fermenting stuck

Because when you have a wine with sg of 1.000, you're talking about a final gravity reading, after fermentation. The PA table, on the other hand talks about the starting gravity of the must. The PA tables are usually calibrated for a water-sugar solution and 1.000 is the sg point of water, so "must" of 1.000 by definition contains no sugar so it has PA of 0%.

Pp

My understanding is the SG 1.000 is calibrated with distilled water at

Zero PA (0PA) is a stable reference point, though it doesn't reflect the true zero PA density of grape must. About 5-10 percent of the total soluble solids in the grape must are from non-fermentable sugars, organic acids, organic acid salts, nitrogen-containing compounds, tannins, pectins and mineral salts.

These other soluble solids are responsible for a lot of the uncertainty in PA calculation from must SG. Alcohol evaporation during fermentation is the other significant variable making PA an inexact predictor of final alcohol content.

I found the following reference about grape maturity useful for getting a good picture of the grape growing practices and analyses on winemaking results.

One topic they discuss is potential alcohol.

Gene

jim wrote:

The scales on a hydrometer are like a snapshot. A picture of a moment frozen in time. That picture is of a prepitch must with no alcohol in it. Once alcohol enters the picture, the claibration on the hydrometer is no longer valid. HTH

Frederick

Now that makes it all coherent for me, thanks Frederick!

Are you sure about that, Frederick?

My understanding is that alcohol doesn't affect the Specific Gravity accuracy, but it does affect the correlation to "potential alcohol" and "degrees Brix". The more alcohol, the bigger the skew error.

I'm aware of only three significant factors that cause errors in specific gravity measurement of wine with a properly calibrated hydrometer.

First, bubbles in the solution cause buoyancy errors (that's why we spin the hydrometer, to knock bubbles off of it). Second, suspended solids (floating grape skins, etc.) which, if touching the hydrometer, cause buoyancy errors. Third is temperature of the juice, for which we have SG correction tables.

Am I missing something here?

Gene

Yup - that's what I meant. Jim's question was about the relationship between SG and PA and alcohol definitely "skews" that up. ;o)

Frederick

Apples and pears contain sorbitol which give them nonfermentable gravity. The effect you describe does occasionally occur with some varietys of pears. Apples the effect is always present but is ususually quite low. Stuck ferments on apple is sometimes due to zinc deficiency Years ago I used to add twenty grams of zinc sulphate and five grams of thiamine to twenty five thousand litres of ferment this was enough in most cases to unstick a stuck ferment.

Bob M

Hi. It's been a while since I participated in this discussion - the old adage of busy life being my crutch. Thanks to the theads I did make a starter to try and re-start this batch. I was successful and this weekend I have started working on getting the batch clear and ready for bottling. It's been a fun experiment though not quite the taste I was hoping for (at least the test glass I couldn't resist trying the other night), I'm sure this will improve with age.

While I haven't taken the time to write in, it's been great fun to follow the threads here and it's done a lot to keep my interest piqued for wine-making adventures.

Cheers and merry Christmas.

Chris Charlottetown, PEI, Canada