A contribution from winemaking author Daniel Pambianchi
Copyright © Daniel Pambianchi 2022
Editor's Note: This is an advanced topic for amateur winemakers intended for those who are more ambitious and technically inclined. It is a companion piece to our two other articles on oxygen management topics in winemaking. Before reading this article, we recommend you start with Managing Oxygen During & After Wine Bottling, and An Experiment to Compare Wine Transfer Methods.
Amateur winemakers have become proficient at managing sulfur dioxide (SO2) in wine, from using analytical techniques to calculating free SO2 (FSO2) needs. But oxygen management is not yet part of winemakers’ quality-control protocol, amateurs and commercial operators alike, even though the spoilage impacts of molecular oxygen (O2) are well known.
Sure, winemakers understand how to protect wines from microbiological oxidation, that is, from excessive oxygen exposure that can enable aerobic acetic acid bacteria (AAB) into converting ethanol into the vinegar-smelling acetic acid – a fault known as volatile acidity, or VA. As spoilage progresses, acetic acid can in turn be esterified into ethyl acetate, a powerful ester that imparts a nail-polish-remover smell. Regular and timely topping up of carboys and barrels and maintaining proper SO2 levels will keep AAB in check.
But chemical oxidation – the interaction of dissolved oxygen with wine compounds – is less understood and therefore not managed, at least not quantitatively, as it should. Wines will show older than their actual age, exhibiting premature browning with possibly a hint of a bruised-apple smell due to acetaldehyde production. It happens all too commonly.
Let’s examine how dissolved oxygen (DO) interacts with wine compounds in chemical oxidation, and how sulfite works against oxidative damage. Then we’ll look at how to implement DO management strategies towards making age-worthy wines.
Red wines are rich in polyphenols, substances that belong to a very broad class of compounds that includes tannins and anthocyanins (pigment molecules that give red wines their color). Tannins and other polyphenols are colorless but which can turn into their brown-colored form, known as quinones, when they react with oxygen. The reaction is typically very slow, but naturally occurring iron and copper ions in wine act as catalysts and can greatly speed up such oxidative reactions, including the conversion of ethanol into acetaldehyde, and shave off a few years from a wine’s shelf life. Those brown-colored quinones and acetaldehyde can polymerize with anthocyanins and other polyphenols to form pigmented polymers that alter the attractive color of reds.
An interesting chemical property of polyphenols is their ability to regenerate themselves back from their quinones into their colorless form in the presence of sulfite. Errant quinones can be intercepted by sulfite and turned into colorless substances too. Sulfite is also able to convert oxygen radicals, namely hydrogen peroxide, resulting from ion-catalyzed oxidation of oxygen, into harmless sulfates. If there is no sulfite to intercept hydrogen peroxide, the latter can go on to oxidize ethanol into acetaldehyde. As winemakers know, the distracting smell of acetaldehyde can be muted with a sulfite addition although wine quality and aging potential have already been compromised.
And so, wine must be sulfited not only for protection against microbial spoilage but also against chemical oxidation. As an illustrative example, if two identical wines are bottled under the same closure but one having two-thirds as much FSO2, the shelf life of the latter can be cut by almost three-quarters.
For protection against microbial spoilage, the amount of sulfite to add is determined based on the pH of wine.
For protection against chemical oxidation, the incremental amount of sulfite to add is based on the amount of dissolved oxygen (DO). We know from SO2–O2 chemistry that theoretically they react in a 4:1 ratio. This means that for every 1 mg/L (ppm) of oxygen (written as 1 mg O2/L) measured in wine, FSO2 should be bumped up by 4 mg/L (ppm); however, in practice, the ratio is closer to 2.5:1 This may not seem like much, but consider that, if you are not careful in your winemaking, your wine can become fully saturated to 8 mg O2/L, which means that up to 20 mg/L of SO2 is spent just dealing with DO. If you sulfited according to pH to a level of, for example, only 20 to 30 mg/L (ppm), your wine will be quickly depleted of FSO2 and no longer protected from either microbial or chemical oxidation.
Measuring DO is relatively simple and inexpensive depending on chosen technology.
DO is measured using a metering device and a special probe equipped with an oxygen sensor using one of three technologies: galvanic (cheapest), polarographic and optical (most expensive). If you already own a Vinmetrica SC-200 or SC-300, you can purchase their galvanic-type DO probe, or you can buy an integrated DO meter.
Vinmetrica SC-200 and SC-300 can be upgraded to test for DO.
The instrument is first calibrated using an aqueous solution treated with sodium sulfite to eliminate all oxygen, and a second vial of water shaken vigorously for several minutes to saturate the sample completely with oxygen, or against air, which contains 21% oxygen. Once calibrated, DO is measured directly in the wine or calculated from your instrument’s measurements, for example, in millivolts (mV) if using a Vinmetrica unit.
Here’s an example of how to adjust your sulfite additions based on DO (and pH) measurement.
For a red wine with a pH of 3.45, the recommended FSO2 is 22.0 ppm at a molecular SO2 level of 0.5 ppom. If you measured 2.0 ppm O2, then you would increase FSO2 by 2.5 times 2.0 ppm for a total addition of 22.0 + 5.0, or 27.0 ppm FSO2. That is the “desired FSO2” value you would plug into your sulfite calculator to determine the amount of potassium metabisulfite (KMS) to add for your wine volume. A handy SO2 CALCULATOR can be found on Daniel Pambianchi’s website at https://techniquesinhomewinemaking.com/winemaking-tools/.
Monitoring DO can help you understand oxygen uptake during your processing, from techniques or equipment you use, and adjust your process accordingly. For example, if you measure DO before and after a racking or filtering operation and you notice a big jump in DO, then you would need to address the cause of excessive oxygen uptake. Perhaps you are splash-racking too vigorously.
It also allows you to identify potential oxidation problems before they start spoilage. If during a routine control you notice a large jump in DO, it may well point to a defective airlock (e.g. split seam) or a bad bung seal on a carboy or barrel that you may not have otherwise detected until spoilage had begun. And if you detect a problem early on, you may not notice elevated DO, but your FSO2 measurement may have dropped significantly more than expected. So always measure FSO2 and DO concurrently during your routine control.
And measuring DO as you uncork a bottle will help you understand how well your closure has performed. Closure performance can significantly alter your wine’s shelf. A closure having an oxygen transfer rate (OTR) double another closure will reduce shelf life in half. (OTR is the amount of oxygen a closure allows into the bottle over a certain period of time.) Double the OTR is actually very small; imagine the shelf life where OTR can vary by one or two orders of magnitude. OTR for closures can be significantly different, so make sure you protect your investment with the best, lowest-OTR closures possible.
Measuring DO should become as common as measuring FSO2. It can only help you make better and more age-worthy wines.
About author Daniel Pambianchi
Daniel Pambianchi is a passionate winemaker, and the author of “Modern Home Winemaking” , “Techniques in Home Winemaking” , and “Kit Winemaking” .
At The Beverage People, this article has been our go-to reference at The Beverage People for understanding and making additions of SO2 in wine or cider.