What affects final gravity?

I am using Beersmith to formulate a Smoked Porter recipe from BYO magazine (changed it to a small batch). I adusted the ingredient quantities and hit the estimated Original Gravity exactly as the recipe stated, 1.065, however the Final Gravity estimate is off. The recipe states 1.015 but Beersmith shows 1.019. I tried adjusting the grains but that changes the OG in addition to the FG. What would affect the FG without changing the OG?

In Beersmith, it's just an estimate. It doesn't take ingredients like some less fermentable items like crystal, or more fermentable items like honey, into account at all. It also doesn't seem to change the expected FG if you change yeast strains to a different attenuation.

What you can do is adjust the recipe in Beersmith to get the right OG, and the desired IBU/Sg ratio. Then, use your yeast strain's typical attenuation rate, and use the attenuation % tool to estimate your likely FG. If you have more unfermentables, you'll probably get less than the typical %, and if you have more (or mash low), you'll probably get a bit more than the typical %. If it's a yeast you're familiar with, it's pretty easy to figure out.

Say I formulate the recipe to the correct estimated OG, what things in the ingredients or process can I control to change the FG? In other words, once you put in all of the ingredients, are you going to get the same FG no matter what?

secinarot said:

Say I formulate the recipe to the correct estimated OG, what things in the ingredients or process can I control to change the FG? In other words, once you put in all of the ingredients, are you going to get the same FG no matter what?

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No, you can do a few things to change the FG. The first is to use different yeast. Some yeast strains will only attenuation 68% or so, giving you a higher FG. Some will attenuation 80%+, giving you a much lower FG. You can mash your grains at a lower temperature (148-151F) to give you a more fermentable wort. You can mash them higher (154-158F) to give you a less fermentable wort. In AG brewing, you have more flexibility to make a mash profile to give you the results you want (thin mash, thick mash, temperature, etc).

Keep in mind too that the final gravity gives you the ABV estimation too. If you have a lower FG you will have higher alcohol content, because more of the sugar was fermented. There are also chemicals you can add when fermentations stalls that get things going again. I like to rock my primary to rouse yeast and get a little more activity out of them, rather then adding chemicals.