DNA reveals the ingredients of garum, the most fragrant and popular sauce of the Roman Empire.

Garum, made from fermented fish entrails, was the most popular sauce in the Roman Empire. With its umami flavor and powerful aroma, it accompanied many dishes to impart a salty taste, much in the same way Asians dip everything in soy sauce or we in the West use salt. The philosopher Seneca described it as a "precious decay of rotten fish," and Pliny the Elder praised the sauce made in Cartago Nova (Cartagena) for its exquisiteness. Its production, in fact, was deeply rooted along the coasts of the Iberian Peninsula.

Some cookbooks from the period have mentioned the sauce's ingredients, but in an ambiguous and incomplete manner. Now, a team of Spanish and Portuguese researchers has managed to reveal the recipe's true composition by obtaining DNA from the residue at the bottom of fish salting vats from the Adro Vello site (3rd century AD) in O Grove (Pontevedra). The findings, published in the journal ' Antiquity', confirm the use of European sardines ( Sardina pilchardus ) in the preparation of garum.

This highly prized sauce was prepared both for local consumption and for long-distance trade, which was possible due to its long shelf life. It was produced in large fish salting plants, known as cetariae, in coastal areas, especially in the Atlantic coasts of Hispania (present-day Iberian Peninsula) and Tingitana (Morocco), famous for their fish sauces.

These facilities processed and preserved fish, primarily through salting and fermentation. Large fish, such as tuna, were cleaned, gutted, and placed in layers of salt in stone vats to extract moisture, thereby preventing bacterial growth and allowing the fish to be stored for long periods.

Small fish, not typically considered high-quality, were used to make fish sauces such as garum. These were prepared by grinding the whole fish and fermenting it in brine. Thus, pelagic fish, such as sardines, sprats, anchovies, and mackerel, were an important component of the human diet in Roman times in the form of fish sauce, while sardines were also used to make fish pastes.

"Fish bones are a frequent find in the archaeological record of the Roman period, but their use in archaeogenomic studies is residual, probably due to the fragmentary nature of the remains, which makes species or even genera identification difficult," says Paula F. Campos of the Interdisciplinary Center for Marine and Environmental Research (CIIMAR) at the University of Porto. In cetariae, the remains found in fish tanks are often so damaged as to be unrecognizable, impossible to identify by observation alone. This is especially true in the case of salting remains, as they were intentionally crushed during sauce production. These grinding and fermentation processes accelerate DNA degradation.

To determine whether ancient DNA studies were possible on these highly processed remains, the researchers extracted and sequenced genetic material from fish remains (bones, vertebrae, and scales) from a salting vat in the Adro Vello cetaria. By comparing the DNA sequences from the archaeological remains with those of modern sardines, the researchers determined that Roman-era sardines were closely related to those found in the same area today, confirming genetic continuity despite the species' high mobility.

Overall, these findings confirm the value of ancient DNA studies for identifying damaged animal remains. This has important implications for the study of archaeological fish remains in the future.

"In this study, we demonstrate that usable DNA can survive in fermentation environments, such as the brines used by the Romans to make garum," Campos concludes. "Despite the persistent conditions that promote DNA degradation, the methods we describe allow species identification from these processed bones," he says.