Does topping off secondary mess up ABV calculations?

[kwiley]

If you top off your secondary after racking with fresh cider, leftover from raw pressing and therefore untreated with sorbate (pasteurized at best, nothing done to it that would affect fermentation), doesn't that invalidate any subsequent SG and ABV calculations? If you then take an SG reading, whether immediately after racking and topping off, or a week later or any subsequent point during secondary fermentation, can't you no longer make relative ABV calculations based on the drop in SG from the SG at the beginning of the primary fermentation?

But I'm not sure about this, since both effects (reducing alcohol and increasing sugar) go the same direction when you top off, so maybe one compensates for the other. Topping off dilutes the true alcohol level (by mixing in cider with no alcohol) while at the same time adding whatever sugar is in the cider you top off with (and therefore increasing SG due to additional sugar). So perhaps increasing the SG of the secondary has the effect that original-SG minus current-SG still gives the true ABV (this assumes you top off with the exact same cider you started fermentation with so that adding it definitely increases the SG, topping off with -- in the extreme case water, with no sugar) would actually pull the secondary's SG down, making it seem like it has additional alcohol when you still clearly diluted the alcohol by adding water to it).

As I'm thinking through this, maybe what you have to do is get the SG of the cider you are topping off with before topping off and combine that SG with the fermenting cider's various SGs in some way.

Thoughts?

 

[Qhrumphf]

If you weight the "original gravity" of your cider to include all the eventual top up water, the math should still work.

If you started with 4 gallons of 1.060 and by the end you've added one gallon of water, your adjusted "og" would be 1.048. Use that to compare to FG.

If you're toping up with juice, treat that the same way.

 

[jseyfert3]

As I'm thinking through this, maybe what you have to do is get the SG of the cider you are topping off with before topping off and combine that SG with the fermenting cider's various SGs in some way.

If you started with 4 gallons of 1.060 and by the end you've added one gallon of water, your adjusted "og" would be 1.048. Use that to compare to FG.

If you're toping up with juice, treat that the same way.

@kwiley in case you don't realize how to do the math they just did, I'll outline it here quickly. That's a total of 5 gallons. 4/5 gallons is 0.8 (80%), 1/5 gallons is 0.2. So it's 0.8*1.060 + 0.2*1.000 = 1.048.

If you instead had, say, 4 gallons of juice at 1.060, and you topped up with 1/2 gallon of juice at 1.040, then you can do: 4/4.5 = 0.889, 0.5/4.5 = 0.111, so we have 0.889*1.060 + 0.111*1.040 = 1.058. You'd then use 1.058 as your "IG" to calculate ABV with the FG.

Alternatively if you're fully stopping fermentation before adding any additional juice, you can use the IG and FG of the juice before diluting and do the calculation on alcohol dilution, but calculating the effective IG by weighing the SG's allows you do calculate ABV even if fermentation continues after adding new juice.

 

[kwiley]

Right,

If you weight the "original gravity" of your cider to include all the eventual top up water, the math should still work.

If you started with 4 gallons of 1.060 and by the end you've added one gallon of water, your adjusted "og" would be 1.048. Use that to compare to FG.

If you're toping up with juice, treat that the same way.

Right, but the only way to do this when topping up with juice or raw-pressed cider is to have the SG of that juice in hand -- before adding it. That way you cam factor it into the averaging out of the SGs as things mix. I guess that makes sense.

 

[Qhrumphf]

Well, you've got the capacity to measure the gravity of your top up juice, do you not? Measure it. Done.

 

[kwiley]

Well, you've got the capacity to measure the gravity of your top up juice, do you not? Measure it. Done.

I was inquiring retroactively, having already racked to secondary and topped off quickly to close up the headspace. I'll know next time to take the proper measurements. Thanks.