2009_Book_FoodChemistry

5.4 Interactions with Other Food Constituents 389
Strecker degradation, oxidation to indolylacetic
acid and decarboxylation. The oxidative cleavage
of skatole yields o-aminoacetophenone
(cf. Formula 5.36), which has an animal odor
and is the key aroma substance of tortillas
and taco shells made of corn treated with lime
(Masa corn). In the case of milk dry products,
o-aminoacetophenone causes an aroma defect
(cf. 10.3.2). Its odor threshold of 0.2µg/kg
(water) is very low. On the other hand, p-aminoacetophenone
has an extremely high odor
threshold of 100 mg/kg (water).
(5.35)
p-Cresol (odor threshold on starch 130 µg/kg)
has been detected as an accompanying substance
of skatole in samples of white pepper having an
aroma defect. It is also formed in citrus oil and
juice by the degradation of citral (cf. 5.5.4).
(5.36)
5.4 Interactions
with Other Food Constituents
Aroma interactions with lipids, proteins and
carbohydrates affect the retention of volatiles
within the food and, thereby, the levels in the
gaseous phase. Consequently, the interactions
affect the intensity and quality of food aroma.
Since such interactions cannot be clearly followed
in a real food system, their study has
been transferred to model systems which can,
in essence, reliably imitate the real systems.
Consider the example of emulsions with fat
contents of 1%, 5% and 20%, which have been
aromatized with an aroma cocktail for mayonnaise
consisting of diacetyl, (Z)-3-hexenol,
(E,Z)-2,6-nonadienol, allyl isothiocyanate and
allyl thiocyanate. The sample with 20% of fat
has the typical and balanced odor of mayonnaise
(Fig. 5.32 a). If the fat content decreases, the
aroma changes drastically. The emulsion with
5% of fat has an untypical creamy and pungent
odor since there is a decrease in the intensities
of the buttery and fatty notes in the aroma
profile (Fig. 5.32 b). In the case of 1% of fat,
pungent, mustard-like aroma notes dominate
(Fig. 5.32 c).
Headspace analyses show that the drastic change
in the aroma of the emulsions is based on the fact
that the concentrations of the fatsoluble aroma
substances (Z)-3-hexenol, allyl isothiocyanate
and allyl thiocyanate in the gas phase increase
with decreasing fat content (Fig. 5.33). Only
the water-soluble diacetyl remains unaffected
(Fig. 5.33).
The concentration of the very aroma active
(E,Z)-2,6-nonadienol (cf. 10.3.6) in the head
space is below the detection limit. However,
this odorant can be detected by headspace
GC-olfactometry (cf. 5.2.2.2). The results in
Table 5.36 show that this alcohol as well as
(Z)-3-hexenol no longer contribute to the aroma
in the 20% fat emulsion. In the emulsion with
1% of fat, (E,Z)-2,6-nonadienol, allyl isothiocyanate
and allyl thiocyanate predominate
and produce the green, mustard-like aroma
(Table 5.36).
A knowledge of the binding of aroma to solid
food matrices, from the standpoint of food aromatization,
aroma behavior and food processing and
storage, is of great importance.