How Traditional Balsamic Vinegar Is Made: Cooked Must, Barrels and Time

A catalogue card in the heritage records of the Emilia-Romagna region describes nine barrels standing in the attic of Villa Fabriani, at Spilamberto, where the museum of traditional balsamic vinegar has its home. The card gives their capacities in litres: 10, 13, 19, 23, 31, 37, 46, 59 and 73. Each barrel has a numbered metal label, and the name of its wood is carved into it. Oak holds the two smallest places and the two largest; chestnut appears twice, and between them stand acacia, ash and cherry.

The Consorteria owns them. The vinegar that passes through a line like this is made in a few operations: a grape must is cooked, left to turn, and then moved from barrel to barrel, once a year, for a decade or more. Several of them rest on a decision that the specifications leave to the maker, and the first is taken in the vineyard: when to pick.

Before the barrels

The specifications for the two protected vinegars allow a long list of vines: Lambrusco in all its varieties, Trebbiano, Ancellotta, Sauvignon, Sgavetta, Berzemino and Occhio di Gatta, along with the vines of the local DOC wines. The museum’s guide names the ones in use: Trebbiano di Spagna, Trebbiano Modenese, Lambrusco and Ancellotta. What the texts do not fix is ripeness. According to the Modena consortium, when to pick, and in what proportions, is the producer’s call. The must has to reach at least 15 on the saccharometer, an instrument that reads the sugar dissolved in the juice.

Picking is normally by hand, the consortium adds, which lets the producer choose bunches and send the fresh must straight to the fire. The timing matters because, as an agronomy article of 2016 puts it, the cooking has to begin before fermentation does. The museum’s guide describes the evening pressing and the morning on which the must is drawn off. A soft pressing, the consortium says, gives a free-run must, sweet, fragrant and low in tannin, and the agronomy article notes that gentle crushing keeps the seeds whole, since broken seeds release raw tannins.

At the fire

The must goes into an open vessel over a direct flame. The cauldron was once copper over wood; the agronomy article says it is now more often stainless steel over gas. The Modena specification asks for atmospheric pressure, an open flame and at least 30 minutes at 80 °C or more. The museum’s guide puts the working temperature at around 90 to 95 degrees, and the Modena consortium says it falls from about 95 to 70 as evaporation thickens the must.

The time spent is far longer than the floor. The agronomy article gives 10 to 16 hours; Jancis Robinson was told 12 to 40 on her visit, until the must had been reduced by about half. The range says something in itself: the cook is working a material, not a timer.

Heat is read with a high-temperature thermometer or by watching how the surface moves, and impurities are skimmed off. The consortium says the cook judges the duration from the harvest and from what the barrels need. The agronomy article spells out the choice. A lightly reduced must leads to higher acidity, a moderately reduced one suits the yearly topping up, and a heavily reduced one is used to correct excess acidity in a set. The must ends with a third to a half of its volume gone.

When fermentation begins

The cooked must cools and settles, then goes into the barrels. A review of the microbiology by Lisa Solieri and Paolo Giudici, published in 2008, describes the production in three broad steps: yeasts convert the sugar to alcohol, acetic bacteria oxidise the alcohol to acetic acid, and the vinegar then ages for at least twelve years. The yeasts and bacteria are those of each loft; a 2023 review adds that no dried yeast is added.

Both specifications bar any substance they do not list. The Modena text lists one, mother vinegar, which seeds a new set with the bacteria of an older one.

The bacteria need air, so the bung holes are covered with gauze rather than sealed, and the plugs are turned in loosely. They also follow the weather. According to the agronomy article, they go dormant below about 10 °C, wake around 15 °C and slow again above 32 to 35 °C. The specifications require lofts with ventilation and exposure to natural changes of temperature.

A line of unequal barrels

A vinegar set, a batteria, is a line of barrels in descending size. At one end stands the largest, the head, which receives the year’s cooked must; at the other the smallest, the tail, from which vinegar is eventually drawn. In the museum’s set that makes the 73-litre barrel the head and the 10-litre barrel the tail. The museum’s guide puts the work of the line in three stages: fermentation and acetic oxidation in the largest barrels, maturing in the middle ones, ageing in the smallest. It gives five as the minimum number of barrels; accounts of family sets run from three to twelve.

The specifications say little about numbers. They require only barrels of differing sizes and woods, each numbered and marked. The Modena application names oak, chestnut, cherry, mulberry and juniper; the Reggio text adds ash and robinia. The museum’s card lists acacia, a name Italian usage also gives to robinia.

The guide offers a reason for the order: hard woods suit the small barrels because they limit evaporation, and soft woods suit the start of the line because they favour it. The Reggio consortium credits chestnut with tannins and a dark colour, and cherry with sweetness. The rule is not rigid. In the museum’s set, oak holds the two smallest places and the two largest, and the agronomy article says families use different sequences.

Once a year

The year’s work is a cascade. A little vinegar is drawn from the smallest barrel, which is refilled from the next one; that barrel is refilled from the one before it, and so on to the largest, which receives the cooked must of the year. Each barrel gives up just enough to refill the one below. Italian has two words for the operations: rincalzo, the topping up, and travaso, the transfer from barrel to barrel. The Modena consortium puts the work in winter. The agronomy article says after winter, while temperatures are still below 10 °C. The museum’s guide names no month.

In Robinson’s account, about a tenth of the smallest barrel is drawn off each year and about a tenth evaporates, so two litres are moved to replace each litre bottled and each litre lost. The work is not left to time alone. The museum’s guide says checks in the acetaia cover the microorganisms responsible for the change and how the sugar concentration and the acetic degree vary in the casks. It lists a densimeter, which reads density and so the sugar, along with copper buckets, funnels of copper or blown glass, and a glass pipette for samples. The museum’s laboratory checks sugar and acidity for each of its sets.

The barrels themselves are never emptied or cleaned, Robinson wrote. Simone Caselli, of Acetaia Caselli, described them to the Boston Globe in 2016 as family heirlooms.

Twelve years and beyond

Nothing is drawn from the smallest barrel before twelve years have passed, the Modena consortium says, and the amount taken each year is left to the producer. In Robinson’s account it is never more than half of the barrel; the agronomy article puts the ceiling at 10 to 15 per cent of its content. The specifications set the minimum at twelve years for both vinegars.

What the years do is physical. Water leaves through the wood and the bung hole, and a 2023 review reports that sugars, acids and polyphenols concentrate, raising density and viscosity. It gives the finished vinegar a sugar content of around 40 per cent, a pH between 2.3 and 3.2, and 2 to 5 per cent acetic acid. Because of the yearly transfers, every barrel holds a blend of vinegars of different ages, the oldest in the smallest. The same review calls the age of such a vinegar not easy to define, and notes that a model published in 2007 by Giudici and Rinaldi estimates it from the volumes drawn off and added.

These changes take place on the loft’s terms. The 1999 application for the Modena name forbids accelerated or artificial ageing, including induced changes in temperature, humidity and ventilation, and neither text, as the consortia publish it, says why twelve. The museum’s guide gives the yield: about 100 kilos of grapes produced about two litres after 25 years.

Wood around wood

A barrel that begins to leak is not replaced. Caselli told the Boston Globe that when one leaks with age, a new one is built around it. Robinson wrote that the oldest barrels are between 100 and 200 years old, and that when one leaks it is covered with another layer of wood.

In a set kept for generations, a barrel that gives way may be wrapped in a younger one, and the line goes on as before, with a little vinegar moving along it each year. The museum’s nine show their woods openly: the name of each is carved on it. What each holds is a blend of years.