Food Chemistry

3.7 Changes in Acyl Lipids of Food 215

Table 3.37.Amino acid losses occurring in protein re-
action 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

aTrp analysis was not performed.
bCystine analysis was not performed.

Table 3.38.Amino acid products formed in reaction
with peroxidized lipid

Reaction system Compounds formed

from amino acids
amino lipid
acid

His Methyl linoleate Imidazolelactic acid,
Imidazoleacetic acid
Cys Ethyl Cystine, H 2 S, cysteic
arachidonate acid, alanine, cystine-
disulfoxide
Met Methyl linoleate Methionine-sulfoxide
Lys Methyl linoleate Diaminopentane,
aspartic acid, glycine,
alanine,α-aminoadipic
acid, pipecolinic
acid, 1,10-diamino-
1,10-dicarboxydecane

• Storage at low temperature in the dark. The
  autoxidation rate is thereby decreased substan-
  tially. However, in fruits and vegetables which
  contain the lipoxygenase enzyme, these pre-
  cautions are not applicable. Food deterioration
  is prevented only after in activation of the en-
  zyme by a blanching process (cf. 2.6.4).
  
  
  • Addition of antioxidants to food.
  
  
  
  

3.7.3.1 AntioxidantActivity.....................................

The peroxy and oxy free radicals formed during

the propagation and branching steps of the autox-

idation 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 res-

onance 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 there-

fore cannot initiate lipid peroxidation. The end-

products formed in reactions 3 and 4 in Fig. 3.35

are relatively stable and in consequence the aut-

oxidation radical chains are shortened.

The reaction scheme (Fig. 3.35) shows that one

antioxidant molecule combines with two radicals.

Therefore, the maximum achievable stoichiomet-

ric factor is n=2. In practice, the value of n is

between 1 and 2 for the antioxidants used. An-

tioxidants, in addition to their main role as radical

scavengers, can also partially reduce hydroperex-

ides to hydroxy compounds.

3.7.3.2 AntioxidantsinFood

3.7.3.2.1 NaturalAntioxidants.....................................

The unsaturated lipids in living tissue are rela-

tively stable. Plants and animals have the neces-

sary complement of antioxidants and of enzymes,

for instance, glutathione peroxidase and superox-

ide dismutase, to effectively prevent lipid oxida-

tion.

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-

Fig. 3.35.Activity of an antioxidant as a radical scav-

enger. AH: Antioxidant