The wine maker may allow fermentation to proceed relying only on the yeast naturally present on the grape skins and in the winery equipment or the wine maker may add extra yeast in a process known as inoculation. Two yeast species are used in fermentation, Saccharomyces cerevisiae or Saccharomyces bayanus (“The Wine Making Process” 2008). Yeast is responsible for the presence of positive but also negative aroma characters in wine. For example, when yeast is under stress it produces a compound called hydrogen sulfide, which smells like rotten eggs. To avoid this undesirable quality, a wine maker may add nutrients to the fermentation tank. The duration of fermentation also influences wine character.