It sounds like you’re doing a lot of things right, especially being that your grape wine has taken off, but your plum wine is just plum tuckered out.
Seems like you’ve got it in a warm enough spot and you’ve added all of the right things — you haven’t over sulfited it and you’d think that since your grape batch was happily fermenting your yeast strain and nutrients were working just fine.
What does stick out to me a bit (and it took a couple of readings of your details plus a little math conversion to work out) is your starting Brix or sugar content. Though you don’t say what the plum’s initial Brix or sugar level was, you do mention that a portion of the plums were cooked with some sugar and that there were 6 lbs. (2.7 kg) of sugar added to 2 gallons (7.6 L). 6 lbs. = 96 oz. = 2.72 kg of sugar. Your volume is 2 gallons = 7.57 L of juice. That works out to be about 31.5–32 °Brix in sugar alone.
Add that on top of the natural sugar present in the plums, which, depending on the variety, could contribute between 10–20 °Brix themselves and you’re looking at a potential °Brix range of around 42–52! If you translate that into potential alcohol, that’d net you somewhere in the region of 24–30% alcohol. I don’t know of any wine yeast strain that can ferment anything higher than 18% alcohol to dryness. K1, BCS103, EC1118 (also known as Prise de Mousse or “PDM”) and a handful of others are good “power yeast” strains and must be used when faced with high Brix situations.
Needless to say, your Brix here is a little on the extreme side. Even if you use some high-Brix, dessert-wine type yeast and inoculation practices (more on that below), at that high of a sugar level you’re frankly going to eventually just encounter too much osmotic pressure from the sugar. The sugar may be too high for the yeast to get much of a foothold and unless you do multiple stepwise inoculations, “feeding” the fermentation with the high-sugar juice as you go, it’s possible the yeast might not even be able to start fermenting in an environment where the sugar content is that high. There’s a reason that jams, jellies, and fruits preserved with sugar manage to keep so well over time.
Yeast cell walls just aren’t built to handle sugar at the level your must appears to be and they’ll become what I call (the super-scientific enological term) “cranky.” When I’ve made dessert wines in the past (I once made Bonny Doon Vineyards’ “Vin de Glaciere” and “Framboise” earlier in my career) I experienced the full gamut of effects that maladjusted yeast can throw at you: Sluggish starts, high volatile acidity (VA), aldehyde production, off-aromas, oxidation, and of course the inevitable stuck fermentation. It takes a lot of winemaking derring-do to avoid these outcomes.
So, what to do in your situation? You have a few choices. First, you could add water (and maybe add a little more acid and yeast nutrient accordingly to make up for the extra volume of water) to dilute the sugar down into a more reasonable level. In my history of making sweet wines, I’ve never attempted a sweet wine (which will have residual sugar) above 32 °Brix. Dilution can affect flavors and aromas, however, and there usually is a point of diminishing returns that is dependent upon the flavor concentration in the fruit concerned.
You could give up on fermentation altogether and just fortify your plum solution with a lot of brandy, rum, or whisky (I’m serious!) to make a sort of alcoholic punch concentrate. You’d want your alcohol level to be at least 20%, so buy some booze and do your algebra accordingly.
This might be lovely spiced up with some cinnamon, orange peel, and nutmeg and bottled as gifts for the holidays. Just dilute with some sparkling water, serve over ice (or mull with orange and cranberry juice) for festive, delicious cocktails. Since you’ve got 2 gallons (7.6 L) of the stuff it seems like it’s a workable volume for fortifying and then bottling up as thoughtful homemade gifts for loved ones.
Or . . . you could attempt a yeast inoculation and fermentation, in which case you’re going to want to follow all of my high-Brix fermentation tips, below:
-Start with a super-robust yeast strain: BCS103, EC1118 (a.k.a., PDM), or K1 are good choices. Basically, you want to choose something with the highest alcohol tolerance you can find. Low VA production is also a plus.
-Use a higher inoculation rate: Look at what the yeast manufacturer recommends and then increase it by 50%. This helps guard against the normal cell debilitation and death that can occur with high-Brix, as well as high osmotic pressure, fermentations.
-Add more nutrient than you normally would: I would do an increase of 30% over what you would normally use for a regular (22–25 °Brix) grape fermentation. This ensures that everyone has a chance to be fed. The risk is residual nutrient remaining in the wine to potentially feed spoilage organisms later, but this can be fixed by filtration and judicious watching and care of your aging wine later.
-Follow good inoculation protocols: Practice scrupulous sanitation. Use filtered (non-chlorinated) water. Measure temperatures accurately. Use a hydration-water yeast nutrient like Go-Ferm or FermStart in order to help build up the strongest cell walls possible. Follow yeast vendor’s directions for that particular organism.
-Pitch differently: Rather than hydrating dry yeast and tossing it all into the must/juice at once, a very sweet wine requires a very different approach. The first step is indeed the same: Hydrating the yeast (with hydration nutrient) in warm water to make sure it’s viable. Then, as opposed to tossing the yeast slurry into your juice/must, you’ll be gradually “feeding” your juice must into your yeast inoculation. Guidelines will differ based on your total volume and starting Brix, but basically the idea is to always make sure you’re never overwhelming your fermentation with sugar levels that are too high.
In your case, since you’ve got only two gallons (7.6 L) and you’d start with a yeast hydration solution of probably ¼ cup volume or so, I’d dilute maybe 25 mL of your juice/must with 50 mL of water, then add that to the hydrated yeast. Wait for good bubbling and activity to show (should be quick, probably about 30 minutes or so). Then proceed to add 50 mL of your juice/must diluted with 50 mL water to that, make sure you’re seeing good activity again (maybe around an hour or so). Then you should be able to feed direct small volumes of your juice/must into the solution, monitoring activity by Brix drop.
Yes, this can be a laborious process and I’ve had sleepless nights doing it in the cellar. Eventually the fermentation will slow down and stick . . . and this is the ending point where you have to add SO2, chill it, settle, and eventually filter it before bottling. The hope is that by this point the VA won’t be too high and you’ll end up with something drinkable.
Which option to choose? I leave it up to you. Super-sweet juices and musts are always difficult to deal with and yours seems like no exception!