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Effect of pretreatment with microwaves on oxidative stability and nutraceuticals content of oil from rapeseed
Food Chemistry 121 (2010) 1211–1215
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Effect of pretreatment with microwaves on oxidative stability and nutraceuticals content of oil from rapeseed Sodeif Azadmard-Damirchi a,*, Fatemeh Habibi-Nodeh a, Javad Hesari a, Mahbob Nemati b, Bahram Fathi Achachlouei c a b c
Department of Food Science and Technology, Faculty of Agriculture, University of Tabriz, Tabriz 51664, Iran Department of Pharmacognosy, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz 51664, Iran Faculty of Agriculture, University of Mohaghegh Ardabili, P.O. Box 56199-11367 Ardabil, Iran
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Article history: Received 13 June 2009 Received in revised form 28 January 2010 Accepted 2 February 2010
Keywords: Cold press Microwave Oil stability Phytosterol Rapeseed oil Tocopherol Vegetable oil extraction
a b s t r a c t Demand for oil extracted by cold press, such as rapeseed oil, is increasing, but oil extraction yield, and nutraceuticals content are lower for cold pressed oil, compared with oil extracted by solvent. In this study, rapeseed was treated with microwaves, to investigate the possibility of enhancing oil extraction yield, oxidative stability and nutraceuticals content. Rapeseed was pretreated with microwaves for two different times (2 min and 4 min) and oil was then extracted with a press. To compare the results, oil was also extracted from untreated rapeseed by solvent and press. Results showed that solvent-extracted oil had the highest phytosterol content. Microwave pretreatment of rapeseed can increase the oil extraction yield (by 10%), phytosterols (by 15%) and tocopherols (by 55%) of the oil extracted by press. Oil extracted from untreated rapeseed by press had the lowest oxidative stability (1 h); this was increased to 8 h by pretreatment of rapeseed with microwaves. Therefore, from the obtained results, it is advisable to treat rapeseed with microwaves before extraction by oil press, because it gives a relatively good recovery of oil, with a high amount of nutraceuticals, and can produce oil with a longer shelf life and enhanced value. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Extraction of oils from oilseeds can be carried out by pressing or with solvent. Solvent oil extraction is usually applied to seeds with low content of oil (<20%), such as soybean. Pressing method is applied for seeds with a high amount of oil, such as rapeseed, but this method is relatively inefficient and a large portion of the oil is left in the meal (Anderson, 1996). However, residual oil in the meal can be extracted afterwards by solvent. Solvent extraction method is the most efficient method with less oil remaining in the meal, but this method has some industrial disadvantages, such as plant security problems, emissions of volatile organic compounds into atmosphere, high operation costs, poor quality products caused by high processing temperatures and a relatively high number of processing steps (Buenrostro & López-Munguía, 1986; del Valle & Aguilera, 1999). Oil extraction by mechanical pressing is simpler, safer and contains fewer steps, compared with oil extraction by solvent (Oyinlola, Ojo, & Adekoya, 2004).
* Corresponding author. Tel.: +98 411 3341316; fax: +98 411 3356005. E-mail addresses: [email protected], [email protected] (S. Azadmard-Damirchi). 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.02.006
There are two types of oil pressing: cold press and hot press. No thermal treatment is used in the cold press method, in contrast to hot press, where seeds are pretreated by heat, this is known as cooking (Anderson, 1996). Oil obtained from cold pressing of oilseeds has generally better preserved native properties. Therefore, there is a growing demand in the market for cold pressed oil, such as cold pressed rapeseed oil. However, cold pressing has a lower oil extraction yield, compared to hot pressing. To overcome the low oil extraction yield problem in cold pressing, it is advisable to seek new pretreatments instead of the usual thermal treatment of seeds before pressing. The new pretreatment should allow for better retention and availability of desirable nutraceuticals, such as phytosterols and tocopherols in the extracted oil. Instead of thermal treatment, microwave radiation of seeds is receiving attention (Takagi & Yoshida, 1999; Uquiche, Jeréz, & Ortíz, 2008). Reduced processing time and energy savings are advantages of microwave radiation because the energy is delivered directly, throughout the volume of the material (Thostenson & Chou, 1999) and it is possible to achieve rapid and uniform microwave treatment (Ayappa, Davis, Davis, & Gordon, 1991; Thostenson & Chou, 1999; Venkatesh & Raghavan, 2004). By using microwave radiation, a higher extraction yield can be obtained because the cell membrane is ruptured. In addition, permanent pores
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are generated as a result, enabling the oil to move through the permeable cell walls (Takagi, Ienaga, Tsuchiya, & Yoshida, 1999). The aim of the present study was to investigate in rapeseed the impact of pretreatment by microwave radiation prior to oil extraction by pressing on the phytosterols, tocopherols and oxidative stability of the extracted oil. To the best of our knowledge, there is no information in the literature on the application of microwave radiation as a pretreatment of oilseed to enhance extraction of nutraceuticals from rapeseed oil.
flask containing distilled water. The oxidation process was recorded automatically by measuring the change in conductivity of the distilled water due to the formation of volatile compounds. The oxidative stability index (OSI) is defined as the point of rapid change in the rate of oxidation, and the results are expressed in hours (h).
2. Materials and methods
The tocopherols content of the oil samples was measured by high-performance liquid chromatography (HPLC) (Cecil Instruments Ltd., Cambridge, England) according to the method described by Fathi-Achachlouei and Azadmard-Damirchi (2009), and Tabee et al. (2008a, 2008b). Approximately, 10 mg of the extracted oil were dissolved in 1 ml n-heptane, and 10 ll were directly injected. The column used was LiChro CART 250-4. Tocopherols were detected by fluorescence detector at wavelengths of 294 and 320 nm for excitation and emission, respectively. According to the retention times of reference samples of tocopherols in the chromatogram, each tocopherol in the analysed oil samples was identified. Quantification was carried out using an external standard method with reference samples of tocopherols. All samples were analysed in triplicate and the results reported are the means of these.
2.1. Sample Rapeseed samples were obtained from the local market (Gorgan, Iran). Solvents were obtained from Merck (Darmstadt, Germany). Tocopherols, 5a-cholestane and sterol standards were purchased from Sigma–Aldrich Co. (St. Louis, MO, USA). 2.2. Microwave pretreatment For each microwave pretreatment of rapeseed, 100 g of seeds were placed in an even layer in Pyrex petri dishes (26 cm diameter) inside the microwave (Model: MW2300 GF, 800 W). Samples were microwave treated at a frequency of 2450 MHz for two times of radiation (120 s and 240 s). Rapeseed sample without radiation (0 s radiation time) was used as a control sample. Three sets of 100 g rapeseeds were used at each radiation time. 2.3. Oil extraction by pressing Oil was extracted from the microwave-treated rapeseed samples and the control by pressing with a hydraulic laboratory-made press at 10 MPa for 10 min according to the method described by Willems, Kuipers, and De Haan (2008). With longer time (more than 10 min), there was no additional oil extraction. Fine particles in the expressed oil were separated by filtration. Experiments were carried out on extracted oil samples as soon as possible after oil extraction; otherwise extracted oil samples were stored at 4 °C for a short time. 2.4. Oil extraction by solvent Oil was extracted from rapeseed samples by solvent, according to the method described by Azadmard-Damirchi, Savage, and Dutta (2005), after slight modification. In brief, powdered rapeseeds (100 g) were extracted with 300 ml hexane at room temperature with vigorous shaking for 1 h in an Erlenmeyer flask covered by aluminium foil. Then, the mixture was filtrated through defatted filter papers, using a Buchner funnel under vacuum. Solvent was removed under reduced pressure at 35 °C. Experiments were carried out on extracted oil samples as soon as possible after oil extraction; otherwise extracted oil samples were stored at 4 °C for a short time. Extraction was carried out for three 100-g sets of rapeseeds. 2.5. Oil stability Oxidative stability of the rapeseed oil samples obtained from the previous steps was determined by Rancimat (Metrohm 743 Rancimat; Metrohm, Riverview, FL, USA), according to the method described by Tabee, Azadmard-Damirchi, Jägerstad, and Dutta (2008a). Oil samples were weighed (2.5 g) into the reaction vessel in triplicate and heated to 110 °C with an air flow of 20 l/h. Volatile products released during the oxidation process were collected in a
2.6. Determination of tocopherols by high-performance liquid chromatography
2.7. Phytosterol analysis 2.7.1. Saponification of oil samples For analysis of phytosterols, oil samples were saponified according to the method described by Azadmard-Damirchi et al. (2005), after minor modification. The weighed oil sample (ca. 30 mg) was mixed thoroughly with 3 ml of 2 M KOH in 95% ethanol in a glass tube, and shaken in a water bath at 90 °C for 15 min. After cooling the tubes, 2 ml of water and 1.5 ml of hexane were added and mixed vigorously. Thereafter, the mixture was centrifuged at 3000 rpm for 5 min and the hexane layer containing unsaponifiables was separated for further analysis. 2.7.2. Trimethylsilyl ether derivatives of the phytosterols Trimethylsilyl (TMS) ether derivatives of phytosterol classes were prepared according to the method described by AzadmardDamirchi et al. (2005). 2.7.3. GC analysis of the phytosterols Phytosterols were determined by GC as TMS-ether derivatives according to the method described by Azadmard-Damirchi and Dutta (2006). In brief, a fused-silica capillary column (DB-5MS, 30 m 0.25 mm, 0.50 lm; J&W Scientific, Folsom, CA, USA) was used. The column was connected to a Chrompack CP 9002 gas chromatograph (Middleburg, The Netherlands) equipped with a flame ionization detector. The analysis conditions were: (a) injector 260 °C; (b) oven 60 °C for 1 min, increased at a rate of 40 °C/ min to a final temperature of 310 °C held for 27 min; (c) helium as the carrier gas and nitrogen as the make-up gas at a flow rate of 30 ml/min and (d) detector temperature of 310 °C. According to the retention times of reference samples of phytosterols in the chromatogram, each phytosterol in the analysed oil samples was identified. Quantification was done relative to the 5a-cholestane as an internal standard. All samples were analysed in triplicate, and means of the results are reported. 2.8. Statistical analysis The statistical analyses were carried out with Minitab 13 (Minitab, Inc., State College, PA, USA).
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3. Results and discussion Effect of microwave pretreatment of rapeseed on oil extraction yield by pressing, nutraceuticals content and oil stability were investigated and compared with untreated rapeseed as a control sample. Results showed that microwave pretreatment can increase oil extraction yield. In addition, it was observed that increasing microwave treatment time can increase oil extraction yield; 2 and 4 min pretreated rapeseed samples gave 18% and 25% oil by pressing, respectively (Table 1). Obtained results concur with previously published results (Uquiche et al., 2008). Uquiche et al. (2008) have reported that pretreatment of hazelnuts with microwaves can increase the oil extraction yield, and increasing the treatment time also had a positive effect on the oil extraction yield. It has also been shown that the time of exposure to microwave radiation has a significant effect (p < 0.05) on oil extraction yield rather than level of potency (400 W and 600 W) (Uquiche et al., 2008). Moisture content of the rapeseed samples before microwave treatment was 7%. It has been demonstrated that the low moisture in the microwave-treated samples can make them more brittle and therefore can achieve a greater rupture of tissue and increase the extraction of oil during the mechanical pressing (Uquiche et al., 2008). In untreated oilseeds because the intact cell wall causes a major resistance to oil extraction by pressing, oil extraction yield is low (Aguilera & Stanley, 1999; Uquiche et al., 2008). It has been shown that microwave treatment cause a modification in the cellular wall and gives greater porosity (Uquiche et al., 2008). In this study, oil was also extracted from untreated rapeseed samples by solvent. Solvent extraction gave higher recovery of oil from rapeseed compared with cold pressing (Table 1). It is known that a low amount of oil (about 1%) remains in defatted material after oil extraction by solvent (Anderson, 1996). Therefore, oil extraction by solvent, because of complete extraction of oil from oilseed, could give an accurate oil content of samples, in order to compare oil extraction yield from treated and untreated rapeseed samples by press. Oxidative stability of analysed oil samples was determined by Ransimat. Oxidative stability of vegetable oils is influenced by many factors, mainly fatty acid composition, antioxidants and minor compounds. The stability of oil samples showed significant differences (p < 0.05), depending on the extraction method and
microwave pretreatment, and followed this order: cold pressed oil < solvent extracted oil < oil extracted from 2 min microwavepretreated seeds < oil extracted from 4 min microwave-pretreated seeds (Table 1). Higher stability of oils extracted from microwavepretreated rapeseeds may arise from their high antioxidant content e.g. tocopherols (Table 2). The obtained results concur with previously published data (Veldsink et al., 1999). It has been reported that oil extracted from microwave-treated rapeseed shows a markedly improved oxidative stability, most likely due to the increase of phenolic antioxidants (Veldsink et al., 1999). Tocopherols and tocotrienols are lipophilic antioxidants present in vegetable oils. Tocopherols occur in four related forms, designated alpha (a), beta (b), gamma (c) and delta (d) on the basis of their chromanol ring. The tocopherols have a saturated side-chain, whereas the corresponding tocotrienols have an unsaturated sidechain. Tocopherols are natural antioxidants and inhibit lipid oxidation in fats and oils by modifying the radical chain autoxidation process. Tocopherol content of rapeseed oil samples was determined by HPLC (Fig. 1, Table 2). Analysed rapeseed oil samples had high amounts of a- and c-tocopherols; no tocotrienols could be detected in rapeseed oils under the experimental conditions used in this study. a-Tocopherol was present at lower levels than c-tocopherol in analysed rapeseed oil samples (Table 2). This result concurs with previously published results (Schwartz, Ollilainen, Piironen, & Lampi, 2008). It has been reported that rapeseed oil has high amounts of a- and c-tocopherols and trace amounts of b- and d-tocopherols (Schwartz et al., 2008). Schwartz et al. (2008) have also analysed several samples of rapeseed oils but could not detect tocotrienols. The amount of individual and total tocopherols varied significantly (p < 0.05), depending on the extraction method (Table 2). Oil extracted with solvent had a higher content of total tocopherol (596 lg/g) compared with oil extracted by pressing from untreated rapeseeds (Table 2). It shows that cold pressing is not sufficient to produce oil with enhanced tocopherols content. Oil extracted by pressing from untreated rapeseeds had the lowest total content of tocopherols (510 lg/g) among the analysed oil samples (Table 2). Pretreatment of rapeseeds by microwave prior to oil extraction by press increased the tocopherols significantly (p < 0.05) in oils (Table 2). These results suggested that damage to the oilseed cell membrane by microwave pretreatment allow increased release
Table 1 Extraction yield (%) and oxidative stability (h) of oils extracted from rapeseeds.A Parameter
Extracted by
Extraction yield Oxidative stabilityB
Extracted by press after microwave pretreatment for
Solvent
Cold press
2 min
4 min
38a 2.5c
15d 1d
18c 5b
25b 8a
(a–d) Denotes statistically significant differences (p < 0.05). A Means of triplicate analyses (CV is generally less than 5%). B Oxidative stability was measured by Rancimat (see Section 2 for details).
Table 2 Tocopherols distribution and content of oil extracted from rapeseed samples. Tocopherol
Extracted by
Extracted by press after microwave pretreatment for
Solvent
Cold press
2 min
4 min
a-Tocopherol c-Tocopherol
204A,Bc (34)C 392c (66)
184d (36) 326d (64)
417a (45) 507a (55)
363b (45) 448b (55)
Total
596c
510d
924a
811b
(a–d) Denotes statistically significant differences (p < 0.05). A Means of triplicate analyses (CV is generally less than 5%). B Values are in lg/g oil. C Values are percentages.
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S. Azadmard-Damirchi et al. / Food Chemistry 121 (2010) 1211–1215 Table 3 Phytosterol distribution and content in oil extracted from rapeseed samples.A Phytosterol
Extracted by
Extracted by press after microwave pretreatment for
Solvent
Cold press
2 min
4 min
Cholesterol Brasicasterol Campesterol Stigmasterol Sitosterol D5-Avenasterol
25Ba (0.31)C 750a (9.2) 2900a (35.7) 35a (0.43) 4100a (50.5) 310a (3.8)
15c (0.23) 510d (7.7) 2110d (32) 26c (0.4) 3720d (56.4) 220d (3.3)
19b (0.26) 680c (9.2) 2560c (34.6) 30b (0.41) 3850c (52) 268c (3.6)
23a (0.29) 710b (9.1) 2750b (35.2) 32ab (0.41) 4008b (51.3) 290b (3.7)
Total
8120a
6601d
7407c
7813b
(a–d) Denotes statistically significant differences (p < 0.05). A Means of triplicate analyses (CV is generally less than 5%). B Values are in lg/g oil. C Values are percentages.
Fig. 1. High-performance liquid chromatogram of tocopherols of rapeseed oil. (1) a-Tocopherol; (2) c-tocopherol.
of tocopherols and enhance their amount in extracted oil. The obtained results concur with previously published data (Ko et al., 2003; Oomah, Busson, Godfrey, & Drover, 2002; Oomah, Liang, Godfrey, & Mazza, 1998). Levels of tocopherols in grape seed oil and hemp seed oil extracted from seeds treated by microwave were significantly increased (Oomah et al., 1998, 2002). It has also been reported that contents of a-, b- and c-tocopherol in rice bran oil were significantly (p < 0.05) increased when the rice bran was subjected to microwave heating for up to 30 s (Ko et al., 2003). However, oil extracted from 2 min microwave pretreatment had a higher tocopherol content, compared with oil extracted from 4 min pretreatment (Table 2). It shows if the time of exposure is high, microwave pretreatment can reduce the tocopherol content of extracted oils. This is probably because tocopherols were decomposed to some extent by elevated temperature caused by relatively long period (4 min) of microwave pretreatment. It has also been reported that longer heating of rice bran with microwave can cause a significant degradation of tocopherols (Ko et al., 2003). Tocopherol distribution was not changed in oils obtained by press or solvent, but pretreatment with microwave could change the tocopherols distribution (Table 3). Contribution of a-tocopherol in total content of tocopherols was increased from 34–36% (in oil
extracted from untreated rapeseeds) to 45% (in oil extracted from pretreated rapeseeds) (Table 2). Phytosterols (plant sterols) are minor components of vegetable oils and form a major proportion of the unsaponifiables (Azadmard-Damirchi et al., 2005). Phytosterols in vegetable oils are important from a nutritional point of view because they contribute to lowering serum cholesterol levels, and are also considered to have anti-inflammatory, anti-bacterial, anti-ulcerative and antitumour properties in humans (Beveridge, Li, & Drover, 2002; Moreau, 2004), as well as contributing to the oxidative and thermal stability and shelf life of vegetable oils (Przybylski & Eskin, 2006). Phytosterols were determined as their TMS-ether derivatives by GC (Fig. 2, Table 3). Six phytosterols – cholesterol, brassicasterol, stigmasterol, campesterol, sitosterol and D5-avenasterol – were detected in the analysed rapeseed oil samples (Fig. 2). Sitosterol was predominant (50–56%), followed by campesterol (32–36%) and brassicasterol (8–9%) (Table 3). These results concur with previously published results (Schwartz et al., 2008). The amount of individual and total phytosterols varied significantly (p < 0.05) depending on the extraction method (Table 3). Oil extracted from rapeseed by solvent had the highest phytosterol content (8120 lg/g) among all analysed samples while phytosterol content of oil extracted by press from untreated rapeseed had the lowest amount (6601 lg/g). These results show that solvent extraction is more efficient in extracting phytosterols in oil. This result concurs with previously published results which reported that phytosterol content in sea buckthorn weed oil extracted by hexane is higher, compared with cold press (Li, Beveridge, & Drover, 2007). Phytosterol content in oil samples extracted by press increased with increasing microwave treatment time (Table 3). Total phytosterol contents of oil extracted by press from untreated oil and oil treated for 2 and 4 min with microwaves were 6601, 7407, and 7813 ppm, respectively. These results show that pretreatment of rapeseed by microwaves is a good tool to enrich phytosterol content of extracted oil. It should be mentioned that some of the phytosterols, such as D5-avenasterol, can act as antioxidants (Kamal-Eldin, Appelqvist, & Yousif, 1992) which might be another explanation for the high stability of oil extracted from microwavepretreated rapeseed (Tables 1 and 3). It has recently been reported that microwave pretreatment of rapeseed could increase the canolol content (Spielmeyer, Wagner, & Jahreis, 2009). Canolol is a compound of rapeseed which can be formed by decarboxylation of sinapic acid (Spielmeyer et al., 2009; Wakamatsu et al., 2005). Roasting of rapeseed by microwave could cause an increase of the canolol content in the extracted oil (Spielmeyer et al., 2009). The temperature influences the amount of canolol formed in the rapeseed. Microwave roasting of rapeseeds for 4.5 min with a final temperature of 135 °C caused an in-
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Fig. 2. GC-FID chromatogram of TMS-ether derivatives of phytosterols of rapeseed oil sample. (1) 5a-Cholestane (internal standard); (2) cholesterol; (3) brassicasterol; (4) stigmasterol; (5) campesterol; (6) sitosterol; (7) D5-avenasterol.
crease in the canolol content from 5.8 lg/g to 340.1 lg/g (Spielmeyer et al., 2009). However, higher temperatures (>160 °C) can cause a reduction in the canolol content (Spielmeyer et al., 2009). Canolol, like tocopherols and phytosterols, has antioxidative and positive health activity and its presence in rapeseed oil is desirable. Solvent-extracted rapeseed oil has to be refined and this is a disadvantage because some of the healthy compounds such as tocopherols and phytosterols can be reduced during refining. In the case of canolol, for example, it has been observed that canolol was not detectable in refined rapeseed oils (Spielmeyer et al., 2009). However, cold-pressed oils with good quality do not need refining processes and canolol has been determined at the level of 9–81 lg/g (Spielmeyer et al., 2009). The main reason to extract oil from oilseeds by cold press is to avoid using heat treatment and solvent which can destroy some nutritive components and also possibly cause chemical contaminant. However, it should be kept in mind that cold pressing gives a low recovery of oil from oilseeds and there is a need to extract residual oil from meal afterwards by hot press or solvent, which is an additional process. Therefore, from the obtained results, it is advisable to treat rapeseeds with microwave before extraction by press, because it gives relatively good recovery of oil with a high stability and high amount of nutraceuticals and promotes formation of canolol. Nutraceuticals such as tocopherols, phytosterols and canolols have been reported to have antioxidative and positive health effects. Therefore, increases of rapeseed oil nutraceuticals content can produce oil with longer shelf life and enhanced value.
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Effect of pretreatment with microwaves on oxidative stability and nutraceuticals content of oil from rapeseed Sodeif Azadmard-Damirchi a,*, Fatemeh Habibi-Nodeh a, Javad Hesari a, Mahbob Nemati b, Bahram Fathi Achachlouei c a b c
Department of Food Science and Technology, Faculty of Agriculture, University of Tabriz, Tabriz 51664, Iran Department of Pharmacognosy, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz 51664, Iran Faculty of Agriculture, University of Mohaghegh Ardabili, P.O. Box 56199-11367 Ardabil, Iran
a r t i c l e
i n f o
Article history: Received 13 June 2009 Received in revised form 28 January 2010 Accepted 2 February 2010
Keywords: Cold press Microwave Oil stability Phytosterol Rapeseed oil Tocopherol Vegetable oil extraction
a b s t r a c t Demand for oil extracted by cold press, such as rapeseed oil, is increasing, but oil extraction yield, and nutraceuticals content are lower for cold pressed oil, compared with oil extracted by solvent. In this study, rapeseed was treated with microwaves, to investigate the possibility of enhancing oil extraction yield, oxidative stability and nutraceuticals content. Rapeseed was pretreated with microwaves for two different times (2 min and 4 min) and oil was then extracted with a press. To compare the results, oil was also extracted from untreated rapeseed by solvent and press. Results showed that solvent-extracted oil had the highest phytosterol content. Microwave pretreatment of rapeseed can increase the oil extraction yield (by 10%), phytosterols (by 15%) and tocopherols (by 55%) of the oil extracted by press. Oil extracted from untreated rapeseed by press had the lowest oxidative stability (1 h); this was increased to 8 h by pretreatment of rapeseed with microwaves. Therefore, from the obtained results, it is advisable to treat rapeseed with microwaves before extraction by oil press, because it gives a relatively good recovery of oil, with a high amount of nutraceuticals, and can produce oil with a longer shelf life and enhanced value. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Extraction of oils from oilseeds can be carried out by pressing or with solvent. Solvent oil extraction is usually applied to seeds with low content of oil (<20%), such as soybean. Pressing method is applied for seeds with a high amount of oil, such as rapeseed, but this method is relatively inefficient and a large portion of the oil is left in the meal (Anderson, 1996). However, residual oil in the meal can be extracted afterwards by solvent. Solvent extraction method is the most efficient method with less oil remaining in the meal, but this method has some industrial disadvantages, such as plant security problems, emissions of volatile organic compounds into atmosphere, high operation costs, poor quality products caused by high processing temperatures and a relatively high number of processing steps (Buenrostro & López-Munguía, 1986; del Valle & Aguilera, 1999). Oil extraction by mechanical pressing is simpler, safer and contains fewer steps, compared with oil extraction by solvent (Oyinlola, Ojo, & Adekoya, 2004).
* Corresponding author. Tel.: +98 411 3341316; fax: +98 411 3356005. E-mail addresses: [email protected], [email protected] (S. Azadmard-Damirchi). 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.02.006
There are two types of oil pressing: cold press and hot press. No thermal treatment is used in the cold press method, in contrast to hot press, where seeds are pretreated by heat, this is known as cooking (Anderson, 1996). Oil obtained from cold pressing of oilseeds has generally better preserved native properties. Therefore, there is a growing demand in the market for cold pressed oil, such as cold pressed rapeseed oil. However, cold pressing has a lower oil extraction yield, compared to hot pressing. To overcome the low oil extraction yield problem in cold pressing, it is advisable to seek new pretreatments instead of the usual thermal treatment of seeds before pressing. The new pretreatment should allow for better retention and availability of desirable nutraceuticals, such as phytosterols and tocopherols in the extracted oil. Instead of thermal treatment, microwave radiation of seeds is receiving attention (Takagi & Yoshida, 1999; Uquiche, Jeréz, & Ortíz, 2008). Reduced processing time and energy savings are advantages of microwave radiation because the energy is delivered directly, throughout the volume of the material (Thostenson & Chou, 1999) and it is possible to achieve rapid and uniform microwave treatment (Ayappa, Davis, Davis, & Gordon, 1991; Thostenson & Chou, 1999; Venkatesh & Raghavan, 2004). By using microwave radiation, a higher extraction yield can be obtained because the cell membrane is ruptured. In addition, permanent pores
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are generated as a result, enabling the oil to move through the permeable cell walls (Takagi, Ienaga, Tsuchiya, & Yoshida, 1999). The aim of the present study was to investigate in rapeseed the impact of pretreatment by microwave radiation prior to oil extraction by pressing on the phytosterols, tocopherols and oxidative stability of the extracted oil. To the best of our knowledge, there is no information in the literature on the application of microwave radiation as a pretreatment of oilseed to enhance extraction of nutraceuticals from rapeseed oil.
flask containing distilled water. The oxidation process was recorded automatically by measuring the change in conductivity of the distilled water due to the formation of volatile compounds. The oxidative stability index (OSI) is defined as the point of rapid change in the rate of oxidation, and the results are expressed in hours (h).
2. Materials and methods
The tocopherols content of the oil samples was measured by high-performance liquid chromatography (HPLC) (Cecil Instruments Ltd., Cambridge, England) according to the method described by Fathi-Achachlouei and Azadmard-Damirchi (2009), and Tabee et al. (2008a, 2008b). Approximately, 10 mg of the extracted oil were dissolved in 1 ml n-heptane, and 10 ll were directly injected. The column used was LiChro CART 250-4. Tocopherols were detected by fluorescence detector at wavelengths of 294 and 320 nm for excitation and emission, respectively. According to the retention times of reference samples of tocopherols in the chromatogram, each tocopherol in the analysed oil samples was identified. Quantification was carried out using an external standard method with reference samples of tocopherols. All samples were analysed in triplicate and the results reported are the means of these.
2.1. Sample Rapeseed samples were obtained from the local market (Gorgan, Iran). Solvents were obtained from Merck (Darmstadt, Germany). Tocopherols, 5a-cholestane and sterol standards were purchased from Sigma–Aldrich Co. (St. Louis, MO, USA). 2.2. Microwave pretreatment For each microwave pretreatment of rapeseed, 100 g of seeds were placed in an even layer in Pyrex petri dishes (26 cm diameter) inside the microwave (Model: MW2300 GF, 800 W). Samples were microwave treated at a frequency of 2450 MHz for two times of radiation (120 s and 240 s). Rapeseed sample without radiation (0 s radiation time) was used as a control sample. Three sets of 100 g rapeseeds were used at each radiation time. 2.3. Oil extraction by pressing Oil was extracted from the microwave-treated rapeseed samples and the control by pressing with a hydraulic laboratory-made press at 10 MPa for 10 min according to the method described by Willems, Kuipers, and De Haan (2008). With longer time (more than 10 min), there was no additional oil extraction. Fine particles in the expressed oil were separated by filtration. Experiments were carried out on extracted oil samples as soon as possible after oil extraction; otherwise extracted oil samples were stored at 4 °C for a short time. 2.4. Oil extraction by solvent Oil was extracted from rapeseed samples by solvent, according to the method described by Azadmard-Damirchi, Savage, and Dutta (2005), after slight modification. In brief, powdered rapeseeds (100 g) were extracted with 300 ml hexane at room temperature with vigorous shaking for 1 h in an Erlenmeyer flask covered by aluminium foil. Then, the mixture was filtrated through defatted filter papers, using a Buchner funnel under vacuum. Solvent was removed under reduced pressure at 35 °C. Experiments were carried out on extracted oil samples as soon as possible after oil extraction; otherwise extracted oil samples were stored at 4 °C for a short time. Extraction was carried out for three 100-g sets of rapeseeds. 2.5. Oil stability Oxidative stability of the rapeseed oil samples obtained from the previous steps was determined by Rancimat (Metrohm 743 Rancimat; Metrohm, Riverview, FL, USA), according to the method described by Tabee, Azadmard-Damirchi, Jägerstad, and Dutta (2008a). Oil samples were weighed (2.5 g) into the reaction vessel in triplicate and heated to 110 °C with an air flow of 20 l/h. Volatile products released during the oxidation process were collected in a
2.6. Determination of tocopherols by high-performance liquid chromatography
2.7. Phytosterol analysis 2.7.1. Saponification of oil samples For analysis of phytosterols, oil samples were saponified according to the method described by Azadmard-Damirchi et al. (2005), after minor modification. The weighed oil sample (ca. 30 mg) was mixed thoroughly with 3 ml of 2 M KOH in 95% ethanol in a glass tube, and shaken in a water bath at 90 °C for 15 min. After cooling the tubes, 2 ml of water and 1.5 ml of hexane were added and mixed vigorously. Thereafter, the mixture was centrifuged at 3000 rpm for 5 min and the hexane layer containing unsaponifiables was separated for further analysis. 2.7.2. Trimethylsilyl ether derivatives of the phytosterols Trimethylsilyl (TMS) ether derivatives of phytosterol classes were prepared according to the method described by AzadmardDamirchi et al. (2005). 2.7.3. GC analysis of the phytosterols Phytosterols were determined by GC as TMS-ether derivatives according to the method described by Azadmard-Damirchi and Dutta (2006). In brief, a fused-silica capillary column (DB-5MS, 30 m 0.25 mm, 0.50 lm; J&W Scientific, Folsom, CA, USA) was used. The column was connected to a Chrompack CP 9002 gas chromatograph (Middleburg, The Netherlands) equipped with a flame ionization detector. The analysis conditions were: (a) injector 260 °C; (b) oven 60 °C for 1 min, increased at a rate of 40 °C/ min to a final temperature of 310 °C held for 27 min; (c) helium as the carrier gas and nitrogen as the make-up gas at a flow rate of 30 ml/min and (d) detector temperature of 310 °C. According to the retention times of reference samples of phytosterols in the chromatogram, each phytosterol in the analysed oil samples was identified. Quantification was done relative to the 5a-cholestane as an internal standard. All samples were analysed in triplicate, and means of the results are reported. 2.8. Statistical analysis The statistical analyses were carried out with Minitab 13 (Minitab, Inc., State College, PA, USA).
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3. Results and discussion Effect of microwave pretreatment of rapeseed on oil extraction yield by pressing, nutraceuticals content and oil stability were investigated and compared with untreated rapeseed as a control sample. Results showed that microwave pretreatment can increase oil extraction yield. In addition, it was observed that increasing microwave treatment time can increase oil extraction yield; 2 and 4 min pretreated rapeseed samples gave 18% and 25% oil by pressing, respectively (Table 1). Obtained results concur with previously published results (Uquiche et al., 2008). Uquiche et al. (2008) have reported that pretreatment of hazelnuts with microwaves can increase the oil extraction yield, and increasing the treatment time also had a positive effect on the oil extraction yield. It has also been shown that the time of exposure to microwave radiation has a significant effect (p < 0.05) on oil extraction yield rather than level of potency (400 W and 600 W) (Uquiche et al., 2008). Moisture content of the rapeseed samples before microwave treatment was 7%. It has been demonstrated that the low moisture in the microwave-treated samples can make them more brittle and therefore can achieve a greater rupture of tissue and increase the extraction of oil during the mechanical pressing (Uquiche et al., 2008). In untreated oilseeds because the intact cell wall causes a major resistance to oil extraction by pressing, oil extraction yield is low (Aguilera & Stanley, 1999; Uquiche et al., 2008). It has been shown that microwave treatment cause a modification in the cellular wall and gives greater porosity (Uquiche et al., 2008). In this study, oil was also extracted from untreated rapeseed samples by solvent. Solvent extraction gave higher recovery of oil from rapeseed compared with cold pressing (Table 1). It is known that a low amount of oil (about 1%) remains in defatted material after oil extraction by solvent (Anderson, 1996). Therefore, oil extraction by solvent, because of complete extraction of oil from oilseed, could give an accurate oil content of samples, in order to compare oil extraction yield from treated and untreated rapeseed samples by press. Oxidative stability of analysed oil samples was determined by Ransimat. Oxidative stability of vegetable oils is influenced by many factors, mainly fatty acid composition, antioxidants and minor compounds. The stability of oil samples showed significant differences (p < 0.05), depending on the extraction method and
microwave pretreatment, and followed this order: cold pressed oil < solvent extracted oil < oil extracted from 2 min microwavepretreated seeds < oil extracted from 4 min microwave-pretreated seeds (Table 1). Higher stability of oils extracted from microwavepretreated rapeseeds may arise from their high antioxidant content e.g. tocopherols (Table 2). The obtained results concur with previously published data (Veldsink et al., 1999). It has been reported that oil extracted from microwave-treated rapeseed shows a markedly improved oxidative stability, most likely due to the increase of phenolic antioxidants (Veldsink et al., 1999). Tocopherols and tocotrienols are lipophilic antioxidants present in vegetable oils. Tocopherols occur in four related forms, designated alpha (a), beta (b), gamma (c) and delta (d) on the basis of their chromanol ring. The tocopherols have a saturated side-chain, whereas the corresponding tocotrienols have an unsaturated sidechain. Tocopherols are natural antioxidants and inhibit lipid oxidation in fats and oils by modifying the radical chain autoxidation process. Tocopherol content of rapeseed oil samples was determined by HPLC (Fig. 1, Table 2). Analysed rapeseed oil samples had high amounts of a- and c-tocopherols; no tocotrienols could be detected in rapeseed oils under the experimental conditions used in this study. a-Tocopherol was present at lower levels than c-tocopherol in analysed rapeseed oil samples (Table 2). This result concurs with previously published results (Schwartz, Ollilainen, Piironen, & Lampi, 2008). It has been reported that rapeseed oil has high amounts of a- and c-tocopherols and trace amounts of b- and d-tocopherols (Schwartz et al., 2008). Schwartz et al. (2008) have also analysed several samples of rapeseed oils but could not detect tocotrienols. The amount of individual and total tocopherols varied significantly (p < 0.05), depending on the extraction method (Table 2). Oil extracted with solvent had a higher content of total tocopherol (596 lg/g) compared with oil extracted by pressing from untreated rapeseeds (Table 2). It shows that cold pressing is not sufficient to produce oil with enhanced tocopherols content. Oil extracted by pressing from untreated rapeseeds had the lowest total content of tocopherols (510 lg/g) among the analysed oil samples (Table 2). Pretreatment of rapeseeds by microwave prior to oil extraction by press increased the tocopherols significantly (p < 0.05) in oils (Table 2). These results suggested that damage to the oilseed cell membrane by microwave pretreatment allow increased release
Table 1 Extraction yield (%) and oxidative stability (h) of oils extracted from rapeseeds.A Parameter
Extracted by
Extraction yield Oxidative stabilityB
Extracted by press after microwave pretreatment for
Solvent
Cold press
2 min
4 min
38a 2.5c
15d 1d
18c 5b
25b 8a
(a–d) Denotes statistically significant differences (p < 0.05). A Means of triplicate analyses (CV is generally less than 5%). B Oxidative stability was measured by Rancimat (see Section 2 for details).
Table 2 Tocopherols distribution and content of oil extracted from rapeseed samples. Tocopherol
Extracted by
Extracted by press after microwave pretreatment for
Solvent
Cold press
2 min
4 min
a-Tocopherol c-Tocopherol
204A,Bc (34)C 392c (66)
184d (36) 326d (64)
417a (45) 507a (55)
363b (45) 448b (55)
Total
596c
510d
924a
811b
(a–d) Denotes statistically significant differences (p < 0.05). A Means of triplicate analyses (CV is generally less than 5%). B Values are in lg/g oil. C Values are percentages.
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Extracted by
Extracted by press after microwave pretreatment for
Solvent
Cold press
2 min
4 min
Cholesterol Brasicasterol Campesterol Stigmasterol Sitosterol D5-Avenasterol
25Ba (0.31)C 750a (9.2) 2900a (35.7) 35a (0.43) 4100a (50.5) 310a (3.8)
15c (0.23) 510d (7.7) 2110d (32) 26c (0.4) 3720d (56.4) 220d (3.3)
19b (0.26) 680c (9.2) 2560c (34.6) 30b (0.41) 3850c (52) 268c (3.6)
23a (0.29) 710b (9.1) 2750b (35.2) 32ab (0.41) 4008b (51.3) 290b (3.7)
Total
8120a
6601d
7407c
7813b
(a–d) Denotes statistically significant differences (p < 0.05). A Means of triplicate analyses (CV is generally less than 5%). B Values are in lg/g oil. C Values are percentages.
Fig. 1. High-performance liquid chromatogram of tocopherols of rapeseed oil. (1) a-Tocopherol; (2) c-tocopherol.
of tocopherols and enhance their amount in extracted oil. The obtained results concur with previously published data (Ko et al., 2003; Oomah, Busson, Godfrey, & Drover, 2002; Oomah, Liang, Godfrey, & Mazza, 1998). Levels of tocopherols in grape seed oil and hemp seed oil extracted from seeds treated by microwave were significantly increased (Oomah et al., 1998, 2002). It has also been reported that contents of a-, b- and c-tocopherol in rice bran oil were significantly (p < 0.05) increased when the rice bran was subjected to microwave heating for up to 30 s (Ko et al., 2003). However, oil extracted from 2 min microwave pretreatment had a higher tocopherol content, compared with oil extracted from 4 min pretreatment (Table 2). It shows if the time of exposure is high, microwave pretreatment can reduce the tocopherol content of extracted oils. This is probably because tocopherols were decomposed to some extent by elevated temperature caused by relatively long period (4 min) of microwave pretreatment. It has also been reported that longer heating of rice bran with microwave can cause a significant degradation of tocopherols (Ko et al., 2003). Tocopherol distribution was not changed in oils obtained by press or solvent, but pretreatment with microwave could change the tocopherols distribution (Table 3). Contribution of a-tocopherol in total content of tocopherols was increased from 34–36% (in oil
extracted from untreated rapeseeds) to 45% (in oil extracted from pretreated rapeseeds) (Table 2). Phytosterols (plant sterols) are minor components of vegetable oils and form a major proportion of the unsaponifiables (Azadmard-Damirchi et al., 2005). Phytosterols in vegetable oils are important from a nutritional point of view because they contribute to lowering serum cholesterol levels, and are also considered to have anti-inflammatory, anti-bacterial, anti-ulcerative and antitumour properties in humans (Beveridge, Li, & Drover, 2002; Moreau, 2004), as well as contributing to the oxidative and thermal stability and shelf life of vegetable oils (Przybylski & Eskin, 2006). Phytosterols were determined as their TMS-ether derivatives by GC (Fig. 2, Table 3). Six phytosterols – cholesterol, brassicasterol, stigmasterol, campesterol, sitosterol and D5-avenasterol – were detected in the analysed rapeseed oil samples (Fig. 2). Sitosterol was predominant (50–56%), followed by campesterol (32–36%) and brassicasterol (8–9%) (Table 3). These results concur with previously published results (Schwartz et al., 2008). The amount of individual and total phytosterols varied significantly (p < 0.05) depending on the extraction method (Table 3). Oil extracted from rapeseed by solvent had the highest phytosterol content (8120 lg/g) among all analysed samples while phytosterol content of oil extracted by press from untreated rapeseed had the lowest amount (6601 lg/g). These results show that solvent extraction is more efficient in extracting phytosterols in oil. This result concurs with previously published results which reported that phytosterol content in sea buckthorn weed oil extracted by hexane is higher, compared with cold press (Li, Beveridge, & Drover, 2007). Phytosterol content in oil samples extracted by press increased with increasing microwave treatment time (Table 3). Total phytosterol contents of oil extracted by press from untreated oil and oil treated for 2 and 4 min with microwaves were 6601, 7407, and 7813 ppm, respectively. These results show that pretreatment of rapeseed by microwaves is a good tool to enrich phytosterol content of extracted oil. It should be mentioned that some of the phytosterols, such as D5-avenasterol, can act as antioxidants (Kamal-Eldin, Appelqvist, & Yousif, 1992) which might be another explanation for the high stability of oil extracted from microwavepretreated rapeseed (Tables 1 and 3). It has recently been reported that microwave pretreatment of rapeseed could increase the canolol content (Spielmeyer, Wagner, & Jahreis, 2009). Canolol is a compound of rapeseed which can be formed by decarboxylation of sinapic acid (Spielmeyer et al., 2009; Wakamatsu et al., 2005). Roasting of rapeseed by microwave could cause an increase of the canolol content in the extracted oil (Spielmeyer et al., 2009). The temperature influences the amount of canolol formed in the rapeseed. Microwave roasting of rapeseeds for 4.5 min with a final temperature of 135 °C caused an in-
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Fig. 2. GC-FID chromatogram of TMS-ether derivatives of phytosterols of rapeseed oil sample. (1) 5a-Cholestane (internal standard); (2) cholesterol; (3) brassicasterol; (4) stigmasterol; (5) campesterol; (6) sitosterol; (7) D5-avenasterol.
crease in the canolol content from 5.8 lg/g to 340.1 lg/g (Spielmeyer et al., 2009). However, higher temperatures (>160 °C) can cause a reduction in the canolol content (Spielmeyer et al., 2009). Canolol, like tocopherols and phytosterols, has antioxidative and positive health activity and its presence in rapeseed oil is desirable. Solvent-extracted rapeseed oil has to be refined and this is a disadvantage because some of the healthy compounds such as tocopherols and phytosterols can be reduced during refining. In the case of canolol, for example, it has been observed that canolol was not detectable in refined rapeseed oils (Spielmeyer et al., 2009). However, cold-pressed oils with good quality do not need refining processes and canolol has been determined at the level of 9–81 lg/g (Spielmeyer et al., 2009). The main reason to extract oil from oilseeds by cold press is to avoid using heat treatment and solvent which can destroy some nutritive components and also possibly cause chemical contaminant. However, it should be kept in mind that cold pressing gives a low recovery of oil from oilseeds and there is a need to extract residual oil from meal afterwards by hot press or solvent, which is an additional process. Therefore, from the obtained results, it is advisable to treat rapeseeds with microwave before extraction by press, because it gives relatively good recovery of oil with a high stability and high amount of nutraceuticals and promotes formation of canolol. Nutraceuticals such as tocopherols, phytosterols and canolols have been reported to have antioxidative and positive health effects. Therefore, increases of rapeseed oil nutraceuticals content can produce oil with longer shelf life and enhanced value.
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