Reductive and oxidative winemaking techniques produce wines that can vary greatly in style, and both have relative benefits and drawbacks depending on the wine being produced. Reductive winemaking processes are those that are undertaken in the absence of oxygen. These techniques include use of a closed press, closed-top stainless steel tank fermentation and aging, use of sulfur dioxide (SO2) and inert gases, and limited racking. Oxidative styles are made with the deliberate (usually controlled) introduction of oxygen, either in small or large doses. These processes include hyperoxidation, micro-oxidation, racking, barrel aging, not topping up, and intentional avoidance or limited use of SO2. The decision to use reductive or oxidative winemaking techniques throughout the winemaking process will depend upon the variety, the regional style of a wine, legislation, and commercial potential.
Reductive and oxidative winemaking techniques begin at fruit reception and at the press. At grape reception, fruit may be protected from oxygen by the addition of SO2 in the form of potassium metabisulfite and/or dry ice. The latter chills the must and sparges oxygen from the fermentation vessel or the press. These reductive techniques serve to preserve freshness of fruit, with the added benefit of protecting the must from unwanted microbes. For white grapes and rosé, pressing technique will have an impact on potential reductive aromas. The use of a closed press, where grapes are pressed and the juice is sent directly to tank without any exposure to oxygen, will produce a bright wine with high aromatic intensity. The use of SO2 is also important at this juncture. Bob Berteau at Chateau Ste. Michelle is challenged with moving large quantities of juice from pressing facilities in eastern Washington to the winery in Woodinville, in western Washington. For Riesling with pHs of 3 and 3.2, the addition of 60 to 80 parts per million of SO2 is used to protect the juice from oxidation during that journey, keeping the juice and resultant wine fresh and reductive in style.
Alternatively, at the press, white must may be subjected to hyperoxidation. The must is deliberately exposed to oxygen with the understanding that oxidative processes will play out early on, reducing opportunities for oxidation post-fermentation. During hyperoxidation, tannins are oxidized into quinones and will fall out of the wine over time. According to Eglantine Chauffour, Technical Winemaker at Enartis USA, oxygen should be carefully controlled at this stage, allowing 5 to 30 milligrams per liter into the must over a couple of hours. These varying techniques at press for white wines are known as the “green school” and “brown school,” respectively. Protected must will remain bright and green, while oxidized must will turn a deep brown, until the oxidized tannins settle to the bottom of the fermentation vessel and the wine returns to its original green color.
Because of the production of carbon dioxide (CO2), oxygen exposure is limited during fermentation, but there are avenues that are more or less oxidative and reductive. Crushing berries prior to fermentation creates a more oxidative environment, with the purpose not of influencing the flavor profile but of putting yeasts in contact with sugar and oxygen. In addition, punchdowns and pumpovers introduce oxygen during fermentation, helping yeasts to multiply and assist in polymerization of tannins for red wines and color stabilization for both reds and whites. Alternatively, carbonic maceration is a reductive process where enzymatic reactions occur within the cell itself, producing alcohol without the action of yeast or oxygen. The result is a fresh, simple style, like Beaujolais Nouveau, that can be consumed early and is enjoyed for the freshness of fruit flavors.
Following fermentation, the élevage vessel provides either a reductive or oxidative environment. Many delicate white varieties like Riesling, Sauvignon Blanc, and Gewürztraminer benefit from élevage in a hermetically sealed stainless steel tank, which protects these varieties from losing their fresh esthery aromas and delicate floral flavors. A more oxidative approach is taken when oak barrels are used for élevage. Oxygen can permeate in small amounts through the staves of a barrel (20 to 45 milligrams per liter, depending on the aging regiment). This process helps to polymerize tannins, stabilize color, and add complex flavors to the finished wines. Additionally, barrel aging tends to mute fruit intensity. It’s important to note that oxidative does not mean oxidized. On the contrary, wines in barrel often rest on their lees, especially when racking is limited, and can become reductive when SO2 levels are maintained and barrels are frequently topped.
Lees aging is a reductive process, while bâttonage is generally a process of oxygenation, and both contribute to flavor profile. Lees aging can add textural complexity to a wine, but can become reductive if heavy lees remain at the bottom of a barrel or tank for months without stirring. This can lead to the formation of off aromas of hydrogen sulfide (H2S) as the lees decompose. To prevent this type of reduction, bâttonage introduces oxygen to the lees and discourages the formation of H2S compounds. Winemaker Dominique Lafon notes that while small amounts of oxygen are introduced during bâttonage, the lees are suspended and will consume most of the oxygen prior to settling. Therefore, while bâttonage is a process of oxygenation, it’s also a process that protects against oxidation.
In the élevage vessel, wines can be protected from oxygen by maintaining appropriate SO2 levels, relative to the wine’s pH, topping up regularly (on average, every 10 to 14 days), and the use of inert gases. Oxidative processes like racking or micro-oxygenation can be used during élevage to add complexity or to break up reductive compounds in wines produced from grape varieties like Syrah that are susceptible to hydrogen sulfide formation. Micro-oxygenation is the addition of controlled and metered doses of oxygen to stabilize color, polymerize tannins, soften mouthfeel, and reduce green flavors.
Racking during élevage and/or prior to bottling can influence the reductive tendency of red wines, and the extent of this is related to SO2 use. Cornas producers Thierry Allemand and Auguste Clape both seek to avoid reductive aromas in their Syrah, but they take different approaches in the cellar. At Allemand, Thierry believes the addition of SO2 during élevage is the cause of reductive aromas. He does not add SO2 until right before bottling, and therefore does not rack during élevage. At Clape, SO2 is added early during élevage, and the wines are racked four or five times before bottling to avoid the formation of H2S. As long as appropriate levels of SO2 are maintained, oxidation during racking can be avoided.
There are styles of wine that are intentionally oxidized, not just “oxygenative” in their production, to express regional style and typicity. In Madeira, heating wine either by the Canteiro or Estufa method, coupled with an extended élevage that results in exposure to oxygen, produces hazelnut and toffee characteristics. Amontillado Sherry begins its journey protected from oxidation with a veil of flor. The flor dies off, the wine is fortified, and the slow process of oxidation begins. Compared to Fino, which is not oxidized, Amontillado is darker in color and has distinct caramel and almond notes. In the Jura, there are two styles: wines that are “topped up” and those that are not “topped up.” By law, wines produced and labeled as Château-Chalon are not topped up and age for six years and three months sous-voile, producing a distinct nutty character that expresses regional typicity.
There is currently a trend toward intentionally producing reductive aromas in white wines, particularly in Burgundy. The aromatic profile of Jean-Marc Roulot’s wines in Meursault is often described as reductive, exhibiting mineral, flinty, or matchstick aromas. While Jean-Marc states that he does not strive for reductive characteristics, after he changed his winemaking technique to include six months aging in stainless steel tank (after barrel aging and prior to bottling), he began noticing more reduction in his wines. Coche-Dury is also known for a reductive style. The winery practices élevage with limited racking and little exposure to oxygen, and the wines have significant lees contact in barrel. Because some top domains in Burgundy are intentionally, or not intentionally, producing reductive styles, reduction is often seen as a sign of quality.
However, there are dangers associated with reductive winemaking. Top among them is the formation of hydrogen sulfide compounds. This can be prevented during fermentation by ensuring yeasts have the appropriate levels of yeast assimilable nitrogen (YAN). If hydrogen sulfide forms during fermentation, it’s likely that a reductive winemaking environment will cause the H2S to reduce to disulfides and eventually to mercaptans, producing a stewed vegetable, onion, and/or cabbage character. Vigorous racking can break up H2S bonds. Hydrogen sulfide can also be treated early in élevage with the addition of copper sulfate.
There are also drawbacks to oxidative winemaking. With the introduction of excessive oxygen, wines will experience loss of freshness, loss of color, and browning. They may also have a propensity toward microbial contamination and the formation of volatile acidity. Therefore, oxidative processes must be carefully controlled. This includes topping up, keeping appropriate SO2 levels in relation to the pH of the wine, and maintaining appropriate storage temperature for the wine. Temperature, turbidity, level of CO2 in a wine, and the level of microbial activity influence oxygen consumption and dissolution. Oxygen is more soluble at cooler temperatures and is consumed more readily with increased turbidity and a higher microbial population. CO2 will aid in sparging O2 from the wine. Monitoring the factors that influence dissolved oxygen (DO) levels is crucial, particularly prior to bottling, when acceptable DO levels are less than 0.2 grams per liter.
The choice to use reductive or oxidative winemaking techniques will depend on the style of wine being produced. The pressing technique, use of SO2 and inert gases, choice of élevage vessel, racking regiment, and pre-bottling techniques will all influence whether a wine is produced reductively or with intentional, controlled oxygen exposure. Wines made in a light style are more appropriate for reductive winemaking practices, while full-bodied wines or those with more polyphenols are well suited to winemaking practices that introduce oxygen.
Hi Amy, have a look, but chateau Chalon is 5 years under veil, 6 years and 3 month total but not all "sous Voile". This is the case for all Vin Jaune in the Jura.
Nicely done and very informative. Thank You!
Fantastically informative and instructive. Thank you. This is wonderfully helpful
Excellent, very complete article! Thank you!
Bose is going to make me read this!