Fresh kernels of the walnut tree (Juglans
regiaL.) show a characteristic and pleasant aroma,
the molecular basis
of which was unknown. The application of an aroma extract dilution
analysis resulted in 50 odor-active compounds. Among them, 37 had
not been reported as fresh walnut kernel volatiles before, including
the two odorants with the highest flavor dilution factors, namely,
fenugreek-like smelling 3-hydroxy-4,5-dimethylfuran-2(5H)-one (sotolon) and oatmeal-like smelling (2E,4E,6Z)-nona-2,4,6-trienal. Quantitations
revealed 17 odorants with concentrations in the walnuts that exceeded
their odor threshold concentrations. Aroma reconstitution and omission
experiments finally showed that the characteristic aroma of fresh
walnuts is best represented by a binary mixture of sotolon and (2E,4E,6Z)-nona-2,4,6-trienal.
Of both, the natural concentration was ∼10 μg/kg. Further
sensory studies showed that the walnut character is intensified when
their concentrations are in parallel increased to ∼100 μg/kg.
This finding may guide the future breeding of new walnut cultivars
with improved aroma.