Headspace, and why it is a safety number
Updated
In every other property in this estate headspace is about appearance and expansion. Here it is part of the pressure calculation.
Not a cosmetic decision
Headspace is the volume the gas has, and gas pressure depends on how much gas is in how much space at what temperature.
Which makes the fill level a variable in whether a bottle survives a warm afternoon.
Too little
An over-filled carbonated bottle has less room for expansion, so the pressure at a given temperature is higher.
In a live product that is still generating gas, a small headspace is consumed quickly and pressure climbs faster.
And liquids themselves expand with heat, so a nearly full bottle is pushing on the closure before any gas is considered.
Too much
More oxygen in contact with the product, which oxidizes flavor and color and shortens shelf life. In an unpasteurized product it also gives aerobic organisms somewhere to work.
More gas comes out of solution into a large headspace, so the drink is flatter at the same total carbonation.
And it looks under-filled, which customers notice and retailers mention.
Setting it
Pick a target from the container supplier's guidance for a carbonated product, and hold it with a tolerance.
Then validate it the way everything else in this property is validated: fill at the target, hold bottles at the warmest realistic temperature for the full shelf life, and see what the pressure does.
Check fill level through a run rather than at the start, because a gravity filler drifts as the reservoir empties.
And where the product is live, remember the headspace is being consumed by a reaction, so the safe fill level for a live drink is lower than for the same drink carbonated and stabilized.