Affects of overpitching yeast

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permo

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So I recently discovered a flaw in the logic I had been using to make starters. I have been using 3.5 oz of DME in a liter of water to make the "first phase" of all my starters. This is a standard one liter starter. Good for brews up to 1.060 or so....

When I have been stepping my starters up I have been adding 7 oz of dme to 32 oz water thinking I had a two liter starter.....wrong! I have the DME for 2 liters being added to the starter. Effectively creating the fermentables of a three liter starter.

I was wondering why my recent brews have been over attenuated and my belgians have been so clean! Overpitching seams to be my culprit.


Just thought I would share.
 
Yeast growth is sensitive to inoculation rate (cells/ml), and yeast growth is inhibited by high gravity starters. Your second starter was 1L with an OG of about 1.070. I'm not sure what effect high gravity has on yeast growth, but it most likely stressed your yeast out.

With a 100% viable smack pack, a 1L simple starter (no stir plate) would double 0.5 times, meaning if you start with 100b cells, in 1L, that's 100m yeast cells per ml, you'll make about 50b new cells, and end up with 150b cells at the end.

If you take that 150b and then pitch into another 1L starter, you'd have 150m cells per ml, and have a reduced growth factor, of around 0.3, which would mean you'd grow an additional 45b cells, giving you 195b cells total.

For an ale, the rule of thumb is 0.75m cells per ml of wort per degree Plato. So for a 1.060 beer (about 15 *P), you'd want 225b cells. So by my calculations you were probably underpitching by at least 30b cells, and more if your smack pack or vial was less than 100% viable.

If you stepped up to a 2L starter, your inoculation rate would've been 75m/ml, which would result in a yield factor of about 0.7, and you'd end up with 255b cells. In a 1.060 ale, that would be overpitching by about 13%, if your yeast was 100% viable, which it almost certainly wasn't.

Source - "Yeast", White and Zainasheff.

Edit: I guess my point is, you probably weren't actually overpitching, so I'd look for other factors that could have affected your attenuation and flavor profile. I would guess fermentation temperature, but it depends on your temp control.
 
Nateo said:
Yeast growth is sensitive to inoculation rate (cells/ml), and yeast growth is inhibited by high gravity starters. Your second starter was 1L with an OG of about 1.070. I'm not sure what effect high gravity has on yeast growth, but it most likely stressed your yeast out.

With a 100% viable smack pack, a 1L simple starter (no stir plate) would double 0.5 times, meaning if you start with 100b cells, in 1L, that's 100m yeast cells per ml, you'll make about 50b new cells, and end up with 150b cells at the end.

If you take that 150b and then pitch into another 1L starter, you'd have 150m cells per ml, and have a reduced growth factor, of around 0.3, which would mean you'd grow an additional 45b cells, giving you 195b cells total.

For an ale, the rule of thumb is 0.75m cells per ml of wort per degree Plato. So for a 1.060 beer (about 15 *P), you'd want 225b cells. So by my calculations you were probably underpitching by at least 30b cells, and more if your smack pack or vial was less than 100% viable.

If you stepped up to a 2L starter, your inoculation rate would've been 75m/ml, which would result in a yield factor of about 0.7, and you'd end up with 255b cells. In a 1.060 ale, that would be overpitching by about 13%, if your yeast was 100% viable, which it almost certainly wasn't.

Source - "Yeast", White and Zainasheff.

Edit: I guess my point is, you probably weren't actually overpitching, so I'd look for other factors that could have affected your attenuation and flavor profile. I would guess fermentation temperature, but it depends on your temp control.
Click to expand...


I agree with you somewhat. But by adding the stepped up 1.070 wort to the original 1 liter of what is now beer, dilutes the entire solution to around 1.050, which by all means should not stress out the yeast.
 
permo said:
I agree with you somewhat. But by adding the stepped up 1.070 wort to the original 1 liter of what is now beer, dilutes the entire solution to around 1.050, which by all means should not stress out the yeast.
Click to expand...

I don't think it hurt the yeast too much, but a 1.050 starter will definitely cause more stress than a 1.030 starter. Whether that stress is perceptibly detrimental or not in the finished product, I couldn't say.

If you haven't checked it out yet, I highly recommend "Yeast."

Here's another snippet: "You also do not want to make a high-gravity starter to grow yeast. The higher the gravity, the more pressure it puts on the yeast. . . In general, when dealing with reasonably healthy yeast, keep the starter wort gravity between 1.030 and 1.040. If you're trying to revive a stressed yeast. . . use a lower gravity starter wort, about 1.020." (White, p.133)
 
Nateo said:
I don't think it hurt the yeast too much, but a 1.050 starter will definitely cause more stress than a 1.030 starter. Whether that stress is perceptibly detrimental or not in the finished product, I couldn't say.

If you haven't checked it out yet, I highly recommend "Yeast."

Here's another snippet: "You also do not want to make a high-gravity starter to grow yeast. The higher the gravity, the more pressure it puts on the yeast. . . In general, when dealing with reasonably healthy yeast, keep the starter wort gravity between 1.030 and 1.040. If you're trying to revive a stressed yeast. . . use a lower gravity starter wort, about 1.020." (White, p.133)
Click to expand...


I agree with the snippet you posted. I always make my starters smaller and lower gravity when reviving yeast from a commercial bottle or an aged washed sample.

I think I should buy that book.
 
What about using a yeast cake from a previous batch to ferment fresh wort? I've done this a few times without any problems. It seems this would be considered "overpitching" by anyone's standard.

Thoughts? Is overpitching overrated?
 
insubordinateK said:
What about using a yeast cake from a previous batch to ferment fresh wort? I've done this a few times without any problems. It seems this would be considered "overpitching" by anyone's standard.

Thoughts? Is overpitching overrated?
Click to expand...

According to the Yeast book, if it's an ale, yeast viability starts to fall after fermentation has been done for 24 hours. Without doing viability testing and cell counting you couldn't know for sure, but here's my guess.

I'll brew a beer of average strength, maybe 1.050-1.060, let it ferment out, leave it in primary for a couple weeks after gravity stabilizes, rack it off, then pitch a high gravity beer (1.090+) onto it. The viability of the cake is falling every day for the 2 weeks it sits.

So I would guess that the actual number of viable cells in the cake is fairly low, and I think you'd have to overpitch by a lot, like 2-3x, to really impact the beer negatively.

So I think you could overpitch, and overpitching could be bad, but you'd really have to try to do it.
 
If your beers are finishing lower than expected, you could possibly have some kind of infection. I use a refractometer when making starters. I always use 1.040 as my SG, no problems yet, knock on wood....
 

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