• Skip to main content
  • Skip to footer
  • Cart
  • Account
  • Login

Denise Gardner Winemaking

  • Scroll
    • Services
    • About
    • Posts
    • Contact
  • Services
    • Services for Winemakers
    • Services for Wine Lovers
  • About
    • Meet Denise
    • Honors and Certifications
    • Client Testimonials
    • Media
    • Terms and Conditions
    • Privacy Policy
  • Learn
    • Articles
    • Cellar Tools
    • Lessons
    • Winemaking Q&A Summaries
    • Training Videos
  • Winemakers’ Blog
  • NOW
  • Search
Home / Winemakers' Blog / Wine Flaw Focus: Hydrogen Sulfide and “Reduction”

Oct 19 2018

Wine Flaw Focus: Hydrogen Sulfide and “Reduction”

Direct causes of hydrogen sulfide and “reduced” sulfur-based off-odors is still a bit unclear. While basic wine production techniques decrease the risk of this wine flaw, many winemakers experience the incidence of this flaw several times over the course of their career.

Hydrogen sulfide, commonly referred to as H2S, is usually correlated with inadequate nutrient management during primary fermentation. Additional contributors include residual sulfur from pesticide treatments and high soluble solid concentrations during primary fermentation.

Winemakers are encouraged to measure the yeast assimilable nitrogen (YAN) before fermentation to reduce occurrence of hydrogen sulfide development and stuck fermentations. With a starting YAN concentration, winemakers can optimize the concentration of nutrients to add, as well as the timing of addition during primary fermentation. While most yeast hydration nutrients add very little nitrogen to the fermentation, complex nutrients and diammonium phosphate (DAP), added later in fermentation, may have a noticeable impact on fermentation health and quality.

Without knowing the juice/must YAN content, winemakers play a guessing game with nitrogen additions. Here, the risk of adding too little or too much nutrient during primary fermentation is a possibility. Both situations can potentially lead to stuck fermentations or hydrogen sulfide development late in the fermentation process. Additionally, high YAN fermentations can pose additional risks for the wine in later stages of production.

Wine Business Monthly recently reported on a red winemaking study conducted by Opus One Winery (Napa Valley, CA) and UC Davis. In this study, the oxidation reduction (redox) potential was used as an indicator for when to integrate oxygen into the fermentation. In addition to making proper nitrogen addition, inhibiting fermentation “reductive” moments was key to minimizing hydrogen sulfide production.

Without proper YAN or redox potential testing equipment, many wineries rely on making standard “recipe” nutrient additions (i.e., adding the same nutrients at the same concentration to each fermentation). Additionally, routine pump-overs and punch-downs are conducted based on routine, instead of need. As one may guess, without treating fermentations with better acuity, either situation can lead to hydrogen sulfide development by the end of primary fermentation.

Conducting a copper screen to assess if a reduced wine can be treated with copper sulfate. Photo by: Denise M. Gardner

Treating Hydrogen Sulfide and “Reduced” Aromas

It’s safe to say that every winemaker will experience hydrogen sulfide or reduced aromas in a wine they make at some point in their winemaking career. In fact, some winemakers opt to avoid measuring YAN because treating hydrogen sulfide is perceived as being quite easy.

With a simple search online, you can quickly find copper sulfate addition remediation strategies.

I encourage winemakers to first conduct a “Copper Screen” with the wine in question. In this bench trial, two wine glasses filled with an equal volume of wine are prepared. One glass receives an overdose of copper sulfate. Both glasses are labeled, capped, and left to sit for about 15 minutes. After 15 minutes has passed, the wine aroma is assessed between the wine without the copper addition and the wine with the copper addition. If the wine containing copper sulfate has improved, then the winemaker can assume a copper sulfate addition will address the reduced aroma problem.

Please note that copper screens are tested using smell only. And, wine samples poured should not be returned to the wine tank of which the samples came from.

Some copper screens will integrate the use of a third glass with a dose of the heavy metal, cadmium. While cadmium is not added to wine, the use of it in this sensory test is to assess if the source of the reduced aroma is by hydrogen sulfide or if it includes additional mercaptans. Cadmium only reacts with hydrogen sulfide (Iland et al. 2012), and so its ability to clear up the aroma of a reduced wine fully indicates that only hydrogen sulfide is present in the wine. As a cadmium addition is not a viable option for winemakers and because disposing of cadmium is challenging, I do not encourage the use of a third glass in the copper screen.

Once a copper screen determines if a copper sulfate addition will help fix a reduced wine, the winemaker can move onto a “Copper Addition Trial.” This bench trial identifies the exact concentration of copper sulfate needed to remediate the hydrogen sulfide or reduce aroma. In practice, winemakers will opt for the minimal dosage rate required to treat the reduced aromas.

Copper sulfate additions are regulated by the Alcohol and Tobacco Tax and Trade Bureau (TTB). According to 27 CFR 24, winemakers cannot add more than 6 mg/L of copper to a wine at any time. Furthermore, the level of residual copper in a finished wine cannot exceed 0.5 mg/L in concentration.

The Problem with Copper Sulfate Additions

While copper sulfate additions may appear as a quick and easy fix for reduced aromas, it is not always the case.

First, copper sulfate reacts with hydrogen sulfide and any other thiols or mercaptans in the wine. Therefore, if you are dealing with a wine variety rich in aromatic varietal thiols (e.g., Sauvignon Blanc), the addition of copper sulfate can actually reduce the wine’s varietal aroma in addition to treating the reduced aroma.

Second, copper sulfate does not treat all forms of reduction. Disulfides, which are commonly grouped as aromatic compounds contributing to reduced aromas, do no react with copper. Therefore, it’s possible to treat a reduced wine with copper sulfate and have the reduced aroma remain. In these instances, the use of ascorbic acid with copper sulfate additions is needed.

This technique does not come without its own potential detrimental side effects to wine quality. The incorrect use of ascorbic acid can quickly cause excessive oxidation in any wine, and winemakers must take necessary precautions to ensure this does not happen during an ascorbic acid addition. Furthermore, the reaction to break the disulfide bonds can be slow, delaying winemakers from progressing the wine through production.

Finally, residual copper in wine is occasionally suggested as being problematic for wine quality as the wine ages. With this in mind, winemakers that make copper sulfate additions are encouraged to measure the residual content. Residual copper levels can be fined from the wine using several different production techniques. After this fining process,re-analyze the wine for residual copper levels to ensure the treatment has been successful.

Additional References

Iland, P., N. Bruer, A. Ewart, A. Markides, and J. Sitters. (2012) Monitoring the winemaking process from grapes to wine: Techniques and concepts, 2nd edition. ISBN: 978-0-9581605-6-8

Written by Denise Gardner · Categorized: Winemakers' Blog · Tagged: H2S, Hydrogen Sulfide, Reduced, Wine Faults, Wine Flaws

Denise Gardner is a winemaking consultant facilitating wineries to improve their production practices, efficiency, quality, and marketability. Want to get darn good at making wine? Subscribe today to our free bi-monthly content:

Get Darn Good

Footer

  • Services
  • Wine Lovers
  • Learn
  • Meet Denise
  • Winemakers’ Blog
  • NOW

info@dgwinemaking.com

  • Facebook
  • Instagram

Copyright © 2025 All Rights Reserved · Denise Gardner Winemaking · Terms and Conditions · Privacy Policy · Cookie Policy · Site by Tempora · Log in

Manage Cookie Consent

We use cookies to optimize our website. By using our services, you agree to our Cookie Policy for managing data.

Functional Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes. The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
Manage options Manage services Manage {vendor_count} vendors Read more about these purposes
View preferences
{title} {title} {title}