By Harold McGee
There are two basic methods for fermenting beer, and they produce distinctive results. One is rapid fermentation at a high temperature with ale yeasts (strains of Saccharomyces cerevisiae) that clump together, trap the carbon dioxide gas that they produce, and rise to the wort surface. The other is slow fermentation at a low temperature with lager yeasts (Saccharomyces uvarum or carlsbergensis) that remain submerged in the wort and fall to the bottom when fermentation is over. These are often called “top” and “bottom” fermentations.