Measuring fruit's effect on gravity

[JustTegIt]

I will be trying a blueberry wit (aiming for Sweetwater Blue - an Atlanta craft brew) soon and and I will be adding 2.5 lbs of frozen (sterilized with campden) blueberries to 2.5 gallons (half batch) in secondary.

How does one go about measuring the effect on gravity of sugars added during secondary - especially since they wont really be dissolved in the wort immediately? Correct me if I am wrong - but if the gravity goes back up during secondary due to added fermentables then the ABV calculation using just the SG - FG will not account for that jump, and therefore the calculated ABV will be too low.

Is my logic incorrect or is there a way around this?

 

[MalFet]

Your logic is mostly correct, but the end result is different. In addition to adding sugar, fruit adds water. So whether a fruit bumps your "inherent" OG up or down depends on your recipe. See this description for more detail.

There are ways of estimating what you are getting, based on the nutritional content information of the frozen blueberries and a few guesses about pulp weight, but it's always going to be a rough estimation at best.

 

[tjpfeister]

A refractometer helps predict the added fermentables, but I find it is still tricky to guess just how fermentables/weight you get from the fruit. I suppose there might be some info if one does some research on wine making, but I have not read up on it as I generally just use the initial ABV calculation and add a "+" to the end of that ;-)

 

[devilishprune]

The main thing to keep in mind is the gravity of the fruit relative to the OG of the beer. If the gravity of the fruit is lower than the OG of the beer, then it's going to dilute it (because of added water), and vice versa.

Check out this link for more info: http://www.themadfermentationist.com/2010/10/adding-fruit-to-beer-increases-alcohol.html

 

[MalFet]

Your logic is mostly correct, but the end result is different. In addition to adding sugar, fruit adds water. So whether a fruit bumps your "inherent" OG up or down depends on your recipe. See this description for more detail.

The main thing to keep in mind is the gravity of the fruit relative to the OG of the beer. If the gravity of the fruit is lower than the OG of the beer, then it's going to dilute it (because of added water), and vice versa.

Check out this link for more info: http://www.themadfermentationist.com/2010/10/adding-fruit-to-beer-increases-alcohol.html

What a good idea!

 

[JustTegIt]

thanks for the replies. That link makes perfect sense! I would have to buy a refractomoter to get the exact Brix of my blueberries - but from my internet research it seems that most blueberries are between 10-15 Brix. Either way it wont be too far off from my OG of 12.5 Brix, so after a calculation (using formula in above link) the change in gravity will be at most +- .3% ABV.

 

[kanzimonson]

Yeah I added 5.5# of peaches to 5.5gal of beer and after racking off the peaches I had an extra quart of liquid... that was a surprise.

Crappy beer though.