By varying the fermentation conditions, the same yeast can be used to produce very different products such as regular and light beers and malt liquors. In brewing, in contrast to the practice in wine making, yeast is virtually always collected after each fermentation and reused in subsequent fermentations. Brewers call the practice of adding yeast to wort 'pitching', although it most often is done by pumping yeast into the cooled, aerated wort stream as it flows to the fermenter. Spontaneous fermentation with endogenous organisms is done rarely, and only with a few specialty products like some of the Belgian Lambics and Geuezes. Lager yeasts throughout the world are remarkably similar and difficult to distinguish other than at the genetic level.
Ale yeasts vary in cell morphology.
Most commercial brewing yeasts are polyploid. At the end of a fermentation yeast flocculates and sinks to the bottom in a lager fermentation or rises to the top in an ale fermentation. Beer is always safe to drink, as no pathogens can grow in it.