ABV when coffee was added after fermenation (w/unknown SG before adding coffee)

[jack13]

The OG was 1.066 on a coffee stout. I let it fully ferment, then racked it (4.75 gallons) onto 1 gallon of coffee in a secondary.

I realize now I should have taken a gravity reading before I transfered, but I did not. Anyway, it's now 83% beer of an unknown SG and 17% coffee.

The FG (with the coffee in there) is 1.014. Now I'm not sure how to figure out my ABV. So...

Question #1. Can I assume regular old dilution rules apply, and the SG before I added the coffee was 1.017?

I came up with 1.017 according the following logic...or "logic". Since I have an 83% beer / 17% coffee solution, with a known weighted average of 1.014, I took a weighted average formula and algebra-ed it to give me the unknown reading of the "original" beer.

1.014 (the known weighted average) = (0.83)X + (0.17)1.000
And so (1.014 - 0.17) / 0.83 = 1.017

Question #2. IF that's correct, it would mean I had 1.066 wort that was fermented down to 1.017 (6.43% ABV). I then diluted that with 0.00 ABV coffee, so then I'd just do 6.43 x 0.83= 5.34% ABV?

 

[xpops]

working through your *logic* - it seems to make sense to me, providing standard dilution rules apply...which i don't see why they would.

i believe there's a dilution tool in beersmith, if you use that. You could plug your numbers in to confirm your math/theory/logic?

 

[doug293cz]

Your method works if coffee has an SG of 1.000. I would just make some coffee of the same strength that you used in the beer, and measure it's SG. If it's different from 1.000, then use the actual SG in the weighted SG formula.

Brew on

 

[jack13]

Turns out my coffee (normal strength, I think) is 1 brix/1.0039, for anyone curious.

 

[brandonnys]

I'm really curious about the goal you're trying to achieve. Just to know what the gravity going into secondary was? Are you trying to figure out the attenuation?

(Please do not misconstrue this as an attack; I'm honestly curious what you'd use that information for.)

 

[doug293cz]

Turns out my coffee (normal strength, I think) is 1 brix/1.0039, for anyone curious.

I'm assuming since you list brix, that you are using a refractometer to measure the SG of your coffee. I wouldn't use a refractometer to measure SG of coffee. The ones we use are calibrated specifically for solutions of sucrose and water. I doubt whatever is in coffee has a refractive index the same as sucrose. I would only trust a hydrometer measurement.

Brew on

 

[jack13]

I'm assuming since you list brix, that you are using a refractometer to measure the SG of your coffee. I wouldn't use a refractometer to measure SG of coffee. The ones we use are calibrated specifically for solutions of sucrose and water. I doubt whatever is in coffee has a refractive index the same as sucrose. I would only trust a hydrometer measurement.

Brew on

Good point. Will do!

 

[jack13]

I'm really curious about the goal you're trying to achieve. Just to know what the gravity going into secondary was? Are you trying to figure out the attenuation?

(Please do not misconstrue this as an attack; I'm honestly curious what you'd use that information for.)

Just trying to figure out the ABV, which would have been easy if I took a gravity reading before racking the beer onto the coffee in secondary. I guess I'm in the habit of doing a gravity reading when I bottle.

 

[brandonnys]

But... I guess I'm not following. The ABV of the liquid you drink, which has coffee in it, is what you're looking for, right? Separating out the liquids that make up the final product (beer) and knowing their individual gravities was what I see you are doing, but I'm curious why?

 

[jack13]

But... I guess I'm not following. The ABV of the liquid you drink, which has coffee in it, is what you're looking for, right? Separating out the liquids that make up the final product (beer) and knowing their individual gravities was what I see you are doing, but I'm curious why?

True, but to figure out the ABV of the final product, I can't simply use the OG (1.066) which did not include coffee, then a reading of the final product that includes the coffee (1.014 in this case). If I undestand correctly, what tells you the ABV is comparing the amount of sugar in the wort, then the amount of sugar left in the beer (the difference tells you the ABV).

BUT, that assumes the difference between the OG and FG readings is all due to the yeast fermenting the sugar into alcohol, not diluting it (e.g., with coffee.)

In other words, some of the difference between my OG 1.066 and my FG 1.014 was due to yeast fermenting sugar, but some was due to dilution by coffee. My "dilemma" (boy, I'm glad I have these problems!) was in parsing that out.

At this point, my pride wants me to have done this right, but my sense of humor wants there to have been a much easier way.

 

[doug293cz]

...

At this point, my pride wants me to have done this right, but my sense of humor wants there to have been a much easier way.

The easier way was to measure FG at the end of fermentation, but once you burned that bridge, the only choice was the way you ended up doing it. You did well to figure out how to recover from your original oversight.

Brew on

 

[brandonnys]

I think I'm still confused.

Let's say you brew a beer, and out of the boil and into primary, your SG is 1.060. You let it ferment until it's done, and it's gravity is 1.016. You add some coffee to the beer and it's now 1.014.

What I'm failing to understand is why the 1.016 gravity reading is necessary. Isn't the gravity of the beer inclusive of all of the adjuncts what you're measuring, not the beer sans adjuncts? I just don't know what the benefit is to knowing what the beer is without the components that make it what it is...

 

[doug293cz]

I think I'm still confused.

Let's say you brew a beer, and out of the boil and into primary, your SG is 1.060. You let it ferment until it's done, and it's gravity is 1.016. You add some coffee to the beer and it's now 1.014.

What I'm failing to understand is why the 1.016 gravity reading is necessary. Isn't the gravity of the beer inclusive of all of the adjuncts what you're measuring, not the beer sans adjuncts? I just don't know what the benefit is to knowing what the beer is without the components that make it what it is...

Look at the math. The approximate formula for ABV is:

ABV = (OG - FG) * 131.25​

If you dilute the finished beer, the SG will go down. But if you substitute the new SG for the FG in the formula, it says your ABV went up. This clearly cannot happen since you have a constant amount of alcohol in the beer, and increasing the volume by dilution can only decrease the ABV. Thus the ABV calculation must be done with the "true" (end of fermentation) FG.

Brew on

 

[brandonnys]

It clicked. Thanks for the explanation.

 

[bigplunkett]

One question. Do you really need to know the abv. Drink a few youll find out roughly.