Abstract: The instant invention relates to processes and systems for the fermentative production of ethanol. The ethanol is produced by fermenting a fermentable carbohydrate with a yeast, whereby acetaldehyde is externally supplied to the yeast cell for reducing glycerol by-product formation by the yeast cell, for improving the performance of yeast at high ethanol levels and/or for suppression of infections during the fermentation. The acetaldehyde that is externally supplied to the fermentation medium can be produced by catalytic oxidation of ethanol. Advantageously, acetaldehyde production from ethanol is integrated in a system for ethanol production. Thus, in another aspect the invention relates to a system for producing ethanol, e.g. an ethanol plant, which system, in addition to the usual means for fermentative production of ethanol, comprises a means for producing acetaldehyde by catalytic oxidation of ethanol.