In the Packaging chapter of American Sour Beers I included formulas designed to estimate the amount of carbonation that will be produced by the microbes when blending similar sour beers of different gravities. While I believe it is important to understand the logic behind the math, it is certainly much easier to simply plug in the numbers rather than solve equations by hand. A few weeks ago someone emailed a question about the formulas, which sparked me to put together an easier method. I finally found time to refine it, so here I present my Blending Priming Calculator spreadsheet! Unlike the formulas in the book, it can determine carbonation for a blend of up to five beers.
These formulas are only exact if the wort for each component is identical. Having the same carbohydrate profile ensures that the remaining dextrins in one batch would be fermentable by the bacteria and Brettanomyces present in another. You’d expect the final gravity of the blend to approach the final gravity of the driest component, which is why this component must be entered in a specific position in the spreadsheet. Even then there is no way to guarantee the accuracy of the calculation because more attenuative microbes could be "hiding" in a younger/sweeter component.
To use the spreadsheet, start by selecting the number of component beers you will be blending from the drop-down list. This selection controls which formulas are used from the hidden Calculations tab.
Input the peak temperature the beer reached after the gravity stabilized (this is the same as all other priming calculators). If you ferment a beer through a warm summer and it continues to ferment into the fall, but stops before winter, you’d note the temperature it was when you stopped seeing the gravity drop (assuming it didn't get warmer after that). If the beer fermented all winter, but aged into the hot summer, you'd note the hottest temperature it reached during the summer.
I’ve noticed anecdotally that long-term aging in a barrel knocks about half of the assumed residual carbonation out of the beer. The "Residual CO2 Volumes" in red will calculate automatically. If you happen to own a capable CO2 meter (aren't you lucky!), you could override these calculation and simply enter the measured volumes of CO2 in row 5.
At this point enter the current gravity reading for each component and the volumes of beer that you plan to include in the blend. With those pieces of information entered you can see how much residual carbonation the blend will contain at bottling, and after it completes bottle-fermentation (assuming no priming sugar).
Input the target volumes of CO2 desired, and the formulas will display how much table or corn sugar would be required to carbonate the beer to that level. As we are assuming the fermentation of dextrins from the blended beers, it may take 6-12 months of cellaring to achieve full carbonation (as is carried out by traditional Belgian gueuze blenders).
I’ll certainly update this spreadsheet as I think of improvements (and hear your suggestions). At a minimum I'll add a metric tab as the metric formulas are already presented in the book.