Chapter 17

Managing Oxygen During Winemaking

Types of Wine Oxidation

There are three basic types of oxidation that can effect wine: enzymatic oxidation, chemical oxidation, and microbial oxidation. Enzymatic oxidation almost entirely occurs in grape must. Non-enzymatic oxidation, also called chemical oxidation of wine, prevails in fermented wine. Microbial oxidation is a result of spoilage microorganisms such as acetic acid bacteria, film yeasts, and Brettanomyces.

Enzymatic Oxidation

During the crushing, pressing, and other processing steps, oxygen comes into contact with the grape juice, leading to the enzymatic oxidation of phenolic molecules. For this to occur the oxidation enzyme, oxygen and the phenolic substrate must be present. The polyphenol oxidases (PPO) of healthy grapes are known as tyrosinase, cresolase, and catechol oxidase, with laccase occurring in Botrytis cinerea infected grapes. Polyphenol oxidase activity is the primary enzyme responsible for oxygen consumption by juices, and this enzyme consumes oxygen at a rapid rate, particularly in the absence of sulfur dioxide.

Conditions Affecting PPO Activity

Generally, if uninhibited by sulfur dioxide, PPO consumes more oxygen than microbial activity early in juice. As microbial populations build, or if the juice contains a high bio-load, there may be more microbial competition with PPO than would occur normally. Thus, the fate of oxygen in juice is dependent upon the relative ratios and activity of the microbes (e.g., bacteria, yeasts, and molds) present and PPO activity.

Non-Enzymatic Oxidation

In wines, the oxidation usually occurs without the mediation of enzymes. This type of reaction is called non-enzymatic oxidation or chemical oxidation. Nonenzymatic oxidation prevails in fermented wine and begin by the oxidation of polyphenols such as catechin, epicatechin, anthocyanins, and other phenols present in grapes.

Microbial Oxidation

Spoilage microorganisms such as acetic acid bacteria, film yeasts (e.g., Candida, Pichia) and Brettanomyces are dependent upon oxygen. In addition, certain wild yeasts belonging to the group Kloeckera and Hanseniaspora in the absence of sulfur dioxide can be abundant in must and juice at the beginning stages of fermentation.

Impact of Oxygen on Wine

When a wine becomes oxidized the first is the development of acetaldehyde, the oxidation product of alcohol. At high levels the aroma is considered a defect and is reminiscent of rotten-apples. The second is the development of another wine fault, volatile acidity (VA). This is when a wine begins to take on a vinegary smell: a little bit of volatile acidity can provide a lift to the nose, but too much is unpleasant, and adds a sweet and sour character to the palate. And there are color changes too: white wines turn a more golden or orange hue, with some browning hints, while reds lose their brightness and become more orange, with a brick red or brown rim. There's also the loss of fruitiness.

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