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Chemical characteristics of cold-pressed blackberry, black raspberry and blueberry seed oils and the role of the minor components in their oxidative stability Quanquan Li, Jiankang Wang, and Fereidoon Shahidi J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b01821 • Publication Date (Web): 20 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016
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Journal of Agricultural and Food Chemistry
For submission to: Journal of Agriculture and Food Chemistry
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Chemical characteristics of cold-pressed blackberry, black raspberry and blueberry seed
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oils and the role of the minor components in their oxidative stability
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Quanquan Li1, Jiankang Wang2, Fereidoon Shahidi1,*
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1
11
A1B 3X9
12
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Departments of Biochemistry, Memorial University of Newfoundland, St. John’s, NL, Canada,
Northern Lights Biotechnology Ltd., St. John’s, NL, Canada, A1C 2H3
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*Corresponding author. Email:
[email protected]; Telephone: +1 (709) 864-8552.
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ABSTRACT: The chemical characteristics of cold-pressed blackberry, black raspberry, and
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blueberry seed oils were evaluated for their fatty acid composition, positional distribution of fatty
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acids, triacylglycerol (TAG) profile and minor components profile. The role of minor
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components, including tocols and pigments on the oxidative stability was also investigated using
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high temperature and fluorescent lighting induced oxidation before and after tested berry seed
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oils were stripped of their minor components. The results indicated that all tested berry seed oils
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contained significant levels of palmitic (C16:0), stearic (C18:0), oleic (18:1), linoleic (C18:2n−6),
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and α-linolenic (C18:3n−3) acids, along with a favorable ratio of n−6/n−3 fatty acids (1.49−3.86);
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palmitic, stearic, oleic and α-linolenic were predominantly distributed on the terminal positions.
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Six triacylglycerols, namely LnLnLn, LnLLn, LLLn, LLL, OLL and OLLn, were the major
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species detected in the tested berry seed oils. Total tocol contents were 286.3−1302.9 mg/kg,
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which include α-, γ-, and δ-tocopherols as well as δ-tocotrienol. Oxidative stability of the three
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berry seed oils was compromised after the removal of tocols under high temperature induced
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oxidation, while the loss of pigments (chlorophylls) led to weak oxidative stability when exposed
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to fluorescent lights.
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KEYWORDS: blackberry see oil, black raspberry seed oil, blueberry seed oil, fatty acids,
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positional distribution of fatty acids, triacylglycerols, tocols, tocopherols, tocotrienols, pigments,
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chlorophylls, carotenoids, oxidative stability
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Journal of Agricultural and Food Chemistry
INTRODUCTION
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Edible oils are essential macronutrients in food and a condensed source of energy. Edible
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oils could also benefit human health by serving as a source of essential fatty acids, fat-soluble
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vitamins and other nutrients and non-nutrient phytochemicals. Berry seed oils, such as
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blackberry, black raspberry, and blueberry seed oils, are considered specialty oils, which may be
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used in different products by the food, nutraceutical and cosmetic industries. These berry seed
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oils are abundant in oleic acid, linoleic acid, and α-linolenic acid, and thus are a rich source of
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essential polyunsaturated fatty acids (PUFA), and their potential health benefit has led to a rapid
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commercial development of these oils in a variety of products and applications.1
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Edible oils are primarily composed of triacylglycerols (TAG), around 95%, and minor
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amounts of other components, including diacylglycerols (DAG), monoacylglycerols (MAG), free
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fatty acids (FFA), phospholipids, tocols (tocopherols and tocotrienols), sterols, pigments, among
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others.2 The fatty acids present in TAG are saturated, monounsaturated, or polyunsaturated, and
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the degree of unsaturation is a main factor that dictates the oxidative stability of the oils. The
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oxidative stability is also influenced by the positional distribution of fatty acids in the TAG, as
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well as the minor components in oils and their storage conditions. The minor components, such
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as tocols including tocopherols and tocotrienols, may play a role in protecting oils from
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oxidation by serving as free radical scavengers, and their mechanisms of action in retarding
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oxidation have been well-studied.3-5 In addition to neutralize free radicals, they may form
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chelation complexes with transition metals, reducing peroxides, and stimulate antioxidative
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defense enzymes in the body.6 Pigments, such as chlorophylls, can act as photosensitizers when
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exposed to light, which leads to rapid oxidation of edible oils.7
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Oxidative stability of cold-pressed blackberry, black raspberry and blueberry seed oils
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has not been thoroughly studied, and the antioxidant/prooxidant effects of minor components in
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the berry seed oils tested have been reported. In addition, there is no literature about the TAG
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profiles of cold-pressed berry seed oils or their impact on oxidative stability. Therefore, the
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objectives of this study were to characterize the cold-pressed berry seed oils by determining their
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fatty acid composition, positional distribution of fatty acids, TAG profile and their minor
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components, as well as to assess the effects of minor components on oxidative stability of the
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oils.
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MATERIALS AND METHODS
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Materials. Cold-pressed seed oils of blackberry, black raspberry and blueberry were kindly
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provided by Fruit Smart®, Inc. (Grandview, WA, USA).
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thiobarbituric acid (2-TBA), gallic acid (3,4,5-trihydroxybenzoic acid), silicic acid powder (mesh
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size: 100-200, acid-wash) and activated charcoal were obtained from Sigma Chemical Co. (St.
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Louis, MO, USA). Tocopherols and Tocotrienols were kindly provided by Planning &
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Development Department of Eisai Food & Chemical Co., Ltd. (Tokyo, Japan). Hexane,
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acetonitrile, methanol, ethanol, sulfuric acid, isooctane, isopropanol, 1-butanol and all other
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chemicals were obtained from Fisher Scientific Co. (Nepean, ON, Canada). All solvents were of
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ACS grade.
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Folin-Ciocalteu reagent, 2-
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Methods
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The Removal of Minor Components from Berry Seed Oils Using Column Chromatography.
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Blackberry, black raspberry and blueberry seed oils were stripped of their minor components
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according to the method described by Abuzaytoun and Shahidi with minor modifications.8 A
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chromatographic column (3.4 cm internal diameter x 40 cm length) was packed sequentially with
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45 g of activated silicic acid, followed by 45g of charcoal and another 45g of activated silicic
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acid. The silicic acid (100 g), before introduction to the solvent, was activated by washing three
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times with a total of 3 L of distilled water. After each treatment, the silicic acid was left to settle
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for 30 min, and then liquid was discarded. The silicic acid was then washed with methanol and
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the supernatant discarded.
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Oil (60 g) was diluted with an equal volume of hexane and passed through the
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chromatographic column. The solvent in the stripped oils was evaporated under vacuum at 50 °C,
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and traces of the solvent were removed by flushing with nitrogen. The column-stripped oils were
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then transferred into 10 mL glass vials, flushed with nitrogen, and kept at -80 °C for up to one
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month prior to performing subsequent experiments.
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Fatty Acid Composition. The fatty acid compositions of non-stripped and column stripped berry
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seed oils were analyzed by gas chromatography-flame ionization detection (GC-FID). Fatty acids
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were converted to fatty acid methyl esters (FAMEs) using 6% sulfuric acid in methanol,
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according to the method of Wanasundara and Shahidi.9 FAMEs were analyzed using a Hewlett-
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Packard 5890 series II gas chromatograph (Agilent, Palo Alto, CA, USA) equipped with a fused
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capillary column. Results were expressed as weight percentage of each fatty acid in total fatty
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acids.
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Triacylglycerol Profiles. The high performance liquid chromatography–photodiode array
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detection–atmospheric pressure chemical ionization-mass spectrometry (HPLC-DAD-APCI-MS)
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determination of triacylglycerols (TAG) in tested seed oils was conducted according to the
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method of Lisa and Holcapek with minor modifications.10 Berry seed oils were dissolved in
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acetonitrile/2-propanol (1:1, v/v) to obtain a 3% (w/v) solution. The elution system was as
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follows: 0 min, 100% A; 106 min, 31% A; 120 min, 100% A.
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Positional Distribution of Fatty Acids
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a. Selective Hydrolysis by Using Pancreatic Lipase. The oil samples were hydrolyzed using
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pancreatic lipase as described by Christie with minor modifications.11 The oil (25 mg) was
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weighed into a glass tube, and then 5.0 mL of Tris-HCl buffer (1.0 M, pH 8.0), 0.5 mL of
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calcium chloride (2.2%), and 1.25 mL of sodium taurocholate (0.05%) were added. Porcine
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pancreatic lipase (5.0 mg; EC 3.11.3) was added into the mixture after it had been kept in a water
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bath for 5.0 min at 40 °C. The glass tube was subsequently placed in a gyratory water bath
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shaker at 250 rpm under a blanket of nitrogen for 1 h at 40 °C. The enzymatic reaction was
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stopped by adding 5.0 mL of ethanol, followed by the addition of 5.0 mL of 6.0 M HCl.
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b. Extraction and Separation of Hydrolytic Products. Diethyl ether (50 mL in total) was used to
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extract the hydrolytic products three times, and then the extract was washed twice with distilled
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water and dried over anhydrous sodium sulfate followed by the removal of the solvent under
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reduced pressure at 30 °C. The hydrolytic products were separated on silica gel thin-layer
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chromatographic plates (Sigma-Aldrich, St. Louis, MO, USA). The plates were developed using
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a mixture of hexane/diethyl ether/acetic acid (70:30:1, v/v/v) for 45−55 min. The free fatty acid
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bands were scraped off and lipids extracted into diethyl ether, which were then used for fatty
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acid analysis as described by Wanasundara and Shahidi.9
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c. Fatty Acid Compositional Analysis of Hydrolytic Products. Fatty acid composition and
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positional distribution of the products were determined following their conversion to the
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corresponding methyl esters. A Hewlett-Packard 5890 series II gas chromatograph (Agilent, Palo
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Alto, CA, USA) equipped with a Supelcowax-10 column (30 m length, 0.25 mm diameter, 0.25
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µm film thickness; Supelco Canada Ltd., Oakville, ON, Canada) was used to analyze the FAMEs.
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The FAMEs were identified by comparing their retention times with those of a known standard
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mixture. Positional distribution of fatty acids at sn-1,3 positions was calculated as (% fatty acid
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at sn-1,3 positions/% fatty acid in triacylglycerols) × 100.
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Determination of Tocols. Tocol content in blackberry, black raspberry, and blueberry seed oils
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were determined by reverse phase high performance liquid chromatography-mass spectrometry
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(HPLC-MS). The analysis was performed using an Agilent 1100 HPLC system (Agilent
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Technologies, Palo Alto, CA, USA), equipped with a UV-diode array detector (UV-DAD).
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Separation was achieved on a C-18 column (4.6mm× 250mm coupled with a guard column,
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Sigma). Tocols were eluted using a gradient solvent system containing methanol-acetonitrile-
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isopropanol. Fifty microliters of each tocol standard and oil samples were used and detection was
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followed at 295nm. The mobile phase started with methanol/acetonitrile/isoproponol (41:59:0,
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v/v/v) at a flow rate of 0.8 ml/min and maintained for 15 min. The mobile phase was then
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gradually changed to methanol/acetonitrile/isoproponol (16.5:23.5:60, v/v/v) from 15 to 25 min,
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followed by gradual increase to 100% isoproponol from 25 to 35 min, which lasted 10 min. In
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order to recondition the column, the mobile phase was changed to its initial state,
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methanol/acetonitrile/isoproponol (41:59:0, v/v/v) in 5 min, and was kept there for 10 min. All
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analyses were performed by using mass spectrometric detector system (LC-MSD-Trap-SL,
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Agilent, Palo Alto, CA, USA) and alternating ion atmospheric pressure chemical ionization
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(APCI). The operating conditions used were 121 V for the fragments, drying temperature of
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350°C, APCI temperature of 400°C, nebulizer pressure of 60 psi, drying gas flow of 7 liter/min.
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Tocopherol and tocotrienol concentrations in samples were calculated based upon standard
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curves and expressed as (mg/kg of oil).
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Measurement of Pigments. Pigments, present in the stripped and non-stripped oil samples were
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determined by measuring the absorbance at 550-710nm for chlorophylls and their derivatives,
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and 430-460 nm for carotenoids.8 The oil sample was mixed with hexane (1:1, v/v) and
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transferred into cuvettes, and the absorbance was read using a 8453A UV-Visible
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spectrophotometer (Agilent Technologies, Palo Alto, CA, USA).
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Oxidative Stability Tests of the Original Berry Seed Oils and Their Stripped Counterparts
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under Accelerated Oxidation Conditions Using Schaal Oven and Fluorescent Lighting. The
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oxidative stability of stripped and non-stripped seed oils was examined under Schaal oven
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conditions and fluorescent lighting respectively. The oxidative stability of the oils was
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determined under Schaal oven (model 2, Precision Scientific Co., Chicago, IL, USA) conditions
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at 60°C for 7 days. Each day (24 h) of storage under such conditions is equivalent to 1 month of
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storage at ambient temperatures. Oil samples were removed from the oven after 1, 3, 5 and 7
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days. For photooxidation under fluorescent lighting, samples were kept in a rectangular
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polyethylene box (70 cm length × 35 cm width × 25 cm height) equipped with two 40 W cool
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white fluorescent lights, which were suspended approximately 10 cm above the surface of the oil
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containers.8 The fluorescent radiation was at a level of 2650 Lux, and the temperature inside the
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container was 27±1 °C. Oil samples were removed from the light box after 1, 3, 6, 12, 24, 48,
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and 72 h. The caps of sample containers were all wrapped with Parafilm, and kept at -80 °C for
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oxidative stability tests within a month.
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Conjugated dienes (CD) of the oil samples were determined according to the IUPAC
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method.12 CD was calculated according to the equation below where A represents absorbance of
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the solution at 234 nm, C is the concentration of the solution in g/100 mL, and d is the cell length
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in 1 cm.
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CD = A/(c × d)
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Oil samples (0.05-0.20g) were analyzed for their contents of thiobarbituric acid reactive
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substances (TBARS) according to the AOCS method.13 The intensity of the resultant colored
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complex was measured at 532 nm using 8453A UV-Visible spectrophotometer (Agilent
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Technologies, Palo Alto, CA, USA). The values of TBARS were reported using a standard line
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prepared with 1,1,3,3-tetramethoxypropane as precursor of malondialdehyde (MDA).
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Statistical analysis and data interpretation. All the experiments were performed in triplicate.
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One way ANOVA (analysis of variance) and Tukey’s standardized test were performed at a level
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of p0.05) from each
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other. The trend of TBARS formation during the photooxidation was not as clear as those for
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primary oxidation products as indicated in their CD values. The Results also indicated that non-
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stripped blackberry and blueberry seed oils had better stability than other oils.
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ACKNOWLEDGEMENT One of us (FS) thanks the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support in the form of a discovery grant.
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Table 1. Fatty acid composition of the original blackberry, black raspberry, and blueberry seed oils and their stripped counterparts. FA(%) C16:0
BKSO 3.3±0.2
SBKSO 3.4±0.1
BRSO 2.0±0.0
SBRSO 1.9±0.0
BESO 5.2±0.2
SBESO 4.9±0.5
C18:0
1.6±0.3
1.7±0.2
0.6±0.0
0.6±0.0
1.3±0.0
1.2±0.4
C18:1
14.4±0.5 15.4±0.3
9.7±0.1
10.5±0.1 22.2±0.6 22.5±0.1
C18:2 n-6
63.4±1.3 63.6±0.5 53.4±0.0 54.7±0.2 41.9±0.4 43.0±0.7
C18:3 n-3
16.5±0.4 13.4±0.1 33.7±0.1 30.4±0.1 28.1±1.5 27.6±0.5
∑SFA
4.9
5.1
2.6
2.5
6.5
6.1
∑MUFA
14.4
15.4
9.7
10.5
22.2
22.5
∑PUFA
79.9
77
87.1
85.1
70
70.6
∑n-3
16.5
13.4
33.7
30.4
28.1
27.6
n-6/n-3
3.8
4.7
1.6
1.8
1.5
1.6
BKSO, blackberry seed oil; SBKSO, stripped blackberry seed oil; BRSO, black raspberry seed oil; SBRSO, stripped black raspberry seed oil; BESO, blueberry seed oil; SBESO, stripped blueberry seed oil; SFA, saturated fatty acids; MUFA, monounsaturated fatty acids; and PUFA, polyunsaturated fatty acids
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Table 2. Triacylglycerol (TAG) composition (relative concentration percentage) of the original blackberry, black raspberry and blueberry seed oils. TAG (%)
BKSO
BRSO
BESO
LnLnLn
3.78±0.16b
13.25±0.15a
7.65±0.22c
LnLLn
12.90b±1.23b 24.29a±0.12a 17.96±0.44c
LLLn
20.16±2.47b
23.83±0.09a
16.81±0.41c
LnOLn
2.95±0.22c
4.98±0.02b
8.18±0.20a
LnLnP
0.97±0.21 b
0.60±0.01b
2.58±0.06a
LLL
20.16±2.17a
12.78±0.09b
6.27±0.15 c
OLLn
7.83±0.90 b
7.23±0.07b
13.24±0.33a
LnLP
2.69±0.23b
1.48±0.06b
4.51±0.11a
SLnLn
0.58±0.02 a
nd
0.90±0.02a
OLL
9.27±0.85a
4.31±0.11c
6.09±0.15b
OLnO
1.10±1.89b
nd
2.82±0.07a
LLP
3.65±0.54a
1.28±0.06c
2.37±0.06b
SLLn
2.11±0.28a
0.83c±0.03c
1.64±0.04 b
LnOP
n.d
nd
1.37±0.04
OLO
1.85±0.02b
0.66±0.04b
2.09±0.05a
SLL
2.64±0.18a
nd
2.10±0.06a
SOLn
n.d
0.42±0.06
n.d
OOO
0.10±0.21a
nd
0.10±0.01a
ALL
0.27±0.01
nd
n.d
22
ACS Paragon Plus Environment
Page 22 of 31
Page 23 of 31
Journal of Agricultural and Food Chemistry
SLO
0.54±0.32a
nd
0.45±0.0a
Total
95.92
95.92
97.13
nd, not detected; values in the same row with different superscripts are significantly different (p