Timing for second round of oxygenation

Sounds like u already hit it with 1 round...rdwhahb

 

I would say to hit it again. In Yeast, the 12 - 18 hour logic is to wait until the cells have done at least one division. Too early and it's wasted, too late and the oxygen will still be helping the little buggers. Oxygenation shouldn't be a problem as the gasping microbes will be metabolizing the oxygen. W & Z also recommend raising the temp of monster beers at the 48 hour mark to keep the yeasties purring along.

 

Never be afraid to give your fermentor a good shake before high krausen fermentation has begun.

 

I would suggest that minimizing osmotic shock in beers like this by increasing the sugar content of your starter when you step it up to approximate that of your main beer and pitching larger amounts of yeast to lessen the log phase are more important than giving it a second round of oxygen, but I don't think it will hurt.

I would say this is unnecessary but probably won't hurt anything as most ester formation occurs during the log phase of a yeast's life cycle and by 48 hours after pitching, you should have solid fermentation.

 

I agree about the esters being formed early, but don't think this is the reason White/Zainasheff make this recommendation. Although no implicitly stated, optimizing the yeast's environment for proper attenuation seems to be the goal here.

For @Scope4Beer , did you set your alarm early and re-oxygenate or let it ride?

 

This is just the opposite of what the authors of Yeast state. They recommend pure oxygen at pitching and at the 12 - 18 hour mark for beers over 1.083. They specifically say "you must aerate with pure oxygen". Not sure of the "oxygen is toxic to yeast" content you quote from Palmer. This is contrary to much of the science on the subject . . . Saccharomyces get along quite well in the presence of oxygen.

I've only brewed one monster beer (1.100) and it did get oxygen after fermentation started. It was a Wee Heavy that turned out positive. I'm aging it now (8+ months) and not a trace of oxidation. Isn't it nice to have the morning to yourself.

 

Just curious, but do they give any clues as to how giving the yeast more oxygen is going to make them attenuate "properly"?

 

Here's the thing. First off, it depends to which genera of yeast you are referring as to how oxygen effects growth and fermentation. If you're talking about Sacch. sp. then they follow something called Pasteur Effect where they grow better in an aerobic environment, but ferment better in an anaerobic one. Since this is the case, giving your yeast too much oxygen would inhibit their fermentation if some is still left in solution after the log phase is over. FWIW, Brettanomyces sp. follow what is referred to as Custers Effect or Negative Pasteur Effect, which means that they both grow AND ferment better in the presence of oxygen. These scenarios, btw, don't have anything to do with desired aromas or flavors in the beverage, just growth rates and yeast health.

 

They don't dwell on this aspect, other than to say an increase in temp will keep them working at their maximum. The implication is to keep 'em healthy so they finish their job. The scenario here is very high gravity beers, the goal is to prevent under-pitching or yeast under-performing.

Your explanation seems to be the point W&Z are making; after first cell division add oxygen so the new generation of cells can grow more efficiently. The goal here is to increase/maintain a growth rate consistent with the higher gravity wort. Any oxygen added at pitching (or adding later) will quickly be taken up by the cells, there will be no O2 lingering. Without getting out my bio charts, I believe the anaerobic phase of fermentation is the only time ethanol (and CO2) are produced.

In my mind this all makes sense . . . add oxygen early, it will be metabolized by the yeast to make them grow faster, the oxygen is virtually eliminated as the exponential growth phase gives us the precious alcohol. Higher gravities need a higher pitch count, more O2, and in extreme cases a boost in temp.

For @Scope4Beer , it would be nice if you followed up with the results when the brew is complete (your OG/FG and yeast strain). Would be interesting as to how it all worked out.

 

Indeed, but more oxygen won't necessarily help that. Acclimating the yeast to their eventual environment will.

The phases aren't really hard and fast, but, yeah, most of the energy taken in by the cells during the log phase of growth is channeled towards replication, not fermentation.

 

I've never heard that oxygen helps yeast to grow faster. But it helps them grow more. The O2 is used to make sterols, which are building blocks of cell walls. Less sterols means the cells can divide less times, because there's not enough cell wall material.