2009_Book_FoodChemistry

388 5 Aroma Compounds
Table 5.35. Tertiary thiols in food
Name Structure Odor threshold Occurrence
(µg/kg, water)
4-Mercapto-4-methyl- 0.0001 Basil, wine (Scheurebe), Grapefruit
2-pentanone
4-Methoxy-2-methyl-2- 0.00002 Olive oil (cf. 14.3.2.1.1),
butanethiol
black currants
3-Mercapto-3- 0.003 Roasted coffee
methylbutylformate
1-p-Menthen-8-thiol 0.00002 Grapefruit
3-mercapto-3-methylbutylformate is undesirable
because it causes off-flavor at concentrations
as low as 5 ng/l. 1-p-Menthene-8-thiol, which
contributes to grapefruit aroma, is a chiral compound.
The (R)-enantiomer exhibits an extremely
low odor threshold shown in Table 5.35. The
(S)-enantiomer has a weak and unspecific odor.
5.3.2.6 Pyrazines
Paprika pepper (Capsicum annum) and chillies
(Capsicum frutescens) contain high concentrations
of 2-isobutyl-3-methoxypyrazine (X in
Table 5.24 for structure). Its biosynthesis from
leucine is assumed to be through the pathway
shown in Formula 5.34.
The compound 2-sec-butyl-3-methoxy-pyrazine
is one of the typical aroma substances of
carrots.
Pyrazines are also produced by microorganisms.
For example, 2-isopropyl-3-methoxypyrazine has
been identified as a metabolic byproduct of Pseudomonas
perolans and Pseudomonas taetrolens.
This pyrazine is responsible for a musty/earthy
off-flavor in eggs, dairy products and fish.
5.3.2.7 Skatole, p-Cresol
The amino acids tryptophan and tyrosine are degraded
by microorganisms to skatole and p-cresol
respectively (cf. Formula 5.40).
The odor thresholds of skatole have been determined
in sunflower oil (15.6µg/kg) and on
starch (0.23 µg/kg). This compound plays a role
in the aroma of Emmental cheese (cf. 10.3.5)
and causes an aroma defect in white pepper
(cf. 22.1.1.2.1). It can probably also be
formed nonenzymatically from tryptophan by the
(5.34)