Page 26 - The Miracles of Smell and Taste

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minute difference between the molecules L-
carvone and D-carvone stems from their at-
oms having different sequences. Despite
this exceedingly close similarity, a human

nose can easily distinguish between the
two, telling us that the former sug-
gests cumin and the latter, spear-
mint. 9
Another property of the
nose that amazes scientists is its
immaculate sensitivity. The mini-
mum concentration of a substance
required for us to recognize its par-
ticular smell is known as the smell
threshold. The analysis mechanism in
our noses in unbelievably sensitive;
some scents in the air can be per-
ceived at concentrations of less than

one part in a trillion! Research has
shown, for example, that the
threshold for perception of butyric
acid is a 10 billion fold dilution of the
pure substance. 10
The more molecules are investigat-
ed, the more marvels of the scent percep-
tion system are revealed. What we perceive as
any single aroma is actually an effect caused by
large numbers of different molecules. For instance,
the “ordinary” smell of white bread actually consists

The Miracles of Smell and
Taste