2009_Book_FoodChemistry

220 3 Lipids
complex heavy metal ions (chelating agents, sequesterants
or scavengers of trace metals). Thus,
initiation of heavy metal-catalyzed lipid autoxidation
can be prevented (cf. 3.7.2.1.6). Results
compiled in Table 3.42 demonstrate the synergistic
activities of citric and phosphoric acids in
combination with lauryl gallate. Whereas citric
acid enhances the antioxidant effectiveness in the
presence of all three metal ions, phosphoric acid
is able to do so with copper and nickel, but not
with iron. Also, use of citric acid is more advantageous
since phosphoric acid promotes polymerization
of fat or oil during deep frying.
The synergistic effect of phospholipids is
different. Addition of dipalmitoylphosphatidylethanolamine
(0.1–0.2 weight %) to
lard enhances the antioxidative activity of
α-tocopherol, BHA, BHT and propyl gallate,
while phosphatidylcholine shows no activity.
The reaction of ascorbic acid with tocopherol radicals
as described in 3.7.3.2 is a synergistic effect.
3.7.3.2.4 Prooxidative Effect
The activity of antioxidants reverses under certain
conditions: they become prooxidants. One
wayinwhichα-tocopherol can become peroxidatively
active is shown in Formula 3.81. Another
way is through the formation of the chromanoxyl
radical in concentrations high enough
to overcome the inertness mentioned in 3.7.3.1
and abstract H-atoms from unsaturated acyl lipids
to a definite extent, starting lipid peroxidation.
This activity reversion, which is also undesirable
from a nutritional and physiological point of
Table 3.42. Synergistic action of citric (C) and phosphoric
acids (P) in combination with lauryl gallate (LG)
on oxidation of fats and oils
Added to AF value after addition of
fat/oil
0.01% 0.01% 0.01% 0.01%LG 0.01%
C P LG + LG
0.01% +
C 0.01% P
0.2 ppm Cu 0.3 0.2 0.9 4.7 4.1
2 ppm Fe 0.6 0.5 0.1 5.7 0.2
2 ppm Ni 0.5 0.6 3.0 7.0 4.4
view, is prevented by co-antioxidants, e.g., vitamin
C (cf. 3.7.3.2.1), which can reduce the chromanoxyl
radical to α-tocopherol.
In the presence of heavy metal ions, e.g., Fe 3⊕ ,
ascorbic acid becomes a peroxidant. It reduces
Fe 3⊕ to Fe 2⊕ , which can produce superoxide
radical anions or hydroxyl radicals with
oxygen or H 2 O 2 (Fenton reaction, cf. 3.7.2.1.8)
Prooxidative effects have also been observed with
carotenoids and flavonoids at higher concentrations.
3.7.4 Fat or Oil Heating (Deep Frying)
Deep frying is one of the methods of food preparation
used both in the home and in industry.
Meat, fish, doughnuts, potato chips or french fries
are dipped into fat (oil) heated to about 180 ◦ C.
After several minutes of frying, the food is sufficiently
tender to be served.
The frying fat or oil changes substantially in its
chemical and physical properties after prolonged
use. Data for a partially hydrogenated soybean oil
Table 3.43. Characteristics of partially hydrogenated
soybean oil before and after simulated deep fat frying a
Characteristics Fresh oil Heated oil
Iodine number 108.9 101.3
Saponification number 191.4 195.9
Free fatty acids b 0.03 0.59
Hydroxyl number 2.25 9.34
DG 1.18 2.73
Composition of fatty acids (weight %)
14:0 0.06 0.06
16:0 9.90 9.82
18:0 4.53 4.45
18:1 (9) 45.3 42.9
18:2 (9, 12) 37.0 29.6
18:3 (9, 12, 15) 2.39 1.67
20:0 0.35 0.35
22:0 0.38 0.38
Other 0.50 0.67
a The oil was heated for 80 h (8 h/day) at 195 ◦ C.
Batches of moist cotton balls containing 75% by weight
of water were fried at 30-min intervals (17 frying
operations/day) in order to simulate the deep frying
process.
b Weight % calculated as oleic acid.