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Question about plastic fermenters

Discussion in 'Homebrewing' started by clearbrew, May 1, 2013.

  1. clearbrew

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    I was recently reading an article in BYO about plastic fermenters and their oxygen permeability. I know that plastic fermenters do have a degree of oxygen permeability, but I'm having doubts about it during fermentation.
    What I mean is this:
    If beer is fermenting or has been fermented, we assume approximately .5 - .8 (from what I remember reading somewhere) volumes of CO2 have been absorbed in the beer. If nothing is disturbed (meaning if the blowoff tube or airlock is not removed) then the contents of the plastic fermenter will remain under a positive pressure. I realize it is not a high degree of positive pressure, but as long as it stays above ambient air pressure, then how does oxygen get in?
    Again, if the beer is moved into a plastic fermenter and pressure/carbonation is lost, then I understand.

    Just a thought that I felt was worth discussion.
     
  2. scurvy311

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    Osmosis - Molecules passing through a semipermeable membrane to form an equilibrium. The rate at which this happens, I cannot vouche for. I have a plastic conical, and have never had oxidation issues...granted the longest I have ever left beer in there was around 5 weeks. Im interested in knowing the ball park number where this would be an issue.
     
  3. JackHorzempa

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    Below is something I have posted in a previous thread which is relevant to this thread:

    “ …the fact that a CO2 blanket is covering the beer and pressure is equalized across the bucket walls should prevent significant O2 migration in”

    That is not how it works. O2 migration is defined by Dalton’s law of partial pressures and Frick’s Law. Below is something I posted previously on this topic:

    The ‘issue’ of oxidation of homebrew in plastic buckets is ‘defined’ by Dalton’s Law of Partial Pressures and Frick’s Law.

    Dalton’s law of partial pressures ‘defines’ that the total pressure within a vessel (a homebrew bucket) is an addition of all of the pressures of the different gases within the vessel. The ‘dominant’ gas in a homebrew bucket during active fermentation is CO2 but there is also Nitrogen, Oxygen, etc. in the wort (in solution) and within the headspace. A ‘notion’ is that during active fermentation there is little concern about Oxygen (and possible oxidation effects) since during the active outgassing some of the other gas molecules (e.g., N2, O2, etc.) are ‘scrubbed out’ along with departing CO2.

    When the active fermentation ceases, then this is where Frick’s Law comes more into play. Within the liquid (beer now) there are gases in solution (N2, O2, CO2, etc.) Outside of the bucket is air. Frick’s Law basically defines that gas types will diffuse from areas of higher concentration to areas of lower concentration; this is done on an individual gas type basis. So, the fact that there is ‘a lot’ of CO2 in the bucket (both in solution in the beer and the headspace) has no effect on oxygen diffusion. If there is more oxygen (in the air) outside the bucket then is inside the bucket (in solution in the beer and headspace) then oxygen will indeed diffuse into the bucket. The rate of diffusion is dependent on the oxygen permeability of the bucket and difference in oxygen concentration on the two sides of the bucket.

    Cheers!
     
  4. Applecrew135

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    God I love chemistry!
     
  5. HerbMeowing

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    The BYO article claimed oxidation reaches perceptible levels after 10 days in a plastic ale pail.

    Seriously?
    If so...a whole lotta people out there are downing quarts and shorts of soggy cardboard-flavored homebrew and coming back time and again for more.
     
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  6. inchrisin

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    Yeah, I find 10 days a really short period as well. I bet it's more perceptible in hoppy beers than in say a session stout. Maybe not 10 days later, but....
     
  7. clearbrew

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    O.K. I did a bit of reading on Frick's law.
    I saw a lot of math. The math had letters in it. When did we start using letters instead of numbers? I think I even saw some triangles in there, but it was about to be eaten by this giant E shaped thing. ;)
    Anyway, I understand Frick's law to be about pressure equalization in an open area. If a plastic bucket is permeable then I agree that the pressures will equalize. What I still don't understand is how the oxygen gets in if the bucket is under pressure with CO2. Are oxygen atoms smaller. Wouldn't the CO2 have to get out first?
     
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  8. JackHorzempa

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    “What I still don't understand is how the oxygen gets in if the bucket is under pressure with CO2. Are oxygen atoms smaller. Wouldn't the CO2 have to get out first?”

    A lot of folks have a difficult time understanding this. For many people, since there is ‘pressure’ inside the vessel from lots of CO2 this ‘pressure’ will keep other gases out.

    I doubt that there is some additional way for me to explain this to you but the situation is as I previously stated it: Frick’s Law basically defines that gas types will diffuse from areas of higher concentration to areas of lower concentration; this is done on an individual gas type basis. So, the fact that there is ‘a lot’ of CO2 in the bucket (both in solution in the beer and the headspace) has no effect on oxygen diffusion. If there is more oxygen (in the air) outside the bucket then is inside the bucket (in solution in the beer and headspace) then oxygen will indeed diffuse into the bucket.

    Maybe there is a ‘better’ physics person out there that can explain this in another manner?

    Cheers!
     
  9. premierpro

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    I'm sooo confused!
     
  10. sarcastro

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    Makes sense to me. Physics, she be a cruel mistress when it comes to brewing.
     
  11. mattbk

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    OK - a good discussion nonetheless - but the science geek in me needs to make a correction - it's Fick's Law. (not Frick)

    http://en.wikipedia.org/wiki/Fick's_laws_of_diffusion

    I will try to explain as simply as I can. What happens if you put a cold bucket of water outside in the summer? Over time, the water will warm up. This is an example of diffusion - heat from the air diffuses into the cold water, until an equilibrium is reached (the water is the same temperature as the air.) Nature likes equilibrium.

    Compare a plastic bucket of cold water to one shrouded in insulation: it will take longer for the insulated one to heat up, even in the summer. Slower diffusion of heat through the bucket, due to the insulation.

    The bucket example is the same. Higher O2 in the air, lower O2 in the bucket. O2 travels in, same as the heat would. This will continue forever until the same amount of O2 is in the air as in the bucket. Equilibrium. Nature.

    The time it takes to reach O2 equilibrium is a function of how permeable the bucket is. You would imagine that a bucket made of very thick steel would be less permeable than a thin plastic bucket; that is, there are more microscopic holes in the plastic for the O2 to travel through than the steel.

    This is why steel makes for better fermenters than plastic buckets, amongst other reasons (eg durability.) The question is: does it matter in your beer? That will be a function of how sensitive your beer is to oxidation (compare, for example, a DIPA to, say, a Mild) and how long your beer is in the fermenter.

    I used to use glass for everything. But plastic is so much simpler. I haven't found a big difference between the two. Yet.
     
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  12. carteravebrew

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    "When did math become f*cking English?! I need a goddam dictionary to do my math homework!"
    -said by a classmate while we were doing calculus homework senior year of highschool...sorry, just made me recall a fond memory...
     
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  13. mattbk

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    Sorry, me too: I have taken a math class that had no numbers in it. Crazy shit.
     
  14. spartan1979

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    Any differences between the type of plastic in a bucket versus a Better Bottle?
     
  15. VikeMan

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    I think you explained it as well as any physics teacher could. (mattbk's analogy is also helpful.)
     
  16. mattbk

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    I'd suspect HDPE buckets and (I think) PET BB's do not have the same amount of permeability. But I have no idea of the differences. Here's a pretty safe answer: probably not enough to matter!
     
  17. VikeMan

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    O2 molecules are smaller than CO2 molecules, but that doesn't affect the phenomenon described by Fick's Law. (Though I suppose it could affect ability of a given gas to cross the membrane (i.e. the plastic bucket). I've never seen anything about the CO2 permeability of plastic buckets).

    The reason the size of the gas molecules doesn't matter from a diffusion/equilibrium standpoint (i.e. why Fick's Law works) is that there is a lot of space between the gas molecules. i.e. they are not a solid, and from a practical standpoint they don't interfere with each other. Someday someone should write a BYO or Zymurgy article called 'The Myth of the CO2 Blanket.' Hmm....
     
  18. JackHorzempa

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    Thanks for correcting my fricking typo!

    Cheers!
     
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  19. JackHorzempa

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    You asked: “Any differences between the type of plastic in a bucket versus a Better Bottle?” The short answer is yes there is a difference.

    Below is a discussion I posted previously which ‘quantifies’ the amount of oxygen permeability of various material via an oxygen permeability coefficient. The lower the value of the O2 permeability coefficient the less permeable the material is to O2 diffusion.

    “I think you will agree with me that for Phase 2 there is a large differential in the oxygen concentration between the outside and inside of the bucket. So, let’s discuss oxygen permeability of the bucket. There was some discussion about the integrity of the bucket lid seal. I obtain a very good seal with my bucket so I am not too concerned about that aspect. What I ‘question’ is the amount of oxygen ingress directly through the plastic material. As I am sure you are aware, a solid still has lots of ‘open space’ and this ‘open space’ permits the oxygen molecules to pass through the bucket wall. Oxygen permeability is characterized by O2 permeability coefficient. Below are some examples for various materials; the units of the O2 permeability coefficient are cm3 mm/m2 day Atm 20° - 25°C:

    • Better Bottle Polyethylene Terephthalate – PET: < 1

    • Typical Bucket High Density Polyethylene HDPE: 44 -91

    • Low Density Polyethylene LDPE: 98 – 138

    • Butyl Rubber: 132-141

    • Silicone Rubber (dimethylsilicone): 40700

    So, you can see that from an O2 permeability coefficient perspective that a HDPE plastic bucket is much more oxygen permeable than a Better Bottle (or glass carboy or keg).”

    I should mention that I primary ferment in HDPE plastic buckets and I have let the beer sit in the bucket for up to 5 weeks for a number of batches. I have never noticed any effects of oxidation in those particular beers but Matt’s comment is relevant here:

    “That will be a function of how sensitive your beer is to oxidation (compare, for example, a DIPA to, say, a Mild) and how long your beer is in the fermenter.”

    For example, I personally would not let a hoppy beer (e.g., IPA or DIPA) sit in my primary bucket for 5 weeks (or longer).

    Cheers!
     
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  20. PortLargo

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    GEEK Alert - - TMI Alarm - - NERD Warning . . . Continue below at your own risk:

    From another forum, a rep from Ale Pail made this figures available on the permeability of their standard HDPE bucket:

    GAS RATE(cc/mil/24hrs/100in^2)

    CO2 345
    Ethane 236
    Hydrogen 321
    Natural Gas 113
    Oxygen 111
    Freon 12 95
    Helium 247
    Nitrogen 53
    Sulfur Dioxide 306

    So yes, the CO2 is struggling to get out, just like the O2 is fighting it's way in. What we picture as a benign bucket of wort is really a hub for gases of all sorts to pass through. We could all rest easier if the gases would just back off, but the Law won't let them (where's a cop when you really need one?).


    Here's how different plastics hold up to the onslaught of diffusion:

    [​IMG]

    Not only are our buckets "leaking" the good gas (CO2) out and the bad gas (O2) in, they are also leaking moisture. Maybe this explains why my mop is one of my most used brew-tools. Interestingly, the PET thumps the HDPE on gas leaks, but lags miserably on moisture.

    If anyone really wants a reference to any of the above and can stand the weirdo stigma . . . just ask.
     
  21. AlCaponeJunior

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    I would highly question the statement that there's a detectable degree of oxidation after ten days in a plastic bucket. I don't know the true time, but I'd say it depends on various things, including the permeability of the bucket, and the environment in which the bucket is stored.

    I know my bro tried to take a looksky at the temperature inside my chest freezer using a lighter. The thing wouldn't light. Not enough oxygen. Presumably the CO2 from the beer pushed a significant amount of air out of the freezer, leaving far more CO2 than normal inside the freezer. Thus, I shant worry too much about it.

    Obviously for extended aging, use glass.
     
  22. billandsuz

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    and for the first time PTFE is not the winner, for everything.

    i'm old school. i like glass. but i do have buckets and they seem to work just fine. just one more thing i need to upgrade i suppose.
    Cheers.
     
  23. mattbk

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    I can't wait until the next time I taste someone's homebrew and tell them "I can detect quite a bit of freon in this beer - you fermented this in plastic didn't you?"
     
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  24. GreenKrusty101

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    That's it, I'm buying a hyperbaric fermentation chamber : )
     
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  25. kjyost

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    Beyond all the other valid points out there, I want to point out that the bucket is barely under pressure, as the airlock essentially keeps the bucket at atmospheric pressure. That said, if CO2 is being produced I would expect it to displace some on the invading oxygen.

    Another decent analogy I would think is a balloon... Even though it is under pressure diffusion occurs into (and out of) the balloon. If you were to find out what was inside the balloon after a week it would not be the same proportion of Helium as there was on Day 1.
     
  26. VikeMan

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    This CO2 production and outward pressure is something you might call the CO2 'Wind,' and it will certainly carry other gasses along with it. But once that pressure is gone, the temporarily remaining CO2 'Blanket' is really just a placebo providing some with peace of mind.
     
  27. clearbrew

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    O.K. this helps explain how the oxygen "gets past" the co2, but doesn't the pressure in the bucket have some influence on the oxygen?
    What I mean is, there may be less oxygen in the bucket than outside, but the oxygen in the bucket is under pressure (not much, but more than ambient). So wouldn't something have to come out of the bucket so that all of the gasses (no matter what type) can reach the same atmospheric pressure?
     
  28. VikeMan

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    A long as there is a significant pressure difference due to CO2 production, the CO2 'wind' can take other gasses along with it. (Thought experiment: Blow through a straw into a tank of oxygen. Is any oxygen getting from the tank to your mouth? Not yet.) But once you get near equilibrium (of total pressures on each side), the phenomenon described by Fick's Law takes over.

    Yes. CO2 comes out, and O2 goes in.
     
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