Fermentation Problems
Stuck Fermentation
Symptoms: Fermentation started but stopped before reaching target SG. Airlock activity has ceased. SG is above 1.000.
| Possible Cause | How to Diagnose | Solution |
|---|---|---|
| Temperature too cold | Check must temperature — below 60°F (15°C)? | Move fermenter to a warmer location (68-75°F). Wrap in a blanket. Give it 24-48 hours. |
| Temperature too hot | Did the must exceed 95°F (35°C)? Yeast may have died. | Cool the must, then re-pitch with a strong yeast (EC-1118). Make a yeast starter first. |
| Yeast nutrient deficiency | Common in low-nutrient musts or high-sugar grapes. | Add yeast nutrient (Fermaid-O or DAP) according to package directions. Stir to aerate slightly. |
| Alcohol toxicity | SG started very high (above 1.110). Yeast reached its alcohol tolerance. | Re-pitch with a high-tolerance yeast (EC-1118, 18% tolerance). Accept that the wine may finish slightly sweet. |
| Excess sulfite | Did you add too much SO2 before pitching? | Aerate vigorously by stirring. Wait 24 hours, then re-pitch yeast. |
💡 The Restart Protocol
For truly stuck fermentation: make a yeast starter by rehydrating EC-1118 in warm water, then gradually adding small amounts of the stuck wine over several hours (acclimatization). This lets the yeast adapt to the hostile environment. Once the starter is actively fermenting, add it to the full batch. This works in the majority of cases.
Fermentation Never Started
| Possible Cause | Solution |
|---|---|
| Too much sulfite added — yeast was killed on contact | Wait 24-48 hours for SO2 to dissipate. Stir vigorously. Re-pitch fresh yeast. |
| Dead yeast (old or improperly stored packet) | Pitch new yeast from a fresh packet. Check expiration dates. |
| Must too cold | Warm the must to 65-75°F before pitching. Yeast is dormant below 50°F. |
| Improper rehydration (water too hot — over 110°F kills yeast) | Re-pitch with properly rehydrated yeast (104°F / 40°C water). |
Off Flavors & Aromas
Hydrogen Sulfide (H2S) — Rotten Eggs
Smell: Rotten eggs, sulfur, struck match.
Cause: Yeast stress — usually nutrient deficiency, high temperature, or sitting on heavy lees too long.
| Severity | Treatment |
|---|---|
| Mild (faint whiff during fermentation) | Add yeast nutrient. Stir/aerate the must. It often resolves on its own. |
| Moderate (noticeable after fermentation) | Rack the wine off lees immediately. Splash-rack (pour from height to aerate). Add 25 ppm sulfite. |
| Severe (persistent after racking) | Copper treatment: add 0.5 ppm copper sulfate (CuSO4). Test on a small sample first. Copper binds with H2S and precipitates out. |
| Evolved to mercaptans (garlic, rubber, sewer) | Very difficult to fix. Try ascorbic acid + copper treatment. If persistent, the wine may be lost. |
⚠️ Act Fast on H2S
Hydrogen sulfide is easily treatable in its early stages but becomes nearly impossible to fix if it evolves into mercaptans and disulfides. If you smell rotten eggs at any point during winemaking, address it immediately. The longer you wait, the worse it gets. Rack off lees, aerate, and add nutrients within 24 hours of detection.
Volatile Acidity (VA) — Vinegar
Smell: Vinegar, nail polish remover (ethyl acetate).
Cause: Acetobacter bacteria converting alcohol to acetic acid. Thrives in warm, aerobic (oxygen-exposed) conditions.
- Mild VA (barely detectable): Add sulfite (50 ppm), ensure no oxygen exposure. Monitor closely.
- Moderate VA: The damage is done — acetic acid doesn't go away. Blending with a clean, low-VA wine can dilute it below the perception threshold.
- Severe VA (obvious vinegar): The wine is lost for drinking purposes. You can let it complete its journey to become actual vinegar — a worthy consolation prize.
Prevention: Keep containers full (no headspace). Maintain adequate sulfite levels. Don't leave the cap exposed to air during primary fermentation. Clean and sanitize religiously.
Oxidation — Sherry-like, Flat, Brown
Symptoms: Browning color, flat/nutty/sherry-like aromas, loss of fruitiness.
Cause: Excessive oxygen exposure — headspace in carboys, insufficient sulfite, leaky airlocks, too many rackings.
| Stage | Treatment |
|---|---|
| Early (slight browning, muted fruit) | Add sulfite immediately (30-50 ppm). Add ascorbic acid (50 ppm) to scavenge oxygen. Top up carboy. Fix airlock. |
| Advanced (very brown, sherry taste) | Mostly irreversible. Can be blended with a fresh, fruity wine to mask. Sulfite will prevent further damage but won't restore lost character. |
Prevention: Maintain sulfite levels at every racking. Keep containers full. Minimize the number of rackings. Use inert gas (CO2 or nitrogen) to blanket wine during transfers.
Brett (Brettanomyces) — Barnyard, Band-Aid
Smell: Barnyard, horse blanket, Band-Aid, medicinal, sweaty saddle.
Cause: Brettanomyces wild yeast contamination. Thrives in low-sulfite, low-acid environments.
- Mild Brett (some winemakers consider this a positive character in small amounts): Increase sulfite to 50+ ppm. Sterile filter (0.45 micron) if possible.
- Heavy Brett: Very difficult to eliminate. Sterile filtration removes the yeast but not the flavors already produced. Prevention is far easier than cure.
Prevention: Maintain free SO2 above 25 ppm at all times. Keep pH below 3.7. Sanitize all equipment thoroughly — Brett lives in scratched plastic, old barrels, and poorly cleaned carboys.
Clarity Problems
Persistent Haze
| Type of Haze | Cause | Solution |
|---|---|---|
| Protein haze (whites) | Heat-unstable proteins | Fine with bentonite (1-2 g/L). Rack after 1-2 weeks. |
| Pectin haze | Insufficient pectin breakdown | Add pectic enzyme. Wait 2-4 weeks. More effective at warmer temperatures. |
| Microbial haze | Yeast or bacteria still in suspension | Add sulfite, fine with chitosan/kieselsol, or sterile filter. |
| Tannin-protein haze | Tannins bonding with proteins | Fine with gelatin (for tannin) or bentonite (for protein). Test on a small sample first. |
| Starch haze (rare) | Unripe grapes or apple additions | Add amylase enzyme. Wait 2-4 weeks. |
Tartrate Crystals
Symptoms: Small, glassy crystals on the bottom of bottles or clinging to the cork. Sometimes mistaken for broken glass.
Cause: Potassium bitartrate (cream of tartar) precipitating out at cold temperatures. Completely harmless and flavorless.
Prevention: Cold stabilize before bottling — chill wine to 28-32°F (-2 to 0°C) for 2-4 weeks. The crystals form and settle in the carboy instead of the bottle.
If already bottled: Nothing to worry about. Decant the wine before serving. Some wine drinkers consider tartrate crystals a sign of a natural, quality wine.
Other Common Issues
| Problem | Cause | Solution |
|---|---|---|
| Wine is too sweet | Fermentation stopped before all sugar was consumed | Restart fermentation (see stuck fermentation above). Or accept it as an off-dry wine. |
| Wine is too dry/thin | All sugar fermented, low body grapes | Back-sweeten: stabilize with sorbate + sulfite, then add sugar or grape concentrate to taste. |
| Wine is too acidic (tart, sharp) | High-acid grapes, no MLF | Allow MLF to soften acidity. Or cold stabilize (removes some tartaric acid). Or add calcium carbonate sparingly. |
| Wine is too flat (low acid) | Over-ripe grapes, warm climate | Add tartaric acid (1 g/L at a time, taste between additions). Blend with a higher-acid wine. |
| Wine is too tannic (bitter, astringent) | Over-extraction, extended maceration, unripe seeds | Time will help — tannins soften with aging. Fine with gelatin or egg whites. Blend with a softer wine. |
| Corks pushing out | Refermentation in bottle (residual sugar + live yeast) | Open all bottles immediately. Pour back into a carboy. Let it finish fermenting. Stabilize properly before re-bottling. |
| Fizzy wine (unintentional) | Residual CO2 or slight refermentation | If mild fizz: degass by stirring vigorously or using a vacuum pump. If refermentation: treat as above. |
🍇 The Winemaker's Triage
When you detect a problem, ask three questions in order: (1) Can I smell it or only measure it? If you can't detect it by taste or smell, it may not matter. (2) Is it getting worse? Take samples over several days. A stable flaw is easier to manage than an escalating one. (3) Is the cure worse than the disease? Aggressive treatments (heavy fining, acidification, copper) can strip character. Sometimes a minor flaw is better left alone than treated into blandness. Not every wine needs to be perfect — it needs to be enjoyable.
Prevention Checklist
- Sanitize everything. The solution to 80% of problems. There is no shortcut here.
- Use a hydrometer. Measure SG at crush, during fermentation, and before bottling. Numbers don't lie.
- Maintain sulfite levels. 25-30 ppm free SO2 at every racking and before bottling. It's your wine's immune system.
- Control temperature. A thermometer on your fermenter costs $5 and prevents the most common fermentation faults.
- Minimize oxygen exposure. Keep containers full. Use airlocks. Don't rack more than necessary. Blanket with CO2 during transfers.
- Taste regularly. Your palate is your best diagnostic tool. Catch problems early by tasting at every racking.
- Keep notes. Record everything: dates, measurements, additions, temperatures, observations. When something goes right (or wrong), your notes tell you why.
When to Give Up
Most wine problems are fixable. But some aren't. If your wine has:
- Obvious, strong vinegar smell and taste (advanced VA)
- Persistent mercaptan/rubber/garlic odor that doesn't respond to copper treatment
- Visible mold growth on the surface
- Geranium off-flavor (from sorbate + MLF interaction — a permanent, irreversible fault)
...then it may be time to dump it. It's disappointing, but every failed batch teaches you something the next batch benefits from. Even the best commercial winemakers occasionally lose a batch. Learn, adjust, and make the next one better.