Guide To Sulphur Management In Wine

Sulphur dioxide is one of the most used additives in the winery and the one most often misunderstood. Add too little and wine oxidises or spoils, add too much and you strip aroma, risk sensory faults and push against legal limits. Getting it right isn’t about a fixed number, it’s about understanding what SO2 is actually doing and adjusting for the wine in front of you.

This guide covers the fundamentals: the forms SO2 takes, why molecular SO2 is the number that matters, and how to think about additions through the life of a wine.

what so2 actually does

Sulphur dioxide does two jobs at once. It’s an antioxidant, mopping up oxygen and slowing the browning and aroma loss that come with it. It’s also an antimicrobial, suppressing spoilage yeast and bacteria that would otherwise take hold. SO2 remains an important tool due to its ability to cover both of these jobs so effectively. The catch is that the same dose behaves differently from one wine to the next, and the reason is chemistry, not guesswork.

free, bound and total

When you add SO2 to wine, it splits into two forms.

Bound SO2 is the portion that has reacted with compounds in the wine (acetaldehyde, sugars, pigments). It’s chemically tied up and contributes little protection.

Free SO2 is the portion still available to do the work. This is the form you manage.

Total SO2 is simply the two added together, and it’s the figure that legal maximums are written against. Limits vary by wine style (higher allowances for sweet wines), so it’s worth confirming the current cap for your style, but total SO2 tells you little about how protected a wine actually is day to day. That comes down to the free fraction, and more precisely, one part of it.

molecular so2 – the number that matters

Not all free SO2 is equally effective. Only a small part of it exists as molecular SO2, the active form that delivers real antimicrobial protection. How much of your free SO2 takes that form depends almost entirely on pH.

This is the single most important concept in SO2 management. The higher the pH, the less of your free SO2 is molecular, and the more you need to add to achieve the same protection. A low pH wine (around 3.2) reaches protective molecular SO2 with relatively modest free SO2, where a high pH wine (around 3.7) needs substantially more free SO2 to get to the same place.

The practical target most winemakers work to is around 0.5 – 0.8 mg/L molecular SO2 for microbial protection, with the lower end often sufficient for wines going through or finished with malolactic fermentation and the higher end for sweet or at risk wines. Rather than doing the calculation by hand each time, use our calculator below.

timing additions from harvest to bottle

SO2 management isn’t a single event, it’s a series of decisions.

At the crusher, a measured addition protects juice from oxidation and knocks back wild flora before fermentation, without over-dosing to the point of inhibiting your chosen yeast. Fruit condition guides the rate, clean fruit needs less and compromised fruit more.

During fermentation, free SO2 is largely consumed and bound, so protection is low by design; this is where good cellar hygiene and oxygen control do the heavy lifting.

Post-malo is the key adjustment point for most wines. Once malolactic fermentation is complete, bringing free SO2 up to a protective molecular level locks the wine in for maturation.

Through maturation and at bottling, top up to hold your molecular target, accounting for the small oxygen pickup that comes with handling and filling.

the products behind it

The workhorse addition is potassium metabisulphite (PMS), which carries roughly 52-55% available SO2 and serves as both antioxidant and antimicrobial. For whites where you want a fast-acting antioxidant boost without adding to the sulphur load, ascorbic acid works alongside SO2 to lock in fresh aromas and prevent browning – though it needs adequate free SO2 to work safely, so the two are partners and not substitutes.

the low sulphur toolkit

Consumer demand for low-no sulphur wines is growing, and so is interest in trimming total additions for quality reasons. SO2 will remain important, but you can lean on it less by covering its two jobs with complementary tools rather than asking sulphur to do everything.

There are options right from the start. The earliest opportunity to reduce SO2 is bioprotection at the crusher, seeding must with a protective non-saccharomyces yeast before fermentation so spoilage organisms never get a foothold. IOC Gaïa is used for exactly this, added during the pre-fermentation stages to suppress spoilage microorganisms at the crusher, letting you hold back on SO2 on the way in. Carrying through to ferment itself, QTL yeast strains such as Lalvin Sensy™, Lalvin ICV Okay™, Lalvin Persy™, Lalvin ICV Opale 2.0™, IOC BE Thiols™ and IOC BE Fruits™ produce limited SO2 during fermentation, giving you a cleaner starting point.

On the antioxidant side, several products help protect must and wine from oxygen so SO2 doesn’t have to carry the load alone. Fullprotect™ is selected to stabilise aroma compounds and protect juice and wine from premature development, useful antioxidant cover early when SO2 is naturally low. Essential Antioxidant™, a pure gall tannin, delivers strong antioxidant protection for grape phenols and aromatics with minimal sensory impact, and a strong capacity to bind laccase enzyme.

On the antimicrobial side, tools like Bactiless™ lowers the bacterial population in wine without affecting the yeast population or leaving a sensory mark. This provides a targeted way to manage microbial risk while keeping free SO2 lower.

These products don’t result in completing abandoning SO2, but rather allow it to be used as a complementary tool where it lets you add less.

the bottom line

Good SO2 management comes down to three habits: measure free SO2 and pH together, work to a molecular target rather than a fixed dose, and adjust at the right moments in the wines life rather than reacting late.

If you’d like help setting up SO2 testing, choosing between addition products, or building molecular targets into your protocols, our technical team is happy to work through it with you. Contact us for more information.