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…