The biomass increase had nothing to do with stirring. Both cultures were stirred. An open culture has more gas exchange and less top pressure than a closed culture. The same dynamic happens in open fermentation, which is why open fermentation is usually used with cultures with high O2 demands (although, some strains have such high O2 demands that post-inoculation aeration is required). A culture that is spun fast enough to create a vortex has more surface area than a gently stirred culture, but it does so by creating more turbulence; hence, the cells experience more shear stress.
A Fishtail Aerating a Fermentation Post-Pitching at Samuel Smith’s Brewery
Here is the $10,000.00 question. If stirring was beneficial to brewing Saccharomyces strains, why are batch fermentation vessels not equipped with impellers? Continuous tower fermentation vessels are equipped with slow moving impellers, but they are there to continuously mix new substrate and O2 into the fermentation in order to keep the medium chemically static.
If you go back to the description for NCYC 1026 that I posted earlier, you will notice that Whitbread B was selected for use in continuous towers. A continuous tower fermentation vessel is a bioreactor for continuous process beer production. A top fermenting strain would throw a monkey wrench into the works because beer is continuously drawn off the top while substrate and O2 are added and yeast is recycled.
Very cool. I love seeing those open fermentation pictures in breweries. I wonder if people need o2 masks to be in the fermentation rooms or the venting is really good, otherwise I’d think it’d be impossible to breath with all the co2 output.
The comments are non contradictory. Both stirred cultures are degassed by stirring. It’s just that the open culture experiences less restriction. CO2 dissolves and comes out of solution based on partial pressure; therefore, any top pressure impacts the ability to degas a solution.
Where are the large sample size peer reviewed studies that demonstrate significant O2 pickup via stirring? From what I can ascertain, there are none. Home brewers are blindly assuming that the results from work performed by one amateur brewer are 100% accurate and hold for all cases. Yet, that work involved a tiny sample size. Science is based on repeatability.
The takeaway from this person’s experiment was almost comical in that the results made others believe that stirring was the reason for the increased growth. What stirring the culture vigorously did was increase the amount of surface area in an otherwise poor choice for a culturing vessel. The same effect can be accomplished without stirring or shaking by using a vessel that is wide enough that the liquid level is very shallow. The key to O2 pickup from air will always be surface area.
Turbulence is just as important. One of the reasons mountain streams contain more dissolved oxygen than wide, flat rivers is turbulent flow. The rate at which oxygen crosses the air-water interface depends on the difference between the concentration of oxygen in the surface water and in the air immediately above - a steep gradient causes faster diffusion across the boundary. If the water is completely still, oxygen builds up in the surface layer, reducing the gradient between air and water and slowing down diffusion. Likewise, if the air is completely still, O2 levels in the air immediately above the water will fall (and CO2 level from yeast will rise). Agitation stops this happening - the O2-rich water at the surface is continually replaced so a steep gradient in O2 concentration between air and water is maintained. Stirring also sets up currents in the air, stopping CO2 from pooling (though bear in mind CO2 does not form an impenetrable barrier as some brewers think - oxygen is continually diffusing through it).
That’s not to say that stir plates don’t inhibit growth through shear stress.
If large surface area without agitation was all you needed, your lungs would take in all the O2 your body needs without breathing. Diffusion on its own isn’t enough.
Intuitively, I don’t see how stirring in the presence of oxygen could fail to increase DO levels. That would be difficult to test for directly with DO probes not being useful during active fermentation. Doubtless the reduction in dissolved CO2 would also be responsible for some of the effect of stirring.
I’m told breathing is more about blowing off co2 than intaking o2. The latest CPR doesn’t even worry about giving breaths. According to my old NAUI instructor, it’s the buildup of CO2 that triggers the breath desire. That’s why hyperventilating before free diving is so dangerous. You blow off too much CO2 and run out of O2 before you realize you need to breath.
Yes breathing rate and volume are set by blood CO2 level measured via pH. But getting O2 into the body is important - brain cells only last a few minutes without it. As I understand it, hypoxia kills you first if you stop breathing, not CO2 poisoning.
Interesting point about hyperventilating. I’ll remember that next time I go snorkelling.
If I remember correctly, NAUI calls it shallow water drowning.
But who knows regarding the correlation to a stired starter. It seems like in a flask with foil over it that not mych o2 would inhale at least not during o2 off gassing. The positive pressure and pooling of co2 probably limits that. Maybe if you simulated the pumping action of breathing by pumping air into the flask.
Yes, but the key to O2 pickup will always be defined by the surface area to volume ratio. All turbulence is to doing is turning over the surface. 1L in a 24" x 24" pan will saturate faster than 1L in a 2L Erlenmeyer flask on a stir palate.
CO2 may not form an impenetrable barrier, but it is causing the inside of the flask to be under positive pressure where the gas flow at the mouth of the flask is accelerating due the conical shape of an Erlenmeyer flask.
Lungs & stirred Erlenmeyer are obviously very different, but the main point is that diffusion across a stagnant air-water boundary is slower than between moving fluids.
Although you can’t see it, the gas above the swirling liquid in an Erlenmeyer flask is also circulating. The turbulence in the gas will generate currents that bring fresh air in as well as expelling stale air, so O2 will be stirred into the gas space.
I don’t know how long a stir plate mixture would take to reach saturation with O2, but bear in mind it’s a dynamic system: yeast are consuming O2 to make ergosterol and so maintaining the gradient that drives diffusion. It’s not just a case of reaching O2 saturation once - you need to keep replenishing it. That’s one drawback with the shaking approach as you only get one hit of O2 and it might not generate enough ergosterol for a 5 gallon brew.
Side note on air columns above an Erlenmeyer flask on a stir plate: Don’t most folks cover the mouth of the flask? I know I’ve certainly never done an open-fermentation stir-plate starter…Thinking that would significantly impact any air vortex, and also serve to keep the CO2 trapped in the flask.
Yes it would disrupt a vortex, but unless it’s airtight there will still be flow. An easy way of seeing how much airflow is generated by stirring would be to drop a lit match into two flasks, cover both, put one on a stir plate and leave the other. When the match goes out smoke will fill the flask. That rate at which the smoke is replaced by air would give you an indication of how quickly O2 is drawn into the flask by the motion.
Ah, but without liquid in the flask it’s an apples to oranges comparison, running a stir bar in air will move a lot more air than a vortex in a liquid.
I’m no aero/fluid dynamics expert, but I don’t see the vortex in liquid creating much of a vortex in the air. While the air vortex may create some motion, I’m doubting it would do much to aerate the wort.
There’s no reason why you couldn’t do the same test with water in the flask.
Friction between the water surface and air would set up rotation in the air. i’m certain about that, but how much gas exchange there is with the outside air via a tight fitting foil cap is debatable.
Friction between the water surface and air would set up rotation in the air. i’m certain about that, but how much gas exchange there is with the outside air via a tight fitting foil cap is debatable.
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Clearly, but the real question is how much of the air rotates? At what height above the liquid is the air column rotation little enough to be meaningless? And remember the very idea behind an Erlenmeyer flask’s shape is it’s ability to contain a swirling liquid. Air can be treated like a liquid.
