Chemical and Functional Properties of Food Saccharides

N
H 2
R
CH COOH
HC
O
C O
R 1
HC
N
R
CH COOH
C O
R1 CO2 HC
C
R 1
processed foods. 63 During the reaction, a-dicarbonyls are transformed to a-aminocarbonyls,
which might liberate ammonia or condense to numerous heterocyclic
compounds such as pyrazines, pyrrols, and others. 64
In the presence of oxygen, the corresponding acids are formed in parallel to
Strecker aldehydes. The formation of acid is further increased in the presence of
traces of transition metals. It has been demonstrated that the acids are not formed
to a significant extent by the oxidation of aldehydes. The mechanism shown in
Reaction 18.15 was proposed to explain their formation. 63 Up to the intermediate
(18.71), the mechanism does not differ from that leading to Strecker aldehydes.
This intermediate may either liberate a Strecker aldehyde or it may be oxidized
to iminoketone (18.72), which on isomeration and hydrolysis yields the corresponding
acid. Some “Strecker” acids are, similarly to Strecker aldehydes, important
odorants of thermally processed foods. 63 Strecker aldehydes can be also
formed directly from the Schiff base (18.1) (Reaction 18.4) or the Amadori compound
(18.4) without passing through the formation of a-dicarbonyls. The mechanisms
proposed for the formation from Amadori compounds include either oxidation
of enaminol (18.3) (Reaction 18.16) or further amination of the Amadori
HO
C
H 2
+
H 2 O
Reaction 18.15
C
HC
© 2004 by CRC Press LLC
CH
NH CH COOH
O
OH
HC OH
CH 2 OH
R 1
18.4 18.3
Reaction 18.16
HO
HC
C
HC
CH
NH CH COOH
OH
OH
HC OH
CH 2 OH
R 1
HC
O 2/Me 2+
C
R 1
-H 2O2 / Me 2+
HO
compound (18.4) (Reaction 18.17). 19,31 An alternative pathway leading to Strecker
N
OH
R
CH
R
N CH OH
O
18.72
HO
OH
O
H 2 O
O 2/Me 2+
HC
C
HC
CH
OH
-
-H 2O2
N
O
OH
HC OH
CH 2 OH
N CH
R 1
HC
HC
R 1
C
C
R 1
R 1
R
NH CH
N
CO 2
OH
18.71
OH
CH COOH
H 2 O
R
C
HO
H 2 O
OH
OH
HO
C
H 2
OH
O
C
R 1
OH
O
NH 2
O
R 1 CHO
RCHO
RCOOH
OH
OH
N COOH
+
R 1
OH
NH 2