2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 215
Table 3.37. Amino acid losses occurring in protein reaction
with peroxidized lipids
Reaction system Reaction Amino acids lost
conditions
protein lipid time T ( ◦ C) (% loss)
Cyto- Linolenic 5 h 37 His(59),Ser(55),
chrome C acid
Pro(53),Val(49),
Arg(42),Met(38),
Cys(35) a
Trypsin Linoleic 40 min 37 Met(83),His(12) a
acid
Lysozyme Linoleic 8 days 37 Trp(56),His(42),
acid
Lys(17),Met(14),
Arg(9)
Casein Linoleic 4 days 60 Lys(50), Met(47),
acid
Ile(30),Phe(30),
ethyl ester
Arg(29),Asp(29),
Gly(29),His(28),
Thr(27),Ala(27),
Tyr(27) a,b
Oval- Linoleic 24 h 55 Met(17),Ser(10),
bumin acid Lys(9),Ala(8),
ethyl ester
Leu(8) a,b
a Trp analysis was not performed.
b Cystine analysis was not performed.
Table 3.38. Amino acid products formed in reaction
with peroxidized lipid
Reaction system
amino
acid
lipid
Compounds formed
from amino acids
His Methyl linoleate Imidazolelactic acid,
Imidazoleacetic acid
Cys Ethyl Cystine, H 2 S, cysteic
arachidonate
acid, alanine, cystinedisulfoxide
Met Methyl linoleate Methionine-sulfoxide
Lys Methyl linoleate Diaminopentane,
aspartic acid, glycine,
alanine, α-aminoadipic
acid, pipecolinic
acid, 1,10-diamino-
1,10-dicarboxydecane
• Addition of antioxidants to food.
3.7.3.1 Antioxidant Activity
The peroxy and oxy free radicals formed during
the propagation and branching steps of the autoxidation
radical chain (cf. Fig. 3.19) are scavenged
by antioxidants (AH; cf. Fig. 3.35). Antioxidants
containing a phenolic group play the major role in
food. In reactions 1 and 2 in Fig. 3.35, they form
radicals which are stabilized by an aromatic resonance
system. In contrast to the acyl peroxy and
oxy free radicals, they are not able to abstract a H-
atom from an unsaturated fatty acid and therefore
cannot initiate lipid peroxidation. The endproducts
formed in reactions 3 and 4 in Fig. 3.35
are relatively stable and in consequence the autoxidation
radical chains are shortened.
The reaction scheme (Fig. 3.35) shows that one
antioxidant molecule combines with two radicals.
Therefore, the maximum achievable stoichiometric
factor is n = 2. In practice, the value of n is
between 1 and 2 for the antioxidants used. Antioxidants,
in addition to their main role as radical
scavengers, can also partially reduce hydroperexides
to hydroxy compounds.
3.7.3.2 Antioxidants in Food
3.7.3.2.1 Natural Antioxidants
The unsaturated lipids in living tissue are relatively
stable. Plants and animals have the necessary
complement of antioxidants and of enzymes,
for instance, glutathione peroxidase and superoxide
dismutase, to effectively prevent lipid oxidation.
During the isolation of oil from plants (cf. 3.8.3),
tocopherols are also isolated. A sufficient level
is retained in oil even after refining, thus, toco-
• Storage at low temperature in the dark. The
autoxidation rate is thereby decreased substantially.
However, in fruits and vegetables which
contain the lipoxygenase enzyme, these precautions
are not applicable. Food deterioration
is prevented only after in activation of the enzyme
by a blanching process (cf. 2.6.4).
Fig. 3.35. Activity of an antioxidant as a radical scavenger.
AH: Antioxidant