PET vs glass bottles

[Terry]

Fair point, but doesn't stop the citizens of Adelaide attending Schuztenfest each year to down copius ammounts of German brews and wursts on an annual basis.

[bigmuzb]

It is not so much the CO2 leaking out but the O2 entering. Remember your gas laws, the concentration of the CO2 matters little to the oxygen, so it is the oxygen wich transfers across the membrane which buggers the beer.

Dogger

Hey guys- I have just started using pet. No experience yet, but I will post anything interesting.
BUT- how can oxygen at 14 psi, permeate a bottle full of beer and co2, at maybe 25 or 30 psi?

I understand migration between gases etc, but if one is pressurised surely that acts as apretty strong barrier to entry of oxygen?

If you said that the co2 could leak out, then I would have to agree with you. And yes, a big frosty glass bottle is nice, but hard to get, I don't feel like drinking anything commercial that comes in longecks, and they are heavy and noisy to bottle when the kids are asleep!

Muz

[Dogger Dan]

Nope,

Doesn't see it , 30 psi CO2 means nothing to the O2. Think of it this way, Lots go to areas of little. You have no/minimal dissolved O2 in your beer so it wants in.

Believe it or not

Dogger

[mahaba]

New to brewing, but interesting to read the comments about PET bottles

Am I to take it that it is OK to ferment in plastic but bottle in glass?

Or would it just be easier to pour your beer from a PET into a glass that has been kept in the freezer?

Seems to be an "each to their own" type question

[Dogger Dan]

Mahaba,

It is my opinion that you should ferment in glass and do away with plastic altogether simply because it unsanitary. I use a plastic primary but the fermentations are so strong the bacteria never get a chance.

Dogger

[bigmuzb]

I had no idea that was the case dogger, so eventually the bottles must burst because of all the oxygen in them- how does soft drink which is heavily carbonated survive?

Muz

[Dogger Dan]

No, the O2 is dissolving into the beer, not the head space.

So if the O2 is atmospheric, there is no increase in pressure

Dogger

[Oliver]

Dogger,

I take you on your word, as I have no idea about this sort of thing and you're the food scientist (or whatever you are ).

But to clarify something for everyone, am I correct in saying that O2 will enter because it's smaller than CO2 and thus squeezes between the CO2 molecules?

Presumably O2 will keep entering until the pressure of the O2 inside the bottle is the same as on the outside, hence a bottle will not "explode" because the pressure does not keep building and building.

Am I correct?

Oliver

[Dogger Dan]

Remember when we were kids and we put the the starch in the plastic bag and sunk it in iodine water to prove diffusion.

This is the same gambit. The O2 moves from high concentration to low across a semipermiable membrane, It doesn't care what the pressure of CO2 is, it dissolves into your beer and you have oxidation.

Now to me, if you have 14 psi CO2 and 10 psi O2, The resulting pressure is 14 psi because the gasses couldn't care less what the other one is about as long as each can reach an equilibrium so which ever is at the greatest pressure is the pressure on the bottle.

Because it is atmospheric O2 going into the beer, the resulting pressure O2 will be atmospheric as the two sides try to reach equilibrium.

The CO2 will remain at 14 or 12 or whatever it is pressurized at therefore the pressure in the bottle is 14 psi.

Is this only making sense to me? Normally I am the thick one, I should get my kids on here and do their Mad Science routine

Open a beer and let it sit, what happens and why do you think that is ponder if you will

Finally, If you really don't give a shit as to why, bottle in glass, because the wine people do it and they have been doing it for years and they say glass is better.

Or bottle in plastic. If you drink it inside of a year you wont notice anything anyway

Dogger

[Oliver]

Just as I thought

Oliver

[Dogger Dan]

I re-read that and sorry if I offended anybody. Normally when everyone looks at me with blank faces it is me who is off tangent, which is what I was thinking here to.

Dogger

[r.magnay]

it is all in the mind, I have been drinking beer for a lot of years, from glass, steel cans and aluminium cans, if you tip the beer into a glass and drink it, it all tastes the same! Since I started homebrewing I have only ever used glass until recently, but because I do a lot of travelling on bush roads, I started using PET bottles for some brews, I carry a stainless steel panikan in my fridge, and pour the beer into that, bloody beautiful! The only time I would not use plastic, is for a stout or something that I intend to put down for an extended period of time. I can't bring myself to drink straight from a pet bottle, but then I don't like the plastic cups you get at some venues, or even the hard plastic "glasses" you can get, we drink from pewter, glass, stainless steel mugs and bloody near anything that will hold beer, my prefrence is glass or stainless steel.
That's my opinion and I'm sticking to it!

[Dogger Dan]

Ross,

If I could get my hands on a stainless tankard all would be well

Dogger

[Mutley]

All makes sense on the science Dogger.

But I'd also add by your logc that the CO2 would diffuse OUT as well as the O2 diffusing IN. Not that it has any impact on the merits of your argument about 02 spoiling beer.

One comment on plastic vs glass, I do notice that the "beer" smell is much harder to remove from plastic than glass once the beer is emptied from the bottle. Even brisk bottle brushing in hot water or soaking in mild bleach solution doesnt seem to work.

Must be due to the porosity of the plastic ? I am concerned that some of this might find its way into the next batch..... possible ?

[r.magnay]

The stainless steel panikan I have is 750ml it is a chinese made thing with a brand 555 on it, I would think you could pick them up from a camping store or an army disposal type store. Coopers actually have a stainless steel mug with their name engraved on it, it is about 350ml and is a double skinned affair with insulation impregnated into it, they gave them away last year as a membership gift, but I know they are now for sale for about $15.00 Australian, they are also pretty bloody good!

[Dogger Dan]

The CO2 is to big to get out, hence the term semi-permiable

Dogger

[munkey]

not that my commet is needed judging by the above posts but ill say my pice none the less.

dogger is absolutly spot on, the differense between plastic and glass is that plastic is made of strands or fibers (polimers) which grip together, (and to make things simple in this case) forming a net.
now this net (POLYETHYLENE TEREPHTHALATE) has a mesh size bigenough for oxygen to fit through and small enough to keep the carbon molocules in .
now seeing as oxygen is a real loanly eliment it wants desparatly to be connected to anything with electrons to make it more stable, it will stop at no lenths to enter the bottle and oxidise anything it can.
regardless of pressure, as exsplaned before the o2 fits in between the moloqules (think of them as balls all fitted together and the space inbetween the balls is ware the o2 will go.

blah blah blah.... rant rant rant.

[bigmuzb]

There you go. Thanks guys for a (now) pretty good explanation.

I drink my beer within a couple months at the longest, so I will quite happily use pet.

Glass Nazis notwithstanding!!!!!

Muz

[Dogger Dan]

I am thinking I need to learn English

Dogger

[mordy43]

Shit dogger i think we all need another brew or 12 after that one