Food Chemistry

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 pro-
tein (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
aSchiffbase reaction with theε-NH 2 groups of
two lysine residues:

(3.80)

TheSchiff base adduct is a conjugated fluo-
rochrome that has distinct spectral properties
(λmax excitation ∼350 nm; λmax 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 practi-
cal 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 hydroly-
sis. 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

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.

3.7.2.4.4 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 re-

tarded by:

• Exclusion of oxygen. Possibilities are packag-
  ing under a vacuum or addition of glucose ox-
  idase (cf. 2.7.2.1.1).