​Oxidation-Type Faults

  • Oxidation
  • Acetaldehyde
  • Volatile acidity (VA)
  • Ethyl acetate
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Oxidation

Aroma Characteristics
  • Loss of volatile aroma compounds.
  • The development of cardboard, straw, and hay aromas. 
  • Extreme oxidation can cause aromas of wet wool, wet dog, or varnish.  
  • Varietal Attributes 
    • White wines from the ‘floral’ varieties, especially Riesling, are prone to oxidation. 
    • Red wines can withstand significant oxidation due to a higher content of phenolic compounds, as they are natural antioxidants.
    • For some wine styles like sherry, oxidation is desired.

Production Mechanism
  • The development of multiple compounds including a range of aldehydes.
  • The volatilization of other desired compounds.

Prevention and Mitigation
  • Refrigeration
  • Inert gas blanketing during the production and packaging operations.
  • Effective sulfur dioxide management.


Acetaldehyde

Aroma Characteristics
Over-ripe bruised apples, sherry, and nutty.

Levels
  • The sensory threshold: 100-125 mg/L
  • Table wines generally have acetaldehyde levels below 75 mg/L

Production Mechanism
The oxidation of ethanol which develops when:
  • Yeast are in oxidative conditions.  This can also produce high levels of acetic acid and ethyl acetate. 
  • Acetaldehyde is an intermediate in the bacterial formation of acetic acid.  
  • During aging, ingress of oxygen into the bottle causes the oxidation of ethanol into acetaldehyde.  
  • Winemaking practices, like the addition of SO2 during fermentation, influence the concentration of acetaldehyde. 


Volatile acidity (VA)

Aroma Characteristics
  • Vinegar like.  Acetic acid (vinegar) is the primary component of VA.

Levels
  • Aroma threshold: 0.1 – 0.125 g/L, depending on the style of wine, the individual, and the presence of ethyl acetate.
  • Typically Detrimental > 0.7 g/L 
  • The maximum legal content 
    • Australian wines, excluding SO2 and expressed as acetic acid: 1.5 g/L.
    • United States: Red Table Wine 1.2 g/L and White Table Wine 1.1 g/L
    • Though a part of some wine regulations, volatile acidity in wine represents no threat to health.

Production Mechanism
The AWRI notes: “Volatile acidity is a measure of the low molecular weight, or steam-distillable, fatty acids in wine.”  Acetic acid constitutes >93% of this metric with the remainder being carbonic acid (from carbon dioxide), sulfurous acid (from SO2), sorbic acid (if added to wine), lactic, formic, butyric, and propionic acids. 

Acetic acid production
  • Yeast during alcoholic fermentation. The quantity produced is dependent on: 
    • Yeast strain or yeast species. Native or wild yeasts of Hansenula, Kloeckera, and Metschnikowia can produce high concentrations of acetic acid before and during the early stages of fermentation, but this typically occurs only with damaged grapes.
    • Yeast stress caused by excessively low or high fermentation temperatures, high sugar musts, nitrogen deficient must, or excessively acidic conditions (pH <3.2). 
    • Brettanomyces can produce elevated levels of VA when grown under aerobic conditions. 

  • Vitamin addition like that of nicotinic acid and thiamine to white grape juice can increase wine’s acetic acid concentration. 

  • Botrytis mold (noble rot) can break open grape skins causing infection by high acetic acid-producing microbes, therefore dessert wines often have higher levels of acetic acid (Neeley, 2004).

  • Heterolactic lactic acid bacteria (LAB) can also produce elevated amounts of acetic acid when growing in the presence of glucose. 

  • Hydrolysis of acetyl groups in the hemicellulose of new oak barrels can cause small increases in acetic acid. 

  • In bottled wines, acetic acid typically only increases if the wines are exposed to air. 

Prevention and Mitigation
 (Neeley, 2004)
Limit the growth of acetic acid bacteria by:
  • Eliminating air, oxygen in particular, from wine containers.
  • Sulfur dioxide addition.  
  • Rejection of moldy grapes. 
  • Inoculation with a low-V.A. producing strain of Saccharomyces.
  • Removal with reverse osmosis. 
  • Dilution by blending a high V.A. wine with a low V.A. wine after the wine has been filtered to remove microbes.

Ethyl acetate

Aroma Characteristics:
  • Low levels: Fruity aroma which can add complexity to the wine. 
  • High levels: Nail polish remover.  
  • Ethyl acetate can also contribute to the sensory perception of volatile acidity. 

Levels
  • Sensory threshold: 12 mg/L
  • Normal range: 30 – 60 mg/L 
  • Defective range: 150 – 200 mg/L 

Production Mechanism
Ethyl acetate is the primary ester produced by yeast. Factors that can influence ethyl acetate concentration include:
  • Yeast strain species and strain. Native or wild yeasts of Hansenula and Kloeckera can produce high concentrations of ethyl acetate before and during the early stages of fermentation. 
  • The temperature of fermentation, the amino nitrogen content of juice, and sulfur dioxide levels. 
  • Acetic acid bacteria growth under low oxygen conditions can lead to higher levels of ethyl acetate.

Prevention and Mitigation
SO2 can be used to reduce the levels of non-Saccharomyces strains. ​

Volatile Sulfur Compounds (VSCs)  [1]

Some volatile sulfur compounds can contribute positive “fruity” characteristics. Thiols in Sauvignon Blanc for example.  However, others contribute undesirable “reductive” aromas of rotten egg, natural gas, or onion.  They can be managed during fermentation by oxygen addition however their presence can be problematic post packaging.  The most common volatile sulfur compound faults are:

Hydrogen Sulfide and Unwanted Sulfhydryls

  • Hydrogen sulfide (H2S)
  • Unwanted sulfhydryls: ​Methanethiol (methyl mercaptan)
  • Unwanted sulfhydryls: ​Ethanethiol (ethyl mercaptan)
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Hydrogen sulfide (H2S) 
Hydrogen sulfide is the primary “reductive” aroma.  Additionally, other reductive aromas are derived from hydrogen sulfide.

Aroma Characteristics:
Rotten egg

Levels
Detection threshold in wine: 1 – 2 µg/L (parts per billion)

Production Mechanism
The various forms of sulfur like sulfate, sulfite, and sulfur-containing amino acids are responsible for yeast biosynthesis. Excess Hydrogen sulfide can be made by yeast during fermentation due to:
  • The presence of elemental sulfur on grape skins (from sulfur sprays).
  • Inadequate levels of free α-amino nitrogen (FAN). 
  • Added SO2.
  • A deficiency of B-complex vitamins (pantothenic acid or pyridoxine), 
  • Unusually high levels of cysteine in the juice.
  • A high concentration of metal ions. 
  • The production of H2S can also be yeast strain-dependent.
  • Stress caused by  nitrogen deficiencies and stuck fermentations.

Prevention and Mitigation
SO2 can convert H2S into Sulfur + H2O2. 
Copper sulfate (CuSO4) fining.  
  • This creates copper sulfide (CuS) which precipitates out and H2SO4. 
  • Laboratory analysis should be performed to prevent copper instability and to stay legally compliant as some countries have a maximum allowable limit,  For example, the United States allows <0.5 mg/L.  For more insight into copper instability: hawaiibevguide.com/wine-stabilization
  • In white wine it is ideal to remove high-density solids that might contain elemental sulfur before they can react with other wine compounds.  This can be done by settling, centrifuging, or filtering the must before fermentation.
  • In red wine aerating at the first racking can volatilize the H2S. This works by oxidizing H2S to elemental sulfur (S). However, the sulfur that is precipitated must be removed by centrifugation or filtration to prevent its conversion back into H2S. 
Unwanted sulfhydryls: Methanethiol/Methyl mercaptan (MeSH)

Sulfhydryls are compounds that contain a -SH group and are also known as ‘thiols’ or ‘mercaptans

Aroma Characteristics:
  • Methanethiol (methyl mercaptan, MeSH): Rotten egg and cabbage 
  • Their presence in wine above the threshold is generally viewed as a defect. However, the odor of these sulfur compounds is vital to many foods.

Levels
  • Methanethiol sensory threshold: 0.02 – 2.0 µg/L.​​

Production Mechanism
  • Methanethiol and ethanethiol are directly formed by yeast metabolism.

Post-bottling 
  • Methyl thioacetate can form Methanethiol 
  • H2S may react with
    • Ethanol to form Ethanethiol 
    • Methanol to form Methanethiol 

Prevention and Mitigation
Copper fining.  Aeration of the wine should not be used as it can oxidize sulfhydryls (mercaptans) into disulfides. These disulfides do not react with copper and therefore cannot be removed by copper fining. 

Unwanted sulfhydryls:  Ethanethiol/ethyl mercaptan (EtSH)

Sulfhydryls are compounds that contain a -SH group and are also known as ‘thiols’ or ‘mercaptans

Aroma Characteristics:
  • Ethanethiol (ethyl mercaptan, EtSH): onion and rubber
  • Their presence in wine above the threshold is generally viewed as a defect. However, the odor of these sulfur compounds is vital to many foods.

Levels
  • Ethanethiol sensory threshold: 1.1 µg/L.

Production Mechanism
Methanethiol and ethanethiol are directly formed by yeast metabolism.

Post-bottling 
  • ​Ethyl thioacetate can form Ethanethiol 
  • H2S may react with
  • Ethanol to form Ethanethiol 
  • Methanol to form Methanethiol 

Prevention and Mitigation
Copper fining.  Aeration of the wine should not be used as it can oxidize sulfhydryls (mercaptans) into disulfides. These disulfides do not react with copper and therefore cannot be removed by copper fining. 

Disulfides

Disulfides can have positive or negative aromas depending on their concentration
  • Dimethyl sulfide (DMS)
  • Dimethyldisulfide (DMDS)
  • Diethyldisulfide (DEDS)
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Dimethyl sulfide (DMS)
Aroma Characteristics:
  • ​Low concentrations: Has aromas of vegetable, truffle, or blackcurrant and may positively contribute to the body of aged white wines. 
  • Higher concentrations: Has aromas of asparagus, cooked corn, cooked tomato, or molasses and perceived as a fault.

Levels
  • Sensory threshold: 30 and 60 µg/L

Production Mechanism
  • DMS: The breakdown effect of a sulfur-containing amino acid called S-methyl methionine during bottle aging (AWRI-Removal of volatile sulfur compounds).

Prevention and Mitigation
Disulfides do not react with copper and therefore cannot be removed by copper fining.
Ascorbic acid and SO2 can convert compounds to methanethiol and ethanethiol (by reduction). Once converted they can be removed by copper fining.
Dimethyldisulfide (DMDS)

Aroma Characteristics:
  • Dimethyldisulfide (DMDS): Onions and cooked cabbage

Levels
  • DMDS sensory threshold: 29 µg/L. 

Production Mechanism
  • ​DMDS and DEDS: Sulfhydryls such as EtSH and MeSH can be rapidly oxidized to produce symmetrical or asymmetrical disulfides.

Prevention and Mitigation
Disulfides do not react with copper and therefore cannot be removed by copper fining.
Ascorbic acid and SO2 can convert compounds to methanethiol and ethanethiol (by reduction). Once converted they can be removed by copper fining.
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​Diethyldisulfide (DEDS)

Aroma Characteristics:
  • Burnt rubber and garlic

Levels
  • DEDS sensory threshold: 4.3 µg/L.

Production Mechanism
  • ​DMDS and DEDS: Sulfhydryls such as EtSH and MeSH can be rapidly oxidized to produce symmetrical or asymmetrical disulfides.

Prevention and Mitigation
Disulfides do not react with copper and therefore cannot be removed by copper fining.
Ascorbic acid and SO2 can convert compounds to methanethiol and ethanethiol (by reduction). Once converted they can be removed by copper fining.
​​​

Brettanomyces/Dekkera Faults

​The yeast brettanomyces can cause unwanted volatile phenolic compounds in wine.  
  • ​4-ethylphenol (4-EP)
  • 4-ethylguaiacol (4-EG)
  • 4-ethylcatechol (4-EC)​
  • Brettanomyces Prevention and Mitigation
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​4-ethylphenol (4-EP)

Aroma Characteristics:
 Band aid®, medicinal or pharmaceutical 

Levels
  • Excessive: 425 µg/L of 4-ethylphenol
  • Aroma threshold in neutral red Australian wine: 368 µg/L
  • 4-EP aroma can be masked or accentuated by other aroma compounds like those contributed by oak.

Production Mechanism
Produced by Dekkera/Brettanomyces in which:
  • p-Coumaric acid, the ester of tartaric acid, can be freed by pectin esterase via postharvest hydrolysis. 
  • p-Coumaric acid is decarboxylated to vinylphenol by the cinnamate decarboxylase enzymes.  These enzymes are found in various yeast strains and are particularly prevalent in Dekkera/Brettanomyces.
  • The 4-vinylphenol is converted to 4-ethylphenol by the enzyme vinyl-phenol reductase, which is present in Dekkera/Brettanomyces but is absent from Saccharomyces.
  • Bacteria, while capable of volatile phenol production under some conditions, produce very small reactions compared to Dekkera/Brettanomyces. For this reason, 4-ethylphenol is a recognized marker compound for the presence of Brettanomyces.
4-ethylguaiacol (4-EG)

Aroma Characteristics: Clove, spicy or smoky 

Levels
  • The aroma threshold (in a neutral Australian red wine): 158 µg/L 
  • 4-EG is typically found in lower quantities than 4-EP in red wines and often 10 times less for Cabernet Sauvignon wines.

Production Mechanism 
Derived in the same fashion as 4-ethylphenol but from a different precursor: ferulic acid.
4-ethylcatechol (4-EC)

Aroma Characteristics:  Horsey 

Levels
The aroma threshold (in a neutral Australian red wine): 774 µg/L 
Concentrations of 4-EC are similar to 4-ethylguaiacol in Pinot and Cabernet wines.

Production Mechanism
4-Ethylcatechol is derived in the same fashion as 4-ethylphenol but from a different precursor (caffeic acid). 
Brettanomyces Prevention and Mitigation
Prevention of development, prevention strategies include: Strict hygiene regimen, monitoring of nutrients, monitoring residual sugars during and at the end of fermentation, temperature control, use of sulfur dioxide, and avoiding the use of old oak barrels or any potentially contaminated equipment.
For more on Brettanomyces: 
UC Davis. (2018, March 20). Dekkera bruxellensis. Department of Viticulture and Enology. Retrieved February 25, 2022, from https://wineserver.ucdavis.edu/industry-info/enology/wine-microbiology/yeast-mold/dekkera-bruxellensis

Mousiness​

  • What They Are
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Aroma Characteristics:
  • Reminiscent of caged mice or sometimes cracker biscuits.
  • A mousy taint is rarely detected by sniffing because the compounds involved are not volatile at wine pH.  Rather the taint is perceived late on the palate and can take a few seconds to build. 

Levels
  • ACTPY range in wine (µg/L): 4.8 – 106
  • ETPY range in wine (µg/L): 2.7 – 18.7
  • ETPY range in wine (µg/L): Trace – 7.8

Production Mechanism
The compounds responsible for ‘mousiness’ are the N-heterocyclic volatile bases:
  • 2-acetyltetrahydropyridine (ACTPY) is the main compound.
  • 2-ethyltetrahydropyridine (ETPY) 
  • 2-acetylpyrroline (ACPY).

Typically produced by microbes including 
Most strains of lactic acid bacteria.  The heterofermentative species of Oenococcus oeni, Lactobacillus hilgardii, Lactobacillus plantarum, and Lactobacillus brevis, in particular, can produce mousiness. 

Dekkera/Brettanomyces may also be capable of producing mousy compounds.
More likely to occur in wines with low concentrations of SO2 and low acidity.

Exposure to air or oxygen may cause mousy taint. However, this is an empirical observation with no known mechanism. 

Prevention and Mitigation
  • Mousy off-flavor can only be mitigated by blending.
  • Sterile filtration can be used to prevent additional mousy character if the cause is microbial. 
  • Preventatively, the AWRI recommends 50 and 80 mg/L total sulfur dioxide (more for wines of high pH) for red wine production if there are doubts about acetic acid contamination.

For more insight into mousy off flavors: 
Australian Wine Research Institute. Avoid mousy off flavors.  (2015, February). Avoid mousy, off-flavours. The Australian Wine Research Institute. Retrieved July 25, 2022, from www.awri.com.au/wp-content/uploads/2018/04/s1694.pdf  

Cork Related Wine Taints [4]

Cork Taint is associated with the aroma compound classes of chloroanisoles and bromoanisoles.  However, the term cork taint can be misleading because corks can cause multiple taints, and the most common taint which provides a moldy, wet cardboard odor, is not exclusively caused by cork.

Below are the main cork-related taints as summarized in:
Cravero MC. Musty and Moldy Taint in Wines: A Review. Beverages. 2020; 6(2):41. 
https://doi.org/10.3390/beverages6020041 
  • 2,4,6-trichloroanisole (TCA)​
  • 2,4,6-Tribromoanisole (TBA)
  • 2-Methoxy-3,5-dimethyl pyrazine (MDMP/ fungal must) 8
  • Other taints produced in cork
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2,4,6-trichloroanisole (TCA)
The main compound responsible for cork taint is 2,4,6-trichloroanisole.

Aroma Characteristics:
  • Mold and wet cardboard
  • It is also found in other foods and beverages including
    • Coffee from Central and South America where it produces The "Rio Defect," described as medicinal, phenolic, or iodine-like.
    • In brewing beer it can create a damp cellar that can contaminate water and other raw materials. 

Levels
Odor detection threshold: 
  • In white wines: 3–7 ng/L 
  • In the red wines: 8–15 ng/L though higher the wine is aged in wood.
TCA has such a low sensory perception threshold it has been a subject of biomedical research. In work by Takeuchi et al (2013), its been found to function by the inhibition of ciliary transduction channels, which in turn causes the sensation of a mold and wet cardboard. For an in-depth look at how TCA produces odors: Takeuchi, H., Kato, H., & Kurahashi, T. (2013). 2, 4, 6-trichloroanisole is a potent suppressor of olfactory signal transduction. Proceedings of the National Academy of Sciences, 110(40), 16235-16240 https://doi.org/10.1073/pnas.1300764110   

Production Mechanism
Chlorophenols, a group of man-made chemicals, are a precursor to TCA.  These can arise in the cork by:
  • Contamination as they were formerly used as a biocide and have accumulated in the environment. 
  • By the chlorination of phenol, a natural component of cork. This occurs from aerial contamination and by sanitizers containing chlorine compounds including sodium hypochlorite (bleach).
  • TCA and related anisoles are formed by the methylation of 2,4,6-trichlorophenol (TCP) and related chlorophenols by fungi that grow on the cork trees. These fungi include Aspergillus, Armillaria mellea, Paecilomyces spp., Penicillium chrysogenum, Penicillium glabrum, and Trichoderma viride.  These molds can also form TCA in oak.

Prevention and Mitigation
  • Avoid usage of chlorine sanitizers like sodium hypochlorite (bleach).
  • Usage of synthetic stoppers or screw caps which can be easily sterilized. 
  • The encapsulation of the closure with a plastic or metallic foil (Echave et al 2021).
  • Regulation EU 2019/934 permits treatment using a filter plate containing zeolites silicate Y-faujasite for the adsorption of haloanisoles. It is subject to the conditions laid down in file 3.2.15 (2016) of the OIV Code of Oenological Practices.
  • Cravero also notes various experimental treatments to mitigate TCA including:
    • The usage of polyanilines and the possibility of using inexpensive polymeric materials as potential fining agents.
    • Using activated charcoal obtained from carbons of coconut. 
    • Aliphatic synthetic polymers (ultra-high molecular weight polyethylene) to lower TCA from food and beverage products.
    • Highly absorbent yeast cell extracts
    • The use of molecularly imprinted polymers and nonimprinted polymers like polyethylene to absorb chloroanisoles through direct liquid contact and via vapor. 
2,4,6-Tribromoanisole (TBA)

​Aroma Characteristics:
  • Musty and moldy, behaves like TCA, and produces a similar aroma in the wine.

Production Mechanism
Tribromophenol (TBP) is the precursor for TBA.  It is found
  • As a flame retardant and fungicide used for wood and wood products. It can be released from treated wood and will circulate in the air and can cling to any surface.
  • Synthesized by some algae.
  • As a component of bromine-containing detergents.
  • On winery surfaces, including barrels, plastics (including synthetic closures), natural corks, and wood structures, including walls, floors, and ceilings. 

Levels
Perception threshold: As low as 4 ng/L.
2-Methoxy-3,5-dimethyl pyrazine (MDMP/ fungal must) [8]
A pyrazine different from those found in Cabernet Sauvignon and Sauvignon Blanc.

Aroma Characteristics:
  • Herbaceous potato, unripe hazelnut, or dirt
  • In raw and roasted coffee it has an earthy aroma.

Levels
  • Aroma threshold in a neutral white wine: 2.1 ng/L

Production Mechanism
  • In cork, it is produced from amino acids by the proteobacterium Rhizobium excellensis. 
  • It can also be found in oak wood however it is destroyed during toasting.   
  • The decontamination techniques that are used for TCA can also be effective at reducing the incidence of MDMP.
  • Other pyrazines: 3-isobutyl-2-methoxypyrazine (IBMP), 3-sec-butyl-2-methoxyipirazine (SBMP), and 3-isopropyl-2-methoxypyrazine (IPMP) have undesired vegetable odors of green pepper, peas or potatoes. 
  • 1-Octen-3-One degradation to 1-Octen-3-Ol: Can also be caused by wood barrels or grapes with Botrytis cinerea. It gives wine an off-flavors of fresh mushrooms or metal in white wines like Sauvignon Blanc, Pinot Meunier, Pinot Gris.  It can be removed from wine by activated carbon filtration. 
  • Guaiacol and 4-Methylguaiacol can be caused by Streptomyces present on cork degrading vanillin to vanillic acid and then to guaiacol.  Other sources of this aroma compound can be found in the section on “Brettanomyces” and the section on “Smoke Taint”.
  • 2-Methylisoborneol (2-MIB) can be produced in cork by Actinomycetes bacteria. It has earthy, mushroom, and damp earth.  While its concentration decreases during wine storage, the final wine may still contain the off-odor. 

Smoke taint [9]

Smoke aromas, while desirable in Islay Scotch Whisky, are often undesirable in wine.  In recent years, wildfires in wine-growing regions like Australia and California have increased due to global climate change.  When this occurs, vineyards and grapes are exposed to smoke and inturn pass on an undesirable smoky characteristic to wines.  The AWRI has a great set of resources on smoke taint which we have summarized.
  • Production Mechanism and Influencing Factors
  • Aroma Characteristics
  • Prevention and Mitigation
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Production Mechanism
When lignin in wood is burnt, it releases free volatile phenols which cause the smoke character. These are primarily:  Guaiacol, 4-methylguaiacol, o-cresol, p-cresol, m-cresol. 

These aroma compounds can be absorbed by grapes by binding to their sugars.  This  produces glycosides that have no smoky aroma until they are released by hydrolysis during fermentation, aging, or 
by the enzyme glycosidases in salvia when being drunk. 

The key markers which are elevated when smoke exposure has occurred are: syringol gentiobioside, methylsyringol gentiobioside, phenol rutinoside, cresol rutinoside (includes o- m-, and p-cresol rutinosides), guaiacol rutinoside and methylguaiacol rutinoside. [10]

Factors that impact smoke taint  [11]
  • The grape growth stage may lead to varying impacts of smoke taint where:
    • Shots and flowering stages: Low
    • Berry growth from peasize to onset of veraison: Variable to high
    • Post-veraison leading up to harvest: High
  • However, the AWRI notes that data from Australian smoke events in 2019/20 showed a significant risk of perceptible smoke characters even prior to veraison.
  • Grape varieties may have different uptakes of smoke compounds, however there is no conclusive research.
  • Vineyard location, as proximity to areas with regular intentional or unintentional fires increases smoke exposure.  For example: 
    • Agricultural burns like stubble burning which is the process of burning the residual stalks of wheat, rice, rye, flax, corn, rice, oats and barley straws. [12]
    • Prescribed burns used to prevent wildfires. [13]
    • Places with regular, unintentional wildfires.


Levels
While many of the volatile phenols that cause smoke taint are also found in the constituents of new toasted oak barrels, it is the excessive levels which separate smoke taint in barrel aged wines, and their presence are an issue in non-barrel aged wines.  The quantity of smoke exposure has yet to be determined. However, it is thought that a low level of smoke exposure where visibility through smoke haze is >10-15 km or the %obscuration/m is <0.05 is unlikely to cause a perceptible smoke character in grapes or wine. 


Aroma Characteristics:
  • While smoke aroma is generally described as smoky, burnt, ashy or medicinal, the particular aroma impact depends on the specific volatile phenol. In particular:
  • Guaiacol: Smoky, medicinal 
  • Cresol (m/p/o isomers): Tar, medicinal, phenolic
  • 4-Methylguaiacol: Vanilla, clove, smoky
  • Syringol/4-Methylsyringol : Smoky, charcoal 


Prevention and Mitigation
  • Pre-fermentation practices to mitigate smoke aroma are primarily centered around minimizing skin contact.  These include:
    • Hand harvesting fruit to minimize breaking or rupturing of skins.
    • Excluding leaf material which can also impart smoke related characteristics
    • Avoiding skin-contact styles of winemaking
    • Keeping fruit cool as it was found that processing at 10°C had less extraction of smoke-related compounds than fruit processed at 25°C
    • Whole bunch pressing has been shown to reduce extraction of smoke-derived compounds particularly in white grapes.
  • Fermenting free run juice and press fractions separately. There is less extraction of phenolic contaminants from smoke in the first 400 L/t fractions, especially when combined with fruit cooling
  • Reverse osmosis or Nanofiltration.  For additional insight:
    www.awri.com.au/wp-content/uploads/2022/06/Treating-smoke-affected-wine-with-nanofiltration.pdf
  • Glycosidase enzymes, which hydrolyze (break apart) the sugar molecule from the volatile phenol, can be used early in winemaking so that the volatile phenol can be removed by activated carbon or nanofiltration. [14]
  • The phenolic glycosides are not hydrolyzed after activated carbon or nanotation can be applied.
  • Dilution by blending an unaffected wine with a smoke tainted wine to reduce the concentration of volatile phenols.  [15]

Additive Related Faults​

  • What they Are
  • Diacetyl (2,3-butane dione)
  • Geranium (ether 2-ethoxyhexa-3,5-diene)
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Sulfur dioxide (SO2)
Aroma Characteristics: 
In excess, it has a pungent penetrating aroma that reacts strongly with receptors in the nose causing sneezing and often a choking sensation

Levels 
No aroma impact: <15 mg/L free sulfur dioxide

Production Mechanism
Excess-free sulfur dioxide.  For more on this see 
www.hawaiibevguide.com/post-fermentation-process-stabilization 

Prevention and Mitigation
Staying within the guidelines of SO2 usage.
Diacetyl (2,3-butane dione)

Aroma Characteristics:
  • Low levels: Adds complexity. Aromas of butter or butterscotch characters as is typified by “buttery chardonnay”.
  • High levels, the aroma may be objectionable

Levels
  • Low levels: 1 – 4 mg/L 
  • High levels: >5 mg/L

Production Mechanism
  • Primarily produced by bacteria during malolactic fermentation through the catabolism of citric acid. Yeasts, while capable of producing diacetyl typically produce less than a 1 mg/L sensory threshold.
  • Concentration is dependent on factors including the bacterial strain, the amount of oxygen in the wine, the citric acid concentration, and temperature. Higher concentrations favor the oxidation of α-acetolactate to yield diacetyl.

Prevention and Mitigation
  • Not allowing or carefully monitoring malolactic fermentation.  

For more on malolactic fermentation: www.hawaiibevguide.com/post-fermentation-flavor-adjustments 

Geranium (ether 2-ethoxyhexa-3,5-diene)

Aroma Characteristics:
  • Crushed geranium leaves

Levels
  • Odor detection threshold: <1 ng/L, which indicates this compound is extremely potent

Production Mechanism
  • Sorbic acid can be used before bottling as an antifungal agent in sweet wines. 
  • The lactic acid bacteria, Oenococcus oeni, metabolizes Sorbic acid to sorbic alcohol (2,4-hexadien-1-ol).  Sorbic alcohol is then rearranged under acidic conditions to 3,5-hexadiene-2-ol.  This, in turn, reacts with ethanol to form ether 2-ethoxyhexa-3,5-diene.

Prevention and Mitigation
  • Not using sorbic acid. 
  • Not allowing the wine to undergo malolactic fermentation. 
  • Given its low aroma threshold, it is practically impossible to remove.

Other Wine Faults

  • Chlorophenol taints
  • Eucalyptol (1,8 Cineole)
  • Indole
  • Geosmin
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Chlorophenol taints are generally produced from the chemical chlorination of phenol.  For example when Hypochlorite solutions (bleach), react with phenols present in plastic, fibreglass, paints and fittings or are used to treat wood like in the production of paper. The main compounds are 2,4-dichlorophenol and 2,6-Dichlorophenol  (2,6-DCP) and they have aromas of plastic, paint, medicinal, and phenolic.
Eucalyptol (1,8 Cineole) is attributed to vineyards planted near large groves of eucalyptus which are thought to release significant amounts of the spicy, mint-like, fresh, cool and medicinal aroma compound into the air.  There it binds with the skins, and for this reason is reportedly more prevalent in red wine.  Eucalyptol can also be produced by the grapes.  
Indole: Implicated in the phenomenon known as untypical (UTA) or atypical (ATA) it has an aroma of chemical, plastic, mothballs, styrene and rubber.  It is observed to  become ‘stuck’ during primary fermentation, therefore yeast stress may be a factor.

Geosmin (trans-1,10-dimethyl-trans-9-decalol): Caused by the mold Penicillium expansum on grapes that develop on grapes pre-contaminated with Botrytis cinerea, it has fungal, moldy or earthy odors.  Grape seed oil can reduce the quantity of geosmin in wine, however, this application can also decrease volatile aroma compounds like esters.

Resources and Suggested Reading

For a video on Wine Faults watch
https://wineserver.ucdavis.edu/multimedia/ven290-microbiological-wine-faults 

For more on the laboratory sensory analysis of wine:
The International Organization of Vine and Wine (OIV) Review document on sensory analysis of wine www.oiv.int/public/medias/3307/review-on-sensory-analysis-of-wine.pdf

  1. The Australian Wine Research Institute. Removal of Volatile sulfur Compounds. (2022, March 27). Retrieved July 20, 2022, from www.awri.com.au/industry_support/winemaking_resources/storage-and-packaging/pre-packaging-preparation/removal-volatile-sulfur-compounds/

  2. Wine Flavours, Faults and Taints. The Australian Wine Research Institute. (2021, April 19). Retrieved July 20, 2022, from www.awri.com.au/industry_support/winemaking_resources/sensory_assessment/recognition-of-wine-faults-and-taints/wine_faults/

  3. Australian Wine Research Institute.  Avoid mousy off flavors. (2015, February). Avoid mousy, off-flavours. The Australian Wine Research Institute. Retrieved July 25, 2022, from www.awri.com.au/wp-content/uploads/2018/04/s1694.pdf

  4. Cravero, M. C. (2020, June 16). Musty and Moldy Taint In Wines: A Review. MDPI. Retrieved July 20, 2022, from 
    www.mdpi.com/2306-5710/6/2/41/htm 

  5. Takeuchi, H., Kato, H., & Kurahashi, T. (2013). 2, 4, 6-trichloroanisole is a potent suppressor of olfactory signal transduction. Proceedings of the National Academy of Sciences, 110(40), 16235-16240 https://doi.org/10.1073/pnas.1300764110
       
  6. CDC.  What are chlorophenols? what happens to chlorophenols in the ... ATSDR. (2022, June). Retrieved July 21, 2022, from www.atsdr.cdc.gov/toxfaqs/tfacts107.pdf

  7. Chatonnet, P., Fleury, A., & Boutou, S. (2010). Identification of a new source of contamination of Quercus sp. oak wood by 2, 4, 6-trichloroanisole and its impact on the contamination of barrel-aged wines. Journal of agricultural and food chemistry, 58(19), 10528-10538. Retrieved from: https://www.winebusiness.com/content/file/2010-Chatonnet-Identification%20of%20a%20New%20Source%20of%20Contamination%20of%20Quercus%20sp_%20Oak%20Wood%20by%20

  8. Chatonnet, P., Fleury, A., & Boutou, S. (2010). Origin and incidence of 2-methoxy-3, 5-dimethylpyrazine, a compound with a “fungal” and “corky” aroma found in cork stoppers and oak chips in contact with wines. Journal of agricultural and food chemistry, 58(23), 12481-12490. www.academia.edu/12147873/Origin_and_Incidence_of_2-Methoxy-3_5-dimethylpyrazine_a_Compound_with_a_Fungal_and_Corky_Aroma_Found_in_Cork_Stoppers_and_Oak_Chips_in_Contact_with_Wines

  9. The Australian Wine Research Institute. Smoke Taint. (2022, June 26). Retrieved July 20, 2022, from www.awri.com.au/industry_support/winemaking_resources/smoke-taint/

  10. The Australian Wine Research Institute. Sensory Impact of smoke Exposure. (2021, January). Retrieved July 21, 2022, from www.awri.com.au/wp-content/uploads/2020/02/Sensory-impact-of-smoke-exposure.pdf

  11. The Australian Wine Research Institute. Smoke Taint Entry Into Grapes and Vineyard Risk Factors. (2021, January). Retrieved July 21, 2022, from https://www.awri.com.au/wp-content/uploads/2012/04/smoke-taint-entry-into-grapes-and-vineyard-risk-factors.pdf

  12. The Australian Wine Research Institute. Stubble Burning – A Possible Source of Smoke Taint In Grapes. (2021, January). Retrieved July 21, 2022, from https://www.awri.com.au/wp-content/uploads/2018/05/Stubble-burning-fact-sheet.pdf 

  13. The Australian Wine Research Institute,  Minimising the Impact of Prescribed Burns On Wine-Grape Production. (2021, March). Retrieved July 21, 2022, from https://www.awri.com.au/wp-content/uploads/2018/06/controlled-burns-fact-sheet.pdf 

  14. The Australian Wine Research Institute.  Treating Smoke-Affected Wine with Glycosidase Enzymes. (2022, JunRetrieved July 21, 2022, from www.awri.com.au/wp-content/uploads/2022/06/Treating-smoke-affected-wine-with-glycosidases.pdf

  15. The Australian Wine Research Institute. Remediation of Smoke-Affected Wine By Dilution. (2021, February). Retrieved July 21, 2022, from www.awri.com.au/wp-content/uploads/2020/03/Dilution-for-smoke-remediation-fact-sheet.pdf 

  16. Lisanti, M. T., Gambuti, A., Genovese, A., Piombino, P., & Moio, L. (2014). Earthy off-flavour in wine: Evaluation of remedial treatments for geosmin contamination. Food chemistry, 154, 171-178.  Retrieved from: www.academia.edu/10398555/Earthy_off_flavour_in_wine_Evaluation_of_remedial_treatments_for_geosmin_contamination