The Antioxidant Activities of Polyphenolic Extracts from Grape Pomace on Seven Types of Chinese Edible Oils

Abstract

Edible oil was the important component of food and easily oxygenated. To inhibit lipid peroxidation, adding synthetic antioxidants were commonly used. However, the safety of the synthetic antioxidants was doubted and it was advised to develop the safe, efficient and natural antioxidant. Grape polyphenols was the important secondary metabolites of grape. In the paper, the antioxidant activities of polyphenolic extracts from grape pomace on seven types of Chinese edible oils was studied. By measuring the peroxide value (POV) and acid value (AV) of edible oils under different conditions to show the antioxidant activities of grape polyphenols. The results showed that when adding amounts of grape polyphenols were 0.02%, the antioxidant effects were obvious. Compared with BHT and TBHQ, the antioxidant activity of grape polyphenols had a better effect on sesame oil. And there was a synergy between grape polyphenols and Vitamin C, and the opimum ratio was 1:4.

Introduction

The oxidative deterioration of edible oils caused rancidity, which is easily influenced by different factors such as light, heat, enzymes, and metal ions (Zanardi et al., 2000). Being deteriorated is common for vegetable oils, especially for unsaturated fatty acids. Once the vegetable oils become rancid, edible oils generate free radicals, peroxides, aldehydes, ketones and other compounds that alter the smell, color and nutrient content of the oil (Kanner et al., 1994). The rancid oil can accelerate aging and may cause cancer and atherosclerosis. The key link between oil production and oil storage is preventing the quality and safety of the oil from oxidation. The outer packaging of oils is one straightforward measure (Nenadis et al., 2003). The most effective means to avoid the oxidation of stored fats and oily foods is adding antioxidants. Natural antioxidants include compounds such as polyphenols, phytic acid, polysaccharide, etc (Wang et al., 2011). Synthetic antioxidants are efficient, stable, and easy to use, and the maximum permitted amount is often added. Currently, the most commonly used synthetic antioxidants are mainly phenolic antioxidants, such as Butylated hydroxytoluene (BHT) and Tertiary butylhydroquinone (TBHQ) (Wang et al., 2005). However, several previous studies showed that synthetic antioxidants endanger human health , and the safety was doubted (Yu et al., 2000). Out of curiosity and considering the consumer spending, some studies are trying to find and develop natural, efficient, non-toxic antioxidants as replacements. In terms of food additives, grape polyphenols can be added as a kind of ideal natural antioxidant of edible oil or meat products to delay the fat oxidation (Sanchez et al., 2008).

Polyphenols contains multiple phenolic hydroxyl plant components (Araujo et al., 2011). Grape polyphenols are important secondary metabolites of grape and mainly exist in grape seeds , skins, brance tendrils and leaves (Sanchez et al., 2003; Yildirim et al., 2005). Presently, the main method of extracting polyphenols is solvent extraction, ultrasound-assisted extraction, microwave-assisted extraction, supercritical fluid extraction, biological enzyme extraction, ion precipitation and membrane technology. Chinese grape and wine industry has developed rapidly in recent years (Wang et al., 2001). The residues generated during grape processing account for a large proportion of the total fruit weight, and the annual pruning of vineyards produces a large amount of trash, Much of the trash is used as fertilizer, feed or firewood (Garg et al., 2012). However, most of residues was discarded as garbage, which leads to a low coefficient of utilization. Grape pomace contains large amounts of polyphenols in the form of plant-based bioactive substances such as resveratrol, phenolic acids, flavonoids, catechin, epicatechin, and procyanidin (Kirimlioglu et al., 2006). These polyphenols is of high antioxidant activity and is regarded as natural antioxidants (Saito et al., 1998). The use of grape pomace to develop natural antioxidants for use in foods would not only make proper use of waste materials but also yield valuable natural, safe antioxidants. The present study measured the ability of polyphenol extracts from grapes to inhibit lipid peroxidation in various edible oils. The results indicated that the potential for grape polyphenols to be developed into natural antioxidants for edible oils.

Materials and Methods

Materials and reagents    The pomace of Cabernet Sauvignon grape (Vitis vinifera L.) was collected from the Yanqi Basin(Xinjiang Province, China). Lard oil was obtained from heated and refined pork suet. Peanut oil, sunflower seed oil and til oil were extracted from the collected material with a small oil press. Rapeseed oil, soybean oil, and sesame oil, filtered but without any additives, were bought from an oil mill.

Gallic acid was from Sigma-Aldrich (Shanghai, China). Sodium was from Tianjin Bodi Chemical Co. Sodium thiosulfate, potassium hydroxide, acetic acid, diethyl ether, ethanol, potassium iodide, chloroform, BHT, TBHQ, ascorbic acid, starch and phenolphthalein were purchased from the Tianjin Kermel Chemical Reagent Co.

Instruments and equipments    UV-1700 spectrophotometer (Shimadzu Corporation), KQ2300D E-type CNC ultrasonicator (Machine Instrument Co. Kunshan), Millipore ZMQS 5001 ultra-pure water meter (Millipore, France), Sorvall RC-5C PLUS superspeed desktop refrigerated centrifuge (the US Kendro company), HH.W21.600S electrically heated water bath (Shanghai Yuejin Medical Instrument), 101-2AB oven (Tianjin Test Instrument Co.) and 6YY-203 automatic rapid hydraulic press (Penguin Zhengzhou Grain and Oil Machinery Co.).

Extraction of polyphenols from grape pomace    3.00 g grape pomace powder and 30 mL of methanol solution (1 mol/L, HCl/methanol/H₂O,1/80/19, v/v/v) was mixed and the mixture was ultrasonicated at 100 W for 30 minutes at 25°C and then centrifuged at 8000 rpm for 15 min at 4°C. The extraction procedure was repeated twice and the supernatant was collected and stored at −4°C.

Measurement of phenolics    Total phenolic content (TPC) was determined by the Folin-Ciocalteu method (Javanmardi et al., 2003). Briefly, 50µL of extract was mixed with 2.5 mL of Folin- Ciocalteu reagent which was diluted 10 times, and 2 mL of sodium carbonate solution (7.5%) was added. The mixture was allowed to react at 25°C in the dark for 15 min, and the absorbance was measured at 765 nm. The results were expressed as mg gallic acid equivalents (GAE) per 100 g of extracts.

The total flavonoid content (TFOC) was measured according to the method of Makris et al. (2007). In a centrifuge tube, 0.1 mL of extract was mixed with 0.3 mL of methanol solution, 0.4 mL of distilled water and 0.03 mL of 5% NaNO₂ in sequence, and the mixtures were incubated for 5 min. Then 0.03 mL of 10% AlCl₃ was added and the mixtures were allowed to stand. After 5 minutes, 0.2 mL of Na₂CO₃ (1 mol L⁻¹) and 0.24 mL of distilled water were added to the reaction system. And the absorbance was measured at 510 nm. The results were expressed by a calibration curve developed using rutin.

The total flavan-3-ol content (TFAC) was determined with the p-DMACA method (Li et al., 1996). 0.1 mL of extract which was diluted 2 times was mixed with 3 mL of p-DMACA solution (0.1% in 1 M HCl in MeOH). And the mixture was allowed to react at room temperature for 10 min, the absorbance was determined at 640 nm. The results were estimated as milligrams of (+)-catechin equivalents per gram of extract.

The total monomeric anthocyanin content (TMAC) was detected by the PH differential method (Meng et al., 2012a). The extract was diluted to the same dilution at PH 1.0 and 4.5 with the buffers. After 15 min, the absorbance was determined at 510 nm and 700 nm in both pH 1.0 and 4.5 buffers and calculated using the following equation:   

The TMAC was expressed in terms of cyanidin-3-glucoside and calculated with the following formula:   

Where A is the absorbance, MW is the molecular weight of cyanidin-3-glucoside (449 g mol ⁻¹), Ve is the extract volume (0.06 L), DF is the dilution factor, ε is the molar extinction coefficient of cyanidin-3-glucoside (29,600), and M is the weight of the sample (3.00 g).

Antioxidant activity determination    DPHH free radical-scavenging capacity was measured according to the method of Meng et al. (2012b). Briefly, 0.1 mL of extract and 3.9 mL of DPPH methanolic solution were mixed and reacted in the dark for 20 min. The absorbance was estimated at 515 nm. The results were expressed as micromoles of trolox equivalents per gram of grape mass.

The reducing power of potassium ferricyanide (PFRA) was measured according to the method of Zhang et al. (2011). In a test tubes, 0.1 mL extract was mixed with 2.5 mL of phosphate buffer (0.2 M, pH 6.6) and 2.5 mL of potassium ferricyanide (1%) in sequence. The mixture was reacted in water bath at 50°C for 20 min. 2.5 mL of trichloroacetic acid (10%) was added after the mixture was cooled in ice water for 5 min. Then the mixture was centrifuged at 6000 g for 10 min. 1 mL of the upper layer was mixed 2.5 mL of distilled water and 0.5 mL of 0.1% ferric chloride (0.1%), after 5 min, the absorbance was estimated at 700 nm. And the absorbance indicated the reducing power of the sample.

Measurement of peroxide value (POV)    In a 250 mL brown glass, 2.00–3.00 g of grape skin and 30 mL of chloroform-glacial acetic acid was added and dissolved. 1.00 mL of saturated potassium iodide was added to the mixture with vibrating gently for 0.5 min, then placed the sample in the dark. After 3 min, the sample was taken out to another glass and mixed with 100 mL of distilled water. Meanwhile, the sodium hyposulphite standard solution (0.0020 moL L⁻¹) was used in titration until the color of the solution was yellow, 1 mL of starch indicator was added to the glass and the titration was over until the blue color disappeared. The blank test was set up with the chloroform-ice acetic acid in the same method. The POV of the samples was calculated using the equation (POV, meq kg⁻¹) = (V₁–V₂)×C×0.1269×78.8×100/m (Eq.3), where V₁ is volume of the sodium hyposulphite standard solution of the sample consumption, V₂ is volume of the sodium hyposulphite standard solution of the blank consumption, C is the concentration of standard titration solution, m is the weight of the sample.

Measurement of acid value (AV)    In a conical flask, 3.00–5.00 g of sample and 50 mL of neutral ether ethanol mixture was added and dissolved. 2–3 drops of phenolphthanlein incicator was added, with potassium hydroxide standard titration solution titration (0.050 mol L⁻¹) until the mixture became reddish color. The AV of sample was calculated using the formula: X=V×c×56.11/m (Eq.4). Where X was the AV of sample (g kg⁻¹), V was the volume of potassium hydroxide standard titration solution of the sample consumption (mol L⁻¹), m was the weight of sample.

Measurement of the antioxidant effects of polyphenolic extracts from grape pomace    The antioxidant effects of different amounts of grape polyphenols on edible oils were determined. Grape polyphenols were added as a percentage of 100 g of grease weight (0%, 0.02%, 0.05%, 0.10%, 0.15%), and the oil was kept in an oven at 70 ± 1°C. After 0, 6, 12, 18, 21, 24, 27, and 30 d, the POV and AV was measured.

Comparison of the antioxidant activity of grape polyphenols, BHT and TBHQ was made. A total of four oil samples of 100 g each were added to 100 mL bottles. Then, either grape polyphenols, BHT or TBHQ were added to 0.02% to the corresponding bottles. After 0, 6, 12, 18, 21, 24, 27 and 30 d at 70 ± 1°C, the POVs were measured.

The synergistic antioxidant effects of grape polyphenols and VC on edible oils was measured. Grape polyphenols and VC were mixed at ratios of 10:0, 1:9, 1:4, 3:7 and 2:3 and added to 100 g of the edible oils to 0.05%. Then, the POV was measured after 0, 6, 12, 18, 21, 24, 27 and 30 d at 70 ± 1°C.

Statistical Analysis

All the experiments were carried out in triplicate and each experiment was repeated three times. The results were expressed as average of three replications of three individual experiments and the data was analyzed by using Microsoft Excel 2010. Datas are averages of three replicates per amount of grape pomace extraces. Error bars are the standard deviation of the mean (n = 3).

Results

Determination of the polyphenol content and antioxidant activity of grape pomace    To provide a theoretical reference for the development of natural, high-performance antioxidants, the total content of phenolic compounds, anthocyanins, flavan-3-ols, flavonoids, DPPH scavenging, and PFRA in grape pomace was measured (Table 1). The results of TPC, TAC, TFAC and TFOC showed that grape pomace contains polyphenols and the results of DPPH and PFRA showed the antioxidant capacity.

Table 1. Polyphenol content and antioxidant activity of grape pomace

TPC (0.01 g kg−1)	TAC (g kg−1)	TFAC (0.01 g kg−1)	TFOC (0.01 g kg−1)	DPPH scavenging (µM Trolox g−1)	PFRA (absorption value)
39.8 ± 0.862	4.372 ± 0.023	199.93 ± 0.778	403.56 ± 0.314	1235.27 ± 18.952	0.30 ± 0.008

Note: Datas are averages of three replicates per amount of grape pomace extraces(± standard error).

Changes in the POV of edible oils    The POV is given as the millimoles of peroxide present in oils and fats. In GB 10146-2005 and GB 2716-2005, which provide national standards, the POV of edible lard and vegetable oils is <15.76 meq kg⁻¹ and <15 meq kg⁻¹, respectively. In this study, these values were called the critical values.

Effects of different amounts of grape polyphenols on the POVs of edible oils    The POV of lard oil reached the critical value between 24–27 days with adding 0.15% grape polyphenols and it was the slowest than others (Fig.1A). When grape polyphenolswas added to colleseed oil, lipid peroxidation occurred later than that in control. An extract concentration of 0.02% had the best antioxidant effect in colleseed oil (Fig.1B). Compared with colleseed oil, the POV of sesame oil had no significant change before 12 days (Fig.1C). In soybean oil, within a certain range of concentrations, with the increase of the extract concentration the POV decreased followed by increased. When 0.02% extract was added, the antioxidant effect on soybean oil was clearest (Fig.1D). The POV of peanut oil changed slowly and it was smaller than other treatments only when 0.10% extract was added. (Fig.1E). In sunflower seed oil, when the addition of 0.10% extract was added, the POV increased much more slowly than the others and it reached the critical value on the 20–21 day (Fig.1F). The addition of 0.10% extract had the strong antioxidant effect on the til oil (Fig.1G).

Fig. 1.

Effect of different doses of grape pomace extracts on the POVs of lard oil, colleseed oil, sesame oil, soybean oil, peanut oil, sunflower seed oil and til oil at 70 ± 1°C.

Antioxidant effects of grape polyphenols, BHT and TBHQ on edible oils    Fig.2A shows the effect of grape polyphenols on lard oil was weaker than that of BHT, and the effect of BHT was weaker than that of TBHQ. The POV treated with grape polyphenols reached the critical value between 18–21 days. After BHT treatment, the POV had barely reached the critical value on day 30. The POV treated with TBHQ was only 7.75 meq kg⁻¹ on day 30. From Fig.2B, The POV treated with grape polyphenols increased slowly than that treated with BHT and TBHQ, we can see the antioxidant effects of grape polyphenols on colleseed oil were significantly stronger than those of BHT and TBHQ. The POV treated with grape polyphenols reached the critical value on day 27, and the POV was small on the day 30. The POV treated with BHT reached the critical value between 27–30 days, and that of the oil treated with TBHQ did not reach the critical value on the day 30. While the POV of sesame oil treated with grape polyphenols changed slowly, and was only 0.86 meqkg⁻¹ on the day 30. The POV treated with BHT and TBHQ also changed slowly, but the antioxidant effects of grape polyphenols on this oil were significantly stronger than those of BHT and TBHQ (Fig.2C). In soybean oil, the POV reached the critical value between 27–30 days after treatment with grape polyphenols and on day 30 after treatment with BHT. But the POV treated with TBHQ did not reach the critical value on day 30. Therefore, the antioxidant effects of grape polyphenols on soybean oil were roughly equal to those of BHT and TBHQ (Fig.2D). The POV of peanut oil treated with grape polyphenols reached the critical value between 12–18 days. After treatment with BHT, the POV reached the critical value on day 18. After treatment with TBHQ, the POV reached the critical value between 18–21 days. Therefore, the antioxidant effects of TBHQ were slightly stronger than others, but the effects were not obvious (Fig.2E). The antioxidant effects of TBHQ on sunflower seed oil were stronger than those of the grape polyphenols, and BHT had the weakest effect (Fig.2F). Grape polyphenols were not suitable to be used as antioxidants in in sunflower seed oil. when treated with grape polyphenols, the POV reached the critical value between the 18–21 days. When treated with BHT ,the POV reached the critical value between 21–24 days. Sunflower seed oil treated with TBHQ reached the critical value between 24–27 days. So the antioxidant effects of TBHQ on sunflower seed oil were stronger than others (Fig.2F). When til oil treated with BHT, the POV reached the critical value between the 21–24 days, whereas til oil treated with grape polyphenols or TBHQ did not reach the critical value by day 30. Therefore, grape polyphenols are suitable antioxidants for til oil (Fig.2G).

Fig. 2.

Effect of different antioxidants on the POVs of lard oil, colleseed oil, sesame oil, soybean oil, peanut oil, sunflower seed oil and til oil at 70 ± 1°C.

The synergistic antioxidant effect of grape polyphenols and VC on edible oils    Fig.3A shows different ratios of grape polyphenols to VC had different antioxidant effects on the edible oils. When the ratio of grape polyphenols to VC was 1:9, the POV reached the critical value between 18–21 days. When the ratios were 1:4, 2:3 and 3:7, the POV reached the critical value between 24–27 days. When the ratio was 10:0, the critical value was reached between 12–18 days. Obviously, the ideal ratio was 1:4. We can see from Fig.3B, when the ratios were 1:9 and 2:3, the POV reached the critical value between 27–30 days, when the ratios were 1:4 and 3:7, the POV did not reach the critical value by the 30th day. When the ratio was 10:0, the critical value was reached on the 27th day. The POV changed more slowly at a ratio of 1:4, making the 1:4 ratio ideal. In sesame oil, the POV for all treatments changed slowly and only reached approximately 2 meq kg⁻¹, which was far less than the critical value. The difference among treatments was not obvious, and the 1:4 ratio had a much stronger effect than the others (Fig.3C). After all of the treatments to soybean oil, the POV reached the critical value between 27–30 days. The 10:0 ratio had the smallest effect, and treatment with the 1:4 mixture led to the smallest POV (Fig.3D). The POV changed rapidly in peanut oil. When the ratio was 1:9, the critical value was reached on day 18. When the ratios were 1:4, 3:7, or 2:3, the critical value was reached between 18–21 days. When the ratio was 10:0, the critical value was reached between 12–18 days. Overall, the 2:3 ratio had the strongest effect (Fig.3E). Fig.3F shows when the ratios were 1:9, 3:7 or 10:0, the critical value was reached between 21–24 days. When the ratio was 1:4, the critical value was reached on day 24. When the ratio was 2:3, the critical value was reached between 24–27 days. Therefore, the 2:3 ratio had the strongest effect. In the til oil, after all treatments, the POV reached the critical value between 21–24 days. A comparison of all of the treatments shows that the 3:7 ratio had the strongest effect (Fig.3G).

Fig. 3.

The synergistic effect of grape skin extracts and Vc on the POVs of lard oil, colleseed oil, sesame oil, soybean oil, peanut oil, sunflower seed oil and til oil at 70 ± 1°C.

Changes in the AV of the edible oils    The AC (AV) is the amount of KOH (in milligrams) required to neutralize all of the acidic constituents present in a lg sample of a petroleum product.

Regulations GB 10146-2005 and GB 2716-2005 show that the national standard for the AC of edible lard oil is <1.5 mg/g and that for the AC of edible vegetable oil is < 4 mg/g. In this paper, these values are referred to as the critical values.

The effect of different amounts of grape polyphenols on the AV of edible oils    Fig. 4A shows that when grape polyphenols were added to lard oil, the lard oil's AV changed rapidly. Therefore, grape polyphenols are not suitable for controlling the AV of lard oil. Fig. 4B shows that when nothing was added to the colleseed oil, the AV did not reach the critical value by day 30. When 0.02% extracts were added, the AV reached the critical value between 21–24 days. When 0.05% or 0.15% was added, the AV reached the critical value between 12–18 days. When 0.10% was added, the AV reached the critical value between 18–21 days. While sesame oil, soybean oil and colleseed oil were similar in that the AV reached the critical value much more quickly when grape polyphenols were added, showing that grape polyphenols did not have an inhibitory effect on the AV of these oils (Fig.4C, 4D). In peanut oil, the AV did not reach the critical value on day 30 by any treatment (Fig.4E). In sunflower seed oil, when 0.02% was added, the AV reached the critical value between 21–24 days. When 0.05% was added, the AV reached the critical value between 18–21 days. When 0.10% or 0.15% was added, the AV reached the critical value between 12–18 days (Fig.4F). The AV of peanut oil, sunflower seed oil and til oil increased with increasing concentrations of grape polyphenols, but none of their AV exceeded the critical value. It suggests that grape polyphenols may not be suitable antioxidants for edible oils, but other indicators should also be comprehensively considered. In the til oil, the AV did not reach the critical value with any treatment, but the AV increased with increasing levels of grape polyphenols (Fig.4G).

Fig. 4.

Effect of different doses of grape skin extracts on the AV of lard oil, colleseed oil, sesame oil, soybean oil, peanut oil, sunflower seed oil and til oil at 70 ± 1°C.

Comparison of the antioxidant activity of grape polyphenols, BHT and TBHQ    Fig. 5A shows that the AC was greater than the critical value with every treatment, indicating that the three types of antioxidants had a weak effect on lard oil rancidity. BHT had a stronger effect than the other two antioxidants. The inhibitory effect of BHT on AC was much greater in colleseed oil and peanut oil. The AV of colleseed oil was close to the critical value on day 21 with TBHQ, whereas the AV of the samples with BHT and grape polyphnols did not reach the critical value by day 30. And the antioxidant effect of BHT and grape polyphnols on colleseed oil was quite (Fig. 5B). In peanut oil, for all treatments, the AV did not reach the critical value by day 30. A comprehensive evaluation showed that TBHQ and grape polyphenols had the stronger inhibitory effect than BHT. However, considering safety and natural origin, the grape polyphenols were more suitable for addition (Fig. 5E). The inhibitory effect of TBHQ on AV was greater in lard oil, sesame oil, soybean oil, sunflower seed oil and til oil. Fig.5C shows the AV of sesame oil was very high after 30 days, and the AV of the sample treated with TBHQ reached the critical value on day 24, TBHQ was superior to other two types of antioxidants. Soybean oil changed similarly as sesame oil did, but the effects of BHT and TBHQ were comparable (Fig.5D). The AV of sunflower seed oil treated with grape polyphenols was near to the critical value on day 21. The other two grape polyphenol-treated samples did not reach the critical value on day 30. The inhibitory effect of BHT was stronger than that of TBHQ (Fig.5F). Til oil changed similarly as peanut oil, and the inhibitory effect of TBHQ was stronger than that of BHT (Fig.5G).

Fig. 5.

Effect of different antioxidants on the AC of lard oil, colleseed oil, sesame oil, soybean oil, peanut oil, sunflower seed oil and til oil at 70 ± 1°C.

The synergistic effect of grape polyphenols and VC on the AC of edible oils    Fig.6A shows that the AC of all of the treatments reached the critical value on day 12 in lard oil. However, on day 30, the best ratio was 3:7. Fig.6B shows that adding VC to the treatment had a much stronger effect. This result suggested that the combination of grape polyphenols and VC had a synergistic effect on the control of colleseed oil's AV. The ratio with the strongest synergistic effect was 1:4, and the AV was close to the critical value on day 24. In sesame oil, the AV of all of the treatments quickly reached the critical value. Therefore, it is difficult to control the AV of sesame oil and a ratio of 1:9 had the strongest effect (Fig. 6C). Soybean oil behaved similarly to sesame oil, and the optimal ratio was 1:4 (Fig. 6D). In peanut oil, some mixtures of grape polyphenols and VC showed negative synergy. When the ratio was 3:7 or 2:3, the inhibitory effect on oil rancidity was clearly less than that of the treatment with only grape polyphenols. For all treatments over the 30 days, the AV did not exceed the critical value and the optimal ratio was 1:4 (Fig. 6E). In sunflower seed oil, When the ratio was 10:0, the AV was higher than the other treatments. When the ratio was 1:4, the AV was the lowest among all the treatment, so the optimal ratio was 1:4 (Fig. 6F). In til oil, the AV was generally low and did not exceed the critical value over 30 days. The optimal ratio was 1:4 (Fig. 6G).

Fig. 6.

The synergistic effect of grape skin extracts and Vc on the AC of lard oil, colleseed oil, sesame oil, soybean oil, peanut oil, sunflower seed oil and til oil at 70 ± 1°C.

Discussion

The influence of the different addition of grape polyphenol to edible oil    Oil rancidity is divided into two parts: One is keto form rancidity, which is expressed as acid value, the other is that the double bond of unsaturated fatty acids in oils were oxidized and cleaved generating free radicals, which is expressed as peroxide value (Becker et al., 2006). When the adding amount was 0.15%, the POV of lard oil changes slowly, while the AV changes fast. The result is consistent with the other studies, which shows that grape polyphenol can prevent the cleavage of the double bond of unsaturated fatty acid or the elimination of free radicals and promote the hydrolysis of oil (Nassu et al., 2003). In this study, the AV of the seven edible oil increased with the adding amount of grape polyphenol extractive growing. Adding lye can reduce the acid value oil (He et al., 1996). A similar study showed that the oxidation resistance of tocopherols in oil was as well as the synthetic antioxidant BHT when the amount of tocopherols in oil was a few, and high tocopherols content would decline the antioxidant capacity (Zhen et al., 2003). The optimal concentration of the phenolic antioxidants is 0.02%, too low or too high concentration both affect the stability of the oil. High concentrations of grape polyphenols may produce the adverse free radicals to the edible oils such as OH⁻, while it has little antioxidant effect. The stability of oil oxidation is affected by not only the amount of antioxidant content, but also the composition of fatty acids (Tsimidou et al., 1992). The oxidation rate of linoleic acid and other unsaturated fatty acids is faster than oleic acid because the double bond of the methylene is very active. The reason why the POV of sesame oil was lower than other edible oil, is antioxidant of the grape polyphenols and the lignin in sesame oil. And the lignin has strong antioxidant capacity (Mohamed et al., 1998). Sesame oil contains about 85% of unsaturated fatty acids (Abou-Gharbia et al., 2000).

The comparison of grape polyphenols or BHT with TBHQ    Grape polyphenols antioxidant has little effect on lard oil or til oil, because the POV treated with grape polyphenols reached the critical value between 18–21 days. After BHT treatment, the POV had barely reached the critical value on day 30. The POV treated with TBHQ was only 7.75 meq kg_1on day 30. But the antioxidant effect of grape polyphenols on colleseed oil and soybean oil is as well as BHT and TBHQ, because the POV reached the critical value between 27–30 days after treatment with grape polyphenols and on day 30 after treatment with BHT or TBHQ. The antioxidant effects of TBHQ on sunflower seed oil were stronger than others. When the sunflower oil treated with grape polyphenols, the POV reached the critical value between the 18–21 days. BHT treatment delayed reaching the critical value between 21–24 days. Sunflower seed oil treated with TBHQ reached the critical value between 24–27 days. On the sesame oil it is best than both BHT and TBHQ and the POV was only 0.86 meq kg⁻¹ on the day 30. The effect of the three kinds of antioxidants on peanut oil is weak and all the POV reached the critical value between 18–20 days, it is hard to controlled. On the other hand, the AV of edible oil added with grape polyphenols is higher than that when the edible oil added with BHT and TBHQ. Moreover, the AV of all the edible oil except for lard oil can basically be controlled with BHT and TBHQ. It shows that grape polyphenolscontains something which will increase the AV of edible oil. And studies show that the antioxidant effect of TBHQ is better than BHT (Zhang et al., 2004; Jayathilakan et al., 2007). But BHT makes an prominent antioxidant effects on the edible oil has been confirmed (Rehman et al., 2006).

The synergy between grape polyphenols and VC    VC as the substance of the synergistic agent of grape polyphenols is advantageous to improve antioxidant of edible oils. When the ratio of the grape polyphenols and VC is 1:4, the antioxidant effect on colleseed oil and soybean oil seems to be the best. When the ratio is 1:4, the POV of lard oil is the smallest. The AV of sesame oil reaches the critical value at the early time, considering all the POVs the optimum treatment is 2:3. The AV of peanut oil is stable and the POV is not stable, and when the ratio is 2:3, it is better than the others. The AV of the til oil is commonly small under the critical value for 30 days and the best processing of POV is 3:7. The AV of peanut oil indicates that the inappropriate adding proportion probably has negative synergy to the edible oils. When the ratio is 3:7 or 2:3, the inhibition of peanut oil's oxidative rancidity is worse than the sample only processing grape polyphenols. It is because that a certain dose of VC plays a critical role in synergy (Olas et al., 2002).

Conclusions

The optimal adding amounts of grape polyphenols for sesame oil, rapeseed oil and soybean oil were 0.02%, for the peanut oil, sunflower seed oil and til oil, the appropriate adding amounts were 0.10%. Compared with BHT and TBHQ, the antioxidant activities of grape polyphenols had a better effect on sesame oil. And there was a synergy between grape polyphenols and VC, and the most appropriate ratio for colleseed oil and soybean oil was 1:4.

Acknowledgment    We thank the Yanqi Basin for providing grape pomace. And also thank the National Technology System for Grape Industry (nycytx-30-2p-04) providing generous financial support for this work.

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