Food Chemistry

220 3 Lipids

complex heavy metal ions (chelating agents, se-
questerants or scavengers of trace metals). Thus,
initiation of heavy metal-catalyzed lipid autoxi-
dation can be prevented (cf. 3.7.2.1.6). Results
compiled in Table 3.42 demonstrate the syner-
gistic 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 advan-
tageous since phosphoric acid promotes polymer-
ization of fat or oil during deep frying.
The synergistic effect of phospholipids is
different. Addition of dipalmitoylphos-
phatidylethanolamine (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 rad-
icals as described in 3.7.3.2 is a synergistic effect.

3.7.3.2.4 Prooxidative Effect

The activity of antioxidants reverses under cer-
tain conditions: they become prooxidants. One
wayinwhichα-tocopherol can become perox-
idatively active is shown in Formula 3.81. An-
other way is through the formation of the chro-
manoxyl 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 undesir-
able from a nutritional and physiological point of

Table 3.42.Synergistic action of citric (C) and phos-
phoric 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%
CPLG+ LG
0 .01% +
C0.01% P

0.2 ppm Cu 0. 30. 20. 94. 74. 1
2 ppm Fe 0. 60. 50. 15. 70. 2
2 ppm Ni 0. 50. 63. 07. 04. 4

view, is prevented by co-antioxidants, e.g., vita-

min C (cf. 3.7.3.2.1), which can reduce the chro-

manoxyl radical toα-tocopherol.

In the presence of heavy metal ions, e.g., Fe^3 ⊕,

ascorbic acid becomes a peroxidant. It re-

duces Fe^3 ⊕to Fe^2 ⊕, which can produce super-

oxide radical anions or hydroxyl radicals with

oxygen or H 2 O 2 (Fentonreaction, cf. 3.7.2.1.8)

Prooxidative effects have also been observed with

carotenoids and flavonoids at higher concentra-

tions.

3.7.4 FatorOilHeating(DeepFrying)

Deep frying is one of the methods of food prepa-

ration 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 suffi-

ciently 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 fryinga

Characteristics Fresh oil Heated oil

Iodine number 108. 9 101. 3

Saponification number 191. 4 195. 9

Free fatty acidsb 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. 342. 9

18:2 (9, 12) 37. 029. 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

aThe 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.

bWeight % calculated as oleic acid.