Common Wine Faults: Oxidation, Volatile Acidity, and Brettanomyces with Jim Duane, Winemaker at Seavey Vineyard
Jim Duane started making beer in high school in Salt Lake City because it was basically the only way to get it. From there he moved to wine kits, then to a job in New Zealand at 22, then to UC Davis for a master’s degree, and then to Napa Valley in 2004, where he has been ever since. He has spent almost 15 years as the winemaker at Seavey Vineyard, a 200-acre estate in Conn Valley with about 40 acres under vine, farmed by an in-house team. He also hosts the Inside Winemaking podcast, which has become a go-to technical resource for winemakers at every stage of their career.
He sat down with Lauren Heindel to work through the winemaking faults that create the most problems in practice: how they develop, how to detect them, and what actually works in managing them. This is Part 1 of a two-part conversation.
The challenge with wine faults is not that the science is obscure. Most winemakers know what oxidation is, know what Brett smells like, know that volatile acidity has to stay in check. The harder problem is knowing exactly when to intervene, what the right intervention is, and what happens when you are working alone in a small cellar with no margin for expensive mistakes. That is the practical level Jim operates at, and it is what makes his perspective on these faults worth listening to.
In this episode, we cover:
How moving from a large winery to an estate operation changed Jim's Winemaking philosophy, and why doing less is often the right call at smaller scale
Why new winemakers consistently make the same mistake with equipment, and what to focus on instead
The narrow window of oxygen protection that fermentation provides, and why oxidation typically develops in the period right after
How to manage SO2 timing around malolactic fermentation, including the tricky period when you cannot add sulfite without killing the bacteria you need
Why volatile acidity cannot be removed, only controlled, and what role bacteria and oxygen play in pushing it out of range
How Brettanomyces develops in barrel, what the first signs are, and the combination of temperature management, SO2, and filtration that brings it under control
Unlearning big winery thinking
Jim spent time at larger wineries before coming to Seavey, working in roles where he was writing work orders and handing them to cellar staff, overseeing the operation but not always directly connected to the vineyards or the individual lots. Coming to Seavey changed that entirely. At an estate winery where he works alone, every decision about every wine is his, and there is no insulating layer between intention and execution.
The shift was not just logistical. It changed how he thinks about winemaking itself. At a larger operation, intervention is often the safe choice, because the risk of something going wrong at scale is high and the cost of a mistake is enormous. At a small estate, the calculus is different. With intimate knowledge of each lot and the ability to catch problems early, you can afford to do less and let the wine express more of what the site and the vintage actually gave you.
I had to ultimately learn to shift my thinking and actually do a lot less work in my Winemaking now at the smaller winery than I would do at a larger winery. I'm just more intimately tied to a smaller volume of wine so I can really stay on top of stuff and it allows me to take more risk through fermentation and just through all of Winemaking.
Jim Duane , Winemaker
Seavey Vineyard
The goal Jim describes is peeling back the layers of protection that larger winery production builds in, things like filtering and other hands-on interventions that are appropriate at scale but can smooth away character in smaller lots. That goal requires being genuinely close to the wine, which at a small estate is structurally possible in a way it cannot be at a winery running 24 hours.
If you are coming to a small winery from a larger operation, Jim's advice is to resist the instinct to import the protocols you already know. The risk profile is different, the volume is different, and the interventions that made sense in a large cellar may be unnecessary or counterproductive at small scale. Start by questioning each step rather than assuming it carries over.
The first mistake new winemakers make
Jim works with a lot of people who are new to winemaking, either through consulting or through his podcast, and he sees the same pattern consistently. The first thing people want to do is acquire equipment: the right tools, the right gear, the right setup. He draws a direct parallel to his daughter getting into tennis and immediately wanting the shoes and the racket before she had learned to hit the ball.
The equipment instinct is understandable. Winemaking has a real gear culture, and there are easy ways to spend a lot of money on tools that feel like they are part of making better wine. But Jim’s position is that the foundation is grapes and clean winemaking, and until those are solid, the equipment does not matter. At the beginning of any winemaker’s learning curve, mistakes are inevitable, and the goal is to keep those mistakes inexpensive.
I really try and temper people to focus on grapes and great clean Winemaking, especially at the beginning. And then as needed, we can graduate and get to some of the fancier, more high-priced tech pieces of equipment. Because most people do make a lot of mistakes, and unfortunately a lot of people, including myself when I first started, have to roll a barrel out to the front and dump the wine down the drain because it's not salvageable. Want to keep those mistakes cheap in the beginning.
Jim Duane , Winemaker
Seavey Vineyard
The same logic applies to lab equipment. Jim has a basic lab setup behind him in his office at Seavey, and he recommends that new winemakers invest in the fundamentals first. The ability to measure free SO2 and to monitor fermentation covers most of what you need in the beginning. The more sophisticated tools can come in as skills progress and the areas where precision actually matters become clearer.
Before adding any new piece of equipment, ask what specific problem it solves and whether you have that problem yet. Equipment that addresses a challenge you have not encountered is sunk cost. Equipment that solves the thing that is actually limiting your wines right now is investment. That distinction is harder to make before you have made a few vintages, which is one reason why restraint at the beginning produces better outcomes than spending early.
Oxidation and the window after fermentation
Oxidation is the wine fault Jim sees most frequently, and it is closely tied to a specific vulnerability in the winemaking timeline. While wine is actively fermenting, the CO2 produced by yeast provides genuine protection from oxygen. That protection is real but it is narrow, and it ends when fermentation does. The period immediately after fermentation, when the wine is still in tank or barrel without the CO2 blanket and before sulfite additions have been made, is when oxidation risk is highest.
The intervention at that moment is SO2. Beyond its role as a preservative, sulfite can scavenge acetaldehyde, the compound responsible for the rotten apple aroma that characterizes oxidized wine. Getting sulfite additions right in timing and amount is one of the more consequential technical decisions in the cellar, and it is also where many new winemakers are least confident.
You can only have this short window of time when wine is really protected from oxygen, when it's actively fermenting. So I see a lot of easy ways to get into oxidation. That's probably one of the main faults that I see.
Jim Duane , Winemaker
Seavey Vineyard
The timing complexity increases significantly around malolactic fermentation. During MLF, you cannot add SO2 because it would inhibit the bacteria doing that conversion. The window between the end of primary fermentation and the completion of MLF is a period when the wine is essentially unprotected from a sulfite standpoint. Jim’s approach is to monitor that window closely, verify that the malic is fully converted to lactic, and then move quickly with sulfite additions once it is complete. Waiting too long after MLF is complete is a common source of problems.
If you are adding SO2 after fermentation, verify that MLF is complete first. Adding sulfite while MLF is still active kills the bacteria you need and stops the conversion partway through, which can leave residual malic acid and create instability in the finished wine. A simple paper chromatography test or a lab analysis confirms completion before you commit to the sulfite addition.
Volatile acidity: managing what you cannot remove
Every wine has some volatile acidity. It is a normal product of fermentation, present mostly as acetic acid, and at low levels it does not register as a fault. The problem starts when concentrations increase past the threshold where it begins to affect the wine’s character, and at that point the winemaker has a limited set of options, because unlike many other faults, VA cannot be taken out of a wine once it is there. The only strategy is keeping it from getting worse.
The primary source of VA is bacteria, not yeast, and bacteria are stimulated by oxygen. Any wine with significant oxygen exposure, whether from a partially full vessel, an open-top tank, or a barrel that was not monitored closely, is at risk of bacterial activity driving VA upward. The most dangerous period is after fermentation ends and during the MLF window, when SO2 protection is limited or absent and bacteria of all kinds are active in the wine.
You can't get rid of volatile acidity. You just need to manage it and control it so it doesn't get out of whack and you end up with vinegar. Managing your SO2 is part of it — the majority of all acidity comes from bacteria, not from yeast. And so you wanna give the yeast a chance to build their populations for fermentation, but you don't really want bacteria running out in front of the yeast building their populations.
Jim Duane , Winemaker
Seavey Vineyard
In practical terms, VA management is largely about eliminating the conditions that allow bacteria to proliferate: keeping vessels full, limiting headspace and oxygen exposure, and adding SO2 as soon as MLF is confirmed complete. For winemakers working with open-top fermenters or vessels that are hard to keep full during aging, the exposure risk is higher and monitoring needs to be more frequent. VA that is caught early, when it is beginning to move upward, is manageable. VA that has been running unchecked for months is not.
When checking your wines for VA, use smell as an early detection tool before running numbers. Acetic acid and ethyl acetate have distinct aromas, and training your palate to notice the earliest signs of VA drift is faster than waiting for lab results. If something smells slightly off in the direction of vinegar or nail polish remover, measure it before the next scheduled check. Early intervention is the difference between a manageable problem and an unrecoverable one.
Brettanomyces: two camps and what to do about it
Jim is clear about where he stands on Brettanomyces in his own cellar: he does not want it. He is equally clear that he actively seeks out wines from Bordeaux and parts of Italy that show Brett character, and he finds that those secondary, savory aromas integrate well with the style of wines made in those regions in a way they typically do not with the more fruit-forward profile of California wine. He considers both positions defensible and does not pretend they are not in tension with each other.
In a practical winemaking context, Brettanomyces is a barrel problem, not a fermentation problem. Jim has never seen it develop before about six months of wine being in oak, and the longer a wine stays in barrel, the greater the exposure risk. The proxy compounds for Brett, the molecules that produce the bandaid and sweaty horse aromas most associated with the fault, are ethyl phenol and ethyl guaiacol. The organism that produces them is a yeast well adapted to wine environments: it tolerates wine pH, moderate free SO2, and typical alcohol levels. Once it is established in a wine, it is not easy to eliminate.
It's not a problem that develops during fermentation, like some of the other faults we discussed. You tend to see Brettanomyces in red wines after — I've never seen it before six months of wine being in barrel. And the longer a wine's in barrel, the more likely you are to see it.
Jim Duane , Winemaker
Seavey Vineyard
The most effective first response when Brett is detected is lowering the cellar temperature, ideally to around 52 degrees. This does not eliminate the infection, but it significantly slows the metabolism of the organism and stops the production of those characteristic aromas from advancing. Jim learned this from a microbiology professor at Washington State University, and it held up in his own cellar. Beyond temperature, bumping free SO2 toward the higher end of what the wine can tolerate provides additional inhibition. But the step that actually removes the infection is filtration. Filtering to one micron removes essentially all of the Brett cells from the wine and stops the progression. The complication is that a wine often still needs time in barrel after filtration, which means potential re-exposure. Monitoring infected lots through lab testing, particularly the scorpion PCR test available through labs like ETS, gives a quantitative picture of how much Brett is present and where.
If you smell anything that raises a flag in barrel, bandaid, smoke, or an unusual savory character that was not there at the last tasting, pull the temperature down immediately while you investigate. The temperature adjustment costs almost nothing and loses you nothing if the smell turns out to be something else. Waiting to confirm the source before acting gives the infection more time to establish itself.
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Frequently Asked Questions about Common Wine Faults
What are the most common wine faults winemakers encounter? +
The most frequently occurring wine faults in production settings are oxidation, volatile acidity, and Brettanomyces. Oxidation typically develops in the period immediately after fermentation ends, when the CO2 protection from active yeast is gone and sulfite additions have not yet been made. Volatile acidity is caused primarily by bacterial activity and cannot be removed once present, only controlled through SO2 management and oxygen limitation. Brettanomyces is a barrel-specific issue that typically does not appear before six months of aging and worsens the longer wine stays in oak.
What causes volatile acidity in wine, and how do you manage it? +
Volatile acidity in wine is caused primarily by bacteria, not yeast, and is stimulated by oxygen exposure. Acetic acid is the main component and is present in all wines at low levels, becoming a fault when concentrations rise to the point of affecting aroma and flavor. VA cannot be removed from a finished wine; the only management strategy is preventing it from increasing. This involves maintaining adequate free SO2 to inhibit unwanted bacteria, keeping vessels full to minimize headspace and oxygen exposure, and moving quickly with sulfite additions once malolactic fermentation is complete. The MLF window is particularly risky because SO2 cannot be added without inhibiting the bacteria needed for that conversion.
How do you detect and manage Brettanomyces in a winery? +
Brettanomyces detection starts with smell: the characteristic aromas of bandaid, sweaty horse, or smokiness in barrel-aged red wines, produced by the compounds ethyl phenol and ethyl guaiacol, are the first sign of infection. Confirmed detection can be done through quantitative PCR testing, available through wine labs like ETS, which measures how many Brett cells are present. Management involves lowering cellar temperature to around 52 degrees to slow the organism's metabolism, maintaining free SO2 at the higher end of the wine's tolerance, and filtering to one micron to physically remove the cells from the wine. Infected lots should be quarantined and monitored separately to prevent cross-contamination.
What should new winemakers prioritize before investing in equipment? +
New winemakers should focus on sourcing quality grapes and practicing clean winemaking fundamentals before investing heavily in specialized equipment. The early learning curve involves unavoidable mistakes, and the goal is to keep those mistakes inexpensive while building foundational skills. Basic lab equipment for measuring free SO2 and monitoring fermentation covers most of what is needed at the start. More sophisticated tools become relevant as specific technical needs emerge, but purchasing equipment ahead of need rarely improves outcomes and adds cost to the inevitable learning-curve losses. Equipment decisions are better made in response to actual problems encountered than in anticipation of problems that may not arise.
How does SO2 work to protect wine from faults? +
Sulfur dioxide serves two protective roles in winemaking. As a preservative, it inhibits microbial activity, including the bacteria that produce volatile acidity and other spoilage organisms. It also scavenges acetaldehyde, the compound responsible for the rotten apple aroma of oxidized wine, rendering it sensory-inactive. The timing of SO2 additions is complicated by malolactic fermentation: sulfite inhibits the bacteria that perform MLF, so additions must be withheld until that conversion is confirmed complete. Once malic acid has been fully converted to lactic acid, moving quickly to protect with SO2 is important, as the window after MLF is complete and before sulfite is added is a period of elevated risk for both oxidation and bacterial spoilage.