With that said, one of the major differences between S. cerevisiae and S. pastorianus is that S. pastorianus is a much more efficient maltotriose fermenter. Maltotriose is a three-part sugar (trisaccharide) that consists of three glucose molecules bound together by a glycosidic bond. This sugar is formed in the mash by alpha amylase via the hydrolysis of starch. S. cerevisiae is much less efficient at breaking the glycosidic bond that holds maltotriose together; therefore, a higher percentage of this trisaccharide remains in solution after fermentation is complete. The efficient fermentation of maltotriose is the major reason why lager beer is less sweet than ale, as maltotriose is the second most abundant sugar in wort.
Another difference is between S. cerevisiae and S. pastorianus is that S. pastorianus produces fewer esters and more sulfur when used correctly (the S. bayanus genetic contribution allows S. Pastorianus to ferment at temperatures where S. cerevisiae gives up the ghost). Most amateur brewers who are unaware of this difference are horrified by the smell of hydrogen sulfide (H2S) gas the first time that they encounter it. However, H2S production is a very common with S. pastorianus. While sulfur is part of the lager taste profile, it needs to be kept in check via good fermentation temperature control and minimizing the production of dimethyl sulfide (DMS) and dimethyl sulfoxide (DMSO) during wort preparation.
