Thank bacteria for the flavour of food

Swiss scientists have solved a Sauvignon conundrum this week with the discovery that part of the flavour of the famous white wine is down to bacteria in your own mouth! The French enologist Emile Peynaud drew attention to the burst of fruity flavours that follows 30 seconds or so after a gulp of the white wine is swallowed. This is known as the retroaromatic effect and similar olfactory phenomena accompany onions, peppers and other fruits and vegetables.

To find out why it happens Christian Starkenmann and his colleagues at the Swiss flavour company Firmenich collected saliva samples from volunteers. Half of each sample was sterilised by gentle heating to kill any bacteria whilst the other half remained pristine. The researchers then added odourless sulphur-containing chemicals called cysteine-S-conjugates, which are the same as those found in wine and other fulsome foods. The saliva-sulphur mixtures were then wafted beneath the noses of a second group of people. A smell was only detected when the mixtures included the pristine saliva, and lasted for between 30 seconds and 3 minutes, just like the taste of the Sauvignon.

To find out why the team cultured bacteria from the saliva samples and succeeded in growing one common anaerobic (oxygen-hating) species called Fusobacterium nucleatum. When this was added to the sulphur compounds dissolved in mineral water it released the same aromas as the intact saliva. The bugs, it turns out, can break down the odourless chemicals to produce aromatic sulphur-containing chemicals called thiols. It suggests, say the team, that this mouth bug is at least partly responsible for the way things taste and smell!