Forked Thread: Dissolving Oxygen in Wort

[biertourist]

In an attempt to keep the "Medal Winners Equipment" thread (http://www.nationalhomebrewclub.com/forum/index.php/topic,9282.15.html
) on topic, I've created this new thread on oxygenation and dissolving oxygen in wort.

Bear with me here for a minute, I've been brewing a few years but still consider myself very much a novice. I have a fair bit of experience of dissolving gases in water from my normal life and I'm going to take an educated guess that the same principals apply to wort as to water.

Oxygen will naturally dissolve into water under atmospheric pressure but there is a limit on the amount, we'll call this 100% saturation.
The amount of oxygen it takes to get to 100% saturation depends on a few factors; temperature, pressure and whatever else is already dissolved in there.

Cold water can hold more oxygen than hot water, fresh water more than salt water, water at sea level more than at the top of a mountain. There are big tables to show you what amount of oxygen water can hold for these given parameters.

We can push the oxygen content over 100% and 'supersaturate' the water, we still measure it as a % of what it will hold under normal conditions.
e.g. the same principal applies with carbon dioxide when we condition our beers, we have 100% saturation of CO2 due to the high pressure inside the vessel keeping it in solution, when we release that pressure the saturation potential of the beer drops sharply and the beer becomes supersaturated and comes out of solution and we have fizz. Simple (I hope).

When it comes to fermenting wort we will not be fermenting it under pressure (or most of us won't anyway) and the temperature will remain constant. By using pure oxygen in the wort it is possible to get the saturation level above 100% and if we don't disturb it it may stay like that for up to 24 hours, but it will come down to 100% on it's own without any other factors in that time period (assuming it behaves the same way as water). This isn't really long enough to have any effect on the yeast I don't think.

The next thing to consider is what happens when the yeast gets active, it produces CO2 which will start to form bubbles once it gets over 100% saturation. This bubbling of the CO2 will strip out any excess oxygen extremely quickly leaving it back at 100% saturation (until the yeast starts to use it up), for practical purposes for homebrewers I'd nearly call it instantaneous.

Aeration (splashing, vigorous stirring, whisking, bubbling air) is actually pretty good at getting water up to 100% saturation but the one the you have to be aware of is the temperature. If you go through your aeration procedure before the wort has fully cooled then you may leave your yeast short of oxygen. You could get your wort to 100% saturation at 30 degrees but then as it cools down to say 21 degrees it may only be 80% saturated making it much harder for the yeast.

What do I do? I get my beer down to 21 degrees and let it slowly cascade into my fermenter, the drop is about 20cm from the tap to the very top of the fermenter so it gets a good drop. I sometimes also give it a very vigorous stir with a whisk just to make sure. I got a loan of an oxygen meter from a friend and measured it in the past and it was 100%, happy days.

With regards to putting air/oxygen through a stone in the wort, to get the best efficiencies they need water pressure above them to force the oxygen into the solution. In practice you need to make sure they are at least 30cm below the surface or most of it will just vent off.

The one advantage using pure oxygen over air will be speed, using pure oxygen will get you to 100% saturation quicker than air but that's personal choice.


Now waiting for Will to shoot me to ribbons and show me up as an idiot! 

Some great points in here, Bogwoppit.

On the cold water vs. hot water subject: Yes, cold liquids have an ability to have more Oxygen (or CO2) dissolved in them, BUT hotter water / wort will absorb that oxygen much faster and when you're oxygenating / aerating with a stone you're applying a very large amount of oxygen in a very short period of time and you need as much absorbed before the bubbles reach the surface of the wort / liquid. -Unlike when you're force carbonating a keg, when you oxygenate you're not going to end up forcing a bunch of oxygen into the head space and having it slowly dissapate down into the wort over time -we need it to be rapidly absorbed. (In practice brewers generally find the opposite - with the same flow rate and oxygenation times they end up with more O2 in the wort at higher temperatures rather than lower temperatures because of the faster absorption rate.)


You covered a great list of factors (temperature and pressure) but turbulence also plays a part. 

There's a lot of great theory in there but when it comes to actual application to oxygenation practices used in breweries most of it isn't applicable. -Example: The idea that the oxygen level will stay where it is for up to 24 hours if the wort doesn't get disturbed doesn't seem to play out at all.  There are a number of brewers and home brewers who use dissolved oxygen meters to measure oxygen levels in wort and they've found the opposite; they recommend to pitch yeast immediately after oxygenating because the O2 levels decline fairly rapidly.


I also find the statement that aeration gets you up to 100% saturation very misleading; it gets you up to 100% saturation based upon the oxygen levels in the air -it will most definitely not get you to the same 100% saturation levels if you're using O2 from a bottle.  8 ppm is shown again and again to be the maximum dissolved oxygen levels you can get from air regardless of how you get that air into solution; even with high gravity worts, within 3 ish minutes at higher flow rates (3 liters per minute) you can get to 20ppm with pure oxygen.  -So I'm curious what this "% saturation" level you keep referring to actually means... Is it just comparing the oxygen levels of the water / wort to the oxygen levels that are in the air?  -If so it's an interesting data point for aeration methods in general but ppm is more applicable to brewing and in making a comparison between oxygenation / aeration methods as it's an absolute measure vs. a relative one.


The size of the bubbles also make a difference when oxygenation / aerating when it comes to speed of getting the oxygen dissolved -you could just attach gas hose / tubing to an O2 bottle and bubble it through the wort and most of the oxygen is just going to float to the top of your fermenter as bubbles and then evolve into the atmosphere.  Use the same amount of O2 bubbled through a 5 micron stone and you'll see far higher DO levels; use a 0.5 micron stone and you'll see further increases in dissolved oxygen.

-Again because in brewing we actually don't want to oxygenate the wort and create staling reactions so we oxygenate quickly then pitch the yeast quickly and the yeast will quickly drop those dissolved O2 levels back down minimizing oxidizing staling reactions.  -This limits us to a narrow band of techniques that are available so we really only need to look at oxygen dissolving under those conditions; most of the theoretical variables don't play a huge part then.


In practice most breweries oxygenate inline during transfer to / from their plate and frame chillers; many have settled on the input / hot side of the chiller as the O2 will absorb more quickly on the hot side and the turbulance generated in the chiller itself also helps the O2 dissolve, the rapidly dropping temperature as the wort passes through the chiller helps it hold more O2 that way, too.  -Some breweries choose to add the O2 stone on the output side of the plate and frame chiller to avoid any hot side oxidation but absorption is going to be slower and they'll probably waste a bit more O2 to reach the same PPM levels that way.  (when the oxygenated wort is only going to stay hot for a couple seconds, I wouldn't worry about it and personally I'd add it on the hot side of my chiller) --Brewers that aerate vs. oxygenate inline with sterile filters and air pumps or even venturi aeration also choose to aerate on the hot side of their chillers as the hot wort can sterilize any microflora that might be introduced into the air stream.



It's also worth noting that oxygen dissolves quite a bit slower in high gravity worts and ironically the yeast need more oxygen for high gravity worts.  Standard practice is to target a minimum of 12-15 ppm for lagers but of course if you're brewing a doppelbock higher is probably better.


I just recently came across some great data points from a home brewer on HomeBrewTalk who was oxygenating with a 0.5 micron stone and tracking dissolved oxygen in PPM for each of his fermentations -he also included temperature, time, o2 speed from the regulator and starting gravity for each fermentation.  His data doesn't plot to a curve super nicely but it's still pretty good.  Let me see if I can go find it...



Adam

[DEMPSEY]

 

[biertourist]

I'll also add that I'm fairly certain that oxygen isn't absolutely required.

Again because in general in brewing we want to get oxygen quickly into and out of the wort to prevent oxidation / staling reactions the typical (good) practice is to dissolve all of the oxygen that we need for the fermentation quickly and then pitch the yeast quickly; BUT it's useful to realize that yeast need an actual total QUANTITY of oxygen and not a particular CONCENTRATION of oxygen dissolved in the wort.  -It sure is easier on the brewer if all that oxygen can be added in one go, especially when you have to sanitize that aeration stone each time, but it's not actually necessary.

-Let me clarify that statement: ppm is a measure of the CONCENTRATION of oxygen; not the total quantity of of oxygen.  What the yeast REALLY need is x liters of oxygen for a given fermentation -obviously that total quantity goes up as the batch size goes up, the amount you need also goes up as the gravity goes up and it varies whether you're brewing a lager or an ale; so it's generally a lot easier, especially given typical brewing procedure of injecting all the O2 at once, to think in terms of PPM because you don't have to adjust those 8 ppm / 12 ppm numbers based upon your batch size.  -BUT if you think in terms of total oxygen or "my yeast for this beer need x liters of oxygen in total" and you know that you're only going to get half of that from aerating with a stone or aerating by shaking @ 8 ppm, technically you can just wait for the yeast to take that oxygen up and then give them more later. (This mental model helps to understand that oxygenation setups aren't actually required and you can just add a "double shot" of oxygenation via aeration because it's total oxygen that matters and not a particular concentration.)

--I'm completely and totally making this number up as an example; it could be ORDERS OF MAGNITUDE off from being accurate: let's say your yeast need 4 liters of oxygen (actually dissolved into the wort so they can use it, not just 4 liters coming out of your O2 bottle) for a 23L batch of 1.085 doppelbock and you know that they'll only get 2 liters of oxygen from shaking the carboy for 10 minutes (which remember puts you at a concentration of 8ppm) -If you wait 2 hours for the oxygen to take up the oxygen and thake the carboy again for 10 minutes and add another 2 liters, your yeast will have the total 4 liters of oxygen that they need to complete the high alcohol fermentation.


All that to say that if you have a fermentation that says it needs 15 ppm (strong lager) and you know that you can get 8 ppm by shaking the carboy / using an aquarium pump and aeration stone, then do that, then add your yeast and wait 2 hours and do it again and the yeast will have the oxygen that they need.  Yeast don't care about concentration but total oxygen.  Concentration is just an easier way to calculate total oxygen that we use as a shortcut.  (Note: A problem with this could be that you might only get 6 ppm of oxygen into solution by shaking a carboy of 1.085 wort as oxygen is less soluable at higher wort gravities.)


Also VERY important: You don't want to wait say 24 hours or longer to do your 2nd aeration addition as you can create off flavors at this point.


Another related interesting idea: Again, it's the YEAST that need the oxygen and not the wort so why not aerate your starter and get your yeast half of their O2 that way -starters are also lower gravity than this theoretical 1.085 wort so more O2 will dissolve into the starter.  Do your starter for 24 hours, let it settle out and then for the 2 or 3 hours you're brewing on brewday put it back on the stir plate so that you're aerating the starter wort and helping those yeasts to build up their sterol reserves.


Last related idea: Yeasts need oxygen to build up lipid / fatty acid reserves for other metabolic processes and building cell walls. You should consider those lipids as a functional equivalent of oxygenation.  -If you naturally pass quite a bit of trub from your boil kettle into your fermenter the yeast will get extra lipids / fatty acids directly from that trub which reduces the need for oxygen (one of the reason high trub fermentations proceed faster).   All I'm saying is you want to think of how much "fatty acids + oxygen" that your yeast need.  If you KNOW that you pass a lot of trub, you will want to drop your O2 levels a bit because too much fatty acids+oxygen results in fusol alcohol production which means beers that are hot or taste more alcoholic than they are; it also reduces esters that you might actually want in your beer.  Fatty acids and oxygen should be thought of together and it is this combination of fatty acid levels+o2 levels that need to be balanced.


</end oxygen brain dump>

Adam

[Bogwoppit]

Interesting points there Adam.

I'll have to wait until I'm back I the office on Monday to do a detailed response to that.

You're obviously fairly experienced with the process from a brewer's point of view, I'm only translating what I know from fish biology but I'm pretty certain the exact same principals apply.

I'd agree pretty much with everything you say but I'll get back on on Monday and expand in my original and bore everyone some more.

Allan