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Equipment

Thermometers and temperature measurement

How should I measure fermentation temperature?

In short

Fermentation is exothermic, so cider in a vessel is warmer than the room around it, and how much warmer depends on the vessel size and the vigour of the ferment.

A room thermometer therefore reads the wrong thing. So does an adhesive strip on the outside of an insulated tank. The reading that matters comes from a probe in the liquid, or from a well that reaches into it.

For small batches an adhesive liquid-crystal strip on a thin-walled vessel is a reasonable compromise; for anything with real thermal mass it is not.

The instruments

Why the number matters

Fermentation rate rises steeply with temperature, and so does the risk of the ferment running away and stalling. Yeast under thermal stress produces more of the compounds nobody wants: higher alcohols, acetic acid, and in some conditions hydrogen sulphide. It also loses the volatile esters that a cool ferment retains, partly because they are made in different proportions and partly because a fast ferment blows them out of the vessel with the carbon dioxide.

Cool fermentation is the standard route to an aromatic cider, and the reason is straightforward: more of the fruit’s own aroma survives, and the yeast’s ester production is favoured over its fusel production. Traditional West Country practice arrives at the same place from a different direction — pressing in late autumn and fermenting in a cold stone building through the winter gives a long, cool, slow ferment without anyone having to control anything.

At scale the arithmetic changes entirely. A large tank cannot lose the heat it generates and will climb well above ambient without cooling. This is not a refinement; it is the difference between a controlled ferment and a stuck one.

Reading discipline

A probe introduced into a vessel is a contamination route. It is sanitised before it goes in, and the sanitiser is one that will not taint the cider at the small carryover involved. The better arrangement, where the vessel allows it, is to take a sample through a tap and read that, or to fit a permanent well.

Readings are taken at the same depth and at the same time of day if they are to be compared, because a tall vessel is not at one temperature. The bottom of a fermenting tank can be several degrees different from the top, and which is warmer depends on whether the vessel is being cooled and where the jacket sits.

Instruments drift. A probe checked occasionally against an ice-water bath and against boiling water — with the local boiling point in mind — is a probe whose readings can be relied on; one that has never been checked is a source of confident numbers of unknown accuracy.

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