A typical fermentation is almost completely anaerobic. The molecular oxygen that is available in the processed must or juice is quickly utilized by any microorganisms present in the medium. Yeasts require oxygen to synthesize and assimilate substances vital to their reproduction. If the must is not aerated, oxygen will become the limiting factor, yeast numbers will be too low to ferment effectively and the fermentation may end prematurely. In these circumstances it is not unusual for the fermentation medium to become reductive and hydrogen sulfide may be formed (Morgan et al., 2006). The modern trend in winemaking is to use large doses of selected yeasts that have been grown in hyperaerated media and are rich in the necessary growth factors. If aeration is required in order to enhance aromatic complexity, exposing the must to oxygen at the beginning of the fermentation is much less risky and more effective then at the end, when the yeast growth factors are depleted. Musts from rotted vintages should be aerated as little as possible due to the presence of oxidative enzymes in solution. Red musts can be aerated to a greater extent to increase the rate of fermentation (Morgan et al., 2006).