As part of my interest in the science of brewing, I thought it might be interesting to write a little bit about sugar and the role it plays in beer.  Beer is composed of roughly four or five ingredients:  malted grain, hops, yeast, water, and occasionally an adjunct (spices, fruit extract, etc.) that adds some flavor.  Given the small number of components, you can imagine that the quality and composition of each has a drastic impact on the final product – poorly chosen grain, impurities in your water, or unviable yeast can all ruin what would otherwise be a great brew.

As you probably know, the malted grain used in beer is primarily a source of sugar – food for the yeast – and enzymes.  The mash process involves heating liquid and soaking the malted grain to extract the sugar, enzymes, and other molecules into the water solution.  The next step is to heat the wort to ~150 °F for a period of time, where an enzyme known as beta-amylase breaks down the starches, or complex carbohydrates (chains of sugars) into simple carbohydrates known as monosaccharides (single sugar units) or disaccharides (two sugar units). This particular temperature is chosen because it is where the hydrolyzing activity of beta-amylase has been shown to be most rapid. The major disaccharide product of the breakdown of starches from barley is maltose, and is where the process of malting gets its name.  Once broken down into simple sugars, the sugars can be eaten by the yeast.

Carbohydrates (also known as sugars or saccharides) are so-named because their molecular formula is (CH2O)n.  In other words, the formula can be thought of as a hydrate of carbon (equal equivalents of carbon and water).  Structurally though, you can see that the picture is much more complicated than that:  glucose, probably the most important sugar, is a quite complex molecule despite having only six carbons, and there is no actual water present in the structure.

Since sugars are the most abundant organic compounds on earth, it is not surprising that they are used a food source.  It turns out that yeast metabolize sugars in two fundamental ways:  respiration and fermentation.  Respiration occurs in the presence of oxygen, while fermentation occurs in the absence of oxygen, or anaerobic conditions.  During the process of fermentation, A sugar molecule is broken down into two molecules of pyruvate.  Following that, the pyruvate is further metabolized into carbon dioxide and ethanol.  So, from each molecule of sugar you get 2 carbon dioxides and two ethanol molecules (note that the number of carbons are conserved).

The yeast will continue to consume sugar and use the energy generated from oxidizing (burning) the sugar until growth is inhibited by high alcohol levels, or a lower level of sugar (depending on the beer).  The percentage of unmetabolized sugar in your beer after fermentation is referred to as attenuation, and often ranges from 65-85% (meaning 65-85% of the sugar is consumed).  Of course, the entire picture is much more complicated than this:  some sugars don’t even have six carbons, and can’t be metabolized by the pathway described.  Other metabolic pathways give rise to fusel alcohols, or alcohols with more than two carbons.  Fusel alcohols can give a solvent-like flavor to beer.  If you allow your beer to condition in a secondary fermenter, these fusel alcohols can be further metabolized into esters, which provide more desirable fruity flavors. So, if as a homebrewer you notice a solvent-like off flavor, one of the ways you might think about combating that is to be more patient and use secondary fermentation for a few weeks before bottling. Go buy some beer to drink in the meantime…

In any case, sugar metabolism is pretty clearly critical to the process of making beer.  There are dozens of different types of sugars in grain, and thousands of different chemical compounds in beer – the picture here is only the most fundamental steps.  This is really the simplest of introductions, but hopefully you learned something new about the role of sugar in beer. The next time you enjoy a brew, just remember you’re drinking billions of years worth of evolution and chemistry.  Then drink 3 more, forget it all, and enjoy the suds.



  1. Kid Carboy Jr. · · Reply

    Now we know! And knowing is half the battle.

  2. Thanks for reading, folks. We now return you to a more typical exchange here at Aleheads:


  3. Ha ha… he called the shit poop.

  4. glucanmaximus · · Reply

    On the subject of sugar, why (for some) Xtreme beer sucks.

    There ar 2 types of Fusel Alcohol’s that may be present in beer, aliphatic & phenol. I believe the good professor made mention of the aliphatic type, which is volatile & may condition out in the secondary. A Phenol alcohol presence usually results in a drain pour, as it usually worsens with age. One of the main reasons for preserving a yeast strain, is the elimination/reduction of fusel alcohol; a tired/mishandled yeast strain is susceptible to mutations which create a myriad of undesirable fermentation byproducts, chiefly among them the dreaded fusels. Aside from shitty yeast, elevated fermentation temps, &/or high original gravity’s (especially above 16 plato) can & will produce gobs of brain punishing fusel alcohol’s. Even though there is a strong correlation between increased sugar content (gravity) & presence of fusels, Amino acids rather than sugar is the culprit. Excessive amino acids are generally a result of excessive protein breakdown in the malting or mashing process. Fusel alcohol’s are metabolized from amino acids rather than sugars, however high wort gravity’s will frequently produce a greater amino acid pool.

    Lagers due to (hopefully) strain purity & low fermentation temps, generally create the least amount of fusel alcohol’s, while the shallow fermentation vessels in the the attic of a belgian abby can make some fairly brain crushing brews due to the wild yeast. The worst abomination in the zymological kingdom, is the addition of adjuncts. The sugar (maltose) content, as Prof. pH Lager pointed out, of malt has it limits. Barring the process of creating an eisenbock (which is considered a form of distilling), adjuncts are the rocket assisted means of taking a beer into the abv stratosphere. Nothing creates fusel alcohol better than adjuncts, especially dextrose monohydrate/C6H12O6 (corn sugar), which is widely used as culture medium for the fermentation industry to produce amino acids, among other products. Furthermore specialty yeast like WLP099 (super high gravity strain), has a propensity to create phenols & esters (nail polish) as gravity’s surpass the 16 plato threshold. In general terms if a brew reeks of solvent/nail polish, be assured there is a strong likelihood that fusel alcohol is present.

    All-malt strong beers, when executed well are indeed awe inspiring. Gobs of malt, hops, & low/no heat are IMHO hallmarks of brewing excellence. What pisses me off is the cheaters sliding adjunct fortified cloying corn oil laden hot ass franken brews badged with the promise of being crafted with the finest ingredients on planet fucken earth, & charging large pizza with all the toppings money for a 12 ouncer.

  5. Speaking of “brain punishing”, my head hurts.

    Thanks for edumacatin’ us, Maximus. As a non-insider, I love reading about this stuff. I find that a lot of “new” microbreweries have a tendency to produce beers with solvent flavors. I don’t know if that’s a function of the breweries still tweaking their recipes or perhaps just learning how to use the new equipment. Generally, I find that the beers and breweries that have been around the block awhile avoid these issues.

    You seem to imply that this may not always be the case, however. I’m curious…if you’re willing to relate a little more insider information, are there any particular breweries or just certain high-gravity brews that you think are poorly crafted as far as inclusion of fusel/solvent flavors are concerned? No worries if you don’t want to trash anyone in the industry. Just wondering if there are certain ale factories or beers we should be “on the lookout for” to get a sense of which beers fail in this regard.

    Thanks, as always, for your insight. Comments like yours are why we do this!

  6. Thanks for the insight glucanmaximus! I’m a homebrewer with formal training in chemistry, so hearing the explanations from someone with a serious background in brewing is fantastic. I know that yeast can generate some fusel alcohols by alternate pathways of metabolizing sugar, but as you mention they aren’t the major source.

    Please keep commenting, and I’ll echo Barley and say I’d love to hear more about what breweries you particularly like, esp. for the high gravity brews. as an amateur I am always looking for ways to improve my own beer, and now I know to avoid those kits that include corn sugar, particularly for the high gravity stuff. Corn sugar is abundant in the cheaper homebrew ingredient kits. Thanks again!

    I’ll see if I can dig up some information on fusel alcohols and write a more informative post in the future.

  7. glucanmaximus · · Reply

    Most “new” microbreweries are subject to the infamous “first brew blues”, a right of passage when learning the “Idiosyncrasies” of a new system. Even established breweries have their fair share of gremlins, so experience is the greatest teacher. If I were to venture a guess as to why your finding solvent flavors in some of these new start up’s, I would place the blame on technical errors or equipment failures (Idiosyncrasies), as opposed to recipe formulation & “tweaking”. Needless to say, learning curves can be a bitch (I could insert diatribe about brewery turn over & product inconsistency here, however I will spare everyone).

    Without going through the alphabet of beer spoilers, I’ll try to stay on topic, focusing on the fusel verity. Fusel alcohol is unacceptable when it’s presence is identified in beer, either by taste or mouth feel. Yeast deemed suitable for the production of beer will generally make 40-130 mg/L of aliphatic alcohol’s, & 10-80 mg/L of phenol alcohol’s, clean ale strains & lager strains will fall on the low end of the range. Wild yeast may see levels well above the 200/300+ range, moreover a wild yeast infections can play hell with fermentation. Purity of the yeast strain in conjunction with a sanitary fermentation vessel is essential. New breweries can be dusty places post construction, a failure to clean house (cellar) is a fairly common source of off-flavors.

    Mashing regiments must reflect the malt, moreover malt analysis. Again looking at contributors of fusel alcohol’s, care must be taken with malt that contains higher than avg. protein levels. It is desirable to dissolve/modify proteins as well as carbohydrates during the mashing process, however excessive protein breakdown may lead to elevated levels of amino acids. Amino acids are essential to yeast health, moreover they provide nitrogen which promotes metabolization, however this nitrogen (free amino nitrogen) in excess may become fuel for fusel alcohol’s.

    Controlled fermentation temperatures are essential to preventing the presence of fusel alcohol (among other byproducts) in a finished beer. Elevated temperatures during fermentation should be avoided as the yeast will rapidly metabolize creating many off flavors, & die prematurely leaving many byproducts in the beer that would otherwise be consumed during the later part of a “controlled” fermentation.

    The “breweries that have been around the block awhile avoid these issues”. By examining the brewing process, or “tweaking” an erratic cooling system, most capable breweries are able to correct the nail polish effect. There will always be fusel alcohol in beer, it’s up to the brewer/brewery as to how much is present. As I mentioned prior, high gravity’s posses the fuel to create high levels or alcohol in both simple (ethyl), & complex (fusel) forms. Many beer enthusiast (Extremophiles) revere the numbers (IBU’s, ABV’s, OG’s) of their favorite beers, & IMHO create a bias, favoring huge numbers. Savvy marketing has exacerbated this condition to the point where many huge, dare I say “Xtreme”, beers chock full of nail polish, &/or ridiculously under attenuated (due to piss poor micro practices), take top spots on “certain rating” sites. Pushing the parameter for the sake of numbers, at the expense of quality is, for lack of better words, is bull shit.

    As for the “certain ale factories or beers” I plead the 5th, however the sugar bombs I’ve made mention to do not hide their character very well. Any non allergy suffer should be able to smell the corn at the moment of lift off. Initially the alcohol will overwhelm the pallet(sp) followed by a harshness that may manifest into bitter/medicinal/solvent tones. Something hop’d to high hell, may help mask all but the alcohol burn. The main issue I have with large amounts of fusel alcohol, is they are considerably more toxic than ethyl, & will induce in many a beer drinker the “add-junk” headache.

    Mc Hops, Prof. pH, you are certainly welcome, I hope I didn’t put anyone to sleep while on the soap box. Over the years I’ve had great & horrible examples of big styles, imperials, barley wines, belgians, & so forth. For strong ales I like to keep the OG in the neighbor hood of 16 plato, trust me a well attenuated 16 plato beer will have you feeling pretty good after a couple pints. As far as large breweries I’ve had pretty good luck with Anchor & Sierra Nevada’s bigger styles in respect to beer cleanliness.

  8. Certainly didn’t put me to sleep — I love this stuff. I’ll have to read this again later and really digest it. In the meantime, I’m going to a bar to have a sierra nevada.

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