Double drop brewing, 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 2-tier system, which adds cost, but which is being used by the new Colchester Brewery as it improves the quality of the finished beer.
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.
All in all it produces a better beer, especially in those years when the quality of malt has been affected by poor barley quality at the time of harvest, as occurred in 2011, which results in higher nitrogen levels and as a consequence unwanted enzymes. Double-drop brewing eliminates these.