Low OG = high FG?

[kmonty]

just brewed a batch of Brown Porter. I put too much liquid in the brew pot resulting in a lower OG of 1.045 (1.051 was the target)
After holding the temp at 67 for two weeks, the next gravity was 1.018. A week later, 1.016. (1.014 was the target)

My question is: if my OG gravity is low, will that mean there aren't enough fermentables to reach to target FG?
Just curious.

Thanks in advance!

 

[budchx]

Actually to be sure the fermentation is done you have to take the FG to days in a row and have the same reading,if it's different that means that fermentation is still going on.

 

[kmonty]

Guess i'll take another reading tomorrow then and see if it's the same...

Thanks!

 

[pericles]

OG and FG are directly related, but only loosely. Here's what I mean:

Beer wort is made up of fermentable sugars and non-fermentable compounds. Normally, when you brew a recipe, the ratio of fermentable sugars to non-fermentable compounds remains pretty constant. That means that a low OG will result in a low FG too, because there's less stuff in the beer overall.

The reason I say that FG is only loosely related to OG is because there are a lot of styles for which you can artificially pump FG up with additives. My Belgian Golden Strong, for instance, routinely finishes at 1.000, but starts at 1.070. That's a pretty high OG and a VERY low FG. You can do the opposite, too, by adding lactose or matodextrine malt to a session beer, in order to create a low alcohol beer with lots of mouthfeel - that would result in a low OG with a high FG.

In your case, I think fermentation just hasn't finished, though. I'd let it keep going, maybe boost the temperature a degree or too, and rouse the fermenter.

 

[kmonty]

Thanks for the thorough answer! That makes a bit more sense to me - specifically the relationship to OG to FG regarding the fermentables and non-fermentables.

I have boosted the temp from 67 to 70 the past couple days, so I will indeed give it some more time to finish out. I used London Ale 1028 and it is supposed to attenuate in the mid-70's so I am hoping it keeps going. (now it's in the mid-60's)