Cider carbonation experiments

[Maylar]

Over the last few months I've been conducting some experiments in cider bottle carbonation, and I thought I’d share my data with the community.

The focus of my experiments is in 3 areas. First, to come up with a way of quantifying the buildup of CO2 pressure rather than using the "squeeze the PET bottle" method. Secondly, I wanted to determine the safe level of carbonation before stovetop pasteurization, and thirdly to document the change in specific gravity when sweetening / priming / pasteurizing.

Being an engineer, I came up with this gizmo to give me some numbers –

It’s just a 60 PSI pressure gage attached to a soda bottle cap. The hardware consists of a 10-32 to 1/8 NPT adapter and the gage, both available from McMaster-Carr.

Stovetop Pasteurization

My first tests were to ferment some sugar water and let pressure build to 25 PSI @65°F (2.4 volumes). Then I plunked the bottle into a pot of 175° water to see what would happen. The towel in the pot is to isolate the bottle from the hot bottom surface;

Unfortunately, when the pressure got to about 40 PSI the bottle “popped” out the bottom:

Bummer. I repeated this 3 times with the same effect. So, I adapted my gage to a capped glass bottle and tried again:

This one had been primed to 1.010 and allowed to reach 22 PSI @ 65F, which should be 2.21 volumes. I put it into a pot of hot water drawn from the tap in my sink, and after 10 minutes at 122°F the pressure was up to 45 PSI. I replenished the tap water and after 7 minutes at 136°F the pressure got up to 55 PSI and was still rising. This is about the maximum safe pressure of a used beer bottle, according to web research I’ve done and I’m nowhere near 180° yet. If anyone has authoritative data on that, please chime in.

Calculations support my numbers to some extent. According to this web page:

http://www.brainlubeonline.com/GasLawsBeer.html

...and this formula:

P = -16.6999 - 0.0101059 T + 0.00116512 T^2 + 0.173354 T V + 4.24267 V - 0.0684226 V^2

There should be 64.3 PSI @ 136° and 2.21 volumes of CO2, and I think my glass bottle would have reached that if I gave it more time.

So, what is a safe level of carbonation that won’t bust bottles at 180°F? That depends on how good your bottles are. From searching the web, it seems that all bottles are not created equal. Some say 4 bar (58 PSI) is the limit, others say that breweries test their bottles to 100 PSI. And according to calculations, a meager 2.0 volumes (mild carbonation IMO for cider) will reach 90 PSI @ 180.

SG After bottle carbonation –

I did another set of tests to take a look at how much sugar (as measured with a hydrometer) is left after bottle conditioning. I was surprised to find that there was very little change in SG before and after. Example, 2 teaspoons of table sugar in 12 oz of water reads exactly 1.010. After carbing to 25 PSI, I measured it again and it was still 1.010, or maybe 1.009. I had 2 bottles in that test and both were the same. The actual carbonation in those samples was just what I like, and 25 PSI is when I stick ‘em in the fridge.

Observations –

-The pressure after exposure to heat did not return to the same level as before, it was higher than when I started. My theory on that is the yeast goes into overdrive and continues to add carbonation up till the point where they “die”.

-Squeezing the PET bottle, 15 PSI was “firm” and 20 PSI was “hard”, according to my feel. 25 PSI is rock hard.

-Stovetop pasteurizing is a risk. (Duh). I like my ciders carbonated to about 2.4 volumes, and there’s no way to get there and safely expose bottles to 180°F.

I hope this info proves to be useful to some of you.

 

[D-west]

Awesome post! Makes me feel a lot better about spending $300 on my kegging setup.... haha I now just counter pressure fill my bottles and keep them in the cold garage.

 

[rocketsan]

This thread reminds me that I'm part of the dummy sector of homebrewing...

 

[Tkeonce]

So how do you carbonate then and store safely?

 

[Maylar]

So how do you carbonate then and store safely?

You can carbonate dry cider by using the correct amount of priming sugar. It's when you want it sweet and carbonated that's the problem.

Sweet can be done with non fermentable sugar, like Xylitol. That's what I use.

Or, sweeten and prime with whatever and refrigerate the bottles when they're carbonated. I'm not sure how long you can safely store cold cider, but I have had bottles in my 40° fridge for over a month that didn't over carbonate.

Or, take your chances with pasteurization and hope that you have strong bottles.

Of course if you can keg and force carbonate, you have other options.

 

[kgressler]

Nothing to add here but to say thanks for all the time it took to do these experiments.
This is why I will not try to pasteurise any bottles any time soon.

 

[Archer]

Great info! Now a dumb question: how did you attach the guage to the beer bottle cap?

 

[phug]

Better yet, how did you cap it with the gauge in place

 

[Maylar]

I capped the bottle, then epoxied a 1/8" thick piece of aluminum to the cap and drilled & tapped it for the 10-32 threads on the adapter.

 

[phug]

Ah that makes sense, no expense spared on the epoxy and none on the Teflon tape I hope