Definition: sulphur - SO2

Bacteria are much more sensitive to sulphur than yeasts. SO2 has long been used in the form of "corking" barrels. When sulphur dioxide is added to must or wine, it quickly reacts with potassium to form salt and, within a few hours, combines with molecules that have an aldehyde or ketone function (sugars, acetaldehyde, etc.). Thus, in musts and wines, there are different forms of SO2:
active SO2 gas, which is part of free SO2: it represents only a very small proportion of SO2, but it is the most effective as an antioxidant, anti-oxidase and antiseptic;
salified SO₂, particularly potassium, which is also part of free SO₂, but is much less effective than active gaseous SO₂;
combined SO2, which is even less effective.
The SO2 content of wines is regulated by European legislation. Excess SO2 can cause headaches in wine consumers. Inhaling it is toxic and irritating to humans. Its presence must be indicated to consumers on wine bottles.

Discussion:
The issue of SO2, or sulphur, sulphur dioxide, bisulphite or sulphur dioxide, is regularly raised because this toxic substance is not viewed favourably by consumers.
What is it? What is it used for? Is it good or bad for your health? Is it better if wine does not contain it?
What is it?
Definition: Sulphur dioxide (or sulphuric anhydride) is a chemical compound consisting of two oxygen atoms and one sulphur atom; its molecular formula is SO2.
The process of adding sulphur dioxide to wine is called sulphiting.
What is it used for?
It stops the growth of bacteria and yeasts, thus preventing further fermentation by sterilising the wine. Sulphur dioxide also fixes the oxygen present, thus protecting oxidisable substances (tannins) and preventing the growth of bacteria that would turn the wine into vinegar.
It is mainly used to protect, sanitise and preserve wine.
Is it good or bad for your health?
In high doses, it is obviously toxic to humans. It is blamed for headaches and heartburn caused by wine.
However, it is authorised in the following amounts:

Type of wine Maximum total SO2 permitted in mg per litre of wine

Red 160

White and rosé 210

Red (sugars > 5 g/l) 210

White and rosé (sugars > 5 g/l) 260

Sweet 300 to 400

High-quality sparkling wine (VMQPRD) 185

Natural wine, fortified wine (sugars > 5 g/l) 200

Is it better if there is none in the wine?
To answer this question:
1. There can be no ZERO SULPHITE wines, because this molecule already exists naturally in wine. Even without any added sulphites, very small amounts are present in wine.
Why? Simply because yeast always produces a tiny amount during fermentation, generally less than 10 ppm or mg/litre. So there are no wines without sulphites. Obviously, if this value is compared with the authorised added doses, up to 210 mg/l for white wine and 400 mg/l for sweet wine, it is understandable to want to say "sulphite-free wines", but the correct statement would be "wines without added sulphites".
2. Current analysis methods do not allow for the desired accuracy. A margin of error of 10mg/l must be allowed. For this reason, regardless of the method of vinification, with or without the addition of sulphur, the statement "Contains sulphites / contains sulfites" is mandatory.
3. As previously stated, its role is to enable the wine to be preserved. Without sulphur, the wine may have to be drunk even more quickly, or even not consumed at all, because it would not withstand the effects of time and external factors.

However, there are very rare exceptions:
When the grapes are of incredible quality and have the correct (rather than low) levels of naturally occurring sulphur dioxide, then the winemaker has a raw material that is worth its weight in gold, as it means they do not need to add any and can produce a high-quality wine.
So wine needs a minimum amount of sulphur dioxide to be of good quality, but without excess.
Regulations require the bottle to be labelled with the words: "Contains sulphites". This applies to wines containing more than 10 mg/l, which is easy to understand given the explanations given above. The words "sulphite-free" are not permitted.

Conclusion:
Sulphite-free wines do not exist.
Being against them is excessive; as in all situations, they should not be abused.
Finding quality wines "without added sulphites" is almost like looking for a needle in a haystack, but it is not impossible.
For example:
We have the opportunity to taste Beaujolais Morgon wines from Domaine Lapierre:
"It was in 1981, under the influence of Jules Chauvet, a well-known and respected researcher at the University of California, that Marcel Lapierre decided to make wine without SO2 or yeast and to cultivate his vines biodynamically: without chemical fertilisers or weedkillers."
When they are well preserved in the bottle, they are sublime. Wine lovers with a cool or air-conditioned cellar will enjoy choosing Morgon Lapierre, which is neither filtered nor sulphited: this very fine wine should be stored at a temperature below 14°C. A slight deposit may sometimes appear, but this does not alter the quality of the wine.
Wine lovers who do not have this storage facility should choose Morgon Lapierre without filtration, which is only sulphited to stabilise it microbiologically.

See also the entry: Sulphur (in wine).

Partial source: Emisommeliere - LABIVIN, a Sommelier at your Service... - Highly recommended blog: