Boil off different for high gravity?

[CascadesBrewer]

I played around with this recipe on BeerSmith Mobile (what I use for recipes). It came up with some different numbers but an overall OG of 1.080. It seemed like BeerSmith treated sugar added to the boil as if 100% of the sugar made it into the fermenter. It did not not seem to subtract out any of the sugars that would be lost in the kettle. I don't know if BeerSmith Desktop treats this any different. Sugar added to the boil should not be treated as the same 75% efficiency (since that also includes your mash efficiency) but seems like it should be adjusted (to maybe 95%) based on kettle losses.

 

[Big Monk]

I played around with this recipe on BeerSmith Mobile (what I use for recipes). It came up with some different numbers but an overall OG of 1.080. It seemed like BeerSmith treated sugar added to the boil as if 100% of the sugar made it into the fermenter. It did not not seem to subtract out any of the sugars that would be lost in the kettle. I don't know if BeerSmith Desktop treats this any different. Sugar added to the boil should not be treated as the same 75% efficiency (since that also includes your mash efficiency) but seems like it should be adjusted (to maybe 95%) based on kettle losses.

There are no sugars lost in the kettle.

Kettle losses don’t affect gravity. X gallons or liters of post boil wort and Y gallons or liters of fermenter wort will have the exact same gravity. Likewise, Z gallons or liters of wort left in the kettle will have the same gravity. The exception would be if for some reason the added sugars in the boil didn’t mix well.

The only 3 stages that actively affect gravity of the wort are the mash/sparge stage, added boil sugars, and boil evaporation.

 

[CascadesBrewer]

There are no sugars lost in the kettle...Kettle losses don’t affect gravity.

Yeah...but with BeerSmith Mobile I get the same gravity if I add 1 lb of sugar to the boil and I leave 0.5 gals in the kettle as I would if I added that 1 lb of sugar to the fermenter. Clearly some of the volume sucked up by the hops and left behind contains some of the sugar that was added to the boil. It is not a huge amount for me since I usually count only about 0.25 gal left behind.

 

[Big Monk]

Yeah...but with BeerSmith Mobile I get the same gravity if I add 1 lb of sugar to the boil and I leave 0.5 gals in the kettle as I would if I added that 1 lb of sugar to the fermenter. Clearly some of the volume sucked up by the hops and left behind contains some of the sugar that was added to the boil. It is not a huge amount for me since I usually count only about 0.25 gal left behind.

If I add 1 lb of sugar to 5.5 gal of boil volume then leave 0.5 gal in the kettle the gravity post boil and in the fermenter will be the same.

If I add 1 lb of sugar directly to 5 gal in the fermenter, post boil gravity and fermenter gravity should not be the same.

1 lb of sugar into 5.5 gal minus 0.5 gal should not equal 1 lb of sugar directly into 5 gal. I’d question your software if that’s what it’s telling you.

 

[jeeppilot]

Remember that the reason that sugar doesn't count 75% towards your efficiency is because the sugars are already there and don't get mashed (like if it was extract or honey or something like that).
So you are really in the ballpark with a difference of .002!

Didn’t even think about that. Perfect!

 

[jeeppilot]

@CascadesBrewer - The late sugar addition was not adjusted to my 75% brewhouse efficiency like the other grains were. @Yooper pointed out this is because the table sugar isn't mashed, just added directly to the boil.

I tend to think of gravity as a percentage... if 30% of your wort is sugar and 70% water, and you leave some wort behind in the kettle or the hops absorb some wort, your remaining wort is still 30% sugar and 70% water, just slightly less volume of wort. Same principal as when people go to Colorado or somewhere high altitude and think they are breathing less oxygen. Nope, still 78% nitrogen, 21% oxygen like at sea level, just less volume of each per breath.

- @RPIScotty, the original OG of 1.082 (I think I typoed 1.081 in the first post) was due to the software thinking I was boiling down to 2.5 gal, all of which made it to the fermenter, when instead I was boiling down to 3 gal and losing .5 gal between the kettle and fermenter. As you said, it is not a matter of leaving sugars behind in the kettle, but a function of greater wort concentration. The manually calculated OG of 1.068 nearly matches the software output of 1.070 when I select the "Use Boil Size and Boil Evaporation Rate" function. I'm not sure if this function is new or I just never saw it till now.

Original OG (OOG)

Recalculated OG

 

[jeeppilot]

Here's the proper way to do the math:

4.8 gal of 1.033 wort has 4.8 * 33 = total 158.4 gravity points

Adding 1 lb of table sugar adds a total of 46 points, so your total points value is: 158.4 + 46 = 204.4 pts

3 gal with 204.4 pts equals 204.4 pts / 3 gal = 68.1 pts/gal, which converts to an OG of 1.068.

The difference between 1.063 and 1.068 is likely attributable to volume measurement errors, which become more important with smaller batch sizes.

Brew on

I've never really gotten into the weeds with the math on this stuff until now. I really have relied on the software up to this point. Good stuff. I've also heard rumors that high gravity beers tend to be less efficient. As much as 10% or more? I didn't account for that and maybe that could explain the difference? Or I measured wrong, or my 8 looked like an 3 when I write it down... who knows.

 

[doug293cz]

I've never really gotten into the weeds with the math on this stuff until now. I really have relied on the software up to this point. Good stuff. I've also heard rumors that high gravity beers tend to be less efficient. As much as 10% or more? I didn't account for that and maybe that could explain the difference? Or I measured wrong, or my 8 looked like an 3 when I write it down... who knows.

High gravity does not affect the calculations in your quoted post. High gravity (larger grain bill) definitely reduces lauter efficiency but not necessarily the conversion efficiency. Since mash efficiency = conversion efficiency * lauter efficiency, lower lauter efficiency also means lower mash efficiency. The chart below shows how beer "size" affects lauter (and therefore mash) efficiency.

And the quoted post is barely the tips of the weeds that are mash/lauter math. The basics of using point values to predict SG's is not too difficult to master. Getting into rigorous mass balance for SG prediction, and lauter simulation for efficiency prediction is quite a bit more involved (but doesn't require calculus or differential equations [unless you want to try to simulate a fly sparge.])

Brew on

 

[jeeppilot]

Just as a follow up to my working through some missed OG issues in this post, I just got this email from Brewers Friend today. In it is a description of their new calculation process. They admit their “old method” calculated OG based on boiling down wort to the volume specified going into the fermenter. This INCLUDED losses to trub, chiller and other, resulting in up to 20% differences! Of course, at higher gravities that percentage is an even larger number. The new calculation seems to be a more accurate reflection of what really happens. See below.

“To understand the efficiency calculation, an example is best.
Let’s say your boil starts at 6 gallons. You have 120 points of fermentables (e.g. 5 pounds of 30 ppg at 80% efficiency). You boil down to 5 gallons. In this example, your system may lose 1 gallon to kettle dead space, hop absorption, losses to the way you transfer wort, wort left in the chiller, and so on. In this case, you up with 4 gallons in the fermenter.
In the old method:
You’d select the fermenter target, and enter 6 gallons as your boil size. Four gallons would be your batch size, since that is the volume going into the fermenter.
The software assumed that the difference from the 6 gallons at the beginning reducing to 4 gallons as the batch size was all boil-off.
As a result, the OG was calculated as: 1 + ( 120 / 4 ) * 0.001 = 1.030
That is clearly wrong because it indicates that all fermentable sugar was condensed into 4 gallons of wort. The reality is that it was condensed into 5 gallons of wort, and some of that was simply thrown away. The loss of wort does not increase specific gravity of the remaining wort, of course. The loss would have the same SG as the wort going into the fermenter.
So in our updated calculation, you would select the batch size of 4 gallons as the target into the fermenter, enter 6 gallons as pre-boil size, and 5 gallons as post-boil size.

You can also let the system calculate from your equipment profile’s boil-off rate.

Now the OG can be calculated correctly:
1 + ( 120 / 5 ) * 0.001 = 1.024
You will notice that it is a 20% difference. That seems small to some, but we wanted to give the most accurate OG predictions possible for our users, both homebrewers and professionals.”