Fermentor options are vast these days, forcing homebrewers to choose between different materials, shapes, and sizes. I’m personally partial to plastic/PET carboys for their ease of use and reduced risk of amputation, though some remain convinced this material is too oxygen permeable and prefer fermenting in glass carboys. Curious whether such claims held any water, we put this one to the test and the results are in!
I prefer my stainless steel conical fermenter to do my primary fermentation in and then will usually transfer to a plastic carboy for the secondary. I use my glass carboy every once in a while but it is just a pain to clean.
For me, the more interesting result was the delay in the PET wort taking off. Was there any difference in BB date on the yeasts? Given the different behaviour of the worts from pitching, it doesn’t come as much of a surprise that there was a noticeable difference in the resultant beer, but it seems unlikely that yeast pitched into wort contained in PET rather than glass would behave so differently initially if that were the only variable at play.
I use a stainless brew bucket most of the time. I have a PET jug (big mouth bubbler knock off) that I use occasionally if the Brew Bucket has a lager going and I want to make an ale in the meantime. Maybe I’m just buying into the hype, but I’ve never made a lager in plastic.
I’d consider each having their own package of yeast to be a second variable, FWIW.
I read this one this morning. He rehydrated two packets together, then split them evenly between the two carboys, so yeast isn’t a factor.
I’m not convinced the foam in the glass carboy was activity so much as residual foam from the transfer from kettle. Can’t be sure though.
It wasn’t clear in the article, so I just edited to make it so-- the yeasts were rehydrated together then evenly split between the carboys.
A couple random thoughts:
From what I can tell from people I know and this forum, the biggest plurality might actually ferment in plastic buckets rather than carboys.
Also, have you ever done an experiment where the participants are given the exact same batch of beer and asked to tell if there is a difference. I bet 70% would identify a difference.
Good thoughts, Pete. I’m extremely skeptical that the PET carboy actually made any difference. I’d want to see a few more repetitions of the experiment with the same result before I could reach that conclusion. As I’m sure Marshall would agree, a single trial provides food for thought, but not a final conclusion.
And if it did make a difference, I sincerely doubt it was because of O2 ingress. This was a 2-week primary fermentation, so there should have been plenty of yeast still active to take up the miniscule extra O2.
I’m still not 100% sold that there’s a difference based on my own experience but if there is, I would bet on either flavor carryover from a prior batch/cleaner residue, or hop oils adsorbing to the plastic leading to a slightly lower hop character in the plastic beer.
What might explain the statistically significant results? Here are my spitballing thoughts:
-oxidation? I’m skeptical that permeability of the container is important in a short fermentation when both batches were open fermented.
-adsorption characteristics? I am fairly certain that the materials have different adsorption and desorption characteristics relative to beer flavor compounds. I can’t say whether such differences are significant, but it could explain reliable, but subtle differences. The effect may differ based upon the beer style as different beer styles have different flavors.
-different pitching levels? This theory would be consistent with what appears to be en earlier krausen in the glass.
-reactor geometry. Plastic carboys tend to be wider and shorter than glass carboys of the same volume. We do know that geometry does make a difference on an industrial scale, but don’t know about the homebrewer scale.
In the discussion on the site, it was pointed out and confirmed by Marshall that the glass carboy was a 6.5G and the PET was a 6G. So there may also be some headspace/geometry things there that could explain the differences in observed foam. The surface texture and adsorption stuff also seems quite plausible.
I agree that oxygen at such a small level over that timeframe seems highly unlikely to cause a noticeable difference, but our bodies are amazingly sensitive instruments. It’s reading the gauges that’s the hard part 
One other thought – if PET is oxygen permeable to some small degree then doesn’t that mean CO2 also escapes to some degree? Again, I’m skeptical that this is enough to make a big difference over such a small timeframe, but curious if anyone has thoughts on what that actually looks like from a feasibility standpoint. I don’t know the right formulas to even begin to approach that mathematically.
If soda bottles are PET (and I think they are) they are definitely CO2 permeable. They will lose carbonation over time.
One of our favorite pizza joints delivers a free liter of soda with every pizza. We don’t drink soda, so when I forget to refuse it the bottles just go into the pantry. Over time, they go flat even when unopened. I couldn’t possibly tell you how long that takes, but it could be some of the bottles are well over a year old.
Soda bottles are pressurized and it is possible that the weak point may be the seal of the cap rather than the PET itself. The rate of O2 or CO2 transfer, I would bet, is much higher at the interface of the bung and opening than it is through the plastic. I think O2 diffusion is a non-starter for this discussion over the course of a two week fermentation.
This is likely a weak limb to try and explain the difference, but what sanitizer was used?
Glass always seems to clean up better than plastic. If star-san was used, it’s possible something was “left behind” in the plastic fermentor that was successfully washed out of the glass one.
Either way, I’d say that cleanliness is more of a concern for most brewers when choosing between glass and plastic, not oxidation. But that’s just my hunch.
I use glass. I can be more aggressive with cleaning techniques without having to worry.
How aggressive do you need to be? A good soak takes most everything off. What’s left can usually be gotten with a soft cloth or rag stuffed through the neck of the fermenter.
If you’re scraping crust off the inside, you need to soak it longer.
I’ve had a couple (long, 4+ week) fermentations were the brown scum was pretty well dried on. After a long soak with Alkaline Brewer’s Wash I still had to break out the carboy brush to get it all off. So, not that aggressive, but I’d be concerned with the effects of a stiff bristled brush on plastic over time.
By the way, I’m not calling the beer infected. I’m thinking more along the lines of brewers yeast from a different batch staying behind.
StarSan. This is an interesting theory, for sure. I make sure to get as much out of my carboys as possible prior to filling with wort, which for my PET carboys involves gently applying pressure to shoot the bubbles out… you can’t do that with glass.
Star San has been noted to be a yeast nutrient in a minor way. If that’s so, it would seem that the plastic batch, assuming it retained StarSan (which is a pretty long shot), would have had an advantage, not disadvantage, over glass.
Sorry, I wasn’t clear-- I’m able to squeeze almost every last bubble out PET, while the glass retained more bubbles because I could only rely on shaking over my sanitizer bucket.