i have a clone recipe Magic Hat #9 that calls for apricot additions. I'm using 3lbs of apricot puree. i found this article about adjusting OG to account for fruit additions, that I found interesting and useful...so thought i would share in case you ever have done it or do it in the future. for my recipe, the clone targets 1.052 OG with abv of 5.1%, so I adjusted my recipe to target OG of 1.053 to account for dilution of the fruit I would be adding. using the formula below, my adjusted OG would be about 1.052, matching the profile for this clone.
To correctly determine the impact on the alcohol content of adding fruit you need to determine the effective brix/plato of the beer. To calculate this you need four pieces of information:
1. Weight of the fruit (not including pits/stems/seeds).
2. Brix/plato of the fruit (from a refractometer or packing info).
3. The original brix/plato of the beer (OG reading).
4. The weight of the beer (based on the current volume, but the density of the wort before fermentation. This can be determined by using the following formula: weight of the beer = original gravity of the beer x volume of the beer x weight of 1 gallon of water)
Effective Brix/Plato = (Weight of beer x Brix/Plato of beer + Weight of fruit x Brix/Plato of fruit) / (Weight of beer + Weight of fruit)
For example if you have 4.5 gallons of 11 P (1.044) beer it would be 1.044*4.5*8.35 = 39.2 lbs of beer onto 10 lbs of 14 P cherries, the effective OG would be (39.2*11+10*14)/(39.2+10) = 11.61 P. That is to say the increase in the effective starting gravity was .61 P, enough to boost the alcohol by .3% ABV assuming the same FG (about the same increase in alcohol from the priming sugar).