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 com-
pound. 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 concen-
trations 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-pyra-
zine is one of the typical aroma substances of
carrots.
(5.34)Pyrazines are also produced by microorganisms.
For example, 2-isopropyl-3-methoxypyrazine has
been identified as a metabolic byproduct ofPseu-
domonas perolansandPseudomonas 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 de-
graded by microorganisms to skatole and p-cresol
respectively (cf. Formula 5.40).
The odor thresholds of skatole have been de-
termined 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 pep-
per (cf. 22.1.1.2.1). It can probably also be
formed nonenzymatically from tryptophan by the