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.
PREMATURE OXIDATION
DESCRIPTION
Oxidation occurs in every wine, ever so slowly during the winemaking process and aging, including bottle aging, but it should not occur prematurely or unexpectedly in young wines, either in bulk or bottled.
Premature oxidation is a condition of uncharacteristic early browning of wine and smell of acetaldehyde (bruised apple) and nutty aromas and flavors, which mute other desirable aromas..
CAUSES
Premature oxidation is the result of chemical oxidation of polyphenols and ethanol due to prolonged exposure to oxygen with very low or no sulfur dioxide (SO2) protection. This is usually a result of poor oxygen and SO2 management, most often due to leaving too much headspace in carboys or other vessels, or excessive processing in the absence of SO2 protection. In bottled wine, it can also result from the use of poor closures with high oxygen transfer rates (OTR).
Aging and storage temperatures are also important factors as warmer temperatures accelerate oxidation reactions.
This fault is often accompanied by volatile acidity (VA) and possibly by a whitish surface film.
ASSESSMENT
A wine affected by premature oxidation will have a very distinctive smell of bruised apple from acetaldehyde, a sure sign that free SO2 (FSO2) has been depleted. If you are able to measure FSO2, confirm to what extent free SO2 has been used up — in all likelihood, it is below the critical 10 mg/L threshold, or possibly completely depleted.
Acetaldehyde is easily detected given its very low detection threshold, in the 500 µg/L (that’s parts per billion, or ppb) range. Once detectable, wine quality is seriously compromised and remedial actions can make the wine drinkable, maybe, or it may have to be discarded depending on the severity. Unfortunately, there is no simple analytical tool for home winemakers to monitor acetaldehyde concentration; you will need to rely on your nose.
Whites affected by premature oxidation will exhibit a dark or brownish color.
In reds, you will need to assess the wine in a glass; tilt the glass and look for an orange, brownish hue at the rim. It can be very difficult, if not impossible, to fix an oxidation problem in a red wine; it is best left alone.
If a whitish film has formed on the surface of the wine, you have advanced oxidation and a surface yeast problem.
REMEDIAL ACTIONS
If you detect acetaldehyde, add potassium metabisulfite (KMS) immediately to bind the acetaldehyde to make it non-volatile. Add KMS to the recommended FSO2 level based on pH. Then taste the wine again, and if you still smell the acetaldehyde, try adding 50% of the amount of FSO2 just added, then repeat another time if the smell persists. For example, if you first added 50 mg/L FSO2, try adding another 25 mg/L and another 25 mg/L. At this point, you would have added twice the recommended FSO2 level; if you can still smell the acetaldehyde, your wine is likely beyond fixing and is best discarded.
For whites affected by limited oxidation and showing a darkish yellow color with light-brown hues, first treat with casein at a rate based on bench trials, then treat with bentonite to assist in settling the casein. If your bench trials conclude that casein strips out too much aromas, try using PVPP instead. You can also try specially formulated casein–bentonite preparations, such as Bentolact S.
Following a successful treatment to an acceptable color without excessive loss of aromas and flavors, add specific inactivated yeast with high antioxidant properties containing polysaccharides and/or glutathione (GSH), such as OptiMUM White or Glutastar, to protect against further oxidation.
If a white wine is affected by advanced oxidation and has a dark-brown color, it is likely not salvageable. As an absolute last resort only and when other treatments have not been effective, you can try removing some of the brown color using activated carbon. Perform bench trials first to make sure the treatment produces acceptable results. Add the activated carbon powder directly to the wine and stir thoroughly; never add more than the maximum because it will strip color excessively and leave an off carbon-like flavor. Add bentonite at the maximum rate of 1 g/L immediately after the activated carbon treatment, rack after a few days, and filter the wine before bottling.
If you have managed to cure the wine, be sure to top up whichever carboy or vessel it was in.
PREVENTIVE ACTIONS
To prevent premature oxidation, avoid prolonged oxygen exposure during processing and particularly during storage and aging. Top up carboys and barrels with the same or similar wine making sure to top up barrels regularly — at least once a month — due to evaporative losses. Although you can use inert gas or create a vacuum to protect wine in partial carboys or tanks — but not in barrels — this is only a short-term strategy; it’s always best to top up.
Maintain adequate FSO2 levels based on pH and molecular SO2 (MSO2) for the type of wine. Never allow FSO2 to fall below 10 mg/L.
For whites and rosés, minimize the amount of phenols extracted during processing and perform a preventive addition of PVPP when fining, and store at cool temperature, ideally around 13 °C (55 °F).
Invest in a dissolved oxygen (DO) meter and make DO monitoring a regular task of your quality control protocol. By monitoring DO (at the same time as pH and FSO2), you will be able to detect a potential problem before acetaldehyde develops to detection threshold levels. You may have a defective bung on a carboy but you may never know that without knowledge of both FSO2 and DO measurements.
For wines that may be more fragile and prone to oxidation, or if you want to reduce the use of sulfite, consider other antioxidants and techniques, such as the use of tannins, glutathione, specific inactivated yeast (SIY), mannoproteins or aging on the lees. There is a vast selection of products specifically designed as preventive against oxidation in white and rosé wines.