Most often, we open a bottle of wine and are greeted with wonderful aromas and flavors that delight our senses and taste buds.  However, on a rare occasion, and sometimes in home winemaking, we encounter less than pleasant aromas and tastes.  Are these considered wine flaws or wine faults – and what is the difference?  What we would most like to know is how to identify these and what causes them. 

To start with, let’s understand the difference between a wine flaw and a wine fault.  A wine flaw is an imperfection in the wine, such as a slight off-odor, minor cloudiness, bubbles or small particles in the wine.  A wine flaw might also include an imbalance with acidity/sweetness, short finish, lack of exceptional aroma or flavor, or color that is slightly off from expectations for that varietal.  These are all considered wine flaws because they are not considered normal for the wine type but they are minor enough that the wine is still drinkable. 

On the other hand, a wine fault is a major deviance from the normal characteristic of the wine and causes it to be undrinkable.  A wine that has developed cork taint is usually so pungent that the wine is undrinkable.  Likewise, a wine that has developed high volatile acidity (VA) will be so sharp and acidic that it too is undrinkable.  These are both examples of wine faults.

The majority of wine flaws and faults can be grouped into:  oxidation, sulfur compounds, microbiological, and environment.  These are often the result of poor winemaking practices or decisions.

Oxidation is the most common cause of wine faults, with oxygen being both a friend and foe in the winemaking process.  During fermentation, oxygen is our friend as it is vital for the yeasts to thrive and perform their job of converting sugar into alcohol.  After primary fermentation, oxygen often becomes more of a foe as it can become the catalyst for numerous reactions including a rise involatile acidity. 

Other common wine faults fall into the Sulfur Compounds category.  Negative sulfur compounds, such as hydrogen sulfide (H₂S), are often associated with the olfactory flaw known as “reduced” notes.  This means that you have a high amount of negative sulfur-based compounds and not enough available oxygen in the wine to mitigate these compounds.  In winemaking, there is a fine balance between a “reductive” environment (“low redox potential”) where H₂S will persist, and an “oxidative” environment (“high redox potential”) where sulfurs will often precipitate out. 

Microbiological faults are often the result of contamination with bacteria or yeasts that have not been inhibited by sufficient sulfur dioxide (SO₂), which serves as an antimicrobial and antioxidant agent in winemaking.

Environmental faults are the easiest faults to avoid by simply ensuring that winemaking and storage facilities follow best practices in environmental controls such as temperature and light.

Below is a description of some of the common wine flaws (if minor) or faults (if excessive):

Oxidation Faults

Flaw/Fault	Sensory Description	Cause
Acetaldehyde	Smells like sherry, nutty, bruised apple or dried out straw.	Wine is exposed to too much air during winemaking/bottling. Also result of film bacteria (Acetobacter) on surface of wine.  Low SO2 also contributes to this development.
Surface Yeast Contamination (Candida)	Smells like musty wet cardboard, acrid, sherry.	Too much headspace in container and oxygen is touching the  wine surface.  Low SO2 contributes to this development.
Acetic Acid / VA (Volatile Acidity)	Smells sharp like vinegar or pickles.	Typically caused by acetic acid bacteria (Acetobacter) but can also be lactic acid bacteria. Excess oxygen in the headspace of tanks, barrels or carboys. Low SO2 levels in wine.
Ethyl Acetate / VA (Volatile Acidity)	Smells sweet or fruity at low levels. Sharp, acetone or nail polish remover at high levels.	Oxidation of wine along with microbial spoilage such as Acetobacter.

Sulfur Faults

Flaw/Fault	Sensory Description	Cause
Sulfur Dioxide (SO2)	Smells like a burnt match head; sharp/acrid, nose burn sensation.	Sulfur dioxide is often added to wine as an antioxidant or antimicrobial agent.  But too much added can cause this sulfur flaw.
Hydrogen Sulfide (H2S)	Smells like rotten egg. Very pungent and offensive but can dissipate with aeration.	Reduction of elemental sulfur residue (from fungicide sprays on grapes, soil). Yeast stress from lack of nitrogen/nutrients or temperature stress. Lack of oxygen during fermentation.
Ethyl Mercaptan	Smells like garlic/onion, cabbage, vegetal, skunk. Very pungent and offensive.	Existing H2S reacts with ethane to form mercaptans. Yeast metabolizing sulfur in the lees (during fermentation) or during aging from H2S that was not removed earlier.
Thiols & Disulfides	Smells like burnt rubber, garlic/onion, canned corn, cooked cabbage. Very pungent and offensive.	Further oxidation and development of ethyl mercaptan (ethane or methane thiols). Difficult to treat at this advancement.

Microbiological Faults

Flaw/Fault	Sensory Description	Cause
Brettanomyces 4-Ethyl-guaiacol (4EG)	Smells of smoky, spicy, cloves.	Contamination of Brettanomyces (spoilage yeast) due to improper sanitation and inadequate SO2 levels.
Brettanomyces 4-Ethyl-phenol (4EP)	Smells like stables, horsey, sweaty-saddles.	Contamination of Brettanomyces (spoilage yeast) due to improper sanitation and inadequate SO2 levels.
Brettanomyces 4-Vinyl-phenol	Smells medicinal like plastic Band-aid bandages.	Combination of both 4EG and 4EP also due to improper sanitation and inadequate SO2 levels.
Yeast / Ongoing Fermentation	Smells yeasty with visible cloudiness and fizziness.	Residual sugar is left remaining in the wine with insufficient SO2 to inhibit and/or lack of sterile bottling.
Lactic Acid Bacteria	Smells like a swampy, stale dishcloth or sauerkraut. Wine may appear turbid and slightly effervescent.	Lactic acid bacteria (LAB) are used to convert malic acid to lactic acid during malolactic fermentation (MLF). Caution must be taken to ensure that MLF is complete before bottling or that MLF has been properly inhibited, followed by sterile filtration.
Diacetyl	Smells like rancid butter, buttery, butterscotch.	Diacetyl is produced by the metabolism of citric acid in the wine during malolactic fermentation (MLF). Can be considered nice when this aroma is desired, but is usually not desired in red wines.
Geraniol	Smells like crushed geranium leaves, floral, sweet, bubblegum.	This fault occurs when lactic acid bacteria reacts with excessive amounts of potassium sorbate (sorbic acid) during malolactic fermentation.

Environmental / Other Faults

Flaw/Fault	Sensory Description	Cause
Cork Taint (Trichloranisole / TCA)	Smells moldy, musty (wet newspapers), and earthy like decayed wood. Fruit aromas are masked.	Caused by a reaction between chlorine (cleaners) or bromophenols (fungicides) with fungus often found in corks.
Heat Damage	Smells like cooked fruits or maderized wine.	Excessive storage temperatures for prolonged periods or excessive temperature fluctuations
Lightstrike	Delicate white wines may take on a wet wool or wet cardboard characteristic.	Caused by excessive exposure of white wines to light. Wine bottles should be dark glass and/or stored in dark environments.
Excessive Oak	Overly oaky and loss of fruit characters.	Wine spent too much time on oak.
Acid Imbalance	Wine tastes flabby.	Too low TA (tartaric acid) and too high pH in wine.
Sediments	Wine smells fine but there are visual sediments in the wine.	Crystals may be due to tartrate instability; small sediments may be due to unfiltered wine; dark sediments in red wine may be due to unstable color (anthocyanin-tannin bonding).
Plastic	Wine smells like plastic or kerosene.	The use of non-food grade plastic containers in winemaking.

Many of these wine flaws/faults can be avoided altogether by:

• Start with clean grapes and sanitized winery equipment in appropriate environment.
• Make sure that you maintain adequate SO₂ levels in the wine.
• Maintain a low pH (higher acidity) which is more resistant to microbial activity.
• Monitor fermentations (primary, MLF) to completion.

Let’s hope that all your wine tasting experiences will be pleasant without experiencing any of these wine flaws or faults!

– Irene Scott, WSET-3, CSWS
UC Davis Winemaking Certificate 2020
OCWS Wine Education Chair
irene@ocws.org