Ways to Automate Sulfur Dioxide Testing in Wine

Free and total sulfur dioxide (SO₂) analysis guides important and consequential decisions in the cellar: is the wine protected, how much to add if not, and when is the wine ready to be racked or bottling ready. Those consistent and repeatable results are imperative, no matter who runs the test and however busy the day is, with many winemakers now aiming to use minimal effective doses. With virtually no room for correction other than time, an accurate result followed by a cellar addition matters at every stage from fermenting must to pre-bottled wine. Automation offers one solution to this problem for winery labs.  This post surveys different options for SO₂ automation, what changes in the lab at each level, and how to choose the one that fits.

Why Automating SO₂ Testing Pays Off in the Cellar

A manual SO₂ test has many points where a result can drift. Pipetting volume, judging an endpoint visually, reaction timing, and volumetric reagent preparation can each lead to inconsistent results from user error. In red wines, subtleties of color change make the endpoint harder to read.

Automation reduces how much the result depends on the operator. The analyzer runs the same way no matter the time of year, and the lab team is available for oversight and evaluation of the data. For the winemaker, the payoff is a number that can be trusted and duplicated from one operator to the next.

Options for Lab Automation

The reference point: aeration-oxidation (AO)

Every analytical method is judged against a benchmark. In U.S. winery labs that benchmark is often considered to be aeration-oxidation, or AO. You will also see it called the Paul-Rankine or Franz-Paul method, and it is often lumped together with the modified Monier-Williams method. 

In an acidified wine sample, volatile molecular SO₂ is carried by air and trapped by hydrogen peroxide as sulfuric acid. The acid is then titrated with sodium hydroxide to quantify Free SO₂.  To measure total SO₂, the sample is heated to release the bound forms first.  

Despite being an OIV method that is referenced, AO is slow, hands-on and manual, occupying 15 to 20 minutes per sample for free SO₂, and another 15 to 20 for total. Accuracy depends on technique: airtight glassware, steady air flow, adequate run time, fresh peroxide, and standardized sodium hydroxide, making between laboratories data differ by up to 9 mg/L below 50 mg/L.  That time trade-off explains why many labs fall back on faster methods for routine work and reserve AO for reference checks.

Automating the endpoint: iodometric (Ripper) titration

Manual Ripper. The Ripper method titrates acidified wine with iodine. Iodine oxidizes the SO₂, and once the SO₂ is used up, excess iodine turns a starch indicator blue, hard to see in red wines.  For total SO₂, sodium hydroxide is added first and left for 5 to 10 minutes to release the bound forms, and the sample is then acidified and titrated the same way. The method is simple, inexpensive, and quick, which is why so many winery labs rely on it.

Automated titration with Flash2. An automatic titrator keeps the Ripper chemistry but takes over the manual steps. On Flash2 by Steroglass, peristaltic pumps add the acid, and for total SO₂ the sodium hydroxide, followed by a timed 5-minute release step. A burette doses the iodine, and a double platinum electrode detects the endpoint by the current that flows once free iodine appears. The instrument calculates the result in mg/L. 

In practice:

• No sample preparation; a 25 mL minimum, or 10 mL with the 30-position micro tray
• Free SO₂, total SO₂, or both on the same sample: the double method titrates free SO₂ first, then releases and titrates the bound fraction, and reports total as the sum
• Flash2 can standardize the iodine titrant itself against sodium thiosulfate as a preprogrammed method
• Walkaway batches on the AS PLUS autosampler, with 18 or 30 positions
• Results stored with titration curves, exported by CSV or USB through Flash2Data software, or sent directly to a LIMS

Once the wine is measured into the autosampler cups, the run is walkaway: Flash2 dispenses every reagent and works through the batch unattended. The endpoint is objective and repeatable, regardless of operator or wine color. Burette reading, calculation, and transcription errors disappear, and staff with little titration experience can run the method. Steroglass reports up to a 90 percent reduction in analysis time compared with manual methods.

Automating the workflow

Discrete colorimetric analysis on BioSystems Y15 or SPICA automates the full sequence. The analyzer dispenses sample and reagents into a reaction cuvette, reads the color by photometry, and calculates the result. No operator has to watch a titration.

Free SO₂ reacts with pararosaniline to form a magenta product in a reaction that is specific for sulfite, but some molecules in red wine also react. BioSystems handles this with a second reaction with a strong oxidizing agent.  The first reading captures sulfite plus interferences, the second captures only the interferences, and the difference is the free SO₂ with a first result in about 3 minutes, with subsequent thereafter.

Total SO₂ After a sample pretreatment that removes interferences, sulfite reacts with DTNB (Ellman's reagent) and reports the first result in about 6 minutes, with further results following in sequence as the analyzer works through the queue. Standard DTNB methods work well in white wine but can show interference in red where the pretreatment is used.

Automated colorimetric analysis increases throughput up to 10 times faster than AO method with good reproducibility and less operator error. Better still, the free sulfite kit was validated versus AO with excellent inter- and intra- laboratory correlation for the same samples.  Reagents and calibrators are liquid and ready to use, with real-time stability of 36 months from manufacture. 

Comparing the options

How the options compare on time

 

Elapsed time per sample is only part of the picture. What matters most in a busy cellar is how long the analyst is tied to the bench. With AO and manual Ripper, that is every minute of every sample. With Flash2 and its autosampler, the only hands-on step is measuring wine into the sample cups; Flash2 dispenses the reagents, titrates with iodine, detects the endpoint, and calculates the result while the analyst returns to other work. A discrete analyzer works the same way: load the samples and walk away while results accumulate.

Matching the Level of Automation to Your Lab

Start where you are, and scale when sample volume justifies it. Three questions help decide which option fits:

• Who runs the test, and how much time can that person give it during peak analysis demands?
• Which wines does the lab handle, including dark colored red wines and white wines with added ascorbic acid?
• Do results go to buyers, regulators, or an accreditation body, and what documentation does that require?

For a small team running a handful of samples, manual Ripper remains a reasonable routine check, with AO in reserve for reference work. When SO₂ testing volume grows, or the person running the test is pulled across the cellar, an automatic titrator such as Flash2 with an autosampler takes the analyst off the bench while keeping the familiar chemistry. A mid-size or larger lab may get more from a discrete analyzer that also runs other wine parameters. Large operations may add data integration needs to the list. The right level is the one that fits today's sample load and leaves room to grow with additional applications.

Even so, any lab should compare a new method against its own current method, on its own wines, before making the switch.

Next Steps for Your Lab

Admeo supplies BioSystems Y15 and SPICA analyzers with BioSystems free and total sulfite reagents, calibrators, and controls, along with Steroglass Flash2 titrators and platinum electrodes for sulfite analysis, all with U.S. stock and application support. If you are weighing a first step or an upgrade, request a quote or talk with our team about which level of automation fits your lab. 

FAQs

Can the Ripper iodometric method be automated? 

Yes. Ripper titration can run on automatic titrators like Flash2 by Steroglass, and a platinum electrode can detect the endpoint in place of a visual operator determination. Automation improves performance and repeatability, but the iodine titrant still reacts with reducing substances, so consideration is needed for dark colored red wines and wines with added ascorbic acid. Flash2 does have a rapid combined free and total sulfite method, and the iodometric titrations are fast and repeatable, if careful consideration is given to sample types.

Can one analyzer run both free and total SO₂? 

BioSystems' SPICA and Y15 analyzers offer capability to run a two cuvette pararosaniline method for free sulfite and a DTNB method for total sulfite with results read photometrically.

Flash2 by Steroglass does have a rapid combined free and total sulfite method and methods for free sulfite and total sulfite all measured with automated reagent dispensing and a platinum electrode.

Does automation change which method is the reference? 

No. Aeration-oxidation (AO) remains most wineries chosen reference, and BioSystems automated methods are validated against it. You may also see AO called the Paul-Rankine, Franz-Paul, or modified Monier-Williams method. Labs should still run their own comparison study before relying on an automated method to understand the differences.