decelerated the oxidation. On the other hand, at higher temperatures,
the rate of peroxide formation of FSO is signi
fi
cantly higher than the
scavenging capacity of the antioxidants present in the FSO and resulted
in high level of oxidation.
Addition of antioxidants also reduced the PV of FSO at all storage
temperatures, and it was signi
fi
cant with TBHQ and ascorbyl palmitate
(p < 0.05). However, the e
ff
ectiveness of antioxidants varied with
their concentrations, storage time and temperatures. The e
ff
ect of an-
tioxidants on day 0 were done after one hour of storage at each tem-
peratures. The e
ff
ectiveness of antioxidants were evident even on day
zero, in one hour of storage. At 25 °C (
Fig. 4
a) on day 0, ascorbyl pal-
mitate displayed highest e
ff
ect followed by tannic acid and TBHQ. The
PV of FSO reduced 5 times with 200 ppm ascorbyl palmitate, whereas
the PV halved with 100 ppm tannic acid and 400 ppm of TBHQ. Ef-
fectiveness of tannic acid however remained unchanged above
100 ppm. The e
ff
ect of ascorbyl palmitate and TBHQ have remained
unchanged over 30 days, but the e
ff
ects of tannic acid diminished (PV
increased). On the other hand, the PV of alpha tocopherol-FSO mixtures
was always higher than that of pure FSO on all days except on day 20,
indicating the pro-oxidant e
ff
ect of alpha tocopherol. At 40 °C (
Fig. 4
b),
the FSO
–
antioxidant mixtures displayed similar behavior as that of
25 °C. At 60 °C (
Fig. 4
c), however, the e
ff
ectiveness of tannic acid and
alpha tocopherol has increased. The tannic acid remained active
throughout 30 days at 60 °C, and it kept the PV of mixtures below the
PV of FSO. However, the e
ff
ectiveness of tannic acid decreased with
increase in concentration above 200 ppm. Alpha tocopherol, which
displayed pro-oxidant e
ff
ect on almost every day at lower temperatures,
displayed pro-oxidant e
ff
ect only on day 30 at 60 °C. On the other hand,
the e
ff
ectiveness of ascorbyl palmitate and TBHQ decreased as a func-
tion of time, although the PV remains lower than that for FSO without
any antioxidants.
The AV of FSO and its mixtures with varied concentrations of dif-
ferent antioxidants measured as a function of time for 30 days at 25 °C,
40 °C and 60 °C are displayed in
Fig. 5
a
–
c. FSO and its mixtures with all
antioxidants except the one with alpha tocopherol displayed negligible
AVs at 25 and 40 °C (
Fig. 5
a and b). The AV of FSO-alpha tocopherol
mixtures increased during storage and a rapid increase in AV was seen
from day 20 at 40 °C, and exceeded the AV of FSO, indicating its pro-
oxidant e
ff
ect. This dramatic transformation of peroxides into sec-
ondary oxidation products is might be the reason for the low PV of
alpha tocopherol-FSO on day 20 (
Fig. 4
).
During storage at 60 °C, even though the PV remained constant, the
AV of FSO increased by 30 unit in 30 days (
Fig. 5
c), indicating a high
rate of formation and transformation of peroxides into secondary oxi-
dation. At this temperature, the FSO-alpha tocopherol mixtures did not
show any antioxidative e
ff
ect, while other antioxidants, depending on
their concentrations and time, reduced the formation of secondary
oxidation products compared to FSO without any antioxidants. The
synthetic hydrophobic antioxidant, TBHQ displayed highest e
ff
ect fol-
lowed by surface active antioxidant, ascorbyl palmitate and then hy-
drophilic antioxidant, tannic acid. The AV of FSO remained unchanged
with TBHQ up to 10 days at 60 °C, and increased gradually thereafter.
However, the AV of antioxidant-added FSO were signi
fi
cantly lower
than that of pure FSO on all days. Even on day 30 at 60 °C, the AV of
FSO with 400 ppm TBHQ was 4.5 times lower than that of pure FSO,
which indicated that the usage of higher concentration of TBHQ has
(a)
(b)
(c)
TBHQ
Time (days)
0
1
10
20
30
Ascorbyl Palmitate
Time (days)
0
1
10
20
30
Alpha Tocopherol
Time (days)
0
1
10
20
30
Tannic Acid
Time (days)
0
1
10
20
30
Peroxide Value (mEq peroxide/
Kg oil)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0ppm
50ppm
100ppm
200ppm
400ppm
Tannic Acid
Time (days)
0
1
10
20
30
Peroxide value (mEq peroxide/
Kg oil)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
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
Peroxide Value (mEq peroxide/Kg oil)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0ppm
50ppm
100ppm
200ppm
400ppm
Alpha Tocopherol
Time (days)
0
1
10
20
30
0ppm
50ppm
100ppm
200ppm
400ppm
Ascorbyl palmitate
Time (days)
0
1
10
20
30
TBHQ
Time (days)
0
1
10
20
30
Fig. 4.
The peroxide 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. No values were recorded for 400 ppm of ascorbyl palmitate since these
values were out of range of the instrument.
A. Mohanan et al.
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