Redox aspects of yeast alcoholic fermentation versus carbonic maceration Valeriu V. Cotea, Cristinel V. Zănoagă, Cintia Colibaba Romanian Academy - Iaşi Branch – Research Centre for Oenology, Aleea M. Sadoveanu 9 700490 Iaşi, Romania Tel/fax: +40-232-228228 oenolab@yahoo.com Key words: redox potential, fermentation, carbonic maceration, ethanol, glycerol Abstract Through fermentation an organism obtains its energetic necessities so that, viewed globally, this is actually an oxidation process. The alcoholic fermentation of must is nevertheless a reductive process, if considering the reductive direction of the redox potential’s evolution. The most important de novo created substances are ethanol and glycerol, so their redox characterization has been obtained. Ethanol – oxidative over a large range of concentrations (1/1,000,000…1/50) – has a reductive behaviour for high values of concentration (1/50…1/1), while glycerol shows a similar behaviour, except in one domain (1/500…1/100), acting as an oxidative agent. In white micro-winemaking of Merlot grapes, an obvious increase of the redox potential has been observed in a first phase, followed by a decrease. A possible explanation of such phenomenon might be that the first fermentation stage is dominated by glycerol formation that, between 1/50 = 0,2% and 1/10 = 1% imparts an oxidative character to the environment. In the second phase, both glycerol and ethanol have a synergic behaviour, being reductive. Fermentation may be provoked by the vegetal cell itself, in an anaerobiosis state. Such a process has been employed as a comparison term for the process of yeast fermentation. Indeed, with the same grapes, subjected however to carbonic maceration, in parallel with the already mentioned alcoholic fermentation, a natural evolution of the redox potential was observed, in oxidative direction, this type of evolution being described by the rH = 23.341 + 0.2125⋅t regression straight line. Introduction Carbonic maceration may be viewed as a non-conventional wine-making process, once the alcoholic fermentation is not produced by yeast but, instead, by the – compulsorily living – very cells of the grape, forced – by its complete isolation from oxygen – to apply the principle of Pasteur and to develop fenmentation, necessary for obtaining the metabolic energy. Starting from the only existing difference, namely the (genetic) heterotrophic character of the yeast and, respectively, autotrophic character of the vegetal cell, the authors initiated a comparative, general analysis of the two processes. To this end, an equally global significant marker has been selected, namely the redox character of the medium within which each process in part is developing. Material and method Fresh grapes of the Merlot kind, harvested in 2005 from the Dealu Bujorului vineyard have been employed in the experiments as biological material.