Unexpected experience fermenting SP

[BernardSmith]

I pitched slurry (stored in the fridge) from a variety of hard ciders I had been making throughout the winter on April 10 and the 3 gallons of SP is still actively fermenting this morning (May 6). The gravity has dropped to about .998 but a) the airlock is still bubbling away very vigorously*** and I can see particles and bubbles very clearly rising and b) I tasted the SP and it tastes pleasantly sweet - not dry but nicely sweet as if the SP should be reading around 1.004 or 5. In other words as if there are unfermentable sugars in this wine.
Apart from the slurry - which was about 1.5 pints - and which almost certainly did contain some apple particulates - the only other fruit was the lemon juice and the table sugar which I added to bring the starting gravity up to 1.085 or a tad higher.
Where is the sweetness coming from? And how come this fermentation seems to be as active as it does at this late stage?
Thoughts?

*** It is as if the colony of yeast is just as large and just as active towards the end of the fermentation as I would expect them to be if there was still about half the sugar remaining to ferment.

 

[sour_grapes]

I tasted the SP and it tastes pleasantly sweet - not dry but nicely sweet as if the SP should be reading around 1.004 or 5. In other words as if there are unfermentable sugars in this wine.

I may have inferred something that you did not mean to imply. However, let me note that unfermentable sugars will raise SG as surely as fermentable ones. Even more, actually, as these have high molecular weight.

I don't have any insight on the rest of your questions. Perhaps you are just a little "miscalibrated" on how sweet a 1.004 wine would be vis a vis a 0.998 one?

 

[GreginND]

What was your starting SG? If it was on the higher side, you certainly could have a significant amount of sugar left at 0.998. The fact that the gravity goes less than 1.000 is due to alcohol being less dense than water. So, the amount of alcohol you have present will skew the gravity given the same amount of sugar present. A wine with 2% sugar can have different SG readings depending on how much alcohol is there. The more alcohol, the less dense the liquid.

Bottom line - I suspect you still have a significant amount of sugar to ferment out. Let 'er finish.

 

[BernardSmith]

Thanks for these quick replies. The starting gravity of the SP was around 1.085 so not very high and the yeasts were a mix of QA 23 and Nottingham: I had been making several batches of hard cider and had saved the lees and washed them and stored them in the fridge for a few weeks and then decided that I wanted to try my hand at Skeeter Pee and thought that I could use the washed yeast. The fermentation took off like a rocket and it is still firing all engines.. although nominally there should be very little fermentable sugar remaining. If it is important - some of the hard ciders I made without any additional sugar (so the gravity was around 1.050) and some I had added brown sugar and the starting gravity was closer to 1.080. I had fed the yeast (when making the cider) with nutrient and had added nutrient to the must of the SP. For the first couple of weeks I had started the SP in the basement where the temperature is about 5 degrees cooler (about 60) and brought the carboy up into the kitchen about 10 days ago where the temperature is about 66.
I guess my surprise is that I have never seen such activity from the yeast so close to the end of the sugar content of the must.

The thought struck me that since I had been making cider might I have accidentally inoculated the lemon juice with malo-lactic bacteria and so have an MLF infection? (Does lemon juice contain significant quantities of malic acid? Could the conversion of malic to lactic acid account for the apparent but very perceptible sweetness? and might an MLF account for this activity? I don't have any way to check for an MLF.

 

[sour_grapes]

I kinda doubt the MLF theory. Just from googling around, evidently there is some malic acid in lemon, but not a lot. Also, MLB are unlikely to grow at pH below 3. I suspect your SP is down there in pH. It all seems unlikely to be MLF.

 

[Ctmaro]

It sounds like the yeast has died out before the sugars are converted

 

[BernardSmith]

It sounds like the yeast has died out before the sugars are converted

But Ctmaro, that would not explain the activity in both the carboy and the airlock. My question was about the surprising duration of the visible activity and not the opposite - a lack of activity. Seems to me that either the yeast is still very vigorous (with no apparent residual sugar) or some bacterial reaction is taking place that has so far not produced any off flavors or aromas but seems to be making the wine taste less acidic

 

[Ctmaro]

Yea you are right, the activity would have stopped.