How to adjust sugar content to reach desired OG when mash contains adjunct solids?

[Dolmetscher007]

I have made many fruit wines, meads, and ciders over the years. Almost always, they were made by mashing up fruit, sometimes adding fruit concentrates, and sometimes adding dried fruit. So by the time I pitch the yeast, there is sometimes a downright thick sludge of purveyed fruit pulp with dried fruit floating around in it along with some fruit juice that has come out of the mashed fruit. I always add pectic enzymes, yeast nutrient, and sometimes acid blend and/or tannins. The pectic enzymes eat at the fruit solids, and by the time the fermentation is over, there is mostly juice, with a surprisingly small amount of the fruit pulp left.

I have always wondered, when I go to take an OG reading of just the juice, there is a lot of sugar locked in the fruit pieces, the dried fruit, the pulp etc. So, unless you absolutely squeeze all of the juice out of the pulp, and not add dried fruit... you will not be taking an accurate OG reading. I'm pretty sure it's not a HUGE difference, but... I'm still just surprised that there is never anything written about this when you read about: original gravity, specific gravity, potential ABV, and final gravity.

In fact... specific gravity doesn't just measure sugar, right? Doesn't it just measure anything that is dissolved into the water? If you took a SG measurement of water, then added a bunch of salt, that too would change the SG reading on a hydrometer, right? If you take cornmeal, for example, and you soak them in a bunch of water, and then you drain off the water the next day... that milky water will have a much different SG than just plain water... but it only has starches in it... and wouldn't ferment into much of anything since those starches haven't been converted into sugar through some kind of enzyme.

Since specific gravity cannot account for sugars locked inside solid fruit, and it is so easily affected by any dissolved solids such as starch, salt, proteins... and whatever else... isn't specific gravity a pretty poor way to judge the exact amount of actual fermentable sugars are dissolved in a volume of water?

 

[RPh_Guy]

specific gravity doesn't just measure sugar, right? Doesn't it just measure anything that is dissolved into the water? If you took a SG measurement of water, then added a bunch of salt, that too would change the SG reading on a hydrometer, right?

Yes, yes, and yes.

isn't specific gravity a pretty poor way to judge the exact amount of actual fermentable sugars are dissolved in a volume of water?

With fruit juice and fruit flavoring, there's no problem at all.
Puree is also pretty accurate to measure since it's contributing almost all the juice (gravity points) immediately after mixing.
Crushed fruit is still fairly accurate because the s.g. of the juice is likely similar your target OG of the wine and won't make a large difference.
Dried fruit might be less accurate, depending on when you measure the OG. If you measure it 24 hours after hydrating the fruit e.g. with sulfite, then they likely would have had a chance the equilibrate due to osmotic pressure... and you'll get an accurate OG reading.

Convention and experience negate the inaccuracy of OG readings for measuring fermentable sugar. We expect FG to finish around 1.000 when fermented "dry", but that still leaves plenty of dissolved solids in the wine.

Hope this makes sense.