is the building block of starch and as such is the primary source of fermentable material in wort and, subsequently, of alcohol in beer. Chemically, glucose is a polyhydroxy aldehyde, with a chemical formula of C6H12O6.

In starch, glucose occurs either as long straight chains (amylose) or as branched chains (amylopectin), in which the glucose molecules are joined by either alpha 1,4 or alpha 1,6 linkages. Brewing yeasts are incapable of metabolizing starches. These must first be enzymatically degraded during malting and mashing into mostly maltose, which is made up of two glucose molecules joined with alpha 1,4 linkages, and maltotriose, which consists of three-linked glucose units. Smaller amounts of glucose are also produced during mashing, particularly toward the end of the process. Typically, all-malt wort contains about 7% glucose, 45% maltose, and 20% maltotriose. The remaining roughly 25% of carbohydrates are sucrose and fructose, as well as unfermentable oligosaccharides and dextrins. During wort boiling some of the glucose reacts with amino compounds (mainly from amino acids) to create colored compounds, which both increase beer color and can impart toffee or caramel-like flavors. See maillard reaction. Glucose can also be added to the boiling process in the form of so-called hydrolyzed starch syrups or glucose syrups. These syrups are derived mainly from corn and wheat, whose starches are converted to a range of carbohydrates, some with high glucose contents. These syrups are used to supplement fermentable sugars derived from malt. In fermentation, maltose and maltotriose are transported into the yeast cell and hydrolyzed (broken down) into glucose. Glucose itself is transported into the yeast cell directly. The intracellular glucose is converted enzymatically into the intermediate compound pyruvate, which, in the absence of oxygen, is then converted to ethanol and carbon dioxide. Glucose is sometimes added to cask-conditioned beers as a priming sugar to initiate a secondary fermentation in the container, generating carbon dioxide to condition the beer and to encourage a small increase in alcohol content. See priming sugar. To the human palate, glucose tastes about 70% as sweet as sucrose and is therefore not likely to contribute excessive sweetness to a beer when it remains in solution in small amounts.