by Andrew Chalk
I just spent a day in College Station in the most enjoyable way possible -- at a Texas A&M University course in “Wine Faults”. Almost everyone was either a winemaker or an academic taking a deep dive into the subject.
Wine Faults are important because, since wine is made (almost invariably) from grapes and the product of their fermentation and aging using universal techniques, all wines are subject to some degree or another to the same faults. Everyone in the industry can recite the names of the ‘Big Six’ but understanding their cause and amelioration is a totally different story. For example, it was brought home to me (as an outsider from the production process) how much ameliorating one fault is a compromise exacerbating another. The most headline-worthy finding was that you don’t need a cork to get cork taint!
The Big Six are: Oxidation, Brettanomyces, Hydrogen Sulfide (H2S), Cork Taint, Volatile Acidity, Malo-Lactic Fermentation in conjunction with high pH (low acid). In this series of articles I will report on each judged through the presentations by the faculty and graduate students who made them. Some of the findings are the results of primary research by the students themselves. The errors are all my own work.
HYDROGEN SULFIDE AND SULFUR AROMAS
Hydrogen Sulfide (H2S) makes a wine smell of rotten eggs. It is the most common of a family of compounds known as volatile sulfur compounds. Others are mercaptan and disulfides. Mercaptans can be split into two types (just to complicate things): Ethyl mercaptans produce a burnt match smell. Methyl mercaptans produce a rotten cabbage or burnt rubber smell. Disulfides produce a variety of vegetable smells. Full details, with a full orgy of subscripts, are shown in Figure 1, reproduced from Ph.D. candidate Cassie Marbach’s presentation.
CAUSES
The good news for the wine retailer or restaurant riddled with guilt that they may have caused this is that they are blameless. The basic cause is a sulfur imbalance that was in the bottle before they touched it. The sulfur could have been elemental sulfur, sulfur dioxide, or amino acids that contain sulfur. The H2S is either excreted by yeast during fermentation when under stress from insufficient nitrogen, or it forms when there is excess elemental sulfur on grape skins, possibly from sulfur sprays.
The actual yeast strain can play a part in the size of these effects. In this regard, the Australian Wine Research Institute and Maurivin developed a low H2S yeast strain it named Platinum.
These faults, a category named reduction-related faults, are the second most common type of fault found in wine, based on data collected at the International Wine Challenge.
PREVENTION
Adequate nutrients during fermentation is the key to minimizing the production of H2S. Nitrogen is the principal nutrient. Winemakers must measure YAN (yeast assimilable nitrogen) through laboratory techniques and adjust accordingly.
Remediation can involve controlled amounts of copper sulfate, Enartis Tan Elevage, and macro oxygenation to eliminate odors.
KEEPING UP TO DATE
Another faults workshop will take place mid spring 2023. For a definitive date and registration details keep an eye on their Facebook page (Texas Viticulture and Enology), where they post updates on upcoming educational programs.