SO2 and Those Pesky Reductones

For years it has been said that the Ripper method for measuring sulfite (SO2) has one drawback: interference from “reductones”. These are substances that react with the iodine titrant as SO2 itself does, causing an overestimation of the true SO2 level. But as we’ll see below, this does not seem to be a huge problem with the Vinmetrica version of the Ripper method, and where encountered, it can usually be corrected for.
The Ripper method uses iodine or iodine-generating reagents to oxidize the SO2 in the test sample (primarily wine, but beer and cider are also examples). It relies on the quantitative reaction of iodine (I2) oxidizing the SO2 in the sample under acid conditions.

In this reaction, SO2 is acting as a “reductant” for the oxidant iodine. When all the SO2 is oxidized by iodine at the endpoint, excess iodine appears in solution. The Vinmetrica system detects this event as a sharp rise in electrochemical current with its SO2 electrode.
Anything in the sample that reacts like a reductant – called a “reductone” – in a Ripper titration will appear to be SO2, leading to an overestimation of the SO2 level. A well-known example of a reductone is ascorbic acid. Ascorbic acid (“AA”, also known as ascorbate or vitamin C) reacts with iodine just as SO2 does, that is

Ascorbic acid can be found in modest amounts in some vinifera varietals, and it can occur in high levels in a number of fruits, like citrus and some berries. When trying to measure SO2 levels in these samples, one must be careful not to get misled by ascorbate parading as SO2!
There are other things present in wines and juices that can act as reductones. In principle, reducing sugars (glucose, fructose) and various polyphenols could act as reductones.

How to correct for reductones:

The standard method for correction of reductones in the Ripper method is to run the SO2 titration twice, once in the normal way, then repeating after adding a scavenger that selectively removes the SO2. The first titration determines SO2 + reductones, while the second determines reductones alone. The difference between these values is the true SO2 level.
Some examples of SO2 scavengers are glyoxal, formaldehyde, and hydrogen peroxide.
Glyoxal and formaldehyde both scavenge SO2 by forming an addition product with its bisulfite form (HSO3-) that can’t react with the iodine. This is the same reaction that occurs when sulfite is bound to aldehydes found in wine.

Most people don’t like to use formaldehyde because it’s foul smelling and toxic. Glyoxal is relatively odorless, quite safe to use and available as a 40% solution (https://www.sigmaaldrich.com/catalog/product/sial/128465?lang=en&region=US)
Hydrogen peroxide (H2O2), readily available at your local drug store as a 3% solution, also selectively removes SO2, but does so by oxidation of bisulfite to bisulfate.

This reaction happens in seconds, whereas the rate of reaction with other reductones is much slower. So by moving quickly you can take out the sulfite and measure the remaining reductones.

Using 40% Glyoxal

1. Titrate a 25 mL wine sample following the standard Vinmetrica method using the SC-100A or the SC-300. Calculate the apparent free SO2 in the usual way:
20 x [volume SO2 Titrant used] = ppm apparent free SO2

2. Titrate a second 25 mL wine sample, but this time, add 2 mL 40% glyoxal first, and let sit for 5 minutes. Then proceed with the standard titration as above. Calculate reductones in the same way as above, but these are free SO2 equivalents:
20 x [volume SO2 Titrant used] = ppm reductones as free SO2 equivalents

3. Subtract the second value from the first to obtain the free SO2 level corrected for reductones.

Using hydrogen peroxide

1. Titrate a 25 mL wine sample following the standard Vinmetrica method using the SC-100A or the SC-300. Calculate the apparent free SO2 in the usual way:
20 x [volume SO2 Titrant used] = ppm apparent free SO2

2. Titrate a second 25 mL wine sample, but this time, add 1 drop of 3% hydrogen peroxide first, then immediately add the Reactant and Acid solution and proceed with the standard titration Calculate reductones as free SO2 equivalents:
20 x [volume SO2 Titrant used] = ppm reductones as free SO2 equivalents

3. Subtract the second value from the first to obtain the free SO2 level corrected for reductones.

We prefer the hydrogen peroxide method because the reagent is cheap, but both methods give similar results.

 

How big a problem are reductones?

In my experience, there’s never been any red or white wines that had appreciable levels of reductones. The only exceptions have been some exotic wines in which vitamin C might be present from the original fruit, or added at some point in the process. In general, when I’ve checked, the reductones might contribute 2-3 ppm additional free SO2 at most. Not enough to even sweat at all! I began to wonder why there is such a persistent belief that reductone interference is a significant problem with the use of the Ripper titration. Perhaps other methods of performing the Ripper titration are more susceptible to this interference than the Vinmetrica method; I don’t know.

But here is some of our data on reductones, and possible reductones, using the Vinmetrica system:

1. Sulfur Dioxide (SO2) is removed by peroxide treatment:
1 mM free SO2 titrates as 62 mg/L (expected 64)
1 mM free SO2 plus 1 drop 3% hydrogen peroxide, waiting 30 seconds, then titrating gives < 1 mg/L as free SO2.

2. Ascorbic acid (Vitamin C) is left behind as a reductone by peroxide treatment:
1 mM ascorbic acid (about 176 mg/L) titrates as 60 mg/L free SO2 (expected 64).
1 mM ascorbic acid plus 1 drop 3% hydrogen peroxide, waiting 30 seconds, then titrating gives 58 mg/L as free SO2.

So ascorbic acid does indeed show up as a reductone, and could be accounted for as described in the methods above.

3. Reductone levels in 2 big reds are negligible:
A 2019 Malbec and a 2016 Syrah, both from our sister company Little Oaks Winery, were titrated for free SO2 in the presence and absence of 1 drop 3% hydrogen peroxide. The phenolic levels in both wines were determined to be about 2100 mg/L by the Folin method, so these really are pretty tannic wines that might be expected to show interferences from the phenolics as reductones.
19 Malbec: 42 mg/L free SO2; 2 mg/L reductones as free SO2 by peroxide treatment
16 Syrah: 24 mg/L free SO2; < 1 mg/L reductones as free SO2 by peroxide treatment

4. When we compare our SO2 results against other labs on the same wines, we get excellent agreement, even though many of those labs are running Aeration Oxidation or other non-Ripper methods. I refer you to a recent post of ours:
https://vinmetrica.com/vinmetrica-compares-well-with-wine-testing-labs/

Briefly, this report shows that Vinmetrica’s SO2 results on 2 sweet red wines are close to the mean of all 92 other labs reporting results by various methods. In fact, we are actually a little lower than that mean, suggesting that we’re not measuring erroneously high values due to reductones.

5. The following substances, considered to have possible reductone properties, did not show any response as SO2 in the standard Vinmetrica titration:
Quercetin (a flavone) 1 mM
Epicatechin (a flavone) 1 mM
Glucose 10 mM
Fructose 10 mM
Phenol 20 mM

So at this point, I’m ready to say that the reductone issue is not significant for the Vinmetrica free SO2 titration, unless you suspect ascorbic acid to be present.  In any case, it’s easy to test for yourself using regular old 3% hydrogen peroxide. Maybe you have some observations you’d like to share:  info@vinmetrica.com