How can potential alcohol go negative?

[Stephen Merriman]

Hello,

I am rather new to wine making and am confused about hydrometer readings. I took a potential alcohol reading before pitching the yeast and it read 11%, which to me means the sugars will yield 11%ABV when fully fermented. After fermentation my hydrometer reads -1% PA, and according to sources online this means my wine is actually 12%ABV (11– -1=12). How is it that I can end up with 12%ABV it the fermentabke sugars at the start are only 11% PA?

 

[bernardsmith]

Hi Stephen Merriman - and welcome. The best way to calculate your ABV (alcohol by volume) is to take your starting gravity, subtract the final gravity and multiply that number by 131.25 (I find it far simpler to multiply the number by 131 because that is "close enough) - So =, if your starting gravity was FOR EXAMPLE - 1.090 and your finishing gravity was 1.000 then you multiply 090 by 131 = 11.79% (Let's call that 12%, an ABV of X.79 sounds silly).
That said, the fixed marks showing "potential ABV" cannot know what your final gravity reading will be (assuming that the fermentation does not stall). It could stop at 1.000 or .998, or .996 or even .994 , so "potential ABV" is itself very approximate - ball park, if you like - and 11% and 12% is for all intents and purposes the same thing - The only time there might be a critical difference is if you are making wine commercially, and you are taxed differently for wines at 12% and above compared to those below 12% - but then you would be required to use a far more accurate method of measuring the amount of alcohol in your wines than using a $10 hydrometer.

 

[S-Met]

And remember that temperature of the sample will alter the reading. Most hydrometers say a calibration temp on them. There's also formulas to adjust your reading if you dont want to adjust your temps.

 

[Mallerstang]

Your hydrometer probably came with a scale, like mine did, translating SG at 20 C to potential alcohol %.

The translation of a single reading to potential alcohol is a bit misleading though - because your end percentage depends on the difference between your start and end SG readings.

Specific gravity just tells you how the current density of your liquid compares to the density of water at 20 degrees C, with water as 1.000. Sugar in solution makes it denser, and alcohol as a component makes it less dense - pure alcohol has SG of less than 0.800.

At the start, you have water plus loads of sugar and no alcohol, so the SG reading is well above 1.000.

At the end if all the sugar has fermented out, you have water plus no sugar and some amount of alcohol, so the SG reading is less than 1.000.

The difference in start and end readings tells you the approximate % alcohol at the end.

 

[bernardsmith]

But my guess that calibrating for temperature is not Stephen's problem. My guess is that the "potential ABV" uses 1.000 as the low point, but beers usually cannot get much below about 1.010-015 whereas wines and mead can often strike .996 without blinking so that "potential ABV" scale is just to give you an idea of the likely alcohol content assuming you already know what to expect... It's all about ball park and not about any number accurate to +/- 1 or 2%.
Does anyone actually ever use that scale? I know I don't because when I make a wine or a mead I know what my "potential" ABV is - That is what I aim for with any recipe I make and so I work to make the starting gravity I need to hit the ABV I want. No surprises.

 

[DoctorCAD]

'Potential" does not mean "absolute". Yeast cannot read a hydrometer, they just work until they cannot any more.