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WineEast Winemaking Strategies to Manage Dissolved Oxygen How to identify practices that prevent unwanted oxygen absorption in wine By T.E. Steiner T he benefits and drawbacks of dissolved oxygen in wine can be discussed at great length. This article will provide an initial overview of some of the benefits and negative aspects regarding the impact of oxygen in wine. However, to extend aging potential and prevent undesirable changes in wine due to excess oxygen, a winemaker must recognize that in most cases oxygen is considered to be detrimental to the production of premium-quality wines. Key areas where oxygen can be introduced into the winemaking process will be identified to help winemakers recognize practices that prevent excess oxygen absorption. Types of oxidation and potential negative effects There are three basic types of oxidation that can have negative effects on wine: enzymatic oxidation, chemical oxidation and microbial oxidation. Enzymatic oxidation: Juice and must oxidation is catalyzed by an enzyme called Wine East HIGHLIGHTS: • olor change and off-aromas can deC velop in wines as a result of enzymatic, chemical or microbial oxidation. • ot all exposure of wine to oxygen is N detrimental. During the initial stages of fermentation, oxygen encourages yeast propagation and prevents stuck or sluggish fermentations. • fter fermentation, it is important to A avoid oxygen exposure during the cellaring process. Oxygen pickup can result from excess headspace and occur during racking, pumping, cold stabilization and filtration. • ottling is a key process where added B dissolved oxygen can negatively impact the aging potential and quality of the wine. 92 W in es & V i ne s AU G U ST 20 13 During trials at the Ohio Agricultural Research and Development Center, oxygen was added to juice or must in order to evaluate the benefits of the process known as hyperoxidation. polyphenol oxidase (PPO). This enzyme oxidizes certain phenolic molecules to produce quinones. These compounds form polymers that influence wine color and flavor along with the loss of varietal character. Typically, these wines develop a brownish tint and may produce an oxidative odor of acetaldehyde (sherry-like aromas). At correct sulfur dioxide (SO2) levels, these enzymes are easily inhibited in musts. It should be noted that their oxidative activity does not occur in wines. Another enzyme to produce oxidative reactions is laccase, which occurs in unsound or rotten fruit. This rot is caused by Botrytis cinerea and is often called bunch rot. Winemakers consider laccase a more serious problem than PPO. This enzyme has more resistance to SO2 and has a wide range of oxidative substrates. When oxygen is present in wines, laccase activity can continue to cause browning with a decrease in varietal aroma. Chemical oxidation: This is the main oxidative process in wines, and it involves the oxidation of polyphenols such as catechin, epicatechin, anthocyanins and other phenols present in grapes. Through a series of reactions with oxygen, certain phenols form quinones and another byproduct, hydrogen peroxidase (H2O2). This oxygen-containing compound is a stronger oxidizing agent than PPO. This oxidizing agent converts ethanol to acetaldehyde, resulting in a Sherry-like aroma in the wine. If wines are not protected with SO2 and from oxygen exposure, chemical oxidation yields several negative sensory notes. These include brownyellow coloration and off-odor formation with aroma degradation. Microbial oxidation: Spoilage microorganisms such as acetic acid bacteria (AAB), film yeasts (Candida) and Brettanomyces ("Brett") are dependent upon oxygen. AAB produces acetic acid (vinegar odor) from ethanol and may also yield acetaldehyde and ethyl acetate (fingernail polish) under certain conditions. In addition, certain "wild" yeasts belonging to the group Kloec-