Home Brew Myths - Part 2

[Bobby]

Let's try and get some evidence based practice back into homebrewing.

So then, what are peoples experience with longterm storage in plastic??

 

[Backlane Brewery]

Well, there must be a real issue somewhere here, or these people wouldn't be plugging their product as follows:

Don't think of Better-Bottle carboys as plastic think of them as flexible glass, because they are made from a special, scientifically tested, type of polyethylene terephthalate (PET), which does not have the disadvantages that make other plastics unacceptable for making wine or brewing beer.
Flexible Glass Unbreakable Negligible oxygen permeability Meets FDA requirements with Kosher statement No taste or odor Clear and colorless Extremely stain resistant Smooth, non-porous, non-absorbing, hydrophobic surface.

Note they only say "negligible" not "nil" permability. And kosher too

 

[thehipone]

Well, who would have ever thought that the Chemical Engineering degree would be useful!
Hopefully this won't be too tough to follow for the non-chemistry types.

WE can look at this as a pseudo steady state diffusion problem with one of 2 assumptions: the formation of staling compounds is reaction rate limited, or mass transfer rate limited. Basically, this means that in the first case, the concentration of O2 in the brew is near the solubility ( I assumed about 8 ppm) or in the second case it is low ( as soon as the O2 gets through it is converted to a staling compound- trans-2-nonenal for papery taste) The difference isnt much, using each assumption I get 0.962 gram/year- .99 gram per year. I used PET diffusion coefficient and 2 mm thickness, with the surface area calculated assuming the fermenter is a cylinder has a height 1.5 times the diameter.

here's where things get a little hairy, the flavor threshold for t2Nonenal is published as 0.11 ppb! So it doesnt take much to taste oxidized. If we assume (key) that it takes 1 molecule of O2 to produce 1 molecule of t2N, then only 5x10^-7 gm of O2 are needed to reach the threshold, or 16 seconds!!!! Now, there are plenty of other reactions that take up O2 too, so the assumption that 1 molecule of O2 leads to 1 molecule of t2N is wrong. If we assume that 1 in a million O2 molecules leads to staling compounds, then it takes ~1/2 year to reach the taste threshold. I'd guess it's closer to the latter case than the former case, but I cant say for sure. It depends on so many factors

unfortunately, I cant say yes or no that this is a myth given the complex chemistry going on inside the vessel. From my experience, reasonably short times are fine (< 2-3 months; haven't tried any longer). Cold temperatures (diffusion coeff. depends on Temp.) will slow things down a bit.

(And I checked my spreadsheet with the example problem on the polymer diffusion page)

 

[Backlane Brewery]

I'd guess it's closer to the latter case than the former case, but I cant say for sure. It depends on so many factors

Obviously, we ALL understood & agreed with everything you said up to this bit, Hipone.
But then you let us all down at the finish.

MAH, you may have to either toss a coin on using plastic over glass, or accept that the variables are never going to let this myth be pinned down one way or another.
That said, Hipone, what info would you need to make a decision case by case?

 

[thehipone]

Well, if you had some sort of mass balance on the O2 getting into the brew you would be able to pin down the ratio and calculate a definitive answer. Basically, you would need an HPLC column to measure the increase in the amount of t2N produced when you dissolve a given amount of O2 into the beer, as well as a measurement of the residual O2 in solution. In fact, I'd be surprised if the big guys don't have some sort of idea on this, they probably know exactly how much they can afford to expose a batch to O2 before observing detrimental effects. Also, the amount and state of lipoxigenase (sp?) enzyme matters, since it accelerates the conversion of lipids to t2N from what I can tell from the relevant literature that I could find.

 

[thehipone]

if anyone is interested, here is some hardcore literature on flavor stability that I came across on my quest to avoid getting anything done today:

Flavor Stability

Still can't find a definitive answer, using the "theoretical minimum amount for damage" from the article you get very short times < 1 day, to get the requisite dose of O2.

Using some other "reccomended" limits I get 180 days+.

Sorry to disappoint if I got anyone excited about answering this, I'm a little bummed that it couldnt be solved..... yet.