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Effectiveness of added natural antioxidants in sunflower oil
By Amalia A. Carelli*, Indira C. Franco and Guillermo H. Crapiste
PLAPIQUI (UNS-CONICET).
La Carrindanga, km. 7, C.C. 717, 8000 Bahía Blanca, Argentina.
[Teléfono: (54-291) 4861700; Fax: (54-291) 4861600; e-mail: acarelli@plapiqui.edu.ar]
RESUMEN
Efectividad de antioxidantes naturales adicionados a
aceite de girasol..
Se investigó la actividad antioxidante de ∝-tocoferol, δ-tocofe-
rol, ácido cítrico y palmitato de ascorbilo en aceite de girasol con
su conteniendo natural de tocoferol. La efectividad de los mismos
fue analizada a través de la medida de la estabilidad oxidativa en
Rancimat y el seguimiento de la oxidación con el almacenamiento
a diferentes temperaturas. Las muestras extraídas periódicamen-
te de la estufa fueron sometidas a los siguientes análisis: índice
de peróxidos, valor de p-anisidina, contenido y distribución de
compuestos polares y contenido residual de tocoferol natural. La
efectividad de cada antioxidante resultó fuertemente dependiente
de la temperatura y método de ensayo. Mientras el ácido ascórbi-
co resultó ser el antioxidante más efectivo según el índice de es-
tabilidad oxidativa medido en el equipo Rancimat, el δ-tocoferol
fue el antioxidante más efectivo en las experiencias de almacena-
miento.
PALABRAS-CLAVE: Aceite de girasol - Antioxidantes
naturales - Autooxidación - Compuestos polares..
SUMMARY
Effectiveness of added natural antioxidants in
sunflower oil.
The antioxidant activity of ∝- and δ-tocopherol, citric acid,
ascorbic acid and ascorbyl palmitate was investigated in
sunflower oil containing naturally occurring tocopherol. The
effectiveness of natural antioxidants in sunflower oil was
monitored by the accelerated oxidative stability test Rancimat and
oxidation development during storage under different conditions.
Samples in storage experiments were periodically removed and
analyzed for peroxide value, p-anisidine value, total content and
distribution of polar compounds, and residual naturally occurring
tocopherol. The effectiveness of each antioxidant was strongly
dependent on temperature and the testing method. While ascorbic
acid appears to be the most effective antioxidant according to the
Rancimat oxidative stability index, δ-tocopherol shows improved
performance when considering storage experiments.
KEY-WORDS: Autooxidation - Natural antioxidants - Polar
compounds - Sunflower oil.
1. INTRODUCTION
The development of oxidative rancidity or
autoxidation is the decisive factor affecting storage
life and the use of edible oils and fats in food.
Autoxidation of lipids is a natural process that occurs
between molecular oxygen and unsaturated fatty
acids through a free-radical chain mechanism that
involves the formation of fat free radicals, peroxide free
radicals and hydroperoxides. The hydroperoxides are
very unstable and decompose to form secondary
reaction products, such as aldehydes, ketones,
alcohols and acids, which cause off-odors and
off-flavors, and affect the quality of the oil. Oxidative
stability and the deterioration of oils depend on the
initial composition, concentration of minor compounds
with antioxidant or prooxidant characteristics and
storage conditions such as temperature, light,
oxygen availability and type of recipient.
Antioxidants are compounds that inhibit or
interfere with the oxidative process and are widely
used to delay lipid oxidation. Nowadays, the addition
of natural antioxidants in edible oils and fats is being
suggested, since the safety of synthetic antioxidants
has been questioned. Antioxidants are classified,
according to their action, into primary or
chain-breaking and secondary or preventive. Primary
antioxidants can react with peroxyl radicals before
they do so with further unsaturated lipid molecules
and convert them into more stable products;
tocopherols are included in this category. Secondary
antioxidants act by other processes such as binding
metal ions, scavenging oxygen, decomposing
hydroperoxides to nonradical products, absorbing
UV radiation, and deactivating singlet oxygen
(Jadhav
et al
., 1996). Common secondary
antioxidants are carotenoids, citric acid, ascorbic
acid and ascorbyl palmitate among others. The
combination of two antioxidants, both primaries or a
primary and a secondary, can result in a synergistic
effect. Besides, some antioxidants can be recycled
by others; for instance, ascorbic acid is capable of
regenerating α-tocopherol from its radicals or
oxidation products (Niki, 1996; Kamal-Eldin and
Appelqvist, 1996; St. Angelo, 1996; Frankel, 1996). In
addition, some antioxidants can have more than one
mechanism of action. Research has been conducted
to find out the antioxidant properties and to better
understand the mechanism of oxidation and
antioxidant action in lipids (Frankel, 1996;
Kamal-Eldin and Appelqvist, 1996; Niki, 1996; St
Angelo, 1996; Frankel, 1998; Crapiste
et al
., 1999;
Grasas y Aceites
Vol. 56. Fasc. 4 (2005), 303-310 303
Yanishlieva
et
al
., 2002). The antioxidative efficacy of
antioxidants is affected by temperature, liquid
composition, physical state (bulk oil phase or
emulsion), and antioxidant concentration, among
others (Kamal-Eldin and Appelqvist, 1996; White and
Xing, 1997; Carelli
et al
., 1998). For this reason,
controversial findings of their effects can be mainly
attributed to the large differences in the substrate
considered, antioxidant concentration, and testing
conditions. Most of these studies have been
carried out using purified oils or triacylglycerols, but
scare information exists about the natural
antioxidant effectiveness in oils containing
endogenous minor components. It has been
claimed that the addition of tocopherols or other
antioxidants to most polyunsaturated oils is
inefficient, because the natural content of the
antioxidants in these oils is almost optimal
(Yanishlieva and Marinova, 2001).
Besides, the methodology used to evaluate
antioxidants and oxidative stability must be carefully
interpreted depending on the conditions of oxidation
and the analytical method used to determine the
extent and end point of oxidation (Frankel, 1993).
Nowadays, a methodology based on a combination
of adsorption and size exclusion chromatography
has been applied to study the alteration of lipids,
enabling the joint evaluation of primary and
secondary compounds (Márquez-Ruiz and
Dobarganes, 1997; Carelli
et al
., 1998; Crapiste
et
al
., 1999; Márquez-Ruiz
et al
., 2000).
The main objective of this work is to study the
effectiveness of natural antioxidants in refined
sunflower oil, comparing the accelerated oxidative
stability test with oxidation measured under different
storage conditions.
2. EXPERIMENTAL PROCEDURES
2.1. Materials
A refined sunflower oil without the addition of any
preservatives was supplied by a local factory.
L-ascorbic acid 6-palmitate (95%), L-ascorbic acid
(99%) and citric acid (99.5%) were purchased from
Aldrich Chemical Company (Milwaukee, WI). In
addition,
α-tocopherol (95%) and δ-tocopherol (90%)
were supplied by Sigma Chemical Company (St.
Louis, MO). Concentrated solutions of antioxidants in
sunflower oil (2000 ppm) were prepared; α-tocopherol
and δ-tocopherol were added directly to the oil and
gently stirred for several minutes; but citric acid,
ascorbic acid and ascorbyl palmitate were added to
the oil as acetone solutions and evaporated to
dryness under nitrogen during stirring. Sunflower oil
samples with different concentrations of antioxidants
(0-800 ppm) were prepared from the concentrated
solutions.
2.2. Storage tests
Oil samples treated with 100 ppm of citric acid,
ascorbyl palmitate, α-tocopherol, δ-tocopherol and
ascorbic acid were stored at 30 + 1oC for 35 days, at
68 + 1oC for 23 days and at 130 + 1 oC for 48 hours.
Experiments at 30 and 68 oC were carried out in
standard laboratory ovens; the samples kept in 100-
mL caramel-colored open glass bottles of 4 cm i.d.
Experiments at 130 oC were performed in the
Rancimat heating block using the equipment vessels.
The ratio oil surface exposed area to oil volume was
approximately 0.2.
In every experiment an oil sample without the
addition of an antioxidant was employed as control.
Samples with the same treatment were prepared in
duplicate in order to obtain two independent
measurements for each time and condition. Samples
were withdrawn periodically from the oven and were
stored at 5 oC under nitrogen atmosphere for further
analysis.
2.3. Analytical methods
Standard methods.
Standard IUPAC (1992) and
AOCS (1993) official methods were used to
determine acidity or free fatty acids (FFA) (IUPAC
2.201), peroxide value (PV) (AOCS Cd 8-53),
p-anisidine value (AV) (AOCS Cd 18-90). The fatty
acid composition was determined by gas
chromatography of the methyl esters according to
IUPAC 2.301-2.302 methods. The oxidative stability
index, represented as induction time in hours, was
measured with a Metrohm 679 Rancimat (Metrohm,
Herisau, Switzerland), at 98oC, 68oC and 20 L/h
airflow. Trace metals (iron and copper) were
measured by flame atomic absorption with a GBC
902 Atomic Absorption Spectrometer (GBC
Scientific Equipment, Victoria, Australia).
Tocopherols.
The content of naturally occurring
tocopherol was measured by high-performance
liquid chromatography (HPLC) using the AOCS Ce
8-89 method (1993). A Varian Vista 5500 HPLC
system with fluorescence detector and a LiChrosorb
Si-60 (250 x 4 mm, 5 m particle size) column (Merck,
Darmstadt, Germany) were used.
Polar compounds.
Polar compounds (PC) were
obtained by solid-phase extraction (SPE) using
Sep-Pack silica cartridges, subsequently separated
by high performance size-exclusion chromatography
and quantified through the internal standard method
according to Márquez-Ruiz
et al
. (1996). For the SPE
step, 1g silica gel SPE cartridges (J.T. Baker Inc.,
Phillipsburg, NJ) were used. Efficiency of the
separation of non-polar and polar fractions by SPE
was checked by thin layer chromatography according
to AOCS method Cd 20-91 (1993). A Waters HPLC,
two 500 and 100 Å PL gel (L=30 cm, d.i. = 7.5 mm,
304 Grasas y Aceites
particle size = 5 µm) (Polymer Laboratories Inc.,
Amherst, MA) columns connected in series, a
refractive index detector (Varian RI-3, Sensibility
1x10-6), tetrahydrofuran as mobile phase at 1
mL/min (10 µl injection), and a Millenium 2010
Chromatography Manager (Millipore Corporation,
Milford, MA) were used in the HPSEC analysis.
2.4. Statistical analysis
The average values of two independent
determinations are reported in tables. The mean
values and their error bars of two independent
experiments are represented in figures. Polar
compounds were analyzed in triplicate (n=3).
Differences in polar compounds between samples
were assessed with Student’s t test, with probability
values of 5% being statistically different.
3. RESULTS AND DISCUSSION
The initial characteristics of the refined sunflower
oil used in this study are shown in Table I. Its anti- and
prooxidant minor components were not eliminated in
order to obtain results with industrial applications.
The compositional information is essential because
of the major and minor oil components’ influence on
the oxidative process. It is known that the rates and
pathways of lipid peroxidation are affected by other
chemical species in the reaction medium as well as
by the physical conditions of the reaction
(Kamal-Eldin and Appelqvist, 1996). The level of
naturally occurring tocopherols (about 700 ppm)
confers intrinsic protection to the oil.
A comparison of antioxidant performance as
measured by the oxidative stability index (OSI) test is
shown in Figure I. The stability of oils treated with
ascorbic acid (AA) increased rapidly as the
antioxidant concentration augmented. The stability
of oils treated with ascorbyl palmitate (AP)
increased significantly up to 400 ppm, although to
a lesser extent in comparison with ascorbic acid,
and slightly from 400 to 800 ppm. The ascorbic
acid has the inconvenience of being oil-insoluble,
but not its esters. Since the metabolism breakdown
of ascorbyl palmitate yields ascorbic acid and
palmitic acid, both normal metabolites, it is
considered together with the ascorbic acid as a
substance which has no restrictions on usage levels
(Giese, 1996).
The absence of linearity in the dependencies of
OSI for sunflower oil on the concentration of ascorbic
acid and ascorbyl palmitate proves that these
antioxidants are consumed not only in chain
termination reactions but also in one or more side
reactions, causing the decrease in their relative
effectiveness with rising concentrations (Yanishlieva
and Marinova, 1992). These antioxidants can act as
synergists with tocopherols by converting oxidized
tocopherol back to the reduced form. The multiple
effects of ascorbic acid and ascorbyl palmitate
include hydrogen donation to regenerate the
antioxidant, metal inactivation to reduce the rate of
initiation by metals, hydroperoxide reduction to
produce stable alcohols by non-radical processes,
and oxygen scavenging (Frankel, 1996).
The OSI values of the oil treated with δ-tocopherol
(DT) raised linearly with the amount of antioxidant.
This linearity could indicate that DT does not
participate in side reactions during accelerated
oxidation at 98oC, being the consumption rate of this
antioxidant practically independent of its
concentration under these experimental conditions
(Yanishlieva and Marinova, 1992).
The remaining naturally occurring antioxidants
did not exhibit significant changes, especially in the
case of citric acid. This seems reasonable since the
α-tocopherol is naturally present in the oil and citric
acid is mainly a chelating agent. Citric acid is a useful
chelating ingredient especially when pro-oxidative
metal ions such as iron and copper are present
(Kamal-Eldin and Appelqvist, 1996; Giese, 1996).
The quantity of those metals in the oil employed for
our tests was relatively low. Tocopherols function as
antioxidants by serving as free-radical terminators
and by scavenging singlet oxygen molecules (St.
Angelo, 1996). When relative tocopherols-antioxidant
properties were compared in oils the following order
δ > γ ≈ β >α was reported in spite of the fact that a
simple structure comparison would suggest the
inverse order (Kamal-Eldin and Appelqvist, 1996).
This difference might be due to the fact that
tocopherols and/or their radicals often undergo side
reactions (reactions other than those with peroxyl
radicals), which may be prooxidative, the α-tocopherol
being more disposed to this (Kamal-Eldin and
Figure 1
Rancimat oxidative stability index at 98°C of sunflower oil with
different antioxidant treatments.
Vol. 56. Fasc. 4 (2005) 305
Appelqvist, 1996). The tocopherol concentration is
an important factor that influences tocopherol
antioxidant activity in bulk oils. Studies in purified
triacylglycerols obtained from sunflower oil showed
that α-tocopherol antioxidant activity is greatest at
lower concentrations (<700 ppm) and loses efficacy
at higher concentrations due to its participation in
side reactions (Yanishlieva
et al.
2002). Since the
content of natural occurring α-tocopherol in the oil
used is near this limit concentration, the addition of
α-tocopherol to the oil might be inefficient, explaining
the poor behavior obtained with samples treated with
α-tocopherol.
To compare the effectiveness of antioxidants a
protection factor (F) defined as the ratio between the
OSI induction time for the sample treated with
antioxidant and the control sample (F=OSIA/OSI0)
was used. The protection factors for ascorbic acid,
ascorbyl palmitate and δ-tocopherol as a function of
antioxidant concentration (ppm) were correlated by
using least square regression to obtain:
FAA = 1.02 + 2.28x10-3 ppm - 1.15x10-6 ppm2, r2 = 0.994 [1]
FAP = 1.02 + 1.152x10-3 ppm - 7.62x10-7 ppm2, r2 = 0.985 [2]
FDT = 0.998 + 3.56x10-4 ppm , r2 = 0.994 [3]
The natural antioxidant performance in sunflower
oil measured by the OSI test can be compared with
Table I
General Characteristics of Sunflower Oil
Analytical determination
Free fatty acids (% oleic acid) 0.16
Peroxide value (meq/kg) 2.7
p-Anisidine value 2.7
Oxidative stability (h at 98°C) 10.0
Fatty acids (%)
C16:0
C18:0
C18:1
C18:2
C18:3
C20:0
C22:0
C24:0
6.58
3.79
19.7
68.4
0.22
0.28
0.75
0.20
Polar Compounds (wt%)
Triglyceride polymers (%)
Triglyceride dimers (%)
Oxidized triglyceride monomers (%)
Diglycerids (%)
Free fatty acids (%)
7.1
3
26
34
29
8
Tocopherols (mg/kg) α:670; β:22; γ:20
Metal content (ppm) Fe:3.2; Cu:0.3
All data reported are arithmetic means of duplicate or triplicate determinations.
306 Grasas y Aceites
the experimental results obtained for synthetic
antioxidants in a previous report (Carelli
et al
., 1998).
The antioxidative activity of δ-tocopherol is
comparable to that of butylated hydroxitoluene
(BHT). Ascorbic acid and ascorbyl palmitate were
much less potent than tertiary butylhydroquinone
(TBHQ) and showed a performance intermediate
between propyl gallate (PG) and BHT. Thus the same
protection factor as that obtained with 200 ppm BHT
(F = 1.11) can be achieved by adding 40 ppm AA, 61
ppm AP or 315 ppm DT. In the same way
approximately 410 ppm AA or 830 ppm AP are
equivalent to 100 ppm PG (F = 1.76). Finally, a
similar perfomance to 50 ppm TBHQ (F = 1.98) is
obtained with 600 ppm AA. Natural antioxidants
present the advantage of having no restriction on
usage levels, while the use of BHT, PG and TBHQ is
restricted to permitted levels, usually a maximum
total antioxidant content of 200 ppm (Giese, 1996).
The deterioration indexes employed to analyze
experimental results from storage experiments are
the peroxide value (PV), the p-anisidine value (AV),
and the total content and distribution of polar
compounds. The peroxide value (PV) is a common
indicator of lipid oxidation, but its use is limited to the
early stages of oxidation. This index accounts for
hydroperoxides, labile intermediate compounds that
decompose into several secondary oxidation
Table II
Deterioration analysis on samples treated with 100 ppm of each antioxidant and stored
at different temperatures
Test TR PV AV PC Polar compound distribution (wt %)
(%) meq/kg (wt%) CV(%) TGP TGD OTG DG FFA
35 days at 30°C
Control 72 84 2.84 12.9 2.3 0.3 1.6 7.8 2.5 0.7
CA 69 79 2.89 15.3 2.6 0.3 2.9 9.0 2.4 0.7
AP 73 87 2.72 7.6a3.9 0.2 1.2 3.7 2.0 0.5
DT 68 81 2.80 8.0a5.0 0.2 1.0 3.8 2.4 0.6
AA 75 85 2.76 15.3 3.2 0.4 3.4 8.1 2.6 0.8
23 days at 68°C
Control 2 108 11.7 21.9 5.5 0.5 4.1 14.5 2.2 0.6
CA 8 99 11.0 18.6 a 4.8 0.3 3.6 12.6 1.6 0.5
AP 7 77 11.1 20.8 8.5 0.4 4.6 13.5 1.8 0.5
DT 19 61 11.1 15.3a2.6 0.3 3.1 9.9 1.5 0.5
AA 8 80 10.4 17.2 a 2.3 0.3 3.6 11.2 1.7 0.4
24 hours at 130°C
Control 20 33 45 22.5 4.0 1.3 9.3 9.7 1.8 0.4
CA 18 41 46 26.3 3.8 1.8 10.8 11.1 2.1 0.5
AP 24 21 30 19.5 a 4.6 0.7 7.8 8.4 2.1 0.5
DT 16 32 39 17.3 a 2.3 0.9 5.7 8.5 1.7 0.5
AA 1 44 60 31.7 6.6 2.8 14.5 11.7 2.0 0.7
a Samples that show an antioxidant effect with respect to control (n=3; P<0.05)
Abbreviations: CA = citric acid, AP = ascorbyl palmitate, DT = δ-tocopherol, AA = ascorbic acid, TR= tocopherol residual,
PV = peroxide value, AV = anisidine value, PC = total polar compounds, CV = coefficient of variation, TGP = triglyceride
pol
y
mers, TGD = tri
g
l
y
ceride dimers, OTG = oxidized tri
g
l
y
cerides, DG = di
g
l
y
cerides, FFA = free fatt
y
acids.
Vol. 56. Fasc. 4 (2005) 307
products. Secondary changes can be measured by
p-anisidine value (AV), an indicator of the aldehyde
content (mainly as 2-alkenals and 2,4-dienals). The
origin, grade and evolution of deterioration can be
evaluated from the polar compound determination.
The oxidized triglyceride monomers (OTG) content is
an indicator of oxidative alteration; the content of
diglycerides (DG) and free fatty acids (FFA) is related
to hydrolytic alteration, and polymeric compounds as
triglyceride dimers (TGD) and triglyceride polymers
(TGP) are useful to assess thermal alteration.
Quantification of oxidized triglyceride monomers and
dimers has been reported as a good measurement
of early and advanced stages of oxidation, since it
provides a direct measurement of primary and
secondary oxidation products (Márquez-Ruiz and
Dobarganes, 1997).
Experimental results after 35 days of storage at
30oC, 23 days at 68oC and 1 day of heating at 130oC
are presented in Table II. The reproducibility from the
mean of two independent measures expressed by
the coefficient of variation were in the ranges
0.2-13% for tocopherol residual (TR), 0.1-5.3% for
PV, and 0.1-4.7% for AV. The polar compound
determination gave a coefficient of variation of
2-8.5% from the mean of three determinations.
PV increased rapidly and AV remained practically
constant during storage at 30 oC. Differences in TR,
PV and AV in relation to the control sample were not
observed at this temperature. However, analysis of
polar compounds showed an antioxidant effect for
AP and DT (P<0.05).
Some differences in TR and PV in relation to the
control sample were observed at 68 oC. No
significant differences in AV were observed between
the samples containing different antioxidantes. PV
values suggested the following order in antioxidant
effectiveness: DT>AP>AA>AC. However, it should be
noted that the use of this index is limited to the early
stages of oxidation, in which no significant
differences with the control sample were observed
(data not shown). The rate of hydroperoxide
decomposition increases with temperature and the
degree of oxidation, the hydroperoxide concentration
reaches a maximum and then decreases at
advanced stages of oxidation (Crapiste
et al.
, 1999).
TR values showed that DT was the only antioxidant
with a protecting effect on the naturally occurring
tocopherol. The PC analysis showed the following
order in antioxidant effectiveness: DT>AA>CA.
Some differences in PV and AV with respect to
the control sample were observed at 130 oC. The AV
index indicated an antioxidant effect of DT and
especially AP at this temperature. PV is not useful to
compare treatments because of the high final
deterioration. Antioxidant effect can be better
assessed by the polar compound analysis when the
naturally occurring antioxidants are consumed
appreciably and the oil oxidation is in an advanced
stage. Under this condition DT and AP showed a
protective effect at 130oC (P<0.05).
Ascorbic acid was the best antioxidant in the
accelerated Rancimat test, but showed a lower
antioxidant activity in storage experiments at 68 oC
and exhibited no effect at 30 oC. This behavior can be
attributed to the very low solubility of AA in oils. It has
been previously found that AA’s protecting efficacy
increased when the continuous airflow facilitates
emulsification (Velazco et al., 2000). The addition of
AA at 130 oC augmented the deterioration as
measured by both PV and %PC, while practically the
whole naturally occurring tocopherol was consumed.
This suggests that AA could deteriorate at high
temperatures inhibiting its antioxidant activity.
It can be observed from Table II that the
free-fatty-acid and diglyceride contents remained
practically constant during all the treatments,
indicating no hydrolitic deterioration. Changes in %
PC at 30 oC were mainly due to the increase of OTG
as a result of the oxidation process. Unexpectedly,
the concentration of DTG in samples with AP and DT
was slightly lower than that obtained for the initial oil.
In contrast, an appreciable increment of DTG was
observed in samples stored at 68 oC. This result has
been previously found, demonstrating that some
polymerization also occurs during autoxidation at
relatively low temperatures (Crapiste et al, 1999). At
high temperatures, oxygen has lower solubility in oils
and as a result the autoxidative peroxide formation
proceeds at lower rates and becomes gradually
replaced with polymerization reactions (Kamal-Eldin
and Appelqvist, 1996). The distribution of polar
Figure 2
Evolution of polar-compound content and α-tocopherol
residual at 68 °C
........... control, -------- treated with 100 ppm of δ-tocopherol.
Abbreviations: PC = total polar compounds, TGP + TGD = total
triglyceride polymers, OTG = oxidized triglyceride monomers,
TR = tocopherol residual.
308 Grasas y Aceites
compounds, with a significant increase in PTG+
DTG, showed that both oxidative and thermal
degradation took place during heating at 130 oC.
Figures II and III compare the evolution of
polar-compound content between the control and the
sample treated with δ-tocopherol at 68oC and 130oC
respectively. The antioxidant effect of DT can be
observed, due to the treated oil having lower
contents of oxidized triglyceride monomers,
especially at the higher times when the oil
deterioration is in an advanced stage. From Figure III
we can infer that DT also acted as an
antipolymerization agent at high temperatures. From
the residual naturally occurring tocopherol it can be
observed that DT showed some inhibition effect,
especially at 68oC. Under some conditions the
tocopherols might be recycled between them,
i.e.
∝-tocopherol was found to regenerate β-, γ- and δ-
tocopherols from their radicals in homogeneous
solutions (Kamal-Eldin and Appelqvist, 1996).
In conclusion, this study provides an insight into
understanding the behavior of added natural
antioxidants on sunflower oil oxidation. While
ascorbic acid appears to be a more effective
antioxidant according to the OSI method, DT shows
better performance when storage experiments are
considered. The effectiveness of the different
treatments was strongly dependent on temperature
and testing methods. Temperature can act in the
oxidative process directly by affecting rates of
different reactions or indirectly by affecting relative
solubility and mass transfer phenomenon of
reactants and products. Oxidation experiments are
useful to evaluate the effectiveness of antioxidants
when they are performed under conditions similar to
those in which the oil will be used or stored.
ACKNOWLEDGMENTS
The authors acknowledge the financial support
from ANPCyT (Agencia Nacional de Promoción
Científica y Tecnológica), CONICET (Consejo
Nacional de Investigaciones Científicas y Técnicas)
and Universidad Nacional del Sur, Argentina.
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Recibido: Febrero 2005
Aceptado: Junio 2005
310 Grasas y Aceites
