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The late Emile Peynaud, considered the father of modern winemaking, remarked that the Sauvignon blanc grape had only a weak taste when one bit into it, but that 20 to 30 seconds later, "you suddenly experience a powerful aromatic rush at the back of the mouth as the Sauvignon fragrance returns." Dr Christian Starkenmann of Firmenich, a fragrance and flavour developer in Geneva, and his colleagues decided to investigate the mechanism behind this "retro-aromatic" effect.
In order to smell something, Starkenmann noted that a volatile substance must come in contact with the olfactory tissue lining the nasal cavity. Other researchers had previously shown that retro-aromatic foods contain odourless sulfur compounds called cysteine-S-conjugates, and that volatile sulfur compounds released from these precursors called thiols gave such foods their scent.
How the study was done
To better understand how the odourless precursors were transformed into their smelly offspring, Starkenmann and his team had 30 trained panellists sample the sulfur thiol and corresponding cysteine-S-conjugate precursor for grapes, onions, and bell peppers. The panellists spat out each sample after five seconds and then reported their perceptions.
While the panellists perceived the scent of the thiols immediately, it took them 20 to 30 seconds to smell the cysteine-S-conjugate precursors. And while the odour of the thiols lasted for only a few seconds, the smell of the precursors persisted for up to three minutes.
To test which components of saliva might be responsible for releasing the thiols from their parent compounds, the researchers incubated the precursors with saliva that contained normal bacteria and with sterile saliva.
Within 24 hours of being in the normal saliva, 80% of the compound had been broken down. But in the sterile saliva, the breakdown was much slower; after four days, less than 15% of the compound had disappeared.
What the findings mean
This suggests, Starkenmann and his colleagues state, that mouth bacteria are responsible for releasing thiols from their precursor compounds contained in retro-aromatic food and drink. Free thiols' shorter taste duration appears to occur because they are quickly absorbed by saliva, they add.
The findings should be useful in helping to develop food products with more complex, longer-lasting tastes, Starkenmann said. In a separate line of research, he noted, he and his colleagues have found that a similar process is at work in producing bad breath and underarm odour. – (Reuters Health, November 2008)
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