Soft PET bottles when cold

[worry wort]

Yes, I've checked the forum and seen some answers but none explain to me why it randomly happens. If the rules of Boyles Law are the answer, why does it not affect a full batch of PET bottles? 

I am on a run. This is my 2nd batch in a row where my PET 750/1250ml, are hard as a rock in the storage rack and when I put them in the fridge, but once they cool only some of them go soft while others hold their pressure. I'm a simple working class man and don't want to wade through pages of chemobabble on it, I am just seeking a plausible, understandable cause and a possible way to avoid it happening. 

I currently have 8 batches down at various ages from 2 weeks to 3 months, all in PET.  I've had this occur before, then it seems to stop happening and the bottles stay hard right through to drinking. 

cheers

WW

 

[jamiek86]

have you ever marked the flat bottles and seen if always same ones doing it?

[Pickles Jones]

53 minutes ago, worry wort said:

I am just seeking a plausible, understandable cause and a possible way to avoid it happening. 

The riddle of the flat PET bottles is still looking for an answer.

I don't think anybody has found an definitive answer which when applied stops the problem which can be quiet random as you have found.

When you have a flat one don't tip it out, reprime it and change the cap for a brand new one and see what happens.

I gave up PETS and moved to glass. 

Edited January 25, 2022 by Pickles Jones

[Classic Brewing Co]

1 hour ago, worry wort said:

Yes, I've checked the forum and seen some answers but none explain to me why it randomly happens. If the rules of Boyles Law are the answer, why does it not affect a full batch of PET bottles? 

I am on a run. This is my 2nd batch in a row where my PET 750/1250ml, are hard as a rock in the storage rack and when I put them in the fridge, but once they cool only some of them go soft while others hold their pressure. I'm a simple working class man and don't want to wade through pages of chemobabble on it, I am just seeking a plausible, understandable cause and a possible way to avoid it happening. 

I currently have 8 batches down at various ages from 2 weeks to 3 months, all in PET.  I've had this occur before, then it seems to stop happening and the bottles stay hard right through to drinking. 

cheers

WW

 

I have been brewing in PET bottles for years, not by choice, purely for convenience or due to a shortage of glass bottles although that changes over time.

Occasionally we all seem to get the odd dud & although it is annoying the number of reasons is endless, it could be the bottle, the cap or the carbonation etc. Marking the bottle would help the elimination process but when I get one I bin the bottle - then it cannot happen again, it pays to replace caps now & again & more importantly after the first use break the seal off on the bottom of the cap before next use to ensure a tight closure.

The obvious answer is move to glass but having said that there are brewers that only use PET bottles, I am sure @MUZZY won't mind me mentioning him but he may also have a few tips for you. If you follow protocol everything should go to plan but nothing is perfect.

I mainly use glass these days but I always fill 2-3 PET from each brew purely for testing the carbonation.

Good Luck.

 

[Aussiekraut]

1 hour ago, worry wort said:

Yes, I've checked the forum and seen some answers but none explain to me why it randomly happens. If the rules of Boyles Law are the answer, why does it not affect a full batch of PET bottles? 

I am on a run. This is my 2nd batch in a row where my PET 750/1250ml, are hard as a rock in the storage rack and when I put them in the fridge, but once they cool only some of them go soft while others hold their pressure. I'm a simple working class man and don't want to wade through pages of chemobabble on it, I am just seeking a plausible, understandable cause and a possible way to avoid it happening. 

I currently have 8 batches down at various ages from 2 weeks to 3 months, all in PET.  I've had this occur before, then it seems to stop happening and the bottles stay hard right through to drinking. 

cheers

WW

 

It's fairly normal to see PET bottles go soft in a fridge. As they cool, more CO2 gets absorbed into the beer and the pressure in the bottle drops. If you take them out of the fridge and let them warm up, they'll go hard again as CO2 is released from solution.

If it doesn't happen to all bottles, I'd suspect differences in the carbonation levels  

[Malter White]

@Classic Brewing Co Thanks for the vote of confidence but I don't have a hard and fast remedy. I just polished off a soft bottle this afternoon in fact.
So sorry @worry wort, I can't offer you much except to say I just drink them and throw away the cap from the offending bottle in the hope it won't happen again. The number of fails I've had has been negligible so I just tolerate them.

[Classic Brewing Co]

21 minutes ago, MUZZY said:

@Classic Brewing Co Thanks for the vote of confidence but I don't have a hard and fast remedy. I just polished off a soft bottle this afternoon in fact.
So sorry @worry wort, I can't offer you much except to say I just drink them and throw away the cap from the offending bottle in the hope it won't happen again. The number of fails I've had has been negligible so I just tolerate them.

Cheers Muzzy, as you know it's all part of the process, given the price we are actually producing 740/750ml's of beer for it is a small price to pay if one doesn't cut it, just throw the offending item out & move on.

[ozdevil]

2 hours ago, Aussiekraut said:

It's fairly normal to see PET bottles go soft in a fridge. As they cool, more CO2 gets absorbed into the beer and the pressure in the bottle drops. If you take them out of the fridge and let them warm up, they'll go hard again as CO2 is released from solution.

If it doesn't happen to all bottles, I'd suspect differences in the carbonation levels  

@worry wort

i have to agree with @Aussiekraut on this .

and others like @jamiek86 and @Classic Brewing Co  have given you good answers to try

but when you opened the soft bottles have they got carbonation




 

[Shamus O'Sean]

20 hours ago, ozdevil said:

but when you opened the soft bottles have they got carbonation

That's the first thing I wondered too.  I am guessing that they are on the flat side though because if the bottles were soft, but adequately carbonated, @worry wort probably would not have raised the question.

Along with the other issues and possible solutions noted above, I reckon slightly increasing your priming sugar rate might help get more consistent results.

[ozdevil]

3 minutes ago, Shamus O'Sean said:

That's the first thing I wondered too.  I am guessing that they are on the flat side though because if the bottles were soft, but adequately carbonated, @worry wort probably would not have raised the question.

Along with the other issues and possible solutions noted above, I reckon slightly increasing your priming sugar rate might help get more consistent results.

true...  i am more on the theory of Aussiekruat on this    

as well as marking the caps on these and bottles  to eliminate them  

you only have to  cross thread one to lose carbanation

[worry wort]

On 1/25/2022 at 4:33 PM, jamiek86 said:

have you ever marked the flat bottles and seen if always same ones doing it?

 A few batches back I started doing this. i'd put an x on the bottle and a small dot on the lid.  I'd open the bottle, drop a sugar cube in, seal with a new lid, give it a shake, then put it back on the shelf for a period and then back in the fridge. Sometimes I would get softies before going to the fridge, usually an issue with the lid or the bottle itself, but it didnt explain to me why the ones that went flat in the fridge held their pressure prior to chilling.

[worry wort]

On 1/25/2022 at 5:09 PM, Pickles Jones said:

The riddle of the flat PET bottles is still looking for an answer.

I don't think anybody has found an definitive answer which when applied stops the problem which can be quiet random as you have found.

When you have a flat one don't tip it out, reprime it and change the cap for a brand new one and see what happens.

I gave up PETS and moved to glass. 

the annoying this is that I've been using plastics since moving to Darwin in 95, no longnecks there, so to brew had to use 1250ml soft drink bottles. Never had an issue over all those years until the past 12 months. I do actually mark the bottles and caps of chilled but now soft bottles, pop in a sugar cube, new cap, shake and leave on shelf for a few weeks then back in fridge. I recently managed to pick up 90 grolsh bottles so I can see at least 2 brews being back in glass. I

[worry wort]

On 1/25/2022 at 8:38 PM, ozdevil said:

@worry wort

i have to agree with @Aussiekraut on this .

and others like @jamiek86 and @Classic Brewing Co  have given you good answers to try

but when you opened the soft bottles have they got carbonation




 

minimal. There is a slight 'pfftt' when opening, beer still taste ok and there is a bit of life in the poured item, but not much, and there is no head. I greatly appreciate the answers given by the guys, but I can't get my head around why if there were an existing issue between bottle and/or cap, why would the gas not escape prior to chilling (and it does on some bottles, which i then deal with by replacing the cap and re-priming with a sugar cube), and if there is a problem with the plastics contracting in the fridge, why is there no leakage of the beer? I still drink it, usually open a 2nd bot and have a 50/50 mix.

 

[worry wort]

On 1/26/2022 at 4:55 PM, Shamus O'Sean said:

That's the first thing I wondered too.  I am guessing that they are on the flat side though because if the bottles were soft, but adequately carbonated, @worry wort probably would not have raised the question.

Along with the other issues and possible solutions noted above, I reckon slightly increasing your priming sugar rate might help get more consistent results.

thanks Shamus, the only issue with the priming sugar amount (I bulk prime) is that those that don't go flat are brilliant, good head and ringss all down the glass, its just one of lifes little mysteries....

 

[Tricky Micky]

7 hours ago, worry wort said:

On 1/26/2022 at 4:55 PM, Shamus O'Sean said:

That's the first thing I wondered too.  I am guessing that they are on the flat side though because if the bottles were soft, but adequately carbonated, @worry wort probably would not have raised the question.

Along with the other issues and possible solutions noted above, I reckon slightly increasing your priming sugar rate might help get more consistent results.

thanks Shamus, the only issue with the priming sugar amount (I bulk prime) is that those that don't go flat are brilliant, good head and ringss all down the glass, its just one of lifes little mysteries....

Hey Dubbya Dubbya, good thread for us PET users. I've had a couple of these happen as well - in fact getting a few almost every batch now...Volume about 46 liters split between two Coopers FV's. I bulk prime in the primary FV, so my question to you would be are you doing the same or do your rack to a secondary and add the sugar mix then?

I pour the sugar dissolved in about 300-400mls of preboiled water into the wort in the primary vessel and stir very gently so not to disturb the trub and so not to aerate the wort...I let that sit for an hour and then bottle. I've done all the discarding of the tops and bottles stuff and it still happens. So I'm thinking of changing my priming method, at least to see if that brings about a change. I've had quite a few of what you're describing - where the bottles seem to lose carbonation whilst in the fridge.....

I'll keep you posted.

[Malter White]

54 minutes ago, Mickep said:

Hey Dubbya Dubbya, good thread for us PET users. I've had a couple of these happen as well - in fact getting a few almost every batch now...Volume about 46 liters split between two Coopers FV's. I bulk prime in the primary FV, so my question to you would be are you doing the same or do your rack to a secondary and add the sugar mix then?

I pour the sugar dissolved in about 300-400mls of preboiled water into the wort in the primary vessel and stir very gently so not to disturb the trub and so not to aerate the wort...I let that sit for an hour and then bottle. I've done all the discarding of the tops and bottles stuff and it still happens. So I'm thinking of changing my priming method, at least to see if that brings about a change. I've had quite a few of what you're describing - where the bottles seem to lose carbonation whilst in the fridge.....

I'll keep you posted.

Hi Mick.
Are your bottles hard before going iton the fridge and then going soft? I doubt your priming method would be the cause of softening. If they're soft before going into the fridge then it could be a possibility your bulk priming isn't consistent through the whole batch.
I guess you could try priming each individual bottle for a batch or two and see if the results change.

Edited January 29, 2022 by MUZZY

[Tricky Micky]

Muzzy,

the bottles I'm talking of are carbonated at ambient temp - placed in a fridge and days later soft as. Just as WW has pointed out. _ We both bulk prime....I'm careful to mix thoroughly when I add the sugar mix to the wort - but again not so much that I aerate the wort or disturb the trub....

I am going to try the sugar cubes straight into the bottles and see if that changes things....I do get the odd flatty here and there during secondary fermentation but nothing to get over excited about.

 

 

[Shamus O'Sean]

19 hours ago, worry wort said:

thanks Shamus, the only issue with the priming sugar amount (I bulk prime) is that those that don't go flat are brilliant, good head and ringss all down the glass, its just one of lifes little mysteries....

 

As you are bulk priming I can understand your frustration.  It does suggest, it is more likely to be something about the bottles/caps.

[iBooz2]

On 1/29/2022 at 3:11 AM, worry wort said:

thanks Shamus, the only issue with the priming sugar amount (I bulk prime) is that those that don't go flat are brilliant, good head and ringss all down the glass, its just one of lifes little mysteries....

Arh Har - there lies your problem, bulk priming and there is a scientific reason I do not do it with beer.

It is almost impossible to completely mix the sugar solution through the beer to get even distribution of the higher gravity sugar in every bottle.  You have a much better chance if you decant the beer off the yeast into another container and be able to mix it more thoroughly.

If you are just stirring the sugar solution in and trying not to disturb your yeast cake then you have pretty much no chance of even sugar amount in every bottle.  The sugar solution being a much higher gravity than your beer (maybe 1.060 or more whereas your FV beer maybe 1.010) will want to go pretty much to the bottom of your FV; the physics alone tells you that.  Stirring will arrest this phenomenon somewhat but not enough.

PET bottles are just plastic balloons and stretch when pressurized.  The problem bottles you are experiencing have not got enough sugar mix to overcome the bottle stretch and also provide enough head space pressure to force CO2 into the beer solution in those bottles and whats more important, maintain that pressure (carbonation) when the beer is subsequently chilled.  The beer in all bottles absorbs much more CO2 from the head space than if they were warm, hence some go soft when cold.

If you were to number and label your bottles at filling from say 1 to 30 you would find the first ones will be the best ones and somewhere near the end say 25 - 30 they will exhibit your problem.  WHY? Because the first filled bottle get slightly more than their fair share of the added sugar and the suspended yeast giving more than enough CO2 pressure to do the job even when chilled.  The last ones get short changed a bit so the head space pressure is not enough to overcome this absorption when chilled. 

If you were to line up your just filled bottles for 1 to 30 and do an accurate temperature adjusted hydrometer check on each one you will find they will all be different.

 

 

Edited January 29, 2022 by iBooz2

[iBooz2]

8 hours ago, Shamus O'Sean said:

As you are bulk priming I can understand your frustration.  It does suggest, it is more likely to be something about the bottles/caps.

I do not remember ever having a bottle or cap fail in any of my brews using PET's, always turned out to be a sugar problem.  If I subsequently added extra sugar and used the same cap = problem solved every time.

I say to @worry wort , forget the bulk prime for a few brews and use measured sugar OR 2 x cubes OR 3 x carb drops for 740 ml PET's (typical pale ale or lager) and you will see the difference.  I think nil, zip, zero soft bottles / duds if you do.

Edited January 29, 2022 by iBooz2

[Tricky Micky]

2 hours ago, iBooz2 said:

Arh Har - there lies your problem, bulk priming and there is a scientific reason I do not do it with beer.

It is almost impossible to completely mix the sugar solution through the beer to get even distribution of the higher gravity sugar in every bottle.  You have a much better chance if you decant the beer off the yeast into another container and be able to mix it more thoroughly.

If you are just stirring the sugar solution in and trying not to disturb your yeast cake then you have pretty much no chance of even sugar amount in every bottle.  The sugar solution being a much higher gravity than your beer (maybe 1.060 or more whereas your FV beer maybe 1.010) will want to go pretty much to the bottom of your FV; the physics alone tells you that.  Stirring will arrest this phenomenon somewhat but not enough.

PET bottles are just plastic balloons and stretch when pressurized.  The problem bottles you are experiencing have not got enough sugar mix to overcome the bottle stretch and also provide enough head space pressure to force CO2 into the beer solution in those bottles and whats more important, maintain that pressure (carbonation) when the beer is subsequently chilled.  The beer in all bottles absorbs much more CO2 from the head space than if they were warm, hence some go soft when cold.

If you were to number and label your bottles at filling from say 1 to 30 you would find the first ones will be the best ones and somewhere near the end say 25 - 30 they will exhibit your problem.  WHY? Because the first filled bottle get slightly more than their fair share of the added sugar and the suspended yeast giving more than enough CO2 pressure to do the job even when chilled.  The last ones get short changed a bit so the head space pressure is not enough to overcome this absorption when chilled. 

If you were to line up your just filled bottles for 1 to 30 and do an accurate temperature adjusted hydrometer check on each one you will find they will all be different.

 

 

Thanks Al, I appreciate the really clear explanation on why bulk priming from the primary FV may not be the way to go. I've read quite a lot about the carbonation problems from BP in the primary FV but your version of the issues which could be at play here are lot easier to understand - so thank-you. I've just bulked primed my latest 46 liter batch and it will possibly be the last. I'll be switching most likely to 2xsugar cubes (2x4.5g =9g/l) that's about where I like my level of carbonation - well at least for the style of beer I produce. 

Of course the other alternative is to rack into secondary vessels and mix the sugar in that way.  

[iBooz2]

20 minutes ago, Mickep said:

Thanks Al, I appreciate the really clear explanation on why bulk priming from the primary FV may not be the way to go. I've read quite a lot about the carbonation problems from BP in the primary FV but your version of the issues which could be at play here are lot easier to understand - so thank-you. I've just bulked primed my latest 46 liter batch and it will possibly be the last. I'll be switching most likely to 2xsugar cubes (2x4.5g =9g/l) that's about where I like my level of carbonation - well at least for the style of beer I produce. 

Of course the other alternative is to rack into secondary vessels and mix the sugar in that way.  

Yep Mick if you want to BP I think you have to rack to a secondary first and accept the risk of oxidation.   Be good if there was a gadget like a circular aquarium air stone that you could put in the bottom of the secondary then push CO2 through it to bubble up, that way mixing the sugars proper and keeping atmosphere out of contact with the beer.   You could then run this until the last bottle was filled.

Way back when I used to make wine it was the only time I did a BP.  Most of those were sparkling wines so any over carbing was a bonus.   Plus you had to rack it off all the skins and crud anyway as its a much more messy affair than beer.

Edited January 29, 2022 by iBooz2

[Tricky Micky]

5 minutes ago, iBooz2 said:

Yep Mick if you want to BP I think you have to rack to a secondary first and accept the risk of oxidation.   Be good if there was a gadget like a circular aquarium air stone that you could put in the bottom of the secondary then push CO2 through it to bubble up, that way mixing the sugars proper and keeping atmosphere out of contact with the beer.   You could then run this until the last bottle was filled.

Way back when I used to make wine it was the only time I did a BP.  Most of those were sparkling wines so any over carbing was a bonus.   Plus you had to rack it off all the skins and crud anyway as its a much more messy affair than beer.

Cheers Al,

Bulk priming beer into a secondary vessel comes with it's own set of problems as you say.  It may solve the dreaded carbonation inconsistencies but then I'd be dealing with the risk of introducing extra oxygen or most likely increasing the risk of an infection as well. In the end 2 sugar cubes in each PET will probably be the way to go for me. I mean at some stage I want to keg anyway when finances pick up a bit.  Been a long couple of years in my industry - smashed by Covid, hopefully all about to pick up again now.

[DavidM]

Fantastic Info.

I switched to bulk priming and was getting good results except for more cloudy beers because of the stir.

I have spare FV's now and have racked off into a secondary FV

Beer looks much clear going into the bottle, will know how it works in a week or so.

Thanks

[stquinto]

On 1/29/2022 at 11:57 PM, Mickep said:

Of course the other alternative is to rack into secondary vessels and mix the sugar in that way.

That's what I do. Dissolve the sugar in to about 300ml of water, cool it down a bit, and put that into a sanitised FV. Then I decant the finished beer directly onto it with a tube from the tap of the FV that has the beer. That way it doesn't splash up too much. I assume that way the beer swirling around it as it fills will mix it in reasonably. Then I bottle.

I only bottle in glass because I have a shed load of them (good , heavy Belgian returnables), and I rarely get any flatties so I am assuming that the priming sugar solution is getting mixed in OK.