Bug with FG ?

Hello,

I've made a recipe of Belgian Tripel on Beersmith :



With a two-steps mashing :



And I get an OG of 1.079, FG of 1.014 and 8.6 % abv :



Everything ok so far.

But when I change the temperature of the 2nd step from 72 deg C to 73 deg C, the abv reaches 10.4 %.
The OG doesn't change but the FG drops to 1.001.




I can't understand this. I think there's some kind of bug here ?

Thanks

Ok I found out the problem was coming from the new FG adjustment tool (Settings > Advanced) to take into account the mash temperature (-1,25 % for each deg F above 153,5).
IMO, there are two different bugs :

- Rising the mash temperature, the FG rises from 1009 at 67,5 deg C to 1014 at 72 deg C, and that's normal BUT on some Mash Profile it stays at 1014 when rising the temperature to 73 deg C or higher (that, I can understand) while on other Mash Profile (such as my two-steps profile) it drops at 1001 when the temperature goes higher than 72 deg C, as if it was 62 deg C or lower.

- The FG drops according to the formula but BS doesn't take into account the max. attenuation of the yeast. In my example when I try with 62 deg C it reaches 1.001, meaning 98 % attenuation although the max attenuation of my yeast (WLP002) is 85 %.

I would have a hard time calling them 'bugs' in the software.  Prediction of FG is a SWAG (Scientific Wild- A$$ed Guess) at the best of times, given the number of parameters in the malts, mashing schedule, yeast type and pitching rate, yeast health, oxygen level of the wort, and fermentation profiles.

Now throw into this the ability to predict fermentability of wort given different mash steps within the prime fermentability range of 63C to 72C with differences of step time and it can be a real chore.  I have not found a good article which defines the ability to predict wort fermentability doing multiple mash steps given all the other parameters involved.

As I understand it, BeerSmith uses a common fermentability model for a single infusion mash.  It does not do well with the multiple steps, as it was not designed for it, and limits out at the temperatures I cited above for the prime fermentability range. 

In terms of the yeast attentuation, I believe based upon running the numbers myself that BeerSmith uses the average attenuation based upon the yeast specification which it is given.  As far as your example, I would need to see the recipe to determine why such a difference.  The addition of simple sugars to a recipe changes the level of attenuation you can get from a particular yeast strain.

Thanks for your answer. Here is the recipe (sorry for the french text).

I agree that the FG adjustment is definitely not an exact science, but here the formula used is clearly described, and it doesn't fit with the results I got.

In my understanding of the formula the higher is the mash temp the higher is the FG. In my example when I change the mash temp from 72 to 73, the FG goes from 1014 to 1001. It really seems to me to be a bug.

I understand it was maybe not designed for multi-steps, and anyway I will be careful next time not to always trust the FG adjustment when the results are weird. But I wanted to share this if it could lead to any improvement.

About yeast attenuation, I used WLP550 which was configured with a max attenuation or 85%. With an OG or 1079 and a FG of 1001 the estimated attenuation is 98%. Again, this doesn't seem right.
When I set the mash temp at 72 instead of 73 the FG is 1014 and the attenuation is 81%, that seems good.

My guess about what is happening is that BeerSmith has to choose a temperature from one of the mash steps to be "THE mash temperature" that it uses to calculate fermentability and FG. Your mash steps are a bit extreme, from very low to very high. When your second step is 72, BeerSmith uses that as the temperature for its calculations, and gets a relatively high FG. When you raise that temp from 72 to 73, BeerSmith's algorithm decides to use the temperature of the previous step instead. That temperature is very low, so it gives a low FG. The jump is because you exceeded the boundary below which BeerSmith will use a temperature to predict FG. What you encountered is not so much a bug as a discontinuity in BeerSmith's simplified mash temperature that shows up because you have such extreme temperatures.

--GF

I think GigaFemto has the right of it.  Add to the confusion regarding the FG, 12% of your fermentables is simple sugars, which ferment out completely.  This increases the apparent attenuation of the yeast, which when combined with the mash temperature extremes, gives you some wild swings.  When I brew something with either very high or very low mash temperatures (I've yet to do a step mash with two steps in the prime mashing range), I usually ignore the FG calculation as I know it will not accurately reflect reality.

Thank you so much for your answers. This is now totally clear for me