2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 193
At room temperature, a radical may inititiate the
formation of 100 hydroperoxide molecules before
chain termination occurs. In the presence
of air (oxygen partial pressure >130 mbar), all
alkyl radicals are transformed into peroxy radicals
through the rapid radical chain reaction 1
(RS-1, Fig. 3.19). Therefore, chain termination
occurs through collision of two peroxy radicals
(RS-8, Fig. 3.19).
Termination reactions RS-6 and RS-7 in Fig. 3.19
play a role when, for example, the oxygen level
is low, e. g. in the inner portion of a fatty
food.
The hypothesis presented in Fig. 3.19 is valid
only for the initiation phase of autoxidation. The
process becomes less and less clear with increasing
reaction time since, in addition to hydroperoxides,
secondary products appear that partially autoxidize
into tertiary products. The stage at which
the process starts to become difficult to survey depends
on the stability of the primary products. It
is instructive here to compare the difference in the
structures of monohydroperoxides derived from
linoleic and linolenic acids.
3.7.2.1.2 Monohydroperoxides
The peroxy radical formed in RS-1 (Fig. 3.19)
is slow reacting and therefore it selectively abstracts
the most weakly bound H-atom from a fat
molecule. It differs in this property from, for example,
the substantially more reactive hydroxy
(HO • ) and alkoxy(RO • ) radicals (cf. 3.7.2.1.8).
RS-2 in Fig. 3.19 has a high reaction rate only
when the energy for H-abstraction is clearly lower
than the energy released in binding H to O during
formation of hydroperoxide groups (about
376 kJ mol −1 ).
Table 3.27 lists the energy inputs needed for
H-abstraction from the carbon chain segments
or groups occurring in fatty acids. The peroxy
radical abstracts hydrogen more readily from
a methylene group of a 1,4-pentadiene system
than from a single allyl group. In the former
case, the 1,4-diene radical that is generated
is more effectively stabilized by resonance,
i. e. electron delocalization over 5 C-atoms. Such
considerations explain the difference in rates of
autoxidation for unsaturated fatty acids and show
why, at room temperature, the unsaturated fatty
Table 3.27. Energy requirement for a H-atom abstraction
D R-H ,(kJ/mole)
H
|
CH 2 − 422
H
|
CH 3 −CH− 410
H
|
−CH−CH=CH− 322
H
|
−CH=CH−CH−CH− 272
acids are attacked very selectively by peroxy
radicals while the saturated acids are stable.
The general reaction steps shown in Fig. 3.19 are
valid for all unsaturated fatty acids. In the case
of oleic acid, H-atom abstraction occurs on the
methylene group adjacent to the double bond,
i. e. positions 8 and 11 (Fig. 3.20). This would
give rise to four hydroperoxides. In reality, they
have all been isolated and identified as autoxidation
products of oleic acid. The configuration of
the newly formed double bond of the hydroperoxides
is affected by temperature. This configuration
has 33% of cis and 67% of the more stable
trans-configuration at room temperature.
Oxidation of the methylene group in position 11
of linoleic acid is activated especially by the two
neighboring double bonds. Hence, this is the initial
site for abstraction of an H-atom (Fig. 3.21).
The pentadienyl radical generated is stabilized by
formation of two hydroperoxides at positions 9
and 13, each retaining a conjugated diene system.
These hydroperoxides have an UV maximum absorption
at 235 nm and can be separated by high
performance liquid chromatography as methyl esters,
either directly or after reduction to hydroxydienes
(Fig. 3.22).
The monoallylic groups in linoleic acid (positions
8 and 14 in the molecule), in addition to
the bis-allylic group (position 11), also react to
a small extent, giving rise to four hydroperoxides
(8-, 10-, 12- and 14-OOH), each isomer having
two isolated double bonds. The proportion of
these minor monohydroperoxides is about 4% of
the total (Table 3.28).