Today I tested the specific gravity, alcohol content and bitterness of my latest homebrew in the lab. According to my refractometer, the original gravity was 13 brix and the final gravity around 5.8 brix. Plugging these numbers into Sean’s refractometer calculator (the best one out there), I get an OG of 1.0505 (12.5°P) and an FG of 1.0097 (2.49°P). The OG value is most likely slightly larger than the real value, since I took the measurement before adding the yeast starter (which will lower the gravity). According to BeerSmith, the calculated IBU of this recipe is only 13.4. But the program doesn’t take into account any isomerization that occurs during the 30 minute whirlpool after the boil (the recipe featured quite a large flameout addition), so I guessed that the actual IBUs would be closer to 30. So what do the Anton Paar DMA 5000 M and Alcolyzer say about my beer:
Measured ABV%: 5.34
Measured Specific Gravity: 1.007646
Estimated Original Extract: 12.02° P
So, this means that the actual gravity is 2 points lower than the calculated, interesting. I also plugged the brix values into Sean’s spreadsheet, which contains the old widely used cubic approximation, a new linear approximation, and the new cubic approximation (the one in the PHP calculator). The linear approximation was closest to the measured value, with 1,0088. If I lower the original brix (so that the gravity matches with the estimated original extract given by the Anton Paar), the calculated final gravity increases further from the measured value. So, this tells me that Sean’s new approximations are at least better than the old one, and despite being slightly off, are accurate enough for me.
So, the calculated IBUs (Tinseth) were only 13.4, but thanks to the large flameout additions and 30 minute whirlpool, the measured IBU was actually 35.0, nice! I performed the IBU assay according to ASBC methods, and measured an absorbance of 0.700 in a quartz cuvette on a Shimadzu UV-1800 spectrophotometer. Really interesting that the flameout additions contributed this much to the bitterness. I don’t have BeerSmith in front of me, but am expecting that the overall effect of the 30 minute whirlpool on my system is approximately adding 10 minutes to the boil time of all hop additions. The actual beer does not taste like 35 IBUs though, at least to my palate, so there might be some truth in the theory that late additions extract a smoother bitterness.