Forced fermentation test with starter beer

[TennBrewer]

I know that you can hold back a small amount of wort from the primary fermenter and dose it with some yeast to determine approximate final gravity, but....if I make a starter for the batch, would I get close enough if I test the beer from the starter? It's the same yeast I intend to use in the all-grain batch, it's just the using the DME-based wort for the starter. I don't know if anyone has checked this before but I'm brewing a small batch soon and the starter is already going, so I am going to check this out. The only significant number I can derive from this is the apparent attenuation. If I apply that figure to my full batch, I should be able to approximate the final gravity. Maybe.

 

[duncan.brown]

The fast fermentation test is to check the fermemtability of your wort. The yeast in the fermentation test will quickly (a couple of days, usually) get to final gravity, so you know what the final gravity of the beer will be. This is particularly useful if you’re spunding to carbonate once you get close to final gravity. Even though your starter is using the same yeast, you won’t have the same sugars in the starter as in your wort so you won’t know how fermentable the wort is.

 

[TennBrewer]

I brewed a one liter British Ale Yeast (Wyeast 1098) starter with OG 1.038 and after 24 hours when it appeared finished, the FG was 1.009. I pitched this yeast starter into a 2.3 gallon batch of pale ale (OG 1.066) and when it finished fermenting the FG was 1.009. Coincidence? Maybe. But I am going to try it again with a 2.5 liter starter of Giga GY054 Vermont Ale yeast OG 1.038 and FG 1.006. PItched into an 11 gallon batch of Pale Ale (similar to the previous batch) and it is currently fermenting away.

 

[rsquared]

I brewed a one liter British Ale Yeast (Wyeast 1098) starter with OG 1.038 and after 24 hours when it appeared finished, the FG was 1.009. I pitched this yeast starter into a 2.3 gallon batch of pale ale (OG 1.066) and when it finished fermenting the FG was 1.009. Coincidence? Maybe. But I am going to try it again with a 2.5 liter starter of Giga GY054 Vermont Ale yeast OG 1.038 and FG 1.006. PItched into an 11 gallon batch of Pale Ale (similar to the previous batch) and it is currently fermenting away.

Definitely a coincidence. Based on those numbers for the Wyeast 1098, the starter had an apparent attenuation of 76%, but the beer made it to 86%.

If you want to know an approximate attenuation range, the yeast manufacturer usually lists those. The point of a forced fermentation test is to determine what that yeast will do on the particular composition of sugars that happens to be in that particular batch of wort.

 

[Vale71]

DME is usally rather low-fermentability compared to an all-grain wort. DME producers usually optimize their production process for yield and speed of conversion and couldn't care less about fermentability. Your starter wort and your actual wort didn't even have the same apparent attenuation, much less the same real attenuation considering that they also had very different OGs to start with.
If you want to trust this coincidence to happen every time then it's your decision but IMHO you'll get much more consistent results by simply using the manufacturer's data as a starting point.

 

[TennBrewer]

DME is usally rather low-fermentability compared to an all-grain wort. DME producers usually optimize their production process for yield and speed of conversion and couldn't care less about fermentability. Your starter wort and your actual wort didn't even have the same apparent attenuation, much less the same real attenuation considering that they also had very different OGs to start with.
If you want to trust this coincidence to happen every time then it's your decision but IMHO you'll get much more consistent results by simply using the manufacturer's data as a starting point.

Manufacturers data for attenuation is usually a range, say 75% to 80% with some margin of error, I suppose. However scientific you want to get, it's still a crapshoot for the FG. The last batch I made, in the previous post, finished out at 1.008, the starter finished at 1.006. However you want to look at it, I think it gets close enough. Not that I'm going to rely on this every time because if I want a reasonably accurate figure, I would perform the forced ferm test. I think the starter FG gets pretty close so far but I will continue to monitor the results. I am only doing this as an experiment, I am not counting on the starter FG for brewing. Thank you all for your comments and expert advice, it's why I love HBT.