If wine leaves a bitter, cotton-like coating on the tongue, don't blame your nose or even your sense of smell, say the authors of a paper in
Chemical Senses.

Instead, blame your nerves. The traditional oak barrel character, also called barrique character, is perceived via the trigeminal nerve, which is responsible for, among other things, pain and temperature perception. 

In an experiment, patients with severed taste nerves who were unable to taste the five flavors sweet, sour, bitter, salty and umami could feel the barrique flavor, also called astringency, on their tongue. The results of team headed by Professor Hanns Hatt from the Faculty of Biology and Biotechnology at the Ruhr-Universität were confirmed by experiments conducted at the Smell and Taste Center Florida in Gainesville: subjects whose taste nerve was temporarily anaesthetised perceived the barrique flavour nevertheless. Once the trigeminal nerve was likewise switched off, however, the barrique flavor disappeared.

Gallic acids responsible for astringency

The cell physiologists from Bochum decoded which exact structural features a material must possess to trigger the barrique flavor. The crucial element is a gallic acid group that consists of one carbon ring, namely a phenol ring, with three adjacent OH groups. The more gallic acid groups a substance contains, the stronger the astringency.

"Gallic acids occur in smaller quantities in the seeds of grapes and in large complexes in oak barrels," explains Hanns Hatt. "In red wine, many of those molecules bond together and have an accordingly strong effect." The RUB team also discovered that larger complexes affect nerve cells more strongly, too, and result in a clearer barrique flavour by humans.

Wine gets its barrique character not only through maturing in an oak barrel, but also through the addition of wood chips or wood flour. According to Hatt, this process has become even simpler: "Our research has made it possible to add the chemical substance that generates the barrique flavor directly to a wine. Such additives are approved for use in the EU and the USA."

Barrique substances activate cells in the trigeminal nerve

The RUB team used cell cultures to study the effect of barrique substances on cells from the trigeminal nerve of mice. In collaboration with the Chair of Food Chemistry at the TU of Munich, the researchers isolated substances triggering astringency from Bordeaux wine. The barrique substances excited the trigeminal cells; they activated a signaling pathway also known from olfactory cells, which runs via a G protein-coupled receptor.

"We don't know yet which receptor exactly is responsible for the barrique flavor, but we are attempting to identify it," says Hatt.