Measuring Brix

My understanding is that only sugar affects the refractive index.

I suppose that makes it theoretically possible to measure the TA indirectly - sort of.

Tom S

Tom, this is a related question: How do you get an accurate hydrometer reading on freshly crushed must? There are obviously lots of suspended solids which affect the reading. Do you run it through a strainer? Even this results in more than just thin juice. Or do you just take into account the fact that there must be a correction factor? I've heard of people just subtracting 1-2 Brix units.

Lee

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Surely it's so small it's insignificant though. The bulk density of tartaric acid is about 1000 g/l so even a TA of 10 g/l would give an increase in volume that's insignificant: adding 10 g of acid to 1 litre gives a volume increase of (a bit less than) 10 ml. That's only a 1% change in volume which surely can't affect the SG much.

Ben

I suspect that any dissolved substance in a liquid which changes that liquid's density will affect both a refractometer and a hydometer, as they both measure the total content of a liquid.

If not so, the widly published conversion tables for Brix and SG would surely have a disclaimer noting any proven variences and the causes.

Both insruments require a like temperature adjustment as temperature directly affects liquid density. I see no reference to "other disolved componenets" adjustments or comensation in either insturment's operational instructions.

I do know that a hydrometer reading is directly affected by the "cleanliness" of the measured liquid. That's why measuring filtered liquid is recommended. I would think the same applies to a refractometer as well, athough my refractometer's instructions make no mention of filtering.

I don't think I plan on standing out in the vineyard with a lab filtering setup when I'm spot checking the grapes with the refractometer.

Perhaps there is a professional chemist amoung us with a learned opinion?

Fred

Well, I don't know about learned, but I am a chemist. You are correct that both refraction and density (what is measured with a hydrometer) is affected by total dissolved substances. How much impurities are a factor can, of course, vary a lot depending on what they are and how much there is. My guess is that the hydrometer scales for potential alcohol have been developed to be reasonably accurate with respect to grape musts and any errors due to average acid levels would be included in that empirically developed scale. I wonder if other fruit musts would have more error? I suspect so, but perhaps it is not of consequence. I have no idea really how accurate the refraction measurements are with respect to sugar and other dissolved substances. I am happy if my measurements let me know the alcohol level withing +/- 1%. So, I usually don't sweat it. I would add also that I recently measured the SG of a pear must with tons of fine pulp in it and compared it to that which had been filtered. The SG measurements agreed within .002. Close enough for me! Of course, undissolved solids are another matter!

I do that first, and then let it settle. The heavy gunk falls out of suspension pretty quickly.

Tom S

Hi, Ben -

I think there's something wrong with your reasoning. The bulk density of tartaric acid and sugar are pretty close to each other, so when dissolved in water will affect the specific gravity similarly.

Tom S

Fair enough and I do grant that it's something to consider. But we're still talking < 10 g/l. Unless you are really going for accuracy (in which case I doubt you'd really be using just a hydrometer) it's doesn't have a significant impact on SG readings for PA/final abv calculations.

Ben

What is an "imperfectly prepared juice sample" (or more important, how does one properly prepare a sample)? [Being serious, not facetious.]

A hydrometer measures specific gravity at a specific temperature. It does a reasonably accurate job of this. I've seen over 12 formulas for converting this measurement to Brix (and over 28 for converting SG and Brix to potential alcohol), all yielding vastly different results. Not all of them take into account suspended solids and other things that may be in solution besides sugar.

I'm not sure percisely what the refractometer measures, but suspect it to be considerably more accurate (for computing Brix and PA) than the hydrometer.

Still, are we planning a surgical missile strike, or making wine? I use a hydrometer, and the formula which makes the most sense to me. A refractometer costs more than 300 pounds of grapes, and doesn't taste as good. Besides, doesn't one have to recalibrate a refractometer (with yet another chemical test kit, with all the possibility of error) periodically?

Well, what I meant by an imperfectly prepared sample is basically a sample with a fair amount of gunk floating in it. In order to properly prepare a sample I would cold settle some juice in the 'fridge to minimize that. BUT, as you mentioned we're not planning a surgical missile strike (though I've made a wine that's bombed a couple of times) so I usually don't bother. It's really an intellectual curiosity that leads me to wonder why the two instruments read differently. The refractometer is measuring the change in index of refraction, and the only calibration adjustment most of them have is a screw to set the reading to zero for pure water. There's no way to calibrate the slope, only the offset as far as I know. I suspect the refractometer may be more accurate than the typical inexpensive winemaking hydrometer pre-fermentation but as others have mentioned, once there is a significant amount of alcohol the refractometer no longer measures sugar accurately and you have to rely on the hydrometer.

Regards,

John

Hi Paul and All,

I was very active in home wine competitions for many years. I went thru my files and recorded harvest pH, TA and Brix values for 76 red wines. All of these wines were fermented between 1983 and 1999 and were made from San Diego, Riverside or San Bernardino County grapes. All 76 wines had received at least one award from a large competition, and many of the wines were entered in more than one competition. These 76 wines had produced 5 - BOS,

29 - gold, 47 - silver and 41 bronze awards.

The average harvest values of pH, TA and Brix calculated for these 76 red wines are 3.86, 0.57 and 24.6. Using the pH * pH * Brix criteria yields an average value of (3.86*3.86*24.6) 366.5 for these 76 wines. Cox's value of

260 may be OK for cold climate grapes. But, I am sure 260 is not an applicable value for warm climate grapes.

I am convinced taste is by far the most important harvest parameter. However, "when to harvest" is such an important decision, why not use all the data and information available? I always cross my fingers too.

lum

I agree that taste is far most important in harvest parameter. I am working with a consultant wine maker / vineyard manager and he uses tastes almost entirely. However, my taste is not as calibrated as his.

A question - did you add any acid to the high pH and relatively high Brix wines you refer to and how were the wines after a couple years in bottle? Not trying to be picky - just curious. I have made some wine from California grapes and did not do any acid adjustments and they now taste sorta like cough medicine rather than wine.