By Magnus von Kageneck, cellar master · Updated: September 2026
For some of you the harvest is already in, for others it is still to come — the plan stays the same, only the dates shift.
Measure first, then move it somewhere cool
Whether fermentation has finished is something you measure — and measure properly: sample into the measuring cylinder, shake out the carbon dioxide vigorously, no coarse lees. Dissolved carbon dioxide pushes the reading down, coarse lees push it up. And the hydrometer is calibrated for 20 °C: for every 3.5 °C below, subtract 1 °Oe; for every 3.5 °C above, add 1 °Oe — on hydrometers with a built-in thermometer these figures are printed on the scale (Measure, don't guess).
A fully fermented table wine sits at around zero degrees Oechsle or just below, a dessert wine at about 25 °Oe. Below zero, an Oechsle hydrometer whose scale starts at 0 °Oe shows nothing — but that is enough to decide: if the hydrometer sinks down to the mark, a table wine is done.
If it reads higher, fermentation is still running: the demijohn stays somewhere warm; below 10 °C fermentation stalls, the sugar stays in and ferments again in spring when things warm up. Leave 10 to 20 per cent headspace, half the vessel when fermenting on the mash — otherwise the foam cap pushes up into the airlock, and a blocked airlock bursts the vessel. If you are still on the mash, press at the end of the fermentation period and ferment the juice on under an airlock.
Once it is done, the vessel goes to the coolest dark spot in the cellar, airlock left on. Exactly how cool, the book does not say — only as cool as possible. And top up now: towards the end of fermentation, or shortly after at the latest, the vessel should be full — otherwise the young wine turns brown before racking. Dawdling does not help: bacteria multiply easily in the lees.
Racking after two to four weeks
After two to four weeks the young wine has to come off the lees. The rule of thumb from the Kitzinger Weinbuch applies: 1 g of potassium metabisulphite per 10 litres — a flat-rate dose, not a laboratory value.
Safety note: potassium metabisulphite is a hazardous substance — signal word "Danger", H318 (Causes serious eye damage), EUH031 (Contact with acids liberates toxic gas). Do not breathe the dust, wear eye protection, never bring it together with acid while dry; the label and the safety data sheet of your product are authoritative.
Dissolve the portion beforehand in a little wine — not in plain water, which would first have to be acidified — and mix it in evenly as you transfer. Have ready: a wine siphon, a cleaned receiving vessel and sulphite powder in 1 g portions, one per ten litres. Do not tip the demijohn; draw it off with the siphon. In the process the wine comes into contact with air: discoloration, even to brown, can appear within days and can no longer be corrected with sulphur. At the end of winter comes the second racking, and possibly a third before bottling.
Full to the bung, not nearly full
"After fermentation the vessels must always be full to the bung," writes Paul Arauner where he describes the care that light, lower-alcohol wines demand (Kitzinger Weinbuch, revised edition 1999, Paul Arauner GmbH & Co. KG, Kitzingen, p. 94). The rule applies just as much to heavier wines: store them full, cool and dark. Top up with a similar wine; otherwise move it into a smaller vessel. Film yeast gets going as soon as the carbon dioxide has left the surface; only wine above 12 to 13 % vol is safe from it, and even then only at cellar temperatures of around 10 to 20 °C (Recognising wine faults).
That applies to vessels under an airlock. If you seal the demijohn tightly or cork it, deliberately leave a small air cushion — anything containing alcohol expands as it warms up. And if fermentation pauses while the cellar cools down, the negative pressure sucks the sealing liquid from the airlock into the wine.
The empty cask is the real winter problem
An empty wooden cask dries out from the inside: the wood gives off water, the joints open up, mould spores come in with the air — and they find plenty to feed on: residues of alcohol, acid and sugar in the wood. The fungus grows into the pores of the cask, and then the cask is ruined.
Short time empty: burn a sulphur strip in it, according to Arauner half to one whole strip per 100 litres of cask volume — meaning the asbestos strips of the time, carrying 2.5 g of sulphur. How much yours carries is stated on the pack. Burning produces sulphur dioxide, a toxic, strongly irritating gas: ventilate well, do not hold your head over the cask opening, leave the room; the label and safety data sheet are authoritative.
Long time empty over winter: store it wet. The number you must not get wrong: Arauner calculates 500 mg of sulphurous acid per litre — 50 g per 100 litres of cask volume; since potassium metabisulphite contains only about 50 % of that, it comes to 100 g of potassium metabisulphite per 100 litres of cask water plus about 40 g of citric or lactic acid. Ten times as much as in wine — and it is cask water, not a drink. The two figures must never get mixed up; the label remains authoritative.
The order: clean the cask, check it for leaks; dissolve the acid in water, pour it in, rinse after it; add the potassium metabisulphite, also dissolved — dry, it will not spread through the full cask; fill to the bung with tap water. Citric acid, as a powder, is easier to weigh out than lactic acid, which only comes as an 80 per cent liquid — there the label applies. Concentrated acid can damage skin and eyes: put on safety goggles, add acid to water, never the other way round. The acidification is intentional: only then does the potassium metabisulphite release the sulphurous acid — this too produces sulphur dioxide, so ventilate.
In spring, drain the water and fill the cask completely with clean water several times, leaving each filling for no more than four to five days — otherwise, according to the book, you carry a stale sulphuric-acid taint into the next wine.
The frost line in the unheated cellar
Cold is good, frost is not. Sugar lowers the freezing point: according to the Kitzinger Weinbuch, such a liquid only freezes between −3 and −4 °C, and for glass demijohns the book gives −4 °C as the lowest limit. Do not rely on it, one figure lies within the range of the other: keep the cellar frost-free, above zero. Whatever freezes expands and bursts the glass — and splinters cause injuries.
The second frost damage is quieter: chilled too far, fermentation stalls, and the sugar ferments again in spring. A wine that has gone quiet in January is not automatically finished — measure the must weight once more before sealing or bottling. Otherwise, according to the book, you risk haze and, worse, bottles bursting or leaking. If it starts bubbling again: draw off part of it and keep the airlock clear.
The winter rule
Too warm, not full, too long on the lees — that is where almost everything goes wrong in the winter cellar. And carbon dioxide is heavier than air: wherever fermentation vessels stand, the room must be ventilated.
Source: Kitzinger Weinbuch, revised edition 1999, Paul Arauner GmbH & Co. KG, Kitzingen, pp. 27 f., 30, 39 ff., 52 ff., 57, 80 f., 94, 156 ff., 172 f., 188. Arauner and Kitzinger are third-party marks.
Magnus von Kageneck is Maischewerk's cellar master character. The texts in this series are written with the support of artificial intelligence and are checked for technical accuracy before publication.
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