The effect of pomegranate peel extract (PPE) on the oxidative stability of corn oil during heating was studied. Oxidation was followed by determining peroxide value (PV),
Se estudió el efecto del extracto de cáscara de granada (ECG) sobre la estabilidad oxidativa del aceite de maíz durante condiciones de calentamiento. La oxidación se siguió mediante la determinación del índice de peróxido (IP), el índice de
Fried foods are very popular because of their desirable flavors, colors and textures. It is well known that frying oils used as a cooking medium at high temperatures in the presence of oxygen and water from the food to be fried are subjected to very complex reactions of oxidation, polymerization and hydrolysis (
In recent years, attention has been focused on industrial waste. Vegetable co-products are interesting because they contain important molecules such as phenolic compounds that include simple phenols, flavonoids and anthocyanins. Many co-products have been studied as a source of antioxidants and their use is encouraged regarding their high biological activities. Pomegranate skin (
Among vegetable oils, sunflower oil has been used as a model to investigate the ability of various plant extracts in preventing its peroxidation (
All chemical standards including phenolic acids and reagents were purchased from Sigma-Aldrich Co. Ltd (St. Louis, MO. USA). Refined corn oil was recovered from the Agrimed group (Sfax, Tunisia).
Fruits were harvested from Gabsi pomegranate trees from the Mahdia region in Tunisia. Fruits were washed and hand-peeled. Fruit peel was sundried and ground using a laboratory mill. The resulting powder was freeze-dried and protected from light under vacuum pack. 0.5 g of sample was extracted with 10 mL of methanol in the dark for 24 hours using an orbital shaker. Extractions were made in triplicate. The extracts were then filtered through a Whatman No.4 paper filter and concentrated under vacuum at 50 °C with a rotary evaporator. The crude extracts obtained where then freeze-dried, vacuum packed and stored under refrigeration until further analyses.
The methanolic extracts of peels were evaluated for the content of: TPC, TOPC, anthocyanins (TAC), flavonoids (TFC), and total condensed tannins (TTC). All absorbance measurements were carried out using a UV-1601 Shimadzu spectrophotometer.
The TPC and TOPC of the methanolic fractions were determined according to the method of
TFC, expressed on a dry weight (DW) basis as mg catechin equivalents (CEQ)/g of sample, were evaluated according to the colorimetric assay developed by
TAC were measured according to the method of
TTC was evaluated according to the procedure reported by
To evaluate the antioxidant activity of PPE, the DPPH test was used according to the method described by
Where A blank is the absorbance of the control reaction (containing all reagents except the tested compound), and A sample is the absorbance of the tested compound.
Refined corn oil, free of synthetic antioxidants, was divided into five portions. Three of them were supplemented with 200, 500 and 1000 ppm PPE. The fourth one was mixed with synthetic antioxidant BHT at the permitted legal limit of 0.02% and was prepared for comparative purposes. The last portion, without additive, served as control. Before supplementation, PPE and BHT were mixed separately with a minimum amount of methanol to ensure dispersion in oil in an ultrasonic water bath. Then they were added to corn oil in brown glass bottles, and mixed for 30 min at 50 °C to get a diffusion of compounds from the PPE to the corn oil (
Samples with PPE, BHT and control samples were induced to oxidation by heating under simulated frying conditions using an incubator maintained at 200 °C. Samples were separately heated for 2, 4, 6, and 8 hours with temperature control using a calibrated chromel-alumel thermocouple (HI 935009, Hanna Instruments). The samples were taken out of the incubator and cooled after each heating time and stored at -20 °C until analysis.
The progress of lipid oxidation was evaluated by measuring standard chemical indices: FFA, PV,
The FFA was determined by the titration of a solution of oil dissolved in ethanol/ether (1:1, vol/vol) with an ethanolic solution of potassium hydroxide (0.1M). The result was expressed as % of oleic acid
The PV was determined using the standard titration method by the American Oil Chemists’ Society (AOCS) (
The
The CD and CT formed were determined using the standard method of
TOTOX was used to estimate the oxidative deterioration of lipids. The TOTOX value was defined as the sum of both values (PV and
All assays were run in triplicate. The results are reported as mean values of three analyses and standard deviation. Data was subjected to statistical analysis using the SPSS program, release 11.0 for Windows (SPSS, Chicago, IL, USA). The one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test were employed to study the differences between individual means and seemed to be significant at p < 0.05. Principal component analysis (PCA) was carried out using XLSTAT (2014) for Windows (Addinsoft, New York, USA).
The total analysis of the compound group defined by the total polyphenol (TPC), total flavonoid (TFC), total
Results are expressed as means ± standard deviation (n = 3). TPC: total plolyphenols (mg GAE/g DW); TOPC: total ortho-diphenol (mg HTE/g D); TFC: total flavonoids (mg QCE/g DW); TTC: total tannins (mg TA/g DW); TAC: total anthocyanins (mg CyE/g DW).
Hydrolysis is one of the most common reactions that causes frying oil degradation and therefore increased free fatty acid content. Therefore, free acidity is an important factor for oil quality (
Results are expressed as means ± standard deviation (n = 3). Bars (mean value ± SD) with different letters after each heating period are significantly different (p < 0.05) according to the one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test.
Supplementation with PPE and BHT markedly reduced the FFA increase and inhibited the development of rancidity. BHT was often used traditionally as an antioxidant in oil products to retard oxidation. However, it is allowed for use within legal limits in the food industry due to its toxic and carcinogenic effects (
Peroxide value is an indicator of the extent of initial oxidation in oil and fats (
At any heating time, the PVs measured in oil samples containing PPE (natural antioxidant) and BHT (synthetic antioxidant) were significantly lower than that of the control. In addition, the PVs for these samples increased but this increase was very slow. Initially, PPE at various doses and BHT at 200 ppm were comparable up to 6 hours of heating, when the PVs for PPE at 500 ppm and 1000 ppm were significantly lower (4 and 4.5, respectively) than that of BHT (
The measurements of CD and CT are indicators of the oxidative deterioration of oils. CD and CT are generated by the oxidation of polyunsaturated fatty acids (PUFA). In fact, during heating PUFA are oxidized with the formation of hydroperoxides and their double bonds suffer a rearrangement and generate CD. CT are formed through the conjugation to include three or more double bonds. The resulting CD exhibit intense absorption at 232 nm and quantified by K232; similarly, CT absorb at 272 nm and quantified by K272. Thus, the higher the proportion of PUFA in the oil, the higher the levels of CD and CT formed during frying (
The CT contents increased with the increase in heating time at a greater rate for the control. After up to 6 hours of heating, CO-1000 showed the lowest level of CT followed by CO-500 followed by BHT. Based on these results, the antioxidant activity of pomegranate peel extract at 500 and 1000 ppm was better than BHT at its legal amounts. These findings are in accordance with those reported by
TOTOX is useful for quantifying oxygen-directed oil degradation. The results in
h | CO | CO-BHT | CO-200 | CO-500 | CO-1000 |
---|---|---|---|---|---|
|
27.51±0.13a | 27.51±0.38a | 25.65±0.24b | 24.63±0.29b | 24.24±0.32b |
|
67.73±0.21a | 48.33±0.70b | 41.28±0.62c | 39.35±0.008c | 34.12±0.04d |
|
92.5±0.49a | 71.63±0.39b | 71.63±0.20b | 69.04±1.02bc | 66.91±0.71c |
|
99.35±0.17a | 77.58±0.21b | 74.83±0.83c | 71.88±0.16d | 71.40±0.37d |
|
118.25±0.58a | 83.48±0.29 b | 80.54±0.24c | 74.89±0.17d | 73.16±0.23e |
Results are expressed as means ± standard deviation (n = 3). For each condition, values in the same row with different letters are significantly different at (p < 0.05) according to the one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test. PPE: pomegranate peel extract; TOTOX: total oxidation value; CO: Corn oil; CO-BHT: oil supplemented with BHT; CO-200: oil supplemented with 200 ppm PPE; CO-500: oil supplemented with 500 ppm PPE; CO-1000: oil supplemented with 1000 ppm PPE.
Phenolic content is a primary parameter for vegetable quality evaluation and directly involved in the prevention of oxidation and oil preservation (
A: Total phenol contents (TPC), B: Total orthodiphenol contents (TOPC). Results are expressed as means ± standard deviation (n = 3). Data with different letters for the same heating time are significantly different (p < 0.05) according to the one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test.
The changes in the TOPC in the different oils during heat treatment are shown in
A significant difference (p < 0.05) between the TOPC of different oils was observed before and after each treatment period. The initial amount of TOPC in the CO (1.16 mg HT/kg oil) was significantly (p < 0.05) lower than that in the PPE supplemented oil. Thus, in CO-1000, it was six times more than the initial amount. TOPC declined as a function of heat treatment in all oils. After 6 h of heating, the TOPC disappeared completely in CO and CO-BHT and decreased by 50% in CO-1000 (3.48 mg HT/kg) and CO-500 (2.71 mg HT/kg) compared to initial values. These contents remained stable until 8 hours of heating. As observed in TPC, the thermal degradation of TOPC was proportional to the initial concentration of PPE added to the oil. CO-200 seemed to be the most stable system to TOPC degradation followed by CO-BHT. In CO-1000 and CO-500, the percent of TOPC loss was similar to that of CO. It can be concluded that, at a level of 200 ppm of PPE, the remaining phenols (TPC or TOPC) would be responsible for protecting the oil but higher than that, the pro-oxidant effect would be observed.
The DPPH radical scavenging activities of CO and supplemented oils significantly decreased as a function of the heating time (
Data with different letters for the same heating time are significantly different (p < 0.05). Results are expressed as means ± standard deviation (n = 3) according to the one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test.
A multivariate statistical analysis of the data was performed using PCA to analyze the oxidative stability of different oil samples (CO, CO-BHT, CO-200, CO-500 and CO-1000) before and after 8 hours heating.
PV: peroxide value, p-AV: p-anisidine value, FFA: free fatty acid value, CD: conjugated dienes, CT: conjugated trienes, TOTOX: total oxidation value, TPC: polyphenol content, TOPC: ortho-diphenols.
PC1 was positively related to TPC, TOPC and DPPH antioxidant activities. PC2 was related more closely to FFA, PV,
This data revealed that the lowest levels for TPC and TOPC were observed in CO and CO-BHT, with the greatest extent of the oxidative deterioration, expressed by PV,
The PCA results confirmed the deterioration effect on corn oil quality due to heating and the effectiveness of various PPE extracts used (200, 500 and 1000ppm) compared to BHT against the formation of primary and secondary oxidation products.
According to the results, it can be concluded that PPE exhibited significant potential to stabilize corn oil under heating conditions. They decreased the thermal deterioration of oil by enhancing its hydrolytic stability, inhibiting double bond conjugation and reducing the loss in polyunsaturated fatty acids. PPE at concentrations of 500 and 1000 ppm have potential stabilization efficiency compared to synthetic antioxidant (BHT). Therefore, PPE, a potential antioxidant source, can be recommended to extend the shelf-life of unsaturated vegetable oils.
The authors would like to thank the administrative and technical staff for their constant support.