Issue link: http://winesandvines.uberflip.com/i/172581
winemaking vate is cleaved to form carbon dioxide and acetaldehyde. In this situation, acetaldehyde rather than O2 serves as the terminal electron acceptor, becoming converted into ethanol. Oxygen has additional roles in the yeast cell and is involved in many biosynthetic reactions that require electron movements. Many key biological reactions are, in fact, oxidation-reduction reactions. Oxygen can be both a direct participant in electron-transfer reactions or used in enzymatic-oxidation reactions in microbial systems. Oxygen addition can enable metabolic activities of non-Saccharomyces microbes present and stimulate the yeastfermentation process. Molecular oxygen is necessary to act as the hydrogen acceptor in the first reaction of sterol biosynthesis, the folding of the precursor compound into the sterol form, catalyzed by squalene oxidase.5 Molecular oxygen is also required as the hydrogen acceptor during introduction of double bonds in the generation of unsaturated fatty acids, by the enzyme desaturase.5 Formation of these unsaturated long-chain fatty acids and sterols are termed "survival factors." Without survival factors, yeast would not be able to overcome the ethanol toxicity of high-ethanol concentrations that develop during fermentation. These components are also needed as growth factors for yeast cells. Thus, oxygen availability will impact metabolic activities of the yeast and yeast cell composition, which, in turn, will impact the contribution of yeast to the sensory profile of the finished wine. These effects are indirect and can be difficult to predict. With respect to microbial activity, oxygen can impact both the viability and metabolic activities of microorganisms. The activity of those microorganisms can have an impact on wine composition independent of any direct role of oxygen. The microbes can also produce endproducts—such as sulfur compounds, alcohols and aldehydes—that can participate in the oxidation-reduction reactions occurring in the juice or wine, or that can be substrates of oxidation such as fatty acids. Considering the potential impact of these metabolites, much more information is needed about how oxygen affects the production of yeast metabolism, especially with respect to compounds with a sensory impact. Impact of juice/must aeration prior to fermentation Many winemaking techniques introduce molecular oxygen into the juice prior to the onset of fermentation. This introduction may be unintentional and a consequence of crush i ng, pressi ng or tank-to-tank transfers—or it may be deliberate as in the case of juice hyperoxidation to force oxidative changes to occur early in the wine.7,9,27 Oxygen exposure in white juice increases browning10,29 but also leads to an increase in perceived bitterness in white wines.23 Hyper-oxidation is generally employed to force polymerization and precipitation of phenolic compounds as insoluble brown pigments in white wine production, thereby stripping the wine of browning potential and protecting the wine against further browning during élevage post-fermentation. 27 In many cases, it seems that the same effect may be achieved by not adding SO2 and allowing unfettered PPO activity.27 Hyper-oxidation and polymerization may also lead to precipitation of bitter and astringent compounds in white juices.27 This is particularly important in over-extracted white juices that may result from mechanical harvesting. Some ® 707-938-1300 70 p r acti c al w i ne ry & v i n e yard O CTO BER 20 13 info@acrolon.com