Leaching-Resistant Carrageenan-Based Colorimetric OxygenIndicator Films for Intelligent Food PackagingChau Hai Thai Vu and Keehoon Won*

Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, 30 Pildong-ro 1-gil, Jung-gu, Seoul 100-715,Republic of Korea

ABSTRACT: Visual oxygen indicators can give information on the quality and safety of packaged food in an economic andsimple manner by changing color based on the amount of oxygen in the packaging, which is related to food spoilage. Inparticular, ultraviolet (UV)-activated oxygen indicators have the advantages of in-pack activation and irreversibility; however,these dye-based oxygen indicator films suffer from dye leaching upon contact with water. In this work, we introducecarrageenans, which are natural sulfated polysaccharides, to develop UV-activated colorimetric oxygen indicator films that areresistant to dye leakage. Carrageenan-based indicator films were fabricated using redox dyes [methylene blue (MB), azure A, andthionine], a sacrificial electron donor (glycerol), an UV-absorbing photocatalyst (TiO2), and an encapsulation polymer(carrageenan). They showed even lower dye leakage in water than conventional oxygen indicator films, owing to the electrostaticinteraction of anionic carrageenan with cationic dyes. The MB/TiO2/glycerol/carrageenan oxygen indicator film was successfullybleached upon UV irradiation, and it regained color very rapidly in the presence of oxygen compared to the other waterproofoxygen indicator films.

KEYWORDS: UV-activated oxygen indicators, carrageenans, water resistance, dye leakage, intelligent food packaging

■ INTRODUCTION

Food packaging has evolved to satisfy consumer demands forquality and safety; an intelligent packaging system senses,communicates, and monitors the conditions of packaged foodto give information about food quality, safety, and historyduring transport and storage. This innovative packaging canprovide great benefits to consumers and the food industry, andthus, it is becoming more important.1−5 Various variablesrelated to food quality and safety are monitored in intelligentpackaging: temperature, oxygen, carbon dioxide, toxins,etc.2,4,6−8 In particular, oxygen has a significant effect on thespoilage process of several products, and hence is removed infood packaging by modifying the atmosphere with gases, suchas nitrogen and/or using oxygen absorbers/scavengers.4,9,10

However, the oxygen level in the package headspace canincrease with time because of poor sealing, air permeationthrough the package materials, and package tampering ordamage during storage and/or transportation. As a result, thedecay of food is accelerated; therefore, the absence of oxygenshould be assured by sensing the amount of oxygen in thepackage. Whereas conventional oxygen-detection methodsrequire expensive instruments and trained operators, colori-metric oxygen indicators are cheap and enable consumers tosense the presence of oxygen in food packages with the nakedeye.11

Among the several types of visual oxygen indicators,ultraviolet (UV)-activated oxygen indicators have beenattracting great attention, because they possess many propertiesof ideal oxygen indicators. For example, they exhibit anirreversible response toward oxygen (i.e., preventing falseindications), and they are not activated until irradiated with UVlight, which allows for in-pack activation and a longer shelf life(even under aerobic conditions). They lose their color rapidly

upon exposure to UV light, remain colorless without oxygen,and regain their original color with oxygen.12,13 Typically, UV-activated oxygen indicator ink {which is composed of a redoxdye [e.g., methylene blue (MB)], a sacrificial electron donor(e.g., glycerol), and an UV-absorbing photocatalyst (e.g., TiO2

nanoparticles)} is spread onto the inner side (i.e., food contactside) of food package films and then encapsulated in a polymerfilm (e.g., zein).11 When it contacts water contained in food,however, the oxygen indicator film suffers from the leaching ofredox dyes from the film, which not only lowers the indicationefficiency but also potentially leads to health problems (e.g.,diarrhea, gastritis, nausea, and vomiting).14,15 Moreover, underthe new regulations, intelligent packaging systems should notrelease their constituents into the packaged food and may beseparated from the food by a barrier to prevent substancesbehind the barrier from migrating into the food.16 Manyattempts have been made to address this problem;17−21 forexample, Mills et al. employed synthetic sulfonated polystyreneand low-density PE as the coating polymer, but these polymersresulted in very slow color recovery under aerobic con-ditions.17,19

In this study, as the encapsulating polymer for water-resistantoxygen indicator films, we introduce carrageenans, which arenatural sulfated polysaccharides extracted from edible redseaweed and used extensively in the food industry as gelling andthickening agents, edible films, and coatings.22−24 We presentthe carrageenan-based UV-activated oxygen indicator, which is

Received: March 30, 2014Revised: June 24, 2014Accepted: June 30, 2014

Article

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characterized by not only resistance to dye leaching but alsorapid color recovery in the presence of oxygen.

■ MATERIALS AND METHODSMaterials. MB, azure A (AA), thionine (Th), titanium(IV) oxide

(TiO2, AEROXIDE P25, nanopowder, 21 nm particle size), glycerol,zein from maize, and kappa (κ)- and iota (ι)-carrageenans werepurchased from Sigma-Aldrich (St. Louis, MO) and used as receivedwithout further purification. The film for coating the oxygen indicatorink was a commercial nylon/polyethylene (PE) vacuum packaging filmof 0.15 mm thickness (Pack4U, South Korea). All experiments wereconducted in triplicate.Carrageenan-Based Oxygen Indicator Film Preparation.

Indicator ink consisted of 40 mg of MB, 1.2 g of TiO2, and 1.2 g ofglycerol. All of these components were added to 3.2 g of aqueousethanol solution (90%) and then mixed thoroughly by a magneticstirrer, followed by 5 min of ultrasonic dispersion (Vibra-Cell VCX-750, Sonics and Materials, Inc., Newtown, CT). The packaging film (4× 4 cm) was spin-coated with the ink at 5000 rpm for 30 s using a spincoater (WS-400B-6NPP-LITE, Laurell Technologies Corporation,North Wales, PA) and dried. The resultant film was then dippedinto ι-carrageenan solutions (0.2, 0.3, 0.4, and 0.5%, w/v) using a dipcoater (KSV-DC, KSV Instruments, Bridgeport, CT) at an immersionand withdrawal speed of 85 mm/min and finally spun at 2000 rpm for30 s.The thickness of the carrageenan-based indicator films was

measured using a thickness gauge (ID-S112B, Mitutoyo, Japan) andpresented with an average of three samples with eight measurementsalong the edge of each film.Zein-Based Oxygen Indicator Film Preparation. Zein (20%)-

based indicator ink was prepared by adding 40 mg of MB, 1.2 g ofTiO2, 1.2 g of glycerol, and 0.8 g of zein to 3.2 g of ethanol solution(90%) and then mixing with ultrasonication. The packaging film (4 ×4 cm) was spin-coated with the ink at 5000 rpm for 30 s and thenallowed to dry in the dark.Dye Leaching of Oxygen Indicator Films into Water. The

oxygen indicator films were submerged in a beaker filled with distilledwater for 1 day, and leached dyes were quantified at 1, 3, 6, 12, and 24h by measuring the absorbance at λmax of each dye with a Varian Cary50 UV−vis spectrophotometer. Dye leakage (%) was defined as a ratioof the amount of dye leaching into the water for a given time to theinitial amount of dye on the film; thus, dye leakage of 100% indicatesthat all of the dye leached from the film into the water. The initial dyeamounts were determined by measuring the absorbance afterimmersing the films in vigorously stirred solutions for 3 h: 70%ethanol for the zein-based films and HCl solution (2 M) for thecarrageenan-based films.Activation and Recovery of Oxygen Indicator Films. For

bleaching of the oxygen indicator film, UV irradiation was carried outusing a CL-1000 UV cross-linker (UVP, Upland, CA) equipped withfive tubes of UVC lamps (8 W each). The irradiation intensitymeasured with a UVX digital radiometer (UVP, Upland, CA) was 5.5mW/cm2. For color recovery, the photobleached film was placed inambient air, and its color change was monitored using a CM-2600dspectrophotometer (Konica Minolta, Tokyo, Japan).Color Measurement. Several systems for expressing color

numerically (i.e., color space) were developed by an internationalorganization concerned with issues of lighting and color, theInternational Commission on Illumination (usually abbreviated CIEfor its French name, Commission Internationale de l’Eclairage). TheL*a*b* color space (also referred to as CIELAB) devised in 1976 isthe most widely used, owing to the uniform distribution of colors.25 Inthis space, L* indicates lightness, ranging from 0 (black) to 100(white), while a* and b* are the chromaticity coordinates that indicatecolor directions: +a* is the red direction; −a* is the green direction;+b* is the yellow direction; and −b* is the blue direction. Because theblue MB-based indicator loses its color during photobleaching andregains the color during recovery, the main color change happens inthe b* axis and can be defined as −Δb*:

−Δ * = * − *b b bt0

where b0* is the initial b* value of samples and bt* is their b* value at aspecific time t. The difference of the b* value has been successfullyused as a measure of the MB color.19

■ RESULTS AND DISCUSSION

Preparation of Carrageenan-Based Oxygen IndicatorFilms. To begin with, we examined whether carrageenans caninteract with redox dyes. Two major classes of carrageenanswere employed: κ- and ι-carrageenans. Carrageenans are linear,partially sulfated galactans that mainly consist of alternating 3-linked β-D-galactopyranose (G) and 4-linked 3,6-anhydro-α-D-galactopyranose (DA); κ-carrageenan (G4S-DA) has onesulfate group covalently coupled via ester linkage to the carbonatom C-4 of the G residue (G-4), while ι-carrageenan (G4S-DA2S) possesses two sulfate groups at the G-4 and DA-2.22

Three types of thiazine dyes were used: MB, AA, and Th. When0.2 mL of each aqueous dye solution (10 mg/mL) was addedto 10 mL of ι-carrageenan solution (2.5 mg/mL), insolublestringy aggregates were formed with all of the dyes tested;however, aggregation with κ-carrageenan was barely observedwith the naked eye in all cases (data not shown). This can beexplained by the difference in the sulfate content of thecarrageenans; typically, commercial κ- and ι-carrageenanscontain 22 and 32% (w/w) sulfate, respectively, althoughthey vary considerably according to seaweed species andbatches.23 It was demonstrated that the sulfate moiety ofcarrageenans is essential for the interaction with cationic dyes,and the binding activity increases when the percentage ofsulfate is increased.26−28 In the present work, only ι-carrageenan was used for further study and carrageenan refersto ι-carrageenan unless otherwise mentioned.We prepared UV-activated oxygen indicator films using

carrageenan as the coating polymer; the packaging film wasspin-coated with ink composed of redox dyes, glycerol, andTiO2 and then dip-coated with carrageenan solutions (see theMaterials and Methods). For comparison, the zein-based filmwas also fabricated in the same manner as the carrageenan-based film. However, little dye remained on the film becausethe dyes dissolved into the zein solution (90% ethanol) duringthe dip-coating step.20 Therefore, the indicator films using zeinas the encapsulation polymer were prepared by spin-coating thepackaging film with the ink comprising redox dyes, glycerol,TiO2, and zein, as described in the Materials and Methods. Forreference, the carrageenan-based film could not be prepared inthis manner because the addition of the dyes to the aqueouscarrageenan solution led to the formation of insolubleaggregates as already mentioned.

Leakage Behaviors of Redox Dyes from the Films.Because MB is readily soluble in water, without properprotective polymers, the dye in the indicator film leaches outvery quickly when immersed in water.17 With the water-insoluble zein polymer (5%), the water resistance of the MB-coated indicator film was investigated by immersing the film inwater for 24 h. As shown in Figure 1 (closed circles), after MBleached out quite quickly within the first hour of immersion,the dye leakage reached 48% and then remained nearlyconstant. The resultant MB/TiO2/glycerol/zein (5%) film wasso light-colored that it was not suitable for a colorimetricoxygen indicator. Although water-insoluble zein covered MB, itcould not completely prevent the small water-soluble ionic

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molecules (about 16.9 Å in length, 7.4 Å in breadth, and 3.8 Åin thickness) from leaking into the water.29

The same experiment was carried out using water-solublecarrageenan (0.2%) as the encapsulating polymer; surprisingly,only 2.1% of MB on the film leached into the water (opencircles in Figure 1). This high resistance to dye leakage isbelieved to be attributed to the electrostatic interaction of theanionic polymer and the cationic dye.26−28 Carrageenan isnegatively charged because of the two sulfated groups on therepeating dimer, whereas MB, generally available as chloridesalts, exists in cationic form in water.The concentration effects of each polymer were examined for

the protection ability against dye leakage: 5, 10, 15, and 20% forzein and 0.2, 0.3, 0.4, and 0.5% for carrageenan. When the zeinconcentration was increased from 5 to 20%, the dye leakagegradually declined to around 30% but it was still high (Figure2a). The dye leakage of the carrageenan-based film slightlydecreased from 2.1 to 1.8% when the concentration wasincreased from 0.2 to 0.5%, indicating that carrageenan iseffective enough at as low a concentration as 0.2% (Figure 2b).We examined whether carrageenan can also prevent dyes

other than MB from leaching from the film into water. Thecarrageenan (0.2%)-based oxygen indicator films were prepared

with AA and Th in the same way as with MB; for comparison,the zein (20%)-based films were also fabricated. The leakages ofthe three dyes after 24 h of immersion in water are shown inFigure 3: zein (open bars) and carrageenan (closed bars).

Similar to MB, the leakages of AA and Th were considerablewith the zein polymer, particularly Th. The reason why Thshowed the highest leakage is not evident, but this could be dueto the larger contact area of Th with water; Th (with no methylgroup) is more hydrophilic than AA (with two methyl groups)and MB (with four methyl groups).30 In comparison to zein,carrageenan diminished the leakage of AA and Th as well asMB to a greater degree because of the electrostatic interactionof the anionic carrageenan with the cationic dyes. In addition,the dye leakage might be further lowered by additionaloptimization (e.g., the employment of carrageenans with higherpercentages of sulfate and/or with other molecularweights).22,23

Photobleaching and Recovery Process of Carra-geenan-Based Indicator Film. An UV-activated indicatorfilm should be bleached when exposed to UV light (photo-bleaching step) and regain its color in the presence of oxygen(recovery step). Because the strong interaction betweencarrageenans and dyes might impede the photobleaching andrecovery processes of the carrageenan-based indicator film, weinvestigated this possibility. The oxygen indicator films wereprepared using MB and carrageenan (0.2, 0.3, 0.4, and 0.5%) asthe redox dye and the coating polymer, respectively. When thecarrageenan-coated films were irradiated with UVC light(intensity = 5.5 mW/cm2) for 4 min, all of the films werecompletely bleached (data not shown). The finely dispersedTiO2 nanoparticles (photocatalyst) encapsulated in thepolymer generate electron−hole pairs upon UV irradiation.The photogenerated holes oxidize glycerol (a sacrificial electrondonor), and the photogenerated electrons reduce MB to itscolorless reduced form.12 The initial rate of photobleaching wasalso measured as a function of the carrageenan concentration.Figure 4a (circles) shows that the bleaching rate decreased asthe polymer concentration increased. This may not be due tothe increase in the thickness of the carrageenan film with theincreasing concentration because the difference in the thicknesswas not significant, as shown in Figure 4a (bars); the reason is aquestion for future research.Figure 4b shows the recovery behavior of the MB/TiO2/

glycerol/carrageenan (0.2%) indicator film in air under ambientconditions in which −Δb* was plotted against recovery time.The oxygen indicator film was recovered within about 8 h, Thecolorless reduced form of MB on the film was reoxidized byoxygen, and the original blue color of the indicator film was

Figure 1. Dye leakage of zein (5%)-based and ι-carrageenan (0.2%)-based oxygen indicator films as a function of the water immersiontime.

Figure 2. Effect of the polymer concentration on MB leakage after 24h of (a) zein-based and (b) ι-carrageenan-based oxygen indicator films.

Figure 3. Leaching behaviors of MB, AA, and Th dyes on zein (20%)-based and ι-carrageenan (0.2%)-based oxygen indicator films.

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restored. This recovery time may not be satisfactory ifcompared to conventional water-susceptible indicator films(∼6 min),12 but it is appreciably short considering that theother waterproof films had recovery times of 5 days usingsulfonated polystyrene17 and 2.5 days using low-density PE.19

The high recovery rate of the carrageenan-based film isprobably attributed to the hydrophilic nature of carrageenans,because hydrophobic films hinder the formation of the chargedspecies, MB+ and OH−, for color recovery.17,19

In conclusion, we have developed an UV-activated oxygenindicator film resistant to dye leaching by introducingcarrageenans, which are natural sulfated polysaccharides. Thedye-binding ability of carrageenans substantially lowered theleakage into water of all of the redox dyes tested (MB, AA, andTh). The MB/TiO2/glycerol/carrageenan oxygen indicatorfilm was successfully bleached upon UV irradiation andregained its color very rapidly in the presence of oxygen.This leaching-resistant colorimetric oxygen indicator film within-pack activation and irreversibility can be applied to verifythat all oxygen is removed using oxygen absorbers/oxygenscavengers, and it can also be used as a seal and leak detectorwith modified atmosphere packaging; it will be essential forintelligent food packaging.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +82-2-2260-8922. Fax: +82-2-2268-8729. E-mail:keehoon@dongguk.edu.FundingThis research was supported by the Agriculture ResearchCenter Program of the Ministry of Agriculture, Food, and RuralAffairs (ARC-710003-03-4-SB120), the New and RenewableEnergy of the Korea Institute of Energy Technology Evaluationand Planning (KETEP) grant funded by the Korea Govern-ment Minis t ry of Trade , Industry , and Energy

(20133030000300), and the Korea Carbon Capture andSequestration (CCS) Research and Development (R&D)Center (KCRC) Program (2013M1A8A1038187) of theMinistry of Science, Information/Communications Technology(ICT), and Future Planning, Republic of Korea.NotesThe authors declare no competing financial interest.

■ REFERENCES(1) Brody, A. L.; Bugusu, B.; Han, J. H.; Sand, C. K.; Mchugh, T. H.Innovative food packaging solutions. J. Food Sci. 2008, 73, R107−R116.(2) De Jong, A. R.; Boumans, H.; Slaghek, T.; van Veen, J.; Rijk, R.;van Zandvoort, M. Active and intelligent packaging for food: Is it thefuture? Food Addit. Contam. 2005, 22, 975−979.(3) Heising, J. K.; Dekker, M.; Bartels, P. V.; van Boekel, M. A. J. S.Monitoring the quality of perishable foods: Opportunities forintelligent packaging. Crit. Rev. Food Sci. Nutr. 2014, 54, 645−654.(4) Pereira de Abreu, D. A.; Cruz, J. M.; Paseiro Losada, P. Activeand intelligent packaging for the food industry. Food Rev. Int. 2012, 28,146−187.(5) Yam, K. L.; Takhistov, P. T.; Miltz, J. Intelligent packaging:Concepts and applications. J. Food Sci. 2005, 70, R1−R10.(6) Kim, K.; Kim, E.; Lee, S. J. New enzymatic time-temperatureintegrator (TTI) that uses laccase. J. Food Eng. 2012, 113, 118−123.(7) Jang, N. Y.; Won, K. New pressure-activated compartmentedoxygen indicator for intelligent food packaging. Int. J. Food Sci. Technol.2014, 49, 650−654.(8) Jung, J.; Puligundla, P.; Ko, S. Proof-of-concept study of chitosan-based carbon dioxide indicator for food packaging applications. FoodChem. 2012, 135, 2170−2174.(9) Feng, S.; Luo, Z.; Shao, S.; Wu, B.; Ying, T. Effect of relativehumidity and temperature on absorption kinetics of two types ofoxygen scavengers for packaged food. Int. J. Food Sci. Technol. 2013,48, 1390−1395.(10) Lee, K.-E.; Kim, H. J.; An, D. S.; Lyu, E. S.; Lee, D. S.Effectiveness of modified atmosphere packaging in preserving aprepared ready-to-eat food. Packag. Technol. Sci. 2008, 21, 417−423.(11) Mills, A. Oxygen indicators and intelligent inks for packagingfood. Chem. Soc. Rev. 2005, 34, 1003−1011.(12) Lee, S.-K.; Mills, A.; Lepre, A. An intelligence ink for oxygen.Chem. Commun. 2004, 1912−1913.(13) Mihindukulasuriya, S. D. F.; Lim, L.-T. Oxygen detection usingUV-activated electrospun poly(ethylene oxide) fibers encapsulatedwith TiO2 nanoparticles. J. Mater. Sci. 2013, 48, 5489−5498.(14) Ghosh, D.; Bhattacharyya, K. G. Adsorption of methylene blueon kaolinite. Appl. Clay Sci. 2002, 20, 295−300.(15) Paul, P.; Kumar, G. S. Targeting ribonucleic acids by toxic smallmolecules: structural perturbation and energetics of interaction ofphenothiazinium dyes thionine and toluidine blue O to tRNAphe. J.Hazard. Mater. 2013, 263, 735−745.(16) Restuccia, D.; Spizzirri, U. G.; Parisi, O. I.; Cirillo, G.; Curcio,M.; Iemma, F.; Puoci, F.; Vinci, G.; Picci, N. New EU regulationaspects and global market of active and intelligent packaging for foodindustry applications. Food Control 2010, 21, 1425−1435.(17) Mills, A.; Hazafy, D.; Lawrie, K. Novel photocatalyst-basedcolourimetric indicator for oxygen. Catal. Today 2011, 161, 59−63.(18) Mills, A.; Lawrie, K. Novel photocatalyst-based colourimetricindicator for oxygen: Use of a platinum catalyst for controllingresponse times. Sens. Actuators, B 2011, 157, 600−605.(19) Mills, A.; Lawrie, K.; Bardin, J.; Apedaile, A.; Skinner, G. A.;O’Rourke, C. An O2 smart plastic film for packaging. Analyst 2012,137, 106−112.(20) Vu, C. H. T.; Won, K. Novel water-resistant UV-activatedoxygen indicator for intelligent food packaging. Food Chem. 2013, 140,52−56.(21) Vu, C. H. T.; Won, K. Bioinspired molecular adhesive for water-resistant oxygen indicator films. Biotechnol. Prog. 2013, 29, 513−519.

Figure 4. (a) Initial bleaching rate and coating thickness of MB/TiO2/glycerol/ι-carrageenan oxygen indicator films as a function of thecarrageenan concentration. (b) Recovery behavior of MB/TiO2/glycerol/ι-carrageenan (0.2%) oxygen indicator film in air underambient conditions with a sampling time of 30 min.

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(22) Campo, V. L.; Kawano, D. F.; da Silva Jr., D. B.; Carvalho, I.Carrageenans: biological properties, chemical modifications andstructural analysisA review. Carbohydr. Polym. 2009, 77, 167−180.(23) De Ruiter, G. A.; Rudolph, B. Carrageenan biotechnology.Trends Food Sci. Technol. 1997, 8, 389−395.(24) Sanchez-Garcia, M. D.; Hilliou, L.; Lagaron, J. M. Nano-biocomposites of carrageenan, zein, and mica of interest in foodpackaging and coating applications. J. Agric. Food Chem. 2010, 58,6884−6894.(25) Wu, D.; Sun, D.-W. Colour measurements by computer visionfor food quality controlA review. Trends Food Sci. Technol. 2013, 29,5−20.(26) Graham, H. D. Quantitative aspects of the interaction ofcarrageenan with cationic substances. I. Interaction with methyleneblue. J. Food Sci. 1960, 25, 720−730.(27) Graham, H. D.; Thomas, L. B. Precipitation of food gums bythiazine, oxazine, azine and other cationic dyes: Specificity of themethylene blue carrageenan reaction. J. Food Sci. 1961, 26, 365−372.(28) Soedlak, H. S. Colorimetric determination of carrageenans andother anionic hydrocolloids with methylene blue. Anal. Chem. 1994,66, 4514−4518.(29) Hahner, G.; Marti, A.; Spencer, N. D.; Caseri, W. R. Orientationand electronic structure of methylene blue on mica: A near edge X-rayabsorption fine structure spectroscopy study. J. Chem. Phys. 1996, 104,7749−7757.(30) Chakraborty, A.; Ali, M.; Saha, S. K. Molecular interaction oforganic dyes in bulk and confined media. Spectrochim. Acta, Part A2010, 75, 1577−1583.

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