Yeast is a chemotactic heterotrophic microorganism, which can directly absorb and utilize various monosaccharide molecules, such as glucose and fructose. Some yeasts can also metabolize and utilize pentose, ethanol or organic acids. Some disaccharides, such as sucrose, can be hydrolyzed into monosaccharides by extracellular enzymes and absorbed and utilized. Yeast cannot directly utilize starch and other polysaccharides. Therefore, in the process of beer brewing, raw wheat must be saccharified before it can be further fermented and utilized by Saccharomyces cerevisiae.