Updated: Oct 1, 2020
When making wine from grapes, you are often presented with many twists and turns. If the wine veers off course too far and cannot be steered back, a wine flaw or wine fault can result. As a winemaker it is important to learn proper prevention and identification of wine problems because one thing is certain... you will encounter them at some point!
Wine Flaw vs Wine Fault
These are the terms for a small or large defect in the finished wine. When your Pinot Noir or Merlot is just a little off target but is still drinkable, this is a wine flaw. A flawed wine can be tasty but just not representative of the intended style. Some examples of this would be a dry wine that is a little sweet, a still wine that is has a little CO2, or an overpowering characteristic like too much oak, too buttery or too musty. Generally a wine flaw is considered a minor defect or deviation but when the defect becomes large enough to render the wine undrinkable, this is a wine fault. A flawed wine goes in your belly, while a faulted wine goes down the drain!
Flaws/Faults caused by Oxidation
What is acetaldehyde? This can be easily identified as the characteristic smell of sherry or port wine. In low concentrations, it can add aged character to wine, but this is a slippery slope that can quickly wreck your wine. Acetaldehyde is caused by the aerobic bacteria, acetobacter which feeds on ethanol. This bacteria is carried on fruit flies and will inevitably find its way onto the skins of your grapes and into your wine... but fear not! Acetaldehyde can be prevented relatively easily.
Acetaldehyde generally rears its ugly head after primary fermentation is complete and the wine is no longer protected by a blanket of CO2. As the wine enters the aging period, it becomes very important to limit the air contact of the wine. This is easiest achieved by keeping tanks and carboys full to the top with as little headspace as reasonably possible. The next line of defense is Sulfur Dioxide or SO2. As oxygen dissolves into the wine it will bind with unbound, free SO2 until there is no free SO2 remaining in the wine. In addition to scavenging oxygen, SO2 is also reasonably good at suppressing microbes and rendering them dormant. With adequate SO2 levels, and topped up wine tanks, you should not have any issues with acetaldhyde.
Volatile Acidity (VA)
VA is most commonly seen in the form of Acetic Acid (Vinegar). Wine naturally wants to become vinegar and it would do so if we didn't do anything to prevent it. VA is the second stage of oxidation by the bacteria acetobacter. In extremely low concentrations, VA can smell fruity and inviting. It can quickly evolve to unquestionable vinegar smell though. A VA flawed wine will often smell just like a jar of pickles or olives.
Prevention of VA is mostly the same as prevention of acetaldehyde and is all about SO2 management and oxygen management. In rare occasions a stressed yeast can create VA but rarely in high enough concentrations to cause the distinct vinegar aroma. Check the data sheet on your specific yeast strain for it's ability to create VA.
This is one of the worst smelling wine defects, with the very distinct smell of nail polish remover or acetone. For Ethyl Acetate to show up, you first need to have VA present in the wine. Acetic Acid reacts with Ethanol to create Ethyl Acetate. A wine tainted with ethyl acetate would usually be considered a faulted wine.
To prevent ethyl acetate, follow good SO2 and oxygen management practices. If you can prevent VA, you will prevent ethyl acetate.
Flaws/Faults Caused by Reduction
Hydrogen Sulfide and Mercaptan
Why does your wine smell like rotten eggs? If yeast become stressed, they can go reductive and begin to create hydrogen sulfide, which smells like rotten eggs or swamp gas. If left untreated, hydrogen sulfide will react with the miniscule amounts of methanol in wine to create the even more pungent methyl mercaptan. Mercaptan smells like cabbage, burnt rubber, or old socks and is added to natural natural gas to intentionally make it smelly. Mercaptan in VERY low concentrations can be acceptable and is commonly found in sauvignon blanc which gives it a little more mouth feel and a little less crispness. In high concentrations, mercaptan will dominate the aroma of the wine and render the wine undrinkable.
To prevent mercaptan, it is important to keep hydrogen sulfide under control during primary fermentation and to keep your yeast happy. The keys to keeping the yeast happy are to stay within the recommended fermentation temperature range for the yeast chosen and to provide adequate nitrogen and air. Nitrogen can be supplemented with a good yeast nutrient that contains diammonium phosphate other foods for the yeast.
Other Wine Flaws and Faults
A buttery wine is the result of malolactic fermentation, where malic acid is converted to lactic acid by malolactic bacteria. In the right wine, this is great, but in a crisp white wine, the buttery aroma can flatten intended sharpness of the wine. The buttery smell is caused by diacetyl, a biproduct of malolactic fermentation. Diacetyl intensity can be controlled intentionally stressing the malolactic fermentation or expediting it. A stressed malolactic fermentation caused by colder temperatures or a little sulfite will produce more diacetyl. A fast malolactic fermentation caused by warmer temperature and efficient bacteria like CH16 or CH35 will create less diacetyl.
In many white wines, malolactic fermentation is prevented all together by SO2 and sterile filtration to remove the bacteria. Diacetyl will rarely cause a wine to be faulted or undrinkable, but it can cause a flaw and an extreme deviation from the intended style. If you are making wine from apples, sulfite quickly after primary fermentation to prevent malolactic fermentation or your wine will taste more like sour milk than apples.
This spoilage yeast can create an unwanted funk in your wine. While a little brett can make a wine "interesting", a lot of brett can be troublesome. The expression of brett is often often as smells of barnyard, cured meat, band aid, leather, wet dog, or horse blanket. You can see how some of these are interesting, while other expressions are not so good.
A small sulfite addition before fermentation can help to stave off any wild brettanomyces that may be hanging out on your grapes. Innoculating with a competitive yeast strain will help assure that any brett in the must will be out competed and eventually wiped out. If you do end up with a little brett in the wine, providing something to bind with can help suppress the aromatics. A reactive tannin blend can help to clean up a slightly funky wine and can be added during the aging period.
What is a corked wine? Cork taint is caused by 2,4,6 Trichloroanisole or TCA which can enter the winery through oak or cork. A corked wine can have muted fruit smells, and subtle smells of wet newspaper or mold.
To prevent TCA, buy cork and oak products from reputable suppliers and smell them before putting to use. Good clean oak and cork should never smell musty and if it does, you probably should not use it. Some suppliers will certify their products as TCA free, but this comes at a price. Use an inline carbon filter on water sources to eliminate chlorine from any cleaning water. TCA is unable to form if chlorine is not present, which is why you should never use bleach when sanitizing wine equipment.
Maderized or Cooked
If a wine becomes over heated, whether in a car or during the fermentation process, many of the fresh fruity aromas can transform into cooked type aromas. Some wines, like Kosher wine is intentionally heated but this is normally not something that you want to do if making a premium wine. To prevent this, just do not let the wine get warmer than about 90°F for any extended period.
Bottle Fermentation or Effervescence
If a wine goes into bottle with any residual sugar, and steps have not been taken to eliminate the yeast or sorbate the yeast then it is likely that the wine will try to ferment again in the bottle. This can blow the corks right off of the bottles or just result in a bit of effervescence depending on the amount of sugar. A dry wine should read between 0.994 to 0.997 on the hydrometer, and anything higher is likely to have a little sugar present. A residual sugar test can be performed, or a test strip can give you a good idea of the residual sugar content if you are unsure.
Too Much of a Good Thing
If any one component of the wine is overpowering, this can be a wine flaw. When making any additions like oak, acid, sugar or tannin just be careful not to over do it. You can always add more, but it is much more difficult to remove components of a wine. If a wine is a little over-the-top in any one aspect, it can often be blended out with another wine with complimentary attributes.
With good winemaking techniques and careful monitoring, most wine flaws and faults can be prevented. If you have experienced any extra crazy wine flaws or faults, please post in the comments below. To learn more, stop by my YouTube Channel!
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