Lager yeast produce less fruity esters than ale yeasts but can produce more sulfur compounds during primary fermentation. Many first time lager brewers are astonished by the rotten egg smell coming from their fermentors, sometimes letting it convince them that the batch is infected and causing them to dump it. Don't do it! Fortunately, these compounds continue to vent during the conditioning (lagering) phase and the chemical precursors of other odious compounds are gradually eaten up by the yeast. A previously rank smelling beer that is properly lagered will be sulfur-free and delicious at bottling time. Speaking of Time...
...Lager comes from the German word "lagern" which means to store. A lager beer is in cold storage while it ages in the conditioning phase. Temperature influences lagers in two ways. During primary fermentation, the cooler temperature (45-55 °F) prevents the formation of fruity esters by the yeast. In addition to producing fewer byproducts during the primary phase, the yeast uses the long conditioning phase to finish off residual sugars and metabolize other compounds that may give rise to off-flavors and aromas.
