beer is due to a combination of its very old complex multi-strain yeast and the dropping method which encourages it to produce the butterscotch-flavoured compound diacetyl. Yorkshire Square A Yorkshire Square vessel is a two-storey fermentation system developed in Huddersfield by Timothy Bentley, proprietor of the Bentley and Shaw brewery. It is characterised by a shallow chamber approximately two metres high, above which is a walled deck. Cooled wort, the liquid extracted from malted barley, is fermented in the lower chamber, while the yeasty head settles on the deck above. During the first stage of fermentation, the fermenting wort is periodically pumped from the bottom of the chamber over the yeasty head, to keep the yeast mixed in with the wort. Later, the mixing is stopped and the wort in the chamber allowed to settle and cool gently. Most of the yeast rises onto the deck, and is left behind when the beer is drained from the chamber. The whole process takes at least six days. However, beer straight from a Yorkshire Square vessel will still have a harsh flavour, so the residual yeast is allowed to ferment any remaining sugar, producing a little extra alcohol and carbon dioxide, which mellows the beer. This conditioning begins in tanks at the brewery and continues after the beer is filled into casks, hence the phrase 'Cask Conditioned'. Historically, the Yorkshire Square vessel was constructed from Yorkshire Sandstone, but Welsh slate quickly proved to be the material of choice. A modern innovation is the so-called 'Round Square' pioneered by the Black Sheep brewery, which is built from stainless steel. The round shape makes it easier to clean out (a task that requires personnel to physically climb into the vessels) between brewing cycles. The Yorkshire Square fermenting system dates back over 200 years ago, and is still used by Samuel Smith's, the Theakston Brewery, Cameron's Brewery, Marston's and the Black Sheep brewery. References Brewing Beer in the United Kingdom