Mash settings don't affect final gravity (FG) calculation

[GnuUser]

BrewTarget doesn't seem to take the mash schedule into consideration when calculating the final gravity (FG) of the beer.

The mash schedule has a major effect on the amount of fermentable sugars versus unfermentable sugars in a wort. However, nothing I do with the mash schedule changes the final gravity calculation in Brewtarget. There's no difference in FG between a single infusion mash at 65C (149F), a single infusion mash at 72C (161F), or a step mash schedule (e.g., 53C protein rest, 63C low saccharification rest, 70C high saccharification rest), and it doesn't matter whether I set the mash time for 30 mins or two hours. These different mash settings should result in major differences in the final gravity calculation, but they don't.

The only thing that changes FG in Brewtarget for me is the yeast selection and yeast attenuation percentage.

[GnuUser]

Here are a couple of screenshots showing what should be a high fermentability mash (resulting in lower FG) and a low fermentability mash (resulting in higher FG), but the FG in both cases is identical.

[matty0ung]

Thanks for the detailed description.

This looks to be similar to https://github.com/Brewtarget/brewtarget/issues/344. (Interestingly it is, or at least has been, an issue raised in other brewing software: eg https://beersmith.com/forum/index.php?threads/final-gravity-estimate-calc-mash-profile-doesnt-change-the-number.1782/ , https://www.brewersfriend.com/forum/threads/final-gravity-mash-temperature.1049/ .)

In order to implement the enhancement, we would need some formula (or similar) that gives an estimate for how the mash profile affects predicted FG. I had a quick search, but didn't find anything super helpful. I don't know if the size of the temperature effect depends too much on other factors (eg yeast strain) for a general formula to be meaningful. If someone had data from a lot of different mash profiles and a lot of different yeasts then we could either deduce a "best fit" formula or do some sort of interpolation on the data.

[GnuUser]

Matt, thank you. I did search but didn't find that (probably used the wrong keywords).

I do have some graphs I use that have been useful to me in setting mash temps. They come from this website (copies attached).

http://www.woodlandbrew.com/2013/01/mash-temperature-theory.html

They don't predict an FG, but I should be able to develop a formula for calculating an FG from them and from my experience using them. For example, I know that if I mash at 65C for 90 minutes I get a highly fermentable wort with an FG of 1.008 using W-34/70 lager yeast. Combining that with the 65.5C (150F) graph would produce a reasonably-predictable FG based on time at that temp. I'll see what I can come up with.

Yeast strain would be something of a multiplier after fermentability is calculated (i.e., for the same sugar profile in the wort, some yeast strains will attenuate more than others). For example, I'm pretty sure that the Mangrove Jack's Belgian Ale yeast I've used would drop the FG down to something like 1.004 for the same mash schedule I've described above (but I haven't tested that).

[GnuUser]

Actually, here's some good data and a formula from the same website:

http://www.woodlandbrew.com/2013/01/measured-mash-temperature-effects.html

Rule of thumb:

1% less attenuation for every degree above 151.  (starting with the yeast's average attenuation)
(Just think of Bacardi 151 to remember the base mash temp)

The equation :
-0.000663temperature^2+0.1964temperature-13.692

Temperature | Attenuation -- | -- 145 | 85% 150 | 85% 152 | 84% 154 | 83% 155 | 82% 156 | 81% 157 | 80% 158 | 79% 159 | 77% 160 | 76%

[waldrond]

There is a similar plot in Palmer's How to Brew taken from a 2005 paper by Evans et al. The plot appears in at least the 3rd and 4th edition of the book. It would be awesome to get this feature added.