This unique vinegar originated in the region around Modena, northern Italy centuries ago and has enjoyed international renown since the late twentieth century. Produced from trebbiano grapes, what sets this vinegar apart from other traditional wine vinegars is its unusual production method.
Firstly, it is not made from wine as such, but from an unfermented must (crushed grape juice). The must is then boiled down, not only concentrating the acids and sugars naturally present in the grape, but introducing caramel nuances that provide the warm, intense "grapey" taste to the vinegar.
The most fascinating part of the acidulating process lies with the aging. Once boiled down, the concentrated must is transferred to a wooden barrel, usually chestnut and then left to mature. Some time later (up to a few years), the vinegar will be transferred to another barrel, smaller this time, to account for evaporation and to introduce stronger wood flavours due to the increased surface area of the barrel itself. This process is repeated several times, moving through barrels made from oak, chestnut, juniper and mulberry, each barrel getting smaller as the aging process continues.
Most balsamic vinegar found in supermarkets is aged only a couple of years at most and although pleasing, provides little insight to the intense flavour of a well aged balsamic, which can be 40 years old or more (I have heard reference to some that is 100 years old). As always there is a catch. Seriously aged balsamic is seriously expensive. In Australia 50 ml (around 1/4 cup) of 30 year old balsamic fetches $AUD180.00. No small amount I'm sure you will agree. There is a trick though, that many chefs use, to approximate the intensity of flavour a true aged balsamic will provide. Grab a cheap bottle of balsamic and pour it into a saucepan. Boil down until it is around a tenth of the original volume and quite syrupy. The sugars will be very intense and the acids a little too strong, but you have a rough (albeit cheap) idea what the really good stuff tastes like.