ABV calculation after adding dry fruit

[ThriceIn5Minutes]

I added ~1.5 lb of dried fruits to my strong ale last week when I tranfered to secondary. It produced an unexpectedly long fermentation (still going though very slowly after 9 days) that will have a big effect on the ABV. Does anyone know how I can estimate the final ABV? What can I do in the future to more accurately esitmate the ABV?

OG = 1.091
SG after primary = 1.026
SG currently = 1.021

 

[FlyingHorse]

Probably depends a little on what kind of fruit, but raisins are about 75% sugar by weight. So if all of the sugars were extracted and fermented (unlikely), your 1.5 lbs of fruit would be about 1.1 lbs of sugar.

In a 5g batch, that's worth about 10 gravity points, or an additional 1.3% ABV, max. You'll probably get less than half that.

So your "adjusted" OG would be something around 1.096, and your ABV at 1.021 is ~9.8%

 

[ThriceIn5Minutes]

1.5 lb is roughly .5 lb each of raisins, currants, and apricots. Do you have a resource that gave the 75% figure for raisin?

 

[McKBrew]

I have a back issue of Brewing Techniques (no longer in print). It has a decent article on fruit in beer, and someday when I have time I will put the pertinent information in a thread.

It is in this issue if anyone has it available.

 

[FlyingHorse]

Do you have a resource that gave the 75% figure for raisin?

The back of a SunMaid raisin box. One serving = 40g; contains 29g of sugars.

 

[david_42]

I suspect the length of the secondary fermentation was driven by the relatively slow rate of sugar diffusion from the dry fruit, not the high sugar content.