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EFFECTS OF ASCORBYL PALMITATE AND METAL IONS ON OXIDATION OF SUNFLOWER OIL UNDER ACCELERATED OXIDATION CONDITIONS.

Byline: A. Basturk, G. Boran and I. Javidipour

Abstract

In this study, oxidative stability of sunflower oil was studied under accelerated oxidation conditions. First, refined commercial sunflower oil was examined for its composition of fatty acids and tocopherols. Then, oil samples were prepared with Fe+2 and Cu+2 ions (0, 0.15 and 0.3 mg/kg) and ascorbyl palmitate (0, 200 and 400 mg/kg). Oil samples were kept at different temperatures (30, 50 and 70AdegC) for 20 days. Oxidation parameters including peroxide value, malonaldehyde concentration and hexanal content were periodically followed during the storage to investigate the effects of metal ions and ascorbyl palmitate on oxidation of sunflower oil. Results showed that temperature had significant effects on elevation of all oxidation parameters studied (P<0.05).

It was also found that the concentrations of metal ions and ascorbyl palmitate significantly affected the oxidation of sunflower oil. Addition of 400 mg/kg ascorbyl palmitate restricted increment of peroxide value in both Fe2+and Cu2+ added samples. While Fe2+ significantly increased the hexanal content, the presence of Cu2+ increased both hexanal and malonaldehyde values in sunflower oil during oxidation. In samples held at lower temperatures, the hexanal content was almost steady but dramatically increased at higher temperatures. It is concluded that hexanal content could be well considered as an indicator of oil oxidation along with malonaldehyde concentration.

Keywords: Sunflower oil, hexanal, malonaldehyde, peroxide value, oxidative stability.

INTRODUCTION

Oil oxidation has been long recognized for many years but its mechanism is still under investigation. Many researchers investigated the effects of storage conditions on oil oxidation. The primary oxidation products are determined to be hydroperoxides, which are generally unstable, then forming the secondary oxidation products including alkanes, alcohols, aldehydes and acids (Shahidi, 1998; Choe and Min, 2006; Porter, 2013). Many of these chemicals are highly reactive and may initiate the oxidation reactions on their own. Oxidation reactions and resulting chemicals may contribute to the pathogenesis of cancer, atherosclerosis, heart and allergic diseases (Halliwell and Gutteridge, 2015). Therefore, oxidative stability of oils is a concern because of economic and quality loss caused by oxidation (Yanishlieva and Marinova, 2001; Matthaus et al., 2010).

In general, the higher level of unsaturation in oils creates more susceptibility to oxidative deterioration. Factors such as oxygen concentration, presence of antioxidants, metal contaminants, hydroxy compounds and enzymes, exposure to the light and elevated temperatures also influence the oxidative stability of oils (Yanishlieva-Maslarova, 2001; Jakeria et al., 2014; Johnson and Decker, 2015). Oxidative stability is the resistance of oils to oxidation during processing and storage (Guillen and Cabo, 2002; Walallawita et al., 2016). Resistance to oxidation can be defined as the time period necessary to attain to a critical point of oxidation, which is associated with an evident sensorial change or an increase in the oxidation rate (Silva et al., 2001).

Oxidative deterioration can be followed by determination of some quality parameters to estimate the shelf life of oils (Hamilton, 1994; Diridi et al., 2016). Generally, effects of oxidation can be evaluated by acid value (AV) representing hydrolytic reactions, peroxide value (PV), conjugated diene and triene acids as indicators of primary oxidation in addition to malonaldehyde (MAD), anisidine value and thiobarbituric acid reactive substances (TBARS) as indices of secondary oxidation products. Recently, aldehydes such as hexanal (HEX) and propanal have been used as indicators of oil oxidation. HEX is reported to be a strong indicator of oxidation in animal fat due to high content of omega-6 fatty acids while propanal is considered as a better indicator in fish lipids due to high content of omega-3 fatty acids (Shahidi, 1998; Ayala et al., 2014). Hexanal have been evaluated as an indicator for late oxidation level.

The effect of microwave heating on hexanal contents of hazelnut, olive, soybean and sunflower oils have been reported by Javidipour et al. (2016). They noted that hexanal could be considered as a parameter for evaluation of the quality of oils exposed to microwave heating.

Ascorbyl palmitate (AP) is a methyl ester of ascorbic acid dissolving in lipids. As it is a powerful antioxidant that naturally hydrolyzes into ascorbic and palmitic acids, its use in food products is favored more than synthetic antioxidants (Yanishlieva-Maslarova, 2001; Upadhyay et al., 2017). Although antioxidative mechanism of AP is not well known, Lee et al. (1997) reported AP's ability of reducing photosensitized oxidation of oils by quenching singlet oxygen. Meanwhile, Coppen (1994) noted AP as trace metal remover or sequester, consequently diminishing formation of peroxides. Upadhyay and Mishra (2015) reported synergistic effect of oleoresin rosemary and AP with increasing oxidative stability of sunflower oil tested at low and high temperatures. As AP recently obtained considerable attention from researchers, its antagonistic effect on oil oxidation was investigated at varying concentrations in the present study.

It is often relied on accelerated oxidation tests to quickly assess the oil stability against oxidation (Coppin and Pike, 2001; Tena et al., 2017). Since oxidation is the major cause of oil degradation, the most of the accelerated tests are designed to speed up this process by exposing oil samples to elevated temperatures in the presence of excessive amount of air or oxygen (Paul and Mittal, 1997; Tena et al., 2017). In this study, the effects of Fe+2, Cu+2, AP, storage temperature and time on oxidative stability of sunflower oil (SFO) were investigated by using response surface methodology (RSM) to be able to evaluate the effects of 4 factors at a time with reduced number of experimental runs and a through assesment of the effects of different factors on oxidation parameters.

MATERIALS AND METHODS

Sunflower oil used in this study was purchased from a local market in Van, Turkey. Hexanal and 2-methyl-3-heptanone were obtained from Aldrich Chemical Corp. (WI, USA), malonaldehyde bis diethyl acetal (97%) was obtained from Acros Organics (NJ, USA) and tocopherol standards were obtained from Riedel-De Haen AG (Seelze-Hannover, Germany). All reagents were of analytical grade.

Preparation of oil samples: SFO was first analyzed for its initial content of Cu+2 and Fe+2 ions. Oil samples were then mixed with 200 and 400 mg/kg of AP. After that, cupric and ferrous sulfates were dissolved in SFO samples at concentrations of 0.15 and 0.30 mg/kg. The recovery ratio for both Cu2+ and Fe2+ were 98% from the samples.

Conditions for accelerated oxidation: Prepared samples were exposed to heat in a convectional oven (EN 400 Y, Nuve, Istanbul, Turkey) for accelerated oxidation. For this purpose, 100 mL of oil samples were poured into transparent glass bottles, the bottles were held in oven set at 30, 50, and 70AdegC for up to 20 days (Basturk et al., 2007). Oil samples were analyzed for PV, MAD and HEX before exposing the samples to the heat and later on 10th and 20th days of storage for determination of the effects of heating on oxidation of sunflower oil.

Fatty acid composition: Fatty acids were first methyl esterified and then profiled using a gas chromatography (GC), Agilent 6890 series GC (Agilent Technologies, Palo Alto, CA). For that, 0.4 g oil sample was dissolved in 4 mL isooctane, and then methyl esterified in 0.2 mL 2 M potassium hydroxide. Analysis of fatty acid methyl esters (FAME) was performed using the GC equipped with a flame ionization detector (FID) and a 60 m capillary column (ID=0.25 mm) coated with 0.25 um of 50%-cyanopropyl-methylpolysiloxane (JandW Scientific, Folsom, CA, USA). Helium was used as carrier gas at a flow rate of 1.5 mL/min and a split ratio of 1:10. Injector and detector temperatures were 250 and 260AdegC, respectively. Oven temperature was set at 120 AdegC for 5 min, then increased to 240 AdegC at a rate of 15 AdegC/min, and hold at that final temperature for 20 min (Basturk et al., 2007). Samples were injected into the column inlet using an Agilent 7683 B series automatic injector.

FAMEs were identified by comparison of their retention time and equivalent chain length with respect to the standard FAMEs (47885-U, Supelco). FAMEs were quantified according to their percentage area (AOAC, 1990).

Tocopherol analysis: In saponification step, 0.5 g of oil sample was placed in a glass tube, mixed with 1.25 mL 60% KOH and pyrogallol (3:10 in ethanol), and held in a waterbath set at 70AdegC for 30 min. Then, the sample was cooled, mixed with 7 mL of 5% NaCl and 5 mL of hexane, and held in dark on ice for 30 min. After that, upper part of the sample was transferred to the vaporization pot. Hexane was added twice and then removed under nitrogen for extraction. The remaining material was dissolved again in dichloromethane and methanol (1:1, v/v) and the extract was placed in vial (Surai et al., 1996). 20 ul of extracted sample was injected into high performance liquid chromatography (HPLC) to determine the tocopherol content. Normal phase was used to analyze tocopherols using a ThermoFinnigan HPLC (Thermo Finnigan, San Jose, CA).

The chromatographic separation was achieved with a Phenomatographic Luna silica gel column (4.6 mm i.d. x 250 mm, 5 um particle size, Phenomenex, Torrance, CA) by using a mobile phase of n-hexane/ethyl acetate/acetic acid (97.3:1.8:0.9 v/v/v) at a flow rate of 1.6 mL/min. Fluorescence detector was utilized for excitation (Ex) and emission (Em) at wavelengths of 295 and 330 nm, respectively (Panfili et al., 2003). Calibration was done using standard solutions of tocopherols and tocotrienols.

Peroxide value and concentration of metal ions: PV and metal ions were analysed according to the AOAC (1990) methods. The concentrationsof Cu+2 and Fe+2 were initially determined in sunflower oil to avoid any misconclusion that might arise from their intial presence. The results showed that SFO were free of any of these ions at the beginning. These ions were added to the samples in a sensitivity level of +-0.005 mg/kg.

Malonaldehyde content: 0.1 g of oil sample was taken into a sample tube. After adding 1 mL of 10 mM phosphate buffer (containing 1.15% KCl), the content was vortexed. 1 mL of this mixture was taken into another tube, then 200 ul of 0.1 mM FeSO4 was added. This mixture was kept in a waterbath at 37AdegC for 1 h. Then, 50 uL of 0.01% buthylated hydroxytoluene (BHT) and 200 uL of 8% sodium dodecyl sulphate were added. After that, 1.5 mL of 20% acetic acid and 1.5 mL of 0.8% thiobarbituric acid were added. The tubes were kept in a waterbath at 95AdegC for 1 h and then cooled in water. 750 uL of this mixture was transferred into centrifuge tube and mixed with 750 uL of methanol. Tubes were centrifuged at 5000 rpm for 10 min and about 600-700 uL of supernatant was taken into vials. 5 uL of this supernatant was used for injection into HPLC equipment. Mobile phase was 50 mM phosphate buffer/methanol (65/35) mixture and flux rate was 1.5 mL/min.

Using an Agilent 1100 HPLC equipped with ODS 2 reverse phase column and fluorescent detector, peaks were recorded at Ex:532 and Em:553 wavelengths. Qualitative and quantitative analysis of MAD were determined according to the MAD standard (97% malonaldehyde bis diethyl acetate) (Surai and Speake, 1998). A diagram of MAD concentration was obtained against to peak area to determine the linearity of the calibration curve prepared for MAD analysis. Regression equation for the calibration curve was 'Y=23.136X-11.479' and the regression coefficient (R2) was 0.99. Repeatability of HPLC method developed for MAD determination was checked by recording more than 10 chromatograms of 3 mg/kg MAD solutions daily prepared at the most convenient experimental conditions. Peak area was recorded on these chromatograms and values determined were regarded as accurate.

According to the results obtained, relative standard deviation was 1.41%, which concluded that the repeatability of the method was sufficient.

Hexanal analysis: Solid Phase Microextraction (SPME) fiber (50 / 30 / 20, divinilbenzene / carboxen / polydimethylsiloxane, 2 cm, Supelco Co., Bellefonte, PA, USA) was used in extraction of volatile compounds for determination of HEX. SPME fiber was first conditioned in injection block of GC equipment for an hour at 270AdegC. For extraction process, 2 g of sample was transferred into 20 mL vial. The vial was closed tightly, mixed and heated by keeping on a magnetic stirrer at 45AdegC for 5 min. After that, SPME fiber was left into the headspace of the vial and allowed to absorb the volatiles for 30 min at 45AdegC. After extraction, SPME fiber was immediately injected into Agilent 6890 model GC equipped with FID. For quantitative analysis of HEX in the presence of 2-methyl-3-heptanone, the previously obtained results by GC-SPME showed that linear relationship between these two compounds could be given by a regression equation 'Y=0.3937X' (R2=0.99).

By this method, the threshold level for determination of HEX was reported to be 1.6 ppb (Javidipour and Qian, 2008).

Statistical design and analysis: In this study, 4-factor 3-level central composite design was utilized by MINITAB statistics software. Independent variables were storage temperature (Temp, AdegC), storage time (Time, day), metal concentration (Met, mg/kg, Fe2+ or Cu2+), and AP concentration (AP, mg/kg) (Table 1). Dependent variables were PV, MAD and HEX content. Measurements were done in duplicate separately for Fe2+ or Cu2+ added samples. A total of 30 experiments were run with 6 central points for each metal ion. Analysis of variance (ANOVA) were used at a significance level of 0.05. For those of experimental combinations that were not run, dependent variables were estimated by regression equations obtained based on the experimental data. Fit of the models were evaluated by regression coefficients and lack of fit values. Regression equations were obtained by using following second degree formula based on the significance level of the linear, quadratic and interaction effects of the dependent variables.

(Equations)

In this equation, Y represents dependent variable, [beta]o constant value, [beta]i coefficients of linear terms, [beta]ii coefficients of quadratic terms and [beta]ij coefficients of interaction terms while Xi and Xj were independent variables.

RESULTS AND DISCUSSION

Composition of fatty acids and tocopherols: Fatty acid composition of original SFO sample was determined. According to the results, there are four major fatty acids in SFO accounting up to 98.4% of the total. These fatty acids were linoleic (C18:2), oleic (C18:1), palmitic (C16:0) and stearic acids (C18:0) in the descending order at a ratio of 62.6, 24.8, 6.3 and 4.7%, respectively. Tocopherol content and composition of SFO was also examined. The results revealed that the total tocopherol concentration of SFO was 531.9 mg/kg. Distribution of the tocopherols was [alpha], I3 and [beta]-tocopherols in the descending order at concentrations of 510, 13.6 and 8.3 mg/kg, respectively. PV, MAD, and HEX values of all samples were given in Table 2 and Table 3 for Fe2+or Cu2+ added samples, respectively. Based on the results, regression equation for each dependent variable was obtained for each metal ion, seperately.

Lack of fit level was not significant for all equations, indicating regression models were sufficient in estimation of dependent variables (Table 4).

Peroxide value: Temperature, storage time and metal ion concentration significantly increased PV compared to that of the control while concentration of AP decreased PV as expected. Statistical evaluation of the data revealed that the most important factors affecting PV were storage time and temperature (P<0.05). The linear effect of AP concentration was reductive on PV but not statistically significant. The interactive effect of temperature and storage time was also significant (P<0.05). In SFO samples containing Fe2+, the interactive effects of Temp*Time, and Time*AP concentration were significant (Table 4). PV of samples stored at 70AdegC was the highest on 20th day of storage, as expected. Samples including 400 mg/kg AP restricted increment of PV in both Fe2+and Cu2+ added samples. Samples containing 400 mg/kg AP showed lower PV compared to that of samples containing 200 mg/kg AP.

Therefore, AP was believed to be effective in preventing PV increment, although statictically not significant. Higher temperature, longer storage time and higher metal concentration significantly increased PV while AP limited this increase.

According to Javidipour et al. (2015) oils with 400 mg/kg AP had higher tocopherol content, and lower PV and MAD levels compared to the control sample with no AP added during chemical interesterification and storage at 60AdegC. The highest PV of Fe2+ containing SFO samples was 181.2 meq O2/kg oil. Comparing the metal ions used, Cu2+ ion was seen to be reducing the oxidative stability and increasing PV more evidently compared to Fe2+ ion. However, Mancuso et al. (1999) found that iron's increased ability to decompose lipid peroxides prevented accumulation and thus led to lower peroxide value. As given in Figure 1, storage temperature and time steadily increased PV as expected, while almost no difference was seen at varying AP concentrations at any storage temperature.

Malonaldehyde content: In Fe+2 added SFO samples, the linear effects of storage temperature and AP concentration were statistically significant on MAD content (P<0.05). The quadratic effects of temperature and time were also significant on MAD of Fe+2 added SFO. In addition, the interactive effect of Temp*Time and Time*AP concentration was significant on MAD of Fe+2 added SFO (Table 4). On the other hand, the interactive effects were mostly not significant on MAD of SFO samples. As seen from Figure 2, MAD seems to be increasing initially with increasing temperature but then being denatured at higher temperatures. In addition, MAD content unsteadily but significantly increased during storage especially at high AP concentrations in both Fe2+ and Cu2+ added samples. The presence of AP effectively increased MAD concentration indicating pro-oxidative effects at levels studied during late stage of oxidation. Storage temperature was also significant in increase of MAD level, as expected.

In Cu2+ containing SFO samples, the linear effects of metal and AP concentration were found significant on MAD content (Table 4). The quadratic effects of all factors were also significant beside with the interactive effects of Time Met and Time AP in Cu2+ containing SFO samples (P<0.05). The concentration of MAD was almost steady during the first 10 days of storage and after that, showed a dramatic increase during the second half of storage at all temperatures (Fig 2a). Use of AP did not show an evident preventive effect on MAD formation. Karabulut (2010) reported a strong synergistic effect for AP in the presence of [alpha]-tocopherol and pro-oxidative effect for AP in the absence of [alpha]-tocopherol for the oxidation of butteroil triacylglycerols. Lower level of AP concentration was even more effective in terms of reducing MAD formation compared to that of higher level of AP in SFO samples. Beddow et al. (2001) noted that AP may be more effective against oxidation at certain concentrations.

Hexanal content: The interactive effects of Temp*Time, Temp*AP, Time*Met and finally Time*AP on the HEX content were significant in Fe2+ added SFO samples (P<0.05). Increasing levels of these parameters caused significant increments in HEX concentration. In Cu2+ containing SFO samples, the HEX content was significantly affected by storage temperature, storage time, Cu2+ and AP concentrations (P<0.05). Some interactive effects of Temp*Time, Temp*AP, Time*Met and lastly Temp*AP were significant on the level of HEX concentration (Table 4). Initially, HEX concentration was steady but then, dramatically increased toward to the end of storage (Fig 3). Sometimes, the first and secondary oxidation products may be simultaneously formed but sometimes, the secondary products may be formed after the primary oxidation products reach to a certain level of concentration (Guillen and Cabo, 2002).

In Cu2+ containing SFO samples, the interactive effect of Temp*Time significantly affected the concentration of HEX in almost a linear relationship. In general, HEX content was higher in the samples held at higher temperatures for longer periods. The linear effects of AP concentration, storage temperature and storage time were significant in addition to the interactive effects of Temp Time and Time AP on HEX content of SFO samples both with Cu2+ and Fe2+ (Table 4). In samples held at lower temperatures, the HEX content was almost steady but dramatically increased at higher temperatures. The concentration of AP showed increasing effect on HEX formation especially at high concentrations. Metal concentration showed some increasing effect on HEX content especially with longer expose to high temperatures. Storage temperature and time seem to be the most important factors increasing HEX content (Figure 3).

On the other hand, AP showed pro-oxidative effect and increased HEX content especially at high temperatures and with longer storage time (Figure 3).

While fatty acid profile is the most important factor determining the oxidative stability of oils (Shahidi, 1998); the effects of natural antioxidants may not be always preventive against oxidation. For example, [alpha]-tocopherol is an effective antioxidant at low concentrations while its higher concentrations might show pro-oxidative effects. Similarly, I3-tocopherol is more effective against oxidation at high concentrations compared to [alpha]-tocopherol while [alpha]-tocopherol is a stronger antioxidant at low concentrations compared to I3-tocopherol (Fuster et al., 1998). With that in mind, tocopherols may be more effective in preventing oxidation when the concentration of hydroperoxides reach to a certain level (Blekas et al., 1995).

Lampi et al. (1997) reported that the amount of unsaturated fatty acids and their degree of unsaturation are the most important factors affecting the oxidation beside other factors such as the position of unsaturated fatty acids in the triacylglcerols and the presence of anti-and pro-oxidants. It is also reported that metal ions may not always show pro-oxidative effects in oils (Rossell, 1998). In addition, all these variations may create even more complex situations with different oils due to diverse fatty acid profiles. Therefore, it is obvious that mechanism of oil oxidation may not be explained via a single model (Adhvaryu et al., 2000). In this study, SFO samples treated with different levels of AP and different metal ions under accelerated oxidation conditions presented distinct developments in oxidation for almost each treatment combinations studied. The overall results showed that AP had reducing effect on peroxide formation while increasing HEX and MAD formation.

AP may partially hold metals and oxygen initially at early stages of oxidation, consequently preventing peroxide formation to some extend, but then may become ineffective while secondary oxidation products form.

When PV, MAD and HEX values of SFO stored at 30AdegC were examined, PV gradually and continuously increased during the storage while MAD content decreased. Similarly, the HEX content increased up to 10 days of storage but then decreased. In SFO samples stored at 50AdegC, PV and HEX continuously increased during the whole period of storage while MAD content decreased up to 10 days of storage and increased later on. In samples stored at 70AdegC, PV continuously increased in SFO samples with both metal ions, but MAD content continuously decreased in Cu2+ containing samples while it increased after 10 days of storage in Fe2+ containing samples. The HEX content of SFO samples initially increased but then decreased in Cu2+ containing samples, and continuously increased in Fe2+ containing samples. Fomuso et al. (2002) did not observe a significant variation in TBARS content during metal-catalyzed oxidation of structured lipid model emulsion.

Since aldehydes formed during oxidation did not respond well in TBARS test (Fomuso et al., 2002), it can be concluded that hexanal content may be a more reliable parameter representing secondary oxidation products in where TBARS is not functional.

Table 1. Independent variables and their levels.

Independent variable###Abbreviation###Levels

Storage temperature (AdegC)###Temp###30, 50, 70

Storage time (day)###Time###0, 10, 20

Metal ions (mg/kg)###Met###0, 0.15, 0.30

Ascorbyl palmitate (mg/kg)###AP###0, 200, 400

Table 2. Experimental and predicted results for oxidation parameters of Fe2+ added SFO.

Sample###Temp Time###Fe2+###AP###PV###MAD###HEX

No###(AdegC) (day) (mg/kg) (mg/kg) Predicted Experimental Predicted###Experimental Predicted###Experimental

###1###30###0###0###0###11.13###12.32###1.405###1.426###0.046###0.047

###2###30###0###0###400###16.26###12.35###1.411###1.431###-0.002###0.048

###3###30###0###0.3###400###16.26###12.83###1.411###1.433###-0.002###0.049

###4###30###0###0.3###0###11.13###13.01###1.405###1.426###0.046###0.048

###5###30###10###0.15###200###17.19###13.76###0.819###0.877###0.158###0.187

###6###30###20###0###400###23.26###17.38###1.421###1.345###0.334###0.339

###7###30###20###0.3###0###18.13###23.01###0.875###0.940###0.254###0.280

###8###30###20###0###0###18.13###19.15###0.875###0.881###0.026###0.118

###9###30###20###0.3###400###23.26###20.21###1.421###1.385###0.562###0.547

10###50###0###0.15###200###8.11###12.54###1.534###1.426###0.094###0.049

11###50###10###0.15###200###48.18###49.35###1.237###1.243###0.340###0.338

12###50###10###0.15###200###48.18###45.63###1.237###1.051###0.340###0.309

13###50###10###0.15###200###48.18###39.72###1.237###1.400###0.340###0.367

14###50###10###0.15###200###48.18###52.64###1.237###1.462###0.340###0.397

15###50###10###0.15###200###48.18###48.01###1.237###1.030###0.340###0.276

16###50###10###0.15###200###48.18###43.89###1.237###1.213###0.340###0.327

17###50###10###0###200###48.18###47.96###1.237###1.218###0.283###0.322

18###50###10###0.15###0###48.18###46.25###1.261###1.265###0.247###0.239

19###50###10###0.3###200###48.18###50.81###1.237###1.229###0.397###0.442

20###50###10###0.15###400###48.18###48.45###1.537###1.541###0.433###0.480

21###50###20###0.15###200###88.26###90.25###1.534###1.651###0.586###0.540

22###70###0###0###0###17.15###11.93###1.389###1.429###-0.016###0.049

23###70###0###0###400###12.02###12.37###1.395###1.436###0.048###0.048

24###70###0###0.3###0###17.15###12.05###1.389###1.423###-0.016###0.050

25###70###0###0.3###400###12.02###12.65###1.395###1.425###0.048###0.050

26###70###10###0.15###200###91.23###97.95###1.063###1.068###0.372###0.340

27###70###20###0###0###170.45###168.30###1.379###1.281###0.404###0.399

28###70###20###0###400###165.32###152.50###1.925###1.929###0.824###0.854

29###70###20###0.3###0###170.45###181.21###1.379###1.364###0.632###0.655

30###70###20###0.3###400###165.32###161.41###1.925###1.994###1.052###1.138

Table 3. Experimental and predicted results for oxidation parameters of Cu2+ added SFO.

Sample Temp Time###Cu2+###AP###PV###MAD###HEX

No###(AdegC) (day) (mg/kg) (mg/kg) Predicted Experimental###Predicted###Experimental###Predicted###Experimental

###1###30###0###0###0###14.62###12.43###1.404###1.412###0.084###0.048

###2###30###0###0###400###14.62###11.97###1.410###1.423###0.018###0.047

###3###30###0###0.3###400###14.62###12.04###1.418###1.426###0.020###0.048

###4###30###0###0.3###0###14.62###12.81###1.412###1.425###0.086###0.049

###5###30###10###0.15###200###22.20###10.18###1.121###1.141###0.258###0.211

###6###30###20###0###400###29.78###27.59###1.470###1.558###0.214###0.193

###7###30###20###0.3###0###29.78###28.73###1.352###1.396###0.246###0.169

###8###30###20###0###0###29.78###22.12###1.088###1.113###-0.028###0.114

###9###30###20###0.3###400###29.78###32.76###1.734###1.823###0.488###0.434

10###50###0###0.15###200###5.00###12.50###1.566###1.426###0.055###0.049

11###50###10###0.15###200###49.77###43.73###1.293###1.319###0.416###0.429

12###50###10###0.15###200###49.77###50.63###1.293###1.137###0.416###0.378

13###50###10###0.15###200###49.77###46.92###1.293###1.491###0.416###0.479

14###50###10###0.15###200###49.77###52.38###1.293###1.399###0.416###0.382

15###50###10###0.15###200###49.77###56.67###1.293###1.238###0.416###0.420

16###50###10###0.15###200###49.77###45.55###1.293###1.258###0.416###0.464

17###50###10###0###200###49.77###41.90###1.054###1.037###0.347###0.310

18###50###10###0.15###0###49.77###44.48###1.384###1.360###0.338###0.323

19###50###10###0.3###200###49.77###51.56###1.190###1.177###0.485###0.468

20###50###10###0.15###400###49.77###42.33###1.578###1.575###0.494###0.507

21###50###20###0.15###200###94.54###99.37###1.566###1.677###0.543###0.523

22###70###0###0###0###17.90###12.37###1.404###1.420###0.022###0.049

23###70###0###0###400###17.90###12.27###1.410###1.426###0.092###0.048

24###70###0###0.3###0###17.90###12.46###1.412###1.429###0.024###0.050

25###70###0###0.3###400###17.90###13.02###1.418###1.431###0.094###0.049

26###70###10###0.15###200###99.87###112.36###1.121###1.073###0.574###0.574

27###70###20###0###0###181.84###189.11###1.088###1.040###0.530###0.447

28###70###20###0###400###181.84###156.90###1.470###1.371###0.908###0.854

29###70###20###0.3###0###181.84###192.65###1.352###1.300###0.804###0.803

30###70###20###0.3###400###181.84###168.56###1.734###1.613###1.182###1.284

Table 4. Regression coefficients and their significance level of Fe2+ and Cu2+ containing oil samples.

###Coefficients

Independent

###Fe2+ containing samples###Cu2+ containing samples

Variables

###PV###MAD###HEX###PV###MAD###HEX

0 (Constant)###47.4073***###1.25033***###0.33193***###49.7675***###1.30622***###0.41831***

1 (Temp)###37.0194***###0.12250**###0.10722***###38.8372***###-0.03411ns###0.15806***

2 (Time)###40.0761***###-0.00472ns###0.24677***###44.7733***###0.00406ns###0.24356***

3 (Met)###1.8294ns###0.01350ns###0.05805***###2.1072ns###0.06778**###0.06911***

4 (AP)###-2.0600ns###0.13800***###0.09211***###-2.7622ns###0.09728***###0.07844***

11 (Temp*Temp)###6.5656*###-0.27779**###-0.06903**###11.2686**###-0.18091**###-0.01211ns

22 (Time*Time)###2.1056ns###0.28821**###-0.03803ns###5.9336ns###0.26359*** -0.11861**

33 (Met*Met)###0.0956ns###-0.02679ns###0.04947ns###-3.2714ns###-0.18091**###-0.01561ns

44 (AP*AP)###-1.9394ns###0.15271ns###0.02697ns###-6.5964ns###0.17959**###0.01039ns

12 (Temp*Time)###36.5738***###0.12625**###0.11043***###37.1969***###-0.03663ns###0.15462***

13 (Temp*Met)###0.8975ns###0.00187ns###0.01131ns###0.2381ns###-0.00300ns###0.03063ns

14 (Temp*AP)###-1.8650ns###0.02287ns###0.02731**###-4.0069ns###-0.01450ns###0.03400**

23 (Time*Met)###1.6837ns###0.01637ns###0.05718***###1.6056ns###0.06375**###0.06738**

24 (Time*AP)###-2.5663*###0.13538***###0.08843***###-2.8769ns###0.09350**###0.07725***

34 (Met*AP)###-0.3175ns###-0.00250ns###0.00406ns###0.4394ns###-0.00313ns###0.01625ns

Regression###***###***###***###***###***###***

###Linear###***###***###***###***###**###***

###Square###ns###**###ns###ns###***###**

###Interaction###***###**###***###***###**###***

R-Sq###0.99###0.90###0.98###0.99###0.89###0.98

R-Sq(adj)###0.99###0.79###0.97###0.98###0.78###0.95

Lack-of-Fit###0.707###0.971###0.489###0.181###0.973###0.269

Conclusions: The results obtained showed that the most important factors affecting oil oxidation were storage temperature and time. Among the dependent variables studied, increasing storage temperature and time significantly increased the HEX content in the presence of Fe+2 and increased both HEX and MAD values in the presence of Cu+2. Comparing the metal ions tested, Cu2+ was seen to be reducing the oxidative stability of SFO more than Fe2+. The addition of AP as an antioxidant was not effectively preventive on oil oxidation as much as expected. The content of natural antioxidants affects oxidation, not always preventive, but in fact, sometimes pro-oxidative. PV values were reduced by addition of AP but this was not the case when MAD and HEX values are considered.

Acknowledgement: The authors are grateful to Scientific Research Fund of Van Yuzuncu Yil University for the financial support given under the project 2009-FBE-D023.

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