2009_Book_FoodChemistry

360 5 Aroma Compounds
Table 5.16. Some Strecker degradation aldehydes a
Amino acid Strecker-aldehyde Odor
precursor
threshold value
Name Structure Aroma (µg/l; water)
description
Gly Formaldehyde CH 2 O Mouse-urine, 50 ×10 3
ester-like
Ala Ethanal Sharp, 10
penetrating,
fruity
Val Methylpropanal Malty 1
Leu 3-Methylbutanal Malty 0.2
Ile 2-Methylbutanal Malty 4
Phe 2-Phenylethanal Flowery,
honey-like
4
a Methional will be described in 5.3.1.4.
by using the general knowledge of organic
chemistry or biochemistry. For an increasing
number of odorants, the proposed formation
pathway can be based on the results of model
experiments. Postulated intermediates have also
been confirmed by identification in a numbers
of cases. However, studies on the formation
of odorants are especially difficult since they
involve, in most cases, elucidation of the side
pathways occurring in chemical or biochemical
reactions, which quantitatively are often not
much more than negligible.
5.3.1 Nonenzymatic Reactions
The question of which odorants are formed in
which amounts when food is heated depends
on the usual parameters of a chemical reaction.
These are the chemical structure and concentration
of the precursors, temperature, time and environment,
e. g., pH value, entry of oxygen and the
water content. Whether the amounts formed are
really sufficient for the volatiles to assert themselves
in the aroma depend on their odor thresholds
and on interactions with other odorants.
Aroma changes at room temperature caused by
nonenzymatic reactions are observed only after
prolonged storage of food. Lipid peroxidation
(cf. 3.7.2.1), the Maillard reaction and the
related Strecker degradation of amino acids
(cf. 4.2.4.4.7) all play a part. These processes are
greatly accelerated during heat treatment of food.
The diversity of aroma is enriched at the higher
temperatures used during roasting or frying. The
food surface dries out and pyrolysis of carbohydrates,
proteins, lipids and other constituents,
e. g., phenolic acids, takes place generating odorants,
among other compounds.
The large number of volatile compounds formed
by the degradation of only one or two constituents
is characteristic of nonenzymatic reactions. For
example, 41 sulfur-containing compounds,
including 20 thiazoles, 11 thiophenes, 2 dithiolanes
and 1 dimethyltrithiolane, are obtained
by heating cysteine and xylose in tributyrin at
200 ◦ C. Nevertheless, it should not be overlooked
that even under these drastic conditions, most
of the volatile compounds are only formed in
concentrations which are far less than the often
relatively high odor thresholds (cf. 5.6). For this
reason, only a small fraction of the many volatile
compounds formed in heated foods is aroma
active.