Preparation and Characterization of Antioxidative

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Aug 19, 2018 - Yingxiang Zhai, Jiangtao Wang, Hao Wang, Tao Song, Weitong Hu and Shujun Li * ID. College of Materials Science and Engineering, ...
molecules Article

Preparation and Characterization of Antioxidative and UV-Protective Larch Bark Tannin/PVA Composite Membranes Yingxiang Zhai, Jiangtao Wang, Hao Wang, Tao Song, Weitong Hu and Shujun Li *

ID

College of Materials Science and Engineering, Northeast Forestry University, Harbin 150040, China; [email protected] (Y.Z.); [email protected] (J.W.); [email protected] (H.W.); [email protected] (T.S.); [email protected] (W.H.) * Correspondence: [email protected]; Tel.: +86-451-8219-1742  

Received: 17 July 2018; Accepted: 15 August 2018; Published: 19 August 2018

Abstract: In order to prepare functional materials for antioxidant and ultraviolet (UV)-protective green food packaging, condensed tannin, previously extracted from larch bark, was mixed with polyvinyl alcohol (PVA), and then the mixture was used to cast composite membranes. An antioxidative assay using 1,1-diphenyl-2-picrylhydrazyl (DPPH)—a free radical scavenger—and starch–potassium iodide oxidation–discoloration analyses showed that the composite membranes have good antioxidative activities. The low UV transmission and protective effect of the composite films on vitamin E indicated the UV protection ability of the composite membranes. Both larch bark tannin and PVA are rich in hydroxyl groups; scanning electron microscopy analysis demonstrated their compatibility. Also, the mechanical and crystallization properties of the composite membranes did not significantly decrease with the addition of larch bark tannin. Keywords: larch bark tannin; composite membrane; antioxidation; UV protection

1. Introduction Food packaging is an important component of food products. Food easily oxidizes and deteriorates. Antioxidation is a key function of active packaging materials, which, therefore, are helpful for extending the shelf life of foods. Ultraviolet (UV) light can also promote the oxidative degradation of foods. Developing food packaging materials with antioxidative and anti-UV radiation properties is essential. Considering the direct contact of certain packaging materials with food, natural products may be preferable for food packaging materials. Examples of natural antioxidants currently used for food packaging include extracts of apple pomace, guarana, rosemary, and cinnamon bark [1,2]. Vegetable polyphenols are well known as natural antioxidants and they also have anti-UV properties. Polyphenols are important secondary metabolites and are widely present in plants [3]. Larch (Larix gmelinii), which is a conifer abundant in temperate zones of the northern hemisphere, contains a large amount of polyphenols in its bark [4]. The polyphenols in larch bark, which include condensed tannin, are highly polymerized compared with other condensed vegetable polyphenols. Although this raw material is inexpensive and easily obtained, larch bark tannin (LBT) has not been extensively used [5,6]. Polyvinyl alcohol (PVA) is a highly polar and high-strength biodegradable polymer with good thermal stability and membrane-forming ability, which makes it ideally suited for blending with natural polyphenols [7,8]. Accordingly, in this work, LBT was extracted and then mixed with PVA to prepare composite membranes. The antioxidative and anti-UV radiation properties of the membranes were then evaluated. The knowledge derived from this study provides a good foundation for the high-added-value application of LBT. Molecules 2018, 23, 2073; doi:10.3390/molecules23082073

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membranes were then evaluated. The knowledge derived from this study provides a good 2 of 11 foundation for the high-added-value application of LBT.

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Resultsand andDiscussion Discussion 2.2.Results

2.1. 2.1.Analysis AnalysisofofMembrane MembraneAntioxidative AntioxidativeActivity Activity Because Becauseofofits itspolyphenols polyphenolscontent, content,LBT LBTpresented presentedhigh high1,1-diphenyl-2-picrylhydrazyl 1,1-diphenyl-2-picrylhydrazyl(DPPH) (DPPH) radical ability in solution (Figure 1). In the LBT/PVALBT/PVA membrane, LBT endowed radicalscavenging scavenging ability in solution (Figure 1). composite In the composite membrane, LBT the membrane good DPPH scavenging ability. When the membrane contact with endowed the with membrane with radical good DPPH radical scavenging ability. Whencame the into membrane came moisture, water molecules entered causing the swelling of the film. a result, LBT As molecules into contact with moisture, waterit,molecules entered it, causing the As swelling ofsome the film. a result, somereleased LBT molecules were released from theOver composite Over time, network were from the composite membranes. time, themembranes. network structure of thethe membrane loosened themembrane maintenance energyand decreased, resultingenergy in a dramatic increase in the of structureand of the loosened the maintenance decreased, resulting in release a dramatic increase inenhanced the release of LBT, DPPH free radical scavenging As shown LBT, which DPPH free which radicalenhanced scavenging [9]. As shown in Figure 2, the [9]. scavenging ratein ofFigure DPPH2,free increased the increased increase ofsignificantly the tannin content in increase the LBT/PVA theradicals scavenging rate ofsignificantly DPPH free with radicals with the of the tannin content in the LBT/PVA membrane. When tannin content from composite membrane. When the composite tannin content increased fromthe 0.25 to 2.00%, the increased scavenging rate0.25 of to 2.00%, the scavenging rate of DPPH radicals increased from 8.43 to 44.44%. DPPH radicals increased from 8.43 to 44.44%.

Figure1.1.Rate Rate 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging by bark larchtannin bark (LBT) tannin Figure of of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging by larch in solution. in(LBT) solution.

2.2.Membrane MembraneSoluble SolublePhenols PhenolsContent Content 2.2. order evaluate effects of different concentrations in the membrane, soluble InInorder toto evaluate thethe effects of different concentrations of LBTof in LBT the membrane, soluble phenols in the membranes were determined, and the resultinisFigure illustrated in the Figure 3. With the inphenols the membranes were determined, and the result is illustrated 3. With increase of LBT increase of LBT concentration in the membranes, the content of soluble phenols increased. This concentration in the membranes, the content of soluble phenols increased. This resulted in improved resulted in improved DPPH radical scavenging, antioxidation in the acidic starch–potassium iodide DPPH radical scavenging, antioxidation in the acidic starch–potassium iodide (KI) discoloration (KI) discoloration reaction, and UV protection. However, thephenol increase in soluble phenol content was reaction, and UV protection. However, the increase in soluble content was not proportional to not proportional to the LBT content. The reason for this was that membranes with different LBT the LBT content. The reason for this was that membranes with different LBT contents were incubated contents were incubated for min), the same limited time (30 min), and the releasewere profiles of in soluble for the same limited time (30 and the release profiles of soluble phenols similar this phenols were similar in this short time frame. Additionally, the release rate was influenced by other short time frame. Additionally, the release rate was influenced by other factors besides the initial LBT factors besides initial area LBT and content, such distance. as diffusion area and diffusion distance. content, such as the diffusion diffusion

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Figure2. Scavengingrate rateof DPPHradicals radicalsin LBT/Polyvinylalcohol alcohol(PVA) (PVA)membranes. membranes.PVA, PVA, Figure Figure 2.2.Scavenging Scavenging rate ofofDPPH DPPH radicals ininLBT/Polyvinyl LBT/Polyvinyl alcohol (PVA) membranes. PVA, LBT/PVA-1,LBT/PVA-2, LBT/PVA-2,LBT/PVA-3, LBT/PVA-3,and andLBT/PVA-4 LBT/PVA-4represent representcomposite compositemembranes membraneswith withLBT LBT LBT/PVA-1, LBT/PVA-1, LBT/PVA-2, LBT/PVA-3, and LBT/PVA-4 represent composite membranes with LBT content of 0.00, 0.25, 0.50, 1.00, and 2.00%, respectively. content of 0.00, 0.25, 0.50, 1.00, and 2.00%, respectively. content of 0.00, 0.25, 0.50, 1.00, and 2.00%, respectively.

Figure 3.3. Contents ofof soluble phenols inin the composite LBT/PVA membranes. Figure Contents soluble phenols the composite LBT/PVA membranes. Figure 3. Contents of soluble phenols in the composite LBT/PVA membranes.

2.3.Effect EffectofofofIncreasing IncreasingAmounts AmountsofofofLBT LBTin theMembrane Membraneon Starch-KIOxidation–Discoloration Oxidation–DiscolorationReaction Reaction 2.3. Effect Increasing Amounts LBT ininthe the Membrane ononStarch-KI Starch-KI Oxidation–Discoloration 2.3. Withthe the increase LBTcontent content inthe thecomposite composite membranes, themembranes membranes became With increase of LBT content in thein composite membranes, the membranes became increasingly With the increase ofofLBT membranes, the became − oxidation −Ioxidation − increasingly resistant to oxidation. The presence of LBT inhibited I to I 2 , thereby reducing resistant to oxidation. The presence of LBT inhibited I to , thereby reducing the amount increasingly resistant to oxidation. The presence of LBT inhibited I oxidation to I2, thereby reducing 2 2 generated. According toFigure Figure Iproduced 2 produced with the blankPVA PVA ofthe Iamount generated. to Figure 4,to the amount ofthe Iamount produced the blank PVA sample was the ofofI2 IAccording generated. According 4,4,the ofofI2with with the blank 2 amount 2 amount sample wasthe thehighest, highest,corresponding corresponding about0.12 0.12 mg/mL. Withincreasing increasing amounts LBTininthe the the highest, corresponding to about 0.12 mg/mL. With increasing amounts ofamounts LBT in the composite sample was toto about mg/mL. With ofofLBT compositemembrane, membrane, the contentofofIdecreased, gradually decreased, reaching minimum (about 0.05 membrane, the contentthe of I2content gradually reaching a minimum (about 0.05 mg/mL) in0.05 the composite 2 I2gradually decreased, reaching a aminimum (about mg/mL)ininthe the LBT/PVA-4 membrane. This implies that the higher theLBT LBT content the composite LBT/PVA-4 membrane. This implies that the higher the LBT content in the composite membrane, mg/mL) LBT/PVA-4 membrane. This implies that the higher the content ininthe composite membrane, thestronger strongerthe theantioxidant antioxidant capacity themembrane. membrane. the strongerthe the antioxidant capacity of the membrane. membrane, capacity ofofthe

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Figure 4. The effect of tannin content in the composite membrane on the amount of I2. Figure Figure4.4.The Theeffect effectofoftannin tannincontent contentininthe thecomposite compositemembrane membraneon onthe theamount amountofofI2.I2.

2.4. Light Transmittance of the Composite Membranes 2.4. 2.4.Light LightTransmittance Transmittanceofofthe theComposite CompositeMembranes Membranes The data presented in Figure 5 indicate that with increasing tannin content in the composite The Thedata datapresented presentedininFigure Figure55indicate indicatethat thatwith withincreasing increasingtannin tannincontent contentininthe thecomposite composite membrane, the transmittance of UV light considerably decreased, meaning that UV absorption by membrane, UV light light considerably considerablydecreased, decreased,meaning meaningthat thatUV UV absorption membrane, the the transmittance transmittance of UV absorption by the membrane increased accordingly [10]. As the tannin content increased from 0 to 2.00%, the by membrane increased accordingly [10]. tannin content increased from to 2.00%, thethe membrane increased accordingly [10]. As As the the tannin content increased from 0 to0 2.00%, the transmittance of UV light (wavelength less than 280 nm) decreased from 80% to less than 10%, while the transmittance of UV light (wavelength 280 nm) decreased 80% tothan less 10%, than while 10%, transmittance of UV light (wavelength less less thanthan 280 nm) decreased fromfrom 80% to less the transmittance of near-infrared (650 to 800 nm) light did not decrease. The transmittance of visible while the transmittance of near-infrared (650 800light nm)did light did not decrease. The transmittance of the transmittance of near-infrared (650 to 800tonm) not decrease. The transmittance of visible (400–500 nm) light also decreased from 90 to ~65%. This indicates that the composite membrane is visible (400–500 nm) light also decreased 90~65%. to ~65%. indicates that compositemembrane membraneis (400–500 nm) light also decreased fromfrom 90 to ThisThis indicates that thethe composite suitable for UV-protective packaging materials. issuitable suitablefor forUV-protective UV-protectivepackaging packagingmaterials. materials.

Figure 5. Ultraviolet–visible (UV–vis) light transmittance of the composite membranes. Figure5.5.Ultraviolet–visible Ultraviolet–visible(UV–vis) (UV–vis)light lighttransmittance transmittanceofofthe thecomposite compositemembranes. membranes. Figure

2.5. Analysis of the Mechanical Properties of Membranes 2.5. Analysis of the Mechanical Properties of Membranes 2.5. Analysis of the Mechanical Properties of Membranes The results shown in Figure 6 reveal that the average maximum force of the blank membrane The results shown in Figure 6 reveal that the average maximum force of the blank membrane The results shownthe in Figure 6 reveal that the average maximum force of the blank was was 53.32 N. After addition of 0.25% LBT, the average maximum force of membrane the membrane was 53.32 N. After the addition of 0.25% LBT, the average maximum force of the membrane 53.32 N. After addition of 0.25% LBT, the of could the membrane decreased to decreased to the 32.12 N. PVA molecules areaverage linear maximum polymers, force which easily organize into decreased to 32.12 N. PVA molecules are linear polymers, which could easily organize into 32.12 N. PVAWhen molecules are linear polymers, could to easily into aggregates. When a aggregates. a small amount of LBT which was added the organize PVA matrix, the well-organized aggregates. When a small amount of LBT was added to the PVA matrix, the well-organized small amount of LBT of was added the PVAresulting matrix, the aggregate of PVA aggregate structure PVA wastoaltered, in well-organized a decrease in its strength.structure However, withwas the aggregate structure of PVA was altered, resulting in a decrease in its strength. However, with the altered, a decrease its strength.force However, the increase of LBT content, increaseresulting of LBT incontent, theinmaximum of thewith membrane increased becausethe of maximum the good increase of LBT content, the maximum force of the membrane increased because of the good compatibility of LBT with PVA. For example, with the addition of 2.00% LBT, the average compatibility of LBT with PVA. For example, with the addition of 2.00% LBT, the average

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force of the membrane increased because of the good compatibility of LBT with PVA. For example, maximum force reached to the maximum force, 66.03 the tensile strength to of the the with the addition of 2.00%66.03 LBT,N. theCompared average maximum force reached N. Compared membranes did the nottensile changestrength much with the increase ofdid LBT tensile strength wasofabout maximum force, of the membranes notcontent. change The much with the increase LBT Molecules 2018, 23, x FOR PEERsamples. REVIEW 5 of 11 90 MPa for all membrane Regarding elongation, the value for the blank membrane content. The tensile strength was about 90 MPa for all membrane samples. Regarding elongation, was the 31.1%for and decreased considerably with the addition of considerably 0.25–2.00% LBT. The reason forofthis could be value the blank membrane was 31.1% and decreased with the addition 0.25–2.00% maximum force reached 66.03 N. Compared to the maximum force, the tensile strength of the that The the reason tanninfor molecules thus molecules limiting the rotation and limiting movement rotation of the PVA LBT. couldcross-linked, be thatwith the tannin membranes did this not change much the increase of LBTcross-linked, content. The thus tensile strengththe was about and molecular chains, which resulted in the elongation decrease [11]. movement theallPVA molecular chains, which resulted in the decrease [11]. 90 MPaoffor membrane samples. Regarding elongation, the elongation value for the blank membrane was 31.1% and decreased considerably with the addition of 0.25–2.00% LBT. The reason for this could be that the tannin molecules cross-linked, thus limiting the rotation and movement of the PVA molecular chains, which resulted in the elongation decrease [11].

Figure6.6.Mechanical Mechanicalproperties propertiesof ofthe thecomposite compositemembranes. membranes. Figure Figure 6. Mechanical properties of the composite membranes.

2.6.Other OtherAnalytical AnalyticalCharacterization Characterization 2.6. 2.6. Analytical TheOther FT–IR spectraCharacterization ofthe thePVA, PVA,LBT, LBT,and andLBT/PVA LBT/PVAcomposite compositemembranes membranes with with different different LBT LBT The FT–IR spectra of −1 was attributed to the hydroxyl contents are displayed in Figure 7. The broad peak at about 3300 cm −1 was attributed Thedisplayed FT–IR spectra of the7.PVA, LBT/PVA composite with different LBT contents are in Figure The LBT, broadand peak at about 3300 cmmembranes to the hydroxyl −1 was −1 was group (–OH). The small peak at about cm attributed the stretching of C–H. contents are displayed in Figure 7. The 2900 broad− peak at about 3300 cmto attributed tovibration the hydroxyl 1 group (–OH). The small peak at about 2900 cm was attributed to the stretching vibration of C–H. The −1 was attributed to the stretching vibration of C–H. −1peak groupat(–OH). The small at about 2900 cmstretching The peak about 1100cm was ascribed to the vibration of the ether bond (C–O). Because − 1 peak at about 1100cm was−1ascribed to the stretching vibration of the ether bond (C–O). Because of Thevery peaklow at about 1100cm was ascribed to the stretching vibration of the maximum ether bond (C–O). Because of the amount of LBT in the composite membranes (the amount was only the very lowvery amount of LBT of in LBT the composite membranes (the maximum amount waswas only 2.00%), of the low amount in the composite membranes (the maximum amount onlypeaks 2.00%), the FT–IR spectra of themembranes compositedid membranes did obvious not show any obvious the FT–IR spectra of the composite not show any peaks corresponding 2.00%), the FT–IR spectra of the composite membranes did not show any obvious peaks to corresponding to LBT [12]. LBT [12]. corresponding to LBT [12].

Figure 7. Fourier transform–infrared(FT-IR) (FT-IR)spectra spectra of membranes. Figure 7. Fourier transform–infrared of LBT, LBT,PVA, PVA,and andLBT/PVA LBT/PVA membranes.

Figure 7. Fourier transform–infrared (FT-IR) spectra of LBT, PVA, and LBT/PVA membranes.

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The SEM images images(Figure (Figure8)8)showed showed that tannin evenly distributed the matrix. PVA matrix. The SEM that tannin waswas evenly distributed in theinPVA Both Both tannin and PVA molecules are rich in hydroxyl groups, which results in their high polarity tannin and PVA molecules are rich in hydroxyl groups, which results in their high polarity and good and good compatibility [13]. REVIEW Molecules 2018, 23, x FOR PEER 6 of 11 compatibility [13]. The SEM images (Figure 8) showed that tannin was evenly distributed in the PVA matrix. Both tannin and PVA molecules are rich in hydroxyl groups, which results in their high polarity and good compatibility [13].

Figure 8. Scanning electron microscopy (SEM) images of the surface of (a) PVA and (b) LBT/PVA-2 Figure 8. Scanning electron microscopy (SEM) images of the surface of (a) PVA and (b) LBT/PVA-2 films and cross section of (c) PVA and (d) LBT/PVA-2 films. 8. Scanning electron microscopy (SEM) imagesfilms. of the surface of (a) PVA and (b) LBT/PVA-2 films Figure and cross section of (c) PVA and (d) LBT/PVA-2 films and cross section of (c) PVA and (d) LBT/PVA-2 films.

The PVA molecular chains contain a large number of -OH groups, which easily form hydrogen The PVA molecular chains contain a Moreover, large number of -OH groups, which easily form hydrogen bonds both intrainter-molecularly. theof ordered molecular arranged by The PVA and molecular chains contain a large number -OH groups, which long easilychains form hydrogen bonds bothbonds intraand inter-molecularly. Moreover, ordered molecular long chains arranged bonds both intrainter-molecularly. Moreover, the ordered molecular long chains arranged by by hydrogen can and form a dense structure. Thethe XRD patterns of LBT, PVA, and LBT/PVA hydrogen bonds can form aform dense structure. Thesharp XRD of LBT, LBT/PVA composite hydrogen bonds can a9)dense structure. The patterns XRD at patterns ofPVA, LBT, PVA, and LBT/PVA composite membranes (Figure showed one peak a 2θ angle ofand 19.6° for the pure PVA ◦ membranes (Figure 9) showed one sharp peak at a 2θ angle of 19.6 for the pure PVA membrane, composite membranes (Figure 9) showed one sharp peak at a 2θ angle of 19.6° for the pure PVA membrane, due to the membrane’s partially ordered and dense structure. Compared with PVA, LBT due to the membrane’s ordered and dense structure. with Compared with LBT due tomembrane, the membrane’s partially and to dense structure. Compared PVA, LBT PVA, isstructure amorphous. is amorphous. With addition ofordered LBTpartially (0.25 2.00%), the partially ordered and dense of is amorphous. With addition of LBT (0.25 to 2.00%), theand partially ordered and densewas structure of With addition of LBT (0.25 to 2.00%), the partially ordered dense structure of PVA not changed PVA was not changed much, as shown by the values of both the 2θ angle and the intensity, which PVA was notbychanged much, as shown by angle the values the 2θ which angle and the intensity, which of much, as shown the values ofdid both the 2θ and of theboth intensity, meant that the addition meantmeant that the addition of LBT not obviously damage theorderly orderlyarranged arranged structure [14]. that the addition of LBT did not obviously damage the structure [14]. LBT did not obviously damage the orderly arranged structure [14].

Figure 9. X-ray diffraction (XRD) patternsofofthe the LBT, LBT, PVA, PVA, and composite membranes. Figure 9. X-ray diffraction (XRD) patterns andLBT/PVA LBT/PVA composite membranes.

Figure 9. X-ray diffraction (XRD) patterns of the LBT, PVA, and LBT/PVA composite membranes.

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2.7. 2.7. Application Application for for Vitamin Vitamin EE Protection Protection According According to to Figure Figure10, 10,without withoutcoating coating protection, protection, vitamin vitamin EE was was degraded degraded by by UV UV irradiation irradiation very 20% vitamin vitamin EE loss lossininthe thefirst first1010min. min.After After of irradiation, very quickly, quickly, with with nearly 20% 60 60 minmin of irradiation, the the remaining vitamin E was of the original amount. When the PVA membrane without remaining vitamin E was onlyonly 60%60% of the original amount. When the PVA membrane without LBT LBT for coating protection, the of rate of vitamin E degradation was similar the was was usedused for coating protection, the rate vitamin E degradation was similar to thattoofthat theof blank blank control. However, using LBT/PVA-4 coating protection,vitamin vitaminEEwas was not degraded control. However, whenwhen using LBT/PVA-4 for for coating protection, degraded significantly. irradiation, only 5.0% of vitamin E was lost.lost. We We concluded that that the significantly.Even Evenafter after6060min minofof irradiation, only 5.0% of vitamin E was concluded composite membrane was was helpful in protecting vitamin E from degradation by UV the composite membrane helpful in protecting vitamin E from degradation bylight. UV light.

Figure Figure10. 10. Protective Protectiveeffect effectof ofthe theLBT/PVA-4 LBT/PVA-4 membrane membrane on on Vitamin Vitamin E. E.

3. 3. Materials Materials and and Methods Methods 3.1. 3.1. Materials Materials Larch Larch bark bark from from Larix Larix gmelinii gmelinii (Rupr.) (Rupr.) Kuzen Kuzen was was ground ground and and sifted sifted through through 40-mesh 40-mesh sieves. sieves. The PVA had an average degree of polymerization of 1750 ± 50 and a sodium acetate content of The PVA had an average degree of polymerization of 1750 ± 50 and a sodium acetate content of less less than than0.5%. 0.5%. Gallic Gallic acid acid and and other other reagents reagents were wereall allanalytical-grade. analytical-grade. 3.2. 3.2. Extraction Extraction of of Tannin Tanninfrom fromLarch LarchBark Bark LBT was extracted using an ethanol/water LBT was extracted using an ethanol/water(1:1, (1:1,v/v) v/v)mixture mixtureas asthe thesolvent, solvent,with withaasolid/liquid solid/liquid ◦ ratio of 1:20, at a temperature of 55 C, for 2.5 h. After filtration, the filtrate was evaporated ratio of 1:20, at a temperature of 55 °C, for 2.5 h. After filtration, the filtrate was evaporated and and vacuum-dried vacuum-dried for for use. use. The The yield yield was was about about 10.52 10.52ggtannin/100 tannin/100 gg larch larch bark. bark. Using Using the the gallic gallic acid acid standard standardcurve curvemethod, method,the thetotal totalpolyphenol polyphenolcontent contentof ofLBT LBTwas wasdetermined determinedas as545.76 545.76mg/g mg/g [15]. [15]. 3.3. Preparation of the Composite Membrane 3.3. Preparation of the Composite Membrane PVA was first dissolved in water (5% w/w) at 90 ◦ C [16]. Then, an LBT ethanol/water (50% v/v) PVA was first dissolved in water (5% w/w) at 90 °C [16]. Then, an LBT ethanol/water (50% v/v) solution was added into the PVA water solution at room temperature with agitation, and the solution was added into the PVA water solution at room temperature with agitation, and the homogeneous mixture was casted into membranes. According to their LBT content of 0.00, 0.25, homogeneous mixture was casted into membranes. According to their LBT content of 0.00, 0.25, 0.50, 0.50, 1.00, and 2.00%, the membranes were designated as PVA, LBT/PVA-1, LBT/PVA-2, LBT/PVA-3, 1.00, and 2.00%, the membranes were designated as PVA, LBT/PVA-1, LBT/PVA-2, LBT/PVA-3, and and LBT/PVA-4, respectively. LBT/PVA-4, respectively. 3.4. Determination of the Antioxidative Activity of LBT and Composite Membranes by the DPPH Assay 3.4. Determination of the Antioxidative Activity of LBT and Composite Membranes by the DPPH Assay An alcohol solution of DPPH free radicals is purple and has a strong absorption at 517 nm. In the An alcohol of DPPH free radicals purple andwill has be a strong absorption atabsorption 517 nm. In presence of somesolution free radical scavengers, DPPH is free radicals scavenged, and the the presence of some free radical scavengers, DPPH free radicals will be scavenged, and the absorption gradually decreases. There is a quantitative relationship between absorption and DPPH

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gradually decreases. There is a quantitative relationship between absorption and DPPH radical scavenging, so this assay is considered one of the standard methods for the evaluation of antioxidative substances [17]. In this work, LBT was dissolved in 50% (v/v) ethanol/water and used for this test. The membrane samples were also used for this test according to the following method. The membrane samples were cut into 1 × 1 cm pieces, immersed in a sample bottle containing 10 mL distilled water, and stirred at room temperature for 30 min. Then, 1 mL of the solution was added to 4 mL of DPPH solution (150 µmol/L), and the solution was mixed by agitation. The mixture was placed in the dark for 30 min to allow the DPPH free radicals to be fully scavenged. Then, a UV spectrophotometer was used to determine the absorbance of the solution at 517 nm [18]. The scavenging rate of DPPH free radicals was calculated according to Equation (1): DPPH radicals scavenging rate (%) = (1 − Asample /Ablank ) × 100

(1)

where Asample is the absorbance of the sample and Ablank is the absorbance of the blank. 3.5. Determination of Soluble Phenol Content The soluble phenol contents in the composite membranes were determined according to the following method. The first step of the membrane DPPH assay was adopted to produce a phenol solution from LBT/PVA composite membrane samples. The membrane samples were cut into 1 × 1 cm pieces, placed in a sample bottle containing 10 mL distilled water, and stirred in a magnetic stirrer at room temperature for 30 min; then, 1 mL of the solution was added into a 10 mL volumetric flask with 10% 5 mL formol reagent. After 5 min of reaction, 4 mL of 75% (w/w) sodium carbonate solution was added. The absorbance value was measured at 765 nm after placing the samples in a water bath for 30 min. The contents of soluble phenols in the composite membranes were calculated on the basis of a standard curve prepared with gallic acid [19]. 3.6. Effect of the Composite Membranse on the Oxidation and Discoloration of Starch-Potassium Iodide (KI) Iodide ions (I− ) can be oxidized into I2 by the oxygen in the air under acidic conditions, through the reaction shown in Equation (2), and the I2 produced will react with aqueous starch and turn it blue. In the presence of the composite membrane, the amount of I2 produced varied, depending on the antioxidative properties of the membranes. 4 I− + O2 + 4 H+ → 2 I2 + 2 H2 O

(2)

The membrane samples were cut into 2 × 2 cm pieces, separately immersed in a sample bottle with 10 mL distilled water, and stirred for 30 min at room temperature. Then, 2 mL of each membrane solution was mixed with 2 mL of KI (1 mg/mL), and then 20 µL of 3% H2 O2 was added to a pH value of 5.5. After 2 h, each solution was titrated with sodium thiosulfate to determine the content of I2 [20]. 2 S2 O3 2− + I2 → S4 O6 2− + 2 I−

(3)

3.7. UV–Visible Light Transmittance of the Composite Membranes A TU-1901 double beam UV–visible spectrophotometer (Persee Universal Instrument Co., Beijing, China) was used to determine the transmittance (T %) of the composite membranes (50 ± 5 µm in thickness) in a scanning range of 200 to 800 nm. 3.8. Mechanical Properties Test The membrane samples were cut into 50 × 10 mm pieces. The maximum force (N), tensile strength (MPa), and elongation rate at break (%) of the membranes under dry conditions were tested with a

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small mechanical test machine (Landmark Sci. & Tech. Co., Beijing, China) at a speed of 20 mm/min. All tests were performed in quintuplicate, and the results expressed as mean ± standard deviation [21]. 3.9. Other Analysis and Characterizations Fourier transform–infrared (FTIR) spectra were recorded with a Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using the attenuated total reflection (ATR) method [22]. The scanning range was from 500 to 4000 cm−1 , and the resolution was 4 cm−1 . Scanning electron microscopy (SEM) micrographs of the samples were taken with a Quanta 200 scanning electron microscope (FEI Co., Hillsboro, OR, USA), with an accelerating voltage of 10 kV. The membranes were frozen in liquid nitrogen and snapped immediately to prepare the cross sections. Prior to observation, the samples were coated with a thin gold layer [23]. X-ray diffraction analysis of LBT and the composite membranes was performed on a D/Max-2400 diffractometer (Rigaku Corporation, Tokyo, Japan) using a scanning angle of 5–80◦ at a speed of 5◦ /min [24]. 3.10. Protective Effect of the Composite Membranes on Vitamin E Vitamin E is a common nutrient in food that oxidizes easily and deteriorates under the irradiation of ultraviolet light [25]. Using an LBT/PVA composite membrane as a food packaging material could protect vitamin E, since the LBT/PVA composite membranes demonstrated excellent UV protection ability. In this paper, a quartz cuvette containing 40 ppm vitamin E n-hexane solution was coated with one of the membranes, and then the solution was irradiated with a 3 W ultraviolet lamp at 254 nm. A non-coated sample served as the blank control. The absorbance of the vitamin E solution was measured at 300 nm, with 10 min irradiation intervals, and the remaining vitamin E was calculated on the basis of the prepared standard curve. 4. Conclusions The condensed tannin fraction that was extracted from larch bark was successfully used to prepare composite membranes with PVA and provided the PVA membranes with antioxidative and anti-UV properties. The DPPH free radical scavenging and starch-KI oxidation–discoloration assays showed that the composite membranes had good antioxidative ability. The low UV transmittance and the protective effect of the composite membrane on Vitamin E indicated that the composite membrane with 2.00% LBT provided excellent UV protection. The addition of the LBT did not cause obvious changes in the mechanical strength and crystallization properties of the membranes due to the good compatibility of tannin with PVA. The LBT/PVA composite membranes have the potential to be used as antioxidative and UV-protective food packaging. Author Contributions: S.L. conceived, designed the research, and revised the manuscript. Y.Z. performed the extraction experiments and wrote the manuscript. J.W. and H.W. prepared and tested the membranes. T.S. and W.H. analyzed the data. All authors contributed to this research. Funding: This research was funded by the National Key Research and Development Program of China (2016YFD0600806), the Fundamental Research Funds for the Central Universities (2572017EB07), and the National Innovation Experiment Program for University Students at Northeast Forestry University (201410225016). Acknowledgments: The authors are very grateful to Shixue Ren and Yanli Ma, who offered considerable technical support. Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of Larch bark tannin and the composite membranes are available from the authors. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).