Carbonation

The ingredient that is not an ingredient, and the one that turns a bottle into a pressure vessel.

Two ways to get gas in

Forced: dissolve carbon dioxide into the liquid under pressure, from a cylinder, before or during filling.

Natural: let fermentation produce it, either in the tank or in the bottle.

The first is controlled and the second is not, and the difference is the whole of the risk.

Volumes of CO2

Carbonation is measured in volumes: how many volumes of gas are dissolved per volume of liquid.

A lightly sparkling drink is around 2, a typical soft drink around 3 to 4, and a highly carbonated one higher.

The pressure that corresponds to a given carbonation rises steeply with temperature, which is the number that matters. A bottle at a comfortable pressure cold can be at an uncomfortable one warm, and nothing about the liquid changed.

Which is why a carbonation specification has to be set against the warmest temperature the product will see, not the coldest.

Forced carbonation

Chill the liquid, which dissolves gas far better cold, apply CO2 at a pressure corresponding to the volumes you want, and give it time.

Then fill under counter-pressure if you can, because filling a carbonated liquid into an open bottle loses gas and foams.

It is reproducible, measurable and it stops when you stop, which is its great advantage.

Natural carbonation is the risky one

Fermentation in the bottle produces gas from residual sugar, and the amount depends on how much sugar is left and how warm the bottle gets.

That is not a specification, it is a range, and the top of the range is what the bottle has to survive.

It also produces alcohol at the same time, which is the other threshold.

Both are controlled the same way: less residual sugar, fewer live organisms, and cold. And both are tested the same way: a bottle held warm for the full shelf life, then measured, behind a screen.

References