Brewing much studied, the preservative nature of the soft resins is not yet fully understood, though it has been observed that unless stored at a cool temperature, the preservative nature will decrease. Brewing is the sole major commercial use of hops. Yeast is the microorganism that is responsible for fermentation in beer. Yeast metabolises the sugars extracted from grains, which produces alcohol and carbon dioxide, and thereby turns wort into beer. In addition to fermenting the beer, yeast influences the character and flavour. The dominant types of yeast used to make beer are "Saccharomyces cerevisiae", known as ale yeast, and "Saccharomyces
Low-alcohol beer brews used at festivities. More recently, the temperance movements and the need to avoid alcohol while driving, operating machinery etc led to the development of non-intoxicating beers. In the United States, non-alcoholic brews were promoted during Prohibition, according to John Naleszkiewicz. In 1917, President Wilson proposed limiting the alcohol content of malt beverages to 2.75% to try to appease avid prohibitionists. In 1919, Congress approved the Volstead Act, which limited the alcohol content of all beverages to 0.5%. These very low alcohol beverages became known as tonics, and many breweries began brewing them in order to stay in business during