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Reduction in Winemakng

Reduction in Wines

NOTE – Some of what is written below is a little technical. It is difficult to discuss reduction in wines, and not use some technical language. The author tried to make it clear, and welcomes comments relative to how to improve that.

100px-Hydrogen-sulfide-3D-vdWWhat is reduction in wine and why is it called that?  Why are “reduced“ wines problematic in most cases?  Sulfur is the short answer.  All wines produce sulfur containing compounds during fermentation.  These compounds come in various sizes and shapes, and have detection levels in the microgram per liter region. Sulfur undergoes what are known in chemistry as reduction-oxidation (Redox) reactions during the process of converting sugars into alcohol.  What this means is that some molecules gain electrons. That’s called reduction. Nature is quite democratic, and for every reduction reaction there must be an oxidation reaction in which electrons are lost. At the end of the chemical day, everything must balance – electron losses and gains must end up even – like a tied football game.  That’s it for the basic chemistry.

What does all this mean to a winemaker, and eventually to the consumer?  Let me tell you. One of the sulfur villains formed most often during fermentation is Hydrogen Sulfide (H2S), the sulfur analog of H2O. This relatively small molecule, formed during fermentation, smells like rotten eggs, and does not add anything to the organoleptic (taste, smell, finish etc.) qualities of the wine. In fact, it may make the wines undrinkable if not detected and removed. This small molecule can be detected by the human nose at concentration of parts per billion. A note of caution: Hydrogen Sulfide in high concentrations is, or can, be toxic.

As is usually the case, H2S is not the only foul smelling sulfur compound that can be formed in wine. A major part of the problem is the large number of sulfur containing molecules that cause wine reduction problems. To make matters worse, their formation is not well understood in spite of a great deal of research done in the United States and in Europe. Most are a detriment and a defect in the wine if they form, and cannot be removed. Occasionally a sulfur containing compound enhances a wine. Sauvignon Blanc is one wine that contains a number of sulfur compounds which enhance the bouquet of this wine.  One which rolls off the tongue is 4-mercapto-4-methyl-pentan-2-one (4-MMP). There are others, but let’s quit while we are ahead.

Volatile sulfur compounds are formed, as was noted above during fermentation. The formation of H2S is controlled by enzymes in the must. These enzymes reduce sulfates in the wine, and eventually make certain sulfur containing amino acids such as cysteine and methionine. The latter is converted to methanethiol – another foul smelling sulfur compound!  There is much more than this going on, and that is dependent to a certain extent on the amino acid composition of the must. A not easy to discuss topic.

Basically, It is difficult for a winemaker to know how a fermentation is progressing, from a perspective of “what sulfur compounds are forming that I cannot see, or hear, but can smell?”  In addition to the “What” there are the “Whys”. What may cause this and why?  Let’s look and a few common issues, and possible ways to reduce (there’s that word again – but in a different context) the problems and produce a good wine?

Assimilable Nitrogen (nitrogen available to the yeast) deficiency in the must causes the yeast cells to stop metabolizing sugars, to slow down fermentation, and sometimes stop “working” altogether. The result of this proteolytic (protein destruction) state can be the release of H2S from sulfur-containing amino acids as the yeast cells degrade.  This breakdown releases nitrogen that the yeast can use–but at a price. That price is production of H2S.  Older yeast strains form H2S as a matter of course during fermentation. Many of the newer strains of yeast  either do not do that, or form so little H2S that is it inconsequential.

Fermentation temperatures also effect H2S production. Generally, less is produced at lower fermentation temperatures. However is must be noted that while H2S production is lower in cooler fermentations, the H2S is more soluble at these temperatures, and less likely to be removed naturally.

An interesting phenomenon is the fact that more H2S is produced in tall fermentation tanks (height to diameter) than in shorter tanks. The design of taller tanks reduces the redox potential for the conversion reactions, and thus makes the formation of H2S – a redox reaction – easier.

Vineyards were often sprayed with elemental sulfur, and/or with other sulfur containing compounds to stop powdery mildew, or other forms of vine pathogens. This may still be done in many places, but there is little reason to do so other than cost, as the newer products do a fine job, and contain no sulfur.

Some H2S may be formed from the sulfur dioxide (SO2) added to must during the processing of the fruit – destemming and crushing. Sulfur dioxide targets pathogens likely to effect the wine.

Can a reduced wine be brought back to its pre-reduced state? The answer is yes, if the malodorous molecule is H2S or a mercaptan or thiols,  and/or a few other small volatile sulfur compounds. How to do that is a winemaking secret!



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