Cautionary tale

[Taylog1]

I've not come across this problem before (entirely of my own carelessness), but I thought I'd share it for information.

I tend to bottle my cider in 500 ml beer bottles with a crown cap. It's fermented to complete dryness and still.

As it can be quite acidic I often leave some of the crates in a side shed to mature for a couple of years - which much improves them.

I fill them by hand with a plastic tube, and so occasionally they might get filled right to the top. Historically I've focussed on not leaving too much of an air gap, not worrying about too little.

I've just came back from holiday, over a period when the temperature in the shed probably got up to near 40C, to find say 5% of the bottles have broken - not exploded, but cracked all around the base - the glass is all there, but the cider has all leaked out.

 I'm pretty sure its the ones that were overfilled - the liquid has expanded in the heat and rather than the cap coming off the bottle has given along the corner, its weak point. 

I've been making cider for 6 years (say 500 litres a year), but this is the first time I've noticed it in any numbers.  

What's the recommended headroom for still cider in a beer bottle, 1-2 inches ?

         

[cghoe...@gmail.com]

Our bottler leaves about 1 - 1.5" of headspace in the bottle.  We force carbonate to 4.78g/L CO2 in our finished cider.  We bottle with starting cider temp at or near 0C then bath pasteurize (~61C) and have only broken one bottle during the pasteurization process that I can recall over several years.  All of that is to say we have significant pressure and temperature in our finished cider without bottle issues.  Did you fill to the actual top with no headspace and were you using new bottles or used?

[Taylog1]

I had filled a few to the absolute top accidentally (ie slight overflow when filling and I didn’t then pour a small amount out). Bottles were new - the thermal expansion of the cider makes sense, it’s the same as bottles breaking when frozen.  I’ll just make sure I leave a 1 1/2 inch gap in future.

[Claude Jolicoeur]

1-1/2 inch is probably overkill. 3/4 to 1 inch is plenty sufficient as volume buffer to counteract the temperature dilatation effect.

The scientific principle is that the liquid is not compressible - this mean that increasing the pressure of the liquid does not reduce its volume.

On the other hand, the air in the top of the bottle is compressible and the volume will reduce as the pressure increases.

So, as the temperature increases, the cider expands and takes some of the volume occupied by the air. The air pressure will thus increase in the same ratio as the reduction of volume.

The gaz law is: P1 V1 / T1 = P2 V2 / T2 ; or: P2 = P1 (V1/V2) (T2/T1)

So, if V2 is half the original air volume, P2 will be 2 ATM if P1 was 1 ATM. (we can neglect the T differences here).

We can see that if V1 was very small, V2 could then tend towards zero, making P2 extremely large, causing a bottle burst.

[Terry Chalk]

For your information (this is a little off-topic but relevant), Andrew Lea has an excellent Carbonation Table (you will probably find it via google or a past post here) which calculates the pressure generated at different temperatures for given volumes of C02 in a sealed bottle.

I use this table and monitor the pressure in my primed and sealed bottles to determine when the CO2 level is where I want to pasteurise. e.g. if I prime to 1.010, when the bottles pressure is between 1.5 and 2.0 bar there will be between 2.0 and 2.5 volumes of CO2. At this point the SG should be around 1.005 or about 10g/L remaining sugar for a semi dry carbonated cider, so it is time to pasteurise. This method is possibly a bit rough for commercial purposes, but I find it is fine for my amateur craft cider making.

[rhand...@rockisland.com]

 My take would be to fill the bottle to its rated volume and use that as a gauge to fill. You might be surprised at the fill level.

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[cghoe...@gmail.com]

Which is why we don't fill to that level; too low in the bottle...

[Ryn Eichenlaub]

Headspace matters! For exactly the reasons Claude mentions - liquid expands when it gets warm but isn't compressible. This is literally the principle behind hydraulics. For other producers it's worth noting that this applies to kegs too. If you fill your kegs to the top and then they get warm (sit on a sunny loading dock) or they get dropped in handling the liquid inside can exert a LOT of pressure and either booger up your CO2 valve or dome out the top or bottom of your keg. If you're a pro who works with kegs and you've been filling them flat on the floor to overflow I recommend getting in touch with micromatic, they've been so great working with us to educate our team and implement GMPs with our kegs. 

No matter the packaging IMO you're better off blowing a little CO2 in before filling, and leaving a little more headspace than topping up.

With regards to the original question, if your bottles have a defined shoulder & neck I would fill until the level just comes into the neck, minimum surface area and maximum headspace. 

Ryn