Stovetop pasteurising bottle pressure?

[Chalkyt]

Has anyone actually measured the pressure that develops in the bottles during stovetop pasteurising?

After a couple of years fluffing about with it, I am going to try stovetop pasteurising. Down here we are at the end of Spring and of course don't have any apples so I thought I could "experiment" with store bought juice (If it goes a bit pear shaped... no pun intended, I haven't lost any precious fresh pressed juice!)

However yesterday in another thread, RPh Guy raised the issue of pressure vs temperature, which I hadn't really thought much about about.

I hope that this doesn't get too technical but going back to thermodynamics 101 ( a loooong time ago), I understand that a modest 2 volumes of CO2 at pasteurising temperature of 65C (149F) should generate a bit over 6 atm (95psi) of pressure.

It seems that beer bottles are considered to be O.K. up to maybe 5 atm (say, 75psi), however the stovetop pasteurising temperatures that people have used successfully (175F ) suggest in theory that the pressure could go well above 6 atm (90psi). So, I wonder if this is really what happens. If these pressures are reached , how close are they likely to be to bottle bomb territory?

I don't want to be a scaredy cat but I also don't want to be beaten up by SWMBO for wrecking the kitchen. All advice and comments are welcome!

 

[RPh_Guy]

I understand that a modest 2 volumes of CO2 at pasteurising temperature of 65C (149F) should generate a bit over 6 atm (95psi) of pressure.

No, it's 68 psi or 4.6 atm

https://www.hopsteiner.com/psi-calculator/

Hope this helps.

 

[Miraculix]

I did this once and many caps showed outgoing foam due to overpressure, they sealed again after cooling down. Two bottles broke, but did not explode, it was more like a falling apart. However, I would do it in a pot big enough to keep all the bottles inside and cover it with a metal or plastic lid (not glass, for obvious reasons).

I think one thing to consider as well it's temperature shock. It's not good for glass to be rapidly heated or cooled, this can make the bottles crack.

 

[Maylar]

Has anyone actually measured the pressure that develops in the bottles during stovetop pasteurising?

Yep, and I posted results here: https://www.homebrewtalk.com/forum/threads/cider-carbonation-experiments.513435/

It's a bit obsolete now because that was back when 180° was the target temp.

 

[Chalkyt]

Thanks for the replies... they help heaps. Even my ebay search for a gauge has been an education. Do I want a BSPT or NPT connection, Kg/cm2, Bar, psi, the mind boggles.

Thanks RPhGuy for that reference, it is easier than trying to find Henry's constant for CO2 at different temperatures then doing the maths.

And the second reference from Maylar is just what I was looking for. I thought I had seen it somewhere but couldn't find it.

Mind you, during my deliberations I decided to open one of the mixed apple ciders that had been sitting for about six months... yum, just the right amount of carbonation and a touch of sweet. All I need to do now is replicate it!

 

[voidyourwarranty]

Yep, and I posted results here: Cider carbonation experiments

It's a bit obsolete now because that was back when 180° was the target temp.

I realize this is really old but I'm curious if you ever ran the tests with flat liquid as well. Curiosity has me on what the results would be.

 

[Chalkyt]

No, I haven't looked at flat cider. I suppose you might be looking at what happens if you heat pasteurise still cider to kill off any pathogens, whereas my interest has been to find an easy way to produce sweet, carbonated cider. i.e. stop fermentation when the SG is at a particular sweetness level (grams of sugar per litre).

Since the original post back in 2019 I came across Andrew Lea's Excel Carbonation Table which calculates bottle pressure for given carbonation levels at different temperatures. I got it from a reference in a paper by Andrew Lea via cider.org.uk/carbonation_table.xls. I have a copy on my laptop and use the table often, but today when I checked to see if it was still on the website, I couldn't find it. Unfortunately we can't attach .xls files to posts on this forum.

Anyhow, to answer your question, the spreadsheet calculates 0.14bar (2psi) for still or flat cider with 1 volume of CO2 at 20C. This is typical bottling temperature and is based on the cider inherently only having 1 volume of CO2 or less (i.e. it is flat). These days a pasteurising temperature of 65C for 10 minutes is considered to be enough to generate 50PU which is the recommended level for pasteurising cider. At 65C the table shows 2.4bar or 35psi will be created in a bottle of uncarbonated cider, which is about the normal pressure for carbonted drinks (beer, soda, etc), so well inside the pressure limit for typical bottles.

If you can't download the table and want to pursue it further, I can post the formulae that Andrew uses in the table so you can make your own.