2009_Book_FoodChemistry

214 3 Lipids
texture, decreases in protein solubility (formation
of cross-linked proteins), color (browning) and
changes in nutritive value (loss of essential amino
acids).
The radicals generated from hydroperoxides
(cf. Fig. 3.33) can abstract H-atoms from protein
(PH), preferentially from the amino acids
Trp, Lys, Tyr, Arg, His and cysteine, in which the
phenolic HO-, S- or N-containing groups react:
RO • + PH −→ P • + ROH (3.78)
2P • −→ P–P (3.79)
In Reaction 3.79. protein radicals combine
with each other, resulting in the formation of
a protein network. Malonaldehyde is generated
(cf. 3.7.2.1.9) under certain conditions during
lipid peroxidation. As a bifunctional reagent,
malonaldehyde can crosslink proteins through
a Schiff base reaction with the ε-NH 2 groups of
two lysine residues:
Fig. 3.34. Reaction of volatile aldehydes with protein
amino groups
reenter the reaction, the condensation process
results not only in discoloration (browning) but
also in a change in aroma.
(3.80)
The Schiff base adduct is a conjugated fluorochrome
that has distinct spectral properties
(λ max excitation ∼350 nm; λ max emission
∼450 nm). Hence, it can be used for detecting
lipid peroxidation and the reactions derived from
it with the protein present.
Reactions resulting in the formation of a protein
network like that oulined above also have practical
implications, e. g., they are responsible for the
decrease in solubility of fish protein during frozen
storage.
Also, the monocarbonyl compounds derived from
autoxidation of unsaturated fatty acids readily
condense with protein-free NH 2 groups, forming
Schiff bases that can provide brown polymers
by repeated aldol condensations (Fig. 3.34). The
brown polymers are often N-free since the amino
compound can be readily eliminated by hydrolysis.
When hydrolysis occurs in the early stages
of aldol condensations (after the first or second
condensation; cf. Fig. 3.34) and the released
aldehyde, which has a powerful odor, does not
3.7.2.4.4 Decomposition of Amino Acids
Studies of model systems have revealed that
protein cleavage and degradation of side chains,
rather than formation of protein networks, are
the preferred reactions when the water content
of protein/lipid mixtures decreases. Several
examples of the extent of losses of amino acids in
a protein in the presence of an oxidized lipid are
presented in Table 3.37. The strong dependence
of this loss on the nature of the protein and
reaction conditions is obvious. Degradation
products obtained in model systems of pure
amino acids and oxidized lipids are described in
Table 3.38.
3.7.3 Inhibition of Lipid Peroxidation
Autoxidation of unsaturated acyl-lipids can be retarded
by:
• Exclusion of oxygen. Possibilities are packaging
under a vacuum or addition of glucose oxidase
(cf. 2.7.2.1.1).