The Oxford Companion to Beer definition of
Glycogen is the major carbohydrate in yeast cells. It comprises multiple-branched molecules of glucose linked alpha 1,4 in chains and alpha 1,6 at branch points. It typically makes up approximately 20% to 30% of the dry weight of yeast cells. Glycogen serves as a store for the yeast’s biochemical energy, which it uses during the lag phase of fermentation, when growth is limited. See lag phase. Ideally, the glycogen content of pitching yeast should be high, because the glycogen store is rapidly depleted during the first few hours after pitching as the yeast converts glycogen to lipids. Low glycogen levels in pitching yeast are indicative of unsatisfactory yeast handling and are associated with low yeast viability, extended fermentation times, and high levels of diacetyl, acetaldehyde, and sulfur dioxide at the end of fermentation. The glycogen content of yeast can be determined by a number of methods but the simplest is to stain the yeast with a tincture of iodine (Lugol’s stain). High glycogen levels stain deep brown, whereas low levels stain a pale yellow.
Russell, Inge, “Yeast.” In Handbook of brewing, 2nd ed. Fergus G. Priest and Graham G. Stewart, eds., 281–332. Boca Raton, FL: Taylor & Francis, 2006.