Ask The Brewmasters

Ok this is a long way round but bear with me. Oxygen utilization is tied to biomass and carbon usage. This can all get pretty complicated, especially if you're running a fed batch strategy. Odds are you're just trying to prop up some yeast OG style (tank, wort, add yeast, agitate, harvest) and have it all run its course fermentatively. Now this all depends on the yeast strain but odds are you don't have the ability to do a true aerobic fermentation since it requires that you limit the available carbon under a certain threshold to prevent anaerobic fermentation. To do this you have to introduce new carbon proportional to the present (and ever increasing) biomass. It gets tricky. If you use pure oxygen the yeast can exhibit oxidative stress/damage/distress and I've always run o2 sensors for that reason. But here's the trick, you can never over-oxygenate with air. It's impossible. When starting out reactors I run on sterile air initially because my biomass is low enough that it just works and I don't "burn" my yeast in the crucial initial exponential growth phase. The other benefit of running air is that it blows contaminates out of the vessel and contributes to agitation. When air wasn't able to keep up anymore (about 20g/L in exponential stage) I would switch over to a pair of oxygen concentrators. In typical yeast propagation like you're doing, you won't get over 10g/L so no need to even consider it. The O2 you're providing is going to be supplemental at best, not critical and definitely not enough to shift yeast over to an aerobic metabolism. 

Another pro tip - yeast cells love high shear agitation. The whole gentle agitation thing just has no real place in reality. I was running rushton turbines for agitation at 2-3000 rpm... you truly don't get much more shear than that and I was able to prop 200-250g of yeast cells/L (we use mass instead of a cell count because it relates to the chemical reactions needed to make it all work. Way easier and yeast cells have all kinds of differences in size but mass is mass - but it's about 5T cells/L)

TL;DR - use air, run it hard as you an and as long as you want. Make sure it's sterile and make sure you can keep up with the cooling needs. Do use violent agitation and try to get that air as dissolved as possible into the medium. IMOHO, don't even bother with oxygen unless you plan on trying to break the 10g/L (500B cells/L) barrier. 

Ok this is a long way round but bear with me. Oxygen utilization is tied to biomass and carbon usage. This can all get pretty complicated, especially if you're running a fed batch strategy. Odds are you're just trying to prop up some yeast OG style (tank, wort, add yeast, agitate, harvest) and have it all run its course fermentatively. Now this all depends on the yeast strain but odds are you don't have the ability to do a true aerobic fermentation since it requires that you limit the available carbon under a certain threshold to prevent anaerobic fermentation. To do this you have to introduce new carbon proportional to the present (and ever increasing) biomass. It gets tricky. If you use pure oxygen the yeast can exhibit oxidative stress/damage/distress and I've always run o2 sensors for that reason. But here's the trick, you can never over-oxygenate with air. It's impossible. When starting out reactors I run on sterile air initially because my biomass is low enough that it just works and I don't "burn" my yeast in the crucial initial exponential growth phase. The other benefit of running air is that it blows contaminates out of the vessel and contributes to agitation. When air wasn't able to keep up anymore (about 20g/L in exponential stage) I would switch over to a pair of oxygen concentrators. In typical yeast propagation like you're doing, you won't get over 10g/L so no need to even consider it. The O2 you're providing is going to be supplemental at best, not critical and definitely not enough to shift yeast over to an aerobic metabolism. 

Another pro tip - yeast cells love high shear agitation. The whole gentle agitation thing just has no real place in reality. I was running rushton turbines for agitation at 2-3000 rpm... you truly don't get much more shear than that and I was able to prop 200-250g of yeast cells/L (we use mass instead of a cell count because it relates to the chemical reactions needed to make it all work. Way easier and yeast cells have all kinds of differences in size but mass is mass - but it's about 5T cells/L)

TL;DR - use air, run it hard as you an and as long as you want. Make sure it's sterile and make sure you can keep up with the cooling needs. Do use violent agitation and try to get that air as dissolved as possible into the medium. IMOHO, don't even bother with oxygen unless you plan on trying to break the 10g/L (500B cells/L) barrier. 

Hi David,

Curious if you have any references documenting how yeast cells benefit from shear mixing.  There are references indicating that yeast cells have a fairly high resistance to shear damage, and plenty of references related to shear damage, but I cannot find any showing benefits.  No question that high-shear mixers are great at increasing the rate of gas transfer into sparged solutions, but there are other options outside of high-shear turbines.

Cheers, Ashton

Ok this is a long way round but bear with me. Oxygen utilization is tied to biomass and carbon usage. This can all get pretty complicated, especially if you're running a fed batch strategy. Odds are you're just trying to prop up some yeast OG style (tank, wort, add yeast, agitate, harvest) and have it all run its course fermentatively. Now this all depends on the yeast strain but odds are you don't have the ability to do a true aerobic fermentation since it requires that you limit the available carbon under a certain threshold to prevent anaerobic fermentation. To do this you have to introduce new carbon proportional to the present (and ever increasing) biomass. It gets tricky. If you use pure oxygen the yeast can exhibit oxidative stress/damage/distress and I've always run o2 sensors for that reason. But here's the trick, you can never over-oxygenate with air. It's impossible. When starting out reactors I run on sterile air initially because my biomass is low enough that it just works and I don't "burn" my yeast in the crucial initial exponential growth phase. The other benefit of running air is that it blows contaminates out of the vessel and contributes to agitation. When air wasn't able to keep up anymore (about 20g/L in exponential stage) I would switch over to a pair of oxygen concentrators. In typical yeast propagation like you're doing, you won't get over 10g/L so no need to even consider it. The O2 you're providing is going to be supplemental at best, not critical and definitely not enough to shift yeast over to an aerobic metabolism. 

Another pro tip - yeast cells love high shear agitation. The whole gentle agitation thing just has no real place in reality. I was running rushton turbines for agitation at 2-3000 rpm... you truly don't get much more shear than that and I was able to prop 200-250g of yeast cells/L (we use mass instead of a cell count because it relates to the chemical reactions needed to make it all work. Way easier and yeast cells have all kinds of differences in size but mass is mass - but it's about 5T cells/L)

TL;DR - use air, run it hard as you an and as long as you want. Make sure it's sterile and make sure you can keep up with the cooling needs. Do use violent agitation and try to get that air as dissolved as possible into the medium. IMOHO, don't even bother with oxygen unless you plan on trying to break the 10g/L (500B cells/L) barrier.