Category Archives: measurement & testing

How Far Can You Trust Your Hydrometer?

Hydrometers measure soluble solids density, and we use this to closely approximate soluble solids content of fruit juice. Because almost all the soluble solids in wine grapes are sugar, we use hydrometers to determine sugar content. I’m not telling you anything you don’t already know, every book on wine making I have says something like this, but did you maybe tuck the “in wine grapes” qualifier into the back of your head and just remember the “… almost all the soluble solids are sugar” part? I know I did, and it was while researching raspberries that I discovered just how much hydrometers can overstate sugar content.

Great data on raspberries upends an old rule of thumb

I was checking university extension offices, googling, and looking anywhere I had found information on fruit composition before when I came across this great paper on raspberries. It’s got excellent data on sugar and acid content, and I highly recommend it if you’re interested in making raspberry wine. I want to zero in on the Brix and Total Sugar measurements:

Brix vs Sugar Content in Raspberries
Varietal Brix Total Sugar Sugar Brix Ratio
Meeker site 1 11 2.83 25.7
Meeker site 2 8.6 1.01 11.7
Meeker site 3 9.2 2.38 25.9
Meeker site 4 9.8 3.28 33.5
Chilliwack 9.6 3.84 40.0
Tulameen 9.5 3.09 32.5
Willamette 8.7 2.31 26.6
Yellow Meeker 10.8 4.60 42.6

Average 9.7 2.92 29.8

Total Sugar is reported as grams per 100 grams, and if the soluble solids were 100% sugar, then Brix would equal Total Sugar. The last column, Sugar Brix Ratio, is my own calculation and expresses sugar as a percentage of soluble solids. I also reported average values in the last row.

We go from soluble solids being “almost all” sugar in wine grapes to less than 30% (on average) for raspberries! I asked Michael Qian, one of the authors, about this. He said that the data were good but some fruit, like raspberries and blackberries, just have a lot of pectin and other non-sugar soluble solids. I really appreciate him taking the time to help me out; I’m sure he’s a busy guy and this was a pretty basic question from someone outside his target audience (so, if you’re reading this – thank you!).

We know more than before – it just doesn’t feel like it

It’s exciting to learn something new, even if it does make things more complicated. When can we rely on our hydrometers? Wine grapes are probably a safe bet. Raspberries and blackberries are not. We need more information about other fruits, and I’ll be looking into that. If you know something about sugar content and soluble solids of other fruits, please say so in the comments.

Alright, what do we do when we know our hydrometers will read high? I don’t have a good solution yet. For raspberries, we might just adjust the reported Brix by 30% – still not accurate, but closer than the hydrometer reading. If you have an idea, I’d love to hear about it.

Jack Keller once said that a hydrometer is like a compass. Maybe it’s like a magnetic compass. It works well enough much of the time, but the question is, how do we find true north when we really need to?

Titratable Acidity: Trouble with the better way?

I’ve mentioned that I’m getting inconsistent results with my CO2 acid testing apparatus, and I’ve been trying to figure out why. I noticed that the measured volume of CO2 increased in the first few minutes. That’s an important problem because the test works by using that figure to determine how much acid was in the wine or must. Ideally, the final volume would increase almost immediately by the volume of the sample plus the volume of liberated CO2 then remain constant. Not only does the volume rise in the first five minutes, but then it falls over the next thirty minutes.

Gathering data

I had some ideas about this, but wanted to get a clear picture of exactly how the final volume changed over time. I’ve been recording six final volume measurements at the same time intervals over the past two months. For example, on March 2, I tested my Merlot. The initial volume was 0.2 ml, and right after adding the sample I measured the final volume as 2.4 ml. At +5 minutes, +10 minutes, +15 minutes, +20 minutes, and +30 minutes I measured:

2.9 ml, 2.9 ml, 2.8 ml, 2.8 ml, and 2.7 ml

Since the initial volume is arbitrary, it’s more helpful to view the data as a difference from the initial volume or as a percentage of it. Here are the same data expressed as a percentage of the initial volume:

120.83%, 120.83%, 116.67%, 116.67%, and 112.50%

I ran this same test nine more times on different wines and musts. Here are the results expressed as a percentage.

Normalized Volume Measurments
Immediate +5 Minutes +10 Minutes +15 Minutes +20 Minutes +30 Minutes
100.00% 120.83% 120.83% 116.67% 116.67% 112.50%
100.00% 98.18% 96.36% 94.55% 92.73% 89.09%
100.00% 116.67% 116.67% 112.50% 112.50% 108.33%
100.00% 103.23% 98.92% 96.77% 93.55% 92.47%
100.00% 104.55% 97.73% 95.45% 90.91% 90.91%
100.00% 95.24% 90.48% 88.10% 85.71% 83.33%
100.00% 110.00% 106.67% 103.33% 100.00% 96.67%
100.00% 107.69% 104.62% 104.62% 104.62% 103.85%
100.00% 121.88% 125.00% 128.13% 128.13% 128.13%
100.00% 96.81% 91.49% 89.36% 88.30% 87.23%

On six occasions, the final volume rose (indicating more acid) five minutes after the initial measurement then fell over the next thirty minutes. Three times (the red text in the table), the final volume steadily fell after the initial reading. And during one test (indicated in gree), the final volume continued rising for fifteen minutes, then held steady. What’s going on? It doesn’t surprise me that the apparatus can’t hold pressure perfectly, so the gradual fall that I see in almost all the cases makes sense.

Am I just being impatient?

The rise in the first five minutes that I saw in most of the tests made me think that the chemical reaction between the acid and the baking soda was slower than I thought. That would explain six of the tests and sort-of explain that (green) one that rose for fifteen minutes and plateaued. But the three (red) tests that showed a steady decline just don’t fit.

Is it something in the water?

I drain the device after each use, and fill it with water just before a test. Most of the time I get water from my kitchen faucet, but sometimes I use a utility sink in the basement. Water is water, of course, but the kitchen faucet has an aerator on it and the utility sink does not. Could the aerated water from the kitchen be releasing air during the test and affecting the final volume? Might there have been some air bubbles in the tubing that I overlooked? That could explain all ten test results. Being especially quick with the test and using kitchen water (or not doing enough to dislodge all the air bubbles in the tubing) might have caused the fifteen minute rise in the green test. The six times that I saw a five minute rise might have been me going at a more normal pace, allowing some air to bubble out while I got ready for the test. Water from the utility sink would not have cause a rise at all, and that would dovetail nicely with the red tests.

Putting it to the test

I didn’t think that my water source would matter, so I didn’t make a note of it when I recorded my data. So I’m going to run more tests and collect more data. This time, I will leave the water in the apparatus between tests. This ought to insure that no air bubbles out and affects my results. If I get the same sort of results, then I can rule out the air in the water. If, on the other hand, all my tests show a gradual decline after the initial reading, like the red tests, then I think I may have solved this mystery.

Titratable Acidity: A Better Way!

A better way to measure titratable acidity?

So what’s that contraption pictured above? I wrote about it back in February, and I think it’s a better way to measure titratable acidity (TA). It works by adding a measured sample of wine to sodium bicarbonate (baking soda). The baking soda reacts with acid in the sample, giving off carbon dioxide gas (CO2) in direct proportion to the amount of acid. This device measures that CO2, and you can use that to determine the TA of the sample.

Cherry Mead: The case of the disappearing acid

I checked the titratable acidity (TA) of my cherry mead the other day, and something didn’t add up. Over six months, three measurements, and two acid additions (totaling 2.6 g/L) the TA fell from 6 to 5.5 g/L.

The most obvious explanation is that I goofed up the titrations. As I added more acid, the TA should have risen, so if the first measurement was accurate, then the second was low by 2.1 g/L (should have read 7.3 instead of 5.2), and the third was low by 3.1 g/L (should have read 8.6 instead of 5.5). I did three titrations that day using the same procedure, with the same chemicals and with the same equipment. I got “good” results from the other two titrations, and by that I mean consistent with my predictions and with past measurements. So maybe this measurement was accurate and the previous two were off.

That would mean the first was too high by 3.1 g/L (should have read 2.9 instead of 6) and the second by 1 g/L (should have read 4.2 instead of 5.2). A 1 g/L error on the second measurement is possible, because I’m measuring the sample and the sodium hydroxide with a syringe that I think is accurate to 0.5 ml, and that would mean only one large anomaly. Everyone makes mistakes, and maybe that just wasn’t my day.

Was the TA really that low? Well, I haven’t got a time machine handy so I can’t redo the test. My wildflower mead, from A Simple Mead Recipe fame, had an initial TA of 3.5 g/L, which isn’t much higher than 2.9, so that fits. Also, that was back when I had started doing titrations, so I might not have had the hang of it yet. I’m chalking this up to one bad measurement – the initial 6 g/L was really about 3.

Mystery solved!

Titratable Acidity: Mystery, Consistency, and too much acid

Cherry Mead: The case of the disappearing acid

Suppose you measure 6 g/L titratable acidity (TA), then add about 1.3 g/L of tartaric acid. After you let it sit for a while you’d expect a TA over 7, right? Me too. You certainly wouldn’t expect just a little over 5 (call it 5.2), would you? I didn’t either, but that’s what happened and that wasn’t the end of it. I’m talking about my cherry mead and after that 5.2 measurement, I added another 1.3 g/L of tartaric acid. When I checked again the TA stood at just over 5.5 g/L, not the 6.5 I was expecting. Over the course of six months, my starting TA fell from 6 g/L to 5.5 g/L as I added 2.6 g/L.

What happened? I don’t know, but a look at pH tells me that the additional acid was affecting the mead, even if I wasn’t detecting it in my titrations. While TA went from 6 to 5.2 to 5.5, the pH went from 3.56 to 3.39 to 3.13. I’m going to have to chew on this for a while. Got any theories? I’d love to hear them.

Honey Apple: Promising, but not ready yet

Compared with my cherry mead, the honey apple is a model of consistency. Yesterday’s measurements:

SG: 0.996, pH: 3.56, TA: 7 g/L

were exactly the same as on 11/15/07. This is reassuring and gives me a (false?) sense of precision. It’s not ready to drink yet; tasting it all I could think of was “tart and young.” The Lady of the House would only say that, yes, it was an apple wine or mead but refused to offer anything more. It’s clear with compact sediment, and the numbers look good, so I think I’ll rack without making any adjustments.

Tomato Wine: Young, tart, and bone dry

It tastes just as harsh as you’d expect it to from these numbers:

SG: 0.990, pH: 2.97, TA: 9- g/L

In addition to being tart, there is an unusual flavor that I wouldn’t recognize if I didn’t know I was drinking tomato wine. I’m not sure whether I like this tomato flavor or not – its hard to get past the harshness of this wine. The Lady of the House knew it was the tomato wine, even though I didn’t tell her. She made a face and said it was young and that there was “an acid thing” going on. This one needs some more time, and I need to neutralize some of the acid.

So, I’ve got a mystery to solve, some acid to neutralize, and some mead to rack. Time to hit the “save” button.

Acidity In Mead: Being rigorous with incomplete data

I talked about five of my meads yesterday, and how I might decide if they were ready to bottle. I looked at clarity and specific gravity (SG) because I didn’t want the mead throwing off sediment or fermenting in the bottle. I tasted, probably the most important test of any wine or mead, and I checked the acidity.

Mead’s peculiar chemistry makes it difficult to measure the titratable acidity (TA). I explain this in more detail here, but the short version is that the common tests, like titration, overstate the TA. That made me think that such tests had no value, but I’ve since changed my mind. Measured TA’s don’t give you a precise value, but they do give you some information. That’s why I reported TA values for all five meads yesterday.

Using titratable acidity values in making mead

What do those values tell us? I began to get an idea about that when I was thinking about my cherry mead. As I said back then, you can use the TA values as upper limits. If you want to make a mead with the acid profile of a white wine, for example, you look up the range of acid values common in white wine then aim for the high end of the range. A good book on winemaking basics will give you that information. I like Daniel Pambianchi’s Techniques in Home Winemaking, he covers the basics really well and has special sections on ice wine, port, and sparkling wine. He says that the TA in white wines will normally range from 5 – 7.5 g/L. Adjusting a mead to 7.5 g/L puts the actual TA somewhere below that. Tasting the adjusted mead, after about a month or so to allow any acid additions to integrate with the mead, will reveal if the acidity is too low. If it is, then a series of add (no more than 0.5 g/L) – wait – taste cycles will nudge it right into the sweet spot.

Trying to improve a good approach

This is a good approach, but I’d like to get better information on the lower limit of a mead’s TA. Since the measurement error stems from the gluconolactone that exists in equilibrium with gluconic acid and that equilibrium depends on temperature and pH, maybe careful measurement of TA, pH, and temperature would yield some information on how much gluconolactone got caught up in the measurement. We might be able to use that to get a lower limit on, or even pin down, the actual TA. I don’t know how to do that yet, but I’ll see if I can find out. Another way to tackle this problem is to find out how much gluconolactone typically exists in honey. Putting limits on gluconolactone concentration will allow us to put limits on the actual TA. Those are my ideas anyway, if anyone can shed some light on them, or has another idea, please leave a comment and let us all know.

Five Meads: Are we there yet?

I looked in on five meads yesterday to see if they were ready to bottle. I was looking for clarity, I tasted them to see if they were pleasant to drink, and I measured the specific gravity (SG), pH, and titratable acidity (TA).

Name SG pH TA (g/L)
2004 Plain Mead 1.001 3.05 5
2005 Apple Mead 0.995 3.39 5.2
2006 Experiment (boiled) 1.000 3.27 6
2006 Experiment (no heat) 1.000 3.29 5.3
2006 Grape Mead 1.000 3.51 5+

Ready or not, this four year old mead is going in a bottle

I tasted sweetness on the 2004 plain mead, despite the low SG. It had that distinctive, pleasant aroma that I’ve come to associate with mead, and the lady of the house thought it was, “a little young, but it’s going to be good.” I’m not sure I’m as patient as she is, so I’m going to bottle it.

This apple mead is the only one not ready to bottle

The 2005 apple mead tasted and smelled of apple, but only a hint. I thought it was a little tart. It was the only one of the lot that I thought wasn’t clear enough to bottle.

Trying to settle a long running debate

The 2006 experiment is a test of the idea that boiling a mead’s honey-water mixture before pitching the yeast impairs the aroma by driving off volatile compounds. I split a batch, boiled one and made the other without heating. That was two years ago, and I think these meads are ready to bottle. I normally age mead for three years though, so I may let them age in the bottle then have a tasting party next February.

Update 10/28/2008 – The results are in!
It was a long running experiment with a little surprise at the end. Follow this link to see the results of my mead boiling test.

The trouble with titration

The 2006 grape mead is made from the pomace of my smallest batch of wine ever. I added honey, water, nutrient, and cream of tartar. I had some trouble checking the TA on this one because I ran short of sodium hydroxide, the base I use to titrate acid in a wine sample. I added 5 ml to the sample, and that brought the pH to 7.4. That’s very close to the end point. If I really had reached the end point, it would have indicated a TA of 5 g/L. It’s a bit more, maybe 5.25 g/L, but since I can’t be sure I just noted “5+”

Hmm, that acid measuring contraption I wrote about the other day just looks better and better.

Titratable Acidity: A Better Way?

A man, his contraption, and a different way

I learned of a different way to test for titratable acidity, the other day. At the last meeting of the Puget Sound Amatuer Wine and Beer Makers club, Don Proctor demonstrated this method using an odd looking device. He used ordinary baking soda (sodium bicarbonate) to neutralize the acid in a test sample. The important thing about this chemical reaction is that it gives off carbon dioxide (CO2) in direct proportion to the amount of acid neutralized. Now his device didn’t look so odd. The stoppers, tubing, glass cylinders, and green liquid were used to measure the amount of CO2, and if you know how much wine was in your sample and how much CO2 was produced, you can find the acidity of your sample.

The difference is in what you measure

This method, and conventional titration, both aim to measure the amount of acid by neutralizing it with a base. In a titration, you add a carefully measured amount of base until all the acid is neutralized. It’s important that you add just enough base to neutralize all the acid – no more and no less – because you determine the amount of acid in the sample from the amount of base that you add. Because you have to measure the base so precisely, it’s best to add it in liquid form. That means you need to have a solution of base at a precise concentration. Now, this is easy to find, but it’s expensive and it has a short shelf life.

Why the new way is better

You need to neutralize all the acid in Mr Proctor’s method too, but you don’t need to know how much base it took to do that. That means you don’t need to determine the end point (no pH meter) and you can use cheap, shelf stable baking soda instead of expensive perishable sodium hydroxide. That’s a big plus, as I found out the last time I ran out of chemicals. I’m going to have to get one of these contraptions!

Update 9/8/2008: A picture is worth a thousand words

If you’re having trouble visualizing it, take a look at this photo.

Calibrating A pH Meter: Buffer Solution

What is a buffer solution?

You need to calibrate your pH meter for it to work properly, but to do that, you need to immerse it in a solution of known pH. Buffer solutions are the way out of that little chicken and egg problem. These are made of precisely measured ingredients that combine to form a solution of known, and highly stable, pH. It would be pretty tough for most home winemakers to make their own buffer solution, so it’s a good thing that they’re widely available at homebrew shops.

Not all buffers are created equal

I was out of pH 4 buffer, and bought some more from a local shop. The first difference I noticed, between the new and old solutions, was the color – the new one was pink and the old one was colorless. There were two other differences, though, that were more significant. Each solution came with a temperature table that indicated the precise pH, to two decimal places, at a given temperature. The old solution listed the pH from 0C to 95C in 5 degree increments, and over that wide range the pH varied from 4.00 to 4.22. The new solution listed the pH at 20C (pH = 4.02) and 30C (pH = 4.99). The detail and temperature stability of the old solution gave me a lot of confidence. The new one is … pink.

How to shop for buffer solution

My pH meter had been pretty steady, often needing no adjustment at all between uses. When it had drifted, it was only by 0.02 or 0.03. So I was taken aback to see my pH meter read 3.74 when I first put it in the new solution. Maybe the meter drifted by that much since the last time I used it, but maybe the pH of these two “pH 4” buffers differed by 0.26. I was out of the old solution, so I couldn’t check this. The new solution is fresher, so it’s possible that the old one drifted over time, but I’ve got my doubts about the new one. Live and learn. The next time I buy buffer solution, and that’s going to be pretty soon, I’ll try to find out when it was made and how much detail is in the temperature table.

Update 5/13/2012 – Hanna buffers are my favorite

I’ve been calibrating my pH meter for five years now, and I’ve bought various brands of buffer solution. I keep going back to Hanna. I’m not a chemist, but the detailed temperature corrections and the small drift give me a sense that theirs is a cut above. They’re readily available at good prices too, so for whatever it’s worth this is what I buy.

Calibrating A pH Meter

pH meter, in a champagne glass about a quarter full of 6.86 buffered solution, reads a pH of 6.17 at 31.4 Celsius.To work properly, a pH meter must be calibrated. You do this by preparing (or buying) buffered solutions of known pH and testing the meter against them. My meter uses a two point calibration. It works by immersing the meter in the first buffer solution (pH 6.86 in this case) then reading the pH and temperature values.

You turn a calibration screw until the meter shows the correct pH for the given temperature (the bottle of buffer solution has a table of temperature and pH values). The pH of the “6.86” buffer solution that I’m using is 6.85 at 30 Celsius, the closest temperature on the correction table to 31.4, so I turned the calibration screw to 6.85. That’s the first “point” of the two point calibration process. The second step is exactly the same but with a different buffer solution (ph 4).

I’ll use my new toy – um, important scientific instrument – along with my simple acid titration kit to analyze my oregano wine. Fermentation has been very slow and I’m afraid the pH of the wine has fallen so low that it’s inhibiting the yeast.