Bottled beer flat after 4 weeks

[Sqid]

After 3 or 4 days of not drinking my homebrew I came back to it and it was flat. I had drunk about 15 litres which were well carbonated but the last 5 bottles had no bubbles what so ever.
I've been using PET coke bottles. I understand that they are porous to a certain extent but that suddenly?
I put the priming sugar solution into the secondary fermentation bucket before bottling. Maybe not mixed up enough.
I used Kveik Voss and just splashed the beer into the fermenter for oxygenation.
I thought maybe I hadn't screwed the top down hard enough but seems unlikely that I had reserved that feat for the last 5 bottles only.

Any ideas would be appreciated


Disclaimer: There were no beer deaths or losses during the making of this (as "flat" was still was still preferable to what I can buy!)

 

[RM-MN]

The lids of the PET bottles need to be tightened more than you think or the seal will leak out the carbonation. Some might still leak even tightened. Develop gorilla arms so you can really tighten them.

 

[IslandLizard]

I put the priming sugar solution into the secondary fermentation bucket before bottling. Maybe not mixed up enough.

You mean: "bottling bucket," right?

I used Kveik Voss and just splashed the beer into the fermenter for oxygenation.

That was right after you brewed the batch, not when you were bottling, I hope?

I thought maybe I hadn't screwed the top down hard enough but seems unlikely that I had reserved that feat for the last 5 bottles only.

That is odd, yes.
Were those 5 bottles "special" in any way? Such as the first or last five you bottled? Or older bottles that were reused a few times already?
Or just totally random?

 

[Sqid]

I shall definitely screw the lids down full force next time. Thanks for the suggestion RM-NM
As I was writing, I was searching for the politically correct term. Of course there is no secondary fermentation going on in the bottling bucket. Apologies, IslandLizard, for using the wrong vernacular!
The selection of bottles were random but some had been used before. Does the permeability decay after several uses - I have considered this. I wonder if there is an extra layer on the inside of new bottles. Even after a year new coke bottles will still have plenty of fizz. Do you have any info about this?
I had something similar happen before. Came home after a 2 week break and the remaining beers were nearly flat. bland tasting and maybe a little darker in colour. They had been stored in the dark but at a high ambient temperature (82 F). This time they were stored in the fridge after bottle carbonation had completed.

 

[IslandLizard]

Coke bottles, soda bottles in general, are made of PET plastic. It's very stable, and very impermeable to CO2 and O2. There is no extra layer inside. Many fermentation vessels and even "kegs" are made out of PET, as large as 8 gallons, they are among the best (aside from stainless steel), and can last and (re)used for many, many years without any issues.

I had something similar happen before. Came home after a 2 week break and the remaining beers were nearly flat. bland tasting and maybe a little darker in colour. They had been stored in the dark but at a high ambient temperature (82 F). This time they were stored in the fridge after bottle carbonation had completed.

That's puzzling, indeed! The only place CO2 can escape is through the lid closure. As @RM-MN said, tighten them more.
Two things I can think of:

1. The plastic seal inside the lids gets worn out or damaged a little after a few closures. If that's the case, maybe you can get new replacement lids, and
2. Check the very top of the bottle's mouth, it should be perfectly flat, no burrs or having 2 tiny elevated "ridges" from where the mold halves meet. If there are 2 (small) ridges you can carefully file or sand them flat. That may solve the issue in #1 too, wearing the lid seals out after a few tightenings.

 

[Sqid]

Thanks for the thoughts Lizard. I've had a look at the lids and the mouth using a 10X but can't see any damage.

It's a little difficult to see very clearly because you need to get incident light to show the details. However it appears that the lids and the mouths use a flat topped ridge that then just distorts itself to perfectly match it's counterpart. Can clearly see where the mould halves meet and there was no problem in that area.

I'm going to get stainless and an oxbar keg to try with my next batch

 

[IslandLizard]

I'm going to get stainless and an oxbar keg to try with my next batch

The best thing about kegs are they're like a large bottle, and only one to fill.
But you do need a way to keep them cool to carbonate and serve, such as a fridge (kegerator) or externally temp controlled freezer (keezer). That means you also need a CO2 tank, regulator, hoses/connectors, taps, etc. Picnic taps and the like work well too. KegLand has some unique products. Study up on what's needed before committing. We have many threads with good info on that topic.

Read up on Oxebar "kegs" before committing to them. They're fairly cheap but have some drawbacks. We have a few threads and posts on them.

Use our search function.

 

[Sqid]

Many fermentation vessels and even "kegs" are made out of PET, as large as 8 gallons, they are among the best (aside from stainless steel), and can last and (re)used for many, many years without any issues.

Read up on Oxebar "kegs" before committing to them. They're fairly cheap but have some drawbacks. We have a few threads and posts on them.

Is there a better PET keg available.

The best thing about kegs are they're like a large bottle, and only one to fill.

Most important point.
Think I'll get a freezer with thermostat for cooling. I prefer to drink outside my home so I'm thinking maybe some smaller PET kegs that I can cool to near freezing and consume in one session without using a jockey box. Do those picnic ta[s work OK or is it worth going for a nukatap

 

[IslandLizard]

Is there a better PET keg available.

Not that I know of. The Oxebars are the first of their kind, quite unique, and affordable. I don't have any, I just have 5 gallon corny kegs.

I'm thinking maybe some smaller PET kegs that I can cool to near freezing and consume in one session without using a jockey box.

That's perfect. You will need a (small) portable size CO2 tank to take with you to dispense.
But for around the brewery you're better off getting a large CO2 tank (5 or 10kg) for routine carbonation, purging headspaces, transfers, etc.
You can easily fill the small portable tank from the large one.

Do those picnic ta[s work OK or is it worth going for a nukatap

Picnic ("Cobra") taps work fine for on the road, or around the house. Kegland makes those that can be connected to push-to-fit fittings (Duotight, John Guest, etc). Those connectors will fit EVA Barrier (beer and gas lines) so you keep oxygen out of your dispensing system.

Nukataps (and other real taps) need to be mounted on or in something, as they need a beer shaft shank to connect to. Do some searches (here or elsewhere), plenty of information, options, and ideas.