Characteristics of flaxseed hull oil

Characteristics of flaxseed hull oil

Food Chemistry 114 (2009) 623–628 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Chara...

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Food Chemistry 114 (2009) 623–628

Contents lists available at ScienceDirect

Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Characteristics of flaxseed hull oil q B. Dave Oomah a,*, Laurie Sitter b a

National Bioproducts and Bioprocesses Program, Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, 4200 Highway 97, P.O. Box 5000, Summerland, BC, Canada V0H 1Z0 Université de Bretagne Occidentale, Institut Universitaire Professionnalisé, 29000 Quimper, France

b

a r t i c l e

i n f o

Article history: Received 17 July 2008 Received in revised form 22 August 2008 Accepted 30 September 2008

Keywords: Hull oil Lipid fractions DSC Antioxidant activity Solvent extraction Supercritical CO2 SDG Flaxseed

a b s t r a c t Oils from two commercial flaxseed hulls extracted by six procedures were evaluated for physicochemical characteristics. Oil yield ranged from 9% to 28% depending on solvent and extraction. Lipid fractionation of crude flaxseed hull oil yielded 92.5% neutral lipids, 3.1% phospholipids, 2.4% acidic lipids and 2.1% free fatty acids. Flaxseed hull oil exhibited three thermal transitions between 35 and 13 °C with solvent dependent polymorphism. Thermal oxidation by differential scanning calorimetry (DSC) revealed three step oxidation of flaxseed hull oil with mean onset and oxidation temperatures at 121 and 150– 253 °C, respectively depending on the extraction procedure. Flaxseed hull oil exhibited two-fold difference (0.6–1.2 lm Trolox equivalent/g) in antioxidant activity measured by a photochemiluminescence (PCL) assay. Supercritical CO2 extracted the most oil with the highest antioxidant capacity of all evaluated procedures resulting in a defatted flaxseed hull containing the highest (53 mg/g) secoisolariciresinol diglucoside (SDG) level. Ó 2008 B. D. Oomah and L. Sitter of the department of Agriculture and Food, Government of Canada. Published by Elsevier Ltd. All rights reserved.

1. Introduction Recent studies have highlighted the numerous health benefits of flaxseed oil for the cardiovascular and skeletal systems (Griel et al., 2007) and in inflammatory conditions such as rheumatoid arthritis, psoriasis, and ulcerative colitis (Mantzioris, James, Gibson, & Cleland, 1994). It can lower blood pressure particularly in middle age dislipidaemic men (Paschos, Magkos, Panagiotakos, Volteas, & Zampelas, 2007), and provides significant improvement in attention deficit hyperactivity disorder in children (Stark, Crawford, & Reifen, 2008 and references therein). Flaxseed oil consumption has significant effect on slowing bleeding time thereby reducing the risk of myocardial infarction in type 2 diabetic Canadian male patients (Barre, Griscti, Mizier-Barre, & Hafez, 2005). In a randomised controlled study, consumption of flaxseed oil significantly increased the n3 (a-linolenic acid, eicosapentaenoic acid and docosapentaenoic acid) fatty acid content in red blood cells and in all tissues except brain (Barceló-Coblijn, 2007). Although the demand for flaxseed oil as the consumers preferred source of a-linolenic acid is being increasingly recognised by industry, regulators and consumers, due to heightened profile of omega-3 health benefits, a good reserve of this oil from flaxseed hulls remains untapped and underdeveloped. q

Pacific Agri-Food Research Centre. * Corresponding author. Tel.: +250 494 6399; fax: +250 494 0755. E-mail address: [email protected] (B.D. Oomah).

Flaxseed hulls, a low-valued co-product obtained from flax processing represents a potential source of value-added healthy products. The hull, including the seed coat and endosperm, constitutes 36% of the total weight of hand-dissected flaxseed or 22% of the seed when obtained mechanically (Oomah & Mazza, 1998). Flaxseed hull is difficult to digest and therefore hinders access to the lipids (Barceló-Coblijn, 2007). Oil content of flaxseed hulls varies from 26% to 30% depending on processing conditions (Oomah, 2003) representing approximately 18% of the total seed oil. This oil from hulls obtained by dry abrasive dehulling contained significantly higher levels of palmitic acid and lowest level of stearic and oleic acids compared to those from the whole seed (Oomah & Mazza, 1997). Information currently available on characteristics of flaxseed hull oil is deficient and full investigations on oils from commercially available flaxseed hull are long overdue. This investigation describes the characteristics of oil obtained by various solvents from flaxseed hulls to increase its value and contribution to the development of new omega-3 products for the functional foods and nutraceutical applications.

2. Materials and methods Flaxseed hulls were obtained from commercial sources (NatunolaÒ Omega-3 flax hull from Natunola Health, Nepean, ON and Pizzey’s FortiGradTM from Pizzey’s Milling, Angusville, MB). Hulls from both sources were ground in a coffee grinder prior to

0308-8146/$ - see front matter Ó 2008 B. D. Oomah and L. Sitter of the department of Agriculture and Food, Government of Canada. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2008.09.096

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oil extraction by six different procedures. Oil from all hull samples was extracted using hexane (50 g sample in 500 ml hexane) or other solvents, as described by Oomah, Mazza, and Przybylski (1996), purged with nitrogen and stored at 20 °C until analysis. Petroleum ether extracted oil was obtained using the Goldfish fat extraction apparatus (Labconco, USA) for 6 h according to AOAC International method (2000). Acetone oil was obtained by extracting hulls (50 g sample in 500 ml 50% aqueous acetone) for 15 h at ambient temperature and the solvent removed by vacuum filtration. The residue was re-extracted with 100% acetone for 1 h, vacuum filtered, acetone removed (vacuum rotary evaporation, 35 °C), purged with nitrogen and stored in the dark at 20 °C until analysis. Ethanol extraction was achieved using 70% ethanol adjusted to pH 2.0 with formic acid for 15 h at ambient temperature followed by solvent removal with vacuum filtration. The residue was re-extracted with hexane three times (1 h each), the supernatants were pooled, vacuum filtered, and hexane removed as above to obtain oil. Supercritical CO2 extraction was carried out based on AOAC International method (2000) without co-solvent for 1 h in a laboratory-scale supercritical fluid extraction system (Thar Technologies Inc., Pittsburgh, PA) with carbon dioxide (99.95% purity, Praxair, Edmonton, AB) compressed to 52 kPa with the vessel (500 ml) temperature controlled at 100 °C. Flaxseed hulls equilibrated to 23% moisture were hydraulically pressed (Carver Press, 280 kg/cm2) to extract cold-pressed oil. Commercial cold-pressed flaxseed oil (Omega Nutrition, Vancouver, BC) was used as control. Extractions were performed in triplicate and analysed separately. 2.1. Analytical procedures Thermal characteristics of oils were measured using a modulated differential scanning calorimeter (Modulated DSC-2910, TA Instruments, New Castle, DE) described previously (Oomah, Dumon, Cardador-Martinez, & Godfrey, 2006). A flow of nitrogen gas (100 ml/min) was used in the cell cooled by helium (150 ml/ min) in a refrigerated cooling system. The instrument was calibrated for temperature and heat flow with mercury (melting point, mp = 38.8 °C, TA Instruments standard), gallium (mp = 29.8 °C, TA Instruments standard) and indium (mp = 156.6 °C, DH = 28.7 J/g, Aldrich Chemical Co.). Oil samples (4–5 mg) were weighed in open solid fat index (SFI) aluminium pans (T70529, TA Instruments). An empty similar pan was used as reference. The sample and reference pans were then placed inside the calorimeter and kept at 70 °C for 5 min. The temperature was lowered from 70 to 65 °C at a rate of 1 °C/min with modulation at a period of 60 s and temperature amplitude of 0.16 °C. Samples were then kept at 65 °C for 1 min, and then raised again at the same rate up to 70 °C. Scans were performed at 10 °C/min. For thermal oxidation, pans were cooled to 10 °C and scanning was done by heating at 1 °C/min to 350 °C in the presence of oxygen (100 ml/min). Thermal oxidation measurements were performed in duplicate. Separation of individual lipid classes was performed using solid-phase extraction cartridge, (Bakerbond amino [NH2] disposable extraction column, 500 mg, J.T. Baker Inc., Phillipsburg, NJ), with aminopropyl packing, essentially as described by Oomah, Ladet, Godfrey, Liang, and Girard (2000) based on the procedure of Carelli, Brevedan, and Crapiste (1997). The cartridge was preconditioned with 2 ml methanol, 2 ml chloroform, and 4 ml hexane before use. A micropipette was used to inject 150 mg of oil dissolved in chloroform. Lipid classes were recovered by sequential elution under vacuum (5–10 mm Hg) with 4 ml each of chloroform/isopropanol (2/1, v/v), diethyl ether/acetic acid (95/5, v/v), and methanol to separate neutral lipids, free fatty acids, and phospholipids, respectively. The eluates were collected, evaporated under nitrogen, weighed, and stored at 20 °C. Acidic lipids were eluted with hexane/isopropanol/ethanol/0.1 M ammonium acetate/formic acid

(420/350/100/50/0.5) containing 5% phosphoric acid according to Kim and Salem (1990), evaporated for 10 min and then extracted with 1 ml of chloroform three times after adding 1 ml of water. The chloroform fractions were combined, evaporated under nitrogen and weighed to determine yield. 2.2. Antioxidant assay The photochemiluminescence (PCL) assay, based on the methodology of Popov and Lewin (1999) was used to measure the antioxidant activity of oils with a PhotochemÒ instrument (Analytik Jena, USA Inc., Delaware, OH) against superoxide anion radicals generated from luminol, a photosensitiser, when exposed to UV light. Oil samples (100 mg) were diluted with hexane (1 ml) prior to antioxidant determination described previously (Oomah, Tiger, Olson, & Balasubramanian, 2006) using the ‘ACL’ kit provided by the manufacturer designed to measure the antioxidant activity of lipophillic compounds. Antioxidant activity was monitored for 180 s and expressed as lm Trolox equivalent/g sample. Antioxidant assay was duplicated for each sample. 2.3. Extraction and analysis of secoisolariciresinol diglucoside The lignan, secoisolariciresinol diglucoside (SDG) in defatted flaxseed hulls was extracted and analysed essentially as described previously (Ho, Cacace, & Mazza, 2007) by high performance liquid chromatography based on the modified procedure of Muir and Westcott (2000). Briefly, defatted flaxseed hulls (0.5 g) were extracted by direct hydrolysis based on the protocol of Eliasson, Kamal-Eldin, Andersson, and Åman (2003) as described by Ho et al. (2007). The hydrolysates were acidified (5 ml of 2 M H2SO4), centrifuged (11,000g, 10 min) and the supernatants (0.6 ml) were mixed with methanol (0.9 ml, 30 min), re-centrifuged (11,000g, 5 min) and filtered (0.45 lm, Gelman Science Inc., Ann Arbor, MI) prior to HPLC analysis. SDG was separated on a Luna C18, 5 lm, 100 Å, 250  3.00 mm column with a C18 Security Guard cartridge (Phenomenex, Torrance, CA) using a Waters HPLC System (Waters Corp., Milford, MA) consisting of a pump (Model 600), an autosampler (Model 717 plus), a degasser (Agilent 1100) and a 996 photodiode array detector operated by the Empower software. The linear gradient elution with mobile phases consisting of 0.025% TFA in water (solvent A) and methanol (solvent B) was carried out at 30 °C at 0.4 ml/min for 70 min (Ho et al., 2007). SDG was detected at 280 nm and quantified based on external SDG (ChromaDex, Santa Ana, CA) calibration curves ranging from 10 to 400 lg/ml. At least three determinations were made for all assays, except for antioxidant activity which was determined in duplicate. Analysis of variance by the general linear models (GLM) procedure and means comparisons by Duncan’s test were performed according to Statistical Analysis System (SAS Institute, 1990). 3. Results and discussion Oil yield from commercial flaxseed hulls (18.2%, Table 1) was lower than the values (28%) reported earlier for hull fractions of various cultivars obtained by dehulling (Oomah & Mazza, 1997; Oomah & Mazza, 1998). Fat content (18% and 27% dry matter) was also reported for flaxseed powder obtained by abrasion for 1 and 2 min, respectively (Myllymaki, 2002). Oil content of flaxseed hull obtained by the Urschel Comitrol Processor (Madhusudhan, Wiesenborn, Schwarz, Tostenson, & Gillespie, 2000) showed a range (15–17%) comparable to values observed in this study. The highest yield was generally obtained from Natunola hulls (20.3%) in accordance with the manufacturer fact sheet (minimum of 20% oil). Differences in extraction methods were statistically significant (P < 0.0002) due mostly to differences observed for Natunola

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B.D. Oomah, L. Sitter / Food Chemistry 114 (2009) 623–628 Table 1 Fractionation of flaxseed hull oils*. Sample

Oil yield (%)

Neutral lipid

Free fatty acid

Phospholipids

Acidic lipids

Acetone Cold press Ethanol Hexane Petroleum Ether Supercritical CO2 Omega Natunola Fortigrad Mean

28.3a 9.10d 12.1cd 19.3b 18.3bc 20.4b na 20.4x 15.7y 18.2

92.9a 93.6a 91.0ab 93.0a 87.8b 93.4a 92.8 93.1 91.8 92.5

2.09 1.66 2.01 2.34 2.56 2.30 1.37 1.73y 2.48x 2.05

2.33 3.24 2.36 2.25 4.35 3.26 3.68 2.94 3.03 3.05

2.70ab 1.45b 3.65ab 2.39ab 5.27a 1.01b 2.16 2.19 2.70 2.43

Means in a column followed by the same letter are not significantly different by Duncan’s multiple range test at the 5% level. na, Not applicable.

hulls. Overall, extraction with acetone and cold press produced the highest (28.2%) and lowest (9.1%) oil yield, respectively, independent of the source material. Variation in oil yield (18.3–20.4%) (Table 1) obtained using hexane and petroleum ether and with supercritical CO2 was not significantly different. Hexane, ether and supercritical CO2 are known to extract simple neutral lipids whereas acetone also extracts polar lipids due to its high polarity leading to increase in oil recovery. The supercritical CO2 method yielded more oil (statistically not significant at P = 0.05) than those with petroleum ether or hexane contrary to lower (10–34%) recoveries reported for flaxseed (Barthet & Daun, 2002; Bozan & Temelli, 2002). A comparative oil yield (20.5%) has been reported for rice bran extracted with supercritical CO2 and hexane (Kuk & Dowd, 1998). Accelerated solvent extraction (ASE 100, Dionex Corporation, Sunnyvale, CA) with default method conditions (10 kPa, 105 °C, 40 min) of Natunola flaxseed hulls produced oil (22.9 ± 0.08%, w/w) similar to that obtained by the lengthy petroleum extraction method (22.3 ± 0.08%, w/w), thereby validating the actual oil content of the material independent of methodology. Flaxseed hull oils consisted primarily of neutral lipids (92.5%; range 87.8–93.6%) with minor amounts of free fatty acids (2.1%; range 1.4–2.6%), phospholipids (3.1%; range 2.25–4.35%) and acidic lipids (2.4%; range 1.0–5.3%) of the crude oil (Table 1). Similar high levels of neutral lipids (95.5%) and free fatty acids (0.3–1.6%) and other lipid fractions (2.9–4.2%) have previously been reported for flaxseed oil extracted from meal (Oomah et al., 1996). The range of free fatty acids and phospholipids in flaxseed hull oils was lower than those generally reported for rice bran oil (Dunford, 2005). Oil extracted from both sources were similar in composition except that average free fatty acid content of Fortigrad samples (2.48%) was significantly (P = 0.04) higher than those of Natunola (1.73%). Petroleum ether extracted significantly (P = 0.05) less neutral lipids, almost twice the phospholipids and acidic lipids amount (albeit not statistically significant) than hexane despite their similar polarity. The differences in free fatty acids were not significant amongst extraction methods indicating similar ability in extracting polar compounds. Supercritical CO2 and cold press extracted oils were similar in composition since they had the highest and lowest phospholipids and acidic lipid contents, respectively. The free fatty acid content of the supercritical CO2 extracted oil from hulls was twice the level reported for those obtained from flaxseed (Bozan & Temelli, 2002). Neutral lipid content of flaxseed hulls showed high negative (r = 0.72 and 0.86; P < 0.0001) correlation with phospholipids and acidic lipids, respectively. Flaxseed hull oil exhibited signals in both reversing and nonreversing components at approximately the same temperatures (between 33 and 14 °C) for the three thermal transitions indicating signal splitting. Two reversing transitions (between 35 and 33 °C) and (between 25 and 24 °C) indicative of crystalline melting were observed corresponding to the a and b polymorphic forms, respectively (Fig. 1). The second thermal transition

(24 to 25 °C) reflects the melting point of flaxseed oil (20 to 24 °C) (Przybylski, 2005). A minor transition occurred at 14 °C. The reversing component of the heat flow was highly sensitive to the initial source material (Table 2). For example, the first endothermic peak (the a-form) of Natunola oil occurred at significantly higher temperature (32.3 °C) suggesting increased stability than that of oil obtained from Fortigrad (34.9 °C). Oil extracted by cold press, acetone, and supercritical CO2 had higher transition temperature than those obtained by other extraction procedures. In the non-reversing component curves to which kinetic events such as crystallisation, crystal perfection and reorganisation are ascribed, three endotherms were observed in the 35 to 13 °C region (Fig. 2). These endothermic transitions are indicative of the crystallisation and recrystallisation of the metastable a form. The first endothermic peak (34 °C with high activation energy of 25 to 19 J/g) suggests kinetic stability implying a first-order transition. The second and third peaks in the 25 to 13 °C region were assigned to b0 and b crystallisation forms, respectively, believed to be due to the transformation of the metastable phase (a) to the more stable b form. Modulated DSC enabled the detection of small transitions in the reversing signal with great clarity. These transitions representing apparent changes in heat capacity in response to modulation were lower than endotherms observed in non-reversing events. Thus, the endotherms of the first and specifically the second and third thermal reversing events were on average only 69%, 30% and 39% of those of the non-reversing heat flow, respectively (Table 2). Extraction procedures and source material had no significant effect on the DHa values for the reversing heat flow, although DHb values for both reversing and nonreversing heat flows of Natunola oils were significantly (P < 0.0001) higher than those of Fortigrad. Flaxseed hull oil exhibited three maxima on the DSC oxidation curves indicating that thermoxidation can be characterised by at

0

Heat Flow (W/g)

*

-0.02

-0.04

Hexane Acetone Ethanol Petroleum ether CO2 Cold Press

-0.06

-0.08 -60

-40

-20

0

20

Temperature (°C) Fig. 1. Modulated differential scanning colorimetry (MDSC) reversing component of Fortigrad flaxseed hull oils.

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Table 2 Thermal characteristics of flaxseed hull oils. Sample

Acetone Cold press Ethanol Goldfisch Hexane Supercritical CO2 Omega Natunola Fortigrad Overall Mean

Reversing heat flow

Non-reversing heat flow

Ta

D Ha

Tb

D Hb

Tc

D Hc

Ta

D Ha

Tb

D Hb

Tc

D Hc

31.8d 33.9d 33.4bc 33.4b 33.5bc 33.6c 33.1y 32.3x 34.9z 33.6

15.8 15.8 15.6 16.8 15.5 14.6 14.9 16.2 15.2 15.7

24.0 25.1 24.0 24.1 24.5 24.4 23.7x 23.6x 25.1y 24.4

2.2ab 2.6abc 2.2a 2.8c 2.7abc 2.2a 2.8y 3.2y 1.8x 2.5

15.1c 13.9b 13.8b 13.9b 13.3ab 14.9c 12.8x 13.3x 14.9y 14.1

0.2a 1.6b 2.3b 0.4a 0.3a 0.2a 3.8y 0.7x 1.1x 1.0

34.0cd 34.2d 33.9abcd 33.6a 33.8abc 34.0bcd 33.7y 32.5x 35.4z 33.9

23.2b 22.7b 19.4a 24.2b 23.5b 24.8b 18.0x 22.2y 23.6y 22.9

24.1ab 25.5c 24.0ab 24.0ab 25.0bc 24.4abc 23.6x 23.6x 25.4y 24.5

6.8ab 8.6c 7.9bc 8.7c 9.0c 8.2bc 6.4x 9.5y 6.9x 8.2

14.4c 14.3c 14.0bc 13.9bc 13.6b 13.9bc 13.0x 13.1x 14.9y 14.0

0.9a 4.5b 5.7b 1.3a 1.7a 1.1a 7.5y 1.8x 3.3x 2.6

300

350

Means in a column followed by the same letter are not significantly different by Duncan’s multiple range tests at the 5% level.

4.5 3.5

Heat Flow (W/g)

Heat Flow (W/g)

0.01

-0.01 Hexane Acetone Ethanol Petroleum ether CO2 Cold Press

-0.03

-0.05

-0.07 -60

-40

-20

0

20

Temperature (°C)

2.5

Hexane Acetone Ethanol Petroleum ether CO2 Cold Press

1.5 0.5 -0.5 50

100

150

200

250

Temperature (°C)

Fig. 2. Modulated differential scanning colorimetry (MDSC) non-reversing component of Fortigrad flaxseed hull oils.

least three step exothermic effects (Fig. 3). These peaks could be considered as an indication of the level of cross-linking. The oil extracted using acetone, hexane, petroleum ether and supercritical CO2 showed an additional peak between 211 and 227 °C indicating instability at this particular temperature. Oxidation of flaxseed hull oil started at 105–163 °C, within the temperatures reported for edible oils (130–180 °C) with mean onset and oxidation temperatures at 121 and 150 °C, respectively and peaked at 194 to 200 °C depending on extracting solvent and sample types (Fig. 3; Table 3). The onset temperature (105–130 °C) corresponded to the flash point (120–130 °C) of flaxseed oil (Przybylski, 2005). The mean oxidation temperature of flaxseed hull oil at 150 °C resembled that of c-oryzanol in rice bran oil (Bucci, Magri, Magri, & Marini, 2003). Oils extracted with petroleum ether and ethanol had the lowest and highest onset, oxidation

Fig. 3. Differential scanning colorimety (DSC) of the thermo-oxidation profiles of Fortigrad flaxseed hull oils.

and peak1 temperatures, respectively. Significant differences (P < 0.001) were observed in onset, oxidation and peak1 temperatures amongst sample types (Omega, Natunola and Fortigrad), with Fortigrad and Omega samples generally having the highest and lowest temperatures, respectively. The third peak at 242 to 267 °C differed significantly (P < 0.0001) amongst the extracting solvents; the oil with the lowest onset, oxidation and peak1 temperatures also had the highest peak3 temperature. The fourth peak at 306–320 °C indicates the inability of oxygen uptake, resulting in complete thermal polymerisation. Flaxseed hull oil showed no significant differences in the temperature of the fourth peak; an observation similar to that reported previously for hempseed and echinacea seed oils (Oomah, Busson, Godrey, & Drover, 2002; Oomah et al., 2006). Comparison of Natunola with

Table 3 Thermoxidation temperatures (°C) and antioxidant capacity of flaxseed hull oils*. Sample

Onset

Oxidation temperature

Peak1

Peak2

Peak3

Peak4

Antioxidant capacity

SDG (mg/g)defatted hull

Acetone Cold press Ethanol Petroleum ether Hexane Supercritical CO2 Omega Natunola (N) Fortigrad (F) Mean (N and F)

123.8b 113.9c 130.0a 105.0d 121.2b 118.7bc 141.1x 128.3y 111.2z 120.9

154.3b 145.8c 162.5a 137.7d 146.4c 147.4c 164.1x 158.6y 140.9z 150.4

198.3ab 194.0c 200.3a 193.8c 196.8bc 197.3ab 203.2x 200.1y 193.9z 197.3

211.0b nd nd 225.5a 227.2a 210.9b nd 225.5x 223.4y 224.3

248.8cd 242.2d 255.9bc 266.6a 259.8ab 251.2c 254.5xy 257.4x 249.0y 253.2

313.5ab 306.0b 318.1a 316.6a 320.3a 318.7a 322.0x 319.0xy 314.2y 317.1

0.94b 0.61c 0.93b 0.55c 0.59c 1.18a 1.13x 0.79y 0.89y 0.84

20.19d 45.93b 27.52c 51.72a 47.71b 53.08a na 31.18y 48.49x 41.02

*

Means in a column followed by the same letter are not significantly different by Duncan’s multiple range test at the 5% level. nd, Not detected. na, Not applicable. Antioxidant capacity expressed as lm Trolox equivalent/g sample.

B.D. Oomah, L. Sitter / Food Chemistry 114 (2009) 623–628

Fortigrad samples revealed that ethanol extract of Natunola and cold press extract of Fortigrad had the highest and lowest onset, oxidation, peaks1, 3 and 4 temperatures, respectively. The antioxidant activity of flaxseed hull oil may be attributed to its content of a-tocopherols since fractionated hull contains approximately 26% of the total tocopherols found in the whole seed (Oomah, Kenaschuk, & Mazza, 1997). Extraction methods had significant (P < 0.0001) effect on antioxidant activity of hull oils with supercritical CO2 extracted oil exhibiting the highest antioxidant activity, whilst those extracted with petroleum ether, hexane, or cold pressed showed no significant differences in antioxidant activity (Table 3). Antioxidant activity of Omega sample types was significantly (P < 0.0001) higher than those of Natunola and Fortigrad. The comparatively high antioxidant activity of the supercritical CO2 extracted oil indicates the mild treatment and selectivity of carbon dioxide in lipid solubilisation without affecting other components. The cold-pressed oil exhibited low antioxidant activity, about half that of the commercial (cold pressed, omega), in spite of its low free fatty acid and acidic lipid content, suggesting that extracting conditions of the same procedure may alter the overall effectiveness of antioxidant extraction. Decreasing the polarity of the solvent (particularly ethanol to hexane) lead to a significant (P = 0.05) decrease (0.90–0.60 lm/g) in oil antioxidant activity. Antioxidant activity of the acetone extract was significantly higher than those extracted by hexane, a finding similar to those observed in wheat bran and oat bran (Oufnac et al., 2007; Sun, Xu, Godber, & Prinyawiwatkul, 2006). A change in solvent polarity is known to alter its ability to dissolve selected group of antioxidant compounds thereby influencing the antioxidant activity estimation (Zhou & Yu, 2004). A case study for recovering residual flaxseed oil by supercritical CO2 from the after-press cake found the process to be technically and economically viable with extraction efficiency greater than 95% (Martinez, 2005). Coincidentally, the supercritical CO2 extraction method resulted in defatted flaxseed hull, the residual secondary product, with the highest SDG content (53 mg/g) (Table 3). The SDG content of flaxseed hull extracted by solvent systems decreased in the following order: CO2 = petroleum ether > hexane = cold press > ethanol > acetone. This order, except for CO2, is the exact opposite to that observed with the antioxidant capacity indicating a negative correlation (r = 0.556; p = 0.017) between SDG content of defatted flaxseed hulls and antioxidant activity of the oil. Flaxseed hull defatted with petroleum ether and supercritical CO2 did not differ significantly (P < 0.0001) in SDG content contrary to reports by Zhang and Xu (2007) where SDG yield by petroleum ether (7.6 g/kg) was lower than those obtained by supercritical CO2 (10 g/kg). This could be due to the differences in the SDG content of the starting material as evidenced in this study with Fortigrad hulls containing 55% higher SDG than Natunola (Table 3). Thus supercritical CO2 extraction is an elegant strategy to obtain oil delivering both a-linolenic acid and antioxidant benefits of flaxseed hulls leaving behind a valuable ‘‘solvent-free” secondary product with the highest SDG content. Preprocessing of flaxseed hulls similar to those used for rice bran (Orthoefer, 2005) may provide further improvement in superior quality oil suitable for valuable nutraceutical products. Acknowledgement We thank David V. Godfrey, for his technical assistance. References AOAC International. (2000). Fat (crude) or ether extract in animal feed; Supercritical fluid extraction (SFE) method. AOAC Official Method 920.39; 999.02. Official Methods of Analysis (17th ed.). Barceló-Coblijn, G. (2007). a-Linolenic acid-enriched diets. A valid strategy to increase n3 fatty acids levels. International News on Fats, Oils, and Related Materials INFORM, 18(11), 719–721.

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