Sucrose, a secondary product of photosynthesis and the primary transport sugar in many plants, is a disaccharide composed of glucose and fructose. These are both fully fermentable hexoses and are isomers (they have the same molecular formula but differing structures) of each other. The molecular formula of each is C6H12O6. Hence, the formula of sucrose is C12H24O12. Sucrose has an extract value of 381 degree l per kilo. Because a 40% solution of sucrose is often used for priming, adding 7 ml of the solution to 1 l of beer will add 1 degree l of fermentable extract.
Brewing yeast hydrolyses sucrose using the enzyme invertase prior to transportation of the products glucose and fructose across the cell membrane. The glucose is preferentially taken into the cell before fructose, leading to a cumulative imbalance in the amount of glucose and fructose in partially fermented worts where both are present. Once inside the cell, both sugars enter glycolysis, the first part of fermentation.
Most sucrose is derived from either the sugar cane plant or sugar beets. It is widely used by homebrewers as a priming sugar for bottle conditioning or to make up some of the extract in wort. It is often said that sucrose imparts a “cidery” or “vinous” flavor to beer and that excessive amounts should not be added to wort or beer to make up a shortfall in fermentable extract.