Expert: Ed Westemeier - 5/13/2011

Question
QUESTION: Hello,
I plan on starting to brew my own beer soon and have already bought all the equipment. I was wondering about the following:

Both WINE and BEER are produced with yeast fermentation. A glass of wine never has a head of foam. So WHY does beer have a head of foam? Is it that there is still some active yeast in the beer, whereas all the yeast in wine has been killed off by the higher alcohol content?
At what percentage alcohol does the yeast die? Also when gas is trapped in a champagne bottle, when it is opened and the gas escapes it never forms a head of foam like a beer. So what is the difference between beer & wine/champagne, where only beer can have a head of foam.

I live in Texas but am currently in South Africa, where I have internet access but no email access. A week from now I'll be back home & will see my emails then. If you can explain my questions I will be very grateful. I really would like to know what causes that head of foam on a beer.

Thank you for your help and time.
Frank Nelte


ANSWER: The head comes from the carbon dioxide which is dissolved in the liquid. Once the pressure is relieved (by opening the bottle, can, or tap), the carbon dioxide starts coming back out of solution in the form of bubbles. This happens with any carbonated drink, beer, sparkling wine, or soda.

The reason beer has a head is because those bubbles are coated in substances in the beer that wrap around the gas bubbles and stretch with them like tiny balloons, due to their high surface tension.

The substances come from natural ingredients in the malt and hops (proteins, polypeptides, polyphenols, etc.). Wine and soda don't have those ingredients.

Many of the big commercial brewers add chemicals to their beer to enhance head retention, but homebrewers and craft brewers can get excellent head retention by choosing their ingredients carefully. The more protein in the malt, and the more hops in the beer, the better the head retention will be.

As for the question about when the yeast dies, that depends partly on the yeast strain used. Some are more alcohol tolerant than others. In general, most brewing yeast can tolerate around 5-10% alcohol by volume, but when they run out of sugar (from the malt) to ferment, they pretty much quit.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply which I find very helpful. Other questions that now come to mind are:

Why does a Guinness have a head of foam that is so much different from the typical German beer? The Guinness head is thick and dense and long-lasting and almost creamy. Is that due to chemicals they have added? Also, would you say that above 10% alcohol BV (i.e. in wine) all the yeast has died off? And when you say that when they run out of sugar (say at 5% or 6% alcohol BV, etc.)that they then pretty much quit, do you mean that the yeast just becomes inactive at that point, while remaining alive in the brew, or do you mean that "when they quit" that the remaining yeast actually dies? I would take the difference to be that in one case the addition of more sugar would rekindle further fermentation, while in the other case the addition of more sugar would not result in any further fermentation and it would simply add sweetness. (I'm not thinking of adding any sugar at that stage ... it is just a hypothetical question to help me understand.) Thanks for your help.

Frank

Answer
The creamy head on a Guinness is due to several factors, all of which could be easily duplicated by other beers. First, they use a mix of gases, mostly nitrogen, instead of the simple carbon dioxide used by most beers to provide carbonation. Since nitrogen is much less soluble in liquid, it comes out of solution rapidly when the beer is opened (or poured from the tap). Couple that with the special tap they use in bars, which has very tiny openings to further force the gas out of solution, along with the widgets in the cans, and you get a rich, creamy head. The other side of the coin is that the beer becomes rather flat under that head.

As to the yeast, much of it dies at a given alcohol level (which differs for different strains of yeast), but some remain alive but dormant. Brewers (especially homebrewers) can take advantage of that by adding a bit of sugar when they bottle the beer. The remaining yeast will wake up and ferment the sugar, providing carbonation in the bottle.