At some point, every winemaker will experience a flaw in their wine. Despite their best efforts to monitor and maintain it, a small portion will develop aromas or flavors that aren't commercially acceptable. Blending can be a viable solution, but it often results in a greater volume of tainted wine.
Prevention is the best solution. But when there is an issue, the ability to identify it and take corrective action is one of the most important parts of modern winemaking. Below are common wine flaws, their identifying traits, prevention steps, and techniques for eliminating them from finished wine.
Sulfur-Like Odors
Sulfur-like odors (SLO) are common in wine, ranging from rotten egg to onion, garlic, cabbage and burnt rubber. All wines contain sulfur compounds, though most are below the threshold of detection. Depending on winemaking technique or the wine's condition, SLO can become prevalent. The amounts needed to detect them are tiny, in the parts-per-billion (µg/L) range. Actual detection levels depend on each wine's composition: phenolics, volatile aromas, polysaccharides and proteins. Excessive SLO can also add a mineral or bitter taste and astringency.
| Table 1: Compound | Sensory description | Threshold (µg/L) |
|---|---|---|
| Hydrogen sulfide | Rotten egg | 0.5 |
| Methanethiol | Stagnant water | 1.5 |
| Ethanethiol | Onion | 1.1 |
| Dimethyl sulfide | Quince, truffle | 10.0 |
| Methionol | Cabbage | 1200 |
| Diethyl sulfide | Ether | 0.9 |
| Dimethyl disulfide | Asparagus | 15.0 |
| Diethyl disulfide | Garlic | 4.3 |
Most sulfur compounds form during fermentation. Yeast starved for nitrogen produce significantly more. Once the available nitrogen is exhausted, yeast break down amino acids, including the sulfur-containing cysteine and methionine, and the byproduct is hydrogen sulfide (H2S): rotten eggs. Much of it blows off with the CO2 during fermentation. If it doesn't, the wine keeps the off-odor, and higher sulfides (thiols, disulfides) may develop that are harder to remove.
To minimize H2S, get must and juice nitrogen right. Grapevines don't make fruit to feed yeast, so supplemental nitrogen is needed for clean ferments, but excess nitrogen causes very fast fermentation and loss of varietal character. Measure ammonia (NH3) and primary amino nitrogen (PAN). Many labs offer one-day turnaround at harvest, and formol titration or enzymatic tests can measure yeast assimilable nitrogen (YAN = NH3 + PAN) in house.
| Table 2: Fruit condition | Sugar (°Brix) | Required YAN (mg/L) |
|---|---|---|
| Clean | 21 | 140 |
| Clean | 24 | 250 |
| Considerable rot | – | 300 |
| – | 28+ | 300 |
YAN comes from proprietary supplements such as GoFerm, Thiazote, Nutristart and Superfood. Diammonium phosphate (DAP) provides only NH3, so consider using other products alongside it for a more balanced fermentation, with primary amino nitrogen and vitamins.
Even a healthy fermentation can develop SLO under reductive conditions. Aging wines need some oxygen; stored with little or none, trace sulfur compounds can form very low-threshold species.
Correcting SLO: start with aeration. A splash rack or similar can release volatile H2S below threshold and reverse the reductive environment, converting low-threshold thiols to higher-threshold disulfides. If aeration doesn't work, or disulfides are detectable, copper is an option. Treat first with ascorbic acid (50 mg/L) and SO2, which converts untreatable disulfides into treatable thiols; the copper binds the sulfur and precipitates. Rack copper-treated wines before further processing. Copper is the last line of defense: by law a wine may contain only 0.5 mg/L copper, and copper fining can strip volatile character and leave a wine flat.
Volatile Acidity
All wines contain volatile acidity (acetic, butyric, formic and propionic acids and ethyl acetate). Excess VA, usually acetic acid, gives aromas of vinegar, salad dressing, ketchup and barbecue sauce and reduces varietal character. VA is detectable at 0.6–0.9 g/L. Legal limits are 1.4 g/L in red wine and 1.2 g/L in white.
Some acetic acid forms during fermentation, but most problematic VA develops during storage and malolactic fermentation. Acetobacter consumes ethanol in the presence of oxygen to make acetic acid, and Lactobacillus consumes residual sugar in stored wine to do the same. Ethyl acetate (nail polish remover) can form alongside acetic acid, or early on from spoilage yeast brought in from the vineyard.
Preventing it: treat musts and juice with SO2 for any cold soak or cold settle, and keep it truly cold (45 °F) so spoilage yeast can't get established before inoculation. If you're unsure you can hold the temperature, inoculate at the destemmer so fermentation starts with the right yeast if cooling is lost. In storage, Acetobacter needs oxygen, so top barrels and gas tanks, and keep SO2 up. Wines with residual sugar should be sterile filtered or treated with a sterilant such as Velcorin before storage.
Correcting it: VA is hard to remove once established. First filter out the bacteria to stop acetic acid production. Blending works, but a typical wine carries 0.4 g/L VA, so it takes a lot of clean wine, and if the bacteria aren't fully gone you risk inoculating a larger batch. Some winemakers swear by fining with clean, oxygenated lees. One proven method is reverse osmosis: a high-pressure membrane separates water, alcohol and acetic acid (the permeate), which is treated with an ion exchange resin and returned, leaving the rest of the wine untouched.
Brettanomyces
Brettanomyces bruxellensis (Brett) is a spoilage yeast that gives aromas of band-aid, antiseptic, barnyard, horse blanket, wet cardboard and wet dog, often with a metallic taste. In some wines and regions, a little Brett is considered "house style" (many Bordeaux reds show it). It's a matter of degree; in large amounts it makes a wine undrinkable.
Brett usually shows up in red wines stored in barrels. Warm conditions and low SO2 are ideal for it (unfortunately also ideal for malolactic fermentation), as are high pH and residual sugar. Presence is confirmed by lab culture or by detecting 4-ethylphenol and 4-ethylguaiacol (4EP/4EG): where there's 4EP/4EG, there's Brett.
Brett comes in from the vineyard. It doesn't compete well with other yeasts and won't ferment juice, but once fermentation is done and the competition is gone, it can take hold. Barrels are ideal for its survival, and wines are typically inoculated by old barrels where Brett established itself while they sat empty.
Preventing it is a matter of cellar sanitation: cool cellars and appropriate SO2. Higher pH wines need more SO2 for the same killing power, and any residual sugar or high pH demands extra vigilance. See barrel cleaning and EBX Barrel Protect.
Correcting it: reverse osmosis can remove "Bretty" aromas. After filtering out the yeast, a membrane with a slightly larger pore than for VA passes molecules as large as 4EP/4EG (MW 152), and a carbon block filter removes them and similar spoilage compounds from the permeate.
Pyrazine
Pyrazines are the aroma compounds in bell peppers, chilies and peas, and they occur in grapes too. They peak at veraison and decline with ripening, so underripe, shaded or unevenly ripened fruit can carry elevated levels. Cabernet Sauvignon and Sauvignon Blanc are naturally higher. Grassy notes are expected in Sauvignon Blanc but off-putting in a red. Thresholds are tiny: 2 ng/L in whites and 10 ng/L (parts per trillion) in reds. Seven mL poured into an ocean-going supertanker of red wine could be detected.
Preventing it starts in the vineyard: ripe, well-sunned fruit is low in pyrazine. Manage the canopy to avoid shading, balance crop load, and consider dropping fruit. In the winery, 53% of grape pyrazines are in the stems and 31% in the seeds, so destem quickly, keep stem pieces out of the fermentor, and drop seeds (délestage) if there's real concern.
Correcting it is difficult, and there's no proven technology yet. Blending, or masking with oak or other aromas, are the options.
Cork Taint
2,4,6-trichloroanisole (TCA) gives a musty, wet-cellar aroma. Like pyrazine it's extremely potent: detectable at 2 parts per trillion in whites and 5 in reds. TCA is produced by a mold that converts chlorine and chlorophenols. Corks were once treated with bleach, and if left wet, mold would consume the chlorine and form TCA. Cork makers have made great strides (see Cork 101), but it's still a risk. Pressure-treated wood contains chlorophenols and is hard to keep dry and clean, an ideal environment for TCA.
Mitigation for a TCA-contaminated cellar usually involves burning it to the ground, so prevention is everything. Eliminate all chlorine cleaning products. Keep exposed wood, especially pressure-treated wood, dry and away from wine. Sanitize barrels and treat them with ozone if possible. Because the threshold is so low, blending usually isn't an option. Some wineries report success running wine over Saran wrap: a large surface area in contact with a small volume can bind TCA and reduce its impact.
Oxidation and Maderization
Traces of oxygen are needed to age wine, but gross oxygenation damages it. Oxidized reds and whites turn brown, fruit is muted, and the wine may show bruised apple, sometimes with VA. Barrels not regularly topped and tanks not regularly gassed usually lead to oxidation. Maderization is similar: literally "cooked," with sherry-like character. A main source is untopped barrels where a yeast film grows on the surface. That Candida film is the same microbe used for sherry, but it's not palatable in table wine.
Preventing it is relatively simple: keep oxygen away from wine. Top barrels and gas tanks, stir lees in barrels (autolyzed yeast is antioxidant), and maintain SO2, which scavenges oxygen.
Correcting it: ultrafiltration can remove brown color and maderized character. A membrane passes low-molecular-weight components and holds back the large browned tannins, leaving a wine with more esters and unpolymerized color and much of the fruit the browning had masked.
Other Taints
Several lactic acid bacteria (LAB) can convert malic acid to lactic acid. Oenococcus oeni gives desirable character; others, the kind that ferment sauerkraut and sausage, don't. Undesirable LAB byproducts include diacetyl (movie theater butter, rancid butter) and butyric acid (spoiled cheese, sweat). Prevent them by inoculating for MLF with a cultured bacteria and its nutrient. Mitigation is difficult: aeration can blow off some aromas, and SO2 plus limiting oxygen stops the microbes so the wine can be filtered and the off-aroma blended away. With diacetyl, it's often wise to wait if MLF is still going, since it's part of the MLF pathway and may dissipate when MLF finishes.
Excessive sulfur dioxide can give a matchstick aroma, from large SO2 additions or lowering the wine's pH. Aeration reduces it as the SO2 binds with oxygen.