I'm ashamed to say that until August I had only ever tasted industrial Balsamic and had no real understanding of how the real thing is made. In fact, I'm one of the scorned Balsamic salad dressing mob, though how the young Italian woman I know whose salad only ever sees salt and Ponti Balsamic would be viewed by the food police I dread to think. So I thought I would write this as a guide for those as ignorant as me.
Anyway, we took a daytrip to Modena and it is a delightful little town centre in itself. The very helpful tourist office fixed us a visit to an Acetaia, gave us a map and off we went. We went to Acetaia Caselli, just outside town in San Vito di Spilamberti. Simone,the current gaffer, speaks good English, certainly more than adequate to convey his passion for this peculiar product, and the technicalities.
There are two kinds of Balsamic. Normal for most is 'industrial' production, using white wine vinegar, with cooked grape must, caramel and other darkly hinted at additives are used to make a thin and cheap and widely available product. We shall speak no more of it.
Production
Balsamico di Modena Tradizionale DOP is the stuff. Only must is used from the beginning, with no addition of wine vinegar at any stage. The production at every stage reminded me of the old definition of yachting; standing in a cold shower tearing up fifty pound notes. The favourite grape is Trebbiano - I never thought I would write that - with the three Lambrusco grapes also available, and all must come from a delimited area around Modena.
The harvest takes place towards the end of September/beginning October to get super ripe grapes. Manual harvesting is used and pressing used to be by foot, but now production is up, a machine which simulates this is used. This has been carefully designed not to crush pips and release bitter oils. Sound familiar? After pressing, the must is cooked down to about half its volume by boiling in a kettle for fifteen to eighteen hours. Caselli used to use a copper vessel stirred and supervised by the family's matriarch and many people still do, firing it with wood. Here, gas-fired stainless steel is now used.
When cooled, the must is put into botti where alcoholic fermentation starts and at the same time acetobacter start their action. Ferment slows and then stops for winter, resuming in spring. When it is finished, the liquid, still called must at this stage, is put into barrels in the attic for a year. These attics are dry and ventilated to promote evaporation, so the vinegar is constantly concentrating and evaporating. After its first year, vinegar enters one or other of the 'lines' - mini solera systems. A private householder probably has but one line, maybe two; enough for family use and perhaps gifts. A commercial artisanal producer like Caselli has twenty lines or more. I didn't count them. Each line consists of one 'mother' barrel of forty or fifty litres and eight baby barrels of ten or twenty litres capacity. 
Every year or so, but not necessarily so often, about three litres is taken from barrel #1 in the line of small casks. This is replaced from #2, which is renewed with vinegar from #3 and so on. #8 is filled from the mother barrel, which is filled in turn from a younger line, or replenished with one year old vinegar from the previous year. Still with me? Of course the barrels are not topped up to promote evaporation. The vinegars are all subject to seasonal heating and cooling.
It struck me, I should get out more, that three different fortified wine techniques are found in Balsamic production:
Before bottling, each vinegar is sent to the consorzio for assessment, both organoleptic and analysis.
Barrels
This is a wood-driven product, so a word about casks. Sizes are mentioned above. Obviously they're much smaller than we're used to in wine terms and the woods used vary a bit too. The botti for fermentation and the forty/fifty litre mother barrels are usually oak, but the babies are made from timbers including chestnut, juniper, cherry and mulberry. New barrels undergo a seasoning process for one year. First they are washed with hot brine, then scalded with hot, high acid vinegar. This is discarded and replaced with cold vinegar, which rests in the barrel for the rest of the year and removes the remaining impurities and flavours from the wood.
Barrels for maturation are never filled more than three-quarters full to promote evaporation, and are not sealed, but covered with muslin and a large, flat cork for the same reason. The hardest woods are used for the smaller barrels, softest for the larger. On a scale from hardest to softest, the progression goes Mulberry - Oak - Chestnut - Juniper - Cherry. Use of the rarest woods often involves mixing in a barrel, for instance you might get a cask with Mulberry staves and Chestnut ends. Barrels last more or less for ever, if they start to leak, a cooper will replace only the faulty stave or end, then the vinegar is replaced into the repaired barrel. "The vinegar is like an old man, you cannot take him from the home he has spent his life in and dump him in a new apartment; he will die!" said Simone.
It is not unusual to find barrels from the mid 19th century, but they need to be looked after and do not tolerate high humidity.
The final barrel the Balsamic spends its life in is the most influential - think Glenmorangie wood finishes.Each family has its own cask sequences and these are its secrets. Finishing with Juniper gives more spiciness, whilst finishing with Cherry wood gives more sweetness. Barrel shape is conventionally a slightly ovalised 'standard' shape but Caselli and some others are playing with some vertical, tapered shaped ones.
Tasting
There are four products in the Caselli range.
Primary, thick, raisiny, sweet, toffee, than at the back of the palate the acidity kicks in.
Big difference in weight and complexity. Deeper flavours, much more to taste and again, that intriguing balance between sweetness, gloopiness and acidity.
More restrained on the nose, a bit muted, rich and full on the palate, very complex with a slight metallic edge which is apparently a sought after element in the 25 year olds.
Raisiny on the nose, raising the stakes in terms of weight too. This has evolved dried fruit flavours, mature and the longest finish of all.
Finally we tried the 25 yo drizzled onto young Parmigiano, the classic old combo.
A fascinating visit and left me a little bit less ignorant about things foodie.
"Life's too short to drink bad wine"
This site looks best in Firefox