Development of a defatted mustard meal-based composite film and its application to smoked salmon to retard lipid oxidation

Food Chemistry 133 (2012) 1501–1509

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Development of a defatted mustard meal-based composite film and its application to smoked salmon to retard lipid oxidation In-Hah Kim a, Hee-Jae Yang a, Bong-Soo Noh a, Seo-Jin Chung b, Sea C. Min a,⇑ a b

Department of Food Science and Technology, Seoul Women’s University, 621 Hwarangro, Nowon-gu, Seoul 139-774, Republic of Korea Department of Food and Nutrition, Seoul Women’s University, 621 Hwarangro, Nowon-gu, Seoul 139-774, Republic of Korea

a r t i c l e

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Article history: Received 23 October 2011 Received in revised form 25 January 2012 Accepted 7 February 2012 Available online 18 February 2012 Keywords: Mustard meal Edible film Antioxidant film Smoked salmon Lipid oxidation

a b s t r a c t An antioxidant composite film was developed from defatted mustard meal (DMM) without incorporating external antioxidants and its applicability as a coating to smoked salmon has been investigated. Filmforming variables included the concentration of xanthan, heat treatment, and high pressure homogenisation. Tensile strength, elongation, water vapour permeability, and water solubility of composite films of DMM and xanthan were 5.1–8.8 MPa, 2.9–5.0%, 1.5–2.4 g mm/kPa/h/m2, and 1.1–1.6%, respectively. The composite film with xanthan at 5% (w/w) demonstrated antioxidant properties and the coating with the solution forming the composite film (5% xanthan) retarded lipid oxidation and significantly reduced volatile changes in smoked salmon during storage at 4, 10, and 20 °C for 21 days. Coated salmon was preferred to uncoated salmon in glossiness and fish smell. The composite coating improved the stability of smoked salmon against lipid oxidation without imparting a negative sensory quality to the salmon. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Functions of edible films include inhibition of migration of moisture, oxygen, carbon dioxide, aromas, and oil in or out of food, as well as enhancement of appearance of food, improvement of mechanical integrity, and supplementation of food additives (Debeaufort, Quezada-Gallo, & Voilley, 1998; Krochta, 1997). Ready-to-eat foods, such as smoked salmon, sliced meat products, fruit cuts, vegetable cuts, and chocolate products, are examples of the foods where edible films and coatings can be applied (Bierhals, Chiumarelli, & Hubinger, 2011; Tzoumaki, Biliaderis, & Vasilakakis, 2009). Edible films and coatings have been investigated to extend microbial and physicochemical stability of fish products. Whey protein isolate (WPI)-based antimicrobial films and coatings and chitosan-based films and coatings were applied to smoked salmon and cod products for their microbial safety and preservation (Jeon, Kamil, & Shahidi, 2002; López-Caballero, Gómez-Guillén, Pérez-Mateos, & Montero, 2005; Min, Harris, Han, & Krochta, 2005). WPI and acetylated monoglyceride (AMG) coatings were also evaluated for effectiveness against moisture loss and lipid oxidation of frozen King salmon (Stuchell & Krochta, 1995). All the reported edible films and coatings, however, were produced from pure biopolymer materials, which may be difficult to be used practically in commercial products due to their costs.

⇑ Corresponding author. Tel.: +82 2 970 5635; fax: +82 2 970 5977. E-mail address: [email protected] (S.C. Min). 0308-8146/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2012.02.040

One of directions in edible film research is to develop materials from agropolymers and byproducts from agricultural or food processes (Kang & Min, 2010; Sablani et al., 2009), which may result in high-value commercial use of waste streams. Defatted mustard seed meal (DMM) is a byproduct left after crushing mustard seeds for oil removal. DMM is known to possess a strong antioxidant property from its endogenous antioxidants, including sinapine, sinapic acid, and flavanoids (Matthäus, 2002; Saleemi, Janitha, Wanasundara, & Shahidi, 1993). Provided an edible film is formed from DMM, the film might have antioxidant properties. Development of an edible film using DMM was reported by Hendrix, Morra, Lee, and Min (2012). The physical characteristics of the film, including tensile properties and water vapour permeability, however, needed to be improved for food applications. Methods improving the physical properties have been sought for DMM-based films, which may be also useful for improving the properties of films made of other agricultural by-products. High-pressure homogenisation (HPH) disaggregates and breaks down biopolymer particles using shear forces and pressure (Bouaouina, Desrumaux, Loisel, & Legrand, 2006; McClements, 2005). This may be used to prepare film-forming suspensions containing adequately small biopolymer particles that form films with tensile and barrier properties in desirable ranges for their commercial applications to food products. One of the conceivable approaches is a biopolymer composite method that forms a film with a biopolymer mixture (Min, Janjarasskul, & Krochta, 2009). Water vapour permeability of whey protein films was improved by forming composite films with waxes (Min et al., 2009). Xanthan, a largely

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used thickening agent and stabiliser in the food industry, has been incorporated into edible films to increase crosslinking between biopolymer molecules of film constituents and thus improve the physical properties (Flores, Costa, Yamashita, Gerschenson, & Grossmann, 2010; Mandala, Palogou, & Kostaropoulos, 2002). The incorporation of xanthan in a carrageenan film and a tapioca starch film increased the tensile strength of the films (Briones et al., 2004; Flores et al., 2010). Heating of a protein-containing film-forming solution modifies the 3-dimensional structure of proteins in the solution, exposing internal sulfhydryl groups hidden in the interior of the molecule. This promotes intermolecular disulfide bonding and hydrophobic bonding and allows formation of a film which is stronger and low in water solubility compared to one prepared without heating (Perez-Gago & Krochta, 2001). Edible films must provide adequate physical and sensory properties for food. Thus, integrated analyses of physical and sensory properties are crucial for development of edible films (Tzoumaki et al., 2009). Vargas, Albors, Chiralt, and GonzálezMartínez (2006) developed an edible coating made of chitosan and reported that the coating protected strawberry from degradation of physical characteristics, including colour, without alternating acidity and the concentration of anthocyanin in strawberry. Therefore, the objectives of the research were (1) to study the effects of HPH, xanthan addition, and heat treatment during film formation on the tensile properties and moisture resistance of a DMM-based film to develop the formulation appropriate for food coating and (2) to investigate the effects of the coating with developed formulation on lipid oxidation and sensory properties of smoked salmon, a selected model food, during storage. 2. Materials and methods 2.1. Materials DMM was supplied from the department of Soil Biochemistry & Environmental Organic Chemistry, University of Idaho (Moscow, ID, USA). The oil in Sinapis alba ‘IdaGold’ seeds was extracted by cold pressing (Borek & Morra, 2005). The meal, remaining after oil extraction, consisted of irregularly shaped flakes approximately 1-mm thick, ranging in diameters from 1 to 3 cm and was not treated in any additional manner prior to use as the film base material. The DMM contained polysaccharides, proteins, and lipids of 45.2, 35.0, and 8.7 g/100 g-meal, respectively (dry basis). The composition analysis was conducted by Korea Food Research Institute (Seongnam, Korea). Xanthan was purchased from Wha Cheon Co. Ltd. (Seongnam, Korea). Polysorbate-20 (hydrophilic–lipophilic balance (HLB) value: 16.7), used as an emulsifier to help distribute biopolymers uniformly in film-forming suspensions, was supplied from Ilshinwells Co. Ltd. (Seoul, Korea). Glycerol, used as a plasticiser to improve film flexibility, was purchased from Sigma–Aldrich Co. Ltd. (St. Louis, MO, USA). Pre-sliced commercially-produced cold smoked salmon (Smoked Salmon Pure, OurHome, Seoul, Korea) was purchased at a local supermarket. The thickness of the slice was approximately 3 mm. The ingredients of the salmon product, listed on the package, were Norway salmon, salt, and white sugar. The salmon product was labeled as ‘no preservatives’.

but with heating the film-forming solution at 90 °C for 30 min (designated as ‘Heating’ film); (2) a film prepared without HPH, but with addition of xanthan (5%, 10%, 15% (w xanthan/w DMM and xanthan)) and heating (‘Xanthan-Heating’ film); (3) a film prepared with HPH at 172 MPa with 3 passes and xanthan addition, but without heating (‘HPH-Xanthan’ film); (4) a film prepared with HPH, xanthan addition, and heating (‘HPH-Xanthan-Heating’ film). The effects of xanthan addition, HPH, and heating were examined by comparing results obtained from the films of ‘Heating’ and ‘Xanthan-Heating’, films of ‘Xanthan-Heating’ and ‘HPH-Xanthan-Heating’, and films of ‘HPH-Xanthan’ and ‘HPH-Xanthan-Heating’, respectively. The mustard meal was ground in a Scienceware Micro mill (BelArt Products, Pequannock, NJ, USA) and then sieved to prepare powder (<300 lm). The 11.9, 12.6, 13.3, or 14.0 g of DMM was suspended and dissolved in 184 g distilled water for 30 min and the mixture was homogenised (T 25 digital Ultra-TurraxÒ, Janke & Kunkel GmbH & Co., IKAÒ Labortechnik, Staufen, Germany) at 20,000 rpm for 5 min. The suspension was further homogenised by HPH (SWU, D.O.S. Inc., Siheung, Korea) at 172 MPa with 3 passes for preparing HPH-Xanthan and HPH-Xanthan-Heating films. The homogenate with or without HPH was mixed with 2 g of glycerol (final of 14.3% (w/w DMM and xanthan)) for 20 min and then mixed with 0, 0.7, 1.4, or 2.1 g of xanthan for 10 min by magnetic stirring. The sum of amounts of DMM and xanthan was 14 g. The emulsifier, Polysorbate-20, was added (1% (w Polysorbate-20/w DMM and xanthan)) and then homogenised at 20,000 rpm for 5 min. Only the mixtures for forming the films of Xanthan-Heating and HPH-Xanthan-Heating were heated in a water bath at 90 °C for 30 min following homogenisation. The mixture was then degassed under vacuum. Films were cast by pipetting the degassed mixture onto Teflon plates (15.5 cm diameter) resting on a leveled granite surface. The amount of the film-forming suspension (34.5 g) pipetted was selected to produce a 0.2 mm-thick film. The film-forming mixtures were dried for 2 days at 23 ± 2 °C/35 ± 5% relative humidity (RH). Dried films were peeled intact from the casting surface and stored in a chamber (Tenney-10 model TTUFR-40240, Tenney Engineering, Inc., Union, NJ, USA) controlled at 23 ± 2 °C/52 ± 2% RH for 2 days to equilibrate before testing. A micrometer (Digimatic micrometer Model CR-200, Mitutoyo Co., Kawasaki, Japan) was used to determine film thickness to the nearest 0.001 mm. Twelve samples of each type of film were measured. The mean value was used in calculations for tensile properties and water vapour permeability.

2.3. Tensile properties The American Society of Testing and Materials (ASTM) standard method D 882-02 (ASTM, 1997) was used to measure tensile strength, percentage elongation at break (elongation), and elastic modulus of films. Films were cut into strips with a test dimension of 50  8 mm. Film samples were analysed using a texture analyser (TA-XT2, Stable Micro System Co. Ltd., Surrey, UK) operated at 23 ± 2 °C with a 25-kg load cell and a crosshead speed of 50 mm/ min.

2.2. Film preparation

2.4. Water vapour permeability

Four types of films were produced to study the effects of addition of xanthan and treatments of film-forming solutions with HPH and/or heat treatment on tensile properties (tensile strength, percentage elongation at break, and elastic modulus), water vapour permeability, and water solubility of a DMM-based film: (1) A DMM-based film prepared without HPH and xanthan addition,

The Gravimetric Modified Cup Method based on ASTM E96-92 (McHugh, Avena-Bustillos, & Krochta, 1993) was used to determine water vapour permeability. Circular test cups were made of polymethylmethacrylate (Plexiglas™). De-ionised water was placed in each cup to create a high RH on the underside of the film sealed in the cup.

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2.5. Water solubility of the films Water solubility was measured following the method of Kang and Min (2010). The solubility (%) of the film was defined as the ratio of the water-soluble solid content to initial solid content (100). 2.6. Antioxidant properties of the DMM-xanthan film The extract for determining antioxidant activity of the film was prepared by the method of Yu, Im, Lee, Ji, and Lee (2006). Antioxidant activity of the HPH-Xanthan film with 5% xanthan was determined. The 5 g of film was extracted three times with 30-mL 80% methanol (Sigma–Aldrich Co., St. Louis, MO). Collected extracts were centrifuged (Supra22 K, High Speed Refrigeration Centrifuge, Hanil Science Industrial Co. Ltd., Incheon, Korea) at 14,811 g at 4 °C for 10 min. The supernatant obtained after centrifugation was evaporated at 55 °C using a rotary vacuum evaporator (RE200, Yamato scientific Co. Ltd., Tokyo, Japan). The concentrate was frozen in a deep freezer (DF8520, Ilshin Lab Co. Ltd., Yangju, Korea) at 70 °C for 7 days and freeze-dried (Lyph LockÒ6 freeze dryer, Labconco, Kansas City, MO, USA) to obtain the extract for determining antioxidant activity of the film. Antioxidant properties of the extract from the film were evaluated measuring scavenging activities of 2,20 -azinobis-(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH), following the methods of Re et al. (1999) and Blois (1958), respectively. The extract was diluted with distilled water and the concentrations of the diluted extract used in the measurement were 0.01, 0.03, and 0.12 mg/ mL for the ABTS assay and 0.01, 0.03, 0.12, 0.20, 0.35, 0.50, 0.65, and 0.82 mg/mL for the DPPH assay. The concentrations of ascorbic acid (Sigma–Aldrich Co.) used for comparison of antioxidant activity were 0.001, 0.005, 0.01, and 0.2 mg/mL. As the blank, the mixture of 50 lL of the ABTS solution and 150 lL of distilled water or 150 lL of the DPPH solution and 50 lL of distilled water was used for the ABTS assay or DPPH assay, respectively. The measurement times of ABTS and DPPH assays were 1 min and 30 min, respectively. Radical-scavenging activity was calculated from the difference in the radical absorbance between a blank and an extract sample by the following equation: Radical-scavenging activity ð%Þ ¼ ½ðabsorbance of the blank absorbance of a sampleÞ=absorbance of the blank 100

The RC50 was determined as the concentration required for 50% reduction of ABTS and DPPH radicals. 2.7. Effects of the DMM-xanthan composite coating on lipid oxidation of smoked salmon Each slice of smoked salmon was trimmed to 10 ± 0.4 g (approximately 6  6 cm) and soaked for 3 s in the film-forming solution that forms the HPH-Xanthan film (5% (w/w) xanthan) and the solution was evenly spread using a glass spreader. The coating solution applied to coated samples was dried inside an oven drier (JSOF-150, JS Research Inc., Gongju, Korea) at 40 ± 5 °C at 10 ± 4% relative humidity (RH) for 1 h. Uncoated salmon samples were placed in a laminar flow biohazard hood at 23 ± 2 °C at 25 ± 5% RH for 1 h. Both coated and uncoated salmon samples were individually packed in sterile bags (18 oz (532 cm3), Nasco WHIRL-PAK, Fort Atkinson, WI, USA) and stored at 4, 10, and 20 °C. The storage temperature of 4 °C was selected because smoked salmon is stored and distributed around this temperature and 10 and 20 °C were chosen since these temperatures accelerate the rate of lipid oxidation and thus the antioxidant effect of the coating might be easily examined at

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these temperatures. The storage temperature of 10 °C also was selected to simulate a temperature abuse that may occur during retail display. Preliminary tests incorporating a blue dye (FD&C Blue No. 1, SENSIENT, St. Louis, MO. USA) confirmed that even surface spreading was achieved with this method. The weight gain on the salmon sample after coating was approximately 1.5 g and the thickness of the coating was approximately 0.2 mm. Thiobarbituric acid reactive substance (TBARS) values of samples were determined as measures for lipid oxidation, following the methods of Heath and Packer (1968). At each time of sampling, the salmon sample was trimmed to make a 4.0 ± 0.2 g sample using a scissors and was placed in a sterile stomacher bag (19  30 cm; Nasco WHIRL-PAK). The sample was mixed with 40-mL 1.25% (w/v) trichloroacetic acid and then pummeled by a stomacher blender (Stomacher Lab Blender model 400, Seward Medical, London, UK) for 2 min at a normal speed (230 ± 5% rpm). The stomached mixture was filtered using Whatman #1 (Whatman Ltd., Kent, UK) and the filtrate was centrifuged at 6800g for 10 min. The 2 mL of supernatant was mixed with 2-mL 20 mM of thiobarbituric acid and heated at 95 °C for 30 min in a water bath. After heating, the absorbance was measured at 532 nm and the TBARS value was calculated using determined extinction coefficients (Kosugi, Jojima, & Kikugawa, 1989). 2.8. Sensory evaluation Sensory attributes of appearance, flavour, and texture of smoked salmon samples with and without coating by the film-forming solution producing the HPH-Xanthan film (5% (w/w) xanthan) were evaluated after either 0 or 7 days of storage at 4 °C. The salmon samples were prepared as described in Section 2.7. The 7-day old samples (coated and uncoated samples) were prepared 7 days before evaluation and stored at 4 °C for those 7 days to conduct the evaluation together with freshly prepared 0-day samples. Four kinds of samples were provided to the panel: Uncoated smoked salmon sample stored for 7 days at 4 °C (‘Uncoated-7-day old’), coated sample with the storage (‘Coated-7-day old’), uncoated sample without storage (‘Uncoated-fresh’), and coated sample without storage (‘Coated-fresh’). Thirty-three panelists participated in the sensory evaluation. Panelists were recruited from the department of Food Science and Technology at Seoul Women’s University (Seoul, Korea). The panelists were initially screened for their consumption frequencies of smoked salmon. Panelists consuming smoked salmon products more than once a month were selected as the panelists. The 30 panelists among participants were female ranging in age from 20 to 26. The panelists were asked to rate the attributes of appearance (colour strength, glossiness), flavour (rancidity, fish smell, flavour strength, aftertaste), and texture (cohesiveness, gumminess, oiliness). The samples with storage were evaluated only for colour strength, glossiness, rancidity, and fish smell without having them tasted. A 9-point intensity scale (scale of 1–9) was used for each attribute where higher numbers represent higher intensity of attributes. Samples were coded with three digit random numbers and served in random order with a cup of water and a piece of non-salted cracker to cleanse palates between samples. 2.9. Volatile profiles Volatile profiles of the salmon samples prepared identically to those used in the sensory evaluation, stored at 5, 10, and 20 °C for 7, 14, and 21 days, were analysed by the method of Hong, Park, Choi, and Noh (2011) using an electronic nose system (SMart Nose300, SMart NoseÒ, Switzerland) installed with an automatic headspace sampler (SMart NoseÒ Autosampler, SMart Nose, Switzerland) and a mass spectrometer (Quadrupole Mass Spectrometer, Balzers Instruments, Switzerland). The number of channels of the

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electronic nose system was 191 (bargraph mode, 10–200 amu). The flow rate of 99.999% nitrogen purge gas was 230 mL/min. Salmon samples (1 g each) were incubated at 70 °C for 15 min before injection at 130 °C. Selected 20–30 ion fragments peaks were analysed by principal component analysis (PCA) using statistics software (SMart Nose, Marin-Epagnier, Switzerland). 2.10. Statistical analysis All the experiments for determining physical and antioxidant properties of the films and volatile profiles of the salmon samples were done in triplicate. Each observation within each replication was determined in duplicate except that of the tensile properties. Eight repeated measurements in each replication were made to determine the tensile properties. The storage study to study antioxidant effects of the coating was done once in triplicate for each kind of samples collected at each sampling period. Data from the storage study were analysed at each sampling period for individual treatments. The sensory evaluation was done in duplicate. An ANOVA test (Duncan’s multiple range tests) was carried using PASW Statistics 18 (IBM Co., Ver. 18.0.0, NY, USA) to estimate the significant difference (a = 0.05). Only the significant differences of flavour and texture attributes were analysed by an independent samples t-test (a = 0.05). 3. Results and discussion 3.1. Tensile properties The effects of HPH, xanthan addition, and the heat treatment during film formation on the tensile strength of the film are presented in Fig. 1a. The Heating film was formed in the same way as a DMM film was produced by Hendrix et al. (2012), except for the concentrations of DMM, glycerol, and emulsifier and the kind of emulsifier used (soy lecithin (HLB: 8) in the study of Hendrix et al., 2012). The concentrations of DMM, glycerol, and emulsifier of the Heating film were 7.0%, 1.0%, and 0.07% (w/w total, wet basis), respectively, while those of the DMM-based film reported by Hendrix et al. (2012) were 3.0%, 1.5%, and 0.06% (w/w total, wet basis), respectively. The Heating film formed without HPH and xanthan addition was considered appropriate as a control to investigate improvement of the physical properties of a DMMbased film by HPH and addition of xanthan. The tensile strength of the Heating film was 3.8 ± 1.0 MPa while that of the HPH-Xanthan-Heating film was 8.8 ± 1.2 MPa, which is the highest. The Heating film had a lower tensile strength than the other films (Fig. 1a). HPH applied to the film-forming solution resulted in the formation of the film with a higher tensile strength than the Heating film, regardless of the heat treatment of the solution after HPH. HPH of film-forming solutions of biopolymers can produce films that have mechanical strength and resistance to water vapour which are better than those of the films formed without HPH. This is due to increased interaction and crosslinking among molecules caused by the reduced sizes of biopolymers and biopolymer particles as a result of HPH and thus increased compactness and rigidity of film structures (Banerjee, Chen, & Wu, 1996). The addition of xanthan significantly increased tensile strength when the concentration of xanthan was 15% (w/w) (P < 0.05). Xanthan in a composite film was reported to interact with other macromolecules and increase in rigidity of the film (Mandala et al., 2002). The heating did not significantly affect the tensile strength of the films (P > 0.05) which could be found when the values of HPH-Xanthan film and HPH-Xanthan-Heating film were compared. Heating denatures proteins promoting formation of disulfide bonds between molecules of proteins and hydrophobic interaction

in film-forming solutions, which results in the formation of strong protein films (Perez-Gago and Krochta, 2001). Denaturation of proteins in the film-forming solution might not be enough to change the tensile strength of the film significantly. This might be due to a low concentration of protein in DMM (35% (w/w, dry basis)). The HPH-Xanthan-Heating films had a higher tensile strength (strongness) than WPI-beeswax composite films (2.5–3.6 MPa, Min et al., 2009) and wheat gluten films (1.4–3.8 MPa, Gennadios, Weller, & Testin, 1993). The tensile strength of the DMM-xanthan composite films was similar to those of potato skin-based films (2.5–9.9 MPa, Kang and Min, 2010) and apple skin-based films (1.68–9.18 MPa, Sablani et al., 2009), but was lower than those of SPI-beeswax composite films (95.7–190.9 MPa, Chao, Yue, Xiaoyan, & Dan, 2010). The DMM-xanthan composite films that possessed tensile strength generally higher than those of many other reported edible films may be applicable to various food products as coatings. The HPH-Xanthan film at 10% (w/w) xanthan had the highest elongation (5.0 ± 1.3%). No obvious tendency in elongation of the film, exhibited by the effect of HPH, xanthan addition, or heating was observed (Fig. 1b). HPH, xanthan addition, or their combination did not improve elongation of the Heating film prepared with heating alone. 3.2. Water vapour permeability The films prepared with HPH showed lower water vapour permeability than the films without HPH (Fig. 1c). HPH resulted in a reduction in water vapour permeability. The rate of migration of water molecules in a biopolymer film decreases with decrease in the size of biopolymers in a film matrix due to increase in tortuosity in the pathway of water molecules, which results in increase in the moisture barrier property of the film (Dangaran, Cooke, & Tomasula, 2006). HPH could reduce the sizes of biopolymers and biopolymer particles constructing the matrix of the film (Hendrix et al., 2012). The water vapour permeability was significantly reduced by the addition of xanthan (P < 0.05), regardless of the concentrations used in this study. This might be due to increased crosslinking between biopolymers by xanthan in the film formulation. The potential increase in crosslinking also can explain the reason for the increased tensile strength by the addition of xanthan (Fig. 1a). The water vapour permeability of the HPH-Xanthan film was lower than those of many other biopolymer films, including a calcium caseinate film (7.91 g mm/kPa h m2, Banerjee & Chen, 1995), a wheat gluten film (4.52 g mm/kPa h m2, Aydt, Weller, & Testin, 1991), a WPI film (glycerol:WPI = 1:1.7) (4.99 g mm/kPa h m2, McHugh, Aujard, & Krochta, 1994), apple skin-based films (4.20– 7.56 g mm/kPa h m2, Sablani et al., 2009), and potato skin-based films (2.99–5.30 g mm/kPa h m2, Kang and Min, 2010). 3.3. Water solubility Potential applications of the films to foods may require low water solubility to enhance water resistance and product integrity. HPH might reduce water solubility by decreasing water vapour permeability and forming a more homogeneous and stable hydrocolloid of a film-forming solution. This would result in uniform packing of biopolymers and thus formation of films with dense matrices that retard saturation of the films with water (Sanchez, Pouliot, Renard, & Paquin, 1999). Heating also could reduce water solubility by denaturing proteins in the film-forming solution (Perez-Gago and Krochta, 2001) and increasing interactions between polymer molecules. The temperature of a heating process above the temperature of conformational transition of xanthan may facilitate the interaction of disordered xanthan chains and other polymers (Mandala et al., 2002). However, the decrease in the solubility by HPH or heating was not obvious with DMM-based

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Fig. 1. Effects of xanthan concentration, high-pressure homogenisation (HPH, 172 MPa, 3 passes), and heating (90 °C for 30 min) on tensile strength (a), percentage elongation at break (b), water vapour permeability (c), and water solubility (d) of the DMM-based films. Different letters indicate statistically significant differences between the treatments at P < 0.05.

films, found by comparing the values for Xanthan-Heating film and HPH-Xanthan-Heating film and the values for HPH-Xanthan film and HPH-Xanthan-Heating film, respectively (Fig. 1d). Incorporation of xanthan, however, reduced water solubility significantly, irrespective of the concentration of xanthan used (P < 0.05) (Fig. 1d). The effect of xanthan addition was likely to have been the major factor in decreasing water solubility in this study. Water solubility of the HPH-Xanthan film was lower than those of the DMM-based films reported by Hendrix et al. (2012) (25.4–34.4%), which were produced without incorporation of xanthan. The solubility was also lower than those of WPI-fish gelatin composite film (19.5 ± 0.7%, Jiang, Li, Chai, & Leng, 2010), potato skin-based films (31.3–43.4%, Kang and Min, 2010), and a pullulan-based film (98.4%, Rhim, Wu, Weller, & Schnepf, 1999). The results from the studies of water vapour permeability and water solubility indicate that the HPH-Xanthan film had a relatively higher water resistance compared to many other biopolymer films and thus showed the potential for such film to be applied to food products over a large range of moisture content. The film with a higher water resistance was considered appropriate to be used for studying effects of the DMM-xanthan composite coating on lipid oxidation and sensory properties of smoked salmon. Thus, the film-forming solution producing the HPH-Xanthan film with 5% (w/w) xanthan was used as the coating solution for those studies.

3.4. Antioxidant properties of the DMM-xanthan film Antioxidant properties of scavenging ABTS radicals of the HPHXanthan film (5% (w/w) xanthan) were 20.1%, 48.2%, and 99.8% at the concentrations of 0.01, 0.03, and 0.12 mg extract/mL, respectively, while those of scavenging DPPH radicals of the film were 4.3%, 11.6%, 22.0%, 21.9%, 31.6%, 43.0%, 50.2%, and 62.2% at 0.01, 0.03, 0.12, 0.20, 0.35, 0.50, 0.65, and 0.82 mg extract/mL, respectively. The values of RC50 for the HPH-Xanthan film were 0.04 and 0.6 mg/mL in the ABTS and DPPH assays, respectively. The ABTS assay used an aqueous system while the DPPH assay used an organic system. The difference in polarity of the assay systems could contribute the 15-fold difference between the values for the film assays. The HPH-Xanthan film exhibited a stronger antioxidant activity in an aqueous system than in an organic system. The antioxidant activity of the film was much less than ascorbic acid that has RC50 values of 0.01 and 0.06 mg/mL in the ABTS and DPPH assays, respectively. However, the activity was higher than those of the edible films that were reported as potential antioxidant films. The RC50 values from ABTS and DPPH assays for a jujube-based edible film were reported as 0.5 and 1.1 mg/mL, respectively (Lee, Yang, Ahn, Lee, & Min, 2011). The DPPH assay-based RC50 value of a galactomannan film incorporating honey locust was reported as 1.4 mg/mL (Cerqueira, Souza, Martins, Teixeira, & Vicente, 2010). The HPH-Xanthan film had an antioxidant activity that might be induced from antioxidants

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I.-H. Kim et al. / Food Chemistry 133 (2012) 1501–1509 Table 1 The effect of the DMM-xanthan composite coating on colour strength, glossiness, rancidity, and fishy smell of smoked salmon with and without storage at 4 °C for 7 days. Sensory attribute

Smoked salmon sampleA Uncoatedfresh

Coatedfresh

Uncoated-7day old

Coated-7-day oldB

Colour strength Glossiness Rancidity Fishy odour

5.8a ± 1.6

5.2a ± 1.6

5.9a ± 1.4

5.1a ± 1.5

4.4b ± 1.3 4.8b ± 1.5 4.9b ± 1.8

6.7a ± 1.0 4.7b ± 1.9 5.0b ± 1.6

4.6b ± 1.9 6.6a ± 1.8 7.2a ± 2.1

5.0b ± 1.7 4.9b ± 1.7 4.8b ± 2.2

A Means with different letters in the same rows are significantly different (P < 0.05). B Coated with the DMM-xanthan film-forming solution and stored at 4 °C for 7 days.

Fig. 2. Changes of thiobarbituric acid reactive substance (TBARS) values of smoked salmon without coating (j, N, and d) and coated with the defatted mustard meal (DMM)-xanthan composite film-forming solution (h, 4, and s) during storage at 4, 10, and 20 °C. n = 6.

of DMM and showed the potential to be applied to foods as an antioxidant film to retard lipid oxidation of the food without incorporating external antioxidants. 3.5. Effects of the DMM-xanthan composite coating on lipid oxidation of smoked salmon The TBARS values of coated samples were significantly lower than those of uncoated samples during storage, irrespective of storage temperatures (P < 0.05) (Fig. 2). The differences in the values of the coated and uncoated samples stored at 20 °C were more than 2 times after storage of 7 days. The DMM-xanthanbased coating retarded lipid oxidation in smoked salmon. This could be due to the antioxidant activity of the coating as demonstrated previously and also reduced exposure of oil of salmon to oxygen by a barrier property of coating. The coating applied to smoked salmon may be able to improve lipid oxidation stability of the product out of a normal storage environment of vacuum packaging and refrigeration. For performing storage studies, coating is more appropriate than wrapping films because of its much more uniform contact to the surface of salmon samples than wrapping due to the rough surface of salmon. The applied volume of the film-forming coating solution to form a coating on the salmon surface was determined to produce identical thickness (0.2 mm) to that of the film. Thus, the results from the film and the coating studies were compatible. Development of antioxidant edible films and coatings and their antioxidant effects have been reported (Caprioli, O’Sullivan, & Monahan, 2009; Han, Hwang, Min, & Krochta, 2008; Min & Krochta, 2007). A whey protein film incorporating ascorbic acid demonstrated the potential to further extend oxidation stability of foods beyond what would be achieved with an oxygen-barrier of whey protein films alone, eliminating oxygen initially present in package headspace by oxygen scavenging action of ascorbic acid (Min and Krochta, 2007). 3.6. Sensory evaluation The results of the sensory evaluation are summarised in Tables 1 and 2. Colour intensity was not different among samples. Neither coating nor storage at 4 °C for 7 days affected colour intensity significantly (P > 0.05). Glossiness was highest with Coated-fresh samples. The DMM-xanthan composite coating improved glossiness of smoked salmon, agreeing with a general statement that

Table 2 The effect of the DMM-xanthan composite coating on flavour strength, aftertaste, cohesiveness, gumminess, and oiliness of smoked salmon. Sensory attribute

Flavour strength Aftertaste Cohesiveness Gumminess Oiliness

Smoked salmon sample Uncoated

Coated

5.2a ± 1.7 5.4a ± 1.4 4.1a ± 1.5 4.5a ± 1.9 4.6a ± 1.3

5.6a ± 1.8 5.9a ± 1.4 4.2a ± 1.6 4.5a ± 1.7 4.3a ± 1.5

Means with different letters in the same rows are significantly different (P < 0.05).

edible films and coatings can increase glossiness of food to which they are applied (Kester & Fennema, 1986). The increased glossiness, however, decreased after storage and no difference in the attribute was observed between coated and uncoated samples after storage for 7 days. The coating did not affect the rancidity and fishy smell of smoked salmon (Table 1). The intensities of both attributes did not change in coated salmon after storage. The rancidity and fishy smell of the Uncoated-7-day old sample were highest. The increase of rancidity and fishy smell might be caused by lipid oxidation. The coating prevented change in these attributes, implying the prevention of lipid oxidation in smoked salmon by the coating. The prevention in lipid oxidation in smoked salmon by the coating during storage at 4 °C for 7 days was demonstrated in the TBARS test (Fig. 2). No significant differences between the coated and uncoated samples were observed across flavour strength, aftertaste, cohesiveness, gumminess, and oiliness (Table 2). The flavour of mustard that could be present in the DMM-xanthan composite coating was less likely to be noticeable. The results also indicate that the coating did not obviously affect the texture of the salmon. Overall, the results from the sensory evaluation demonstrated that the coating on the smoked salmon increased the glossiness, reducing lipid oxidation, without altering the texture of smoked salmon. 3.7. Volatile profiles Volatiles of smoked samples stored at 4, 10, and 20 °C for 7, 14, and 21 days were analysed for differences in profiles and the results demonstrated with PCA plots are shown in Fig. 3. PCA is a multivariate statistical analysis method used to effectively summarise and visualise multivariate data sets (Pravdova, Boucon, Jong, Walczak, & Massart, 2002). This procedure extracts dominant patterns in the data matrix in terms of a complementary set of scores and loading plots. PCA allows achieving a reduction of dimensionality, a data exploration finding relationships between objects, estimating the correlation structure of the variables and investigating how

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Fig. 3. Principal component analysis (PCA) plots of the results from the analysis of smoked salmon samples stored for 7 days (a), 14 days (b), and 21 days (c) using the mass spectrometer-based electronic nose. The 4, 10, and 20 °C represent the storage temperature of the coated and uncoated samples. The O and X indicate coated samples and uncoated samples, respectively.

many components (a linear combination of original features) are necessary to explain the greater part of variance with a minimum loss of information. The number of significant principal components is determined by a scree plot. When PCA is performed on auto-scaled matrix data, the principal component loadings are eigenvectors of the correlation matrix (Kallithraka et al., 2001;

Rodriguez-Delgado, González-Hernández, Conde-González, & Pérez-Trujillo, 2002). PCA is often used in the data organisation of electronic nose systems (Capone et al., 2001; Hong et al., 2011). The PCA was applied to analyse and visualise the effect of the variables of coating and storage temperature on volatiles in this study. The first principal component (PC1) expressed 70.59%, 99.83%, and

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97.28% of the variability of data of 7, 14, and 21 days, respectively, which was induced by storage temperature and coating/uncoating, while the second principal component (PC2) expressed 28.58%, 0.16%, and 1.92% of the variability of data of 7, 14, and 21 days, respectively (Fig. 3). The high PC1 score indicates that PC1 was highly correlated with storage time depending upon storage temperature and coating/uncoating. The volatile profiles of coated and uncoated samples stored at 4 °C for 7 days were not distinguishable while those of coated and uncoated samples stored at 10 °C or 20 °C for 7 days were different from each other (Fig. 3a). This is consistent with the results from the TBARS test that the TBARS values of coated and uncoated samples were not different only with those stored at 4 °C for 7 days. Lipid oxidation was likely to have been a factor making the profiles distinguishable. Differences in the intensities of rancidity and fishy smell of 7-day old coated and uncoated salmon samples observed in the sensory evaluation were not reflected in the profiles and do not seem to be have resulted in differences in overall volatile profiles. The volatile profiles of coated and uncoated samples stored for 14 days at 10 or 20 °C were distinguishable while those of the samples stored at 4 °C were not (Fig. 3b). The profiles of the coated samples overlapped more and were less dispersed than those of uncoated sample, indicating less change in volatile profiles of coated samples than uncoated samples at different storage temperatures. The profiles of coated and uncoated samples stored at 20 °C for 21 days were distinctly different from each other (Fig. 3c). The large difference between the samples also was observed in their TBARS values (Fig. 2). More changes in the profiles by temperature were observed with uncoated samples than coated samples stored for 7, 14, and 21 days, suggesting that the coating reduced changes in volatiles of smoked salmon. 4. Conclusions The application of HPH in the preparation of the film-forming solution and the incorporation of xanthan in the formulation of the DMM-based film improved tensile strength and moisture barrier properties of the film. These methods may increase the opportunity for a film of DMM, an agricultural byproduct, to be applied for food products as a coating. The DMM-xanthan composite film at 5% (w/w) xanthan exhibited antioxidant properties and the coating with the film-forming solution for producing the film effectively reduced lipid oxidation and changes of volatiles in smoked salmon during storage. The antioxidant DMM-xanthan composite coating did not negatively alter the sensory properties of the salmon. The coating has demonstrated the potential for the application to commercial salmon products and other fish products as an antioxidant coating without incorporating any external antioxidants, with suitable physical and sensory properties. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 20090077723). References ASTM. (1997). Standard test method for tensile properties of thin plastic sheeting. D822-01. Annual book of ASTM. Philadelphia, PA: American Society for Testing and Materials. Aydt, T. P., Weller, C. L., & Testin, R. F. (1991). Mechanical and barrier properties of edible corn and wheat protein films. Transactions of the American Society of Agricultural Engineers, 34, 207–211. Banerjee, R., Chen, H., & Wu, J. (1996). Milk protein-based edible film mechanical strength changes due to ultrasound process. Journal of Food Science, 61(4), 824–828.

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