The fermentation of fruit and grain into vinegar is one of the oldest transformations humans have deliberately managed. A broken jar from a Sumerian storehouse, dated to around 3000 BCE, still carried residues of acetic acid, the compound that gives vinegar its sharp taste. From the Mediterranean to East Asia, almost every agricultural society developed its own vinegars, and most of them also developed elaborate customs governing when and how the liquid should be used.

The chemistry behind the transformation was not understood until the work of Louis Pasteur in the mid-nineteenth century. Before then, vinegar was thought to appear spontaneously in weak wine that had been exposed to air. Pasteur demonstrated that the acid was in fact the product of a specific group of bacteria, which he named Acetobacter. These bacteria oxidise the ethanol in wine or cider, converting it to acetic acid and water. The reaction requires oxygen, and this explains why traditional vinegar makers kept their barrels only partly full and turned on their sides, so that a large surface was always exposed to the air. Pasteur's work suggested a method of quality control: by inoculating fresh wine with a cultured strain of Acetobacter, a producer could shorten the fermentation from months to a few weeks.

Not every vinegar followed this route, however. In the Shanxi province of northern China, a distinctive dark vinegar is still made from sorghum and other grains, using a process in which a cake of wild moulds and yeasts is allowed to initiate the fermentation before the Acetobacter become active. The finished liquid is then aged for at least three years in open jars arranged in the sun and shade of a courtyard. Evaporation concentrates the flavour and deepens the colour to near black. Chinese writers of the Ming dynasty praised such vinegars as medicinal, and modern analysis has shown that they are rich in amino acids produced by the moulds.

The best-known aged vinegar of Europe is the balsamic vinegar of Modena, a product whose history has been the subject of considerable romantic invention. Contrary to popular belief, the syrupy liquid sold in supermarkets as balsamic is rarely the same as the traditional product; most commercial balsamics are ordinary wine vinegars darkened and sweetened with caramel. The authentic preparation, known formally as balsamico tradizionale, is made from the cooked and concentrated juice of Trebbiano grapes, which is transferred through a series of barrels made from different woods over a period of at least twelve years. The Italian food historian Francesca Bellini has described the batteria, or set of barrels, as a family heirloom, sometimes passed between generations along with house and land.

The role of wood in vinegar production has attracted growing scientific interest. Small amounts of vanillin, tannins, and other compounds leach from the barrel into the liquid, and each wood contributes a distinct profile. Researchers at the University of Reggio Emilia analysed a century-old batteria and identified more than two hundred volatile compounds, some of which had not previously been reported in any food. The same study noted that the microorganisms living in old barrels differ from those in new ones; over time, a stable community establishes itself and, in the view of the researchers, contributes characteristic flavours that cannot be reproduced in new wood.

Vinegar has long been valued for more than its taste. Because the acetic acid it contains makes an environment hostile to many spoilage organisms, vinegar is among the oldest preservatives. Roman soldiers carried a weak vinegar-and-water mixture called posca, which was cheap, easy to transport, and much safer to drink than untreated water. In medieval Europe, fish and vegetables preserved in vinegar remained edible for many months. The Japanese application of vinegar to cooked rice, which eventually gave rise to modern sushi, began as a preservation method for raw fish; the rice was originally discarded, and only later did it become a valued part of the dish.

The nutritional properties of vinegar are also the subject of active research, though the claims made by popular writers are not always supported. Controlled studies have shown that consumption of a small amount of vinegar with a meal can slow the rise of blood sugar in some individuals, which may be useful in managing type 2 diabetes. Claims of weight loss, cardiovascular benefit, or detoxification remain unproven. Bellini has warned that the romantic associations of vinegar make it unusually vulnerable to misleading marketing, and that the most expensive bottles are not necessarily the most beneficial to health.

Across its history, vinegar has played the roles of preservative, medicine, seasoning, and cultural symbol. In each of these roles it has depended on the same simple biology: a colony of bacteria slowly transforming alcohol into acid in the presence of air. The variation among the world's vinegars reflects the variation in the starting materials, the containers, and the patience of their makers.