroma In".

Some brewers offer specific pouring instructions that show use of a dry glass, that is their choice and doesn't change physics:

Other brewers recommend a wet glass:

If a glass wasn't clean and dry why would you put it in the cupboard, who would leave it out when it's wet, mouth up, to collect dust and debris. Glasses go face down on a drying table after washing and into a chiller after drying. Sometimes places have regulations requiring that the disinfection stage go last, in which case it's necessary to spray the inside with clean water immediately before use. Through chillers (door front and back) and lots of customers ensure that glasses aren't sitting around for long.

Different beers have different foaming characteristics, different parts of the world and different bars therein have different standards and customs. Some brewers want their beers served with more or less head than others and using their glasses. As pointed out in Bamforth's book, there's a 287 page explanation on foam alone.

If water on a glass affects foaming, one might reasonably expect that the first drops of beer on the glass would have the same effect.

With the wet glass technique the entire inside of the glass is sprayed with non-carbonated water, all nucleation points are wetted and dust or detergent (that should never have been present) is removed.

With the dry glass technique the "first drops of beer" are carbonated (and sometimes nitrogenated too) and pass over dry glass exposing carbonated liquid to nucleation points on the glass. That permits CO₂ and N₂ to be released beneath the head. With N₂ there is less of an increase in bubble size and less draining, with CO₂ that's not the case.

This smells like an urban legend to me. Is there any science that a wet beer glass has any effect on foaming?

Yes. See above, Bamforth's book quotes many papers and other books.

Additional references:

Some seem to say it increases foaming.

I don't think that is what they are saying. They say that the pour is clean and that the glass is cooled.

Point of interest: The second quote with a bit more context is:

"However, you might be asking yourself, “Why doesn’t the bar just prepare clean glasses beforehand? Why rinse them right before serving me my beer?” Well, the glass cleanser [star cleaner] doesn’t give your glass a thorough cleaning. Rather, it’s used to blast away away lingering dish soap, or other crud that accumulates when you’ve left a glass out for awhile.

But this sink actually serves a greater purpose than just cleaning your glass.

When you rinse a beer glass, it becomes more slippery, and there’s less friction when beer fills it. This allows a more even, clean pour and a substantial, fragrant head. As we’ve established before, beer foam is actually a good thing, because it carries a great deal of your beer’s aromas, and smelling is half the fun. The other benefit of rinsing a beer glass is that the spritz can cool the glass down, because we know that pint glasses often have the tendency to prematurely heat your beer up.".

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