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More importantly, how can the process or recipe be adjusted to reduce this effect?

I'm not trying to be crude. Actually, I'm trying to be less crude.

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hahaha...well stated. –  Brandon Nov 24 '10 at 3:35
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4 Answers

I have heard that beers containing high levels of residual complex sugars can contribute to additional unwanted gas production as the bacteria inside our digestive system breaks the sugars down. However, no doubt there would be other contributing factors, such as what else you were eating & drinking.

Based on the residual sugar line of thinking, you could try anything that results in higher attenuation of the beer, eg, using more simple sugars, mashing at a lower temperature, making sure you pitch the correct amount of viable yeast, and otherwise ensuring that the yeast can work in the best conditions.

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It's the alcohol, but the answer gets more complicated:

Beverages with a weak alcoholic content (<5% ABV) are mild stimulants of acid secretion. Interestingly, stronger concentrations of alcohol, such as standard 80-proof liquors (vodka, whisky, rum) don't stimulate gastric acid secretion or release of gastrin. The powerful stimulants of gastric acid secretion present in beer, which are yet to be identified, are thermostable and anionic polar substances.

I doubt it's sugar, because then cake, cookies, and sweets would cause flatulence, which isn't the case.

One possible solution is to brew bigger, stronger beers. (As if you needed a reason to brew big beers!) Personally, I've noticed I can drink barleywines, imperial brews, trippels, and other beasts with no ...problems... the next day, but if I have more than 3 light beers, things get ugly.

source:
PubMed Central (http://www.ncbi.nlm.nih.gov/pmc/)

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Cakes, cookies and sweets contain simple sugars which are easily digested. Beer contains a far more complex mix of sugars, some of which may be large enough to survive into the lower intestine, where they make a tasty snack for gas-producing critters. –  rainoftoads Nov 24 '10 at 18:47
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The sugars that cause gas are lactose, fructose, raffinose, and sorbitol. Hopefully you don't have the last two in your beer. Fructose is completely fermentable, and the only beers with lactose are milk stouts, which only cause gas in those deficient in the lactase enzyme. Why does everyone think it's the sugar? –  Brandon Nov 24 '10 at 19:00
    
I always thought it was yeast driven. But I totally agree its not sugars, unless you have some really weird sugars as suggested. –  brewchez Nov 26 '10 at 3:04
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Flatulence is caused by yeast. I first experienced this when drinking German beer in Germany. The beer is not pasteurized and therefore contains active yeast. This is also true of homebrewed beer. Unless you are pasteurizing your beer, it contains active/living yeast which continue to ferment (produce gas) after you consume it.

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What can be done about this, short of pasteurizing? –  spoulson Nov 24 '10 at 13:47
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Filtering, using highly floccing yeast. Or, just deal with it and toss back another delicious hefeweizen! –  DHayes Nov 24 '10 at 14:00
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I would have thought that stomach acid would kill any yeast pretty quickly. –  Fishtoaster Nov 24 '10 at 15:10
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Flatulence from beer is caused by the presence of complex sugars. While simple sugars are easily broken down and metabolized by either the yeast our yourself, the complex sugars persist until your lower intestine. There, they are fermented and consumed by bacteria and other gut flora, producing gas as a side effect.

You can actually add an enzyme like Beano to the brew, but it may result in a loss of some flavor. (Though I have heard it suggested for making a low-carb beer.)

The best measure is prevention - careful control of mashing temperature such that the natural enzymes in the malt have time to break down these complex sugars. Allowing for a rest at each of the key active temperatures for the various enzymes give these enzymes a chance to break down these starches and sugars - while just one spike above a critical temperature is enough to denature and inactivate the enzymes, leaving more complex sugars behind.

A similar principal applies for for cooking beans. Beans are best soaked overnight - the old-fashioned method - to give the enzymes a chance to work. Many recipes today call for a quick rehydration by boiling - which immediately kills off the enzymes and makes for an uncomfortable experience for many. As with brewing, patience and some advance planning pays off.

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If using off the shelf malt extract, what preventative measures could be taken? –  spoulson Nov 24 '10 at 16:28
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