Scientists from the Swiss company’s Corporate R&D Division studied the effects of oral microflora on a series of sulphur-containing compounds found commonly in foods such as wine and fruit.
Christian Starkenmann and co-workers report that these cysteine-S-conjugates are transformed by bacteria in the mouth, and that this transformation “explains long-lasting sulfury odours in the mouth that give a second dimension to the flavour perception of food products”.
How consumers sense food is crucial knowledge for a food industry constantly organising the building blocks of new food formulations.
Explaining the significance of the research Dr Starkenmann told FoodNavigator.com: “This is the first time we were able to prove that the mouth act like a reactor and therefore we can modulate the odour perception. A free thiol has a high impact but short, the corresponding cysteine conjugate will produced a delayed impact and stay longer in the mouth. We are now able to make more authentic flavor systems, like in nature.”
The study also shows how critical saliva and the enzymes, proteins, and bacteria it contains, is critical for taste and flavour perception.
“Saliva is produced in the mouth by three major glands, the parotid, sublingual, and submandibular, and by other minor glands,” explained the researchers. “Minor lingual saliva glands are crucial in taste perception because they irrigate taste buds. The role of saliva is to provide a coating on the mouth epithelium as a buffering system.”
The researchers selected three cysteine-S-conjugates to test: S-(R/S)- 3-(1-hexanol)-L-cysteine (found in wine), S-propyl-L-cysteine (found in onions), S-(2-heptyl)-L-cysteine (found in bell peppers). These compounds are precursors for thiol compounds, which were also tested.
Thirty trained panelists sampled the compounds and reported that they could immediately detect the scent of the thiols, but it took another 20 to 30 seconds for them to smell the cysteine-S-conjugate precursors.
The delayed detection of the precursors lasted for as much as three minutes, said the researchers, while the thiols lasted only a few seconds.
In order to get a better understanding of what was happening in the mouth, Starkenmann and his co-workers incubated the cysteine-S-conjugates with sterile saliva or saliva containing Fusobacterium nucleatum, a gram-negative anaerobe present in the mouth.
After 24 hours, the F. nucleatum-containing saliva was associated with an 80 per cent breakdown of the precursor compounds, whereas the sterile saliva was associated with a 15 per cent breakdown after four days.
“The cysteine−S-conjugates are transformed in free thiol by anaerobes,” wrote the authors. “The mouth acts as a reactor, adding another dimension to odour perception, and saliva modulates flavours by trapping free thiols.”
Potential for bad breath
The research also has implications for halitosis, said Dr Starkenmann, a condition mainly due to the degradation of cysteine and methionine coming from food proteins which sticks between your teeth.
“In this respect the population of bacteria was studied and F. nucleatum was recognized as being responsible for the generation of hydrogen sulphide and other mercaptans,” he said. “Our goal is to stop bad bacteria to produce bad odors.
“Interestingly these "bad" bacteria are also producing nice aroma from these cysteine conjugates. This was not known before.”
Source: Journal of Agricultural and Food Chemistry
Volume 56, Issue 20, Pages 9575-9580, doi: 10.1021/jf801873h
“Olfactory Perception of Cysteine-S-Conjugates from Fruits and Vegetables”
Authors: C. Starkenmann, B. Le Calvé, Y. Niclass, I. Cayeux, S. Beccucci, M. Troccaz