Should you oxygenate/aerate dry yeast?

[brazilhead]

My copy of Chris and Jamil's yeast book still hasn't arrived. Can anyone say if it address this topic specifically? I'm also hoping Brew Strong will get eventually around to discussing this topic as I e-mailed them it several weeks ago.

[spiderwrangler]

Isn't the oxygen being utilized largely for sterol formation, which aids in the ability of yeast to bud? I've seen some mention of oxygen being used in aerobic respiration in reference to the period of time before there are classic visible signs of fermentation (bubble formation), but aerobic respiration produces MORE carbon dioxide than fermentation does. Granted, the first amounts of CO2 being produced will tend to stay in solution until it reaches saturation, but an aerobically respiring yeast will produce three times as much CO2 as a fermenting one.
Other places it seems that folks refer to yeast growth as only happening during aerobic respiration, but it seems to me that reproduction and budding must also be happening during anaerobic conditions.

[spiderwrangler]

From Danstar's FAQ:
"I always aerate my wort when using liquid yeast. Do I need to aerate the wort before pitching dry yeast?
No, there is no need to aerate the wort but it does not harm the yeast either. During its aerobic production, dry yeast accumulates sufficient amounts of unsaturated fatty acids and sterols to produce enough biomass in the first stage of fermentation. The only reason to aerate the wort when using wet yeast is to provide the yeast with oxygen so that it can produce sterols and unsaturated fatty acids which are important parts of the cell membrane and therefore essential for biomass production.
If the slurry from dry yeast fermentation is re-pitched from one batch of beer to another, the wort has to be aerated as with any liquid yeast."

[Thirsty Boy]

The idea that yeast re-produce when there is oxygen, then stop and ferment when the oxygen runs out..is old fashioned, simplistic & completely wrong.

Yeast reproduce constantly during a fermentation and continue to do so until they hit a growth limit. Actively fermenting yeast are healthily reproducing yeast. The limit that your yeast should encounter that stops them reproducing... is a lack of nutrients in the form of fermentable sugars. One of the limits they might encounter, is lack of sterols, which could happen because they didn't get enough oxygen during the lag phase... Then you might get fermentation issues.

For optimum flavour, you probably want your yeast to reproduce about 3-5 times while they ferment your beer.... If you pitch the right amount, then thats what will happen, and you are probably going to get away without oxygenating your wort for dried yeast. But if you don't pitch the right amount, or you are planning on re-pitching that yeast, then you need to add oxygen to the mix.

Add some air... It absolutely won't hurt, and it might help. So where is the downside? Now fair enough... If you are using dry yeast you might not want to invest the money and effort required for an oxygen cylinder or aquarium pump - & you probably don't need to - but splash your wort into your fermenter a bit, and invest half a minute of your life giving it a bit of a shake. The yeast will love it even if they don't need it... And if it just so happens that they did need it, you might have made the difference between a suspect and a good batch of beer.

People who wear a belt AND braces are seldom caught with their pants down.... Unless they want to be.

TB

[Spelt]

The Pasteur Effect dictates that the presence of oxygen stimulates yeast cell growth and inhibits the process of fermentation. Its not so much that the cells either grow or ferment, its more an issue of what they are "mostly" doing at relative levels of oxygen. At high oxygen concentrations, fermentation is minimized. Glycolysis and energy production (in the form of ATP) is maximized. As the oxygen is consumed, the yeast do continue to grow but anaerobic metabolism is favored and fermentation begins. Oxygenating a fermenting wort is sufficiently inhibitory to actually shut the fermentation down. So yes, yeast continue to grow, but the growth that we can influence with oxygen is the initial growth before fermentation becomes the predominant metabolic process.

[Thirsty Boy]

I disagree with (what i understand is) your interpretation. The crabtree erect means that yeast are basically always following the fermantative pathway in a wort environment. The wort would be in respiratory phase, but there is too much glucose. You can screw up a fermentation and knock the yeast into respiratory phase by adding oxygen mid fermentation, because by that time, all the glucose has been taken up and fermented. Yes indeed yeast are taking up oxygen at a furious pace in the first period of time after they are pitched.... But they have basically completely taken up all the oxygen within hours (probably less) of being pitched, and yet they don't go into logarithmic growth phase, nor does the Plato begin to significantly drop, till well after that. Yeast don't "continue" to grow during the main fermentation phase.... They primarily grow during the main fermentation stage.

You just have to look at any of the charts that plot cell count vs time vs Plato... In a wort that has been properly oxygenated and has the proper micronutrients, amino acids etc, Cell growth continues, and continues at a very high rate, until the the sugar is basically all gone, it's not the rate of growth, or growth at the start of fermentation we can influence by initial oxygenation - It's the limit to the growth that occurs constantly during the fermentation. And when the yeast stop actively growing the rate of fermentation essentially ceases. Which is (one of the reasons) why you get stuck fermentations - yeast stops reproducing when it hits a growth limit (for this argument lack of sterols and/or fatty acids) and then it basically stops fermenting and metabolizes only enough to keep itself alive. Yeast's purpose in life isn't to ferment sugar - it's to make more yeast. Yeast only ferments sugar so it can reproduce, and it mostly doesn't bother when it can't - it just goes dormant, ticks over and waits for the situation to improve. It does not however, sit there, not growing, but chugging it's way through a lot of sugar.

Separating growth from fermentation, even in degree, is basically just a myth.

[Spelt]

The Crabtree effect does not get free-rein. If yeast were always on the fermentative (anaerobic) pathway in a glucose rich environment, then there would not be any point to oxygenating your wort right at the time of pitching...ever. Our greatest glucose concentration is present at the time of pitching. If we believed in the singular power of the crabtree effect, we would believe that oxidative metabolism could not happen under such a heavy glucose load.

In reality, we know that early and complete oxygenation makes a huge difference in cell growth. Most impressively in high-gravity/high-glucose worts. Apparently oxidative metabolism, cell growth, and sterol formation are not being suppressed by the glucose present during the lag phase. In other words, the crabtree effect is not in play early on...oxidative metabolism is what's happening...not fermentation. Logarithmic growth begins within the first few hours of the pitch, when oxygen is high and fades as both oxygen and then glucose and the other saccharides are consumed. Admittedly oxygen is the first resource to disappear and this lack of oxygen forces the yeast to function as a facultative anaerobe (not its preferred mode of metabolism).

You've indicated that its always a good idea to aerate your wort. Why aerate if the glucose present is suppressing oxidative metabolism?

[Thirsty Boy]

Of course the crabtree effect doesn't get free rein. Nothing in the process of a fermentation is fully on or off as you pointed out earlier - but it does fundamentally effect the early stages - During lag phase, oxygen is rapidly and pretty much completely deleted from the wort, and virtually nothing else happens, cell count doesn't increase, sugars dont decrease, alcohol doesn't increase - its lagging. The yeast isn't particularly using the oxygen for the aerobic pathway... its using it to mobilise and dissipate its glycogen reserves and begin the process of building up its sterol levels.

As far as making it all the way through.. the oxygen metabolism doesn't (in any significant way) happen in fermenting wort, realistically, the Pastuer effect to all intents and purposes, is something that happens in specific laboratory conditions and doesn't in fact happen in real fermentations, oh - in a limited way of course its there, but as far as something that is a prime driver of the fermentation process... nope.

Yeast takes up the oxygen in the very first period after pitching, basically during the lag phase, its gone after less than 20hrs, and probably a hell of a lot sooner than that - during which time biomass will hardly have increased at all. Absolutely we know that oxygen at the start of a fermentation will make a huge difference in yeast growth - but not because the yeast only or even mainly grow while they are uptaking oxygen or while oxygen is present - but because intake of oxygen allows them to build up the internal reserves, which allow them to multiply in the absence of oxygen during the exponential/logarithmic growth phase and the stationary growth phase... both of which happen concurrently with what most brewers would consider to be active fermentation. Actively growing yeast, produce 20-30 times more alcohol than do static yeast - once again - it is during active fermentation that yeast does most of its growing.

And - away from our little contest to see who has the least worst terribly basic and probably flawed understanding of yeast metabolism and back to the original topic - that's why you might well be able to get away with not oxygenating a pack of dried yeast. The stuff is manufactured in a such a way that the yeast already has sufficient reserves of sterols and fatty acids to multiply about 3 times before it hits its limit - and if you pitch an appropriate amount of yeast, 3 generations of growth to hit terminal gravity is approaching the optimum for good beer flavour. Maybe you want 3? maybe you want 5? but its in that ballpark. Thus - no need for oxygen and yet yeast growth to 8 times the original cell count happily occurs.

The yeast is of course completely knackered after this - its sterols are out, you didn't give it enough oxygen to make any more and now its done. If you have any intention of re-using your yeast - you need to aerate, not just the new batch, the first batch!

And like I said - a bit of air cant hurt and might well help - why not give it some?

I back up my bullshitting with action - less than two hours ago I pitched a nice Best Bitter with a pack of Windsor dried yeast. The aeration stone stayed in its box and the wort was simply splashed into its fermenter, I gave it a bit of a shake for 30 or so seconds, and have repeated that once or twice since (if you count the one I am about to do) - that's enough for dried yeast, but also enough to make sure disaster (probably) wont strike even if everything was not 100% perfect with that particular pack of yeast.