Author Topic: Top Cropping a Higher Gravity Wort  (Read 970 times)

Offline MerlinWerks

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Top Cropping a Higher Gravity Wort
« on: November 02, 2015, 02:13:03 PM »
I know conventional wisdom recommends not harvesting yeast from worts > 1.060 due stress, but I assume this also means that you harvested after they have stressed themselves fermenting to terminal gravity.

I have a 1.072 wort that I pitched Saturday evening with ~85 ml of top-cropped WY1318, harvested last week. By Sunday morning things were well on their way. Assuming the ferment has not reached terminal gravity by tonight, does anybody forsee any issues with taking a crop of this yeast for future use? I would only harvest ~ 1/3 or so of the cake to ensure that there is enough left to get the job done. FWIW, at this point I can see that the "dirty" layer/braunhefe has already been pushed to the sides of the fermenter.

The assumption here is that the ferment isn't completely finished so the yeast shouldn't be too stressed yet.

S. cerevisiae

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Re: Top Cropping a Higher Gravity Wort
« Reply #1 on: November 02, 2015, 03:14:22 PM »
Yeast cells endure two major types of stress when pitched into high gravity wort.  As you have mentioned, one is ethanol stress.  However, an equally damaging stress occurs the moment that the cells are pitched.  That stress is known as osmotic pressure.  Osmotic pressure is a phenomenon where water is drawn the to side of a semi-permeable membrane with the highest solute content.  In the case of high gravity wort, the wort has a much higher solute content than the fluid inside of the yeast cells.  Osmotic pressure causes dehydration and the loss of something known as turgor pressure (ethanol basically does the same thing because ethanol is hygroscopic).  Turgor pressure is the pressure inside of a cell that pushes the plasma membrane (a.k.a. cell membrane) against the cell wall.   Lost of turgor pressure causes wrinkles to form in the plasma membrane and shrinkage/deformation of the cell wall, which, in turn, makes it more difficult for the cells to take in nutrients and expel waste.

Here's what happens when yeast cells when they lose turgor pressure:






« Last Edit: November 02, 2015, 04:39:10 PM by S. cerevisiae »

Offline MerlinWerks

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Re: Top Cropping a Higher Gravity Wort
« Reply #2 on: November 02, 2015, 03:58:38 PM »
Thanks Mark.

Well at least I saved enough of last weeks crop to do a starter...

Offline denny

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Re: Top Cropping a Higher Gravity Wort
« Reply #3 on: November 02, 2015, 04:58:23 PM »
Being a yeast abuser, I push the ROT and often reuse yeast from beers in the low 70s OG range.
Life begins at 60.....1.060, that is!

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S. cerevisiae

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Re: Top Cropping a Higher Gravity Wort
« Reply #4 on: November 02, 2015, 05:48:13 PM »
Some yeast strains act like that were pitched into normal gravity wort when pitched into low-seventies wort.  Others do not perform as well.  I ran a repitching experiment with NCYC 1108 earlier this year.  I repitched yeast cropped from a 1.071 rye ale into a 1.052 ale.  The result was lower attenuation.  I took the yeast that I cropped from the 1.052 ale and double-dropped it eight weeks later in starter.  The yeast performed better in terms of attenuation, but the flavor performance had degraded, so I did not go another round.
« Last Edit: November 04, 2015, 06:42:33 AM by S. cerevisiae »

Offline brulosopher

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Re: Top Cropping a Higher Gravity Wort
« Reply #5 on: November 04, 2015, 03:50:00 AM »

Being a yeast abuser, I push the ROT and often reuse yeast from beers in the low 70s OG range.
Man, I like your style!

Offline denny

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Re: Top Cropping a Higher Gravity Wort
« Reply #6 on: November 04, 2015, 05:17:35 PM »

Being a yeast abuser, I push the ROT and often reuse yeast from beers in the low 70s OG range.
Man, I like your style!

Back atcha, bro!
Life begins at 60.....1.060, that is!

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The best, sharpest, funniest, weirdest and most knowledgable minds in home brewing contribute on the AHA forum. - Alewyfe

"The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts." - Bertrand Russell

Offline mchrispen

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Re: Top Cropping a Higher Gravity Wort
« Reply #7 on: November 04, 2015, 06:17:06 PM »
That picture. How I feel on really cold water.


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