Managing Oxygen During Winemaking
Hyper-, Macro-, and Micro-Oxygenation in Wine Production
There are several critical times during the winemaking process that dissolved oxygen in must and wine can be beneficial. These include hyper-oxygenation of juice or musts, macro-oxygenation of fermenting must in red wines, and the use of the micro-oxygenation to soften and increase color stability in red wines.
Hyper-Oxidation
Hyper-oxygenation is a pre-fermentative technique where oxygen or air is added to a non-sulfited must and juice where phenolic compounds react with oxygen to produce quinones. These compounds in turn react with more oxygen to yield brown-colored products that fall out as a precipitate and are racked off the juice prior to fermentation.
Practical Considerations
The effectiveness of hyper-oxygenation depends on the amount of oxygen, pH, and temperature. Hyper-oxidation can be achieved by tank-to-tank or within-tank transfers using an air diffuser, or if a technique such as flotation is used for juice clarification, air can be introduced instead of nitrogen (Section 4.4). The juice should be sparged with pure oxygen or compressed air. Typically, the removal of sufficient flavonoids can be achieved through hyperoxygenation with one oxygen saturation (6 to 8 mg/L oxygen), if the juice did not receive any skin contact, but up to three saturations are needed if the juice did have contact with the skin (du Toit et al., 2006).
Macro-Oxygenation
Macro-oxygenation is only used in red winemaking as it involves vigorous aeration of wine during cap management and after fermentation but before malolactic fermentation. This could be especially helpful for high phenolic wines, especially press wine. Macro-oxygenation during fermentation has a softening effect on the astringent tannins and enables tannins to rapidly react with anthocyanins, forming stable complexes, thus stabilizing color.
Micro-Oxygenation
Micro-oxygenation (sometimes referred to as MO or MOX) refers to the diffusion of air or oxygen into the wine using stainless-steel tanks at a rate equal to or slightly less than the wine's ability to consume that oxygen(Figure 17.1). This technique mimics the wine maturation that occurs in oak barrels, through a slow oxygenation process. Micro-oxygenation represents a good alternative, producing wines comparable to those aged in barrels but in less time and at a lower cost.
Chemical and Sensory Effects of Micro- Oxygenation
Micro-oxygenation induces oxidation reactions with phenolic compounds. These reactions encourage the formation of short-chain polymers between the anthocyanins and the tannins resulting in a more stable color while improving mouthfeel and structure. Since most of the benefits of MOX are related to the reactions of polyphenols, the initial phenol content of the wine is one of the most important conditions at the time of designing a MOX treatment.
Recommended Parameters for Micro- Oxygenation
There are some parameters, including dissolved oxygen rate, turbidity, temperature, free sulfur dioxide, and volatile acidity, that should be monitored during the process. Tasting is also important in terms of determining dose and timing of oxygen.
Stages of Winemaking
The stage of wine production at which micro-oxygenation is started can also influence rate and quantity of added oxygen.
Monitoring the Progress of Micro-Oxygenation
There are several measurements for monitoring the progress of MOX. Monitoring micro-oxygenation requires tasting the wine on a regular, if not daily, basis. Wine analysis of volatile acidity (VA), dissolved oxygen (DO), acetaldehyde, and sulfur dioxide are required to monitor dosage and progression of the MOX treatment.
Micro-Oxygenation Systems
All MOX systems consist of an oxygen source, at least one precision gas regulator capable of measuring out gas at very low flow rates on the order of a few milliliters of gas per month, at least one diffuser, and enough tubing. The pore size of the spray heads must not exceed 10 micrometers to ensure that the oxygen is applied as fine beads. The regulator may be controlled manually or by a small specialpurpose computer (PLC) that is built into the MOX unit or by a centralized controller that can control multiple MOX units at the same time. Calculation formulas are often supplied with dosage apparatus to ensure that the dose is correct. There are two key systems for the controlled addition of oxygen into wine:
Use of Oak Chips
The practice of macerating wine with oak chips is often employed since MOX systems do not provide either the flavors or the tannins that contact with the oak barrels provide. Oak chips provide similar tastes, aromas, and wooden notes to the wine as those obtained with traditional barrel aging, but much faster and at a lower cost.
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