By Harold McGee
Traditional balsamic vinegar begins with wine grapes: white Trebbiano, red Lambrusco, and a number of other varieties are used. Their juice is boiled until the volume is reduced by about a third. Boiling removes enough water to concentrate the juice to around 40% dissolved sugars and acids, and begins the sequence of browning reactions between sugars and proteins that generate both rich flavor and color. The juice is then placed in the first of a sequence of progressively smaller barrels, often made from a variety of woods (oak, chestnut, cherry, juniper), which are kept in an attic or other location where they’re exposed to the variations and extremes of the local climate. In summer heat, the concentrated sugars and amino acids react with each other to produce aroma molecules more commonly found in roasted and browned foods, and the fermentation products and by-products react with each other to form a heady mixture. As evaporation continues to remove water and concentrate the must (about 10% of the barrel disappears each year), each barrel is replenished with must from the next younger barrel. Finished vinegar, whose average age must be a minimum of 12 years, is removed from the oldest barrel. According to one estimate, it takes about 70 lb/36 kg of grapes to make 1 cup/250 ml of traditional balsamic vinegar.