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Preparation and characterization of whey protein isolate films reinforced with porous silica coated titania nanoparticles
Accepted Manuscript Preparation and Characterization of Whey Protein Isolate Films Reinforced with Porous Silica Coated Titania Nanoparticles Dattatreya M. Kadam, Mahendra Thunga, Sheng Wang, Michael R. Kessler, David Grewell, Buddhi Lamsal, Chenxu Yu PII: DOI: Reference:
S0260-8774(13)00076-9 http://dx.doi.org/10.1016/j.jfoodeng.2013.01.046 JFOE 7284
To appear in:
Journal of Food Engineering
Received Date: Revised Date: Accepted Date:
23 November 2012 22 January 2013 27 January 2013
Please cite this article as: Kadam, D.M., Thunga, M., Wang, S., Kessler, M.R., Grewell, D., Lamsal, B., Yu, C., Preparation and Characterization of Whey Protein Isolate Films Reinforced with Porous Silica Coated Titania Nanoparticles, Journal of Food Engineering (2013), doi: http://dx.doi.org/10.1016/j.jfoodeng.2013.01.046
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1
Preparation and Characterization of Whey Protein Isolate Films
2
Reinforced with Porous Silica Coated Titania Nanoparticles
3
4
Short Title: Characterization of WPI Films Reinforced with TiO2@@SiO2 Nanoparticles
5
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Dattatreya M. Kadam1,4, Mahendra Thunga2, Sheng Wang3, Michael R. Kessler2, David
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Grewell4, Buddhi Lamsal5*, Chenxu Yu4* 1
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BOYSCAST Fellow/Visiting Scientist from Central Institute of Post-Harvest Engineering and
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Technology, Ludhiana-141004 (Punjab, India) 2
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Department of Materials Science and Engineering, Iowa State University, Ames, IA, 50011 USA
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College of Materials and Textile, Zhejiang Sci-Tech University, Hangzhou, P. R. China
Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA, 50011,
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USA 5
Department of Food Science and Human Nutrition, Iowa State University, Ames, IA, 50011, USA
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*Corresponding Authors
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Dr. Buddhi Lamsal, email: [email protected]
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Dr. Chenxu Yu, email: [email protected]
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1
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ABSTRACT
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Whey protein isolate (WPI) films embedded with TiO2@@SiO2 (porous silica (SiO2)
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coated titania (TiO2)) nanoparticles for improved mechanical properties were prepared by
22
solution casting. A WPI solution of 1.5 wt% TiO2@@SiO2 nanoparticles was subjected to
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sonication at amplitudes of 0, 16, 80 and 160µm prior to casting in order to improve the film
24
forming properties of protein and to obtain a uniform distribution of nanoparticles in the WPI
25
films. The physical and mechanical properties of the films were determined by dynamic
26
mechanical analysis (DMA), thermogravimetric analysis (TGA), and tensile testing. Water vapor
27
permeability (WVP) measurements revealed that the water vapor transmission rates are slightly
28
influenced by sonication conditions and nanoparticle loading. The DMA results showed that, at
29
high sonication levels, addition of nanoparticles prevented protein agglomeration. The thermal
30
stability of the materials revealed the presence of 3-4 degradation stages in oxidizing the protein
31
films. The addition of nanoparticles strengthens the WPI film, as evidenced by tensile stress
32
analysis. Sonication improved nanoparticle distribution in film matrix; such films can potentially
33
become effective packaging materials to enhance food quality and safety.
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Keywords: Biopolymer films; Nanoparticles; Protein isolates; Thermal properties; Mechanical
37
properties
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INTRODUCTION
40
Whey protein isolate (WPI), an abundant by-product in the cheese-making industry, has
41
been successfully employed as a raw material for preparation of biodegradable polymers
42
(Mulvihill and Ennis, 2003). WPI has desirable film forming and barrier properties, which
43
compare well to petrobased products (McHugh et al., 1994; McHugh and Krochta, 1994; Fairley
44
et al., 1996; Sothornvit and Krochta, 2000; Fang et al., 2002; Hong and Krochta, 2003;
45
Khwaldia et al., 2004; Perez-Gago et al., 2005; Gounga et al., 2007; Pallas-Brindle and Krochta,
46
2008; Min et al., 2009). However, the low tensile strength and high water vapor permeability
47
(WVP) of WPI films limit its applications in food-related packaging. Nanoscale fillers (e.g.,
48
nanoclays) have been used as reinforcements to strengthen whey protein polymer matrices (Zhao
49
et al., 2008).
50
Nanoscale fillers need to be well dispersed in the polymer matrix for better film forming
51
properties. High-power ultrasonication has been shown to create better dispersions of nano-
52
particles (Li et al., 2005) during mixing. Sonication is a highly system-specific dispersion
53
procedure involving a variety of concomitant complex physicochemical interactions. These
54
interactions can result in cluster breakdown or further agglomeration, as well as other effects
55
including chemical reactions (Taurozzi et al., 2010). Ultrasonic waves are generated in a liquid
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suspension either by immersing an ultrasound probe or “horn” into the suspension (direct
57
sonication), or by introducing a sample container with the suspension into a liquid bath through
58
which ultrasonic waves propagate (indirect sonication).
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Efforts are being made to find new uses for whey proteins, including edible films (Anker
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et al., 1998; Kokoszka et al., 2010) and films with antimicrobial functionality for more effective 3
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protection of food products, which would reduce the risk of outbreaks of foodborne diseases
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(Appendini and Hotchkiss, 2002). Unlike chitosan films, whey protein films do not inherently
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possess antimicrobial activities but incorporation of antimicrobial agents, such as sorbic acid, p-
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aminobenzoic acid (Cagri et al., 2001), and lysozyme have been reported (Kristo et al., 2008).
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Another potential method of imparting antimicrobial functionality to packaging films is through
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the utilization of antimicrobial nanoparticles such as titania (TiO2) nanocomposites. Titania is a
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nontoxic and inexpensive material exhibiting bactericidal activity against a wide variety of
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microbes due to its photocatalytic activity. When incorporated into a polymer matrix, TiO2 can
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provide protection against foodborne microorganisms as well as odor, staining, deterioration, and
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allergens in the presence of relatively low-wavelength radiation near the ultraviolet region (Rajh
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et al., 1999; Zhou et al., 2009). However, TiO2 particles easily aggregate and thus deteriorate the
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properties of the film (Zhou et al., 2009). In addition, the direct contact between TiO2 and the
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polymer matrix may lead to rapid degradation of the polymer films themselves. Recently, we
74
developed novel core (TiO2 and shell (SiO2) nanoparticles (TiO2@@SiO2) with a void layer
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between the titania core and the porous silicia outer layer as a reaction/enrichment zone for free
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radicals. This void layer acts as a phase-selective photo-catalyst for the photodecomposition of
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gas phase organic pollutants without any damage to their solid phase organic supports (Wang et
78
al., 2008). With the improvement in film properties, the nanoparticle-embedded films can
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potentially become more effective packaging materials to enhance food quality and safety. The
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main objectives of the present work are to develop WPI films embedded with TiO2@@SiO2
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nanoparticles by utilizing sonication to achieve uniform distribution of nanoparticles inside the
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WPI film, and to characterize the effects of different levels of sonication used. Furthermore, the
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effect of film thickness on the physical and mechanical properties of the films is also of interest.
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Material and Methods
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Materials
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TiO2@@SiO2 nanoparticles (particle size between 100-180nm) were made following a
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previously reported procedure (Wang et al., 2008). Briefly, TiO2 nanoparticles were coated by
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glucose (i.e., carbon) to form particle complex denoted as C@TiO2. The C@TiO2 particles were
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then coated with a layer of silica via a sol-gel method to form SiO2@C@TiO2. Finally, the
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SiO2@C@TiO2 nanoparticles were calcined at an elevated temperature (~773K for 3h) to
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remove the interior carbon layer and create pores on the outer layer to yield nanovoid core/shell
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nanoparticles denoted as SiO2@@TiO2. A schematic illustration and TEM image of the
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TiO2@@SiO2 nanoparticles used in the WPI films are shown in Figure 1.
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Whey protein isolate (WPI) was purchased from Pure Bulk (Roseburg, Oregon, USA)
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which was processed using cold micro-filtration. WPI protein analysis was done using the
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Dumas Nitrogen Combustion method which revealed an 86.01% protein content (as-is basis)
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with a moisture content of 6.96 wt%. All other reagents were of analytical grade. Deionized and
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distilled (DD) water was used for preparation of all samples.
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Preparation of WPI-Based Films
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The WPI films were formed following the method described by Kim and Ustunol (2001)
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with some modifications. WPI and glycerol at 5 wt% each were dissolved in DD water (90 wt%)
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with continuous stirring to obtain a film-forming solution. The pH of the film forming solution
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was adjusted to 8.0 with 2 M NaOH. Then, the solution was heated to 90±2C for 30min in a
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water bath while being stirred continuously, and rapidly cooled in an ice bath for 10-15min to 5
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avoid further denaturation. The solution was then filtered through two layers of muslin cloth to
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remove any coagulation. The filtered WPI solution was subjected to ultrasonication levels of 0,
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10, 50 and 100% sonication (i.e. 0, 16, 80 and 160µm amplitudes), denoted as A, B, C and D
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respectively. The film forming solutions of 10, 15 and 20g were cast on sterile polystyrene petri
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dishes (Fisher brand, USA). These were dried at 35±1C for 24h in a hot air dryer having 24%
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relative humidity (RH), then kept in a 50±2% RH chamber for at least 24h before the films were
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peeled and tested at room temperature (24±1C).
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Film Thickness
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The film thicknesses were measured by a digital micrometer (Model: IP-65, Mitutoya
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Corp., Tokyo, Japan) to the nearest 0.001mm. Measurements were taken at five positions along
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the rectangular strips cut for the mechanical properties determination. Thickness measurement
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for water vapor permeability (WVP) was done at five random positions around the perimeter of
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each film prepared in petri dish. The mechanical properties and WVP were calculated using the
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average thickness of each film.
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Film Surface Wettability/ Contact Angle
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The contact angle measurements were carried out with a Ramé-hart 100-00 115 NRL
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contact angle goniometer (Ramé-hart instrument co, NJ). A droplet of distilled water (4μL) was
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deposited on the leveled film surface to determine the contact angle of water on the biopolymer
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films. All films were pre-conditioned in a humidity chamber at 50±2% relative humidity to avoid
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interference due to competing moisture exchange at the surface around the droplet (Marcuzzo et
125
al., 2010). 6
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Water Vapor Permeability
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Water vapor transmission rate (WVTR), a film barrier property, was evaluated using a
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gravimetric method per ASTM E 96-95 with modifications described by Anderson et al., (2012).
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An open area of film with a diameter of 62 mm was placed on top of a flat-lipped glass beaker
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containing 3±0.01g of anhydrous calcium sulfate (Drierite) to maintain nearly 0% RH inside the
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cell. The lip of the beaker was vacuum-greased and the film was sealed with a custom-made
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flange and an O-ring to secure the film between the beaker and flange. A saturated solution of
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calcium nitrate was used to maintain 95±2% RH outside the beakers in a closed chamber. The
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films were left in the closed chamber for 48h. Two different thicknesses of films, formed using
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10 and 20g of film forming solution, were tested at 24±1°C. The WVP was calculated by plotting
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the weight gain by the film over time using the following equation (Anderson et al., 2012):
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= WL/tAP
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where, is the water vapor permeability (i.e. 10-11g m/m2 s Pa), W is the amount of the water
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gained by the cup (g), L is the film thickness (m), t is the elapsed time (s), A is the film cross
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sectional area (m2), and P is the difference in pressure between the inside and outside of the vial
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(Pa).
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Dynamic Mechanical Analysis
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The DMA measurements were performed on rectangular film samples with dimensions
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(l × w × t) of 131mm × 7±1mm × actual thickness, respectively, using a Q800 DMA (TA
146
Instruments New Castle, DE, USA). Samples were evaluated in tension mode using a dynamic
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temperature sweep to measure the storage and loss moduli as a function of temperature at a fixed
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frequency (1Hz).
149
cooling followed by a constant heating rate of 5°C/min to 150C.
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Determination of Mechanical Properties
The specimen’s temperature was equilibrated at -120C using liquid N2
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The tensile testing of the mechanical properties of the biopolymer samples were as
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performed with an Instron Universal Testing machine (Model 5569, Instron Engineering Corp.,
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Canton, MA) at an extension rate of 50mm/min using a 2kN load cell. The samples for the
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tensile tests were prepared by stamping dog-bone shaped specimens from the biopolymer films
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according to ISO 527 type 5A. The strain in the sample was measured by keeping a gauge length
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of 30mm between the clamps. The Young´s modulus was measured at 0.1 to 0.5% strain using
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the Bluehill software supplied with the machine. For each film type, five specimens were cut
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from a single petri plate film sample and the highest four results were averaged. The test cross
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section was measured for length, width and thickness using a digital caliper.
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Thermogravimetric Analysis
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The thermal stability of the WPI films with and without TiO2@@SiO2 was investigated
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using a thermogravimetric analyzer, TGA model Q50 (TA Instruments, DE, USA) in a nitrogen
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atmosphere. The mass of the samples used was 15±1mg. The samples were heated from room 8
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temperature to 800C at a heating rate of 10C/min to measure weight changes as a function of
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temperature and/or time under a controlled atmosphere and to determine the material’s thermal
166
stability and composition.
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Statistical Analyses
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A single-factor completely randomized experimental design (CRD) was used to
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determine significant differences among the samples at p<0.05. Analysis of variance (ANOVA)
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tests (Fisher’s LSD) were used to determine the effect of the nano particles in the films.
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RESULTS AND DISCUSSION
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The whey protein isolate (WPI) films were transparent, smooth, flexible, homogeneous,
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and without observable pores or cracks. Changes in appearance were observed for films
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embedded with nanoparticles at various ultrasonication levels of 0, 10, 50 and 100% (i.e. tip horn
175
amplitudes of 0, 16, 80 and 160µm, respectively). The appearance of the film side facing the
176
casting plates was shiny while the other one was slightly dull.
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Film Thickness
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The average film thicknesses of WPI films with and without nanoparticles are presented
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in Table 1. It is clear from the Table 1 that the addition of nanoparticles increases the thickness
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of the cast films. Indeed, the same weight of film-forming solution was casted in 100mm
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diameter petri dishes, but the dry matter of the solutions increases with the addition of
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nanoparticles.
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Surface Wettability
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Surface hydrophobicity has been used as an important indicator of a protein film’s
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sensitivity to water or moisture; it could be evaluated indirectly using contact angle (CA)
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measurement. The hydrophobic interactions on the surface of our film specimen may be
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influenced by both sonication and nanoparticles. The CA of WPI films without nanoparticles was
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40±3, decreasing slightly as the level of sonication amplitude increases. This decrease in the
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contact angle could be due to the restructuring of the proteins at higher sonication levels that lead
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to less exposure of hydrophobic regions of the protein to the surface (Chandrapala et al., 2011).
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WPI film swelling was observed during the contact angle measurements using a water droplet.
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Although the observed trend was relatively small in pure WPI films, there was significant
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variation of CA in WPI nanocomposite films. It can be observed that the contact angle increased
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dramatically from 16 to 74 with an increase in sonication amplitude from 0 to 160µm i.e. 0 to
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100% (Fig. 2). A fine distribution of the nanoparticles is achieved by sonication and results in
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covering the whole surface of the WPI film. Consequently, the improvement in the contact angle
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after sonication could be attributed to the domination of surface functionality of the nanoparticles
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compared to the pure WPI film. Generally, protein films with higher contact angles exhibit a
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higher surface hydrophobicity (Tang and Jiang, 2007). The contact angles decreased with an
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increase in measurement time from 60s to 300s irrespective of sonication level and nanoparticle
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loading (Fig. 2). The slight decrease in the contact angle with time could have resulted either
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from the equilibrium time required to show a stable contact angle or due to the slight absorption
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of water into the films.
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Water Vapor Permeability
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The WVP for WPI films for all levels of sonication, thickness, and nanoparticle loadings
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ranged between 210-11 and ~5 10-11g m/m2 s Pa (Fig. 3). The thicker WPI films and desiccant
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gained more weight compared to the thinner films, irrespective of sonication levels and
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nanoparticle loadings. A higher WVP is one of the major limitations when using protein-based
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films as food packaging materials. Therefore, a reduction in WVP is desirable for potential
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applications in food packaging. The RH gradient is an important parameter when calculating the
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WVP (McHugh et al., 1993). The actual thickness of the film was mainly responsible for the
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increase or decrease in its WVP values (Fig. 3) and the same was taken into consideration while
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calculating WVP.
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The WVP is dependent on the number of polar groups present in the polymer film. The
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whey protein films are hydrophilic; they are composed of highly polar amino acids. Further,
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whey powder contains lactose sugar, which is a highly hydrophilic material. The barrier
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properties of films are influenced by the hydrophobic or hydrophilic nature of the polymer, the
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presence of voids or cracks, the type, level and compatibility of incorporated plasticizer, and the
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steric hindrance in the film structure (Santosa and Padua, 1999; Lawton, 2004). Denaturation of
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the cowpea protein at higher pH values exposes the hydrophilic residues on the protein surface
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(Hewage and Vithanarachchi, 2009). This enhances the absorption of migrating water molecules
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to polar groups, facilitating the transport of water and causing the penetration of more water
225
vapor through the film (Nathalie et al., 1992). The WVP of the cowpea protein film increased
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with an increasing plasticizer concentration (Hewage and Vithanarachchi, 2009). The plasticizer
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molecules increase the intermolecular spacing between the protein polymer chains, allowing the
228
water molecules to pass through the films (McHugh et al., 1994). 11
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Dynamic Mechanical Analysis
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Thermo-mechanical analysis was used to study the influence of sonication and
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TiO2@@SiO2 nanoparticles on the phase behavior of the WPI films. Figures 4a and 4b depict the
232
change in elastic loss modulus (E``) and storage modulus (E`) over a temperature range of -
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120C to 140C. The peaks in the loss modulus curves reveal the presence of two distinct
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transition temperatures. The strong peak at a lower temperature (<0C) corresponds to the
235
transition temperature of the sorbitol phase (Ts) in the WPI bulk sample, whereas the shoulder-
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like peak at 90C corresponds to the α-relaxation (Tα) of WPI (Zinoviadou et al., 2009). The α-
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relaxation (Tα) is primarily observed due to the glass transition in WPI. A shift in the relaxation
238
temperatures (Ts and Tα) to higher values were observed in pure WPI samples after sonication.
239
Apart from sonication, Ts and Tα also shifted to higher values with the addition of TiO2@@SiO2
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nanoparticles. The shift of the relaxation peaks towards higher temperatures in the biopolymer
241
films is primarily attributed to a change in the molecular level interactions that can influence the
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segmental mobility in the protein. Sonication of the pure WPI resulted in disordering of
243
intramolecular forces in the protein, whereas sonication at high amplitude for an extended period
244
of time could lead to the formation of agglomerations from the disordered phase. Such
245
agglomerations could act as rigid constraints to hinder the relaxation of proteins near Ts and Tα,
246
resulting in a shift of the relaxation peaks to higher temperatures. However, in the case of WPI
247
nanocomposites, the shift in Ts and Tα is mainly attributed to the presence of the TiO2@@SiO2
248
nanofillers. The presence of these rigid fillers in soft protein films could potentially act as a
249
barrier to the protein flexibility.
12
250
The presence of titania agglomerates in sonicated samples at higher power levels could
251
be reconfirmed from the intensity difference of the Ts relaxation peak. Generally, the intensity of
252
the relaxation peak corresponds to the amount of soft phase in the bulk sample. A decrease in the
253
intensity of the Ts relaxation of WPI-D when compared to WPI-A implies a partial conversion of
254
the overall soft phase to the hard agglomerated phase. However, in contrast to the general trend,
255
the intensity of the Ts relaxation peak in the nanocomposite film WPI-NP-D was higher than in
256
WPI-NP-A. This difference could be attributed to the ability of TiO2@@SiO2 fillers to protect
257
disordered proteins against forming agglomerations.
258
The variation in the storage modulus (E`) with respect to temperature for all the WPI
259
samples is presented in Figure 4(b). The glassy modulus corresponding to the E` below the Ts (~
260
-60C) decreases after sonication in the case of pure WPI samples, whereas the same increases in
261
nanocomposite films. This change in the glassy modulus could be attributed to a partial
262
agglomeration phenomenon at higher sonication levels. Over the temperature range studied, the
263
elastic moduli of the nanocomposite samples were higher than for pure WPI films. The observed
264
high modulus values could be due the reinforcing effect from the rigid TiO2@@SiO2 fillers in
265
the soft protein matrix. Thus, the TiO2@@SiO2 nanoparticles not only improved the mechanical
266
strength of the samples over a broad temperature range, but also prevented self-aggregation of
267
WPI during sonication at high amplitudes.
268
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Mechanical Properties
270
The stress-strain curves for WPI films with and without nanoparticles are shown in
271
Figure 5. A slight scattering was observed between the stress-strain curves of different specimens
272
in the same batch. The stress-strain curves shown in Figure 5 correspond to one of the five
273
specimens tested in each batch. The stress-strain profiles of all the WPI films show a yielding
274
behavior followed by cold drawing prior to failure. The amount of scattering in the stress-strain
275
behavior between different specimens in the same batch of samples can be estimated from the
276
error bars on the Young’s modulus and the tensile strength data in Figure 6. The significant
277
influence of sonication on the stress-strain behavior of the WPI films can also be seen from
278
Figure 5. With the addition of nanoparticles, the modulus of the pure WPI increased from 19MPa
279
to 35MPa (Fig. 6a) and the tensile strength measured from the stress-at-break also increased
280
from 1.03 MPa to 1.18 MPa (Fig. 6b). The increase in tensile properties is more prominent after
281
sonication i.e. the modulus seems to be independent of sonication level in pure WPI films when
282
compared to the obvious increase in the nanocomposite films (Fig. 6a). The tensile strength also
283
showed a similar trend between samples with and without nanoparticles. From the results, it is
284
worth mentioning that both ultra-sonication and TiO2@@SiO2 can collectively contribute to
285
enhance the tensile mechanical properties of the WPI films. Sonication helps in breaking the
286
nanoparticle agglomerations and disperses them finely in the protein matrix. Such well-dispersed
287
nanofillers could improve the reinforcing efficiency of nanoparticles, enhancing the material’s
288
mechanical properties. On the other hand, from Figures 5 and 6c it can be observed that the
289
nanoparticles also affect the strain at break in the samples. In pure WPI films, due to the flexibly
290
of the protein, the material can dissipate more energy resulting in high strain at break; whereas,
291
with the incorporation of reinforcing fillers, the stiffness of the films increases with a 14
292
simultaneous reduction in film flexibility. Thus, the observed difference in the strain at break
293
between pure WPI and WPI nanocomposite films is due to the change in film flexibility.
294
Thermogravimetric Analysis
295
The thermal stability of the WPI films with or without nanoparticles was investigated
296
using Thermogravimetric Analysis (TGA) in an air atmosphere. It can be seen from Figure 7 that
297
the films experience 3-4 noticeable thermal degradation steps in the temperature range of 30–
298
850C. Different stages of degradation can be distinctly observed from the prominent peaks in
299
the derivative weight % curves. The temperature range for the first step of thermal degradation is
300
between 50 –150C, which may correspond to the loss of water from the films. The temperature
301
range for the second step of thermal degradation is between 150 –300C, and this may be due to
302
the decomposition of WPI proteins and the loss of glycerol from the film. The temperatures for
303
the onset of maximum degradation (Tmax) taken from the peak temperature of derivative weight
304
% curves are listed in Table 2. The Tmax of pure WPI is at 222C, which reduced by only 5C
305
after 100% sonication. However, the Tmax of the nanocomposites samples was significantly
306
reduced to 206C and 201C for samples before and after sonication, respectively. The decrease
307
in the Tmax in the samples containing nanoparticles could be attributed to the onset of oxidation
308
of the surface functional groups on the nanoparticle within the temperature limits. An additional
309
third step of thermal degradation in the temperature range of 450-550C was observed, which
310
may be due to oxidation of partially decomposed proteins. With the addition of nanoparticles, the
311
nanocomposite films (WPI-NP-A and WPI-NP-D) exhibit a significant delay in the weight loss
312
at temperatures above 300oC. The difference in the weight loss between WPI and WPI-NP
313
samples at 450oC is listed in Table 2. Similar results are also reported by Kumar et al., (2010) for 15
314
soy protein isolate (SPI) and montmorillonite (MMT) bio-nano-composite films. Because the
315
TiO2@@SiO2 nanoparticles are stable above 650C, the residue remaining after the
316
decomposition of nanocomposite samples at 650C confirms the presence of a similar weight %
317
of nanoparticles in the investigated samples.
318
Conclusion
319
It was observed that whey protein isolate films could sustain their structural integrity
320
under the application of sonication processing, and the incorporation of TiO2@@SiO2
321
nanoparticles helps to improve their mechanical properties. The optical properties of the WPI
322
films were changed dramatically by the incorporation of nanoparticles, changing from a
323
transparent appearance to opaque. Embedded nanoparticles contributed effectively to increase
324
the thickness of films and to alter the contact angle of the film surface, which may have
325
implications in regard to the hydrophobicity of the films. Film WVP values (10-11g m/m2 s Pa)
326
are shown to be dependent on the film thickness. The presence of nanoparticles in the film was
327
shown to improve certain mechanical properties, but it also caused nearly a 50% reduction in
328
elongation. The storage modulus (E`) increased with an increasing level of sonication amplitude,
329
but the presence of nanoparticles led to a reduction in E`, possibly due to phase separation. There
330
were 3–4 steps of thermal degradation of the films in the temperature range of 30–850C. The
331
glass transition temperature (Tg) was between -95C and -60C, and significant differences were
332
observed in the DMA results. The WPI films embedded with nanoparticles have great potential
333
for application in food packaging for extending the shelf life, improving quality, and enhancing
334
the safety of food packaged with them.
335 16
336
Acknowledgements
337
We acknowledge the Department of Science and Technology (DST) Government of India, for
338
the BOYSCAST Postdoctoral Fellowship to Dr. Dattatreya M. Kadam allowing him to carry out
339
this work at Iowa State University. We also acknowledge the Director of the Central Institute of
340
Post-Harvest Engineering and Technology (CIPHET), Ludhiana and Indian Council of
341
Agricultural Research (ICAR), New Delhi for granting deputation to Dr. Kadam.
342
17
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References
344
Anderson, T. J., Ilankovan, P., & Lamsal, B. P. (2012). Two fraction extraction of α-zein from
345
DDGS and its characterization. Industrial Crops and Products, 37 (1), 466–472.
346
Anker, M., Stading, M., & Hermansson, A. M. (1998). Mechanical properties, water vapor
347
permeability, and moisture contents of beta-lactoglobulin and whey protein films using
348
multivariate analysis. Journal of Agricultural and Food Chemistry, 46, 1820–1829.
349
Appendini, P., & Hotchkiss, J. H.(2002). Review of Antimicrobial Food Packaging, Innovative
350
351 352
Food Science & Emerging Technologies, 3 (2), 113-126.
Ashley, A. J. (1994). Permeability and plastic packaging, in: Comyn, J. (Ed.), Polymer permeability, Chapman and Hall, London, pp. 269–308
353
Cagri, A., Ustunol, Z., & Ryser, E. T. (2001). Antimicrobial, mechanical, and moisture barrier
354
properties of low pH whey protein-based edible films containing p-aminobenzoic or
355
sorbic acids. Journal of Food Science, 66, 865–870.
356 357
358 359
Campos, C. A., Gerschenson, L.N., & Flores, S. K. (2011). Development of edible films and coatings with antimicrobial activity. Food and Bioprocess Technology, 4 (6), 849–875.
Cha, D. S., & Chinnan, M. S. (2004). Biopolymer-based antimicrobial packaging: a review. Critical Reviews in Food Science and Nutrition, 44, 223–237.
360
Chandrapala, J., Zisu, B., Palmer, M., Kentish, S., & Ashokkumar, M. (2011). Effects of
361
ultrasound on the thermal and structural characteristics of proteins in reconstituted whey
362
protein concentrate. Ultrasonics Sonochemistry, 18(5), 951–957. 18
363
Fairley, P., Monahan, F. J., German, J. B., & Krochta, J. M. (1996). Mechanical properties and
364
water vapor permeability of edible films from whey protein isolate and sodium dodecyl
365
sulfate. Journal of Agricultural and Food Chemistry, 44, 438–443.
366
Fang, Y., Tung, M. A., Britt, I. J., Yada, S., & Dalglei, D. G. (2002). Tensile and barrier
367
properties of edible films made from whey proteins. Journal of Food Science: I67 (1),
368
188–193.
369
Gennadios, A. (2002). Protein-based Films and Coatings. CRC Press, Boca Raton, FL.
370
Gounga, M.E., Xu, S.Y., & Wang, Z. (2007). Whey protein isolatebased edible films as affected
371
by protein concentration, glycerol ratio and pullulan addition in film formation. Journal
372
of Food Engineering, 83, 521–530.
373
Hewage, S., & Vithanarachchi, S. M. (2009). Preparation and characterization of biodegradable
374
polymer films from cowpea (Vigna unguiculata) protein isolate. Journal of the National
375
Science Foundation of Sri Lanka, 37 (1), 53-59.
376 377
Hong, S. I., & Krochta, J. M. (2003). Oxygen barrier properties of whey protein isolate coating on polypropylene films. Journal of Food Science, 68, 224–228.
378
Jiang, M., Liu, S., Xin, Du., & Wang, Y. (2010). Physical properties and internal microstructures
379
of films made from catfish skin gelatin and triacetin mixtures. Food Hydrocolloids, 24,
380
105–110.
381
Khwaldia, K., Perez, C., Banon, S., Desobry, S., & Hardy, J. (2004). Milk proteins for edible
382
films and coatings. Critical Reviews in Food Science and Nutrition, 44, 239–251. 19
383 384
Kim, S. J., & Ustunol, Z. (2001). Thermal properties, heat sealability and seal attributes of whey protein isolate/lipid emulsion edible films. Journal of Food Science, 66(7), 985–990.
385
Kokoszka, S., Debeaufort, F., Lenart, A., & Voilley, A. (2010). Water vapour permeability,
386
thermal and wetting properties of whey protein isolate based edible films. International
387
Dairy Journal, 20 (1), 53–60.
388
Kristo, E., Koutsoumanis, K. P., & Biliaderis, C. G. (2008). Thermal, mechanical and water
389
vapor barrier properties of sodium caseinate films containing antimicrobials and their
390
inhibitory action on Listeria monocytogenes. Food Hydrocolloids, 22, 373–386.
391
Kumar, P., Sandeep, K. P., Alavi, S., Truong, V. D., & Gorga, R. E. (2010). Preparation and
392
characterization of bio-nanocomposite films based on soy protein isolate and
393
ontmorillonite using melt extrusion. Journal of Food Engineering, 100, 480–489
394 395
Lawton, J. W. (2004). Plasticizers for zein: Their effect on tensile properties and water absorption of zein films. Cereal Chemistry, 81 (1), 1-5.
396
Li, J., Kim, J. K., & Sham, M. L. (2005). Conductive graphite nanoplatelet/epoxy
397
nanocomposites: Effects of exfoliation and UV/ozone treatment of graphite, Scripta
398
Materialia, 53, 235–240.
399
Marcuzzo, E., Peressini, D., Debeaufort, F., & Sensidoni, A. (2010). Effect of ultrasound
400
treatment on properties of gluten-based film. Innovative Food Science & Emerging
401
Technologies, 11(3), 451–457.
20
402
McHugh, H., Avena-Bustillos, R., & Krochta, J. M. (1993). Hydrophilic edible films: Modified
403
procedure for water vapour permeability and explanation of thickness effects. Journal of
404
Food Science, 58 (4), 899–903.
405 406
McHugh, T. H. & Krochta, J. M. (1994). Water vapor permeability properties of edible whey protein-lipid emulsion films. Journal of the American Chemical Society, 71, 307–312.
407
McHugh, T. H., Aujard, J. F., & Krochta, J. M. (1994). Plasticized Whey Protein Edible Films:
408
Water Vapor Permeability Properties, Journal of Food Science, 59, 416–419, 423.
409
McHugh, T. H., Aujard, J. F., & Krochta, J. M. (1994). Plasticized whey protein edible films:
410
Water vapour permeability properties. Journal of Food Science, 59(2), 416 – 419.
411
Min, S., Janjarasskul, T., & Krochta, J. M. (2009). Whey protein bees wax layered composite
412
film tensile and moisture barrier properties. Journal of the Science of Food and
413
Agriculture, 89, 251–257.
414
Mulvihill, D. M., & Ennis, M. P. (2003). Functional milk proteins: Production and utilization, in
415
Fox, P. F., McSweeney, P. L. H. (Eds.), Advanced dairy chemistry-Vol. 1. Kluwer
416
Academic, New York, pp. 1175–1228.
417
Nathalie G., Guilbert S., & Cuq J. L. (1992). Edible wheat gluten films: Influence of the main
418
process variables on film properties using response surface methodology. Journal of
419
Food Science, 57(1), 194 – 195.
420 421
Pallas-Brindle, L., & Krochta, J. M. (2008). Physical properties of whey protein-hydroxypropyl methy lcellulose blend edible films. Journal of Food Science, 73, E446–E454. 21
422
Perez-Gago, M. B., Serra, M., Alonso, M., Mateos, M., & Del Rio, M. A. (2005). Effect of whey
423
protein- and hydroxypropyl methylcellulose-based edible composite coatings on color
424
change of fresh-cut apples. Postharvest Biology and Technology, 36, 77–85.
425
Rajh, T., Nedeljkovic, J. M., Chen, L. X., Poluektov, O., Thurnauer, M. C., & Thurnauer, M. C.
426
(1999). Improving optical and charge separation properties of nanocrystalline TiO2 by
427
surface modification with vitamin C. The Journal of Physical Chemistry B, 103(18),
428
3515–3519.
429
Santosa, F.X., & Padua, G. W. (1999). Tensile properties and water absorption of zein sheets
430
plasticized with oleic and linoleic acids. Journal of Agricultural and Food Chemistry,
431
47(5), 2070-2074.
432
Sothornvit, R., & Krochta, J. M. (2000). Oxygen permeability and mechanical properties of
433
films from hydrolyzed whey protein. Journal of Agricultural and Food Chemistry, 48,
434
3913–3916.
435
Tang, C. H., & Jiang, Y. (2007). Modulation of mechanical and surface hydrophobic properties
436
of food protein films by transglutaminase treatment. Food Research International, 40,
437
504–509.
438
Taurozzi, J. S., Hackley, V. A., & Wiesner, M. R. (2010). Preparation of nanoparticle dispersions
439
from powdered material using ultrasonic disruption. In: CEINT/NIST Protocol for
440
preparation of nanoparticle dispersions from powdered material using ultrasonic
441
disruption, Ver. 1 June 3, 2010. National Institute of Standards and Technology, US
22
442
Department of Commerce, USA. Pp: 1-10. http://www.goodnanoguide.org/tiki-
443
download_wiki_attachment.php?attId=36
444
Wang, S., Wang, T., Chen, W., & Hori, T. (2008). Phase-selectivity photocatalysis: a new
445
approach in organic pollutants’ photodecomposition by nanovoid core (TiO2)/shell(SiO2)
446
nanoparticles. Chemical Communications, 32, 3756–3758.
447 448
Zhao, R. X., Torley, P., & Halley, P. J. (2008). Emerging biodegradable materials: starch- and protein-based bionanocomposites. Journal of Materials Science, 43, 3058-3071
449
Zhou, J. J., Wang, S. Y., & Gunasekaran, S. (2009). Preparation and characterization of whey
450
protein film incorporated with TiO2 nanoparticles. Journal of Food Science,74(7), 50–56.
451
Zinoviadou, K. G., Koutsoumanis, K.P., & Biliaderis, C. G. (2009). Physico-chemical properties
452
of whey protein isolate films containing oregano oil and their antimicrobial action against
453
spoilage flora of fresh beef. Meat Science, 82(3), 338–345.
454
455
456
457
458
459
23
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Figure Captions
461
Fig. 1. View of TiO2@@SiO2 nanoparticles (TEM picture Wang et al., 2008) used in WPI films
462 463 464
to improve mechanical and antimicrobial properties. Fig. 2. Effect of sonication levels on water contact angles of 0.23mm thick pure WPI films and WPI films containing nanoparticles.
465
Fig. 3. Water vapor permeability of WPI films as a function of sonication level, nanoparticles
466
and film thickness. Note: A, C, and D represent 0, 80 and 160m sonication amplitudes,
467
respectively (film thickness, mm)
468
Fig. 4. DMA used for determination of (a) Loss modulus and (b) Elastic modulus of WPI films
469
with or without nanoparticles prepared using different sonication levels. Note: It is one
470
representation of few measurements.
471
Fig. 5. Typical Tensile stress (M Pa) – Tensile strain (%) curves for WPI films at different
472
sonication levels with and without nanoparticles. Note: WPI is whey protein isolate and
473
WPI NP is whey protein isolate with nanoparticles. A, B, C and D represent the 0, 10, 50
474
and 100% sonication levels, respectively. Note: It is one representation of few
475
measurements.
476
Fig. 6. Effect of sonication level and with or without nanoparticles on Young’s modulus, tensile
477
stress and tensile strain of WPI films. The error bars show the standard deviation.
478
Fig. 7. Effect of sonication level and nanoparticle loadings on TGA of 0.23mm thick WPI films.
479
Note: It is one representation of few measurements.
24
480
Table Captions
481
Table 1. Thicknesses of whey protein isolate films with and without nanoparticles.
482
Table 2. TGA investigation on WPI films with and without nanoparticles.
483
25
1
2 3
4
5
6
7 8
Fig. 1. View of TiO2@@SiO2 nanoparticles in WPI films to improve mechanical and antimicrobial properties.
9
10
26
90 80
60 sec 300 sec
Contact Angle (o)
70 60 50 40 30 20 10 0 A
B
C
D
A
WPI
1 2 3
B
C
D
WPI NP
Sonication Level (%)
Fig. 2. Effect of sonication levels on water contact angles of 0.23mm thick pure WPI films and WPI films containing nanoparticles.
27
6E-11
5E-11
WVP (g m/m2 s Pa )
4E-11
3E-11
2E-11
1E-11
0 A A C C D D (0.14) (0.37) (0.13) (0.36) (0.14) (0.30)
A A C C D D (0.14) (0.31) (0.22) (0.29) (0.16) (0.30)
WPI
1
WPI NP Sonication level, % (Film thickness, mm)
2
Fig. 3. Water vapor permeability of WPI films as a function of sonication level,
3
nanoparticles and film thickness. Note: A, C, and D represent 0, 80 and 160m
4
sonication amplitudes, respectively (film thickness, mm)
28
1 2
4 5 6 7
Loss Modulus E`` (MPa)
1000
3
WPI A WPI D WPI-NP A WPI-NP D
100
10
1
8
a 0.1 -100
9
-50
0
50
100
o
Temperature ( C) 10 11 12 13 14 15
Elastic Modulus E` (MPa)
10000
WPI A WPI D WPI-NP A WPI-NP D
1000
100
10
1
16
b 0.1
17
-100
-50
0
50
100
150
o
Temperature ( C)
18
Fig. 4. DMA used for determination of (a) Loss modulus and (b) Elastic modulus of WPI
19
films with or without nanoparticles prepared using different sonication levels. Note:
20
It is one representation of few measurements.
21 29
WPI A WPI B WPI C WPI D WPI-NP A WPI-NP B WPI-NP C WPI-NP D
2.0 1.8
Tensile Stress (MPa)
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
Tensile Strain, %
1 2
Fig. 5. Typical Tensile stress (MPa) – Tensile strain (%) curves for WPI films at different
3
sonication levels with and without nanoparticles. Note: WPI is whey protein isolate
4
and WPI NP is whey protein isolate with nanoparticles. A, B, C and D represent the
5
0, 10, 50 and 100% sonication levels, respectively. Note: It is one representation of
6
few measurements.
7
30
70 60
Youngs Modulus
50 40 30 20 10 0 A
A NP
B
B NP
C C NP
D D NP
Sonication Level (%)
1
Tensile Stress at Break (%)
2.5 2
1.5 1
0.5 0 A A NP
B B NP
C C NP
D D NP
Sonication Level (%)
2
Tensile Strain at Break (%)
80
3 4 5 6
70 60 50 40 30 20 10 0 A
A NP
B
B NP
C C NP
D D NP
Sonication Level (%) Fig. 6. Effect of sonication level and with or without nanoparticles on Young’s modulus, tensile stress and tensile strain of WPI films. The error bars show the standard deviation. 31
1 2 1.2 100
1.1
90
1.0
WPI A WPI D WPI-NP A WPI-NP D
80
5 70
7 8 9
Weight (%)
6
60
0.8 0.7 0.6
50 0.5 40
0.4
30
0.3
20
0.2
10
0.1
10 11
0.9
o
4
Deriv. Weight, %/ C
3
0.0
0
-0.1
12
40
80 120 160 200 240 280 320 360 400 440 480 520 560 600 640 o
13 14 15
Temperature ( C)
Fig. 7. Effect of sonication level and nanoparticle loadings on TGA of 0.23mm thick WPI films. Note: It is one representation of few measurements.
32
1
Table 1. Thicknesses of whey protein isolate films with and without nanoparticles.
Film WPI
WPI NP
Film forming Solution, g
Average Thickness (mm)
Std Dev (mm)
10
0.1456
0.0136
15
0.2276
0.0056
20
0.3084
0.0145
10
0.1593
0.0071
15
0.2329
0.0299
20
0.3189
0.0189
2
33
1
Table 2. TGA investigation of WPI films with and without nanoparticles. Sample
Tmax of derivative
Weight loss (%) at
Residual weight (%) at
curve
450C
650C
WPI–A
222
14.84
0.77
WPI –D
218
16.40
1.76
WPI NP –A
206
22.36
6.64
WPI NP –D
201
20.51
6.96
2 3
34
Highlights WPI films embedded with TiO2@@SiO2 nanoparticles were prepared by solution casting. Films can sustain their structural integrity under the application of sonication processing and nanoparticles. The addition of nanoparticles strengthens the WPI film, as evidenced by tensile stress analysis. Embedded nanoparticle contributed effectively to increase the thickness of films and improve certain mechanical properties Films embedded with nanoparticles have a great potential for application in food packaging
S0260-8774(13)00076-9 http://dx.doi.org/10.1016/j.jfoodeng.2013.01.046 JFOE 7284
To appear in:
Journal of Food Engineering
Received Date: Revised Date: Accepted Date:
23 November 2012 22 January 2013 27 January 2013
Please cite this article as: Kadam, D.M., Thunga, M., Wang, S., Kessler, M.R., Grewell, D., Lamsal, B., Yu, C., Preparation and Characterization of Whey Protein Isolate Films Reinforced with Porous Silica Coated Titania Nanoparticles, Journal of Food Engineering (2013), doi: http://dx.doi.org/10.1016/j.jfoodeng.2013.01.046
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Preparation and Characterization of Whey Protein Isolate Films
2
Reinforced with Porous Silica Coated Titania Nanoparticles
3
4
Short Title: Characterization of WPI Films Reinforced with TiO2@@SiO2 Nanoparticles
5
6
Dattatreya M. Kadam1,4, Mahendra Thunga2, Sheng Wang3, Michael R. Kessler2, David
7
Grewell4, Buddhi Lamsal5*, Chenxu Yu4* 1
8
BOYSCAST Fellow/Visiting Scientist from Central Institute of Post-Harvest Engineering and
9 10
Technology, Ludhiana-141004 (Punjab, India) 2
3
11 12
Department of Materials Science and Engineering, Iowa State University, Ames, IA, 50011 USA
4
College of Materials and Textile, Zhejiang Sci-Tech University, Hangzhou, P. R. China
Department of Agricultural and Biosystems Engineering, Iowa State University, Ames, IA, 50011,
13 14
USA 5
Department of Food Science and Human Nutrition, Iowa State University, Ames, IA, 50011, USA
15
*Corresponding Authors
16
Dr. Buddhi Lamsal, email: [email protected]
17
Dr. Chenxu Yu, email: [email protected]
18
1
19
ABSTRACT
20
Whey protein isolate (WPI) films embedded with TiO2@@SiO2 (porous silica (SiO2)
21
coated titania (TiO2)) nanoparticles for improved mechanical properties were prepared by
22
solution casting. A WPI solution of 1.5 wt% TiO2@@SiO2 nanoparticles was subjected to
23
sonication at amplitudes of 0, 16, 80 and 160µm prior to casting in order to improve the film
24
forming properties of protein and to obtain a uniform distribution of nanoparticles in the WPI
25
films. The physical and mechanical properties of the films were determined by dynamic
26
mechanical analysis (DMA), thermogravimetric analysis (TGA), and tensile testing. Water vapor
27
permeability (WVP) measurements revealed that the water vapor transmission rates are slightly
28
influenced by sonication conditions and nanoparticle loading. The DMA results showed that, at
29
high sonication levels, addition of nanoparticles prevented protein agglomeration. The thermal
30
stability of the materials revealed the presence of 3-4 degradation stages in oxidizing the protein
31
films. The addition of nanoparticles strengthens the WPI film, as evidenced by tensile stress
32
analysis. Sonication improved nanoparticle distribution in film matrix; such films can potentially
33
become effective packaging materials to enhance food quality and safety.
34
35
36
Keywords: Biopolymer films; Nanoparticles; Protein isolates; Thermal properties; Mechanical
37
properties
38
2
39
INTRODUCTION
40
Whey protein isolate (WPI), an abundant by-product in the cheese-making industry, has
41
been successfully employed as a raw material for preparation of biodegradable polymers
42
(Mulvihill and Ennis, 2003). WPI has desirable film forming and barrier properties, which
43
compare well to petrobased products (McHugh et al., 1994; McHugh and Krochta, 1994; Fairley
44
et al., 1996; Sothornvit and Krochta, 2000; Fang et al., 2002; Hong and Krochta, 2003;
45
Khwaldia et al., 2004; Perez-Gago et al., 2005; Gounga et al., 2007; Pallas-Brindle and Krochta,
46
2008; Min et al., 2009). However, the low tensile strength and high water vapor permeability
47
(WVP) of WPI films limit its applications in food-related packaging. Nanoscale fillers (e.g.,
48
nanoclays) have been used as reinforcements to strengthen whey protein polymer matrices (Zhao
49
et al., 2008).
50
Nanoscale fillers need to be well dispersed in the polymer matrix for better film forming
51
properties. High-power ultrasonication has been shown to create better dispersions of nano-
52
particles (Li et al., 2005) during mixing. Sonication is a highly system-specific dispersion
53
procedure involving a variety of concomitant complex physicochemical interactions. These
54
interactions can result in cluster breakdown or further agglomeration, as well as other effects
55
including chemical reactions (Taurozzi et al., 2010). Ultrasonic waves are generated in a liquid
56
suspension either by immersing an ultrasound probe or “horn” into the suspension (direct
57
sonication), or by introducing a sample container with the suspension into a liquid bath through
58
which ultrasonic waves propagate (indirect sonication).
59
Efforts are being made to find new uses for whey proteins, including edible films (Anker
60
et al., 1998; Kokoszka et al., 2010) and films with antimicrobial functionality for more effective 3
61
protection of food products, which would reduce the risk of outbreaks of foodborne diseases
62
(Appendini and Hotchkiss, 2002). Unlike chitosan films, whey protein films do not inherently
63
possess antimicrobial activities but incorporation of antimicrobial agents, such as sorbic acid, p-
64
aminobenzoic acid (Cagri et al., 2001), and lysozyme have been reported (Kristo et al., 2008).
65
Another potential method of imparting antimicrobial functionality to packaging films is through
66
the utilization of antimicrobial nanoparticles such as titania (TiO2) nanocomposites. Titania is a
67
nontoxic and inexpensive material exhibiting bactericidal activity against a wide variety of
68
microbes due to its photocatalytic activity. When incorporated into a polymer matrix, TiO2 can
69
provide protection against foodborne microorganisms as well as odor, staining, deterioration, and
70
allergens in the presence of relatively low-wavelength radiation near the ultraviolet region (Rajh
71
et al., 1999; Zhou et al., 2009). However, TiO2 particles easily aggregate and thus deteriorate the
72
properties of the film (Zhou et al., 2009). In addition, the direct contact between TiO2 and the
73
polymer matrix may lead to rapid degradation of the polymer films themselves. Recently, we
74
developed novel core (TiO2 and shell (SiO2) nanoparticles (TiO2@@SiO2) with a void layer
75
between the titania core and the porous silicia outer layer as a reaction/enrichment zone for free
76
radicals. This void layer acts as a phase-selective photo-catalyst for the photodecomposition of
77
gas phase organic pollutants without any damage to their solid phase organic supports (Wang et
78
al., 2008). With the improvement in film properties, the nanoparticle-embedded films can
79
potentially become more effective packaging materials to enhance food quality and safety. The
80
main objectives of the present work are to develop WPI films embedded with TiO2@@SiO2
81
nanoparticles by utilizing sonication to achieve uniform distribution of nanoparticles inside the
82
WPI film, and to characterize the effects of different levels of sonication used. Furthermore, the
83
effect of film thickness on the physical and mechanical properties of the films is also of interest.
4
84
Material and Methods
85
Materials
86
TiO2@@SiO2 nanoparticles (particle size between 100-180nm) were made following a
87
previously reported procedure (Wang et al., 2008). Briefly, TiO2 nanoparticles were coated by
88
glucose (i.e., carbon) to form particle complex denoted as C@TiO2. The C@TiO2 particles were
89
then coated with a layer of silica via a sol-gel method to form SiO2@C@TiO2. Finally, the
90
SiO2@C@TiO2 nanoparticles were calcined at an elevated temperature (~773K for 3h) to
91
remove the interior carbon layer and create pores on the outer layer to yield nanovoid core/shell
92
nanoparticles denoted as SiO2@@TiO2. A schematic illustration and TEM image of the
93
TiO2@@SiO2 nanoparticles used in the WPI films are shown in Figure 1.
94
Whey protein isolate (WPI) was purchased from Pure Bulk (Roseburg, Oregon, USA)
95
which was processed using cold micro-filtration. WPI protein analysis was done using the
96
Dumas Nitrogen Combustion method which revealed an 86.01% protein content (as-is basis)
97
with a moisture content of 6.96 wt%. All other reagents were of analytical grade. Deionized and
98
distilled (DD) water was used for preparation of all samples.
99
Preparation of WPI-Based Films
100
The WPI films were formed following the method described by Kim and Ustunol (2001)
101
with some modifications. WPI and glycerol at 5 wt% each were dissolved in DD water (90 wt%)
102
with continuous stirring to obtain a film-forming solution. The pH of the film forming solution
103
was adjusted to 8.0 with 2 M NaOH. Then, the solution was heated to 90±2C for 30min in a
104
water bath while being stirred continuously, and rapidly cooled in an ice bath for 10-15min to 5
105
avoid further denaturation. The solution was then filtered through two layers of muslin cloth to
106
remove any coagulation. The filtered WPI solution was subjected to ultrasonication levels of 0,
107
10, 50 and 100% sonication (i.e. 0, 16, 80 and 160µm amplitudes), denoted as A, B, C and D
108
respectively. The film forming solutions of 10, 15 and 20g were cast on sterile polystyrene petri
109
dishes (Fisher brand, USA). These were dried at 35±1C for 24h in a hot air dryer having 24%
110
relative humidity (RH), then kept in a 50±2% RH chamber for at least 24h before the films were
111
peeled and tested at room temperature (24±1C).
112
Film Thickness
113
The film thicknesses were measured by a digital micrometer (Model: IP-65, Mitutoya
114
Corp., Tokyo, Japan) to the nearest 0.001mm. Measurements were taken at five positions along
115
the rectangular strips cut for the mechanical properties determination. Thickness measurement
116
for water vapor permeability (WVP) was done at five random positions around the perimeter of
117
each film prepared in petri dish. The mechanical properties and WVP were calculated using the
118
average thickness of each film.
119
Film Surface Wettability/ Contact Angle
120
The contact angle measurements were carried out with a Ramé-hart 100-00 115 NRL
121
contact angle goniometer (Ramé-hart instrument co, NJ). A droplet of distilled water (4μL) was
122
deposited on the leveled film surface to determine the contact angle of water on the biopolymer
123
films. All films were pre-conditioned in a humidity chamber at 50±2% relative humidity to avoid
124
interference due to competing moisture exchange at the surface around the droplet (Marcuzzo et
125
al., 2010). 6
126
Water Vapor Permeability
127
Water vapor transmission rate (WVTR), a film barrier property, was evaluated using a
128
gravimetric method per ASTM E 96-95 with modifications described by Anderson et al., (2012).
129
An open area of film with a diameter of 62 mm was placed on top of a flat-lipped glass beaker
130
containing 3±0.01g of anhydrous calcium sulfate (Drierite) to maintain nearly 0% RH inside the
131
cell. The lip of the beaker was vacuum-greased and the film was sealed with a custom-made
132
flange and an O-ring to secure the film between the beaker and flange. A saturated solution of
133
calcium nitrate was used to maintain 95±2% RH outside the beakers in a closed chamber. The
134
films were left in the closed chamber for 48h. Two different thicknesses of films, formed using
135
10 and 20g of film forming solution, were tested at 24±1°C. The WVP was calculated by plotting
136
the weight gain by the film over time using the following equation (Anderson et al., 2012):
137
= WL/tAP
138
where, is the water vapor permeability (i.e. 10-11g m/m2 s Pa), W is the amount of the water
139
gained by the cup (g), L is the film thickness (m), t is the elapsed time (s), A is the film cross
140
sectional area (m2), and P is the difference in pressure between the inside and outside of the vial
141
(Pa).
142
7
143
Dynamic Mechanical Analysis
144
The DMA measurements were performed on rectangular film samples with dimensions
145
(l × w × t) of 131mm × 7±1mm × actual thickness, respectively, using a Q800 DMA (TA
146
Instruments New Castle, DE, USA). Samples were evaluated in tension mode using a dynamic
147
temperature sweep to measure the storage and loss moduli as a function of temperature at a fixed
148
frequency (1Hz).
149
cooling followed by a constant heating rate of 5°C/min to 150C.
150
Determination of Mechanical Properties
The specimen’s temperature was equilibrated at -120C using liquid N2
151
The tensile testing of the mechanical properties of the biopolymer samples were as
152
performed with an Instron Universal Testing machine (Model 5569, Instron Engineering Corp.,
153
Canton, MA) at an extension rate of 50mm/min using a 2kN load cell. The samples for the
154
tensile tests were prepared by stamping dog-bone shaped specimens from the biopolymer films
155
according to ISO 527 type 5A. The strain in the sample was measured by keeping a gauge length
156
of 30mm between the clamps. The Young´s modulus was measured at 0.1 to 0.5% strain using
157
the Bluehill software supplied with the machine. For each film type, five specimens were cut
158
from a single petri plate film sample and the highest four results were averaged. The test cross
159
section was measured for length, width and thickness using a digital caliper.
160
Thermogravimetric Analysis
161
The thermal stability of the WPI films with and without TiO2@@SiO2 was investigated
162
using a thermogravimetric analyzer, TGA model Q50 (TA Instruments, DE, USA) in a nitrogen
163
atmosphere. The mass of the samples used was 15±1mg. The samples were heated from room 8
164
temperature to 800C at a heating rate of 10C/min to measure weight changes as a function of
165
temperature and/or time under a controlled atmosphere and to determine the material’s thermal
166
stability and composition.
167
Statistical Analyses
168
A single-factor completely randomized experimental design (CRD) was used to
169
determine significant differences among the samples at p<0.05. Analysis of variance (ANOVA)
170
tests (Fisher’s LSD) were used to determine the effect of the nano particles in the films.
171
RESULTS AND DISCUSSION
172
The whey protein isolate (WPI) films were transparent, smooth, flexible, homogeneous,
173
and without observable pores or cracks. Changes in appearance were observed for films
174
embedded with nanoparticles at various ultrasonication levels of 0, 10, 50 and 100% (i.e. tip horn
175
amplitudes of 0, 16, 80 and 160µm, respectively). The appearance of the film side facing the
176
casting plates was shiny while the other one was slightly dull.
177
Film Thickness
178
The average film thicknesses of WPI films with and without nanoparticles are presented
179
in Table 1. It is clear from the Table 1 that the addition of nanoparticles increases the thickness
180
of the cast films. Indeed, the same weight of film-forming solution was casted in 100mm
181
diameter petri dishes, but the dry matter of the solutions increases with the addition of
182
nanoparticles.
183 9
184
Surface Wettability
185
Surface hydrophobicity has been used as an important indicator of a protein film’s
186
sensitivity to water or moisture; it could be evaluated indirectly using contact angle (CA)
187
measurement. The hydrophobic interactions on the surface of our film specimen may be
188
influenced by both sonication and nanoparticles. The CA of WPI films without nanoparticles was
189
40±3, decreasing slightly as the level of sonication amplitude increases. This decrease in the
190
contact angle could be due to the restructuring of the proteins at higher sonication levels that lead
191
to less exposure of hydrophobic regions of the protein to the surface (Chandrapala et al., 2011).
192
WPI film swelling was observed during the contact angle measurements using a water droplet.
193
Although the observed trend was relatively small in pure WPI films, there was significant
194
variation of CA in WPI nanocomposite films. It can be observed that the contact angle increased
195
dramatically from 16 to 74 with an increase in sonication amplitude from 0 to 160µm i.e. 0 to
196
100% (Fig. 2). A fine distribution of the nanoparticles is achieved by sonication and results in
197
covering the whole surface of the WPI film. Consequently, the improvement in the contact angle
198
after sonication could be attributed to the domination of surface functionality of the nanoparticles
199
compared to the pure WPI film. Generally, protein films with higher contact angles exhibit a
200
higher surface hydrophobicity (Tang and Jiang, 2007). The contact angles decreased with an
201
increase in measurement time from 60s to 300s irrespective of sonication level and nanoparticle
202
loading (Fig. 2). The slight decrease in the contact angle with time could have resulted either
203
from the equilibrium time required to show a stable contact angle or due to the slight absorption
204
of water into the films.
205
10
206
Water Vapor Permeability
207
The WVP for WPI films for all levels of sonication, thickness, and nanoparticle loadings
208
ranged between 210-11 and ~5 10-11g m/m2 s Pa (Fig. 3). The thicker WPI films and desiccant
209
gained more weight compared to the thinner films, irrespective of sonication levels and
210
nanoparticle loadings. A higher WVP is one of the major limitations when using protein-based
211
films as food packaging materials. Therefore, a reduction in WVP is desirable for potential
212
applications in food packaging. The RH gradient is an important parameter when calculating the
213
WVP (McHugh et al., 1993). The actual thickness of the film was mainly responsible for the
214
increase or decrease in its WVP values (Fig. 3) and the same was taken into consideration while
215
calculating WVP.
216
The WVP is dependent on the number of polar groups present in the polymer film. The
217
whey protein films are hydrophilic; they are composed of highly polar amino acids. Further,
218
whey powder contains lactose sugar, which is a highly hydrophilic material. The barrier
219
properties of films are influenced by the hydrophobic or hydrophilic nature of the polymer, the
220
presence of voids or cracks, the type, level and compatibility of incorporated plasticizer, and the
221
steric hindrance in the film structure (Santosa and Padua, 1999; Lawton, 2004). Denaturation of
222
the cowpea protein at higher pH values exposes the hydrophilic residues on the protein surface
223
(Hewage and Vithanarachchi, 2009). This enhances the absorption of migrating water molecules
224
to polar groups, facilitating the transport of water and causing the penetration of more water
225
vapor through the film (Nathalie et al., 1992). The WVP of the cowpea protein film increased
226
with an increasing plasticizer concentration (Hewage and Vithanarachchi, 2009). The plasticizer
227
molecules increase the intermolecular spacing between the protein polymer chains, allowing the
228
water molecules to pass through the films (McHugh et al., 1994). 11
229
Dynamic Mechanical Analysis
230
Thermo-mechanical analysis was used to study the influence of sonication and
231
TiO2@@SiO2 nanoparticles on the phase behavior of the WPI films. Figures 4a and 4b depict the
232
change in elastic loss modulus (E``) and storage modulus (E`) over a temperature range of -
233
120C to 140C. The peaks in the loss modulus curves reveal the presence of two distinct
234
transition temperatures. The strong peak at a lower temperature (<0C) corresponds to the
235
transition temperature of the sorbitol phase (Ts) in the WPI bulk sample, whereas the shoulder-
236
like peak at 90C corresponds to the α-relaxation (Tα) of WPI (Zinoviadou et al., 2009). The α-
237
relaxation (Tα) is primarily observed due to the glass transition in WPI. A shift in the relaxation
238
temperatures (Ts and Tα) to higher values were observed in pure WPI samples after sonication.
239
Apart from sonication, Ts and Tα also shifted to higher values with the addition of TiO2@@SiO2
240
nanoparticles. The shift of the relaxation peaks towards higher temperatures in the biopolymer
241
films is primarily attributed to a change in the molecular level interactions that can influence the
242
segmental mobility in the protein. Sonication of the pure WPI resulted in disordering of
243
intramolecular forces in the protein, whereas sonication at high amplitude for an extended period
244
of time could lead to the formation of agglomerations from the disordered phase. Such
245
agglomerations could act as rigid constraints to hinder the relaxation of proteins near Ts and Tα,
246
resulting in a shift of the relaxation peaks to higher temperatures. However, in the case of WPI
247
nanocomposites, the shift in Ts and Tα is mainly attributed to the presence of the TiO2@@SiO2
248
nanofillers. The presence of these rigid fillers in soft protein films could potentially act as a
249
barrier to the protein flexibility.
12
250
The presence of titania agglomerates in sonicated samples at higher power levels could
251
be reconfirmed from the intensity difference of the Ts relaxation peak. Generally, the intensity of
252
the relaxation peak corresponds to the amount of soft phase in the bulk sample. A decrease in the
253
intensity of the Ts relaxation of WPI-D when compared to WPI-A implies a partial conversion of
254
the overall soft phase to the hard agglomerated phase. However, in contrast to the general trend,
255
the intensity of the Ts relaxation peak in the nanocomposite film WPI-NP-D was higher than in
256
WPI-NP-A. This difference could be attributed to the ability of TiO2@@SiO2 fillers to protect
257
disordered proteins against forming agglomerations.
258
The variation in the storage modulus (E`) with respect to temperature for all the WPI
259
samples is presented in Figure 4(b). The glassy modulus corresponding to the E` below the Ts (~
260
-60C) decreases after sonication in the case of pure WPI samples, whereas the same increases in
261
nanocomposite films. This change in the glassy modulus could be attributed to a partial
262
agglomeration phenomenon at higher sonication levels. Over the temperature range studied, the
263
elastic moduli of the nanocomposite samples were higher than for pure WPI films. The observed
264
high modulus values could be due the reinforcing effect from the rigid TiO2@@SiO2 fillers in
265
the soft protein matrix. Thus, the TiO2@@SiO2 nanoparticles not only improved the mechanical
266
strength of the samples over a broad temperature range, but also prevented self-aggregation of
267
WPI during sonication at high amplitudes.
268
13
269
Mechanical Properties
270
The stress-strain curves for WPI films with and without nanoparticles are shown in
271
Figure 5. A slight scattering was observed between the stress-strain curves of different specimens
272
in the same batch. The stress-strain curves shown in Figure 5 correspond to one of the five
273
specimens tested in each batch. The stress-strain profiles of all the WPI films show a yielding
274
behavior followed by cold drawing prior to failure. The amount of scattering in the stress-strain
275
behavior between different specimens in the same batch of samples can be estimated from the
276
error bars on the Young’s modulus and the tensile strength data in Figure 6. The significant
277
influence of sonication on the stress-strain behavior of the WPI films can also be seen from
278
Figure 5. With the addition of nanoparticles, the modulus of the pure WPI increased from 19MPa
279
to 35MPa (Fig. 6a) and the tensile strength measured from the stress-at-break also increased
280
from 1.03 MPa to 1.18 MPa (Fig. 6b). The increase in tensile properties is more prominent after
281
sonication i.e. the modulus seems to be independent of sonication level in pure WPI films when
282
compared to the obvious increase in the nanocomposite films (Fig. 6a). The tensile strength also
283
showed a similar trend between samples with and without nanoparticles. From the results, it is
284
worth mentioning that both ultra-sonication and TiO2@@SiO2 can collectively contribute to
285
enhance the tensile mechanical properties of the WPI films. Sonication helps in breaking the
286
nanoparticle agglomerations and disperses them finely in the protein matrix. Such well-dispersed
287
nanofillers could improve the reinforcing efficiency of nanoparticles, enhancing the material’s
288
mechanical properties. On the other hand, from Figures 5 and 6c it can be observed that the
289
nanoparticles also affect the strain at break in the samples. In pure WPI films, due to the flexibly
290
of the protein, the material can dissipate more energy resulting in high strain at break; whereas,
291
with the incorporation of reinforcing fillers, the stiffness of the films increases with a 14
292
simultaneous reduction in film flexibility. Thus, the observed difference in the strain at break
293
between pure WPI and WPI nanocomposite films is due to the change in film flexibility.
294
Thermogravimetric Analysis
295
The thermal stability of the WPI films with or without nanoparticles was investigated
296
using Thermogravimetric Analysis (TGA) in an air atmosphere. It can be seen from Figure 7 that
297
the films experience 3-4 noticeable thermal degradation steps in the temperature range of 30–
298
850C. Different stages of degradation can be distinctly observed from the prominent peaks in
299
the derivative weight % curves. The temperature range for the first step of thermal degradation is
300
between 50 –150C, which may correspond to the loss of water from the films. The temperature
301
range for the second step of thermal degradation is between 150 –300C, and this may be due to
302
the decomposition of WPI proteins and the loss of glycerol from the film. The temperatures for
303
the onset of maximum degradation (Tmax) taken from the peak temperature of derivative weight
304
% curves are listed in Table 2. The Tmax of pure WPI is at 222C, which reduced by only 5C
305
after 100% sonication. However, the Tmax of the nanocomposites samples was significantly
306
reduced to 206C and 201C for samples before and after sonication, respectively. The decrease
307
in the Tmax in the samples containing nanoparticles could be attributed to the onset of oxidation
308
of the surface functional groups on the nanoparticle within the temperature limits. An additional
309
third step of thermal degradation in the temperature range of 450-550C was observed, which
310
may be due to oxidation of partially decomposed proteins. With the addition of nanoparticles, the
311
nanocomposite films (WPI-NP-A and WPI-NP-D) exhibit a significant delay in the weight loss
312
at temperatures above 300oC. The difference in the weight loss between WPI and WPI-NP
313
samples at 450oC is listed in Table 2. Similar results are also reported by Kumar et al., (2010) for 15
314
soy protein isolate (SPI) and montmorillonite (MMT) bio-nano-composite films. Because the
315
TiO2@@SiO2 nanoparticles are stable above 650C, the residue remaining after the
316
decomposition of nanocomposite samples at 650C confirms the presence of a similar weight %
317
of nanoparticles in the investigated samples.
318
Conclusion
319
It was observed that whey protein isolate films could sustain their structural integrity
320
under the application of sonication processing, and the incorporation of TiO2@@SiO2
321
nanoparticles helps to improve their mechanical properties. The optical properties of the WPI
322
films were changed dramatically by the incorporation of nanoparticles, changing from a
323
transparent appearance to opaque. Embedded nanoparticles contributed effectively to increase
324
the thickness of films and to alter the contact angle of the film surface, which may have
325
implications in regard to the hydrophobicity of the films. Film WVP values (10-11g m/m2 s Pa)
326
are shown to be dependent on the film thickness. The presence of nanoparticles in the film was
327
shown to improve certain mechanical properties, but it also caused nearly a 50% reduction in
328
elongation. The storage modulus (E`) increased with an increasing level of sonication amplitude,
329
but the presence of nanoparticles led to a reduction in E`, possibly due to phase separation. There
330
were 3–4 steps of thermal degradation of the films in the temperature range of 30–850C. The
331
glass transition temperature (Tg) was between -95C and -60C, and significant differences were
332
observed in the DMA results. The WPI films embedded with nanoparticles have great potential
333
for application in food packaging for extending the shelf life, improving quality, and enhancing
334
the safety of food packaged with them.
335 16
336
Acknowledgements
337
We acknowledge the Department of Science and Technology (DST) Government of India, for
338
the BOYSCAST Postdoctoral Fellowship to Dr. Dattatreya M. Kadam allowing him to carry out
339
this work at Iowa State University. We also acknowledge the Director of the Central Institute of
340
Post-Harvest Engineering and Technology (CIPHET), Ludhiana and Indian Council of
341
Agricultural Research (ICAR), New Delhi for granting deputation to Dr. Kadam.
342
17
343
References
344
Anderson, T. J., Ilankovan, P., & Lamsal, B. P. (2012). Two fraction extraction of α-zein from
345
DDGS and its characterization. Industrial Crops and Products, 37 (1), 466–472.
346
Anker, M., Stading, M., & Hermansson, A. M. (1998). Mechanical properties, water vapor
347
permeability, and moisture contents of beta-lactoglobulin and whey protein films using
348
multivariate analysis. Journal of Agricultural and Food Chemistry, 46, 1820–1829.
349
Appendini, P., & Hotchkiss, J. H.(2002). Review of Antimicrobial Food Packaging, Innovative
350
351 352
Food Science & Emerging Technologies, 3 (2), 113-126.
Ashley, A. J. (1994). Permeability and plastic packaging, in: Comyn, J. (Ed.), Polymer permeability, Chapman and Hall, London, pp. 269–308
353
Cagri, A., Ustunol, Z., & Ryser, E. T. (2001). Antimicrobial, mechanical, and moisture barrier
354
properties of low pH whey protein-based edible films containing p-aminobenzoic or
355
sorbic acids. Journal of Food Science, 66, 865–870.
356 357
358 359
Campos, C. A., Gerschenson, L.N., & Flores, S. K. (2011). Development of edible films and coatings with antimicrobial activity. Food and Bioprocess Technology, 4 (6), 849–875.
Cha, D. S., & Chinnan, M. S. (2004). Biopolymer-based antimicrobial packaging: a review. Critical Reviews in Food Science and Nutrition, 44, 223–237.
360
Chandrapala, J., Zisu, B., Palmer, M., Kentish, S., & Ashokkumar, M. (2011). Effects of
361
ultrasound on the thermal and structural characteristics of proteins in reconstituted whey
362
protein concentrate. Ultrasonics Sonochemistry, 18(5), 951–957. 18
363
Fairley, P., Monahan, F. J., German, J. B., & Krochta, J. M. (1996). Mechanical properties and
364
water vapor permeability of edible films from whey protein isolate and sodium dodecyl
365
sulfate. Journal of Agricultural and Food Chemistry, 44, 438–443.
366
Fang, Y., Tung, M. A., Britt, I. J., Yada, S., & Dalglei, D. G. (2002). Tensile and barrier
367
properties of edible films made from whey proteins. Journal of Food Science: I67 (1),
368
188–193.
369
Gennadios, A. (2002). Protein-based Films and Coatings. CRC Press, Boca Raton, FL.
370
Gounga, M.E., Xu, S.Y., & Wang, Z. (2007). Whey protein isolatebased edible films as affected
371
by protein concentration, glycerol ratio and pullulan addition in film formation. Journal
372
of Food Engineering, 83, 521–530.
373
Hewage, S., & Vithanarachchi, S. M. (2009). Preparation and characterization of biodegradable
374
polymer films from cowpea (Vigna unguiculata) protein isolate. Journal of the National
375
Science Foundation of Sri Lanka, 37 (1), 53-59.
376 377
Hong, S. I., & Krochta, J. M. (2003). Oxygen barrier properties of whey protein isolate coating on polypropylene films. Journal of Food Science, 68, 224–228.
378
Jiang, M., Liu, S., Xin, Du., & Wang, Y. (2010). Physical properties and internal microstructures
379
of films made from catfish skin gelatin and triacetin mixtures. Food Hydrocolloids, 24,
380
105–110.
381
Khwaldia, K., Perez, C., Banon, S., Desobry, S., & Hardy, J. (2004). Milk proteins for edible
382
films and coatings. Critical Reviews in Food Science and Nutrition, 44, 239–251. 19
383 384
Kim, S. J., & Ustunol, Z. (2001). Thermal properties, heat sealability and seal attributes of whey protein isolate/lipid emulsion edible films. Journal of Food Science, 66(7), 985–990.
385
Kokoszka, S., Debeaufort, F., Lenart, A., & Voilley, A. (2010). Water vapour permeability,
386
thermal and wetting properties of whey protein isolate based edible films. International
387
Dairy Journal, 20 (1), 53–60.
388
Kristo, E., Koutsoumanis, K. P., & Biliaderis, C. G. (2008). Thermal, mechanical and water
389
vapor barrier properties of sodium caseinate films containing antimicrobials and their
390
inhibitory action on Listeria monocytogenes. Food Hydrocolloids, 22, 373–386.
391
Kumar, P., Sandeep, K. P., Alavi, S., Truong, V. D., & Gorga, R. E. (2010). Preparation and
392
characterization of bio-nanocomposite films based on soy protein isolate and
393
ontmorillonite using melt extrusion. Journal of Food Engineering, 100, 480–489
394 395
Lawton, J. W. (2004). Plasticizers for zein: Their effect on tensile properties and water absorption of zein films. Cereal Chemistry, 81 (1), 1-5.
396
Li, J., Kim, J. K., & Sham, M. L. (2005). Conductive graphite nanoplatelet/epoxy
397
nanocomposites: Effects of exfoliation and UV/ozone treatment of graphite, Scripta
398
Materialia, 53, 235–240.
399
Marcuzzo, E., Peressini, D., Debeaufort, F., & Sensidoni, A. (2010). Effect of ultrasound
400
treatment on properties of gluten-based film. Innovative Food Science & Emerging
401
Technologies, 11(3), 451–457.
20
402
McHugh, H., Avena-Bustillos, R., & Krochta, J. M. (1993). Hydrophilic edible films: Modified
403
procedure for water vapour permeability and explanation of thickness effects. Journal of
404
Food Science, 58 (4), 899–903.
405 406
McHugh, T. H. & Krochta, J. M. (1994). Water vapor permeability properties of edible whey protein-lipid emulsion films. Journal of the American Chemical Society, 71, 307–312.
407
McHugh, T. H., Aujard, J. F., & Krochta, J. M. (1994). Plasticized Whey Protein Edible Films:
408
Water Vapor Permeability Properties, Journal of Food Science, 59, 416–419, 423.
409
McHugh, T. H., Aujard, J. F., & Krochta, J. M. (1994). Plasticized whey protein edible films:
410
Water vapour permeability properties. Journal of Food Science, 59(2), 416 – 419.
411
Min, S., Janjarasskul, T., & Krochta, J. M. (2009). Whey protein bees wax layered composite
412
film tensile and moisture barrier properties. Journal of the Science of Food and
413
Agriculture, 89, 251–257.
414
Mulvihill, D. M., & Ennis, M. P. (2003). Functional milk proteins: Production and utilization, in
415
Fox, P. F., McSweeney, P. L. H. (Eds.), Advanced dairy chemistry-Vol. 1. Kluwer
416
Academic, New York, pp. 1175–1228.
417
Nathalie G., Guilbert S., & Cuq J. L. (1992). Edible wheat gluten films: Influence of the main
418
process variables on film properties using response surface methodology. Journal of
419
Food Science, 57(1), 194 – 195.
420 421
Pallas-Brindle, L., & Krochta, J. M. (2008). Physical properties of whey protein-hydroxypropyl methy lcellulose blend edible films. Journal of Food Science, 73, E446–E454. 21
422
Perez-Gago, M. B., Serra, M., Alonso, M., Mateos, M., & Del Rio, M. A. (2005). Effect of whey
423
protein- and hydroxypropyl methylcellulose-based edible composite coatings on color
424
change of fresh-cut apples. Postharvest Biology and Technology, 36, 77–85.
425
Rajh, T., Nedeljkovic, J. M., Chen, L. X., Poluektov, O., Thurnauer, M. C., & Thurnauer, M. C.
426
(1999). Improving optical and charge separation properties of nanocrystalline TiO2 by
427
surface modification with vitamin C. The Journal of Physical Chemistry B, 103(18),
428
3515–3519.
429
Santosa, F.X., & Padua, G. W. (1999). Tensile properties and water absorption of zein sheets
430
plasticized with oleic and linoleic acids. Journal of Agricultural and Food Chemistry,
431
47(5), 2070-2074.
432
Sothornvit, R., & Krochta, J. M. (2000). Oxygen permeability and mechanical properties of
433
films from hydrolyzed whey protein. Journal of Agricultural and Food Chemistry, 48,
434
3913–3916.
435
Tang, C. H., & Jiang, Y. (2007). Modulation of mechanical and surface hydrophobic properties
436
of food protein films by transglutaminase treatment. Food Research International, 40,
437
504–509.
438
Taurozzi, J. S., Hackley, V. A., & Wiesner, M. R. (2010). Preparation of nanoparticle dispersions
439
from powdered material using ultrasonic disruption. In: CEINT/NIST Protocol for
440
preparation of nanoparticle dispersions from powdered material using ultrasonic
441
disruption, Ver. 1 June 3, 2010. National Institute of Standards and Technology, US
22
442
Department of Commerce, USA. Pp: 1-10. http://www.goodnanoguide.org/tiki-
443
download_wiki_attachment.php?attId=36
444
Wang, S., Wang, T., Chen, W., & Hori, T. (2008). Phase-selectivity photocatalysis: a new
445
approach in organic pollutants’ photodecomposition by nanovoid core (TiO2)/shell(SiO2)
446
nanoparticles. Chemical Communications, 32, 3756–3758.
447 448
Zhao, R. X., Torley, P., & Halley, P. J. (2008). Emerging biodegradable materials: starch- and protein-based bionanocomposites. Journal of Materials Science, 43, 3058-3071
449
Zhou, J. J., Wang, S. Y., & Gunasekaran, S. (2009). Preparation and characterization of whey
450
protein film incorporated with TiO2 nanoparticles. Journal of Food Science,74(7), 50–56.
451
Zinoviadou, K. G., Koutsoumanis, K.P., & Biliaderis, C. G. (2009). Physico-chemical properties
452
of whey protein isolate films containing oregano oil and their antimicrobial action against
453
spoilage flora of fresh beef. Meat Science, 82(3), 338–345.
454
455
456
457
458
459
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460
Figure Captions
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Fig. 1. View of TiO2@@SiO2 nanoparticles (TEM picture Wang et al., 2008) used in WPI films
462 463 464
to improve mechanical and antimicrobial properties. Fig. 2. Effect of sonication levels on water contact angles of 0.23mm thick pure WPI films and WPI films containing nanoparticles.
465
Fig. 3. Water vapor permeability of WPI films as a function of sonication level, nanoparticles
466
and film thickness. Note: A, C, and D represent 0, 80 and 160m sonication amplitudes,
467
respectively (film thickness, mm)
468
Fig. 4. DMA used for determination of (a) Loss modulus and (b) Elastic modulus of WPI films
469
with or without nanoparticles prepared using different sonication levels. Note: It is one
470
representation of few measurements.
471
Fig. 5. Typical Tensile stress (M Pa) – Tensile strain (%) curves for WPI films at different
472
sonication levels with and without nanoparticles. Note: WPI is whey protein isolate and
473
WPI NP is whey protein isolate with nanoparticles. A, B, C and D represent the 0, 10, 50
474
and 100% sonication levels, respectively. Note: It is one representation of few
475
measurements.
476
Fig. 6. Effect of sonication level and with or without nanoparticles on Young’s modulus, tensile
477
stress and tensile strain of WPI films. The error bars show the standard deviation.
478
Fig. 7. Effect of sonication level and nanoparticle loadings on TGA of 0.23mm thick WPI films.
479
Note: It is one representation of few measurements.
24
480
Table Captions
481
Table 1. Thicknesses of whey protein isolate films with and without nanoparticles.
482
Table 2. TGA investigation on WPI films with and without nanoparticles.
483
25
1
2 3
4
5
6
7 8
Fig. 1. View of TiO2@@SiO2 nanoparticles in WPI films to improve mechanical and antimicrobial properties.
9
10
26
90 80
60 sec 300 sec
Contact Angle (o)
70 60 50 40 30 20 10 0 A
B
C
D
A
WPI
1 2 3
B
C
D
WPI NP
Sonication Level (%)
Fig. 2. Effect of sonication levels on water contact angles of 0.23mm thick pure WPI films and WPI films containing nanoparticles.
27
6E-11
5E-11
WVP (g m/m2 s Pa )
4E-11
3E-11
2E-11
1E-11
0 A A C C D D (0.14) (0.37) (0.13) (0.36) (0.14) (0.30)
A A C C D D (0.14) (0.31) (0.22) (0.29) (0.16) (0.30)
WPI
1
WPI NP Sonication level, % (Film thickness, mm)
2
Fig. 3. Water vapor permeability of WPI films as a function of sonication level,
3
nanoparticles and film thickness. Note: A, C, and D represent 0, 80 and 160m
4
sonication amplitudes, respectively (film thickness, mm)
28
1 2
4 5 6 7
Loss Modulus E`` (MPa)
1000
3
WPI A WPI D WPI-NP A WPI-NP D
100
10
1
8
a 0.1 -100
9
-50
0
50
100
o
Temperature ( C) 10 11 12 13 14 15
Elastic Modulus E` (MPa)
10000
WPI A WPI D WPI-NP A WPI-NP D
1000
100
10
1
16
b 0.1
17
-100
-50
0
50
100
150
o
Temperature ( C)
18
Fig. 4. DMA used for determination of (a) Loss modulus and (b) Elastic modulus of WPI
19
films with or without nanoparticles prepared using different sonication levels. Note:
20
It is one representation of few measurements.
21 29
WPI A WPI B WPI C WPI D WPI-NP A WPI-NP B WPI-NP C WPI-NP D
2.0 1.8
Tensile Stress (MPa)
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
10
20
30
40
50
60
Tensile Strain, %
1 2
Fig. 5. Typical Tensile stress (MPa) – Tensile strain (%) curves for WPI films at different
3
sonication levels with and without nanoparticles. Note: WPI is whey protein isolate
4
and WPI NP is whey protein isolate with nanoparticles. A, B, C and D represent the
5
0, 10, 50 and 100% sonication levels, respectively. Note: It is one representation of
6
few measurements.
7
30
70 60
Youngs Modulus
50 40 30 20 10 0 A
A NP
B
B NP
C C NP
D D NP
Sonication Level (%)
1
Tensile Stress at Break (%)
2.5 2
1.5 1
0.5 0 A A NP
B B NP
C C NP
D D NP
Sonication Level (%)
2
Tensile Strain at Break (%)
80
3 4 5 6
70 60 50 40 30 20 10 0 A
A NP
B
B NP
C C NP
D D NP
Sonication Level (%) Fig. 6. Effect of sonication level and with or without nanoparticles on Young’s modulus, tensile stress and tensile strain of WPI films. The error bars show the standard deviation. 31
1 2 1.2 100
1.1
90
1.0
WPI A WPI D WPI-NP A WPI-NP D
80
5 70
7 8 9
Weight (%)
6
60
0.8 0.7 0.6
50 0.5 40
0.4
30
0.3
20
0.2
10
0.1
10 11
0.9
o
4
Deriv. Weight, %/ C
3
0.0
0
-0.1
12
40
80 120 160 200 240 280 320 360 400 440 480 520 560 600 640 o
13 14 15
Temperature ( C)
Fig. 7. Effect of sonication level and nanoparticle loadings on TGA of 0.23mm thick WPI films. Note: It is one representation of few measurements.
32
1
Table 1. Thicknesses of whey protein isolate films with and without nanoparticles.
Film WPI
WPI NP
Film forming Solution, g
Average Thickness (mm)
Std Dev (mm)
10
0.1456
0.0136
15
0.2276
0.0056
20
0.3084
0.0145
10
0.1593
0.0071
15
0.2329
0.0299
20
0.3189
0.0189
2
33
1
Table 2. TGA investigation of WPI films with and without nanoparticles. Sample
Tmax of derivative
Weight loss (%) at
Residual weight (%) at
curve
450C
650C
WPI–A
222
14.84
0.77
WPI –D
218
16.40
1.76
WPI NP –A
206
22.36
6.64
WPI NP –D
201
20.51
6.96
2 3
34
Highlights WPI films embedded with TiO2@@SiO2 nanoparticles were prepared by solution casting. Films can sustain their structural integrity under the application of sonication processing and nanoparticles. The addition of nanoparticles strengthens the WPI film, as evidenced by tensile stress analysis. Embedded nanoparticle contributed effectively to increase the thickness of films and improve certain mechanical properties Films embedded with nanoparticles have a great potential for application in food packaging