Food Chemistry 140 (2013) 52–56

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Food Chemistry

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Short communication

Novel water-resistant UV-activated oxygen indicator for intelligent food packaging

Chau Hai Thai Vu, Keehoon Won ⇑Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, 30 Pildong-ro 1-gil, Jung-gu, Seoul 100-715, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 August 2012Received in revised form 8 February 2013Accepted 13 February 2013Available online 26 February 2013

Keywords:UV-activated oxygen indicatorAlginateDye leachingWater resistanceIntelligent food packaging

0308-8146/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.foodchem.2013.02.056

⇑ Corresponding author. Tel.: +82 2 2260 8922; faxE-mail address: keehoon@dongguk.edu (K. Won).

For the first time, alginate polymer has been applied to prevent dyes from leaching out of colorimetricoxygen indicator films, which enable people to notice the presence of oxygen in the package in aneconomic and simple manner. The dye-based oxygen indicator film suffers from dye leaching upon con-tact with water. In this work, UV-activated visual oxygen indicator films were fabricated using thionine,glycerol, P25 TiO2, and zein as a redox dye, a sacrificial electron donor, UV-absorbing semiconductingphotocatalyst, and an encapsulation polymer, respectively. When this zein-coated film was immersedin water for 24 h, the dye leakage was as high as 80.80 ± 0.45%. However, introduction of alginate(1.25%) as the coating polymer considerably diminished the dye leaching to only 5.80 ± 0.06%. This isbecause the ion-binding ability of alginate could prevent the cation dye from leaching into water. Thisnovel water-resistant UV-activated oxygen indicator was also successfully photo-bleached and regainedcolour fast in the presence of oxygen.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Oxygen, which is essential for many chemical and biologicalprocesses, is also responsible for most food deterioration and thusshould be removed in food packaging. For oxygen removal, theatmosphere within food package is modified with gases such asnitrogen/carbon dioxide or oxygen scavengers/oxygen absorbersare used (Ahvenainen, 2003; Cecchi, Passamonti, & Cecchi, 2010;Lee, Kim, An, Lyu, & Lee, 2008; Zerdin, Rooney, & Vermue, 2003).However, the oxygen level in the package headspace can increasewith time due to poor sealing, air permeation through the packagematerials, and the package tampered with or damaged duringstorage and/or transportation. As the result, the food can becontaminated with oxygen and spoiled. Whereas conventionaloxygen detection methods require expensive instruments andtrained operators, visual oxygen indicators are cheap and enableconsumers to detect the presence of oxygen in the food packagewith naked eyes. Oxygen indicators are used in intelligent foodpackaging, which monitors the condition of packaged food to giveinformation on the food quality during transport and storage(Ahvenainen, 2003). Some colourimetric oxygen indicators weresuccessfully commercialised (e.g., Ageless Eye� produced by theMitsubishi Gas Chemical Company).

In recent years, many works on UV-activated colorimetricoxygen indicators have been reported (Lee, Mills, & Lepre, 2004;Lee, Sheridan, & Mills, 2005; Mills & Hazafy, 2008; Roberts, Lines,Reddy, & Hay, 2011). This is because they have many attractive

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properties which an ideal oxygen indicator should possess. Forexample, this type of oxygen indicator is not activated until it isirradiated with UV light, which allows in-pack activation and alonger shelf-life (even under aerobic conditions), and exhibits irre-versible response toward oxygen (i.e., prevent the false indication)(Mills, 2005). Typically, they comprise a redox dye (D), a sacrificialelectron donor (SED), a UV-absorbing semiconducting photocata-lyst (SC), and a coating polymer. This UV-activated oxygen indica-tor ink is coated onto the inner side of food package films, andindicates oxygen in the package; the ink-coated film loses its col-our rapidly upon exposure to UV, remains colourless in the absenceof O2, and regains its original colour in the presence of oxygen inthe food package. However, this conventional oxygen indicatorhas a serious problem: the redox dyes encapsulated in the water-insoluble polymer film (e.g., zein) leach out when in contact withwater which food may contain. Because the indicator is placedinside the package, the leached dye can not only stain food unde-sirably, but be also a potential health hazard (Gillman, 2011; Paul& Kumar, 2010).

In order to develop oxygen indicators resistant to dye leakage toallow direct contact with water without loss of dye, sulfonatedpolystyrene was introduced as an encapsulating polymer, and itcould provide striking water stability for a dye (Mills, Hazafy, &Lawrie, 2011). However, the recovery of the original colour wasvery slow (5 days in ambient air at room temperature), and anexpensive platinum catalyst was employed in order to increasethe low recovery rate (Mills & Lawrie, 2011).

Alginate is a linear anionic polysaccharide containing blocks of(1,4)-linked b-D-mannuronate (M) and a-L-guluronate (G) residuesfrom brown seaweeds and bacterial species, and has been applied

C.H.T. Vu, K. Won / Food Chemistry 140 (2013) 52–56 53

to various areas including biomedicine, food, and biocatalyst due toits biocompatibility, low toxicity, low price, and mild gelation byaddition of divalent metal ions such as Ca2+ (Fernández-Pan, Igna-cio, & Caballero, 2011; Kanmani et al., 2011; Lee & Mooney, 2012;Mongkolkajit, Pullsirisombat, Limtong, & Phisalaphong, 2011;Rehm, 2010; Won, Kim, Kim, Park, & Moon, 2005).

We have paid attention to the cation-binding ability of alginatebecause most of the redox dyes used for oxygen indicators are cat-ions, and discovered that alginate can form water-insoluble com-plex with a redox dye. Based on our discovery, in the presentstudy, we introduce alginate as the coating polymer so that itcan bind to a redox dye and thus prevent the dye from leachinginto water. This novel alginate-based UV-activated oxygen indica-tor, which is not only highly resistant to dye leakage, but also fastin the colour recovery, is presented in this work for the first time.

2. Materials and methods

Unless otherwise mentioned, all the chemicals were purchasedfrom Sigma–Aldrich (USA) and used without any further purifica-tion. A supporting film for oxygen indicator ink was commercialnylon/PE vacuum packaging film (thickness: 0.15 mm) fromPack4U (Seoul, South Korea). All the experiments were conductedin triplicate.

2.1. Alginate-based oxygen indicator film preparation

Indicator ink comprised thionine acetate (40 mg), glycerol(0.6 g), and P25 TiO2 (0.6 g) as D, SED, and SC, respectively. All ofthese components were added to 90% ethanol solution (3.2 g)and then dispersed by 5 min of ultrasonication (Vibra-Cell VCX-750). The resultant ink (0.5 mL) was spin-coated onto the packag-ing film (4 cm � 4 cm) at 5000 rpm for 30 s using a spin coater(Laurell, WS-400B-6NPP-LITE). After drying, the coated film wasdipped in alginate solutions (0.25%, 0.75%, 1%, and 1.25%) using adip coater (KSV, KSV-DC) at an immersion and withdrawal speedof 85 mm/min, and then spun at 5000 rpm for 30 s.

2.2. Zein-based oxygen indicator film preparation

Ink for conventional zein (5%)-based indicator films was pre-pared by mixing D (40 mg), SED (0.6 g), SC (0.6 g), and zein(0.17 g) in 90% ethanol solution (3.2 g) and by dispersing themthoroughly with ultrasonication. This ink was spin-coated ontothe supporting film and allowed to dry in the dark. For exact com-parison, zein-based oxygen indicator films were also prepared inthe same manner as described for the alginate-based indicatorfilm: the packaging film was spin-coated with ink consisting ofD, SED, and SC and then was dipped into zein solution.

Fig. 1. Percentage of the remained thionine in the zein-based (s) and the alginate-based (d) oxygen indicator films after the dip coating.

2.3. Dye leaching behaviour of oxygen indicator film

The oxygen indicator film was immersed in distilled water andthionine leaching was quantified at 1, 6, 12, and 24 h by measuringabsorbance at 599 nm with the UV/Vis Cary 50 spectrophotometer.In order to determine the initial amount of thionine coated on thefilm, film was immersed in vigorously stirred solution for 3 h: eth-anol (70%) for the zein-based film and sodium acetate solution(0.5 M) for the alginate-based film (Bajpai & Sharma, 2004). Thedye leakage (%) was defined as a ratio of the dye amount leachinginto water for a given time to the initial amount of the coated dye.The dye leakage of 100% means that all the dye leached from thefilm into water.

2.4. UV irradiation and colour measurement

For bleaching, the oxygen indicator films were irradiated usingthe UV crosslinker (UVP, CL-1000) equipped with 5 tubes of UVBlamp (8 W) for 5 min. The intensity measured with the UVX digitalradiometer (UVP) was 2.5 mW/cm2. Film reflectance spectra wererecorded using the CM-2600d portable spectrophotometer (KonicaMinolta, Japan). The reflectance of the dye on the film was ex-pressed in the Kubelka–Munk function (Mills & Hazafy, 2008):

h ¼ f 0ðR01Þ ¼ð1� R01Þ

2

2R01¼ abC

where R01 is the percentage reflectance relative to a white standardreflectance plate; a is the molar absorption coefficient of the dye; Cis its concentration; b is a constant.

3. Results and discussion

3.1. Coating method

Conventional zein-based oxygen indicator films were preparedby coating ink comprising D, SED, SC, and zein onto films. However,the alginate-based oxygen indicator film could not be prepared inthis manner because thionine in alginate solution resulted inaggregate formation. Therefore, a different coating method wasemployed: the packaging film was spin-coated with ink consistingof D, SED, and SC and then was dipped into alginate solutions of0.25%, 0.75%, 1%, and 1.25% (see Section 2). For comparison,zein-based indicator films were prepared by dipping the filmcoated with D, SED, and SC into zein solutions. However, as shownin Fig. 1, less than 20% of thionine remained on the film after thedipping step because the dye on the film dissolved in 90% ethanolduring the dip-coating; after the dip-coating, the zein-based filmslost their colour and were almost white in the air so that they can-not be used as oxygen indicator films. By contrast, more than 80%of thionine was retained in the case of alginate. In conclusion, algi-nate-based and zein-based oxygen indicator films were preparedin two different coating methods.

3.2. Redox dye leaching behaviour

Dye leakage of the conventional oxygen indicator film based onwater-insoluble zein polymer (5%) was examined by immersingthe film in distilled water. The leaching of thionine into water asa variation of time is indicated in Fig. 2 (open circle). The dye

Fig. 3. Effect of polymer concentration on the dye leaching behaviour after 24 h of(a) zein-based and (b) alginate-based oxygen indicator films.

54 C.H.T. Vu, K. Won / Food Chemistry 140 (2013) 52–56

leached from the film into water quite rapidly in the first hour ofimmersion, and after 6 h the dye leakage reached 80.80 ± 0.45%.After the immersion in water, the zein-based film became solight-coloured even in the air that it was not suitable for oxygenindicators. Even though zein is insoluble in water except in thepresence of alcohol or high concentration of alkali (pH 11 or above)(Shukla & Cheryan, 2001), thionine, cationic dye is so soluble inwater and small in size that it can diffuse out readily. The new oxy-gen indicator film was prepared using alginate solution (0.25%) andthen tested in the same manner. As shown in Fig. 2 (closed circle),the dye leakage was only 19.65 ± 0.42% even after 24 h in water. Itwas very interesting that alginate, hydrophilic natural polymercould prevent the dye from leaching out much better than hydro-phobic zein polymer. This is attributed to ability of alginate to forminsoluble gels with divalent cations (Fernández-Pan et al., 2011;Lee & Mooney, 2012; Mongkolkajit et al., 2011; Rehm, 2010;Won et al., 2005).

Thionine molecules contain three nitrogen atoms: one in thequinone-imino group, one primary amino nitrogen, and one ring(tertiary) atom. Hence, thionine can have total 4 different configu-rations depending on the pH: trivalent ThioH2

+++; divalent ThioH++;monovalent Thio+, and Thio base (Epstein, Karush, & Rabinowitch,1941). ThioH2

+++ and Thio base may exist only in very strong acidicand basic solutions, respectively. The equilibrium between ThioH++

and Thio+ was studied and confirmed with the pKa value of6.3 ± 0.1 (Faure, Bonneau, & Joussot-Dubien, 1967; Sommer & Kra-mer, 1971). In mild acidic solutions such as our system, monova-lent thionine (Thio+) accepts one proton and thus divalentthionine (ThioH++) becomes present. Based on the experimentalevidence, we can reason that this form of thionine is responsiblefor the formation of water-insoluble complex with alginate.

Effect of the polymer concentration on the dye leakage wasinvestigated; the dye leakage after 24 h immersion in water wasdetermined with increasing polymer concentration. When the zeinconcentration was increased from 5% to 20%, the thionine leachinginto water after 24 h was hardly diminished as shown in Fig. 3(a).In contrast, an increase in the alginate concentration from 0.25% to1.25% significantly decreased the leakage down to 5.80 ± 0.06%(Fig. 3(b)).

3.3. Bleach and recovery of alginate-based oxygen detector film

We examined whether the alginate-based film could function asUV-activated colorimetric oxygen indicator, the working mecha-nism of which was reported elsewhere (Mills, 2005). Typical UV-activated oxygen indicators have bleaching and recovery process.

Fig. 2. Dye leakage of the zein (5%)-based (s) and the alginate (0.25%)-based (d)oxygen indicator films immersed in water for 24 h.

In brief, this indicator is activated only when exposed to UV lightto provide photons that have higher energy than SC’s band gap en-ergy. Upon activation, electron–hole pairs are formed in well-dis-persed SC. The photogenerated holes are reduced readily andirreversibly by SED, leaving photogenerated electrons to reduce re-dox dye to its usually colourless form: bleaching process. Therecovery process is triggered only by oxygen and the reduced re-dox dye is oxidised back to its highly-coloured form. All of thesesubsequent redox reactions happen spontaneously upon the differ-ence in redox potentials.

The oxygen indicator film was fabricated using alginate solution(1.25%) and then irradiated for bleaching with 5 � 8 W UVB lamps(intensity = 2.5 mW/cm2) for 5 min. The alginate-based oxygenindicator film was successfully bleached despite the thionine inter-action with alginate (data not shown). For the recovery, the UV-bleached film was placed in ambient air and the film reflectancespectra were monitored with time. As shown in Fig. 4(a), the spec-tra were developed, demonstrating the thionine was oxidised andregained colour irrespective of the dye-alginate complex. However,the wavelength to show the maximum h (i.e., kmax) of this film wasobserved around 500 nm with the gradual shift to the shorterwavelength during the recovery, whereas that of the zein-basedfilm was at 640 nm (data not shown). This colour change whendyes interact with specific substances is known as metachromasy(Bergeron & Singer, 1958; Bradley & Wolf, 1959). This phenomenonis generally because of aggregation of the dye molecules, which

Fig. 4. (a) Film reflectance spectra and (b) h values at the kmax during the recoveryof the alginate (1.25%)-based oxygen indicator in ambient air for 10 h.

C.H.T. Vu, K. Won / Food Chemistry 140 (2013) 52–56 55

produces several new species with absorption spectra havingpeaks at shorter wavelength than the absorption spectrum of thesimple dye molecules (Kay, Walwick, & Gifford, 1964). Lai, Dixit,and Mackay (1984) reported that the absorption maximum of thi-onine shifted gradually and continuously to shorter wavelength asthe thionine concentration was increased. This blue-shifted transi-tion of the absorption maximum was explained by the formation ofhigher aggregates of thionine molecules. Based on this theory andfindings, the thionine on the film might be aggregated through theformation of thionine–alginate complex, so that the kmax moved toshorter wavelength. In addition, the first (bottommost) spectrumin Fig. 4(a) reveals that the photo-bleached film exhibits a smallabsorption peak around 500 nm, which means the dye was notcompletely reduced, and this may be due to the interaction withalginate. However, this absorption is not high enough to noticewith naked eyes. It should be noted that an ideal oxygen indicatorshould not require expensive instruments such as spectrophotom-eters (Mills, 2005).

The h values of the alginate-based oxygen indicator at the kmax

were plotted against the recovery time. Fig. 4(b) reveals that about4 h is required for recovery at room temperature. Considering thatthe water-resistant oxygen indicator using expensive platinumcatalyst (1.52 wt.%) and sulfonated polystyrene requires 12 h to re-cover (Mills & Lawrie, 2011), the alginate-based indicator inventedin this work showed fast recovery behaviour. Further studies on ef-fects of oxygen concentration and other anionic natural polymersare in progress in our laboratory.

4. Conclusions

UV-activated oxygen indicator films were first fabricated usingalginate as a coating polymer. When this alginate-coated film wasimmersed in water for 24 h, the thionine leakage was 19.65 ±0.42%, which was much lower than that of conventional zein-coated films (80.80 ± 0.45%). The increase in the alginateconcentration to 1.25% considerably decreased the dye leachinginto water down to 5.80 ± 0.06%. This high water-resistance isattributed to insoluble complex of the dye with alginate. Thewater-resistant oxygen indicator was successfully photo-bleachedand regained colour fast in the presence of oxygen.

Acknowledgements

This research was supported by the Agriculture Research Center(ARC, 710003-03-3-SB120) program of the Ministry for Food, Agri-culture, Forestry and Fisheries, Korea.

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