Yeast is a microscopic fungus which feeds of the fermentable sugars in the wort, producing alcohol and carbon dioxide (the little bubbles in your brew.) In the warm sugary liquid, yeast cells divide and grow rapidly forming first a scum then a yellowy-brown crust on the surface. British ales are brewed with the yeast Saccharomyces cerevisiae, at a temperature of 18-22 C.

Each specific yeast variety produces different flavours, and brewers will go at great lengths to preserve their own uncontaminated yeast variety. The remaining yeast continues to turn the sugar into alcohol, and helps purge the beer of rough after-tastes. The final treatment is the essential stage that determines whether the beer is a traditional cask conditioned ale (or 'real ale'), or a brewery conditioned product. All of TSA's beers are treated naturally, producing wonderfully fresh 'real ales'.