Slightly specialized question. Short version: would you expect fruit sugars to be fully fermentable, and if not, would you expect amyloglucosidase additions in the fermenter to make any difference? Long version: for a bunch of reasons that seemed to make sense at the time, I'm brewing a sour cherry brut IPA - basically a poor man's kriek, or a Shit Kriek, I guess. My process was to brew a regular low-bitterness IPA, ferment with US-05, add 1ml of amyloglucosidase when the gravity seemed to be stabilising (at about 1.012), wait for the gravity to stabilise again (at 0.998), add about a kilo of dried sour cherries, rehydrated at about 160F in case of bugs, watch the gravity kick up to about 1.012 again from the fruit sugars and then wait for the gravity to stabilise a third time. That's where I'm at now - the next step is to add dry hops (erm, and some oak shavings because if you're going to be silly then you might as well go the whole hog), leave for a few days and package. This has mostly gone okay, but after the cherry addition, about 10 days after brewday, the gravity seems to have stabilised at 1.005 rather than right back at 0.998. I was hoping for this to finish as dry as possible, and was expecting the fruit sugar to be fully fermentable. Is there likely to be something more complex in there that the yeast can't handle? Or is the yeast just conking out from too much effort? Is more yeast or more amylo likely to make any difference? Or should I just accept that it's finished and get it dry hopped and packaged? Cheers!
Most fruits contain fructose, glucose, and sucrose. These are all completely fermentable. But fruits generally also contain some carbohydrates that are not sugars. The exact profile depends on the fruit, of course. Then there's the water contribution from the fruit. Assuming this was a 5 gallon batch with an OG of 1.065 (wild guess), the interim FG of 0.0998 you mentioned, with a kilo of tart cherries added afterward, I'd expect this to (coincidentally) come in at about 0.998 FG, given the sugar, non-sugar carbs, and water content of typical tart cherries. (I ran this through the upcoming BrewCipher fruit calc feature).
Thanks! It was actually a half-sized batch with an OG of about 1.050. Checking the nutritional information on the fruit packet, my kilo would have brought 500g of carbohydrates including 350g of sugars. Oh, and I also took a proper gravity reading yesterday, rather than relying on an uncalibrated Tilt and the stable gravity is more like 1.009, and the beer appears bright. Given all of the above, is it plausible that the yeast has conked out, wither due to the low gravity before the cherries went in, or the low pH afterwards? Any thoughts on whether it'd be worth reyeasting, either with more US-05 or a red wine strain? Cheers!
Regarding the very low initial FG, that shouldn't have any impact on the fermentation of the sugars from the cherries, unless you were bumping up against the ABV tolerance of the yeast. pH-wise, that's hard to say without actually measuring pH. But my gut says the cherries shouldn't have lowered the pH so much that the yeast quit. One thing about fruit, though... the listed carbohydrate and sugar content isn't always accurate. A lot depends on the particular batch of fruit and its ripeness (or lack thereof). This makes predicting OG/FG contributions a little bit of a crapshoot. Is this batch going to be kegged? If so, I wouldn't hesitate to keg it now. If bottling, I'd consider waiting it out a little longer. You used US-05, so you shouldn't be anywhere near its ABV tolerance level yet. Did primary fermentation seem healthy and vigorous?
Yeah, ABV is very unlikely to be an issue. The primary seemed healthy enough to start with, the enzyme-assisted fermentation was a bit slower, and the post-fruit fermentation barely got started. Intuitively I could believe that it's a combination of factors - yeast floccing out at the end of various fermentation steps and reduced pH hindering the small amount that's left. But I'm not an expert yeast wrangler so I don't know whether that's a thing. And it's going to be bottled, so I would like to make sure that it isn't going to suddenly wake up once it's in the bottle! I guess the flip question is, could chucking in an extra packet of US-05 or CBC-1 do any harm? Because if not, the inclination is to just do it and see what happens... Thanks!
