2014 batch 1: Cyser (3) Possible nutrient deficiency

My fermentation did not exactly finalize as I had hoped/expected. The rate of bubbling in the airlock just kept slowing down over a much longer period than I anticipated.

I tried to make plot of the fermentation events. I assumed that the amount of CO2 produced is proportional to the conversion going on and also proportional to the rate of bubbles from the airlock. Short time intervals corresponds to fast fermentation, so in inverted the time between bubbles and put it in % of the fastest bubble rate. For gravity I expected a drop of 0.107, so I calculated the gravity drop in percent of the expected maximum loss in gravity.

Then the plot look like this:

It is clear that after 5 days the fermentation slows day quite dramatically but it does not come to a halt immediately. Interestingly only half of the "fermentable gravity" has been used at this point. I assume that some sort of deficiency sets in - either free ammonium or potassium. The apple pulp was gone and only a clear precipitate of yeast was visible through the fermentation bucket. The liquid is about as clear as milk, so I still assume that there are plenty of amino acids available.

The mead was moved to a new fermentation bucket after 14 days to remove the bottom layer of yeast and this was repeated one week after when even more yeast had precipitated out. 

In regards to gravity I started out with an OG of 1.116 and when the fermentation stopped the gravity was 1.018, ie. a drop of 0.098. This corresponds to an ABV of about 13% using the quick formula: 

(OG-FG)/0.0075.

The fermentation started with a pH of 3.21 of the must. Now after the fermentation the pH is 3.60. This is quite good since the pH of the final mead should ideally be around 3.8. So almost there.....