signi
fi
cantly improved the stability of the oil. The AV of ascorbyl pal-
mitate-FSO mixtures were also remain unchanged up to 10 days at
60 °C, however, it increased rapidly thereafter, but remained lower than
that of the AV of pure FSO. With 400 ppm of ascorbyl palmitate, the
oxidation of FSO can be signi
fi
cantly prevented up to 20 days at 60 °C
(p < 0.05). However, it should be also noted that, the e
ff
ectiveness of
the ascorbyl palmitate was lower than TBHQ only because the number
of molecules of ascorbyl palmitate at all concentrations were 2.5 times
less than that of TBHQ. If they were added on the molar basis, ascorbyl
palmitate would have shown the same e
ff
ectiveness as that of TBHQ.
As we can see, the increase in secondary oxidation products a
ff
ected
the change in their PV values during storage. The PV of FSO and its
mixtures with antioxidant started increasing or decreasing depending
on the rate of formation of secondary oxidation products. For instance,
at 60 °C, the AV and PV of FSO mixture with ascorbyl palmitate started
increasing together after 10 days. While, in FSO-alpha tocopherol
mixtures, the PV started decreasing with increase in the AV, and on day
20 there was a dramatic drop in PV value due to the faster rate of
transformation of peroxides to secondary oxidation products than the
rate formation of peroxides. The mixtures of FSO and TBHQ were also
displayed a similar drop in their PV value on day 10, at which the AV of
the mixtures started increasing. On the other hand, after 10 days the AV
of all mixtures of tannic acid has increased dramatically and reached
almost same value as that of pure FSO. Likewise the PV of these mix-
tures were attained the same value as that obtained for pure FSO. Even
though there was a signi
fi
cant amount of secondary oxidation products
formed, on day 30 the PV of all the mixtures were similar to the value
obtained on day 1, no matter whether the PV decreased or increased
while storage, which indicates that the peroxides were forming while
they were transforming to secondary oxidation products in FSO.
It should be noted that when the PV of FSO or its mixtures with any
antioxidants were less than 2 milli equivalents per grams of oil (
Fig. 4
),
there was not any secondary oxidation products detected (
Fig. 5
). This
result indicates that, there should be a minimum amount of peroxides
present in the FSO prior to their transformation into secondary oxida-
tion products. Therefore, an antioxidant which could prevent the for-
mation of peroxides in FSO might be more suitable for the protection of
FSO.
3.4. Mechanism of action of natural antioxidants in FSO and the criteria for
their selection
Contradicting to polar paradox, ascorbyl palmitate with inter-
mediate polarity and hydrophobic TBHQ displayed higher e
ff
ectiveness
in preventing the oxidation of FSO than hydrophilic tannic acid in all
the studies conducted, while the hydrophobic antioxidant alpha toco-
pherol displayed pro-oxidant e
ff
ect. These results indicate that in this
case, it was not the polarity that determined the antioxidant activity,
rather a closer look at the minor components of FSO and presence of
moisture would be necessary.
It has been known that the e
ff
ects related to the molecular or-
ientation and self-assembly of minor components such as mono and
diacylglycerols (MAGs and DAGs), free fatty acids, phospholipids,
sterols, hydroperoxides etc. can signi
fi
cantly alter the mechanism of oil
oxidation (
Chaiyasit, et al., 2007; Kittipongpittaya, et al., 2014;
Kittipongpittaya, et al., 2016
) and e
ff
ectiveness of antioxidants (
Cui &
Decker, 2016
). The FSO used in this study contained DAGs, sterols,
phospholipids, and moisture (
Table 1
), which all together may have
altered the antioxidant activity of added antioxidants di
ff
erently. For
example, the presence of signi
fi
cantly high amount of DAGs in the FSO,
(a)
(b)
(c)
Tannic Acid
Time (days)
0
1
10
20
30
P-Anasidine Value
0
5
10
15
20
25
30
35
40
45
50
0ppm
50ppm
100ppm
200ppm
400ppm
Alpha Tocopherol
Time (days)
0
1
10
20
30
Ascorbyl Palmitate
Time (days)
0
1
10
20
30
TBHQ
Time (days)
0
1
10
20
30
Tannic Acid
Time (days)
0
1
10
20
30
P-Anasidine Value
0
5
10
15
20
25
30
35
40
45
50
0ppm
50ppm
100ppm
200ppm
400ppm
Alpha Tocopherol
Time (days)
0
1
10
20
30
Ascorbyl Palmitate
Time (days)
0
1
10
20
30
TBHQ
Time (days)
0
1
10
20
30
Tannic Acid
Time (days)
0
1
10
20
30
P-Anasidine Value
0
5
10
15
20
25
30
35
40
45
50
0ppm
50ppm
100ppm
200ppm
400ppm
Alpha Tocopherol
Time (days)
0
1
10
20
30
Ascorbyl Palmitate
Time (days)
0
1
10
20
30
0ppm
50ppm
100ppm
200ppm
400ppm
TBHQ
Time (days)
0
1
10
20
30
Fig. 5.
The p-anisidine value of
fl
axseed oil and its mixtures with di
ff
erent concentrations of di
ff
erent antioxidants at (a) 25 °C, (b) 40 °C and (c) 60 °C measured as a
function of time for 30 days. Names of antioxidants are provided in the corresponding panels.
A. Mohanan et al.
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