Temperature is the one fermentation variable that changes everything else. It sets how fast the yeast works, how much aromatic compound survives, how much colour and tannin the wine takes from the skins, and whether the ferment finishes at all.

The ranges themselves are not controversial. What separates cellars is whether they can hold one.

The limits

Two numbers bound the problem.

Above roughly 35 °C (95 °F), yeast comes under serious stress. Ferments stick, and a stuck ferment at the peak of vintage is among the more expensive things that can happen in a cellar. Push further and the wine takes on a cooked, stewed character that no later handling removes.

Below roughly 5 °C (41 °F), yeast goes dormant and alcoholic fermentation effectively stops.

Everything useful happens between those two figures, and heat is not a neutral occupant of that range — it is a catalyst. The warmer the ferment, the faster sugar becomes alcohol and CO₂. A cool ferment is a slow ferment, by design.

The working ranges

Red wine: 20–32 °C (68–90 °F). The upper half of that band is where colour and tannin extraction happen most readily, which is why reds are fermented warm. The cost is aromatic: volatile compounds are driven off at the same time. Most red programmes therefore move within the range rather than sitting at one point — cooler at the start to protect fruit, warmer through the active phase to extract, then down.

White wine: 12–22 °C (54–72 °F). Cooler, slower, and for a specific reason: low temperature preserves volatile aromatics and favours the fermentation esters that give a young white its lift. Aromatic whites fermented at the bottom of this band can take weeks — occasionally months — to finish, and that slowness is the point, not a delay to be engineered away.

Variety, site and intended style move these figures. A winemaker who wants a broader, less overtly fruity white will ferment warmer on purpose. The ranges are where the decisions are made, not a specification to be met.

Measuring it

Three methods, in ascending order of usefulness.

A sanitised probe thermometer dropped into the tank works and costs nothing, but it means opening the tank, and every opening is oxygen and a contamination risk. Fine occasionally; poor as a routine.

Adhesive temperature strips on the tank wall are contact-free and instant. They read the wall, not the wine, and they are approximate — but during vintage, knowing a tank is running hot within a degree or two is often the information you actually need.

A thermowell with a fixed probe is the only method that gives a continuous, accurate reading of the wine itself without opening anything. It costs more per tank and is worth it on any vessel where the ferment matters.

The practical answer in most cellars is a combination: probes where the decisions are made, strips where a glance is enough.

Controlling it

Knowing the temperature is the easy half.

Cooling the room

Placing fermenters in a cool cellar, or conditioning the fermentation hall, is the traditional approach and remains the only option for wine fermenting in barrel. It works — slowly, imprecisely, and for every tank in the room at once. A cellar with one hot ferment and one cool one cannot use it.

Cooling the tank

A jacketed tank with glycol circulating against the wall changes the problem from managing a room to setting a number, per vessel. This is what makes modern fermentation programmes possible: two tanks side by side, one held at 14 °C and one at 28 °C, both finishing as intended.

Jacket design is where this stops being generic. What matters is not whether a tank has a jacket but whether the jacket reaches the wine that needs it:

  • Coverage against working volume. A jacket sized for a full tank does less at a partial fill, which is exactly the condition during the fill of a red ferment. The OT Conical Fermenter puts cooling on the conical base for this reason — the first load is cooled from the first day, not once the tank is full.
  • Coverage against the cap. In red fermentation the heat is generated in and around the cap, at the top. A jacket that only wraps the lower body is cooling the coolest part of the tank.
  • The other eleven months. The same jacket that manages a ferment in March is doing cold stabilisation in June and holding a temperature through summer. A tank specified only for its fermentation duty is under-specified.

Glycol capacity, pipe routing and tank placement then have to resolve as one problem rather than three — which is the subject of Thermal Control Systems.

The short version

Reds 20–32 °C, whites 12–22 °C, nothing above 35 °C, nothing below 5 °C. Then spend the effort on the harder question: whether your cellar can hold a chosen number in one tank while holding a different one in the tank beside it. Everything else in this note is a means to that.

Formats and jacket options across the range are set out under tanks.