Last week I transferred 4 gal of beer (OG 1.050, FG 1.010) onto 9 lbs of peach purée. The purée has an SG listed on the can of 1.036-1.048. Is that SG that high because of the solid component? I can't imagine that I added 360 fermentable gravity points (assuming a 1.040 SG) to this beer that would ratchet up the alcohol more than twofold. Any thoughts? What would my ABV really be?
The specific gravity is only a measurement of how 'fluid' a liquid is in comparison to water, which is at 1.000. How much additional alcohol will be produced from the fruit will depend on how much fermentable sugar is present in the fruit.
You should have your OG and pre-fruit FG. If you have volumes of those, you can approximate the gravity of the mixture if you know the volume of the puree. Then you can measure the post-fruit FG. You can then use the change in gravity with the fruit addition to estimate your corrected OG and you can measure your FG. If the gravity points are coming from solids in suspension/solution they will still be there after fermentation.
I used to compute the gravity and ABV impact of fruit additions. But countering the additional sugars is the additional volume of water in the fruit. Once you take that into account, I found that my fruit additions always had minimal impact on the ABV, so now I don't bother.
It's a density measurement. It has nothing to do with viscosity or whatever you are implying by using the word fluid. SG is the density as compared to water. An SG of 1.100 is 10% more dense than water. Sugar is more dense than water, alcohol less dense than water.
+1. I have read that in many cases the fruit contributes about the same amount of water as it does sugar, so you end up with roughly the same ABV. With puree, I'm not as sure. Its interesting that a SG is listed. Assuming you used something like Vintner's Harvest puree which comes in 49oz (3lb) cans, that would be 147oz of 1.042 puree + 512oz of 1.010 beer. A weighted average would give you 1.017, which doesn't seem too unrealistic.
I'm probably missing something, but why would you want to add the puree's OG to the finished (before fruit) beer's FG? The numbers you need to compute the ABV are the total effective OG (the weighted average of the wort OG and the puree OG) and the FG after the puree's sugars have fermented out. Also (OP): be careful about "SG" specs on purees, syrups, and similar products. Sometimes they assume that's with 'X' amount of the product added to water to make 'Y' amount of wort (similar to the meaning of a grain's PPG number), and not the actual gravity of the product itself.
You're right. The part I forgot to mention is to add those 7 points (1.010 up to 1.017) to the OG to come up with your new effective OG. Would have made more sense to use the OG in the first place. The real question is what the puree is adding, which I wish I could answer. Someone else asked the same question: http://www.beeradvocate.com/community/threads/puree-gravity-question.93751/
