Troubleshooting Your Wine

The faults and flaws and their analysis and resolution presented below are from my latest book, Modern Home Winemaking, where you can find additional information should you wish to explore these further.

VOLATILE ACIDITY (VA)

DESCRIPTION

Volatile acidity (VA) in wine is due primarily to one acid, acetic acid, which imparts a distinctive vinegar-like smell. It is most often accompanied by another volatile substance, ethyl acetate, which confers a smell of nail polish remover and imparts a particularly hot sensation on the tongue.

All wines have a little VA; it adds organoleptic complexity. Wines produced from late-harvest or botrytized grapes will have higher levels as shriveled or (good) moldy grapes will produce VA within affected berries on the vines.

VA magnifies the taste of fixed acids and tannins but, itself, is masked by high levels of sugar and alcohol.

Acetic acid detection threshold is around 600–700 mg/L, depending on the type of wine and a taster’s sensitivity, and becomes defective as it approaches 1200 mg/L; wine is considered spoiled at 2000 mg/L. Ethyl acetate detection threshold is in the 150–200 mg/L range. The significance of these detection thresholds is that they are fairly high, meaning once detected, wine contains large amounts of acetic acid and ethyl acetate, and is perhaps beyond repair. Unfortunately, here too there are no simple analytical tools for home winemakers to monitor VA and ethyl acetate.

CAUSES

Acetic acid and VA are the result of ethanol oxidation by acetic acid bacteria, which thrive in the present of oxygen (air), hence why this fault is common in wines in poorly topped up carboys, tanks, and particularly barrels. Acetaldehyde is an intermediate of ethanol oxidation, and therefore, if you detect acetaldehyde, you likely have an imminent VA problem. Ethyl acetate is due to esterification of acetic acid. Esterification is a chemical reaction between alcohol and an acid — ethanol and acetic acid here — to form an ester, a volatile substance.

VA is a common problem in poorly managed wines aging in barrels. Due to evaporative losses in barrels, a headspace is created above the wine volume. Although the headspace is a vacuum, it is not a perfect vacuum and still has some oxygen, making it a good breeding ground for acetic acid bacteria. VA in barreled wines will creep up a little, this is normal, but it should never be excessive or detectable by smell — never.

VA is also a small by-product of AF, and therefore, expect VA to increase a little from juice to wine, but again, it should not be detectable by smell; if it does, then the wine was affected by acetic acid bacteria during AF. Yeast produces more VA during a stressful AF, for example, when the AF is conducted at very low or excessively high temperatures, or when nutrients have been depleted.

VA can also result from the degradation of citric acid by lactic acid bacteria during MLF, or from a spoilage reaction involving indigenous lactic acid bacteria. Small inconsequential amounts of citric acid occur naturally in grape juice, but because of the spoilage risks, never add citric acid to acidify wines that will go through MLF. If citric acid is metabolized into acetic acid, it will also produce excessive amounts of diacetyl, a substance that imparts a buttery aroma.

And if you manage your own vineyard, beware of fruit flies; they can cause a disease known as sour rot, which shrivels grape berries causing whole bunches to turn brown and ooze with a distinct vinegar smell due to acetic acid being formed.

ASSESSMENT

Home winemakers are not equipped to measure VA or ethyl acetate, and therefore, you need to rely on your ability to smell and detect these. If you do detect VA or ethyl acetate, you may want to send a sample to a certified lab for evaluation to give you a sense of how big a problem you have.

REMEDIAL ACTIONS

VA is impossible to remove except by reverse osmosis — it involves equipment outside the domain (and budget) of home winemaking.

If the wine is objectionable, discard it — there is no sense in keeping VA-affected wine in your winemaking area as it is a source of further contamination. And it is never a good idea to blend a faulty wine with a perfectly good wine.

If you detect acetaldehyde, it is a sign of imminent VA spoilage; sulfite immediately to bind the acetaldehyde, as per instructions described under premature oxidation.

If high levels of VA are detected in a barrel, discard the barrel too. Yes, it hurts, but it is very difficult to eradicate acetic acid bacteria from a barrel, and you will simply be infecting any wine you transfer into that barrel. If VA is only slightly detectable, there is hope in salvaging the barrel. Fill the barrel with a 300 mg/L SO₂ (about 500 mg/L KMS) solution and let soak for two days to extract VA out of the wood. Empty the barrel out and rinse abundantly with water, then smell the barrel. If there is any hint of VA, it is best to discard the barrel.

PREVENTIVE ACTIONS

To avoid VA infections and spoilage:

Reject damaged or rot-affected fruit.
Use a low-VA producing yeast; consult the yeast manufacturer’s specifications — they will usually include a qualitative descriptor of how much VA (e.g., low, medium, high) a yeast produces.
Avoid prolonged oxygen exposure, top up carboys, tanks and barrels.
Maintain adequate FSO2 levels based on pH, MSO2 and DO.
Do not store wine above 15 °C (60 °F), at least not for extended aging.

Fungi-derived chitosan is another option to prevent VA problems, particularly when working with high-pH wines. Chitosan works by reducing the population of acetic acid bacteria, which therefore also reduces the need for SO₂. Bactiless, BactiControl and EnartisStab Micro are examples of commercial chitosan products used for preventive microbial control.