Dry hop secondary container

Your memory is pretty good, at least from what I can remember. My point was that although O2 migration will happen across a headspace of CO2 (as/after it slowly penetrates the bucket/airlock/etc.), the whole oxidation process is slower than if you had racked into atmospheric air. i.e. starting with mostly CO2 in the headspace is better than starting with lots of O2 in the headspace.

 

Definitely. But to a lesser degree. i.e. less of the CO2 is directly replaced with O2. Ultimately, it's still a losing battle if the secondary or primary is extended for a very long period. Just a question of how soon we surrender.

 

“ …primary (1 week) to keg (dry hopped for 2 weeks, remove hops, chill and serve).”

Matt, it sounds like you have a plan! I personally do not keg my beers but lots of BAs have posted that they just keep the dry hops in the keg for the duration of serving the beer. That seems like an option for you as well.

Good luck with your next dry hopped batch!

Cheers!

 

I've dry hopped with pellets in a PET Better Bottle, 7 days dry hopping, cold crash for 3-4 days and the hop material settles on the bottom. I'm able to rack almost all the beer off it without any kind of filtration.

 

I'm a chemical engineer and, by mass transfer and materials science principles, this is very true. In theory, anyway, reality is always different due to uncontrollable variables, but it is reasonable to make the necessary assumptions used here for our purposes.

Dalton's Law holds true (assuming Ideal Gas behavior), and, since CO2 exists in higher concentration (dissolved and ambient), for that reason there is a greater pressure of CO2. CO2 also has more mass than the major components of air, meaning that it would 'sink', so to speak, and form a uniform layer below the oxygen and nitrogen. Assuming laminar flow (reasonable, considering there is virtually no 'flow' of the gas), the O2 and N2 would be expelled as CO2 pushes up on them while it is generated and seeks to escape (remember, it's pressure is greater than that of its surroundings). N2 doesn't really matter, anyway, since it's chemically inert. Given this, when the lid is opened, there should be a dense layer of CO2 with little O2. If you are using a bucket, you probably have a solid 4-5 in. of headspace at least, so anything convected away will be from the top and the layer of CO2 should remain in tact, thus protecting the surface of the beer. Any air introduced would still sit on top, because it's lighter. With a carboy (plastic or otherwise), this is even less of an issue due to the extremely small opening (relative to the surface of the beer). The only problem would be the headspace in the new vessel, but again, CO2 is heavier so it should come into contact with the beer first. If you are paranoid, you can always purge CO2 into your secondary before and during racking your beer.

TL;DR: Unless you have a fan blowing air into/over your fermenter or you are doing this outside on a breezy day, introducing air by opening the fermenter is a minimal risk. Purge CO2 into the new fermenter if you are anal/extremely paranoid about oxidation.

The only problem I see with what you said about diffusion is that Fick's Law typically is used to describe diffusion into a material, not through it. It can be sort of modelled that way, but it isn't exactly accurate, since the diffusion coefficients change at the boundary layer between the bucket membrane and the beer. To know how much O2 is passing through; figure out the outside concentration of O2, the wall thickness, the diffusion coefficient (for PET in this case), and assume the inside concentration of O2 is zero or some other (arbitrary) desired concentration. This will give you the flux, or volume of O2 carried through the PET membrane (your fermenter). Luckily, I found some lecture notes online (here: http://people.pwf.cam.ac.uk/jae1001/CUS/teaching/materials/M6_Lecture_6.pdf) that list the O2 permeability of several polymers. Let's say that LDPE is close enough, since HDPE isn't listed, and can be our worst case scenario. It has a permeability of ~ 9 Barrer, which means that (at standard temperature and pressure, and per square centimeter of surface area), a 0.6 cm (0.25 in) thick bucket or better bottle will have about 1.3x10^-4 cm^3 of O2 move through it every day. That is a minuscule amount of O2, when you consider that it is also being continuously purged.

TL;DR: O2 permeability is so small for a worst-case, most permeable polymer, that it most likely is being removed by purging CO2 faster than it is coming in. This is especially true when you consider that only so much O2 can dissolve in the beer, considering another gas (CO2) is already saturating the solution and forcing the less concentrated and less dense (more mobile) O2 to precipitate out.

Sorry for completely geeking out here and rambling on, I get a little too excited about science.

 

Firstly, thanks for your post.

You state: “That is a minuscule amount of O2, when you consider that it is also being continuously purged.” I believe that this is true while fermentation is active. What about when fermentation is complete? For example, when you conduct dry hopping?

Cheers!

 

The key thing here is that the fermentation isn't really complete at that point, no reaction ever goes to 100% completion over any timeline. We say it is because the reaction kinetics have slowed down enough that we don't have a vigorous or visible reaction anymore. Yeast is still in suspension, however, and it is still putting out CO2 (kinda like we put out gas long after we've stopped eating ) even though no more active digestion is said to occur. Plus, I'm sure you have had primary fermentations that show little/no airlock activity. So, ignoring the biochem kinetics aspect and going strictly by that, it is totally reasonable to think that if something as vigorous as primary can go by with no sure signs of outgassing, there could still be digestion occurring.

 

Glad to lend a hand, it's always nice to run into other homebrewing engineers.

I'd guess that you hit the nail on the head. While Palmer didn't write his book that long ago, polymer quality has improved and (like you mentioned) they started putting gaskets in buckets like these. Even so, in his book, Palmer says that it doesn't really matter what you use, just do it and see what works for you, because he has had great beers from either type of fermenter.

As for me, I've never used a bucket as a secondary, but because it's been tied up as my primary. I've always used it for that so as to have a little extra headspace for krausen. I was thinking about doing it the other way around for a bitter I want to dry hop, though, for the convenience factor involved.

 

The main issue with using a bucket as a secondary is not the permeability of the plastic, it's the head space, which is unacceptably large IMO.

When I first started brewing years ago and knew nothing of head space and oxidation I racked a small batch of beer (4 gallons) into a bucket for secondary and definitely had problems with oxidation. An extreme case due to the low volume of beer, but the point remains that the bigger the head space the more oxygen there is, and the more oxygen there is the worse off you are.

 

You state: “Yeast is still in suspension, however, and it is still putting out CO2 …” I agree that after the vigorous signs of fermentation are complete that yeast will still put of C02.

The thrust of my question is that for the OP there are basically two phases:

· Phase 1: Primary fermentation (e.g., the first week) when there is outgassing

· Phase 2: The two weeks after primary fermentation is ‘complete’ in which dry hopping occurs.

You ‘asked’: “I'm sure you have had primary fermentations that show little/no airlock activity.” I have brewed 290 batches of homebrew (so far) and I have never experienced a primary that didn’t have vigorous outgassing.

As I stated in my first post: “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.”

I think you and I are in agreement that for Phase 1 there is little to no concern about the potential for oxidation since there is vigorous CO2 activity.

I still question what ‘occurs’ during Phase 2. While there may be some CO2 being produced by the yeast there is no ‘scrubbing’ action. The fact that there is ‘a lot’ of CO2 both in solution and in the headspace does not affect the diffusion of oxygen into the bucket. As I mentioned previously, what affects the rate of oxygen ingress into the bucket is:

· The difference in concentration of oxygen between the air outside the bucket and the amount of oxygen within the bucket

· The oxygen permeability of the bucket

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).

For Phase 2 where the dry hops are ‘sitting’ in a relatively inactive state for two weeks, what is the amount of oxygen ingress into the bucket?

Cheers!

 

Sorry to get a little off topic from this scientific brewing course, but I wanted to know if anyone had an opinion on using carboys for both primary and secondary fermentation? I'm talking mostly IPAs or bigger beers I would dry hop. Do I really need to secondary or can I just add the hops into my primary vessel after most of fermentation has completed? I've heard mixed opinions on the subject, just trying to get another perspective.

This is coming from someone who has yet to get into kegging and is sick of dealing with the dreaded plastic bucket. The plastic just has me paranoid as far as cleaning and sanitation standards go. And you guys aren't making me want to go back to plastic any more with the oxidation facts I've attained.

Ever since I purchased a carboy cleaner, glass carboy is the only fermentation vessel I want to use until I learn more about kegs and get a 2nd fridge to store them. I am not worried about the ease of adding and removing hops, as I don't use a hop sack, I just crumble the pellets up and drop them through a funnel into the carboy. At bottling time (after 7 to 14 days), I just tie a muslin bag to the end of my siphon tubing and it catches the hop junk.

Any opinions on glass carboys? Does it make sense to secondary from 1 carboy to another? Does it matter? Just looking for some knowledge.

 

Well, I will offer my opinion. I personally do not think there is a need for conducting a secondary for the instance of dry hopping. From my perspective, racking from a primary to a secondary has the potential for two risks:

· Infection

· Additional oxidation

I personally just add my dry hops to my primary once signs of active fermentation are complete.

I would recommend that you just add your dry hops to your primary (carboy) once primary fermentation is complete.

Cheers!

 

Yes, we do entirely agree on Phase 1.

I only included the point about non-vigorous outgassing during primary because I recently had a batch do that (first time).

As for Phase 2:
I was suggesting that, by the volume of CO2 in the vessel, it would only be a problem if CO2 were to be overtaken by oxygen in terms of concentration, scrubbing action or no. This would be because a quasi-barrier layer of CO2 would theoretically keep the O2 away from the beer surface. As to whether or not an HDPE bucket is fine, is a question of exactly how long you plan to secondary for. Obviously, and I think we would both agree here, PET carboys would be better than an HDPE bucket in terms of O2 permeability. But, if you are holding for only a few days, maybe even a week, as is the case for dry-hopping or other fermentation additions, would it allow enough O2 to pass through the bucket?

That volume of CO2 is quite small. Typical process gas purges to ensure an environment rich in a particular gas are kept at unobstructed flows of several cm^3 per minute. Let's say 3 sccm is a benchmark we will use, that's 4,320 cm^3 of gas per day to ensure the vessel is very rich in that gas. A standard HDPE bucket fermenter has surface area of ~ 0.066 m^2 and a thickness of ~ 6 mm. Take the average permeability (~ 67) and multiply by the surface area and divide by the thickness (67 x 0.066 / 6 = vol. O2 per day in cm^3) and you get 0.737 cm^3 of O2 passing through the bucket wall per day. Per week that is about 5.2 cm^3 (~ 0.0052 L) of O2. By molarity, this is 0.000047 mol O2, or 0.0015 g of O2 per week. That is barely measurable.

So, I would guess that the biggest oxidation risk to a beer would be racking it into an O2 filled chamber with agitation from the transfer, even if mixing is minimized by proper hose usage. But this risk is inherent, regardless of secondary vessel. Also, as Homebrew42 mentioned earlier, extra headspace in a bucket could be trouble; but, again, that is due to the oxygen already in the bucket, not the diffusion of O2 into the bucket. Larger batches could avoid the headspace problem, though, and I can't think of anybody who would complain about having to make more beer.

 

Sure, filling a 6.5 gallon secondary bucket to near the top could solve that problem, but then you'd need an 8-ish gallon primary bucket (and possibly a new, bigger boil pot).

 

Thank you for doing the math to arrive at the answer of: “Per week that is about 5.2 cm^3 (~ 0.0052 L) of O2. By molarity, this is 0.000047 mol O2, or 0.0015 g of O2 per week. That is barely measurable.” I was uncertain on how to ‘apply’ the O2 Permeability Coefficient so I very much appreciate your help in this matter.

It seems to me that there is very little oxygen ingress into a plastic bucket via simple diffusion. For me, and my process of dry hopping within the primary bucket, that is very good news!

Cheers!

 

You can rack into CO2 purged vessels using CO2 to push the beer from primary. That is what I do for my pale lagers.

Oxidation can take on other forms than the usual sherry (usually in dark beers) or cardboard. You can get a honey aroma in light beers that is an early sign (I see this often in imported German Pilsners). You can get a sweet/toffey flavor in some old beers that is not in fresh examples, that is oxidation. The bittering in Pilsners can go from smooth to rough with time, that is oxidaiton.

As far as infections in a secondary, well it can happen, but I don't worry about it. You can get infections making a starter, or just using room air to aerate. I don't worry about those, either.

 

I tend to dry hop in primary. I don't use bags for the hops, so I just rack off the hops into a secondary for a few days, and bottle. Keeps less hop material in the bottles.

I'm going to start kegging soon, and I'll probably go to dry hopping in the keg. Seems to work better from the beers I've had dry hopped in the keg vs. primary or secondary.

 

It's so much easier, I predict you will wonder why you waited so long to switch.