Wine defects

Wine is a sophisticated beverage that offers a wide variety of aromas. However, even the best wine can have some defects that can negatively affect its aroma and taste, masking the perception of its varietal and territorial olfactory characteristics.

Olfactory defects in wine can occur at various stages of the production process and are often due to processes of deterioration or contamination: problems due to insufficient grape ripening, incorrect fermentation management, poorly controlled oxygen contact, or improper use of sulfur dioxide.

Wine production is a complex process that presents numerous risks to be faced and avoided by winemakers. In most cases, defects affect the organoleptic characteristics of the wine. Fortunately, we have a powerful tool at our disposal to detect these problems: our sense of smell

Below are the main olfactory defects in wine:

One of the most well-known defects is CORK TAINT, which occurs when the cork used to seal the bottles is contaminated with a molecule called Trichloroanisole (TCA), which has very low perception thresholds (a tiny drop can be detected in an entire room) and is therefore a difficult defect to control. TCA can develop for various reasons, including the presence of mold or bacteria in the cork or the use of chlorinated chemicals during processing. This defect can alter the taste and aroma of the wine, giving it a characteristic cork smell. As it originates from various chemical compounds, it can have different olfactory nuances, including mold, fungus, earth, smoke, and burnt. Although this defect is relatively rare, when encountered, it negatively affects the olfactory and gustatory tasting experience and the overall quality of the wine. It is important to note that TCA can contaminate not only corks but also old barrels or treated wood, thereby affecting the entire batch of wine. Wine experts constantly work to identify solutions and minimize the incidence of this olfactory defect in wine bottles, but it is impossible to completely eliminate this risk of olfactory defect. 

Another common olfactory defect is MOLD, caused by the presence of microorganisms such as mold and bacteria due to poor hygiene in cellar materials such as tanks or barrels. This defect manifests as a persistent aroma and taste of damp soil/humidity/undergrowth, which, if present in large quantities, can significantly alter the aromatic characteristics of the wine, making it less enjoyable and compromising its overall quality. Wine producers can prevent the occurrence of this defect by performing the wine making procedures through rigorous quality controls in clean and sanitized environments. Despite these efforts, mold can occasionally occur, even in the most attentive productions. 

It is interesting to know that one of the molecules responsible for the aroma of fungus/mold, 1-octen-3-ol, is often found in big amounts in Ice Wines. Ice Wines, known as "Eiswein" in Germany and Austria, are produced from Riesling and Moscato Ottonel grapes. In Canada, they are called Icewine and are primarily made from Vidal, a French hybrid grape variety. Ice wines are produced by pressing frozen grape clusters that are left on the vine until December-January, allowing intermittent frosts to concentrate the sugars, acids, and aromatic compounds of the grapes. However, the significant delay in harvesting and the high humidity can promote the growth of molds on the berries, enriching the must with 1-octen-3-ol, and generating an intense aroma of porcini mushrooms in the wine. It is important to note that high-quality Ice Wines do not have this fungal aroma.

Another molecule, Geosmin, responsible for the characteristic intense earthy smell that is perceived in the air after rain following a period of drought or during a sudden summer storm that quickly wets the soil, can be found as a defect in wines contaminated by a fungus from the Penicillium family.

OXIDATION is another factor that can lead to olfactory defects in wine. When wine comes into contact with air incorrectly and oxidizes, various olfactory and gustatory defects can occur, including MADERIZATION (reminiscent of stewed apple), VINEGAR, GLUE, and WAX. An excess of oxygen can cause a sensory decline in wine, compromising its olfactory quality, neutralizing the identity of the aromas, and thus homogenizing the smell of all the bottles, nullifying the varietal and territorial characteristics of the wine. Oxidation can be controlled and reduced through preventive measures, such as the use of protective winemaking techniques and the use of antioxidant agents. However, if the state of oxidation persists, temporary symptoms can evolve into irreversible damage. There are cases where oxidation is sought after and desired, such as in the oxidative aging of some French Vins Doux Naturels like Rasteau or Banyuls Rancio, fortified wines like Port, Sherry, Marsala, Madeira, Vin Jaune du Jura, Vin Santo, and others. In these cases, wines are intentionally aged in the presence of air, often in barrels or demijohns made of glass, exposed to air and atmospheric conditions. Fortified wines are supplemented with alcohol, which  kills the microorganisms that can damage the wine. These wines develop a bouquet characterized by notes of oxidized apple, dried nuts, honey, caramel, and others.

  • The defective odor of MADERIZATION, or stewed apple, oxidized apple, is precisely due to the exposure of wine to oxygen without protection. It is caused by the abundant presence of Acetaldehyde, a molecule produced by oxygen. The odor of stewed apple and nuts overwhelms the primary and secondary aromas of the wine, neutralizing them. It is caused by the oxidation of ethanol in the wine, transformed into acetic aldehyde, leading to a decrease in alcohol content and an increase in acidity. To avoid the defect of stewed apple, it is important to protect the wine from oxygen, using techniques such as proper closure of containers and the use of inert gases. It is important to note that acetaldehyde not only causes an unpleasant aroma in oxidized wine but is also responsible for the headaches associated with excessive alcohol consumption.
  • The smell of VINEGAR is also a defect in wine, caused by harmful bacteria for the wine. Acetic acid is present in all wines, but in excess, it constitutes a defect that imparts an acidic taste and reduces the alcohol content of the wine. It is an alteration caused by the contact of the wine with air, such as in poorly sealed barrels or during incorrect topping up.
  • The presence of the olfactory defect of GLUE in wine is associated with the presence of ethyl acetate, an ester that forms from the reaction between acetic acid and ethyl alcohol during the winemaking process. This compound gives the wine an unpleasant aroma reminiscent of glue or solvent. The formation of ethyl acetate can occur under certain unfavorable conditions, such as low alcohol content in the wine, low fixed acidity, and high volatile acidity. Furthermore, prolonged contact of the wine with air and poor cellar hygiene can promote the development of this defect. The olfactory defect of glue is considered severe and irreversible. To prevent it, it is essential to maintain good cellar hygiene and adopt winemaking practices aimed at achieving adequate fixed acidity and reducing the presence of acetic bacteria. If the level of volatile acidity exceeds certain thresholds, the wine cannot be marketed and is destined for vinegar production or distillation.
  • The olfactory defect of WAX in wine is associated with the presence of salts, known as soaps, derived from fatty acids produced during fermentation. This defect can negatively influence the aromatic profile of the wine, imparting notes reminiscent of soap, candle wax, paraffin, and stearin. The olfactory defect of wax is particularly found in white wines, even in the early stages of maturation. Its manifestation can be reduced through appropriate winemaking practices and careful control of production, maturation, and wine storage phases.

There are "tertiary" aromas, resulting from oxidation and long bottle aging periods, which contribute complexity to the wine when they are subtle and do not overpower other aromas. However, they are considered defects if they are too pronounced. This is the case with the aroma of fenugreek (due to the molecule sotolone) in white wines or overripe red fruit and cooked apple in red wines.


In conclusion, determining the ideal amount of oxygen that a wine should have to maximize its aromatic qualities and sensory identity without causing oxidation or reduction defects is an extremely difficult task and represents one of the most challenging endeavors in modern winemaking research.

Olfactory defects due to REDUCTION, the "reduction notes," are instead the result of processes in which oxygen is removed during the fermentation or maturation stages of the wine. Wines that are deprived of air too early can develop a characteristic odor of being "closed," which resembles boiled cabbage, garlic, onion, sulfur water, and rotten eggs. The odors resulting from reduction are caused by sulfur compounds. However, sulfur dioxide added to the must in the form of potassium metabisulfite has important functions during winemaking: it acts as an antioxidant and antiseptic, prevents browning in white must, facilitates must clarification, inhibits wild bacteria and yeasts (promoting Saccaromyces), and aids in the solubilization of many substances present in grape skins. These defects can negatively affect the wine's aroma, altering its organoleptic qualities.

  • One of the most common olfactory defects due to reduction is known as the ROTTEN EGG SMELL, caused by the compound hydrogen sulfide (sulfuric acid). This unpleasant odor originates from the breakdown of sulfur-containing compounds present in the wine, such as amino acids, during fermentation. Hydrogen sulfide generates a strong odor of rotten eggs or sulfur gas, which can mask the desirable fruity and floral aromas of the wine.
  • Another olfactory defect due to reduction is represented by the COOKED CABBAGE, GARLIC. Dimethyl sulfide (DMS) is formed as a result of anaerobic fermentation processes or storage in the presence of specific yeasts or bacteria. It is a chemical compound with a powerful aroma found in many foods and beverages. It is found in wines and beers, as well as in cooked meat, cheeses, tomatoes, citrus fruits, melons, truffles, cereals, and vegetables. This defect can be particularly bothersome and can mask the complex and desirable aromas of the wine, negatively impacting the tasting experience. However, in some cases, and at very low concentrations, DMS can be a virtue rather than a defect. In fact, at low concentrations, DMS can also form in bottles of mature Bordeaux reds after many years of aging, imparting a subtle truffle aroma that is considered a characteristic virtue of these wines.

To prevent or mitigate olfactory defects due to reduction, it is important to adopt appropriate winemaking practices and properly manage fermentation and storage conditions. Adequate aeration during fermentation, the use of selected yeasts, temperature control, and the use of dark glass bottles (which protect the wine from light) can help reduce the formation of these unpleasant reduction odors.

Another olfactory defect in wine is the smell of STALL/STABLE or BARNYARD, which can be attributed to the presence of the molecule 4-ethylphenol, derived from phenols. Phenols are important natural substances in the composition of wine aroma as they contribute to giving vanilla, wood or smoky notes, as well as animal odors. In low concentrations, ethyl-4-phenol contributes to a pleasant leather note in aged wines. However, when the concentration is high, this leather note turns into an unpleasant odor. The yeast responsible for producing the 4-ethylphenol molecule is called Brettanomyces (commonly known as Brett). The barnyard odor it produces is often associated with wine aging in wood, but it is now known that this yeast can colonize the walls and all materials present in the cellar. Cellar hygiene, temperatures below 15 degrees, high alcohol content, and the use of sulfur dioxide can make it difficult for this microorganism to proliferate. Currently, managing this molecule in wines is complex because its formation does not seem to be directly related to the activity of lactic acid bacteria during malolactic fermentation, but rather to the spontaneous growth of unidentified lactic bacteria. Sterile filtration of wines, which guarantees the elimination of bacteria, could avoid the onset of the unpleasant odor of horse manure but would also result in the loss of the possibility of obtaining the pleasant leather note. In conclusion, managing this molecule in wines represents a challenge because understanding its formation can contribute to preventing or mitigating the defect and preserving the aroma and quality of the wines.

The last olfactory defect we will discuss in this blog is the HERBACEOUS ODOR (boiled vegetables, cut grass), caused by the molecule trans-2-esenale, which can be considered unpleasant or undesirable in wines if strongly present. Trans-2-esenale is a volatile chemical substance that forms during the maturation or aging process of wine. The presence and intensity of the olfactory defect caused by trans-2-esenale depend on several factors, including under-ripe grapes, inadequate destemming, or excessive pressing during harvest. To mitigate or prevent this defect, winemakers adopt various strategies, including avoiding prolonged contact between the must and stems during vinification. Other strategies include the use of specific yeast strains, careful fermentation temperature management, the choice of aging practices, and the use of filtration or clarification techniques. Proper management of trans-2-esenale in wine is crucial to preserve its aromatic quality and ensure a better sensory experience. The goal is to obtain a balanced and harmonious wine in which the characteristic aromas of the grape variety and winemaking stages are expressed to the fullest, without the interference of unwanted vegetal notes.


In conclusion, olfactory defects due to oxidation, reduction, sulfur dioxide, fungi or bacteria, and incorrect winemaking practices represent a problem that can be found in wine, negatively affecting its aroma and compromising the tasting experience. Paying attention to proper winemaking practices and appropriate storage conditions can help prevent or reduce the occurrence of these defects, allowing the wine to fully express its desirable aromatic qualities.

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