refined to separate any expelled beer from the wasted yeast, allowing it to flow back into the casks to continue fermentation. The Burton Union is credited with maintaining the strains of yeast used in Burton brewing in a relatively stable state until the mid-20th century. While not widely used in commercial beer production any more, the Burton Union principle is often looked on as a technical challenge for advanced homebrewers, and a commercial approximation of the design is available for experimenters. Marston's Pedigree is the only beer in the UK to use this method of production, while Firestone Walker in California uses a patented variation of the system. Barrique Brewing company in Tennessee uses an unmodified Burton Union system to produce all of its beers. Double dropping Double dropping, also known as the dropping system is a brewing method used for the production of ales. During the early 20th century it was the most popular method of clearing trub (inactive yeast and excess, staling and haze-forming protein from the malted barley) during fermentation for English ales. It is less commonly used today as it requires additional brewing vessels in a two-tier system. During the double dropping process the wort (newly brewed, fermenting beer) is first fermented for a period of time before being transferred, under gravity or by other means, into a lower vessel where it continues fermentation. The dropping process has two primary effects on the beer being fermented: the trub that has settled during the first period of fermentation will be left behind, leaving a cleaner beer and a cleaner yeast to crop from the beer for the next fermentation; the second effect is the aeration of the wort, which results in healthy clean yeast growth, and in certain circumstances butterscotch flavours from the production of diacetyl. Breweries using the double dropping process include Wychwood Brewery who contract brew Brakspear branded beers, and Flack Manor. Marston's use the name Double Drop for one of their beers as they use the related brewing method of the Burton Union system. Wychwood transfers the wort the morning after the day fermentation started – typically about 16 hours later. This process originally took place at the Brakspear brewery in Henley. When Brakspear moved to the Refresh UK's brewery in Witney, a new brewery was built to include the original double dropping system. Brakspear state that some of the flavour common to its