How does OG effect FG

[wsmith1625]

I just switched from extract brewing to BIAB and I missed the original gravity on my first brew. I made a pale ale with a target OG of 1.054 and a FG of 1.012. My OG was 1.047. I'm not sure what my target final gravity should be now. Is it still 1.012 or will it be proportionately lower, 1.005? Could someone explain how this works? Thanks!

 

[eagle23]

OG is your starting gravity, how much sugar is in solution pre-fermentation. FG is the final gravity, how much sugar is in solution post-fermentation. OG and FG relate in the sense that the higher your starting gravity is, the higher your final gravity will be, but that is more dependent other factors.

To guesstimate your FG you'll want your OG and as well as the attenuation of the yeast you used (what percentage of the sugars the yeast will consume). If you took a batch of wort and split it, then used two yeasts with different attenuation's in each of them. The beer fermented with the yeast with the higher attenuation will have a lower final gravity than the other, but the alcohol content will be higher since more yeast was fermented.

Here is a calculator to calculate ABV. It will tell you what your beer's ABV is and also the attenuation of the fermentation.

When you get into all grain brewing, there are some other factors and variables that you can tweak to affect your final gravities, such as mash temp and time. But for BIAB, I think just calculating your ABV and making sure it lines up with your attenuation will tell you what you need. This is a lot of info but I hope it answered your question.

 

[wsmith1625]

Thanks for the info. it helps, but I think I'm struggling with what I think I know compared to actual beer science. Grain had a potential amount of sugar available, and your efficiency calculates how much sugar you collect during your mash. The original gravity is the calculation of the collected sugar in the volume of water. Since I missed the original gravity, the yeast has less sugar to convert, so lower abv. Regardless of the original starting gravity, the yeast should run out of sugar at the same ratio to water...same final gravity. Is that correct, or am I confusing things? Thanks again!

 

[McKnuckle]

You're over-simplifying and missing a couple of factors.

First, attenuation is expressed as a percentage. The recipe's targets, 1.054 fermented down to 1.012, would represent 77.8% attenuation. If you missed the OG by 7 points and got 1.047, then the proportional FG is not 1.005. That's just subtracting the same number of points from the FG as you did from the OG. Instead, you have to apply the same attenuation percentage to the new OG. So, 1.047 fermented down 77.8% becomes 1.0104 (1.010).

Another factor is that yeast tend to exhibit higher attenuation in lower alcohol worts, all things being equal. With only a 5 point difference in OG, this is probably really insignificant for you, but it is still a factor. So I'd anticipate maybe 1.009 as an FG. Of course, you might not hit the expected attenuation at all, due to temperature and enzymatic behavior in your particular mash. That's why they are targets, not specifications.

Hope this helps. Brewing is a science, but it's a science with lots of variables, both obvious and not so much.

 

[dmtaylor]

I just switched from extract brewing to BIAB and I missed the original gravity on my first brew. I made a pale ale with a target OG of 1.054 and a FG of 1.012. My OG was 1.047. I'm not sure what my target final gravity should be now. Is it still 1.012 or will it be proportionately lower, 1.005? Could someone explain how this works? Thanks!

Apparent attenuation should be about the same regardless. Attenuation calculations use the gravity points, which basically ignore the "1.0" in front but then put it back on at the end. So, the fastest calculation you can do on this is: 12 / 54 * 47 = 10.4, so in other words, you can expect your FG to be about 1.0104, or 1.010.

 

[grampamark]

You’re overthinking this.

Yeast are living organisms with little respect for calculations. Your FG will be what it is. At .047 OG you have a potential ABV of just under 6.4% at a theoretical attenuation of 100%. At 75% attenuation that would put your ABV at 4.8%. That would require a FG of .010. But, your attenuation might be 67% or 82% or whatever. So, leave your projected FG where it is and deal with the actual FG when fermentation is complete.

I see a couple of others have posted while I was typing. We all came up with the same numbers. Do we win the trifecta?

 

[wsmith1625]

Yep, figured I was oversimplifying and wanted a better understanding of what's going on in the bucket. All your replies were really helpful. I'm not going to keg my beer until the final gravity stabilizes. Thanks.

HBT is an invaluable resource. I learn so much here. Awesome community.

 

[bgmac]

Grain crush size matters when using BIAB. Go with a very fine crush. Look for a corn meal consistency with lots of flour. Your OG will go way up. I usually get 85% efficiency. If you have a bunch of dimensional chunks then it is too course.

If you are crushing the grain, go with a tight setting, about the thickness of a credit card. If your local home brew supply store is crushing for you, ask for a double crush or if they can tighten up their mill. Using the typical crush size for traditional mash tun method will not get you the standard 75% efficiency because the mash thickness is typically much thinner (2.0 - 3.0 Qts/Lb or more) with BIAB.

Are you dunk sparging, no sparging, sparging? You have to take that in to account. No sparge means you need a heavier grain bill since you will be leaving sugars behind in the grain bed after the bag is removed. Dunk sparging will help wash out some of the sugars. I find a long slow sparge and letting the bag hang over the kettle for a long time to drip dry for like 20+ minutes helps get all of those sweet sugar particles into the boil. Dont be afraid to give the bag a good squeeze before moving the bag out of the hanging postion. Keep your sparge water around 165F, dont go over 170F. Squeezing wont release tannins, its a myth rooted in fiction. Large breweries have hundreds and hundreds of pounds of soaking wet grains crushing down in the mash when they drain the mash.

 

[wsmith1625]

Grain crush size matters when using BIAB. Go with a very fine crush. Look for a corn meal consistency with lots of flour. Your OG will go way up. I usually get 85% efficiency. If you have a bunch of dimensional chunks then it is too course.

If you are crushing the grain, go with a tight setting, about the thickness of a credit card. If your local home brew supply store is crushing for you, ask for a double crush or if they can tighten up their mill. Using the typical crush size for traditional mash tun method will not get you the standard 75% efficiency because the mash thickness is typically much thinner (2.0 - 3.0 Qts/Lb or more) with BIAB.

Are you dunk sparging, no sparging, sparging? You have to take that in to account. No sparge means you need a heavier grain bill since you will be leaving sugars behind in the grain bed after the bag is removed. Dunk sparging will help wash out some of the sugars. I find a long slow sparge and letting the bag hang over the kettle for a long time to drip dry for like 20+ minutes helps get all of those sweet sugar particles into the boil. Dont be afraid to give the bag a good squeeze before moving the bag out of the hanging postion. Keep your sparge water around 165F, dont go over 170F. Squeezing wont release tannins, its a myth rooted in fiction. Large breweries have hundreds and hundreds of pounds of soaking wet grains crushing down in the mash when they drain the mash.

I ordered double milled, but it was still too coarse for BIAB. I bought a "cereal killer" grain mill which I'm going to use going forward. I'm also planning on doing a dunk sparge next brew. I know I can fit the full grain bill in my 10 gal. kettle, but I would rather save 2 gallons for a dunk sparge and hopefully pick up some extra efficiency points as a bonus.

I read a lot of threads on sqeezing the bag and most squeezers agree that it doesn't release tannins. The reason I'm not going to squeeze it because I'm not convinced it's necessary yet. Also, I read an article where they did an identical brew with and without the squeeze. Flavor was basically identical, but the non-squeezed batch had better clarity. I'll reconsider this down the road as I get a few more brews under my belt. Thanks for the feedback.

 

[dmtaylor]

I read a lot of threads on sqeezing the bag and most squeezers agree that it doesn't release tannins. The reason I'm not going to squeeze it because I'm not convinced it's necessary yet. Also, I read an article where they did an identical brew with and without the squeeze. Flavor was basically identical, but the non-squeezed batch had better clarity. I'll reconsider this down the road as I get a few more brews under my belt. Thanks for the feedback.

True. However, squeezing does result in addition of unconverted starches; hence the extra haze.

FWIW, I use BIAB but I "sparge" or rinse, never squeeze. It works well. Efficiencies of 80-90% are common this way, with less haze.

 

[bracconiere]

FG is unfermentable sugars, all you have to do is add glucoamylase to EVERY batch and your FG will always be 1.000...

 

[bgmac]

I ordered double milled, but it was still too coarse for BIAB. I bought a "cereal killer" grain mill which I'm going to use going forward. I'm also planning on doing a dunk sparge next brew. I know I can fit the full grain bill in my 10 gal. kettle, but I would rather save 2 gallons for a dunk sparge and hopefully pick up some extra efficiency points as a bonus.

I read a lot of threads on sqeezing the bag and most squeezers agree that it doesn't release tannins. The reason I'm not going to squeeze it because I'm not convinced it's necessary yet. Also, I read an article where they did an identical brew with and without the squeeze. Flavor was basically identical, but the non-squeezed batch had better clarity. I'll reconsider this down the road as I get a few more brews under my belt. Thanks for the feedback.

Sparging makes a big difference. Don't forget to let the bag hang and drip dry while your boil is heating up. I used to move the bag to a large plastic tote as soon as I finished the sparge. I noticed about 1/2 gallon of wort pooling up in the bottom of the tote. I took a gravity reading on the wort in the tote and it was like 1.089! when my target OG was around 1.050. Of course my actual OG ended up a few points low because I left a good amount of sugar in the bag. Convinced me to let it drain for a long time over the boil kettle. Shot my OG way up. Squeezing is not necessary but I do give it a little pressure by hand right before I move the bag out of the picture.

I order my grain double crushed and it is hit or miss if its been milled fine enough. I usually run it through my homemade mill if it is too course from the LHBS.

 

[Denny]

I just switched from extract brewing to BIAB and I missed the original gravity on my first brew. I made a pale ale with a target OG of 1.054 and a FG of 1.012. My OG was 1.047. I'm not sure what my target final gravity should be now. Is it still 1.012 or will it be proportionately lower, 1.005? Could someone explain how this works? Thanks!

There is no given relationship. It all depends on the fermentability of wort and the health of the yeast

 

[Denny]

True. However, squeezing does result in addition of unconverted starches; hence the extra haze.

FWIW, I use BIAB but I "sparge" or rinse, never squeeze. It works well. Efficiencies of 80-90% are common this way, with less haze.

I both sparge and squeeze

 

[wsmith1625]

There is no given relationship. It all depends on the fermentability of wort and the health of the yeast

Thanks Denny.