Temperature control: glycol, coils and a cold room

The standard commercial arrangement is a chiller producing cold glycol, a pump circulating it through insulated pipework, and a jacket on each tank with a solenoid valve controlled from a temperature probe in a thermowell in the cider. The jacket is a dimpled or channelled sheet welded to the outside of the shell, so nothing enters the product and nothing has to be cleaned. It is the cleanest solution in every sense and the most expensive.

An immersion coil is the cheap version: a stainless coil lowered into the vessel with cold water or glycol pumped through it. It works extremely well, because it puts the heat transfer surface directly in the liquid rather than behind a wall, and it can be moved between vessels. It has one real drawback, which is that it is a complicated shape in contact with the cider that must be cleaned and sanitised like anything else, and a coil that is put away dirty is a contamination source with a large surface area.

A third arrangement pumps the cider out through a plate chiller and back in. It cools quickly and needs no vessel modification, and it is the least gentle of the three: every pass is a pumping operation with its shear, its potential for air ingress and its cleaning burden. It is a reasonable way to knock a runaway ferment down and a poor way to hold a temperature continuously.

A cold room, an insulated shipping container with a refrigeration unit, or a cellar that is simply cold does something the tank-side methods do not: it controls everything in it at once, and it goes on doing so through maturation, cold stabilisation and storage. For a producer with a dozen small vessels rather than three large ones, that is a much better match to the problem.

It responds slowly, which cuts both ways. A room takes hours to change temperature, so it will not chase a ferment that has taken off; equally it does not overshoot, and it rides out a warm afternoon without anyone doing anything. The fermenting cider will still sit above room temperature by an amount that depends on vessel size, so the room is set below the target rather than at it.

The traditional version of this is the stone cider house in a cold autumn, and it is worth saying plainly that it works. West Country practice of pressing late and fermenting through the winter in an unheated building produces the long, cool ferment that modern equipment is bought to reproduce. Where a producer has such a building, the equipment question is largely about summer storage rather than about the ferment.

The last two rows are not jokes. Evaporative cooling and a water bath with ice in it are what a great many small producers actually use, and a water bath in particular is effective out of proportion to its cost, because water has a high heat capacity and surrounds the vessel completely. What they cannot do is hold a temperature without someone attending to them.

The circulating fluid in a jacket or coil is normally water with a freezing-point depressant, and which depressant matters. Propylene glycol is the one used in food and beverage plant where accidental contact with product is conceivable; ethylene glycol is toxic and has no place in a product area at all. Concentration, corrosion inhibitor package and the resulting freeze point and viscosity are the fluid supplier’s specification, and a system run at the wrong concentration either freezes in the chiller or transfers heat badly. This is a supplier conversation, not a remembered number.

Two arrangement details decide whether the control works. The first is where the temperature probe sits: a probe in a thermowell in the liquid measures the cider, a sensor on the outside of an insulated tank measures the insulation, and a thermostat on the wall measures the room. A control loop is only as good as what it is reading. The second is where the cooling is applied — a jacket low on a tank cools the bulk and sets up convection, and a tall tank cooled only at the top will stratify and hold a warm layer at the bottom where most of the yeast is.

It is also worth saying that cooling a ferment too hard is a real failure mode. Dropping the temperature quickly can shock the yeast into sluggishness or drop it out of suspension altogether, and a ferment that has stalled from a cold shock is harder to restart than one that was simply cool throughout. Control means holding a temperature, not chasing one.