Carbonation stones and in-line carbonation

Two variables govern the result and both have to be held. Raising the pressure above the cider pushes more gas into solution; lowering the temperature does the same, because gas solubility in a liquid falls as the liquid warms. A cider carbonated cold and then allowed to warm will push gas back out of solution and raise the pressure in whatever is containing it, which is the mechanism behind a great many surprises at the tap and in the pack.

The relationship between pressure, temperature and the resulting carbonation is well characterised, and equipment suppliers and gas suppliers publish charts for it. Those charts are the right source: they are specific to the units and conventions being used and they are attached to the equipment being operated, which is not true of a figure recalled from elsewhere.

The third variable is time. Gas does not dissolve instantly — it dissolves at the gas-liquid interface, and the rate depends on how much interface there is and how long the gas spends in contact with liquid that is not yet saturated. That is why the stone matters at all: it is a device for manufacturing interface.

A carbonation stone is a cylinder or disc of sintered stainless steel — metal powder fused into a rigid porous body — with pores fine enough to produce a mist of very small bubbles. Small bubbles have far more surface area per unit of gas than large ones and they rise more slowly, so they spend longer in the cider and dissolve almost completely on the way up. Gas fed through an open pipe instead simply produces large bubbles that surface and join the headspace, doing very little.

The stone is fitted low in the tank, on a wand through a top port or on a fitting in the wall, so that the bubbles have the full depth of liquid to travel through. Gas is fed slowly: pushing more gas through the stone than the cider can absorb just makes a bigger bubble and vents it. Carbonating is a patient operation measured in hours, and the tank is usually chilled first because cold cider absorbs more and foams less.

The tank must be able to hold pressure. This is where the most dangerous confusion in the whole subject lies: many fermentation vessels — open-topped tanks, variable-capacity tanks, plastic drums, glass carboys — cannot hold pressure at all, and connecting a gas supply to one is not carbonation but a way of bursting a vessel. Carbonation happens in a rated pressure vessel with a relief device, and nowhere else.

Stones are consumables and they blind. The same fine pores that make them work fill with yeast, protein and cleaning residue, and a stone that has blinded delivers gas unevenly or not at all. They are cleaned in place where the design allows, backflushed, and replaced when the pressure needed to pass gas has climbed.

On a packaging line the tank method is too slow and too imprecise. In-line carbonation injects gas into cider that is already flowing, through a nozzle, a porous element or a venturi, in a ratio set by measuring both flows. Downstream of the injection point the cider passes through a length of pipe, a static mixer or a contact vessel that gives the gas time to dissolve before it reaches the filler, and the whole line is held under enough back pressure that nothing breaks out on the way.

The advantages are control and speed. The carbonation level is a setting rather than the outcome of a long tank operation, it can be changed between products without emptying anything, and there is no waiting. The costs are the instrumentation — flow meters, control valves and usually an in-line carbonation measurement to close the loop — and the fact that a fault produces a whole run at the wrong carbonation rather than one tank.

The same principle appears in a much simpler form on small kit, where gas is introduced into the transfer line from a keg or a pressurised vessel to the filler. It works, and it is far less controllable, so most small producers carbonate in the vessel and then fill under counter-pressure rather than trying to carbonate on the way.

The last two rows are included because they are the alternative to owning any of this equipment, and because the trade is a real one. Fermentation-derived carbonation costs almost nothing in plant and leaves sediment and a slower, less exact process; forced carbonation costs a gas supply and a pressure vessel and delivers a clear, repeatable, adjustable product. Neither is the better cider, and both are made deliberately.

Over-carbonating is the commoner error and the more expensive one. A tank taken too far has to be vented, which loses gas and, if it is vented quickly, foams and loses cider as well; and cider that arrives at a filler over-carbonated fobs, fills slowly, and puts air into the neck as the foam collapses. Warm cider makes all of this worse, which is why carbonation and filling are done cold.

Under-carbonating is quieter and shows up in the pack. A cider filled below its intended carbonation cannot be corrected afterwards without opening it again, and a keg that pours flat because the equilibrium was never reached is usually diagnosed as a dispense fault when it is a carbonation fault.

Both are avoided the same way: carbonate to a measured value rather than to a time, keep the temperature steady while doing it, and measure the finished cider rather than assuming the chart. Carbonation can be measured in tank and in package, and a producer selling a carbonated cider needs some way of knowing what is in it.