### Author Topic: calculate gravity drop from large starter addition  (Read 1038 times)

#### brewsumore

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##### calculate gravity drop from large starter addition
« on: February 15, 2016, 10:22:09 PM »
I had 5.2 gal of 1.085 wort to which I added 2 liters (one half gallon) of 1.035 starter wort.  I'm wondering if anyone knows of a calculator, or has the formula for determining the OG following the starter addition.

I realize it should be simple algebra but it's been a long time since I studied that.

I know, that's an awfully large starter to pitch whole for an ale... my excuse is old liquid yeast and a high OG.
« Last Edit: February 15, 2016, 10:24:14 PM by brewsumore »

#### brewsumore

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• Spokane, WA
##### Re: calculate gravity drop from large starter addition
« Reply #1 on: February 15, 2016, 10:33:28 PM »
I just realized that I can convert to plato and do an average.... maybe - let me work on that.

edit: Yeah right.    Yep, I haven't renewed my advanced mathematics license in wayyy too long.
« Last Edit: February 15, 2016, 10:47:56 PM by brewsumore »

#### Stevie

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##### Re: calculate gravity drop from large starter addition
« Reply #2 on: February 15, 2016, 10:36:57 PM »
1.080

(Volume of wort * gravity of wort + volume of starter * gravity of starter) / total volume.

20.8qt * 85 = 1768
2.1qt * 35 = 73.5
70 + 1768 = 1841.5
1841.5 / 22.9 = 80.4

Must use the same unit of measure for it to work.

#### brewsumore

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##### Re: calculate gravity drop from large starter addition
« Reply #3 on: February 15, 2016, 10:38:12 PM »
1.080

(Volume of wort * gravity of wort + volume of starter * gravity of starter) / total volume.

20.8qt * 85 = 1768
2.1qt * 35 = 73.5
70 + 1768 = 1841.5
1841.5 / 22.9 = 80.4

Must use the same unit of measure for it to work.

Awesome.  Thanks Steve!

#### rayzab64

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##### Re: calculate gravity drop from large starter addition
« Reply #4 on: February 16, 2016, 01:10:50 AM »
I imagine you starter wort wasn't at 1.035 by the time you pitched it though? Probably more like 1.010 to 1.012?

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#### brewsumore

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##### Re: calculate gravity drop from large starter addition
« Reply #5 on: February 16, 2016, 01:43:22 AM »
I imagine you starter wort wasn't at 1.035 by the time you pitched it though? Probably more like 1.010 to 1.012?
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Of course you're right, but the point is to be able to calculate the FG correctly.  Since what is lacking in gravity points of a full krauesen starter is put into balance by addition of ABV, I figure the number I'm looking for is the starter OG.

#### RPIScotty

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##### calculate gravity drop from large starter addition
« Reply #6 on: February 16, 2016, 02:27:49 AM »
I imagine you starter wort wasn't at 1.035 by the time you pitched it though? Probably more like 1.010 to 1.012?

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If he pitched at HK I'd doubt you'd get such a large amount of attenuation.

Starters are about propagation not fermentation.

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« Last Edit: February 16, 2016, 02:29:26 AM by RPIScotty »

#### brewsumore

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##### Re: calculate gravity drop from large starter addition
« Reply #7 on: February 16, 2016, 03:57:46 AM »
I guess I've normally thought in those terms but how do you get around the fact that when yeast eat sugar that they in turn release the "waste" products of ethanol and co2?

Certainly the smell of a full krauesen starter is yeasty, with no alcohol aroma.

#### RPIScotty

• Guest
##### Re: calculate gravity drop from large starter addition
« Reply #8 on: February 16, 2016, 08:59:15 AM »

I guess I've normally thought in those terms but how do you get around the fact that when yeast eat sugar that they in turn release the "waste" products of ethanol and co2?

Certainly the smell of a full krauesen starter is yeasty, with no alcohol aroma.

Let's say you take one of the above posts at face value:

1.035 --> 1.012 = ~65% attenuation. Seems unlikely in the 8 hrs or so it takes to get to HK.

I could be wrong though.

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#### pete b

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##### Re: calculate gravity drop from large starter addition
« Reply #9 on: February 16, 2016, 12:22:28 PM »
I look at it this way: before pitching yeast in the starter you have the gravity of the whole volume of wort, starter plus the main wort. It doesn't matter that one portion started fermenting before the other. For the final analysis you want the total amount of wort's gravity and the FG and Steve's formula gives the gravity of the whole volume before any fermentation took place. Normally we don't bother because the volume of starter isn't as significant but maybe when using the pitch entire starter at HK we should if we want that accuracy.
Don't let the bastards cheer you up.

#### emcfarden

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##### Re: calculate gravity drop from large starter addition
« Reply #10 on: February 16, 2016, 10:02:19 PM »
What's the benefit of pitching the entire starter?

My last brew I cold crashed the starter and poured 90% of the liquid off the top of the yeast cake then let the cake come up to wort temp and pitched. Then you don't have to worry about any (very very little) drop in OG right?

#### brewsumore

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##### Re: calculate gravity drop from large starter addition
« Reply #11 on: February 17, 2016, 01:06:42 AM »
What's the benefit of pitching the entire starter?

My last brew I cold crashed the starter and poured 90% of the liquid off the top of the yeast cake then let the cake come up to wort temp and pitched. Then you don't have to worry about any (very very little) drop in OG right?

The best answer in a short reply is "that's why you should read the book 'Yeast' by Chris White and Jamil Zainasheff."  But I need to and will look again at my copy.  edit:  without having looked at the book, let me give a brief reply based on my understanding (which is somewhat limited, but well-practiced):

Because yeast is an organism you need to "ranch" it to have the best possible outcomes.  In sufficient quantity and recent cell reproduction, and due to the ability to create a large population thru the "full ferment and decant" approach to yeast starters, you will be successful.  But also, when yeast are at the height of their reproductive activity (FK starter) and again in sufficient numbers they will immediately go to chewing on the sugars of the wort, resulting in a fast initial reproduction/growth cycle that will shorten lag time and thereby ensure a good start to the large task of working through a high gravity wort without leading to poor health, population or mutations at critical phases of the ferment.  I see it as insurance through a short lag time.  Both approaches work.  For me, doing a next-day impulse brew using old yeast (all available the day before brew day) in limited supply, a larger than normal full krauesen starter was the best option.  And it worked, with minimal negative side.  It's chugging very well in the ferment chamber I am happy to report.
« Last Edit: February 17, 2016, 01:17:21 AM by brewsumore »

#### emcfarden

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##### Re: calculate gravity drop from large starter addition
« Reply #12 on: February 17, 2016, 05:17:54 PM »
I do need to read that book (and many others for that matter).

I understand the importance of a yeast starter. What I was specifically wanting to know is if there is a performance related difference to pitching yeast with the starter wort as opposed to decanting the wort and pitching only the yeast cake. Or is it solely depending on the time/preference of the brewer?

#### pete b

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##### Re: calculate gravity drop from large starter addition
« Reply #13 on: February 17, 2016, 06:54:49 PM »
I do need to read that book (and many others for that matter).

I understand the importance of a yeast starter. What I was specifically wanting to know is if there is a performance related difference to pitching yeast with the starter wort as opposed to decanting the wort and pitching only the yeast cake. Or is it solely depending on the time/preference of the brewer?

You can look up "shaken not stirred" starter on this forum for lots of detail. The idea of doing it that way is that the yeast is pitched when the starter is at high krausen (about 18 hours after starting) so that the yeast is at its most active, giving it a head start in the wort. Since much of the yeast that is active is in suspension at that point, the whole volume needs to be pitched. The more traditional method is to cold crash and decant the spent wort and pitch the slurry. They both work.
Don't let the bastards cheer you up.

#### emcfarden

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• Posts: 10
##### Re: calculate gravity drop from large starter addition
« Reply #14 on: February 17, 2016, 10:52:22 PM »
I appreciate the info!