From chuckmw@mcs.com Fri Jul 28 15:10 CDT 1995 Date: Fri, 28 Jul 95 15:12 CDT From: chuckmw@mcs.com (Charles Wettergreen) To: jkane@execpc.com Subject: Corney kegs 01 To: jkane@execpc.com ********** Original To: ALL * CARBON * was By: CHUCKMW * COPY * posted: On: MCSNET ********** Conf: 0002 - rec.crafts.brew ----------------------------------------------------------------------- In <3v88hd$uvq@big.aa.net>: Jeff Kane (jkane@execpc.com) wrote: JJ> pata@aa.net wrote: JJ> : lose some carbonation, and you introduce some oxygen, and there is the JJ> : inherent problem with the PET bottles letting oxygen JJ> : permeate in...just don't use the PET bottles for long term storage. JJ> I am sorry, I don't mean to pick on you personally. I see this statement JJ> all the time. I have yet to see some proof that this occurs. Please, JJ> Either give a chemical reason for the permeation of O2 thru the poly(x), JJ> or stop permeating this rumor! While the extracts below from the Homebrew Digest aren't 100% conclusive, they're pretty darn persuasive. They also contain, IMO "a chemical reason for the permeation of O2 thru the poly(x)." JJ> In HBD # 1632 Date : Fri, 13 Jan 1995 12:48:48 -0500 (EST) From : "NAME SEAN O'KEEFE, IFAS FOOD SCIENCE" <SFO@gnv.ifas.ufl.edu> Subject : Oxygen permeability of plastic s Data for oxygen permeability; plastics: material ml o2/(day*mil*sq. meter*atm) PE (polyethylene) 6000-15000 HDPE 1500-3000 Saran 10-350 Mylar 50-100 Foil laminate 0 Plastic laminate 10-400 Data for permeability ratios (relative ability to permeate) material H2O/O2 CO2/O2 Polypropylene 30 4 Polystyrene 100 5 PET (polyethylene terephthalate) 5000 3 "Normally, carbon dioxide permeates four to six times faster than faster than oxygen , and oxygen four to six times faster than nitrogen. Since carbon dioxide is the largest of the three gas molecules, one expects its diffusion coefficient to be the lowest, and so it is. Its solubility coefficient, however, is highest, because its solubility, S, in polymers is much greater than that for other gasses." Karel, M. 1975. Protective pckaging in foods, Ch 12 In "Physical principles of food preservation", O.R. Fennema Ed., Marcel Dekker. I'm not sure how all this relates to the Coke type 2L bottles, but I have stored beer for 6+ months in these 2L beauties (at 5C) with no ill effects noticible. However, I would recommend avoiding root beer bottles, or clean then very well. Residual root beer flavor does not go well with beer. Date : Sat, 14 Jan 95 12:15:30 -0800 From : danpack@grape-ape.che.caltech.edu (Dan Pack) Subject : PET bottles Maybe I can provide some chemical engineering insight into this question. "Dr. J" says: >the permeability of 2L plastic bottles can be solved by a little common >sense. When a brew is bottled in a closed container it builds up a >positive pressure, hopefully :). I would think that the pressure in the >container makes the permeability irrelevent because the lower >atmospheric pressure could not possibly enter the container unless it is >opened or cooled significantly below the saturation pressure of the It's not obvious, but actually there are two ways for a gas to enter the container: convection (bulk flow) and diffusion. The pressure difference between the inside of the bottle and the surrounding air will, in fact, prevent _flow_ (or convection) of oxygen into the beer. However, diffusion is driven by the difference in _concentration_ of oxygen in the beer and air. Since there is, hopefully, very little oxygen in the beer to start with there is a relatively strong driving force for diffusion of oxygen through the plastic . Also, the consumption of oxygen by reaction with the beer (oxidation) ensures that the concentration of O2 inside the bottle will remain low. So, the relevant question is whether PET provides a barrier for oxygen transport. Since PET is used to keep CO2 _in_, and since CO2 is a bigger molecule than O2, I would say it is very likely that O2 is free to diffuse through the plastic . The conventional wisdom and the tongues of talented tasters such as Lee Bussy seem to support this conclusion. Dan Date : Sun, 15 Jan 1995 11:04:25 +1100 (EST) From : David Draper <David.Draper@mq.edu.au> Subject : O2 in PETs/Color conversion/Max efficiency Dear Friends, comments on a few things in today's digest. Dr. J. makes the intuitive conclusion (as many before him have) that the positive pressure of CO2 inside a bottle will keep any O2 out. The amazing truth is that the pressure will *not* prevent oxidation. The process by which O2 gets inside the bottles is diffusion, and diffusion typically takes place in response to a gradient in chemical potential. The chemical potential for this instance is largely composition-dependent--that is, because there is so much more oxygen outside the bottle than inside, there is a "driving force" for oxygen to want to get inside. This happens with glass as well as plastic , but the amount of time it takes for oxygen to diffuse through a glass bottle is many orders of magnitude longer than that for plastic (because of vessel thickness and composition-- plastic vs. glass). JJ> In HBD # 1633 Date : Sun, 15 Jan 95 00:29 CST From : pgravel@mcs.com (Philip Gravel) Subject : Re: Plastic permeability misconception ===> Joseph E. Santos comments about PET bottles and oxygen permeability: > As a happy receiver of vast amounts of brew knowledge I feel >obligated to return some of the wisdom to all. I think the discussion of >the permeability of 2L plastic bottles can be solved by a little common >sense. When a brew is bottled in a closed container it builds up a >positive pressure, hopefully :). I would think that the pressure in the >container makes the permeability irrelevent because the lower >atmospheric pressure could not possibly enter the container unless it is >opened or cooled significantly below the saturation pressure of the >liquid within the container. In conclusion: A brew can be stored in a >container without concern as long as there remains a positive CO2 >pressure in the container. The only worry I can see is the possibility of >reducing the carbonation level in the beer, which would take an extremely >long time as experienced by the 2L of soda that I have had in my house >for over a year. I hope this clears up some of the discussion, if I have >missed something most obvious please feel free to correct me. No problem, here's the correction. Although it may seem to be counter intuitive, your thesis is wrong. The important consideration here is the partial pressure of the individual gasses, not of the total gasses. Air is approximate 20% oxygen and air pressure at sea level is about 14.7 psi. Thus the partial pressure of oxygen in air is about 3 psi (.147 x 20). Assuming the bottled beer contains no oxygen , there will be an oxygen pressure gradient of 3 psi across the plastic bottle from the outside in. It matters not what the carbon dioxide pressure on the inside is. The oxygen will diffuse in over time. - -- Phil _____________________________________________________________ Philip Gravel pgravel@mcs.com Date : Sun, 15 Jan 1995 21:27:24 -0500 (EST) From : "Joseph E. Santos" <jesantos@WPI.EDU> Subject : Re: Misconceptions about misconceptions Thanks Jeff and Domenick, In my previous post, I put out the idea that the pressure exerted in a plastic bottle would prevent the Oxygen permeating into the brew. As an engineering student I should have known that common sense does not always rule. Thanks to some quick responses I had the opportunity to review my basic chemistry and correct the misconceptions I had. To clarify the issue, the pressure of the CO2 in the bottle has no bearing on the O2 transfer between bottle and atmosphere. Because the concentration of O2 is close to nil O2 WILLpermeate the plastic until equilibrium is attained between the concentration in the beer and the atmospheric concentration surrounding the bottle. I have learned my chemistry lesson of the day! BTW, If I understand this correctly the pressure in the plastic bottle would increase due to the excess O2 pressure. Could this be a cause for concern? DR J Just another happy homebrewer!