!I
`
`El.SEVIER
`
`Journal of Food En&incerin11 43 (2000) 91-96
`
`JOURNAL OF
`FOOD
`ENGINEERING
`
`www.clsevier.com/locate/jfoodeng
`
`Microwave drying effects on properties of whey protein isolate edible
`films
`Sevim Kaya, Ahmet Kaya •
`Food Engineering Departme-nl, Uni11ers11y o/Ga:iantep, 273/0 Ga:iantep. Turkey
`Received II Dctembcr 1998; received in revised form 9 August 1999; aa:cpted 13 September 1999
`
`Abstract
`
`Whey protein isolate (WPJ) edible films were dried using microwave drying or at room conditions. The drying time of the films
`required 5 min in microwave oven and 18 hat room conditions. Water vapor permeability (WVP), mechanical properties, gloss and
`haze ofWPJ based edible films were determined. Water vapor transmission rate (WVTR) increased with increasing temperature, but
`the results showed that WVP did not show a similar trend. Microwave drying and drying at room conditions gave similar results for
`the WVP. Application of microwave increased the elongation and tensile strength values. © 2000 Elsevier Science Ltd. AU rights
`reserved.
`
`1. Introduction
`
`Edible films and coatings may have potential appli(cid:173)
`cations in the food industry ($iimnil & Baymd1ril, 1995).
`Edible films and coating are generally fonned from a
`solution or dispersion of the film-forming agent, fol(cid:173)
`lowed by any of the several means to separate the film.
`forming agent from the fluid carrier, or by solidification
`of the film-forming material from a melt (Kester &
`Fennema, 1986). Films and coatings may be differenti(cid:173)
`ated on the basis of an application method. A film can
`be preformed and applied to a food at any time, much
`like a synthetic package, whereas a coating must be
`applied in liquid fonn to a food directly (Sherwin, 1998).
`The specificity of the edible films needs further studies to
`improve the mechanical and barrier properties (Kester
`& Fennema, 1986).
`Whey protein, a by product of cheese manufacture, is
`produced in large quantities and has excellent functional
`properties and could potentially be used for edible films.
`Barrier and mechanical properties of whey protein iso(cid:173)
`late based films have been studied by some researchers,
`but generally they have been studied at 25"C and 100-
`0% relative humidity (RH) gradient (McHugh &
`Krochta, 1994a; Chen, 1995; Fairley, Monahan, Ger(cid:173)
`man & K.rochta, 1996; K.rochta & De Mulder-Johnston,
`1997). WPI films can be made from 8% to 12% whey
`
`• Com:sponding author.
`
`protein solutions (McHugh & K.rochta, 1994b). It was
`found that below 8%, WPI intact films were not fomed,
`presumably due to lack of intermolecular interactions
`upon film dehydration (McHugh, Aujard & K.rochta,
`1994). On the other hand, after 11% of WPI, whey
`protein solutions become gel during heating. Plastic(cid:173)
`izers, such as glycerol and sorbitol, are used generally
`for WPI-based edible films to enhance the film flexibility
`and extensibility (McHugh & K.rochta, l994b; McHugb,
`Avena-Bustillos & K.rochta, 1993; Banerjee & Chen,
`1995; Mate, Frankel & K.rochta 1996).
`Most types of edible films arc prepared by air drying
`(ca. 24 h) after spreading film solution over the plate, a
`fairly rapid film formation is generally required for in(cid:173)
`dustrial reasons. As microwave drying is one of the
`fastest drying methods, it was thought that microwave
`drying could be applied to dry films.
`One of the most useful functions of edible films is
`their ability to act as water, gas (mainly, oxygen and
`carbon dioxide) and oil barriers. Water vapor pcnnc(cid:173)
`ability is one of the most important and widely studied
`property of edible films. Mechanical properties are as
`important to edible films as barrier properties arc. Ad(cid:173)
`equate mechanical strength ensures the integrity of a
`film and its freedom from minor defects, such as a pin
`hole, which ruin the barrier property (Chen, 1995).
`Tensile strength expresses the maximum stress devel(cid:173)
`oped in a film during a tensile test and offers a measure
`of integrity and heavy duty use potential for films and
`percentage elongation at break is a quantitative
`
`0260-8774/00/S • see front matter O 2000 Elsevier Scitnc:c Ltd. All rights reserved.
`PU: S 0 2 6 0 - 8 7 7 4 ( 9 9) 0 0 I 3 6 • 3
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`92
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`S. Kaya, A. Kaya I Juumal uf Fuud Engineering 41 (2000) 9/ 96
`
`cul into 3.2 cm diameter circles and thickness of each
`film was measured at six random positions around the
`film following WVP tests. Mean values were used for the
`calculations (SD ± 0.02).
`
`2.4. Water vapor permeability
`
`Water vapor transmission of films was measured
`using procedures described by some researchers (Kam(cid:173)
`per & Fennema 1984; Aydt, Weller & Tcstin 1991; Park
`& Chinnan, 1995; Sherwin, 1998). Circular glass test
`cups with a diameter of 3 cm and a depth of 3 cm were
`used. After placing 10 ml of distilled H20 in each cup,
`they were covered with the edible films. Films were cut
`circularly with a diameter slightly larger than the di(cid:173)
`ameter of the cup and then they were scaled using
`melted paraffin. The cups were weighed with their con(cid:173)
`tents and placed in a desiccator containing saturated
`Mg{N01)i solution at the bottom. The relative humidity
`value of saturated Mg(N03) 2 solution at each temper(cid:173)
`ature range studied was found as 0.5916, 0.5438 and
`0.514 at 4°C, 20°C and 30°C, respectively (Labuza,
`1984). The desiccators were kept in the incubator (Niive
`ES 500) at 4°C, 20°C or 30°C. Cups were weighed up to
`30-40 b. Three replicates of each film were tested.
`Height of air gap between film and desiccant in cups was
`measured initially and finally and relative humidities
`and WVP values were calculated using WVP Correction
`Method.
`
`2.5. The water vapor permeability calculations
`
`The relative humidity inside the cup was provided as
`100% by placing the water into the cup. The relative
`humidity outside the cup was around 50% by placing
`into
`the desiccator.
`saturated Mg(N03) 2 solution
`Weight loss graphs were plotted with respect to time,
`slopes of them (correlation coefficients of them were
`larger than 0.997) obtained linear least-square method
`used
`lo calculate water vapor
`transmission rate
`(WVTR) in the following Equation (Chinnan & Park,
`1995):
`
`representation of a film's ability to stretch (Gennadios,
`Weller & Testin, 1993).
`The aim of this study is to apply microwave drying
`during preparation of edible films to shorten film drying
`time. The most important factor for applicability of
`different preparation techniques is obtaining similar or
`better properties than usual drying methods. Therefore,
`WVP, tensile strength, elongation, gloss and haze of the
`films were determined. Since the aim of this study is not
`studying drying characteristics of WPI films dried with
`microwave or air drying, the drying characteristics were
`not investigated. Further studies are needed for im(cid:173)
`proving the microwave application and analyzing drying
`characteristics.
`
`2. Materials and methods
`
`2.1. Materials
`
`WPI (BiPro, 98.1% protein) used to make films was
`supplied by Davisco International, Le Seur, MN. All
`chemicals used were reagent grade and the water was
`doubly distilled.
`
`2.2. Film formation
`
`Aqueous solutions of 8% and 100/a (w/w) WPI were
`prepared and heated with stirring to 90 ± 2°C for 15 min
`(total beating time is 30 min) over a hot plate. Solutions
`were cooled to room temperature and vacuum was then
`applied to remove dissolved air. Glycerine (G) was
`added as an equal weight of WPI originally dissolved to
`provide 50% WPU50% G films, total solids basis. So(cid:173)
`lutions containing 2.6 g total solids were pipetted per
`glass plate (15cmx15 cm) to minimize thickness vari(cid:173)
`ations between treatments. Ten plates were cast per
`fonnula. The solutions were spread evenly with a glass
`rod (Avena-Bustillos & Krochta, 1994) allowed to dry at
`room conditions (20 ± 2°C and 40 ± 5% RH) overnight
`and dried films were peeled from casting surface. Then
`dried films were left at 23 ± 2°C and 45 ± 5% RH for
`conditioning for one day.
`In the case of microwave drying, the emulsions
`spread over glass plates were dried in a microwave oven
`(Ar~elik, ARMD 580, with power output 700 W, oper(cid:173)
`ated at 2450 MHz) for 5 min. The boiling and bubbling
`were not observed during drying. The dried films were
`peeled-off and the same conditioning used for the mi(cid:173)
`crowave dried films was applied.
`
`2.3. Film thickness
`
`Thickness of films was measured with a micrometer
`(R&B cloth thickness lester, James H. Heal, Halifax,
`England) having a sensitivity of 0.001 mm. Films were
`
`(1)
`
`Slope
`WVTR = Film area
`
`'
`
`g
`h m2
`where Slope= weight loss vs. time. Film area is the cup
`test mouth area.
`Two WVP values were determined by using the
`methods (defined as classical methods throughout the
`study) described by Chinnan and Park (1995) and Ay(cid:173)
`ranci and (:etin ( 1995), and corrected method suggested
`by Gennadios, Weller and Gooding (1994a). Eq. (2) was
`used to find WVP in the classical method used as ne(cid:173)
`glecting gap resistance inside the cup between the solu(cid:173)
`tion and film layer,
`
`DRL - EXHIBIT 1017
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`S. Kaya, A. Kaya I Journal of Food Engineering 43 (2000) 91 96
`
`93
`
`(2)
`
`WVP- WVTRL,
`Pz - Pi
`where WVP is the water vapor permeability, Pt the ap(cid:173)
`parent pressure (kPa) inside the cup, p2 the water vapor
`partial pressure (kPa) at the film outer surface in the
`system. L the average film thickness (mm). The Pt and P2
`values were calculated from the product of vapor pres(cid:173)
`sure of pure water and the relative humidity of the
`medium at the defined temperatures and given in
`Table I. In the classical methods, vapor pressure of
`water inside (p1) the cup were assumed as pure water
`vapor pressure: 0.817, 2.346 and 4.246 kPa, at 4°C, 20°C
`and 30°C, respectively.
`The corrected WVP values were calculated as based
`on the reported methods by Gennadios et al. (1994a).
`First of all true pressure {Pi) of the film underside was
`calculated.
`
`Pi =Pr - (Pr - Po) exp
`(
`
`WVTR{RT)Az)
`PrD(MW)
`'
`
`(3)
`
`where Pr is the total atmospheric pressure (1 atm), Po
`the partial pressure (atm) of water vapor in air at the
`surface of the solution (or desiccant) in the cup, p 1 the
`partial pressure (atm) of water vapor at the underside
`of the film. Az the mean stagnant air gap height (mm).
`R the gas constant (82.1 x 10-6 m3 atm/g mol K), T
`the absolute temperature (K), D the diffusivity of water
`(cm2/s)
`in air
`in
`vapor
`found
`the
`literature
`at each temperature, MW the molar weight of water
`
`(18 gig mol). From Eq. (3), calculated Pt' values
`(Table 1) were employed in Eq. (2) to calculate the
`corrected WVP.
`
`2.6. Tensile strength, percent elongation, elastic modulus,
`gloss and haze
`
`Mechanical properties were determined using four
`films cast from each solution. Eight strips, 15 cm x 2
`cm, were cut from each type and after conditioning at
`23 ± 2°C and 45 ± 5% RH for at least 48 h prior to tests,
`samples were tested for tensile strength and percent
`elongation according to ASTM Standard Method D 882
`(ASTM, 1993). A TIRATEST 2602 (TIRA Ma(cid:173)
`schinenbau GmbH Raunstein, Germany) was used to
`measure tensile strength, percent elongation and elastic
`modulus of the sample. Initial grip separation and cross(cid:173)
`head speed were set at 100 and 500 mm/min, respec(cid:173)
`tively. The mean of thickness of these films was 0.075
`mm. Haze of samples was measured by using EEL(cid:173)
`Spherical Hazemeter BS 2782 London, England
`(ASTM, 1970a). Gloss was reported in percentage at 45°
`from a line normal to the surface ASTM, l 970b (micro(cid:173)
`TRI-gloss, BYK-Gardner, Silver Spring, MD). The
`gloss and haze values reported were based on four
`samples and four measurements per sample. These tests
`were applied at room conditions (23 x 2°C and 45 ± 5%
`RH). It is necessary to test the mechanical properties of
`the films at controlled temperature and relative humidity
`in order to achieve good reproducibility. Since all kinds
`
`Table 1
`WVTR, WVP and corrected RH (%) values, measuml at different temperatures, of B'Y· and 10% WPJ:G film dried using a microwave or room
`conditions (±SD)'
`
`Dryin1
`Methods
`
`Tempcraturc
`("C)
`
`Thickness
`(mm)
`
`WVTR
`(c/h m2)
`
`RH inside
`Cup(%)
`
`WVP
`(g mm/ kPa h m2)
`
`Microwavcb
`
`Room conditionsb
`
`Microw11vc"
`
`Room condition~
`
`4
`20
`30
`
`4
`20
`30
`
`4
`20
`30
`
`4
`20
`30
`
`0.12
`0.11
`0.12
`
`0.11
`0.10
`0.12
`
`0.10
`0.12
`0.13
`
`0.11
`0.13
`0.13
`
`4.38 (±0.1)
`20.38 (±0.1)
`40.S3 (±0. I)
`
`4.BS (±0.1)
`20.31 (±0.1)
`40.SI (±0.1)
`
`3.11 (±0.1)
`9.87 (±0.2)
`30.01 (±0.1)
`
`3.06 (±0.3)
`9.23 (±0.2)
`30.66 (±0.1)
`
`84 (±0.3)
`1S (±0.8)
`73 (±0.5)
`
`82 (±0.9)
`75 (±0.7)
`73 (±1.0)
`
`88 (±0.9)
`86 (±0.7)
`77 (±I.I)
`
`89 (±0.9)
`87 (±0.8)
`76 (±0.8)
`
`aassical
`
`Corrected
`
`1.64 (±0.10)
`1.90 (±0.10)
`2.36 (±0.07)
`
`1.59 (±0.08)
`2.09 (±0.11)
`2.4S (±0.10)
`
`0.97 (±0.0S)
`1.10 (±0.07)
`1.92 (±0.10)
`
`1.09 (±0.06)
`1.10 (±0.07)
`2.03 (±0.10)
`
`2.68 (±0.12)
`4.32 (±0.13)
`5.40 (±0.11)
`
`2.81 (±0.14)
`4. 70 (±0.12)
`S.66 (±0.13)
`
`1.32 (±0.12)
`I.SI (±0.14)
`3.62 (±0.17)
`
`1.49 (±0.11)
`1.53 (±0.13)
`3.69 (±0.20)
`
`"Thickness arc mean valucs(SD ± 0.02). Relative humidity al the inner surface of the film and corrected WVP values were calculated as described by
`Gennadios ct al. (1994). RH outside cups wns 50"/o,
`b 8% WPI films.
`c I 0% WPI films.
`
`DRL - EXHIBIT 1017
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`

`
`94
`
`S. Kaya, A. Kaya I Journal of Food Engineering 43 (2000) 91- 96
`
`or films were measured at the same room condition, it
`was possible to compare the tensile strength and percent
`elongation or the films.
`
`2. 7. Statistical analysis
`
`Sigma Plot V. 3.0 (Jandel Scientific Graphing Soft(cid:173)
`ware) was used for all statistical analysis. Analysis of
`variance (ANOVA) procedures were used to analyze
`data. Duncan's Multiple Range Test (p < 0.05) was used
`to detect differences in film property mean values.
`
`3. Results and discussion
`
`3.1. Water vapor permeability
`
`The relative humidity gradient is an important pa(cid:173)
`rameter in calculation of WVP (McHugh et al., 1993),
`generally in the litemture 100-0%RH {inside-outside the
`cup) was applied (McHugh et al., 1994; Mate et al.,
`1996).The relative humidity gradient was selected as
`100-50% since most of the foods have high water ac(cid:173)
`tivity (>0.95) and environmental RH is generally 50%.
`WVTR values obtained from the slopes of the lines
`(regression coefficients of the lines were >0.993 at
`p < 0.05), obtained from weight loss of cups covered
`with 8% and 10% WPI films prepared with either mi(cid:173)
`crowave drying or room conditions, were represented in
`Fig. 1. The increasing WVTR values with increasing
`temperature were observed. It was generally accepted
`that WVTR increased with an increase in temperature
`(Kamper & Fennema, 1984; Gontard, Guilbert & Cuq,
`1993) as it was observed in this study.
`Classical and corrected WVP of the 8% and 10%
`WPI:G films, dried at room conditions or in microwave
`oven, are tabulated in Table 1. It was observed that
`there was no significant difference between WVP values
`
`of films dried by using these drying methods (p < 0.05).
`Classical WVP methods gave lower WVP values than
`calculated values using corrected methods, as it was
`expected, because classical methods ignore air resistance
`between film and solution inside the cup (Gennadios
`et al., l994a). It was found that microwave drying or
`drying at room conditions gave similar results in WVP,
`so it was possible to use a microwave for drying of WPl
`films (Table 1). Mean WVP of8% WPI fihns were higher
`than of 10% films, but it was found that there was no
`significant difference in WVP values (.p < 0,05). McHugh
`et al. (1994) reported that there was no significant dif(cid:173)
`ference in WVP at 8% and 10%1 WPI:G concentrations.
`They used 37.5% sorbitol as plasticizer and 0-100% RH
`(out/in) and found that WVP values of the WPI films
`(8% and 10%) were the same (2.7lg mm/kPa h m2) and
`they also reported that glycerol addition instead of
`sorbitol increased WVP. The result of this study was in a
`good correlation with their result.
`The WVP values of 8% and 10% WPI:G films, dried
`either in a microwave oven or at room conditions, with
`respect to temperature were given in Fig. 2. Increasing
`WVP with the increasing temperature were observed.
`The different observations were given in the literature
`with the case of temperature dependency or WVP of
`hydrophilic films, while decrease in WVP with increase
`in temperature was observed for wheat gluten and soy
`protein isolate (Gennadios, Branderburg. Park, Weller
`& Testin, 1994b}, increase in WVP with increase in
`temperature was observed for methyl cellulose and hy(cid:173)
`droxypropyl methyl cellulose films (Chinnan & Park,
`1995).
`
`3.2. Tensile strength, elastic modulus, percent elongation.
`gloss and haze
`
`Guilbert (1986) suggested that rapid drying can
`cause some undesirable mechanical problems such as
`
`B'lloW'llMm
`10'11olM'ltlm
`
`g
`
`-
`
`) '
`
`50j
`~ 40
`~
`§ 30
`-~
`
`! :1_:
`
`~
`
`0
`
`rl"
`C>
`
`11·
`
`...,..
`15
`
`to
`
`20
`
`25
`
`30
`
`35
`
`Temperature ('C)
`
`B'llo-1:0
`tO'Mo-1.0
`
`B r
`:c- 5
`:. ...
`l .
`~
`
`~ 3 I 2 ~
`
`1
`
`0
`
`5
`
`to
`
`15
`
`20
`
`25
`
`30
`
`35
`
`Temperature ("CJ
`
`Fig. l. EITt:et of temperature on WVTR values or 8% and I 0% WPl:G
`film dried using microwave(•) or room coodition (0).
`
`Fig. 2. Effect of temperature on WVP values or 8% aod 10% WPl:G
`film dried usiog microwave(•) or room condition (V).
`
`DRL - EXHIBIT 1017
`DRL004
`
`

`
`S. Kaya, A Kaya I Journal of Food Enginefring 43 (2000) 9/- 96
`
`95
`
`Table 2
`Some physical properties of films prepared with 8% and 10"/o WPI dried using a microwave or room conditions (±SD)"
`
`Drying methods
`
`Haze
`
`Gloss
`
`Tensile strength
`(MPa)
`
`Modulus of elasticity
`(MPa)
`
`Elongation ("lo)
`
`Microwavc:A
`Room conditions'
`Microwave•
`Room conditions•
`
`2.1 (±0.2)b
`2.9 (±0.lf
`1.8 (±0.1)'
`3.2 (±0.2)b
`
`95 (±2)b
`89 (±1)'
`96 (±2)b
`87 (±2)'
`
`2.23 (±0.2)'·b
`1.94 (±0.2)'
`2.43 (±0.1 )b
`2.28 (±0.J)'·b
`
`27.08 (±9.5)'
`20.87 (±8.3)'
`20.17 (±7.0)'
`18.91 (±5.3)'
`
`36.1(±5.3)'
`26.1(±2.4)'
`35.9 (±4.3)'
`26.5 (±3.1 )'
`
`·Numbers with different following letters differ at P - 0.05 level.
`"8%WPI.
`11 10%WPI.
`
`brittleness. However there have been some studies
`conducted at higher temperatures rather than at am(cid:173)
`bient conditions (ca. 23°C). For examples, Kamper and
`Fennema (1984) dried fatty acid and hydroxypropyl
`methylcellulose bilayer films at 90°C for 1 S min, and
`Gennadios and Weller (1991) dried soymilk protein
`films at l00°C for 1 h. It was planned to dry films by a
`microwave, a rapid dryer, and to control the possible
`increase in brittleness due to rapid drying by knowing
`mechanical properties.
`The tensile strength, percent elongation and elastic
`modulus results were given in Table 2. The 8% and
`10% WPI:G-based films gave similar results, but mi(cid:173)
`crowave dried films had higher tensile strength and
`elongation values than dried films at room conditions.
`If the results of this study (Table 2) were compared
`with the results given in literature, the elongation
`values have been found in the similar range (4.10%
`and 30.8% for WPI:G 5.7:1 and WPI:G 2.3:1, respec(cid:173)
`tively (McHugh & Krochta, 1994b)), but the tensile
`strength values of this study were lower than of the
`liter.iture (9.20 and 13.9 MPa for WPI:G 3:1 (Fairley
`et al., 1996) and WPI:G 2.3:1 (McHugh & Krochta,
`1994b), respectively. Modulus of elasticity is the ratio
`of stress to strain over the linear range and measures
`the intrinsic stiffness of the film (Chen, 1995). Al(cid:173)
`though the most frequently reported tensile properties
`of edible films are tensile strength and elongation,
`nowadays modulus of elasticity has been given by
`some reporters (Chen, 1995; Fairley et al., 1996).
`Modulus of elasticity values of WPI films dried using
`microwave or room conditions arc given in Table 2. It
`was observed that there was no significant difference
`between modulus of elasticity of the films dried using
`both drying methods. Fairley ct al. (1996) reported
`that in all types of edible films, a small increase in
`glycerol level results in a large drop in tensile strength
`and an increase in elongation. Their tensile stress,
`elongation and modulus of elasticity (Young's modu(cid:173)
`lus) were 9.2 MPa, 13.7% and 401 MPa, respectively.
`Since they used WPI:G composition 3:1, the lower
`tensile strength and higher elongation values observed
`in this study than their report could be due to the high
`glycerol amount in the solution.
`
`On the other hand, it was known that specular gloss is
`used mainly as a measure of the shiny appearance of
`films and surfaces, and the measurement of haze pro(cid:173)
`vides some information on the homogeneity of the sur(cid:173)
`face and internal defects which can contribute to the
`diffusion or deviation of light. So, the main aim of
`measuring haze and gloss values was to control the
`possible invisible physical degradation of microwave
`drying on films, actually there was not any visible de(cid:173)
`gradation.
`The gloss and haze of the WPI:G films dried using
`microwave or room conditions, given in Table 2, were
`comparable with the synthetic films, sucli as gloss
`(measured at 45°) and haze values of the synthetic
`polypropylene films (Si.iper film-Biaxially oriented
`polypropylene film) are 90% and <1.5%, respectively.
`The gloss and haze values of microwave dried films are
`better also than those dried at room temperature. So,
`microwave drying could be applied for drying of WPI
`based edible films.
`
`4. Conclusion
`
`Application of microwave drying to WPl:G based
`edible films did not affect the water vapor permeability
`characteristics. The effect of microwave drying on the
`mechanical properties should be studied in more detail,
`but it was possible to indicate that microwave drying
`could be applied for drying of WPI films. It was im(cid:173)
`portant to indicate that gloss and haze properties of
`WPl:G based edible films dried by both metliods have
`been found as good as the synthetic films.
`
`Acknowledgements
`
`This study was supported by University of Gazian(cid:173)
`tep. Davisco International is greatly acknowledged for
`supplying WPI (BiPro) used throughout this study. The
`authors thank Mr. Necdet Kileci for his help during
`measurements of mechanical parameters of films at
`Si.ipcr Film, Sanko.
`
`DRL - EXHIBIT 1017
`DRL005
`
`

`
`96
`
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`DRL - EXHIBIT 1017
`DRL006