Help to understand what determines FG

I Just bottled a batch of IPA using a recipe that I designed in Beersmith.

I had 3.3# LME and 1.5# DME in a 2.75 Gal batch. I used a whole pack of US-05 yeast. The program predicted a Starting Gravity of 1.075 and my measured SG was 1.077. The program predicted an FG of 1.020 but I ended up at 1.015. The ferment temp started out @62 and slowly crept up over the 2 weeks ending @ 68. This was a clone of Great Divides TITAN and I wnted a full malty finish.

I guess I have 2 questions.

1. Why did this beer finish so much lower than predicted?

2. I know that I can manipulate fermentables to achieve a desired SG but don't understand what detemines the FG. Can anyone explain in general what determines where a beer finishes?

Thanks,

Jim

Different yeast straing have different attenuatiosn rates. That is the % of fermentables that they can process basically. So if a yeast has an attenuation of 75%, then count on getting 75% of the way from your gravit to 1.000. That is my understanding.
75% would take your 1077 to 1020. The expected. You got around 80%.

This thread at probrewer.com suggests others with the same issue with S05 at one point.

With extract, it's hard to determine the amount of fermentation because you don't know the mash temp. Mash temp is a big factor in determining fermtables vs non fermentables.

Thanks,

I will probably continue with the same brand of extract making that a constant.

So if I wanted it to finish higher I would need a different type, not less yeast?

Jim

Since you're brewing extract beers, you don't really have any control over how fermentable the wort is. So to change your attenuation, using a different yeast strain is pretty much your only option. If you want to continue with dry yeast for now, try the S04 strain.

But there are other things to be aware of in switching strains - it's not like the beer will taste exactly the same except with less alcohol. A little less attenuation also can translate to a heavier mouthfeel, a little extra sweetness, and/or more perceived maltiness. Frequently some of the less attenuative strains also produce more esters and other compounds so you'll have a generally more flavorful beer. None of these things is necessarily bad or good - just personal preference.

I use a less attenuative strain in all my beers because I like these characteristics, but most people on HBT like to get VERY high attenuation. People here are afraid of sweetness in their beers because they believe them to be less drinkable. I can sympathize with that, but I still choose my malt-bombs over watered-down, bland California ale yeast.

I did have some steeped Crystal 60L and some pale 2 row in the grain bill for flavor but yeah it is primarily an extract batch.

Sounds like it would be worth trying the US-04. I want the heavier mouth feel and extra maltiness for this heavily hopped IPA.

Thanks,

Jim