Stopping fermentation and sweetening wine — safe and stable

By Magnus von Kageneck, cellar master · Updated: August 2026

In the summer of 2004 I heard a sound from the cellar that no winemaker ever wants to hear: a dry bang, then the trickle of glass. Two bottles of my currant wine had blown themselves apart, and in three others the cork had crept right up to the wire. I had bottled in spring and, because the wine struck me as too tart, had given it two spoonfuls of sugar per bottle beforehand. It was clear, it tasted good, and I thought the matter was settled. It wasn't: every bottle still held living yeast cells, which in the warmth of July did exactly what nature built them for.

If you want to stop fermentation and sweeten wine, you are working against a living organism. That's no black magic, but it demands a fixed order. Stick to it, and your wine stays sweet and still. Skip it, and you're stacking bottle bombs on your shelf.

Why “just add more sugar” goes wrong

Yeast eats sugar; that is its entire purpose in life. A wine fermented to dryness may have no sugar left, but it is full of yeast cells that are merely resting for lack of food. Even a perfectly clarified wine still contains millions of them; they are simply too small for the eye to see.

If you give this wine sugar, three things happen: first, it tastes wonderful. Then it sits cool in the cellar, and apparently nothing happens. And in the first warm month the yeast wakes up, finds the buffet and starts all over again, this time without an airlock through which the carbon dioxide could escape.

The reverse case is even trickier: the batch you want to halt mid-fermentation because the residual sweetness is just right for you. Frankly, I advise against it in the home cellar. Better to back-sweeten the fully fermented wine afterwards than to choke off a fermentation in progress.

Stopping fermentation: what really works and what only looks like it does

Cold

Below about 10 °C, wine yeast stops working, sinks to the bottom and falls into a kind of hibernation. That is useful: cold wine clears faster and can be racked cleanly. But cold doesn't kill; it only puts the yeast to sleep. Anyone who thinks their wine is “done” because nothing bubbles any more in December will witness the resurrection in May.

Racking and clarifying

The cheapest step, and the one most people tick off too early. Every racking, that is, drawing the clear wine off the sediment with the racking siphon, takes a considerable share of the yeast cells with it. Two or three clean rackings and proper fining bring the cell count down by orders of magnitude. How to go about it, and in which order silica sol and gelatine work together, is covered in detail in the article Fining like a cellar master. Only this groundwork makes the later stabilisation reliable: it works well against few cells and not at all against many.

Sulphur (potassium metabisulphite)

Sulphur dioxide is the backbone of shelf life: it protects against oxidation and keeps wild yeasts, lactic acid bacteria and acetic acid bacteria in check. What it can not do is stop a fermentation in progress; that would take amounts far beyond what is permitted and what is drinkable. In sweet wine, sulphur is nonetheless indispensable: residual sweetness makes the wine more vulnerable to everything that ferments and sours, and without free SO₂ the sorbate from the next section becomes a nuisance. As for the amount, I stick to what I wrote in the article Sulphuring without fear: conservatively and according to the package instructions. The law allows sweet wines somewhat higher total levels, and they need them, too.

Potassium sorbate

Potassium sorbate is the actual fermentation stopper in the home cellar, but it works differently than most people assume: it doesn't kill yeast, it merely prevents the cells present from multiplying. Against an active, vigorous fermentation it is ineffective, and the fewer cells remain in the wine, the more reliable it becomes. First rack and clarify, then add sorbate.

And it has a catch you need to know about: lactic acid bacteria can break down sorbic acid, producing a penetrating smell of geranium leaves, the notorious geranium taint, which spoils a wine beyond rescue. This is exactly what the free SO₂ present at the same time protects against. That is why sorbate never goes into the wine on its own, but always together with a proper dose of sulphur. The EU wine regulation permits a maximum of 200 mg/l of sorbic acid; how many grams of potassium sorbate that means for your number of litres is stated on the package, and that's where you stay.

Ready-made preparations such as ParaVin combine sorbate and benzoate in one bottle and spare you weighing out small amounts of powder in your own cellar. Dose according to the manufacturer's instructions; the sulphur dose remains mandatory all the same.

The heavy artillery

Sterile filtration is the only method that truly gets the yeast cells out of the wine, but it requires fine filter sheets; ordinary hobby filters clarify, they do not sterilise. Pasteurisation kills the yeast with heat and costs freshness. And high alcohol stops any fermentation by itself: most wine yeasts give up between 14 and 16% vol, which is why fortified sweet wines in the style of port stay stable.

The sequence, step by step

1. Let it ferment out. Wait until the wine is truly dry, and measure rather than trusting the silent airlock: with the Oechsle hydrometer, which should then read 0 °Oe or below, or with a residual sugar quick test. A refractometer is no use at this stage; the alcohol distorts the reading.
2. First racking. Draw the clear wine off the yeast lees, fill the demijohn to the brim, no air cushion.
3. Chill and clarify. A few days in the cold, then fine, then rack again. The order and amount of fining agents follow the package instructions. The goal is a brilliant wine with as little yeast as possible.
4. Sulphite. Dissolve the potassium metabisulphite in a little wine and stir it in evenly; never tip dry powder into the vessel. Work in a well-ventilated room and do not breathe in the dust.
5. Add sorbate. Add the potassium sorbate, likewise dissolved, on the same day as the sulphur dose, and mix well.
6. Only now sweeten. In small steps, tasting in between. Sweetness comes across weaker in cold wine, so taste at cellar temperature, not ice-cold.
7. Watch for two weeks. The wine stays in the demijohn under an airlock, and at room temperature, not in the coldest corner of the cellar. Is it beading, turning cloudy? Then it is fermenting again, and a bottle would be the wrong place for it.
8. Bottle. Clean bottles, fill level one to two centimetres below the cork, because a bottle filled to the brim pushes the cork out at the first warmth. Then store cool and dark.

What to sweeten with

Table sugar is the classic choice and perfectly fine, provided steps four and five are done. If you want to gain in aroma, use sweet reserve: unfermented juice or fruit concentrate of the same fruit, which brings back sweetness and fruit. Both are fermentable sugar and therefore depend on sulphur and sorbate.

The second route sidesteps the problem: non-fermentable sweetness. Wine yeast can do nothing with xylitol or ready-made sweetening preparations; there is simply no food left for it. This is the safest option for anyone who wants to work without sorbate. With birch sugar (xylitol), bear in mind that in larger amounts it has a laxative effect and is poisonous to dogs.

The honest residual risk

Let's talk about pressure. As a rule of thumb from sparkling-wine making, around 4 g of fermented sugar per litre produces about 1 bar of overpressure. A sparkling-wine bottle is built for a multiple of that: thick glass, wire cage, a deep punt in the base. An ordinary wine bottle with a natural cork is not. If even just 10 g of residual sugar per litre referments there, the arithmetic gives about two and a half bar. Enough to drive out the cork, and in the worst case enough for the sound of 2004.

Hence the two weeks of observation in the demijohn: they cost nothing and are the only check you have. And if you remain unsure, store the sweetened wine cool and drink it soon, rather than keeping it for three years.

Frequently asked questions

Can I halt a fermentation in progress to keep residual sweetness?

In the home cellar only with difficulty and never entirely safely. Against a highly active yeast population, sorbate is ineffective and sulphur in permitted amounts too weak. The reliable route is the reverse: let it ferment dry, stabilise, then back-sweeten.

Is potassium sorbate enough on its own?

No. Without an accompanying dose of sulphur, geranium taint looms, and against oxidation sorbate does nothing at all. The two belong together, and both go into wine that has already been clarified.

How long do I wait after sweetening before I can bottle?

Around two weeks in the demijohn, preferably at 18 to 20 °C rather than ice-cold: only in the warmth does a refermentation that is getting going show itself at all. If the wine stays still and brilliant, you can bottle.

And what if it referments after all?

Careful, the bottles are under pressure. Chill them first, wear safety goggles when opening, hold a cloth over the cork and keep your face away from the bottle. Then pour the wine back into the demijohn and let it ferment to the end. After that, start again cleanly from the beginning: rack, clarify, sulphite, add sorbate, sweeten. When adding sorbate the second time, include the amount already added: the permitted maximum applies to the total. Nothing is lost except time.

From the cellar, Magnus

Safety note: when handling sulphur, acids and enzyme preparations, wear safety goggles and gloves. What to do in case of contact with skin, eyes or airways is explained on our page Working safely in the cellar.

Related in the shop: Making Wine and Fining & Clarifying Agents