Effect of Micro-oxygenation on Tannat

Summary

Micro-oxygenation is a controlled use of oxygen. Skillful application of this tool can lead to better color, structure, and sensory properties in red wines, but any addition of oxygen also comes with risks. In this experiment, a single lot of Tannat wine with notable astringency was split into two lots post-malolactic fermentation. One lot received no additional oxygen beyond normal cellar operations (racking and barrel aging). The other lot was treated with 5 mL/L/month for 10 hours every other day for 10 days. After SO2 addition and several months of aging, there was no notable difference in basic wine chemistry. The micro-oxygenated wine had lower measured values for monomeric and total anthocyanins, and higher color hue. Sensory analysis was not conducted due to COVID-19 restrictions.

Introduction

Winemakers have a complicated relationship with oxygen. At times, they fiercely protect juice and wine from its presence, excluding it in any way possible. At other times, they invite it in and hope it does its work. It all depends on the wine, and the situation. Micro-oxygenation is a controlled use of oxygen. Skillful application of this tool can lead to better color, structure, and sensory properties in red wines, but any addition of oxygen also comes with risks. 

There are several steps during the winemaking process when oxygen is introduced into juice and wine. Oxygen is introduced almost every time the wine is moved and even during storage (Table 1). After introduction to the wine, oxygen is quickly bound by various components, such as ethanol (to form acetaldehyde), phenolics (tannins and anthocyanins), SO₂, aroma and flavor compounds, which can have a profound effect on astringency and aroma. 

Table 1: Oxygen introduction during winemaking operations^1,2

Unlike the uncontrolled addition of oxygen during cellar operations, micro-oxygenation (MOX) is the process of intentionally adding small amounts of oxygen in a measured way to induce favorable changes in color, aroma, and texture³. The technique was first developed in France in the 1990’s as a way of aging tannic red wines in stainless steel tanks rather than oak barrels. It is now also used to accelerate aging, stabilize color, and treat reduction and herbaceous/green character. Rather than shortening the life of a wine, as oxidation normally does, MOX  can increase the anti-oxidative power and potential aging of the wine by stimulating phenolic reactivity¹.

When oxygen is introduced to wine, it diffuses randomly and encounters many chemical compounds in the wine. Oxygen is generally highly reactive and will show the most effects in those chemicals that are found in the highest concentration in wine, such as ethanol, anthocyanins, tannin subunits, and tannins themselves. Oxygen will also interact with molecules that are responsible for aroma and flavor. Though a winemaker may be targeting one component, oxygen will interact with them all.

One of the most common reasons to micro-oxygenate wine is to help structure tannins. Phenolics such as anthocyanins and tannins are responsible for the color and mouthfeel (astringency) of red wine. Micro-oxygenation can alter the polymerization rate and bonding arrangement of tannins and pigments, leading to less astringency and more color stability. Micro-oxygenation can also reduce the perception of reduced compounds such as H₂S (sewer gas, rotten eggs) and methyl mercaptan (cabbage, skunk)^1,4.

When it was first introduced, reports showed that micro-oxygenation also led to significant decreases in the sensory perception of herbal/vegetal character in wine. However, repeated measurement of the chemical methoxypyrazine, a principle chemical component of “veggie”, did not change. Further study revealed that the perception of “veggie” included a combination of pyrazine as well as some reduced sulfur compounds (AKA thiols) with vegetal aromas (such as canned green beans). The reduction of the perception of “veggie” after micro-oxygenation was due to oxidation of these reduced compounds^4,5.

Along with its potential benefits, micro-oxygenation also has some risks. Allowing micro-oxygenation to go on too long can lead to excessive polymerization, making tannins dry and harsh⁴. Oxygenation at too high of a rate can leave excess oxygen in the wine that reacts with anthocyanins to cause browning and oxidation of varietal aromas and flavors⁶. Oxygen availability in wine can also lead to microbial spoilage. Acetobacter will use available oxygen to produce acetic acid. Brettanomyces growth is facilitated by the presence of oxygen⁴(though it is thought that at low levels, micro-oxygenation poses less of a threat than standard racking³). Zoecklein ¹ likens the management of micro-oxygenation to piloting an ocean liner. It must be done carefully and pro-actively with the understanding that one action begets a chain reaction that is not easy to control or fully predict. A firm understanding of the underlying mechanisms at play and careful monitoring of progress are key to success.

From: Enology Notes #134, Zoecklein (2007)⁵

To micro-oxygenate wine, an oxygen supply (purchased from the gas supplier) is connected to a dosing chamber (such as the Stavin Ox Box). The dosing chamber acts as a high precision dosing regulator that can be set for very small amounts of oxygen flow. Oxygen exits the box through small tubing into miniature sparging tips with very small (10 um) pore size to produce very fine bubbles⁷. Soluble solids and lees have been shown to interfere with MOX effects⁸, and can contain spoilage microbes, so the tubing is configured so that the sparging tips are suspected about an inch above the bottom of the tank to avoid disturbing any remaining lees.

Figure 1: Schematic representation of a micro-oxygenation system. From Gomez-Plaza and Cano-Lopez (2010)³

MOX can be applied at any time during vinification and maturation, with total doses up to 600 mL/L for tannic red wines over the lifetime of the wine⁸. The effects of MOX will differ depending on the timing and dose rate of the addition. MOX has been used during fermentation as a means to encourage yeast health and reduce off aromas during fermentation. MOX prior to malolactic fermentation (MLF) has been shown to be effective in improving wine structure⁸. After malolactic fermentation, it is thought that at least some polymerization has already occurred, so MOX has less of an effect on tannin structure. However, due to the high number of wines needing attention during harvest, applying MOX after MLF may be more practical. It is not uncommon for MOX to be done at higher rates before malolactic fermentation, stopped while malic acid bacteria are active, then applied again at lower rates after malic acid has been depleted^3,8. 

In this experiment, micro-oxygenation was used to soften tannins in Tannat. Treatment was applied post fermentation, prior to malolactic fermentation. Chemical properties were assessed after aging.

Methods

Grapes were hand harvested and refrigerated before destemming to TBins for fermentation. Bins were inoculated with 0.17 g/L RBS yeast rehydrated in 0.15 g/L Go Ferm with daily monitoring of Brix and temperature and twice daily punch downs. Nutrients (0.6 g/L Superfood and 0.46 g/L DAP) were added on the third day of fermentation. All bins were pressed together after 17 days of maceration. Wine was allowed to settle for one week prior to racking to a tank, then splitting the lot. One portion was racked to barrel (control) while the other portion was racked to a 1200L cylindrical stainless steel tank (with at least 4 feet of vertical distance) for micro-oxygenation. Turbidity was measured after racking with a target of less than 200 NTU to prevent aerobic spoilage organisms and particulate binding sites for oxygen. A lead line from the Stavin Ox Box was threaded into the treatment tank so that the dosing stone was suspended 6 inches from the bottom of the tank (to avoid lees). The line remained in place for the duration of the experiment to avoid oxygen addition from disturbing the surface. 

Due to the small volume of the treatment tank (1125 L), continuous dosing was not possible from the Ox Box. Instead, a discontinuous rate was set targeting 5 mL/L/month, at a dose rate of 3 mL/min for 10 hours roughly every other day. Temperature was checked prior to each oxygen dose. Throughout the experiment, the treatment lot was monitored weekly for sensory attributes (specifically tannin structure, offensive odors, and oxidation). Treatment was halted according to the discretion of the winemaker based on this assessment, in this case after 10 days. Wine was then transferred to barrels after one month. After one additional month, 50 ppm sulfur dioxide was added to each lot. SO₂ was checked monthly and maintained to a common target. 

Results

Fruit was harvested on 9/17 (Table 2). After racking to separate vessels, the wines for the two treatments had very similar basic chemistry (Table 3). Wine chemistry after treatment and sulfur dioxide addition was also very similar, however, neither wine completed malolactic fermentation (Table 4).

One concern with micro-oxygenation is the potential accumulation of acetaldehyde that can lead to additional binding of SO₂. Neither lot received SO₂ at crush. Both lots received 50 ppm SO₂ on 12/20. In-house measurement of SO₂ on 1/23 led to additions of 30 ppm to the micro-oxygenated lot and 40 ppm to the control. The control did not require additional SO₂ through April (the end of measurement for this report), however the micro-oxygenated lot required 10 ppm addition on 3/5 and a 20 ppm addition on 4/2, indicating this lot may be binding SO₂ faster than the control lot. 

There was little difference in color intensity for the two wines (Figure 1), however, there was a shift in hue from 0.65 in the control to 0.73 in the micro-oxygenated wine. This indicates a shift from the red to the yellow end of the spectrum, a shift also seen in aged wines. Micro-oxygenated wines have notably lower levels of anthocyanins without increases in polymeric anthocyanins (Figure 3). Phenolic measurements were largely the same between lots (Table 5). The softening of tannins cannot be assessed in overall quantity of phenolics, as this process is a re-organization rather than gain or loss. Sensory analysis is the best way to assess these changes. Unfortunately, due to public health measures put in place during the COVID-19 pandemic, sensory analysis was not completed prior to blending of these wines.

Table 2: Tannat fruit chemistry at harvest (in-house data)

Table 3: Post-fermentation/pre-treatment wine chemistry for two lots of Tannat (in-house lab)

Table 4: Finished wine chemistry for two treatments of Tannat (ICV labs)

Figure 2: Color metrics prior to SO₂ addition for two treatments of Tannat (ICV labs)

Figure 3: Anthocyanins after aging in two treatments of Tannat (mg/L)(ETS labs)

Table 5: Phenolic measurements for two treatments of Tannat (mg/L) (ETS labs)

 

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References

(1) Zoecklein, B. W. Winemaking Topics; Micro-Oxygenation. Virginia Tech Wine/Enology Grape Chemistry Group, n.d.

(2) Waterhouse, A. L.; Laurie, V. F. Oxidation of Wine Phenolics: A Critical Evaluation and Hypotheses. American Journal of Enology and Viticulture 2006, 57 (3), 306–313.

(3) Gómez-Plaza, E.; Cano-López, M. A Review on Micro-Oxygenation of Red Wines: Claims, Benefits and the Underlying Chemistry. Food Chemistry 2011, 125 (4), 1131–1140. https://doi.org/10.1016/j.foodchem.2010.10.034.

(4) Zoecklein, B. W. Current Theory and Applications: Microoxygenation. Practical Winery and Vineyard2007, November/December.

(5) Zoecklein, B. W. Factors Impacting Sulfur-like Odors in Wine and Winery Operations Part 2. Enolgoy Notes #134, 2007.

(6) Singleton, V. L. Oxygen with Phenols and Related Reactions in Musts, Wines, and Model Systems: Observations and Practical Implications. 1987, 38 (1), 69–77.

(7) McCord, J. Micro-Oxygenation: A Treatise. Stavin, 2009.

(8) Lesica, M.; Košmerl, T. Microoxygenation of Red Wines. Acta agricultura Slovenica 2009, 93 (3), 327–336.