Faults and Flaws in Wine

A fault is worse than a flaw. The point or level at which a flaw becomes a fault and makes a wine undrinkable, varies with each wine drinker’s perception threshold of that odor or taste and their tolerance for it as good or bad.  Not all wine flaws are objectionable. Sometimes in low concentrations they can add complexity to wine.

Flaws and faults are not always caused by the winemaker. They can arise from situations beyond the winemaker’s control. At the current state of technology, we have not identified, nor can we measure, all of the chemicals associated with flaws and faults.

Cork floats and none of the alcohols precipitate. Tartaric acid crystals, anthrocyanin pigments and tannins may cause sediment but none are harmful. They just produce an unexpected mouth feel in the wine.

Cloudy wine is almost always a fault. This is different, however, from a red wine that is so opaque that you cannot see through it. If cloudiness  is due to any microbiological contamination, the wine will be faulty on aroma and/or taste.  Tannins and tartaric acid crystals drop to the bottom of a glass so do not contribute to a cloudy wine.

Continued fermentation (controlled amounts) in the bottle is a method used to give sparkling wines their bubbles.

Sulfites combine with oxygen in wine but it stays in solution. Yeast can produce aldehydes, but in the process they consume oxygen and do not release any carbon dioxide.

A deep golden color can sometimes alert you to the possibility that a white wine is oxidized, but it can also indicate a low acid wine or a very ripe wine of certain varietals. Therefore, it is not a fault on its own. If the color were brownish yellow or brown, however, that would definitely be a fault.

Many red wines can take on a garnet color with aging due to the micro oxygenation that has taken place over years. This does not necessarily mean the wine is oxidized. On its own, the garnet color is not a fault. If the color is more toward a brown hue, however, it would be a fault just on color alone.

Green pepper tastes are not infrequent in red wines from cool climate grapes that have difficulty ripening. East coast U.S. and West Coast South America (Chilean) wines sometimes have this problem. A small amount is not always a flaw. Some people find it adds complexity especially in a white wine like sauvignon blanc.

Cat’s urine smell in Sauvignon Blanc is not a methylpyrazine; it is a chemical thiol compound called p-mentha-8-thiol-3-one. Lactic acid bacteria are associated with the smell and taste of sauerkraut.

Merlot or Cabernet grapes from a cool climate are often associated with green pepper tastes, although rarely it can come from insects such as lady beetles included in the grapes at harvest. The best preventative measure is leaf removal around the grapes as soon as possible so they get maximum sun before they change color (veraison).

Lack of must nitrogen can result in reductive aromas of hydrogen sulfide (rotten egg smell) and its metabolites. Picking grapes long past their ripeness runs the risk of the fungus, botrytis cinerea (gray mold), causing a musty, earthy or rotten flavor.

The sensory threshold for acetaldehyde, which causes the oxidized smell, rotten apple/sherry-like aroma, is typically 100 to 125 mg/L (ppm).

A sour and tangy smell is from wine turned to vinegar (volatile acidity) by acetobacter bacteria while a bubble-gum smell is usually due to excess sorbate that was added to stop additional yeast fermentation in a wine with residual sugar.

Oxidation almost always occurs because the wine is exposed to too much oxygen such as barrels during aging that have too much air at the top or a bottle of wine that has been open and exposed to oxygen for several days.

Acetic acid (vinegar) is the most dominant component of volatile acidity. Acetic acid is perceptible at 0.8 g/l. It gives a definite vinegar flavor and smell above 1.5 g/l. The US limit is 1.1 g/l for whites and 1.2 g/l for reds.

While ethyl acetate is also a volatile compound, it results from an excess of acetic acid that is esterified with ethanol. It smells like nail polish, however, acetic acid is still the major component of any volatile acidity. Ethyl acetate is detectable above 0.5 mg/L. No limits have officially been set for this compound in wine.

Aldehydes can be volatile and have a strong smell like sherry, nuts or rotten apples, but they do not contribute to volatile ACIDITY. Aldehydes are from oxidation of ethanol. Acealdehyde, the most common aldehyde found in wine, is perceptible at 100-125 mg/L. Generally any acetaldehyde level below 100 mg/L is allowable.

Almost always, excess acetic acid in wine can be prevented by adding sulfites after fermentation is complete to inactivate any remaining acetobacter bacteria before aging and then making sure to add them again at time of  bottling (if the wine is not sterile filtered of bacteria and yeast).

In the presence of alcohol, acetic acid (vinegar) can get esterified, or converted, to ethyl acetate which imparts nail polish remover, glue, or varnish type aromas.

In low doses, these mixtures of Brettanomyces caused odors can be sensed to be earthy aromas and are desirable for some wine drinkers. In high doses, they are more pungent and become flaws and faults. Animal, sweaty and band aid smells come from 4-ethyl phenol (4 EP).  Smoke, spice, clove, and bacon come from 4-ethylguaiacol (4-EG). Barnyard, horsey, rancid and cheesy come from isovaleric acid. These chemicals will be present in various ratios if Brettanomyces is active during the fermentation or the aging process.

4-EP is the marker chemical for a Brett contamination since other microorganisms do not produce it.

A buttery smell is associated with excess diacetyl. Burnt matchstick is associated with excess sulfites added to the wine.

Corked wine containing TCA has a characteristic odor, variously described as resembling a moldy newspaper or cardboard, wet dog, damp cloth, or damp basement. In almost all cases of corked wine, the wine’s native fruit aromas are also significantly reduced or absent.

Band-aid/barnyard smell would be from Brettanomyces contamination and a sulfur odor would come from a lack of nutrients in the fermentation where active yeast must break down dead yeast cells to get their nitrogen, releasing sulfur in the process.

At least 10 different species of fungi associated with wine cork production have been identified which can produce 2,4,6-TCA primarily from the O methylation of 2,4,6-trichlorophenol (2,4,6-TCP) which is present is harvested cork: Acremonium, Chrysonilia, Cladosporium, Fusarium, Mortierella, Mucor, Paecilomyces, Penicillium, Trichoderma, and Verticillium. The fungi themselves probably grow on the cork during the drying and storing process before actual wine cork production.

Álvarez-Rodríguez ML, López-Ocaña L, López-Coronado JM, et al. Cork Taint of Wines: Role of the Filamentous Fungi Isolated from Cork in the Formation of 2,4,6-Trichloroanisole by O Methylation of 2,4,6-Trichlorophenol. Applied and Environmental Microbiology. 2002;68(12):5860-5869. 

The threshold for TCA is extremely low in the range of  1-250 ng/L or parts per trillion. Note that some individuals can have up to a 250 fold or higher threshold than others.

Some people, however, very rapidly accommodate to the cork taint smell and after sniffing it about 3-4 times, they can no longer discern it as a fault.

These faults suppress or overwhelm any fruit esters so that you cannot detect any basic fruit aromas.

The most commonly described odor is that of smelling a match immediately after lighting it and then quickly blowing it out. Sulfite smell is different than hydrogen sulfide gas which smells like rotten eggs.

If there is not enough nutrient (nitrogen) available to yeast for fermentation,  the yeast has to break down amino acids to get the nitrogen and sulfur is released at the same time. The sulfur smells are called “reductive” odors. These ARE NOT the same smells as the burnt match odor from excess sulfites added for preservative.

On occasion, however, sulfur smells can come from grapes that were recently sprayed right before harvest with sulfur or copper sulfate.

Sulfur released from yeast breaking down amino acids to get nitrogen, can undergo many further chemical changes. Initially is may just be a “rotten egg” smell but later chemical changes result in a myriad of “off smells” due to further reactions of hydrogen sulfide with wine compounds to form mercaptans and dimethyl sulfides. Mercaptans give onion, rubber, and skunk type odors; dimethyl suflides produce cooked cabbage, canned corn, asparagus or truffle odors.

The smells from volatile sulfur compounds suppress or overwhelm any original fruits smells in the wine.

Amounts of residual sugar over 1.5% are perceived as slightly sweet or sweet by all people. Some can detect sweetness above 0.5% (5 gm/L). The average threshold for sweetness is 1%.

Small amounts of residual sugar make a wine more savory and seem fuller bodied, rounder. It seems to counteract  or blunt any increased acidity or bitterness.

Potassium sorbate is added to wine as a bottling preservative if there is residual sugar in the wine. It keeps any yeast present from fermenting the residual sugar in the bottle which might cause effervescence or even an exploding bottle. If too much potassium sorbate is added, it gives a bubble gum taste. Sometimes wine kits for amateur winemakers have an excess of potassium sorbate added to the grape concentrate which can then be detected in the residual wine.

Geranium smells are produced if sulfites are not added to kill off malolactic bacteria at the same time potassium sorbate is added to stop yeast fermentation of residual sugar.

The rancid butter smell is usually caused by malic acid bacteria working on citric acid after all malic acid is exhausted. The cure is to measure malic acid left in the wine and when it approaches zero, sterile filter the wine to less than 0.5 microns or to arrest any malic acid bacteria with sulfites before bottling the wine. This fault is more common in Chardonnay wines that have had malolactic bacteria added during fermentation.

Wines exposed to ultraviolet light, such as that from fluorescent lights, can undergo a fault called light struck. Light struck wines have a cheesy, plastic smell due to dimethyl disulfide.

Colored glass does not prevent or retard oxidation changes, which are primarily dependent on excess oxygen. A buttery, diacetyl smell is caused by malic acid reacting with citric acid in the wine.

While a slight petrol smell can be normal in aged Rieslings, it may be considered a fault in young Rieslings and as a flaw or fault if the level is too high in aged Rieslings for the preferences of the person judging the wine.

The taste is due to bacterial conversion of benzyl alcohol to benzaldehyde, which tastes like bitter almonds. The precursor, benzyl alcohol, comes from grapes, from gelatin used as a fining agent and/or from cement tanks lined with epoxy resin.

Contamination of wine, such as with lactic acid bacteria, can produce “off odors” by oxidizing lysine and alcohol. Tasters describe these odors as “funky” or  “mousiness”. There is probably a range of “off odors” that vary by which microorganisms are present, and that we have not yet adequately defined.