Chemical Composition, Oxidative Stability and Antioxidant Capacity of

Article pubs.acs.org/JAFC

Chemical Composition, Oxidative Stability and Antioxidant Capacity of Oil Extracted from Roasted Seeds of Sacha-Inchi (Plukenetia volubilis L.) Fausto H. Cisneros,*,† Daniel Paredes,† Adrian Arana,† and Luis Cisneros-Zevallos‡ †

Departamento de Ingeniería Agroindustrial y de Agronegocios, Universidad San Ignacio de Loyola, Av La Fontana 550, Lima12, Perú Department of Horticultural Sciences, Texas A&M University, College Station, Texas 77843-2133, United States

‡

ABSTRACT: The effect of roasting of Sacha-inchi (Plukenetia volubilis L.) seeds on the oxidative stability and composition of its oil was investigated. The seeds were subjected to light, medium and high roasting intensities. Oil samples were subjected to hightemperature storage at 60 °C for 30 days and evaluated for oxidation (peroxide value and p-anisidine), antioxidant activity (total phenols and DPPH assay), and composition (tocopherol content and fatty acid profile). Results showed that roasting partially increased oil oxidation and its antioxidant capacity, slightly decreased tocopherol content, and did not affect the fatty acid profile. During storage, oxidation increased for all oil samples, but at a slower rate for oils from roasted seeds, likely due to its higher antioxidant capacity. Also, tocopherol content decreased significantly, and a slight modification of the fatty acid profile suggested that α-linolenic acid oxidized more readily than other fatty acids present. KEYWORDS: oil, oilseed, oxidative stability, tocopherol, fatty acid profile, antioxidant capacity, roasting, Inca peanut, Sacha-inchi, Plukenetia volubilis

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INTRODUCTION There is growing scientific evidence that omega-3 fatty acids are beneficial to human health. However, the diet of the western world is deficient in these types of fatty acids. Therefore, there is interest in finding new sources rich in omega-3 fatty acids. The oil from the seed of Inca peanut or Sacha-inchi (Plukenetia volubilis) has one of the highest contents of omega3 fatty acids in the form of α-linolenic fatty acid (∼45%).1,2 One major problem associated with Sacha-inchi oil, besides its beany flavor, is its potential oxidative instability due to the highly unsaturated nature of its oil. However, unrefined vegetable oils are known to naturally contain antioxidant compounds that may give the oil protection against oxidation. Furthermore, the roasting of safflower, sesame, and canola seeds has been shown to increase the oxidative stability of the extracted oil.3−8 At present, Sacha-inchi oil is extracted commercially in Peru without prior roasting of seeds, even though kernels are roasted for direct consumption as a snack in order to eliminate astringent off-flavors and possible antinutritional factors. Thus, there is need to determine if Sacha-inchi oil stability improves from the roasting of the seeds prior to oil extraction. To our knowledge, this is the first report that addresses this issue. The objective of this study was to determine the effect that roasting of the seeds has on the properties of the extracted Sacha-inchi oil, including the fatty acid profile, tocopherol content and composition, oxidative stability, and antioxidant capacity during accelerated storage.

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Figure 1. Visual appearance of Sacha-inchi seeds (Pinto Recodo variety) at different degrees of roasted kernels: unroasted (A), slightly roasted (B), medium-roasted (C), and highly roasted (D). a few days in a ventilated place until processing time. Commercial flaxseed oil from Barlean’s Organic Oils (Ferndale, WA, U.S.A.) was used as reference due to its high α-linolenic acid content. The seeds were first shelled manually with the aid of pliers (∼64% kernels and 35% shells). The kernels were roasted the following day

MATERIALS AND METHODS Received: Revised: Accepted: Published:

Raw Material and Roasting of Seeds. Approximately 60 kg of Sacha-inchi seeds (Pinto Recodo variety) (Figure 1A) were donated by the Instituto de Investigaciones de la Amazonia Peruana (Tarapoto, Peru). The seeds were received in December 2006 and were stored for © 2014 American Chemical Society

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Journal of Agricultural and Food Chemistry

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DPPH (2,2-Diphenyl-1-picrylhydrazyl) Assay. This analysis was based on the method used by Thaipong and others.12 In this method, antioxidant activity was determined using DPPH as free radical. The procedure is as follows: A 150 μL aliquot of phenolic extract and 2.85 mL of DPPH solution were mixed. After a 24-h period of incubation in the dark, the absorbance was measured at 515 nm with a spectrophotometer. A blank was prepared using methanol instead of phenolic extract. A linear standard curve was built using trolox as the reference reactant in a range from 25 to 800 μM. Antioxidant capacity was expressed as μg trolox equivalents/g oil. A higher trolox equivalent is indicative of a greater antioxidant potential or reducing power. Tocopherol Content. Sample preparation to be analyzed by chromatography was based on the method used by Rodas-Mendoza and others,13 which is described briefly. A 0.5 g oil sample was diluted in 2 mL methanol and mixed for 1 min, afterward, 500 μL hexane was added and mixed in a vortex. The mixture was then centrifuged (2500g × 10 min) and a 1 mL aliquot of supernatant was removed and filtered with a PTFE filter (pore size 0.2 μm) before 20 μL volume injection into the HPLC. The samples were run using a Waters Millennium 3.2 software (Milford, MA, U.S.A.). The HPLC system was equipped with a binary pump system (Waters 515, Milford, MA, U.S.A.), an autoinjector (Waters 717 plus, Milford, MA, U.S.A.), a photodiode array (PDA) detector (Waters 996, Milford, MA, U.S.A.), and a column heater (SpectraPhysics SP8792, San Jose, CA, U.S.A.). The column used was a Tracer Spherisorb ODS2 C18, 250 × 4.6 mm (i.d.), particle size 5 μm (Tracer Analitica, Barcelona, Spain) protected by a column guard of the same composition (Waters Corp., Milford MA, U.S.A.). Methanol was used as the mobile phase, and elution took place at a flow rate of 1 mL/min. The column temperature was kept at 50 °C. Detection was done at 295 nm. Chromatographic peaks were identified by comparing sample retention times with those of standards, and quantification was achieved by external standardization. Fatty Acid Profile. Sample preparation for GC analysis was based on the method used by Misir and others.14 This method consisted in the formation of methyl esters of fatty acids from the corresponding fatty acids. The procedure used is as follows: 0.15 g of oil was diluted in 3 mL of diethyl ether in a test tube to which 0.2 mL of 20% tetramethylammonium hydroxide was added. The mixture was stirred for 2 min and allowed for phase separation. One drop of thymol blue indicator was added, followed by dropwise addition of 0.5 N HCl methanol solution until the color changed from blue to yellow. Finally, 0.5 mL methanol was added to make the samples homogeneous. The chromatographic method used was that described by Misir and others.14 The fatty acid methyl esters were analyzed with a Varian CP 3800 gas chromatograph (Palo Alto CA, U.S.A.) equipped with a Varian CP 8200 automatic injector and a flame ionization detector (FID). A Varian FAME fused silica capillary column (100 m × 0.25 mm, i.d.) (Varian CP-Select). The oven temperature had the following program: 0−30 min at 185 °C and 30−45 min at 235 °C, with increments of 20 °C/min. The FID temperature was 270 °C. The flow rates of helium, air, and hydrogen were 1.6, 300, and 35 mL/min. Statistics. Results are reported as mean values of three replicates and their corresponding confidence intervals at a 95% confidence level (p < 0.05).

using a commercial gas-fired roaster (IMSA, Lima, Peru). Batch size was 4.5 kg approximately Three roasting conditions were applied to obtain the corresponding degrees of roasted kernels: lightly roasted (75−81 °C × 9 min), medium roasted (83−86 °C × 10 min), and highly roasted (99−102 °C × 10 min). The light roasting conditions correspond to the minimum roasting level that eliminates the astringent flavor. Weight loss during roasting ranged from ∼5.5− 6.6%. After roasting, the kernels were immediately cooled with the built-in fan of the roaster. Oil Extraction, Clarification and High Temperature Storage. Oil extraction was achieved by pressing the kernels the following day after roasting. The unroasted and the three roasted samples were pressed with a laboratory Komet expeller (IBG Monforts GmbH, Monchengladbach, Germany). The press temperature of 100−110 °C was measured by inserting a thermocouple in the cake mass that emerged from the expeller. The extracted oil with fine suspended solids was immediately placed in amber glass bottles (0.5 L) and filled to the top to minimize headspace. The bottles were stored in the dark and under refrigeration (5 °C) until clarification by centrifugation. The nonclarified oil yield increased with roasting temperature with values of ∼42, 49, and 65% by weight for lightly, medium and highly roasted samples, respectively. Oil samples were clarified by centrifuging at 29 000g for 15 min in a laboratory centrifuge Beckman Model J2−21 (Fullerton, CA, U.S.A.). The clarified samples were placed in amber glass bottles (120 mL) and flushed with nitrogen and stored in the dark under refrigeration at 5 °C until testing time. For high temperature storage studies, oil samples (50g) were placed in 100 mL beakers (in triplicate) and stored in an oven at 60 °C in the dark for 30 days. Samples were analyzed for peroxide value, panisidine, tocopherols, and fatty acid profile at 0, 8, 18, and 30 days of accelerated storage while antioxidant capacity, by measuring total phenols and DPPH, were done on samples at days zero and 30. All analyses were performed in triplicate. Peroxide Value and p-Anisidine Value. The peroxide value was determined using the AOCS Cd 8−53 official method.9 This method determines all substances in the oil that oxidize potassium iodide (KI) under test conditions. The assumption is that these substances are peroxides or similar compounds that resulted from lipid oxidation. Results are expressed as peroxide milliequivalents/1000 g oil. The panisidine value was determined using the AOCS Cd 18−90 official method.9 This method quantifies the amount of aldehydes (especially, 2-alkenals and 2,4-dienals) present in the oil through the reaction between aldehydes and p-anisidine in an acetic solution. Absorbance was measured at 350 nm with an 8452A photodiode array spectrophotometer (Hewlett-Packard, Palo Alto, CA). Antioxidant Capacity. Two methods were used to measure antioxidant capacity: Total phenols and DPPH assay. Previously, phenolic extracts, to be used for both methods, were prepared from oil samples. Phenolic extracts were prepared according to a procedure by Ninfali and others10 with some modifications: 0.5 g oil were diluted in 25 mL 80% methanol−water and vortexed for 1 min, then stirred using a gyratory shaker model G2 (New Brunswick Scientific Co., Inc., NJ) at 200 rpm × 1 h. The mixture was centrifuged (29 000g × 15 min) using a Beckman Model J2−21 centrifuge (Fullerton, CA, U.S.A.) and an 8 mL aliquot of supernatant was taken. The solvent of the supernatant was removed using a rotovapor and then a laboratory freeze-dryer FTS Systems Inc. (Stone Ridge, NY) for ∼8 h. The solid residue was redissolved in 500 μL methanol (900 μL for samples from intensely roasted seeds). These extracts were used for total phenols and DPPH analyses. Total Phenols Using the Folin Assay. A procedure was used based on the method by Swain and Hillis.11 Four milliliters of distilled water was added to a 250 μL aliquot of phenolic extract and mixed for 30 s in a vortex. Folin-Ciocalteu reagent (250 μL) was added and allowed to react for exactly 3 min. Then 500 μL 1N Na2CO3 solution was added, and the mixture was allowed to react in the dark for 2 h, after which absorbance was read at 725 nm in a spectrophotometer. Chlorogenic acid was used as standard and results were expressed as mg chlorogenic acid/g oil.

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RESULTS AND DISCUSSION

Roasting Process Effect on Sensorial Characteristics of Kernel and Oil. The varying degrees of roasting of Sachainchi seeds resulted in changes in seed color and kernel taste. The color changed from cream in unroasted kernels to light brownsimilar to roasted peanutsin lightly roasted Sachainchi kernels, to brown and dark brown, in medium and highly roasted kernels, respectively (Figure 1). The typical astringent taste of unroasted kernels disappeared with roasting; lightly roasted kernels had no astringent taste but rather a slightly sweet taste; medium roasted kernels had no sweet taste but rather a slightly burnt taste; highly roasted kernels had a bitter and burnt taste. The color of Sacha-inchi oil from lightly

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detected between roasting temperatures in the range 160−220 °C. It is important to keep in mind that different roasting temperatures may have different oxidation and transformation processes reflected in peroxides and aldehydes with completely different behaviors, thus the need to elucidate the specific compounds formed. p-Anisidine Value. Results for p-anisidine showed a welldefined trend in relation to roasting intensity (Table 1). The highest value corresponded to Sacha-inchi oil from the highly roasted seeds, 12.32, followed by medium roasted, slightly roasted, and unroasted seed oils, 9.40, 3.72, and 0.43, respectively, whereas the p-anisidine value obtained for flaxseed oil was zero. These results indicate that oil from seeds subjected to increased roasting show increasing amounts of secondary compounds of oxidation as a result of decomposition of hydroperoxides. This is in agreement with the results obtained on sesame seed oil by Yoshida4 and by Yoshida and Takagi.5 In relation to the increased p-anisidine values and low PV observed before in highly roasted seed oil, we surmise that early in the roasting process, kinetics favors hydroperoxide scission into secondary oxidation products; whereas, in the latter part of the roasting process, kinetics favors the reaction of the antioxidant compounds generated with hydroperoxides. Total Phenols. Oils from unroasted and lightly roasted seeds exhibited similar total phenol values, 2.32 and 2.55 mg chlorogenic acid/100g oil, respectively (Table 1). However, total phenol values increased with roasting intensity; thus, oils from lightly, medium, and highly roasted seeds had total phenol values of 2.55, 6.31, and 13.85 mg/100g oil (Table 1). These results could suggest that as roasting intensity increases, the formation of phenolic compounds, with stronger antioxidant capacities, is favored. Phenolic compounds have antioxidant properties, which can be increased by specific changes in the ring substitution groups.16 An example of changes in phenols with roasting that lead to the formation of phenols with more potent reducing capacities is the conversion of sinapic acid into vinylsyringol (canolol) in rapeseed (canola).7,8 Total phenol values can be correlated with oil color: the higher the phenol value, the darker the oil color (Figure 2). DPPH Assay. Roasting increased the antioxidant capacity of Sacha-inchi oil as measured by DPPH. Roasting intensity increased DPPH values further. The values obtained were 18.2, 23.8, 47.6, and 95.0 μg TE/g oil for oils from unroasted, slightly, medium, and highly roasted seeds, respectively (Table 1). These results suggest that roasting of Sacha-inchi seeds increases the antioxidant capacity of the corresponding oils, with roasting intensity further increasing the antioxidant capacity. As mentioned in the previous section, this could be

roasted seeds did not differ visually from that of unroasted seeds. However, oil from medium and highly roasted seeds exhibited darker oils (Figure 2).

Figure 2. Visual appearance of Sacha-inchi oil samples at day 0 from unroasted (A), slightly roasted (B), medium-roasted (C), and highly roasted seeds (D).

Roasting Process Effect on Oil Oxidation, Composition and Antioxidant Activity. Peroxide Value. Oil peroxide value (PV) did not follow a distinct trend with seed roasting (Table 1). Oil from unroasted seeds gave a value of 0.57 mEq/ kg oil. PV increased with light and medium roasting to 3.35 and 4.09, respectively; and then decreased with high roasting to 0.5 mEq/kg oil. As a comparison, flaxseed oil had a PV of 0.47. All these values are below the maximum acceptable PV value of 15 set by the Codex Alimentarius15 for nonrefined oils. According to the lipid oxidation theory, hydroperoxides begin to form at the initial stages of oxidation until reaching a maximum, and then decline at the latter stages of oxidation because of decomposition to secondary products (aldehydes, ketones, etc.). In light of the results obtained, one might be tempted to suggest that oils from lightly and medium roasted seeds are on the ascending hydroperoxide curve, and therefore, on the initial stages of oxidation; whereas, oils from highly roasted seeds are on the descending hydroperoxide curve, and therefore, on the latter stages of oxidation. However, an alternative explanation would be that roasting produced two opposing effects. On the one hand, it might have favored lipid oxidation, which is reflected in an increase in PV; but on the other hand, it could have favored production of phenolic compounds with potent free radical scavenging abilities, especially in highly roasted seeds (as will be explained later), which resulted in low PV in oils from highly roasted seeds. These results differ from those of Yoshida,4 who studied the effect of roasting seeds on sesame oil, where PV is reported to increase with roasting intensity, especially when roasting temperature surpasses 200 °C. Alternatively, Yoshida and Takagi5 observed an increase in PV in roasted sesame seeds compared to unroasted seeds; however, no PV differences were

Table 1. Roasting Effects on Sacha-Inchi Oil Oxidation, Antioxidant Capacity, Tocopherol Content and Fatty Acid Profile at Day 0a tocopherol content (mg/100g oil) oil sample from unroasted seeds lightly roasted seeds medium roasted seeds highly roasted seeds commercial flaxseed a

peroxide value (meq/kg oil) 0.57 3.35 4.09 0.50 0.47

± ± ± ± ±

0.01 0.04 0.02 0.01 0.02

p-anisidine value 0.43 3.72 9.40 12.32 0

± ± ± ±

0.04 0.06 0.1 0.31

total phenols (mg/100g oil) 2.32 2.55 6.31 13.81 3.79

± ± ± ± ±

0.06 0.18 0.32 0.23 0.32

DPPH (μg trolox equiv/g oil) 18.2 23.8 47.6 95.0 17.5

± ± ± ± ±

1.3 1.4 1.5 2.8 1.7

γ 70.6 65.5 64.4 66.2 19.9

± ± ± ± ±

δ 0.27 0.18 0.28 0.32 1.0

12.6 12.5 12.3 12.3 0.1

± ± ± ± ±

0.1 0.02 0.07 0.07 0

The ± values indicate the 95% confidence interval for the respective mean. 5193



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Table 2. Changes in Fatty Acid Profile of Sacha-inchi Oils from Unroasted and Roasted Seeds during High-Temperature Storage at 60 °C for 30 Daysa % oil sample

16:0

18:0

18:1

18:2

18:3

day 0 unroasted lightly roasted medium roasted highly roasted flaxseed

4.7 4.8 5.0 4.7 5.3

± ± ± ± ±

0.2 0.3 0.3 0.1 0.1

3.3 3.2 3.2 3.1 3.6

± ± ± ± ±

0.1 0.1 0.1 0 0

unroasted lightly roasted medium roasted highly roasted flaxseed

4.7 4.8 4.7 5.0 5.4

± ± ± ± ±

0.2 0.2 0.1 0.3 0

3.3 3.1 3.1 3.1 3.7

± ± ± ± ±

0 0 0 0 0


5.0 5.2 4.9 4.9 6.4

± ± ± ± ±

0.1 0.4 0.1 0.2 0.2

3.5 3.3 3.2 3.2 3.8

± ± ± ± ±

0.1 0.1 0 0 0.2


5.1 5.1 5.2 5.1 6.7

± ± ± ± ±

0.2 0 0 0.1 0.2

3.6 3.4 3.4 3.3 4.6

± ± ± ± ±

0 0 0 0 0.1

8.9 8.2 8.3 8.4 15.8

± ± ± ± ±

0.1 0.3 0.2 0 0

34.1 34.2 34.3 34.3 16.6

± ± ± ± ±

0.1 0.2 0.2 0.1 0.0

48.2 48.8 48.4 48.1 57.5

± ± ± ± ±

0.4 0.45 0.2 0.2 0.1

9.3 8.7 8.6 8.4 15.9

± ± ± ± ±

0 0.1 0 0 0

34.6 34.7 34.7 34.5 16.7

± ± ± ± ±

0.1 0.2 0.1 0.1 0

47.2 47.7 47.6 48.0 57.2

± ± ± ± ±

0 0.1 0.1 0.1 0.1

9.1 8.6 8.8 8.6 16.2

± ± ± ± ±

0.1 0.3 0 0 0.1

34.2 34.5 34.8 34.6 17.1

± ± ± ± ±

0.3 0.3 0 0 0.5

47.2 47.6 47.2 47.6 56.2

± ± ± ± ±

0.2 0.3 0.1 0.1 0.4

10.1 9.4 9.3 9.2 19.3

± ± ± ± ±

0.1 0 0 0.1 0.1

35.1 35.1 35.3 35.1 17.2

± ± ± ± ±

0.1 0.1 0 0.2 0.1

44.3 45.5 45.5 45.7 50.0

± ± ± ± ±

0.4 0.1 0.1 0.1 0.3

day 8

day 18

day 30

a

The ± values indicate the 95% confidence interval for the respective mean.

not have any major effect on the fatty acid profile of Sacha-inchi oil, which is in agreement with previous results obtained for other oilseeds.3−5 High Temperature Storage and Roasting Intensity Effect on Oil Stability. Peroxide Value. The PV of all oil samples increased continuously during storage at 60 °C for 30 days (Figure 3). Sacha-inchi oils from roasted seeds had lower PV than that from unroasted seeds during storage, especially at the end of the storage period (30 days). Roasting intensity slowed down the PV rate of increase of oils during storage, although at the end of the storage period (30 days) the difference in PV between the oils from the lightly and the

due to the formation of phenolic compounds with more potent antioxidant capacities. However, Maillard reaction products, which form during roasting of seeds, are also known to possess antioxidant capacities.16 Furthermore, the formation of oxidative products with reducing power like aldehydes,17 as reflected by the increase in p-anisidine in the present study could also have contributed partially to the observed increase in antioxidant capacity. Tocopherol Content. Sacha-inchi oil, variety Pinto Recodo, showed a high tocopherol content, compared to flaxseed oil (Table 1). Only γ- and δ-tocopherols were detected in Sachainchi oil, 70.6 and 12.6 mg/100g oil, respectively. Oil from roasted seeds had a slightly lower γ-tocopherol content than oil from unroasted seeds; whereas, δ-tocopherols were unaffected by roasting. Oils from seeds roasted to different degrees had similar tocopherol contents (Table 1). These results indicate that only γ-tocopherols were slightly affected by roasting, however, no further decrease in content was detected by increasing roasting intensity. Fatty Acid Profile. The fatty acid profile of Sacha-inchi is shown in Table 2 (day 0). The most abundant fatty acid corresponds to α-linolenic acid (18:3), 48.2%; followed by linoleic acid (18:2), 34.1%; oleic acid (18:1), 8.9%; palmitic acid (16:0), 4.7%; and stearic acid (18:0), 3.3%. These results confirm the high content of α-linolenic acid in Sacha-inchi oil reported previously by Hamaker and others.2 However, the αlinolenic acid content of flaxseed oil obtained in our study is ∼57.5% (Table 2, day 0). It is worth noting the very low content of saturated fatty acids (8.0%) and the very high content of unsaturated fatty acids (91.2%) of Sacha-inchi oil. As a reference, flaxseed oil had 8.9% saturated and 89.9% unsaturated fatty acids. Roasting did

Figure 3. Peroxide values of Sacha-inchi oil unroasted and roasted samples during high-temperature storage at 60 °C for 30 days. Bars for each mean value indicate the 95% confidence interval. 5194



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highly roasted seeds was not significant. The explanation for the lower peroxide content in Sacha-inchi oil from roasted seeds during storage, compared to that from unroasted seeds, seems to be due to the antioxidative protection of the phenolic compounds formed during roasting. Although residual lipoxygenase activity could have been a factor in the oxidation of oil from unroasted seeds, it is unlikely, because the temperature reached at the expeller for the unroasted seeds was around 110 °C. It has been previously shown that this temperature would be high enough to inactivate lipoxygenase.18 p-Anisidine Value. All Sacha-inchi oil samples showed an increase of p-anisidine values during high temperature storage (Figure 4). These values increased rapidly up to day 18, and

Figure 5. Total phenol values of Sacha-inchi oil unroasted and roasted samples during high-temperature storage at 60 °C for 30 days. Bars for each mean value indicate the 95% confidence interval.

Figure 4. p-Anisidine values of Sacha-inchi oil unroasted and roasted samples during high-temperature storage at 60 °C for 30 days. Confidence intervals (95%) are so small that bars do not appear.

from there up to day 30, the increase in p-anisidine values slowed down. Prior to high-temperature storage, Sacha-inchi oil from unroasted seeds showed lower p-anisidine values than oil from roasted seeds. However, during and at the end of the storage period, the reverse is true, indicating that in oils from roasted seeds, p-anisidine values increase at a slower rate than in oil from unroasted seeds during storage (Figure 4). The higher resistance to oxidation of oils from roasted seeds could be attributed to the formation of phenolic compounds with higher antioxidant activity, during the roasting process, possibly derived from precursor compounds with lower antioxidant activity, such as was observed in sesame and canola oil.5−8 Maillard reaction products with antioxidant properties, generated during roasting, may also contribute to the higher stability of oil to oxidation.16,19−21 Total Phenols and DPPH Antioxidant Assays. Sacha-inchi oil samples from unroasted and lightly roasted seeds increased their antioxidant capacitymeasured as total phenols and DPPHduring the high-temperature storage period. However, the antioxidant capacity decreased in oils from medium and highly roasted seeds, especially from the latter (Figures 5 and 6). We surmise that this increase in antioxidant activity in unroasted and lightly roasted oils during storage would be associated with the formation of oxidative products with reducing power as reported previously for aldehydes.17 Alternatively, the decrease in antioxidant activity in medium and highly roasted oils could be due to reactions taking place

Figure 6. DPPH antioxidant activity of Sacha-inchi oil unroasted and roasted samples during high-temperature storage at 60 °C for 30 days. Bars for each mean value indicate the 95% confidence interval.

between phenols and hydroperoxides and possibly through partial regeneration reactions of oxidized tocopherols.22 This is in agreement with the observation that roasting reduces the formation of hydroperoxides and the loss of tocopherols during storage (Figures 3, 7). It is worth noting that total phenols and DPPH exhibited the same trend in all samples. Tocopherol Content. Tocopherol content (both γ- and δtocopherol) was notably reduced during storage at high temperatures in all oil samples (Figure 7). However, tocopherol degradation rate was slower in oils from roasted seeds compared to oil from unroasted seeds; moreover, tocopherol degradation rate was slower in oil samples from more intensely roasted Sacha-inchi seeds (Figure 7). This lower degradation rate of tocopherols in Sacha-inchi oil from roasted seeds seems to result from additional protection from oxidation due to roasting; specifically, higher antioxidant capacity stemming from the formation of compounds with higher antioxidant properties during roasting of seeds. Fatty Acid Profile. Slight changes in the fatty acid profile of all Sacha-inchi oil samples were observed during the hightemperature storage period (30 days) (Table 2). There is a slight trend for the α-linolenic acid relative content to decrease, 5195



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reaction products or new phenolic compounds formed during seed roasting that seem to favor the oil protection.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +(511) 317-1000; e-mail: [email protected]. Funding

We thank the financial support from CONCYTEC, Peru (Contract 142-2006-OAJ). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Mr. Dante Cachique who kindly donated the Sacha-inchi seeds, and to Mr. Gaston Vizcarra for allowing the use of his expeller.

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REFERENCES

(1) Guillen, M. D.; Ruiz, A.; Cabo, N.; Chirinos, R.; Pascual, G. Characterization of Sacha Inchi (Plukenetia volubilis L.) Oil by FTIR Spectroscopy and H NMR Comparison with Linseed Oil. J. Am. Oil Chem. Soc. 2003, 80, 755−762. (2) Hamaker, B. R.; Valles, C.; Gilman, R.; Hardmeier, R. M.; Clark, D.; García, H. H.; Gonzales, A. E.; Kohlstad, I.; Castro, M.; Valdivia, R.; Rodríguez, T.; Lescano, M. Amino acid and fatty acid profiles of the inca peanut (Plukenetia volubilis). Cereal Chem. 1992, 69, 461−463. (3) Lee, Y.-C.; Kim, I.-H.; Chang, J.; Rhee, Y.-K.; Oh, H.-I.; Park, H.K. Chemical composition and oxidative stability of safflower oil prepared with expeller from safflower seeds roasted at different temperature. J. Food Sci. 2004, 69, C33−C38. (4) Yoshida, H. Composition and quality characteristics of sesame seed (Sesamum indicum) oil roasted at different temperatures in an electric oven. J. Sci. Food Agric. 1994, 65, 331−335. (5) Yoshida, H.; Takagi, S. Effects of seed roasting temperature and time on the quality characteristics of sesame (Sesamum indicum) oil. J. Sci. Food Agric. 1997, 75, 19−26. (6) Yen, G.-C.; Shyu, S.-L. Oxidative stability of sesame oil prepared from sesame seed with different roasting temperatures. Food Chem. 1989, 31, 215−224. (7) Wakamatsu, D.; Morimura, S.; Sawa, T.; Kida, K.; Nakai, C.; Maeda, H. Isolation, identification, and structure of a potent alkyl radical scavenger in crude canola oil, canolol. Biosci. Biotechnol. Biochem. 2005, 69, 1568−1574. (8) Spielmeyer, A.; Wagner, A.; Jahreis, G. Influence of thermal treatment of rapeseed on the canolol content. Food Chem. 2009, 112, 944−948. (9) American Oil Chemists’ Society (AOCS). Official Methods and Recommended Practices, 5th ed.; AOCS Press: Champaign, IL, 1998. (10) Ninfali, P.; Bacchiocca, M.; Biagiotti, E.; Servili, M.; Montedoro, G. Validation of the ORAC parameter as a new index of quality and stability of virgin olive oil. J. Am. Oil Chem. Soc. 2002, 79, 977−982. (11) Swain, T.; Hillis, W. E. The phenolic constituents of Prunus domestica. IThe quantitative analysis of phenolic constituents. J. Sci. Food Agric. 1959, 10, 63−68. (12) Thaipong, K.; Boonprakob, U.; Crosby, K.; Cisneros-Zevallos, L.; Byrne, D. Comparison of ABTS, DPPH, FRAP and ORAC assays for estimating antioxidant activity from guava fruit extracts. J. Food Comp. Anal. 2006, 19, 669−675. (13) Rodas-Mendoza, B.; Morera-Pons, S.; Castellote-Bargalló, A. I.; López-Sabater, M. C. Rapid determination by reversed-phase highperformance liquid chromatography of vitamin A and E in infant formulas. J. Chromatogr. A 2003, 1018, 197−202. (14) Misir, R.; Loarveld, B.; Blair, R. Evaluation of a rapid method for preparation of fatty acid methyl esters for analysis by gas-liquid chromatography. J. Chromatogr. 1985, 331, 141−148.

Figure 7. Tocopherol content of Sacha-inchi oil unroasted and roasted samples during high-temperature storage at 60 °C for 30 days. (A) γtocopherol and (B) δ-tocopherol. Bars for each mean value indicate the 95% confidence interval.

while the percentage of all other fatty acids (linoleic, oleic, palmitic, and stearic acids) showed a slight increasing trend. From these results, it can be inferred that the most unsaturated fatty acid, α-linolenic acid, was the fatty acid most susceptible to oxidation during high-temperature storage. This is in agreement with the fact that the higher the unsaturation of a fatty acid the more readily it is oxidized.23 In conclusion, the roasting process of Sacha-inchi seeds caused a slight increase of the oxidation indicators, such as the peroxide value and p-anisidine value of the oil; however, it also increased the antioxidant capacity of the oil as expressed by total phenols and DPPH, which seemed to provide some protection to the oil against oxidation during its storage at high temperatures. During storage stability studies, oils from roasted seeds exhibited lower oxidation and higher tocopherol content compared to oil from unroasted seeds. The intensity of roasting had an important effect in enhancing the protection against oxidation during storage. The fatty acid profile was not affected by roasting. In general, we have shown in this study that roasting favors the stability of Sacha-inchi oil and is recommended for extending its shelf life. Further studies are needed to elucidate the specific compounds formed due to oxidation as well as to characterize the formation of Maillard 5196



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Chemical Composition, Oxidative Stability and Antioxidant Capacity of

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