Microbial role: Bacteria in the mouth may be important for developing the flavours we recognise in bell peppers, onions, grapes and wine.
Credit: iStockphoto
Diagram showing the chemical transformation of odourless cysteine compounds to aromatic thiols.
Credit: Journal of Agricultural and Food Chemistry
SYDNEY: A new study has found that bacteria can chemically alter food in our mouths, producing new compounds that are an important component of the flavours we recognise.
A team of Swiss scientists has found that bacteria react with amino acids called cysteines, in foods such as grapes, wine, onions and bell peppers (capsicums), to create new odours from previously odourless parts of the food.
The study is published in a recent issue of the Journal of Agricultural and Food Chemistry.
"Second dimension" to flavours
"This transformation explains long-lasting sulphury odours in the mouth that give a second dimension to the flavour perception of food products," said lead researcher Christian Starkenmann at Firmenich SA, a chemical manufacturer of perfumes and flavourings based in Geneva.
Because what we perceive as flavour is mostly a result of smell, the find explains why we can only taste some substances fully after swallowing them. The phenomenon, known as a 'retroaromatic' effect had been difficult to explain until now.
Cysteines are found in volume in wines and many high-protein foods. They are odourless, until they react with saliva including bacteria, anti-bacterial agents and enzymes.
There are around 100 million bacteria in every millilitre of saliva. Many of these are anaerobic bacteria, which do not use oxygen to respire, and digest cysteines to form thiols, which have a strong odour.
Taste tests
To confirm the effect of the bacteria, Starkenmann's team completed a range of sensory tests on 30 tests subjects to determine the changing intensity of thiols produced from odourless components of foods.
The participants donated saliva to determine types and concentrations of bacteria. They then tasted cysteines dissolved in mineral water and rated them on strength of taste and time needed for that strength to become apparent.
The researchers found that it took 20 to 30 seconds for the aroma of the thiols to develop and that these flavours persisted for up three minutes.
The ability of bacteria to develop flavours in the mouth isn't always a good thing though.
This study "is interesting because [it shows how chemistry] adds to the quality of eating good food," commented Russell Keast, a sensory scientist at Deakin University in Victoria, Australia. "But it also leads to the possibility of the production of off-flavours and bad odours."
