The co2 "blanket"

[bob1852]

Again, if someone is making beer they love and see no need for improvement, I am their biggest cheerleader

Yooper has been a cheerleader for a beer or two of mine.

 

[p_p]

Check this out:
https://www.homebrewtalk.com/showthread.php?t=580532

...using sulphites to reduce oxygen's effects on beer.

Thanks for the link, I'll have a read.

 

[Yooper]

Yooper has been a cheerleader for a beer or two of mine.

Oh, well, you being a Ninkasi winner and all, I did manage to choke a few down..........................

Seriously the most surprising beer of yours that I thought was outstanding was the habanero IPA. I never had a pepper beer that I enjoyed all that much, and you manage to do that extraordinarily well.

The thing that is surprising to new brewers is that you can make fantastic beer in a bucket and a pot, and you don't really need all that fancy beer. I've had some wonderful beers made with rustic equipment, and awful beers out of sabco brewmagics. It's really all about the technique and skill and attention by the brewer.

Minimizing oxygen exposure is important, but only one of the keys to great beer. Other things, like yeast pitching rate, temperature control, etc, are equally important. If one is ignored, the beer will suffer.

 

[Alt]

There was an eyeopening study done on closures and better bottles that really has surprising results:
http://www.better-bottle.com/pdf/CarboyPermeabilityStudy.pdf (carboy stuff)
http://www.mocon.com/assets/documents/PPS_Article_highq.pdf
This was done to test the oxygen permeability of the better bottles, but the best part of what I learned was in the closures part.

Wow, this has me wondering about the permeability of the silicon gasket on my SsBT Chronical fermenter. Fortunately any effect for me would be tempered by primary durations of three weeks or less. I do my aging in kegs.

Alt

 

[LKABrewer]

I agree with Yooper that the #1 flaw I see in homebrew competitions that keep good beers from being great is oxidation. I bet if you polled some of the award-winning homebrewers in your area and asked them how they handle oxidation, few would answer 'I don't worry about it'. There is no co2 blanket, and every step in your process matters. It is up to each brewer to decide how much oxygen is acceptable in their wort/beer vs. the amount of time/effort needed to avoid it.

If you are bottle conditioning your beer, yeast is a great anti-oxidant and will absorb the headspace oxygen to keep any further oxidation from happening. It will not absorb oxidation already done to your beer from HSA/cold crashing/dry hopping/etc.

 

[p_p]

I agree with Yooper that the #1 flaw I see in homebrew competitions that keep good beers from being great is oxidation. I bet if you polled some of the award-winning homebrewers in your area and asked them how they handle oxidation, few would answer 'I don't worry about it'. There is no co2 blanket, and every step in your process matters. It is up to each brewer to decide how much oxygen is acceptable in their wort/beer vs. the amount of time/effort needed to avoid it.

If you are bottle conditioning your beer, yeast is a great anti-oxidant and will absorb the headspace oxygen to keep any further oxidation from happening. It will not absorb oxidation already done to your beer from HSA/cold crashing/dry hopping/etc.

Thanks for the comment.
I am at peace (for now) with the process up to the point where the beer reaches FG. From there I think I am picking up more oxygen than I should, considering beer happens in a bucket.

Based on a PDF posted in this thread, I was thinking of adding some sodium metabisulphite to scavenge some of the oxygen picked up to the point where the beer gets bottled. Would that be any good? I reckon my water has about 20ppm of sodium and was thinking of adding about 10ppm smb which would scavenge roughly 1.5ppm oxygen without significantly increasing the amount of sodium.

Don't know, the idea is probably nonsense... Just another stepping stone in the road of beer education.

 

[TheHairyHop]

Thanks for the comment.
I am at peace (for now) with the process up to the point where the beer reaches FG. From there I think I am picking up more oxygen than I should, considering beer happens in a bucket.

Based on a PDF posted in this thread, I was thinking of adding some sodium metabisulphite to scavenge some of the oxygen picked up to the point where the beer gets bottled. Would that be any good? I reckon my water has about 20ppm of sodium and was thinking of adding about 10ppm smb which would scavenge roughly 1.5ppm oxygen without significantly increasing the amount of sodium.

Don't know, the idea is probably nonsense... Just another stepping stone in the road of beer education.

It's not nonsense, although I don't know how rapidly the bucket would absorb CO2. You may want to check on the German brewing forums if you're interested in reducing DO

 

[brewbama]

...if someone is making beer they love and see no need for improvement, I am their biggest cheerleader. I think that's great, and I think they should absolutely continue what they are doing. So I'm not trying to be a naysayer here, I'm just saying that often flavor is in the tastebuds of the beerholder.

+1. Ditto. IMO, this is the whole point of the hobby. Cheers!

 

[wishingiwasfishing]

Oh, one other thing I forgot to mention that I think is important is that long primaries with large headspace could be troublesome as well, for those who are doing those 4 week (or longer) primaries thinking that it preserves this mythical c02 blanket. There was an eyeopening study done on closures and better bottles that really has surprising results:
http://www.better-bottle.com/pdf/CarboyPermeabilityStudy.pdf (carboy stuff)
http://www.mocon.com/assets/documents/PPS_Article_highq.pdf
This was done to test the oxygen permeability of the better bottles, but the best part of what I learned was in the closures part.

I started looking around at this when I had a wine that got a little bit of oxidation in a glass carboy, with one of those orange "carboy caps".

Also, keep in mind that even under airlock, oxygen still enters the fermenter through cracks in the airlock, and even through the water or vodka in the airlock. That's one reason winemakers are so meticulous about always topping up their wine, as the wine spends much more time in the carboy than a typical beer. There is maybe an inch or so of headspace in the narrowest part of my carboys, so that there is perhaps an inch of wine exposed to the area just below the bung. This has a protective effect on the wine, but many winemakers use sulfites routinely in the amount of 50 ppm as an antioxidant. With beer, that is rarely done.

I have to admit I'm a bit surprised at the two articles. So, what are the best options we have to implement in our homebrewing, to combat the permeability issues with airlocks?


Thanks,
Dan

 

[Dfitz]

Let me say what I know about a CO2 blanket. If you are fermenting and the top to your fermenter is not air tight (we're talking a rather large fermente better than 20 gal) it'll hold til you run your batch provided you appreciate what your doing.

 

[Yooper]

I have to admit I'm a bit surprised at the two articles. So, what are the best options we have to implement in our homebrewing, to combat the permeability issues with airlocks?


Thanks,
Dan

Well, I started using my kegs even for long term storage of wine. A keg is a perfect secondary vessel, since you can purge and top up the keg with c02.

Some people will do all transfers with co2, from fermenters to packaging.

I don't have the ability to do that, so I just rack the beer to the keg as best I can to avoid oxygen pick up. I don't store beers for a long time, but if I did I'd definitely store in a keg for aging and transfer with a beergun or something like that if I was bottling, to avoid excess oxidation. I think some oxygen pick up is inevitable, but we want to avoid it whenever possible.

 

[wishingiwasfishing]

Well, I started using my kegs even for long term storage of wine. A keg is a perfect secondary vessel, since you can purge and top up the keg with c02.

Some people will do all transfers with co2, from fermenters to packaging.

I don't have the ability to do that, so I just rack the beer to the keg as best I can to avoid oxygen pick up. I don't store beers for a long time, but if I did I'd definitely store in a keg for aging and transfer with a beergun or something like that if I was bottling, to avoid excess oxidation. I think some oxygen pick up is inevitable, but we want to avoid it whenever possible.

Thanks for the thoughts. I've only got 4 kegs right now, so I don't think I'm set up to CO2 transfer and store beers in extra kegs, but I do like the thought. I did just pick up a glass airlock to start using.

I've seen people say that homebrewers are scrutinizing the oxidation issue too much. I don't think so. If you've got a source of oxidation in your brewing you try to take care of it in a sensible manner. Looking for better materials and better methods certainly isn't going overboard in my book.

Thanks again,
Dan

 

[Yooper]

Thanks for the thoughts. I've only got 4 kegs right now, so I don't think I'm set up to CO2 transfer and store beers in extra kegs, but I do like the thought. I did just pick up a glass airlock to start using.

I've seen people say that homebrewers are scrutinizing the oxidation issue too much. I don't think so. If you've got a source of oxidation in your brewing you try to take care of it in a sensible manner. Looking for better materials and better methods certainly isn't going overboard in my book.

Thanks again,
Dan

Well, some people have said to me "I have NO oxidation in MY beers", and the truth is that perhaps they don't really have the world's best palate. The most common flaw that I see in competitions I judge is oxidation.

That's not a problem if you're brewing for yourself, and you can't taste it or it doesn't bother you- so sure, those brewers probably believe some of us are practicing overkill.

 

[lhommedieu]

I look at it from a dual perspective:

Being able to transfer wort or beer from one container to another in a closed (or mostly closed) system using CO2 and pumps is easier than doing it by hand (lifting, siphoning, carrying, pouring, etc.) and has the added benefit of reducing contact with oxygen.

That is not to say that one cannot significantly reduce contact with oxygen using manual methods if one is focused and careful - or increase contact with O2 using an automatic system if one is not.

I no longer transfer from glass carboys because using stainless steel conicals and cornys is easier, safer, and more efficient. I followed the usual safety standards, and while I don't think that I was ever in any danger of breaking a carboy (the seal on the carboy cap would always come off first if pressure built up), I found the system to be clunky and time-consuming. Plus, it's much easier fill a corny with CO2 and purge it, move it around, etc.

 

[techbrau]

Closed transfers still are not good enough if you want to target commercial standards. I measured between 0.3 and 0.4 ppm pickup with my DO meter when closed transferring from my fermenting keg into a serving keg that was purged with more than 20 times its volume worth of co2 by hooking it up in line during fermentation, so that the fermentation co2 flowed into the liquid dip tube and out the gas post via a spunding valve set to a low pressure. There are just way too many tiny hiding spots for air in your transfer lines, disconnect hardware, kegs, etc. It literally takes milliliters worth of air to raise the DO of a batch of fermented beer from zero to 0.2-0.3 ppm.

The one and only way I have found to ensure a DO under 0.1 ppm in the package is to induce another fermentation in the packaging vessel (e.g. by adding krausen, priming sugar, racking with residual fermentable extract, etc).

And yes, going from 0.4+ ppm DO in the package to under 0.1 ppm is what makes the difference between an IPA losing hop aroma/flavor over the course of 3 weeks vs 6 months.

 

[lhommedieu]

Closed transfers still are not good enough if you want to target commercial standards. I measured between 0.3 and 0.4 ppm pickup with my DO meter when closed transferring from my fermenting keg into a serving keg that was purged with more than 20 times its volume worth of co2 by hooking it up in line during fermentation, so that the fermentation co2 flowed into the liquid dip tube and out the gas post via a spunding valve set to a low pressure. There are just way too many tiny hiding spots for air in your transfer lines, disconnect hardware, kegs, etc. It literally takes milliliters worth of air to raise the DO of a batch of fermented beer from zero to 0.2-0.3 ppm.

The one and only way I have found to ensure a DO under 0.1 ppm in the package is to induce another fermentation in the packaging vessel (e.g. by adding krausen, priming sugar, racking with residual fermentable extract, etc).

And yes, going from 0.4+ ppm DO in the package to under 0.1 ppm is what makes the difference between an IPA losing hop aroma/flavor over the course of 3 weeks vs 6 months.

Is this a problem if you've transferred to the keg with enough sugar in the wort for the yeast in suspension to continue fermentation? I like the idea of a spunding valve to maintain appropriate CO2 levels while periodically venting excess pressure - but once that pressure has been maintained, where is the risk of oxygenation?

 

[techbrau]

Is this a problem if you've transferred to the keg with enough sugar in the wort for the yeast in suspension to continue fermentation? I like the idea of a spunding valve to maintain appropriate CO2 levels while periodically venting excess pressure - but once that pressure has been maintained, where is the risk of oxygenation?

No, it's not a problem. Sorry, I should've been more specific in my previous post - when I measured the 0.3-0.4 ppm DO pickup after doing a closed transfer, that was with beer that was fully fermented out in the primary vessel prior to racking.

It really takes barely any sugar at all (as in 1 gravity point or less) to clean up whatever oxygen is in the packaging vessel, be it a bottle or keg. Nowadays, I am actually allowing my primary fermentation to build pressure during the final 3 or so days (my lager primaries are about 9-10 days long, pitching at 43 F and fermenting at 48 F). This way, the beer is mostly carbonated when it's ready to rack to the lagering/serving keg, and the majority of the yeast has already dropped out of suspension. I'm still carrying over 1-2 gravity points worth of sugar into the lagering keg along with just a little bit of active yeast in suspension. I finish fermenting out these last couple points in the lagering/serving keg with a spunding valve attached.

I learned the hard way that it's not good to carry over too much yeast into the lager keg. I had autolysis take hold within 6-8 weeks, verified not only by the taste but also by the beer pH rising above 4.5.

Personally I am thinking that the ultimate in oxygen control for the homebrewer is to abandon kegging altogether and go back to bottle carbonating. As long as you have an airtight primary fermentation vessel without a lot of headspace (like a keg), you could probably get away with fermenting out in primary, giving the yeast a couple days to mostly settle out, and then dosing each bottle with a carbonation tab or a scant teaspoon of sugar syrup before bottling. Bottle carbonation will get you close to zero free oxygen in the package, resulting in an extremely long shelf life as long as the beer wasn't oxygen damaged prior to bottling, and provided you don't carry too much yeast into each bottle.

 

[ferm.adventures]

Lots of interesting stuff here... Really explains why that awesome IPA I made recently turned to dog crap in three weeks. Luckily I had already drank 80% of it by then!

 

[55x11]

Personally I am thinking that the ultimate in oxygen control for the homebrewer is to abandon kegging altogether and go back to bottle carbonating. As long as you have an airtight primary fermentation vessel without a lot of headspace (like a keg), you could probably get away with fermenting out in primary, giving the yeast a couple days to mostly settle out, and then dosing each bottle with a carbonation tab or a scant teaspoon of sugar syrup before bottling. Bottle carbonation will get you close to zero free oxygen in the package, resulting in an extremely long shelf life as long as the beer wasn't oxygen damaged prior to bottling, and provided you don't carry too much yeast into each bottle.

I am sorry but zero is a very small number when we are talking about PPMs of oxygen and how it takes about 12 mL of oxygen at atmospheric pressure to result in 1ppm of oxygen in a keg.

How many ppms is the oxygen content in bottles after conditioning? Or, put it differently - how much oxygen do the yeast consume while reducing gravity by 1 point?

I am sure yeast will scavenge some *oxygen*, but how much - and how much oxygen was introduced at bottling that has to be cleaned up, is a big question - at least to me.

Finally, a keg is just a giant bottle. Whatever happens in a bottle will also happen in a keg, even if its x50 volume. Conditioning happens in both vessels. You could make an argument that adding 0.5 volumes of CO2 equivalent as sugar to the keg (about 1 gravity point) should - supposedly - create an absolutely oxygen free-beer. Then naturally carbed casks and kegs and slightly sugar-carbed kegs (or 1 point gravity unfinished fermentation - which could be a big fraction of fermentations) will be completely oxygen free.

I am pretty sure most homebrewers practicing closed transfer introduce an order of magnitude less oxygen in their beer as compared to brewers who bottle.

You may be talking about 1 mL in 5Gallon keg, but that's the same as 20 micro-liters (20 millimeters cubed) per bottle.

What's the difference between keg conditioning and bottle conditioning?

 

[orangehero]

I measured between 0.3 and 0.4 ppm pickup with my DO meter when closed transferring from my fermenting keg into a serving keg that was purged with more than 20 times its volume worth of co2 by hooking it up in line during fermentation, so that the fermentation co2 flowed into the liquid dip tube and out the gas post via a spunding valve set to a low pressure.

Curious what DO meter you are using?

 

[55x11]

Thanks for the thoughts. I've only got 4 kegs right now, so I don't think I'm set up to CO2 transfer and store beers in extra kegs, but I do like the thought. I did just pick up a glass airlock to start using.

I've seen people say that homebrewers are scrutinizing the oxidation issue too much. I don't think so. If you've got a source of oxidation in your brewing you try to take care of it in a sensible manner. Looking for better materials and better methods certainly isn't going overboard in my book.

Thanks again,
Dan

Doing CO2 transfers (a very sensible idea) - does not depend on how many kegs you own. If you have a CO2 tank and some accessories (some stoppers, caps and some tubing), you and everyone else can do a closed transfer. And I believe you always should.

finally - not sure what "glass airlock" has to do with it. If you mean regular airlock we all use, but made out of glass, I seriously doubt that should be anywhere near top 20 priorities in terms of oxidation.

 

[p_p]

Slightly off subject question ... when we dry hop, we have to open the fermentation vessel which inevitably ends up with some air inside. If by the time we do that there isn't any sugars left for the yeast to consume, is there any merit in adding a bit of sugar to get the yeast going back again and take in the oxygen we have introduced in the process?

 

[Yooper]

Slightly off subject question ... when we dry hop, we have to open the fermentation vessel which inevitably ends up with some air inside. If by the time we do that there isn't any sugars left for the yeast to consume, is there any merit in adding a bit of sugar to get the yeast going back again and take in the oxygen we have introduced in the process?

I don't buy that a small fermentation "erases" oxygen pick up. sure, the yeast may metabolize the 02, but the o2 has already been in contact with the beer and then it's too late.

I'm a long-time winemaker and one of the things we do as a matter of course is to use sulfites, around 50 ppm, in our wine. The s02 binds to the wine, so that oxygen cannot during transfers and bottling. This happens on molecular scale, and once the o2 is in there, a small bottle fermentation won't 'erase' it.

When I dryhop, I just do the best I can to avoid allowing too much oxygen into the vessel, either by transferring onto the dryhops or by quickly and gently opening the primary at the tail end of fermentation. It is not ideal, but I'm satisfied that I'm doing the best I can at this point without a way to do closed transfers. Others are more particular than I am; others may be less particular. My philosophy is to do the best I can and I'm satisfied with that. If I was doing a lot of aged beers, I may look at it differently but most of my APAs and IPAs are consumed rather quickly at my house.

 

[h22lude]

I don't buy that a small fermentation "erases" oxygen pick up. sure, the yeast may metabolize the 02, but the o2 has already been in contact with the beer and then it's too late.

I'm a long-time winemaker and one of the things we do as a matter of course is to use sulfites, around 50 ppm, in our wine. The s02 binds to the wine, so that oxygen cannot during transfers and bottling. This happens on molecular scale, and once the o2 is in there, a small bottle fermentation won't 'erase' it.

When I dryhop, I just do the best I can to avoid allowing too much oxygen into the vessel, either by transferring onto the dryhops or by quickly and gently opening the primary at the tail end of fermentation. It is not ideal, but I'm satisfied that I'm doing the best I can at this point without a way to do closed transfers. Others are more particular than I am; others may be less particular. My philosophy is to do the best I can and I'm satisfied with that. If I was doing a lot of aged beers, I may look at it differently but most of my APAs and IPAs are consumed rather quickly at my house.

What would adding sulfites to beer at dry hopping do? Would that help remove any O2?

 

[wishingiwasfishing]

Doing CO2 transfers (a very sensible idea) - does not depend on how many kegs you own. If you have a CO2 tank and some accessories (some stoppers, caps and some tubing), you and everyone else can do a closed transfer. And I believe you always should.

finally - not sure what "glass airlock" has to do with it. If you mean regular airlock we all use, but made out of glass, I seriously doubt that should be anywhere near top 20 priorities in terms of oxidation.

Hi 55x11,

I was referring to doing co2 transfers when using a keg as the primary also. The airlock statement was in reference to the article, posted in this thread, which referred to Oxygen Transfer Rates for Carboys and Airlocks.


I'm trying to to use better practices and reduce oxidation in areas where I'm able without an extremely high cost.

I do many of the same things that I believe most homebrewers do. I purge the keg, I purge any dryhop additions as best I can, I purge my transfer hoses. I will certainly start doing closed transfers on my next batch. But additionally, I'll continue to entertain, even if I don't implement, other means of reducing oxidation...such as using the keg as a primary.

While it would be beneficial to have a top 20 list of oxidation issues, I believe the best I may be able to do is to make incremental reductions in oxidation in a wide variety of different areas.

Thanks for the tips and advice,
Dan

 

[techbrau]

I don't buy that a small fermentation "erases" oxygen pick up. sure, the yeast may metabolize the 02, but the o2 has already been in contact with the beer and then it's too late.

I'm a long-time winemaker and one of the things we do as a matter of course is to use sulfites, around 50 ppm, in our wine. The s02 binds to the wine, so that oxygen cannot during transfers and bottling. This happens on molecular scale, and once the o2 is in there, a small bottle fermentation won't 'erase' it.

When I dryhop, I just do the best I can to avoid allowing too much oxygen into the vessel, either by transferring onto the dryhops or by quickly and gently opening the primary at the tail end of fermentation. It is not ideal, but I'm satisfied that I'm doing the best I can at this point without a way to do closed transfers. Others are more particular than I am; others may be less particular. My philosophy is to do the best I can and I'm satisfied with that. If I was doing a lot of aged beers, I may look at it differently but most of my APAs and IPAs are consumed rather quickly at my house.

You are correct that yeast cannot "erase" oxidative damage once it has happened, but how fast the damage happens depends a lot on temperature. At mash temperatures, the oxidation of malt compounds happens within seconds to minutes, whereas at fermentation temperatures it's more at the scale of several hours to days. I actually do use about 50 ppm sulfite in my mash water which has already been preboiled to drive off the dissolved oxygen that was already present; if you haven't seen this yet, I would encourage you to read it (I'm one of the authors):

http://www.germanbrewing.net/docs/Brewing-Bavarian-Helles-v2.pdf

Even a small amount of yeast + sugar will metabolize free oxygen extremely fast, on the order of minutes. Check this out:

http://forum.germanbrewing.net/viewtopic.php?f=50&t=355

As long as beer is not exposed to oxygen for more than a few minutes before fresh sugar/yeast is added, the free oxygen picked up during bottling will be scavenged by the yeast much faster than it can react with the beer itself.

 

[techbrau]

Curious what DO meter you are using?

ExTech DO600

 

[techbrau]

I am sorry but zero is a very small number when we are talking about PPMs of oxygen and how it takes about 12 mL of oxygen at atmospheric pressure to result in 1ppm of oxygen in a keg.

How many ppms is the oxygen content in bottles after conditioning? Or, put it differently - how much oxygen do the yeast consume while reducing gravity by 1 point?

I am sure yeast will scavenge some *oxygen*, but how much - and how much oxygen was introduced at bottling that has to be cleaned up, is a big question - at least to me.

Finally, a keg is just a giant bottle. Whatever happens in a bottle will also happen in a keg, even if its x50 volume. Conditioning happens in both vessels. You could make an argument that adding 0.5 volumes of CO2 equivalent as sugar to the keg (about 1 gravity point) should - supposedly - create an absolutely oxygen free-beer. Then naturally carbed casks and kegs and slightly sugar-carbed kegs (or 1 point gravity unfinished fermentation - which could be a big fraction of fermentations) will be completely oxygen free.

I am pretty sure most homebrewers practicing closed transfer introduce an order of magnitude less oxygen in their beer as compared to brewers who bottle.

You may be talking about 1 mL in 5Gallon keg, but that's the same as 20 micro-liters (20 millimeters cubed) per bottle.

What's the difference between keg conditioning and bottle conditioning?

You are absolutely spot on with respect to how little air/oxygen it takes trapped in the package to damage the beer, and you are also correct that naturally carbonating inside the keg is initially as good as carbonating in a bottle, perhaps even better if you've done a good job purging the keg first and performed a closed transfer.

The problem I have with corny kegs is the hardware we connect to them. Especially to the gas post.

Take a look at one of your grey quick disconnects. Turn it upside down, then fill it with water until the water level covers the poppet valve and reaches the o-ring. There's several ml there. When you connect the QD to the gas post, that space is filled with air which gets trapped by the o-ring, and enters the keg through the gas post once you force the disconnect on and the poppet valves open. Every time you disconnect or reconnect the QD, you are injecting several ml of air into your keg.

So why not just connect the gas once and leave it connected? Because your gas lines are oxygen permeable. To what extent depends on the material they're made of, but nearly all flexible plastic tubing has some oxygen permeability. It's not a fast diffusion, mind you, but a month or more is a very, very long time and your gas line has quite a bit of surface area. We are talking about 2 or 3 milligrams worth of oxygen being all it takes to damage the beer.

Then there's the issue of the purity of the CO2 source itself if you're using it to push the beer out. This has been covered in a few other threads here as well as on the German brewing forum, but food grade (99.5% pure) isn't good enough. Ideally the purity you want is 99.998% or better.

I think kegs are fine if you are running a bar or hosting a party or if you can empty them within 2 weeks or so of tapping them. Since I don't drink that fast, I'm considering going back to bottling.

Finally I want to emphasize that if you're happy with your beer, you don't have to go through all of this trouble. This stuff is really most important for extremely oxygen sensitive beers (like light lagers brewed with the low oxygen process from the paper I linked to, or a NE IPA whose hop character you want to last for more than 2 weeks).

 

[TheHairyHop]

Slightly off subject question ... when we dry hop, we have to open the fermentation vessel which inevitably ends up with some air inside. If by the time we do that there isn't any sugars left for the yeast to consume, is there any merit in adding a bit of sugar to get the yeast going back again and take in the oxygen we have introduced in the process?

from what I've read, yes. I added a bit of SMB to my beer while hop steeping and then added some dissolved DME to carb when I added my "keg dry hop". Fantastically hoppy IPA going on 6 weeks. Obviously a small sample size there, but the articles I've read back it up

 

[wishingiwasfishing]

You are correct that yeast cannot "erase" oxidative damage once it has happened, but how fast the damage happens depends a lot on temperature. At mash temperatures, the oxidation of malt compounds happens within seconds to minutes, whereas at fermentation temperatures it's more at the scale of several hours to days. I actually do use about 50 ppm sulfite in my mash water which has already been preboiled to drive off the dissolved oxygen that was already present; if you haven't seen this yet, I would encourage you to read it (I'm one of the authors):

http://www.germanbrewing.net/docs/Brewing-Bavarian-Helles-v2.pdf

Even a small amount of yeast + sugar will metabolize free oxygen extremely fast, on the order of minutes. Check this out:

http://forum.germanbrewing.net/viewtopic.php?f=50&t=355

As long as beer is not exposed to oxygen for more than a few minutes before fresh sugar/yeast is added, the free oxygen picked up during bottling will be scavenged by the yeast much faster than it can react with the beer itself.

Thanks for the links. Good reading for a homebrewer!