Thoughts on Reduction...

Thoughts on Reduction…

The term “reductive” is often thrown around haphazardly, even in the most elite wine circles. Reduction in regards to screw caps has become a hot topic, drawing more attention as the stelvin revolution spreads.  Characteristics in wine that have long been considered as the mysterious work of “terroir” are even being attributed to reduction. So what is reduction, and why do we care?

In very simple terms, reduction is the opposite of oxidation. It occurs when an element or compound gains electrons. When a wine is not exposed to air, it can be said to be in a “reductive” state, compared to a wine that is exposed to oxygen and therefore in an “oxidative” state. 

The trouble with this reaction is that during the process of winemaking, it is necessary for the wine to be under both reductive and oxidative conditions at certain stages. And wine in either of these extreme states can be quickly ruined. So winemakers walk this potentially disastrous fine line carefully.

Aeration is the controlled and deliberate exposure of wine to oxygen, and has positive connotations. Oxidation, on the other hand, is considered a wine fault (with the exception of wines such as Madeira and Sherry) caused by excessive exposure to air. Oxygen is necessary at the beginning of fermentation, helping the yeast to multiply and get off to a healthy start. Red wines require controlled exposure to oxygen during the ageing process in order to build tannin structure, stabilize colour and polyphenols, as well helping the wine to develop a rounder and more polished texture. Aeration is considered to be such an important part of the elevage of red wines that the technique of micro-oxygenation was developed to give winemakers more control over the process, enabling them to set the amount of oxygen and the rate at which it is introduced.  

In the past, winemaking was mostly oxidative, simply because they did not have the technology and tools for it to be otherwise. Since modern developments such as stainless steel tanks, temperature control, use of inert gas, sulphur and protective handling of juice/must, winemaking has become a much more reductive process. White wines especially benefit from being treated reductively, showing fresher and cleaner aromatics as well as avoiding any browning or oxidized character that is generally undesirable. White wines are more susceptible to oxidation because of their lower polyphenol content, which acts as an anti-oxidant. The use of reductive winemaking methods has been vital for the rise in quality of regions such as Friuli and Trentino-Alto-Adige where crisp, clean whites have soared in popularity to the demise of their oxidized predecessors. Indeed, reductive wine making has been the trademark of new world producers such as those in Australia and California.

The benefits of reductive winemaking are obvious. The problems associated with it are not as straightforward. When wine is kept in anaerobic (reductive) conditions, volatile sulphur compounds can form if not enough attention is paid to the health of the wine. The first risk winemakers face is during fermentation. If the yeast are stressed, usually because of nutrient deficiency, they produce Hydrogen Sulphide (H2S), a compound that smells like rotten eggs or sewage. This problem can be taken care of if it is caught early on, often by simply aerating the wine. If it is not corrected quickly, the trouble really starts. When there is no oxygen, the H2S will react with other things to form volatile sulphur compounds such as mercaptans, organic sulphides and thiols that are not so easy to get rid of. The term “reduced” is often imprecisely used to describe aromas presumed to be sulphur compounds. In reality, there are many compounds that belong to this family. 

Dimethylsulphide (DMS) is responsible for cooked vegetable aromas in wine. At high levels, they are perceived as cooked corn or canned tomato. At low levels, DMS may contribute a blackcurrant aroma.

Mercaptans are the cause of the majority of the aromas we refer to as “reductive”. They form when H2S interacts with alcohol. Aromas such as cabbage, rubber/burnt rubber, struck flint, onion and garlic indicate the presence of mercaptans. 

Mercaptans however are not always undesirable. A few compounds have been identified that are responsible for some of the character found in Sauvignon Blanc. 

4-mercapto-4-methylpentan-2-one = Cat pee, eucalyptus, box tree

3-mercaptohexal-1-ol = Passion fruit, grapefruit

3-mercaptohexyl acetate = Passion fruit

Benzenemethanethiol = Smoke, flint stone

Some of these fruity mercaptans can also be found at varying levels in other varieties such as Gewurztraminer, Muscat, Colombard and Petit Manseng.

Considered blasphemous by many, it has been suggested that reduction may be the cause of the characteristics commonly associated with terroir. Minerality has long been the poster-child for terroir, despite the fact that there is no scientific proof supporting the literalist theory that the soil mineral composition is directly translated to the grapes and resulting wine. In his book “The Science of Wine”, Jamie Goode includes some interesting quotes; Sam Harrop MW has voiced his hypothesis that “perhaps these qualities are derived in the winery and not the vineyard”. James Healey has said that he too believes that “minerality and reduction are related”. Goode even manages a quote from Dr. Richard Smart, who replies “This is an absolute nonsense” in response to Jamie’s question of whether there is “direct translocation of flavour molecules from the soil to the grapes, and hence the wine”. A link between reduction and flint character has also been proposed.

 Some wines are more at risk of developing reductive issues than others. When elemental sulphur is used in the vineyard, mainly to prevent powdery mildew, and the residue is present on the grapes and leaves, the formation of H2S during the fermentation is almost certain. The choice of yeast is also important, as certain strains can reduce sulphur dioxide (SO2 – used extensively in the winery for its antioxidant/antibacterial properties) to hydrogen sulphide, also encouraging the formation of mercaptans. When yeast are stressed due to nutrient deficiency they naturally produce H2S as a byproduct, which is why a healthy fermentation is so vital.

Some grape varieties are known for lending themselves more to either reduction or oxidation. Syrah and Mourvèdre are two varieties that are considered to be in this first category, while the majority of white varieties and reds such as Grenache and Pinot Noir have a tendency towards oxidation, which is why they benefit from reductive winemaking.

Wines left on their lees are especially susceptible to the formation of volatile sulphur compounds because the dead yeast if unmanaged can create a reductive environment, which is why batonnage is used for instance with Chardonnay in barrel. Juice with high sugar content is more likely to develop sulphur compounds during fermentation because the yeast are more stressed. Hot climates in general may be more at risk because of this, in areas where the potential alcohol levels are very high.  It is wine under screw cap however that has drawn the most attention regarding their potential for complications due to reduction. 

It has been argued that wines under screw cap are more likely to show reductive character on the basis that it forms a less permeable seal. It is a common misconception that it is air that permeates the cork that interacts with the wine. The amount of air however that actually does permeate the cork is negligible. It has also been proven that good quality cork can perform equal to a stelvin and form just as tight a seal.  It is in fact the air in the headspace of the bottle, aka ulluge, which interacts with the wine. Based on these facts, wines under cork are just as much at risk as wines under screw cap, provided good quality cork is being used.

So the next time you drink that screw-capped Australian Shiraz and notice the characteristic rubbery, eucalyptus aromas, or more controversially, a nice minerally Burgundy which has been aged on the lees in barrel, you may start to wonder if it’s actually terroir that you are tasting….


References:

Bird, D., Understanding Wine Technology (Newark, 2005).
Goode, J., Wine Science (London, 2005).
Rankine, B., Making Good Wine (Sydney, 2004).
Stelzer, T., Taming the Screw (Brisbane, 2005).

 Lecture: Dr. Pascal Chatonnet, Wine Faults Workshop, London International Wine & Spirits Fair, London, UK, May 21 2008.