Troubleshooting Guide for Home Winemaking

A perfect stranger wanting to strike up a conversation about winemaking once came up to me and said, “Making wine is really easy. Don’t believe anyone who says otherwise.”

Making wine is easy when all is “perfect,” but nothing is ever perfect in winemaking. Making great wine is a challenge more often than not given the multitude of problems, faults and spoilages that inevitably happen. The degree of difficulty lies in our skills in overcoming those problems and still craft great wines, or at least salvaging all that time and money we invested and not having to dump the wine.

This troubleshooting guide lists the most common types of problems, faults and spoilages you may encounter, how to fix them, and how to prevent them in the first place.

In trying to zero in on a probable cause, first clearly identify the symptom(s) and try to narrow it down to one most likely root cause using visual and organoleptic cues, and then take corrective action. Not all problems can be corrected; in some cases, the only course of action is to discard the wine down the sewer, and so, prevention is the best cure.

(Note: In the following article, “must” refers to unfermented grape juice with or without grape solids; once fermentation has started, it is referred to as “wine.” And, 1 hL equals 100 L or approximately 25 gallons, so divide the rates by 5 to determine the amount required per 5-gallon (19-L) carboy.)


The Theory

Wine yeasts are fastidious microorganisms that only thrive under favorable growth conditions. If any condition is not conducive to growth and reproduction, the alcoholic fermentation (AF) can become sluggish, as indicated by the Brix or specific gravity (SG) not dropping as expected. Or, it can stop altogether (and is said to be “stuck”), where there is no “bubbling” activity in the airlock.

Fermenting your wine with only indigenous yeasts is not recommended, not only because you don’t know what yeasts are present in your must and therefore cannot predict the outcome, but because these are the most sensitive to the harsh growth conditions found in wine. Most indigenous yeasts cannot ferment beyond 2–5% alcohol by volume (ABV) and are very sensitive to sulfur dioxide (SO2). Cultured strains of Saccharomyces cerevisiae are much more robust and ferment with predictable results. For more information, consult our online Yeast Strains Chart to choose a suitable yeast strain for your wine.

Possible Causes And Remedial Actions

If fermentation is sluggish or stuck, first confirm that the measured potential alcohol (PA) level is not beyond the yeast’s alcohol tolerance — the high sugar content can cause excessive stress on yeast cells and inhibit fermentation. If it is, conduct a progressive fermentation by first inoculating a small volume (e.g. 5%) of must with a strong fermenting yeast strain and an equal amount of complex nutrients. Reserve the remaining volume of must in a sealed container in a fridge. Once yeast activity starts and fermentation becomes vigorous, double the wine volume by adding an equal amount of reserved must (warmed up to within a few degrees of the fermentation temperature) to the fermenting wine. Continue this doubling process until all the must is used up.

Some winemakers prefer to add water to dilute the must down to a desired Brix/SG and PA, but that’s exactly what you will be doing — diluting aromas and flavors. Keep in mind also that water has a bleaching effect (beyond the effect of the dilution) on reds.

If fermentation produced 15–16% ABV and is stuck, the alcohol content has exceeded the yeast’s capacity. It will be very difficult restarting fermentation with that much alcohol; it would be best to leave the wine as is and look for ways to balance it (e.g. blending with a lower-alcohol wine or increasing acidity) to make it palatable.

Another common cause of a stuck fermentation is too cold of a fermentation temperature. To remedy this, simply raise the temperature of the must or wine into the 64–72 °F (18–22 °C) range and stir to distribute the temperature evenly and to get the yeast back into suspension.

If you conclude that fermentation is stuck because of high sulfur dioxide (SO2) from excessive sulfite additions, try aerating (by racking) the wine or doing pump-overs to try and reduce (bind) the free SO2.

In most cases when all conditions in the must or wine are normal, an addition of fresh yeast supplemented with nutrients should restart fermentation.

Preventive Measures

Before crushing, sort and remove moldy or rotten grapes; the microorganisms living on them could deprive your yeast of much needed nutrients.

At crushing, add sulfites to the must to achieve between 25–50 mg/L. Sulfite on the low side of the range if grapes are in excellent condition or if you intend to carry out a malolactic fermentation (MLF); sulfite on the high side if there is considerable grape damage or if there are signs of rot, or if you don’t intend to put the wine through MLF.

Before inoculating with yeast, make any adjustment to the must chemistry, including temperature, so as to be compatible with the selected yeast strain.

When preparing the yeast starter, be sure to allow the cultured yeast to hydrate for the recommended duration (usually 15 minutes) and water temperature (usually between 104–109 °F, or 40–43 °C). Fermentation should start within 24 to 36 hours. Sprinkling dried yeast pellets on the surface of the must is not a recommended practice.

When inoculating, gently stir the must to introduce oxygen, and stir once or twice daily to promote a healthy fermentation. Supplement the yeast with yeast nutrients, particularly if you suspect some deficiency, e.g. grapes have considerable mold. The must will also benefit from a lysozyme treatment when dealing with affected grapes. And always replenish with more yeast nutrients at one-third sugar depletion.


The Theory

Lactic acid bacteria of the kind employed in a MLF (sometimes called malolactic bacteria, or MLB) are very fastidious microorganisms that only thrive under very strict growth conditions. When active, the MLB convert malic acid into lactic acid. If any condition is not conducive to the bacteria reproducing, the MLF can become sluggish or stuck without the full conversion of malic acid, exposing bottled wine to future refermentation and spoilage.

Here too indigenous bacteria are not recommended; most are of the spoilage kind. Only use cultured Oenococcus oeni strains, and follow the manufacturer’s rehydration and inoculation instructions; some must be rehydrated while others can be added directly to wine.

Possible Causes And Remedial Actions

If the malolactic fermentation temperature is too low or too high, raise/lower your fermentation area’s temperature to 64–72 °F (18–22 °C) and very gently stir the MLB culture back into suspension. Do not introduce too much air as most MLB strains are anaerobic and therefore sensitive to oxygen.

If you suspect that the MLB was overly exposed to air and caused MLF to become stuck, re-inoculate the wine with a fresh culture, stir very gently, and ensure that the carboy is properly topped up and protected with a fermentation lock. Avoid any oxidation-promoting operations, such as racking, during malolactic fermentation.

If MLF is stuck because of high SO2 from excessive sulfite additions, try aerating (by racking) the wine or doing pump-overs to try and reduce (bind) the free SO2 until it is within the strain’s specs. You will need to prepare and re-inoculate with fresh MLB.

Only use cultured Oenococcus oeni strains, and follow the manufacturer’s rehydration and inoculation instructions;

If your wine’s pH is too low (most MLB require a pH of at least 3.2), deacidify the wine until the pH is in the range 3.1–3.6, ideally. You may need to prepare and re-inoculate with fresh MLB.

If the alcohol content has exceeded the MLB tolerance, there is not much you can do to complete a stuck MLF. If you expect to ferment to a high alcohol level, inoculate the wine concurrently with the alcoholic fermentation, a technique known as co-inoculation.

Preventive Measures

Ensure that your wine’s pH, temperature, alcohol and SO2 are within the malolactic bacteria’s specifications, and supplement with nutrients and/or carry out the malolactic fermentation on the nutrient-rich fine lees. Stir the lees very gently to avoid excessive oxygen intake.

For more difficult conditions, you can first condition the MLB inoculum using commercial apple juice (manufacturers usually provide instructions). In addition, for the alcoholic fermentation, choose a yeast strain that favors MLF; some yeast strains are known to inhibit malolactic bacteria.


The Theory

Except for red varietals known as teinturiers, which have red pulp and juice, the pulp and juice can range from a light yellow to a golden yellow color.

In reds, color comes from grape skin anthocyanins (color pigments) extracted during maceration and fermentation. Some varietals inherently have greater anthocyanin content, which is also influenced by viticultural practices and growing conditions. For example, the thicker-skinned Cabernet Sauvignon will have a much deeper color than the thin-skinned, cool-climate-grown Pinot Noir. The color exhibited by anthocyanins is pH dependent; the higher the pH, the lighter and the less stable the color. And longer maceration/fermentation results in more extraction and deeper color.

When whites are left to age for too long, they progressively darken to a golden color and then a brownish color. Reds will turn to an orange then brick-red and brownish color, and become lighter as a result of anthocyanin polymerization with other anthocyanins and tannins.

It is to be expected (and normal) that fining and filtering operations will lighten color; but overfining can excessively strip color.

Possible Causes And Remedial Actions

An almost-colorless white wine may be the result of excessive fining or using an unsuitable fining agent. The same is true of lightly colored reds, although common causes are insufficient color development in grape skins, because of a poor vintage or viticultural practices, or too short
of a maceration.

In addition, some grape varieties have inherently low pigmentation, and no amount of maceration can produce a deeply colored wine.

Don’t be tempted to alter color by artificial means; you will only disrupt the wine chemistry, which will destabilize color and possibly impact quality.

For reds, it is best to blend a light wine with a deeper colored one. You may want to keep some highly pigmented teinturiers, such as Alicante Bouschet, on hand expressly for this purpose.

Preventive Measures

If you expect reduced color extraction from your reds, you can add natural yeast derivative nutrients in conjunction with macerating enzymes before the start of fermentation. Use sulfite judiciously as SO2 can have a bleaching effect on anthocyanins. And also, choose a suitable agent for fining your wine.


The Theory

Browning in juice results from oxidation of phenols by naturally occurring enzymes (including polyphenol oxidase) in grapes; this reaction begins upon grapes being crushed or pressed (or if damaged at harvest or in transport). This can be alarming when white juice quickly turns a brownish color; this is due to the low phenol content in white varietals. The brown compounds will eventually settle and be separated through racking.

Browning in wine is a result of oxidation of phenols due to a reaction with oxygen absorbed during wine processing activities or aging. Reds are much better protected by virtue of their significantly higher polyphenol content.

If browning starts unexpectedly or prematurely — particularly if the wine is still in bulk — this points to a serious oxidation problem where ethanol is being oxidized into acetaldehyde with the wine taking on nutty aromas and flavors, akin to Sherry wine.

Possible Causes And Remedial Actions

Oxidation can be due to defective or improper use of equipment, excessive exposure to air during processing or storage, or inadequate sulfite protection. Browning is very difficult to correct in oxidized wine, and the acetaldehyde smell can only be attenuated with sulfite — SO2 binds to acetaldehyde.

Deeply browned whites may be undrinkable and may have to go the way of the sewer. For light browning in whites, try a casein treatment (50–100 g/hL) by first performing bench trials starting with the lower rate as casein can strip wine of aromas, or a PVPP treatment (25–75 g/hL) followed by a bentonite treatment (25–100 g/hL). For reds, it is best to leave the wine alone.

Preventive Measures

Check all your equipment, especially fermentation locks, on a regular basis to ensure no faults. Limit oxygen intake in post-fermentation operations, keep all carboys topped up, and maintain a nominal SO2 level based on wine pH.

Add natural yeast derivative nutrients at the onset of fermentation to avoid browning from oxidation; add gallotannins (which are highly antioxidative) to low-phenol reds either at the crusher or post fermentation.


The Theory

Once wine has completely fermented, it should be protected from extended exposure to oxygen with adequate SO2; otherwise, harmful (aerobic) Acetobacter will start feeding on alcohol and produce acetic acid. Acetic acid is the primary acid in volatile acidity (VA), and imparts an unappealing vinegar smell.

Acetic acid can be further esterified into ethyl acetate and impart an aroma reminiscent of nail polish.

Possible Causes And Remedial Actions

This is most often the result of defective equipment, such as a poor seal from bungs or dry fermentation locks, or a poor topping regimen, and excessive exposure to air, particularly at high pH.

Wine affected by advanced acetic spoilage cannot be cured and is best discarded. If only slightly affected, remove any white film that may have formed on the surface, then sulfite to the maximum level using 1.0–2.0 mg/L molecular SO2. Bottle immediately, and drink the wine as soon as possible.

Preventive Measures

Check all your equipment, especially fermentation locks, on a regular basis to ensure no faults.

Discard moldy grapes at the crusher, which could otherwise increase VA.

And add tartaric acid instead of citric acid when acidifying as the latter will be converted to acetic acid during malolactic fermentation.

Keep carboys and barrels topped up, and maintain a nominal SO22 level based on wine pH. Discard oak barrels affected by VA.


The Theory

Wine left exposed to air will develop a white film on the surface and expand across the surface to take on a whitish-green color as rogue surface yeasts thrive under the aerobic conditions.

Possible Causes And Remedial Actions

The most likely cause is either defective equipment that allowed ingress of air or wine that has not been adequately topped up during aging.

Spoilage begins on the surface and works its way down the volume of wine, and so, first determine how far down the problem has traveled. Use a wine thief and retrieve a sample from just below the surface being careful not to disturb the film. Smell and taste the wine; if it is off, pull another sample from lower down, and repeat until you are satisfied with the integrity of the sample. Then, rack the wine to the point you determined above; discard the remaining wine. Sulfite the wine, and process to bottle as soon as possible.

Preventive Measures

Before using any winemaking equipment, ensure its integrity and proper functioning. Check all fermentation locks specifically looking for any cracked seams; be sure to always have water in the lock. For floating-lid tanks, test the bladder and hand pump for leaks, and fix or replace defective parts.


The Theory

Sulfur dioxide (SO2) is the most effective preservative for winemaking. Free SO2 (FSO2) can be easily detected by its distinctive pungent burnt-match smell. If detected in wine, it is considered a fault and can actually become overpowering and irritating to the nose at high concentrations, and points to over-processing with sulfite. Yeasts produce small amounts of SO2 during fermentation but not enough to drive the level over the detection threshold.

Possible Causes And Remedial Actions

If you detect a sulfur smell in the fermenter, try aerating the wine by successive vigorous rackings. If you detect a sulfur smell in the bottle, aerate the wine by decanting it several times.

Alternatively, for wines with high levels of FSO2, up to 100 mg/L, you can treat these effectively with a dilute 3% hydrogen peroxide (H2O2) solution at a rate of 4 mL per carboy to reduce FSO2 by 10 mg/L. Run bench trials on a small sample before treating an entire batch. Measure precisely and carefully as excessive H2O2 can actually oxidize wine quickly, making it undrinkable. If FSO2 is well beyond 100 mg/L, there might not be any hope to salvage the wine as bound SO2 would become too high to be considered safe.

Preventive Measures

Avoid stressing the yeast during fermentation and only add the recommended amount of sulfite based on wine pH to maintain the desired FSO2 level at 0.8 mg/L molecular SO2.


The Theory

A stinky smell of rotten eggs or burnt rubber is the result of volatile sulfur compounds formed from sulfur-containing precursors. Hydrogen sulfide (H2S) is the culprit for rotten egg or sewer smells while mercaptans are responsible for burnt rubber or rotten cabbage smells. Even at low concentrations, these compounds can mask key fruit aromas.

Possible Causes And Remedial Actions

Elemental sulfur on grapes is a common cause when grapes have been over-treated or treated too close to harvest with sulfur-based vineyard sprays. Reds are more prone to H2S as the juice is macerated extensively with the grape solids. In white winemaking, the juice is separated from the sulfur-contaminated particles after a 24-hour settling period before initiating the alcoholic fermentation.

Excessive use of sulfite prior to fermentation can cause similar problems as well as prolonged contact of wine with burnt sulfur deposits in oak barrels. H2S can also form during fermentation if yeast is deprived of key nutrients, for example, in nitrogen-deficient juice from underripe grapes. But the most common causes are wine left on the gross lees for too long a period and wine subjected to high fermentation temperature.

The success of any corrective action depends on the amount of H2S produced and the intensity of the smell.

If H2S is only slightly detectable, it can be satisfactorily eliminated by racking the wine. This has the drawback of accelerating oxidation and has to be assessed against the severity of the H2S problem.

If H2S is quite noticeable, do not aerate the wine; otherwise, this can further compound the problem by irreversibly transforming H2S into mercaptans and disulfides — foul smelling compounds that cause wine to spoil. Treat the wine with a dilute 1% copper sulfate (CuSO4) solution.

Wine afflicted with mercaptans and disulfides is best discarded.

Preventive Measures

Don’t spray grapes too close to harvest, minimize the use of SO2 to recommended levels, and don’t stress yeasts — feed them lots of nutrients, and rack the wine shortly after the end of alcoholic fermentation to avoid extended contact with the gross lees. Be sure to choose a yeast strain with low H2S production.


The Theory

Once the alcoholic fermentation and MLF have completed, particulates start precipitating and the wine “self-clarifies.” Given enough time, wine will clear on its own; fining and/or filtration are used to hasten clarification. But a perfectly crystal-clear, fined/filtered wine can also turn cloudy from protein instability if exposed to excessive heat. Naturally occurring proteins react with polyphenols in reds, and so, these are usually not a problem; however, whites have negligible polyphenols and therefore are highly prone to protein (heat) instability.

Possible Causes And Remedial Actions

The wine was not protein/heat stable and was then subjected to high temperatures.

Test for protein stability by subjecting wine samples to various temperatures for various durations. Pour a fine-filtered sample (e.g., 200 mL) of wine into a non-reactive container with a high-range thermometer. If the sample is not adequately filtered, other forms of precipitation confound the results. Heat the sample to 176 °F (80 °C) and maintain at that temperature for 10 minutes. Turn the heat off, remove the pot from the heat source, and let cool for 15 minutes. Transfer the sample to a laboratory flask and place it in a freezer for several hours without freezing. Retrieve the flask and allow the sample to warm up to room temperature. If the sample shows any sign of haze or precipitation, then the wine is not protein stable and requires a bentonite treatment.

Another cause of cloudiness is high pectin content, usually a more common occurrence in fruit and country wines. Test for the presence of pectin by adding 50 mL of wine to 200 mL of methanol. If heavy whitish sediments form, the wine contains excessive pectin and it should be treated with pectic enzymes. Follow the treatment with medium and fine filtrations of the wine.

Preventive Measures

For whites, it is recommended to treat the wine with bentonite prior to bottling, especially if a heat stability test cannot be performed. For reds and fruit wines, add pectolytic enzymes at the crusher.


The Theory

Tartrate crystal deposits, considered a minor aesthetic fault, are the result of wine being subjected to cold temperatures for an extended period of time causing potassium and bitartrate ions to become insoluble and crystallize. The rate and amount of crystallization depend on potassium and bitartrate ion concentrations, temperature, and duration of cold storage.

Possible Causes And Remedial Actions

The wine is not cold stable and has been stored at cold temperatures, and formed tartrate crystals. These are easily separated from wine in bulk containers by racking. If they form in bottles, simply decant or pour the wine carefully.

Preventive Measures

To prevent tartrate crystals from forming in the bottle, cold stabilize the wine by chilling down to (water) freezing temperature for several days or by adding metatartaric acid.


The Theory

Table (still) wines need a little carbon dioxide (CO2) gas for freshness and to help release volatile aromas, but, in most styles of wines, it should never be detected on the tongue. When a wine has fizz or imparts a prickly sensation, the wine has excessive CO2 gas — this is considered a fault.

Possible Causes And Remedial Actions

Excessive CO2 can be the result of rushed winemaking (e.g. insufficient degassing), conditions often seen in wines produced and bottled too quickly, or incomplete alcoholic fermentation or malolactic fermentation (MLF) where wine starts refermenting, which can be particularly dangerous if it occurs in bottles.

If the wine is not bottled, first assess by hydrometry and paper chromatography whether the wine has completed both alcoholic fermentation and MLF. If not completed, let it run its course by taking corrective actions described above.

When the alcoholic fermentation and MLF are complete, there will still be some CO2 dissolved in the wine, but it will dissipate over time; otherwise, simply degas using your favorite method until there is no perceptible gas if bottling soon after.

If CO2 is detected in bottled wine, pour all the bottles from that batch into a carboy. Follow the instructions earlier for completing either or both fermentations.

Preventive Measures

Use your hydrometer to ensure that your wine has fermented to dryness, and invest in a paper chromatography kit to test for MLF. Never bottle wine with incomplete fermentation or with residual sugar. These must be properly stabilized with sulfite and potassium sorbate, followed by fine filtration. (Do not use potassium sorbate in any wine that has undergone MLF). Likewise, do not bottle any wine that has any residual malic acid. These have to be properly stabilized using sulfite, followed by fine filtration.


The Theory

In red winemaking, a small amount of beneficial (“good”) tannins are extracted from grape skins during maceration and fermentation. Undesirable harsher (“bad”) tannins are extracted from grape seeds as well as from stems from undestemmed grape bunches. Wines matured in oak barrels also acquire beneficial tannins from the wood.

Tannins are said to be aggressive when the wine is young, meaning that they are bitter-tasting and astringent. As the wine ages, the tannins polymerize into larger tannin complexes with a commensurate reduction in bitterness — they are said to be softer or rounder. High acidity or alcohol will cause increased tannin bitterness.

Possible Causes And Remedial Actions

A wine that tastes overly bitter or astringent is often a result of excessive good tannins from maceration or from extraction of bad tannins from green stems and/or grape seeds from crushing, maceration or pressing, or simply from poor balance between wine components.

An effective way to reduce tannins is to perform a fining with egg white, PVPP or gelatin, and possibly add gum arabic at bottling, blend with a softer wine.

If the wine is bitter and astringent because of an imbalance in wine components from, for example, high acidity due to high malic acid from a poor vintage, or high alcohol, you can try and reduce the total acidity (TA) which will make the high alcohol more palatable; this will make the wine much more balanced even if the alcohol content is relatively high. Typically, you will not want to (cannot) alter the alcohol content. To deacidify, put the wine through MLF if high TA is due to high malic content or use potassium bicarbonate if high TA is due to high tartaric content.

Preventive Measures

Always destem grapes, and reduce maceration time and ease up on polyphenol extraction procedures, such as punch-downs and pump-overs, to reduce tannin extraction to a more palatable level. Do oxygenate reds post-fermentation to soften tannins.


Winemaking may not always be easy, but many problems occur consistently enough that we can anticipate when they might occur and avoid them with proper preventative measures. And — although it is better to prevent problems than to have to deal with them — for some problems, there are remedies. If we lived in a world full of perfect wine grapes, winemaking would be easy. I hope this guide makes your winemaking, if not easy, at least a little easier in our world.