Have I been miscalculating FSU?

[Tom Bugs]

I think I may have made an oops...

I made a spreadsheet where I recorded Original Gravity & date fermentation began.

I took the formula in Claude's book FSU = 100,000 (SG1 - SG2) / N to mean that SG1 was the original gravity and N was the total number of days elapsed..

Just noticed that, for example, my SV Dabinett has only dropped from 1.022 to 1.0215 between 23/3 & 27/4 - my FSU calc reads 25 because OG was 1.056 & now 137 days total have elapsed.

But if I calculate based on the very small drop over 37 days.. maybe the FSU is actually 1.35!

So maybe actually I have quite a sweet cider - not quite sure how I feel on this - it did taste good! The Yarlington barrel has had a similar very small drop but is nearer 1.01. While the keeved dabinett & a Dab/Yarl/Norman blend are both moving slowly at about 15-20 FSU (new cal - old calc was 18-28).

[Tom Bugs]

PS - natural fermentations (with 1/2 dose sulphite) from an old cider orchard, unfertalized for sure.

[Claude Jolicoeur]

Le mercredi 27 avril 2022 à 18:41:39 UTC-4, Tom Bugs a écrit :

Just noticed that, for example, my SV Dabinett has only dropped from 1.022 to 1.0215 between 23/3 & 27/4 - my FSU calc reads 25 because OG was 1.056 & now 137 days total have elapsed.

But if I calculate based on the very small drop over 37 days.. maybe the FSU is actually 1.35!

It is normal that the FSU decreases as fermentation goes.

What we see here is that your FSU has been 25 in average since the beginning of the fermentation. At the start it was probably at 100 or more, and as the fermentation was going, it decreased.

The average FSU calculated since the beginning of the fermentation is not a very useful figure. More useful is to monitor the FSU on a shorter period of time (for example every 2 or 3 weeks) and see how it decreases with time. This is what gives you a better understanding of how your fermentation is going.

For the last 37 days, we see the cider has been very stable with a drop of only half a point. In other words, your fermentation is stuck! This indicates you obviously had very little natural nutrients in your must at the start of the fermentation.

At this point, there are 2 things you can do:

1- bottle the cider and you'll have a medium sweet that will remain stable.

2- add a dosage of DAP (about 15 ppm) to restart the fermentation so the SG will go down further before bottling.

[Tom Bugs]

Thanks for confirming, Claude - my key misunderstanding being the average FSU rather than spot.

Because this is my first time trying to calculate FSU I was looking too much at that & hadn't quite noticed the comparison of SG between readings - I knew things were moving, but hadn't initially clicked that the Dab & Yarls had dropped so little the last month. And you know why they dropped so little? Because I racked them just after the last reading in March! So that's provided a good stabilisation racking invervention, but perhaps now I think on it, I could have left the Dab a bit later - the Yarl seems like a good level of residual sweetness, so I'm happy on that!

Looking at the Dabs data - it has always been moving at 25-40FSU throughout (no fast initial fermentation) until the stabilisation racking slowed it right down - yes, I may consider some DAP & will read up in your book about your approach. Maybe I'll look to keep half 'as is' and unstick the other half.
The Yarl batch was slightly faster, but again stabilised after the one racking.
The Dab/Yarl/Norman mix has moved at a similar rate to the Dabs, but hasn't slowed as much as the Dab batch (SV Normans fermented fairly quickly to dryness already) - it is now at 1.017 with a 6 point drop since racking, so I reckon I'll do another racking quite soon on it & see how that behaves.

The Keeved batch, I'm curious to see how it goes.. No racking so far - it started at 1.055 and has dropped 6-9 points each month or so - still up at 1.034 but still moving slowly....

Thanks Claude!