Beer is produced through steeping a sugar source (commonly Malted cereal grains) in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BC, and archeological evidence suggests that this technique was used in ancient Egypt. Descriptions of various beer recipes can be found in Sumerian writings, some of the oldest known writing of any sort. Brewing is done in a brewery by a brewer, and the brewing industry is part of most western economies. In 19th century Britain, technological discoveries and improvements such as Burtonisation and the Burton Union system significantly changed beer brewing. The methods used to produce beer may be unique to a beer style, geographic region, or company. Barrel aging Barrel aging is a process used to add maturity and character and additional flavour to a beer. Beers are aged for a period of time in a wooden barrel. Typically, these barrels once housed wine, rum, whiskey, bourbon, tequila, and other wines and spirits. Beers are sometimes aged in barrels to achieve a variety of effects in the final product. Sour beers such as lambics are fully fermented in wood (usually oak) barrels similar to those used to ferment wine, usually including microflora other than Saccharomyces cerevisiae. Other beers are aged in barrels which were previously used for maturing spirits. Stouts (particularly Russian Imperial Stouts) are sometimes aged in bourbon barrels. Goose Island's Bourbon County Stout was one of the first bourbon barrel-aged beers in the U.S., but the method has now spread to other companies, who have also experimented with aging other styles of beer in bourbon barrels. By the early twenty-first century, the method of aging beer in used wine barrels had expanded beyond lambic beers to include saison, barleywine, and blonde ale. Commonly, the barrels used for this had previously aged red wine (particularly cabernet sauvignon, merlot, and pinot noir). Some breweries produce exclusively barrel-aged beers, notably Belgian lambic producer Cantillon, and sour beer company The Rare Barrel in Berkeley, California. In 2016 "Craft Beer and Brewing" wrote: "Barrel-aged beers are so trendy that nearly every taphouse and beer store has a section of them. "Food & Wine" wrote of barrel-aging in 2018: "A process that was once niche has become not just mainstream, but ubiquitous." In 2017 Innis & Gunn decided that barrel aging didn't need to take place in a barrel and could be done in