Brewed from grain, most commonly from malted barley, the most modern beer features hops, which act as a natural preservative and add bitterness and other flavors to the beer. Beer may use a warm fermentation process to yield sweeter, more intensely flavored ales; or, a cooler fermentation process may be used for milder, maltier lagers. Variations on these formulas have produced a wide range of beers enjoyed worldwide for hundreds of years.