Chitosan Based Edible Films and Coatings

Review

Chitosan based edible lms and coatings: A reviewa, b,c

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. . . .an withtin basenate an. . . .. . . .

5. Gas permeation properties of edible coati

Materials Science and Engineering C xxx (2013) xxxxxx

MSC-03802; No of Pages 23

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering C

j ourna l homepage: www.e lsev ie r .com/ locate /msec8. Increasing the shelf life of foods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 09. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

Abbreviations: DD, degrees of deacetylation; EMC%, Equilibrium moisture content; CH, high molecular weight chitosan; CM, mediummolecular weight chitosan; CL, low molecular

weight chitosan; MD, machine direction; TD, transverse dbacterial cellulose; FS, Film solubility; MW, molecular weessential oil; TVC, total viable counts; BO, Bergamot essenacid); PBTA, poly(butylene terephthalate adipate); PBSA,GO, Garlic oil; PS, potassium sorbate; N, nisin; AM, antiacid-soluble chitosan; WCS, water-soluble chitosan; PVDC Corresponding author. Tel.: +20 26352316, +20 1

E-mail address: [email protected] (M.Z. Elsabe

0928-4931/$ see front matter 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.msec.2013.01.010

Please cite this article as: M.Z. Elsabee, E.S.ngs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0san blends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06. Effect of electric eld on lm formation .7. Antibacterial activity of chitosan and chitoContents

1. Introduction . . . . . . . . .2. Chitin and chitosan . . . . . .

2.1. Blending . . . . . . . .2.1.1. Blending of chitos2.1.2. Chitosan and gela2.1.3. Chitosan with algi

3. Chitosan/essential oil lms . . .4. Chitosan and clay . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0d edible lms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0d carageenan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0Water vapor and gas permeabilityAntibacterial and antifungal propertiesNanoclayMaher Z. Elsabee , Entsar S. Abdoua Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egyptb Food Packaging and Engineering Department, Food Technology Research Institute, Agriculture Research Center, Giza 12613, Egyptc Chemistry Department, Faculty of Science and Humanities, Salman bin Abdulaziz University, Hotet Bany Tamim 11941, Saudi Arabia

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

Article history:Received 9 August 2012Received in revised form 11 December 2012Accepted 9 January 2013Available online xxxx

Keywords:ChitosanBlendsEssential oils

Chitosan is a biodegradable biocompatible polymer derived from natural renewable resources with numer-ous applications in various elds, and one of which is the area of edible lms and coatings. Chitosan hasantibacterial and antifungal properties which qualify it for food protection, however, its weak mechanicalproperties, gas and water vapor permeability limit its uses. This review discusses the application of chitosanand its blends with other natural polymers such as starch and other ingredients for example essential oils,and clay in the eld of edible lms for food protection. The mechanical behavior and the gas and watervapor permeability of the lms are also discussed. References dealing with the antimicrobial behavior ofthese lms and their impact on food protection are explored.

2013 Elsevier B.V. All rights reserved.irection; TS, tensile strength; CS, chitosan; Ec, elastic modulus; E, Elongation at the break; BCC, bacterial cellulosechitosan; BC,ight; WVP, water vapor permeability; M, (14)-linked--D-mannuronate; G, (14)-linked--L-guluronate; CEO, cinnamontial oil; FFD, Film-forming dispersions; TTO, tea tree oil; PCL, poly-caprolactone; PBS, poly(butylene succinate); PLA, poly(lacticpoly(butylene succinate adipate); WVT, Water vapor transmission rate; MC, methylcellulose; AFM, Atomic force microscopy;microbial; LPSSD, low-pressure superheated steam drying; PDA, potato dextrose agar; CWC, Chinese water chestnut; ACS,, polyvinyl dichloride; TVC, total viable counts; MAP, modied atmosphere packaging; CMC, carboxymethyl-cellulose.006680474 (mobile).e).

rights reserved.

Abdou, Mater. Sci. Eng., C (2013), http://dx.doi.org/10.1016/j.msec.2013.01.010

2 M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxx1. Introduction

Recently, considerable research has been conducted to develop andapply bio-based polymersmade from a variety of agricultural commod-ities and/or of food waste products [1]. This increased interest was in-tensied due to concerns about limited natural resources of the fossilfuel reserve and the environmental impact caused by the use of non-biodegradable plastic-based packaging materials [2]. Such biopolymersinclude starches, cellulose derivatives, chitosan/chitin, gums, proteins(animal or plant-based) and lipids [3]. These materials offer the possi-bility of obtaining thin lms and coatings to cover fresh or processedfoods to extend their shelf life.

Edible lms and coatings offer extra advantages such as edibility,biocompatibility, esthetic appearance, barrier to gasses properties,non-toxicity, non-polluting and its low cost [4]. In addition, biolmsand coatings, by themselves or acting as carriers of foods additives(i.e.: antioxidants, antimicrobials), have been particularly consideredin food preservation due to their ability to extend the shelf life [5].

Seafood products are more perishable than chicken or red meat asthey contain relatively large quantities of free amino acids and vola-tile nitrogenous bases compared with other meats [5]. During storage,sh quality is quickly reduced as chemical and enzymatic reactionslead to the initial loss of freshness, while microbial spoilage producesthe end of the shelf life. The increasing demand for high quality freshseafood has intensied the search for new methods and technologiesfor better sh preservation. One of the possibilities, not muchexplored, is the application of an edible lm or coating, in combina-tion with other microbial stress factors, on the fresh sh muscle.Gomez-Estaca et al. [6] reported that a gelatinchitosan-based ediblelm together with refrigeration and high pressure have lowered themicrobial growth of cold-smoked sardine in comparison to uncoatedsamples.

Over the past several decades, several biopolymers have receivedincreased attention for their applications in chemical, biomedical,and food industries, [7]. For example, chitin suture is resorbable inhuman tissues fromwhich chitosancollagen composites for an arti-cial skin are commercially produced [8]. These polymers are not onlybiodegradable, but also edible. Another area of growing interest is thepreparation of antimicrobial edible lms and coatings [2,9] wherechitosan plays an important role due to its well-documented antimi-crobial properties [10].

2. Chitin and chitosan

Chitin is an abundant naturally occurring biopolymer and is found inthe exoskeleton of crustaceans, in fungal cell walls and in other biolog-ical materials. It ismainly poly(-(14)-2-acetamido-D-glucose), whichis structurally identical to cellulose except that a secondary hydroxyl onthe second carbon atomof the hexose repeat unit is replaced by an acet-amide group. Chitosan is derived from chitin by deacetylation in analkaline media [11]. Actually, chitosan is a copolymer consisting of-(14)-2-acetamido-D-glucose and -(14)-2-amino-D-glucose unitswith the latter usually exceeding 60%. Chitosan is described in termsof degree of deacetylation and averagemolecular weight and its impor-tance resides in its antimicrobial properties in conjunction with itscationicity and lm-forming properties.

The potential of chitosan to act as a food preservative of natural originhas beenwidely reported on the basis of in vitro trials as well as throughdirect application on real complex matrix foods [1217]. Chitosan is alsoan excellent lm forming material [18]. Chitosan lms have a selectivepermeability to gasses (CO2 and O2) and good mechanical properties.However, the fact that chitosan lms are highly permeable to watervapor limits their use as being an important drawback since an effectivecontrol of moisture transfer is a desirable property for most foods, espe-cially in moist environments. Therefore, several strategies have been

used to improve the physical properties of biopolymer based lms.

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2Among them, promising results in increasing the hydrophobicity havebeen obtained by addition of neutral lipids, fatty acids waxes [19,20]and clay [21] although often compromising their mechanical and chem-ical stability and/or of their organoleptic attributes. Moreover, variouschemical and physicalmeans have been demonstrated as good strategiesto improve their mechanical properties, such as the addition ofcross-linking agents, irradiation and ultrasonic treatments [22].

The antifungal and antimicrobial activities of chitosan are believed tooriginate from its polycationic nature [23]. The antimicrobial action ofchitosan is hypothesized to be mediated by the electrostatic forcesbetween the protonated amino group (NH2) in chitosan and the negativeresidues at cell surfaces [24]. The number of protonated amino groups(NH2) present in chitosan increases with increased degrees ofdeacetylation (DD) which inuences the antimicrobial activity [25]. Liuet al. (2004) [25] state that the bactericidal activity of chitosan is causedby the electrostatic interaction between NH3+ groups of chitosan and thephosphoryl groups of the phospholipid component of the cell mem-brane. However, it was found that water-soluble chitosan promotedthe growth of Candida albicans even in acidic media whereas water-insoluble chitosan exhibited inhibitory effect [26]. In addition, a stronginteraction between microbial proteins and chitosan at very acidic pHvalues is low and adsorption of chitosan to Escherichia coli cells increasedstrongly with increasing pH. This means that the protonated NH3+ is notthe predominant factor in the antibacterial capacity of chitosan. Park etal. suggested that the antimicrobial activity of chitosan is not proportion-al to its DD value [27]. As the water soluble chitosan was not efcient asantibacterial agent it is thus conceivable that chitosan molecules havethe ability to interact with bacterial surface compounds, and is absorbedon surface of the cells. However, physiological pH in the cell is aroundneutral, so water-insoluble chitosan molecules can precipitate, andstack on the microbial cell surface, thereby forming an imperviouslayer around the cell and blocking the channels, which are crucial for liv-ing cells. Such a layer can be expected to prevent the transport of essen-tial solutes and may also destabilize the cell wall beyond repair therebycausing severe leakage of cell constituents and ultimately cell death[26]. In support of this last idea, it was found that the cell membranesof Gram-negative and Gram-positive bacteria showed signicant mor-phological changes and shrinking after contactwith chitosan treated cot-ton fabrics [28].

2.1. Blending

The functional properties of chitosan-based lms can be improvedby combining them with other hydrocolloids [2931]. Chitosan/pectinlaminated lms have been developed by the interaction of the cationicgroups of chitosan with the anionic groups of pectin. A decrease inwater vapor transmission rates (WVTRs) by combining chitosan withtwo thermally gelatinized corn starches has been observed [31].

An alternative way to improve the mechanical and physical proper-ties of these biolms is by combining proteins (e.g. milk proteins, soyprotein, collagen and gelatin) with polysaccharides (e.g. starches, algi-nates, cellulose and chitosan). Chitosangelatin blend lms have beenshown to be homogeneous due to the good miscibility between bothbiopolymers [3234] leading to improved material properties of theblend lms as compared to those obtained from the pure polymers.This is explained by the formation of electrostatic interactions betweenthe ammonium groups of the chitosan and the carboxylate groups ofthe gelatin. On the other hand, chitosan/soy protein blended mem-branes [33], are not completely miscible. The blended membranesbecame more brittle with increasing soy protein content, and showeda rougher surfacemorphology, this is probably related to phase separa-tion among blend components. Chitosan/sodium caseinate lms havealso been studied; in this case no phase separation was observed dueto the complexation of the two polymers within the blend lm matrix[34]. Some polysaccharidewhey protein lms have also been prepared

and characterized [35]. Also, the addition of Pullulan to a whey protein

013), http://dx.doi.org/10.1016/j.msec.2013.01.010

of 70 C under stirring until viscous and transparent solution wasobserved. After homogeneously mixing for 10 min this solution waspoured into 5 mm thickness acrylic mold with removable edge stripsand allowed to dry freely at room temperature. After air drying, theedges of the mold were removed and four sides of the lm were sealedwith adhesive tape to prevent the underneath of starch lm from gettingcontact with chitosan coating solution in the next step. Chitosan (variedfrom 1 to 4% (w/v)) was dissolved in 2% (v/v) acetic acid solution thenltered and poured onto the starch lm and the coating was carriedout by an automatic lm coater with wire bar coating rod. After coating,the acrylic mold support containing chitosan-coated starch lm wastaken out from the automatic lm coater and stored at room tempera-ture, allowing the coated lm to dry for at least 72 h. Three surface prop-erties of the obtained lms were studied; gloss, transparency andhydrophobicity.

Gloss is one of the esthetic factors enhancing general appearanceas well as consumer acceptance. The gloss values of chitosan-coatedstarch lms including free starch lm and free chitosan lm aregraphically shown in Fig. 2. The gloss values of free chitosan lmsare found in the range of 132.5145.6 units, indicating a highly glossylm probably due to a smooth surface. On the other hand, the glossvalues of free starch lms are between 52.4 and 60.1 units, reectingthe likelihood of uneven lm surface.

For the coated lms, it can be seen that the gloss values increasewith an increase in the chitosan coating solution content. From theresults, only 1 wt.% chitosan coating solution brings about a signi-cant increase in gloss values of lm compared with those of free

Fig. 1. Equilibrium moisture content (EMC%) of high molecular weight chitosan (CH),medium molecular weight chitosan (CM), low molecular weight chitosan (CL), amy-lose corn starch (Hylon VII) starting materials (A), and chitosanHylon VII lms (B)stored at different relative humidity for 9 days [38].

3M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxlm has shown to decrease water vapor and oxygen permeability, al-though these barrier properties got worse as the amount of polysaccha-ride increased [36]. Chitosanwhey protein lms have been prepared atpH 6 with different protein concentrations, in the absence or presenceof transglutaminase as a cross-linking agent [37]. The chitosan wasthemainlm component and its amountwas kept constant, the proteinwas froma spraydriedwhey product still rich in lactose and the amountof whey protein did not exceed the proportion of 1:9 (protein:chitosan)in the nal lms. In more recent work [2], lms of chitosanwhey pro-tein blend with a high amount of protein has been prepared in order toobtain a blend with new functionality out of the interaction of the cat-ionic polyelectrolyte chitosan with protein. The study was aimed alsoto prepare edible lm-forming material with anti-microbial properties.

2.1.1. Blending of chitosan with starchThe main differences between starch and chitosan are the glucoside

linkage: (1, 4) for starch and (1, 4) for chitosan and, the hydroxylgroup of the second carbon is replaced by the amine group whichappears acetylated in the case of the natural polymer chitin.

Fernandez et al. [38] studied the physical stability and moisturesorption of aqueous chitosanamylase starch lms plasticized withpolyols. They used high, medium and low molecular weight chitosanwith amylose-rich corn starch as a co-lm former in the presence ofglycerol and i-erythritol.

In comparison to regular corn starch which contains approximately28% amylose, Hylon VII is a corn hybrid containing approximately 70%amylose. Since amylose is a linear polymer, it can closely align or associatethrough hydrogen bonding. This characteristic of amylose is primarily re-sponsible for the gelling and lm-forming ability of starches. Since HylonVII contains more than twice as much amylose as regular corn starch itcan form more rigid gels and contribute to the formation of strongerand tougher lms. ChitosanHylon VII solutions plasticized with glycerolor erythritol were prepared in a high-pressure reactor equipped with ablade mixer.

The steady-state moisture in the starting materials was measuredafter 9 days of storage of the samples at different relative humidity.As seen in Fig. 1A, the moisture increase of low molecular weight chi-tosan was lower than that of high and mediummolecular weight chi-tosan at a relative humidity of 95%. The moisture increase of Hylon VIIwas lower than that of other starting materials from relative humidityof 5295%. The storage of the erythritol at a high humidity resulted ina signicant increase in water uptake, causing a liquefaction of thesubstance even higher than that of the chitosan. Starch and chitosanare hydrophilic and retain a considerable amount of water whichdepends on the relative humidity. At least in chitosan, there existthree predominant absorption sites such as the hydroxyl group, theamino group, and the polymer chain end. The polymer chain end issupposed to be composed of a hydroxyl or an aldehydic group [39].

Usually, the amine content increases with increasing molecularweight. In the case of chitosan, the water is bound to the hydroxylgroup more strongly than to the amine group. Therefore, the releaseof water molecules could preferentially occur via the amine group.

The crystallinity of various lm samples plasticized with erythritolstarted to increase after 2 months. The crystallinity of the lms storedat 25 C/60% RH was higher than those of the respective lms stored at40 C/75% RH. The diffraction pattern of the 40 C/75% RH sample after2 months has a strong amorphous background and only two reectionsof crystalline erythritol at about 24.6 and 28.3 (2). Until 3 months, thediffraction pattern has a strong amorphous background and three reec-tions at about 19.6, 20.3, and 37.58 (2), while the diffraction patterns ofthe 25 C/60% RH samples after 2 and 3 months showed a slightly amor-phous background and almost all of reections of crystalline erythritol.

Bangyekan et al. [40] prepared chitosan-coated starch lm by coatingchitosan solution on cassava starchlm containing glycerol as a plasticiz-er. Amixture of cassava starch dispersion (6%w/v in water) and glycerol

used as a plasticizer was heated at the starch gelatinization temperature

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2starch lm. The optimum gloss value is achieved when applying


at maximum load and tensile modulus, of coated starch lms, where-as % elongation at break tends to decrease. In addition, with increas-ing CS coating concentration, a remarkable decrease in % elongation

Fig. 2. The relationship between gloss and chitosan coating content of coated lmscontaining 2, 3, 4, 5, and 6 wt.% glycerol [40].

4 M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxx4 wt.% chitosan coating solution indicating a complete coverage bychitosan layer with an increase in chitosan coating concentration. Itcould be concluded that the smoother surface of chitosan lmenhanced the regularity of coated lm surface leading to increasingin the gloss value.

Transparency may be affected by various factors including lmthickness. The percent transmittance values of coated lms includingstarch and chitosan free lms are presented in Fig. 3. The transmit-tance of chitosan lm is slightly higher than that of starch lm. Thesmoother surface combined with relatively more amorphous struc-ture of chitosan lm (from X-ray evidence) may be responsible forthis transparency change.

The surface hydrophilicity of the chitosan-coated starch lm wasevaluated by means of contact angle determination. The contact anglesof coated lms including starch and chitosan free lms are shown inFig. 4.

When considering the particular glycerol content, the free starchlm exhibits the smallest contact angle. It can be clearly seen thatan increase in concentration of chitosan coating solution broughtabout a signicant increase in contact angle values of the coatedlms. These results indicate that the wettability of the coated lmsdecreased with an increase in the chitosan coating concentration.This phenomenon was attributed to the higher hydrophobicity of chi-tosan surface layer, which was attributed to the role of availableFig. 3. The relationship between % transmittance and chitosan coating content of coat-ed lms containing 2, 3, 4, 5, and 6 wt.% glycerol [40].

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2residual hydrophobic acetyl groups present in chitosan chain. Thisnding suggests that chitosan lm, with this particular degree ofdeacetylation, was more hydrophobic than starch lm.

Mechanical properties are very important for edible lms andcoating to improve mechanical handling of foods [41] or pharmaceu-tical products [42].

The effect of CS coating contents on the tensile properties of CS-coatedcassava starchlms in bothmachine direction (MD) and transverse direc-tion (TD) has been investigated [40]. As shown in Fig. 5, it was found that,at individual glycerol content in the coated lm, there is a signicantchange of the tensile stress at maximum load and tensile modulus inboth the MD and TD upon increasing the amount of CS coating.

The tensile stress and tensile modulus values in MD were higherthan those values in TD. This is probably because polymer chain ofchitosan aligned along the MD of automatic lm applicator during ap-plying force to wire bar coater. The % elongation at break values inboth directions tended to be lower than that of the uncoated or freestarch lms. In addition, % elongation at break in MD was found tobe lower than in TD. Therefore chitosan improves the tensile stress

Fig. 4. Effect of chitosan coating contents on contact angles of coated lms containing2, 3, 4, 5, and 6 wt.% glycerol [40].at break compared to coated lms containing 3, 4, 5, and 6 wt.% glyc-erol may be attributed to the less plasticizing effect due to the mini-mized concentration of plasticizer in starch base lm, including theeffect of brittleness of chitosan lm.

It is observed that there is a little effect on tensile strength of thecoated lm containing 5 and 6 wt.% glycerol upon increasing CScoating concentration. Fig. 6 presents the effect of CS coating contentson tensile properties of coated lm containing 5 wt.% glycerol in bothdirections. At 6 wt.% glycerol, although the improved tensile proper-ties of coated lms in MD and TD are attributed to an increase in CScoating concentration, the greatest tensile strength values obtainedfrom 4 wt.% CS coating are relatively low, i.e., in MD, the tensile stressat maximum load and tensile modulus are about 1.2 and 11.7 MPa,respectively. The % elongation at break in both directions of coatedlms containing high glycerol content, especially in TD, tended to in-crease with increasing the amount of chitosan coating.

Mathew and Abraham [43] modied starchchitosan blend lmsby incorporating ferulic acid, to nd possible application as an ediblelm or coating.

Ferulic acid ((E)-3-(4-hydroxy-3-methoxy-phenyl)prop-2-enoicacid) is an abundant phenolic phyto-chemical found in plant cell wall


%gly

5M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxcomponents, like many phenols, it is an antioxidant in the sense that itis reactive toward free radicals such as reactive oxygen species, it alsomay have pro-apoptotic effects in cancer cells, thereby leading to theirdestruction andmay be effective at preventing cancer induced by expo-sure to the carcinogenic compounds. The starchchitosan blend lmswere fabricated by means of a casting/solvent evaporation method.Ferulic acid was oxidized by adding different concentrations of ferulicacid (25, 50, 75, and 100) to hydrogen peroxide solution (0.1%, v/v)and kept at room temperature for 1 h under stirring.

Thermal analysis of the prepared lms was conducted to investi-gate the effect of ferulic acid on the stability of the lms since this isa vital property to be considered for their application in food andpharmaceutical industry as the edible lms may be subjected toheat processes during their preparation, processing or consumption[43]. The TGA curves of starchchitosan control lm, ferulic acid andferulic acid incorporated blend lms are shown in Figs. 7 and 8. Itcan be seen from these gures that incorporation of up to 100 mgof ferulic acid did not affect the thermal stability.

The surface of the control blend lms and ferulic acid incorporatedstarchchitosan lms were found to be relatively smooth andhomogenous.

The tensile strength (TS) values of ferulic acid incorporated starchchitosan composite lms have also been investigated [43]. Comparedto the control lm, the TS values of the blend lms increased to a

Fig. 5. Effect of chitosan coating contents on tensile properties of coatedlms containing5 wt.value of 62.71 MPawith the incorporation of ferulic acid at a concentra-tion of 75 mg as shown in Fig. 9. However, there was a slight reduction

Fig. 6. Effect of chitosan coating contents on tensile properties o

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2in strength as the ferulic acid concentration reached 100 mg. Increase inTS could be due to the formation of a stable network on account of thecross linkages introduced by ferulic acid. Ferulic acid can enhance thecross linking between polysaccharides through several mechanisms;through free radical mediated cross linking, by esterication with thehydroxyl groups of chitosan and starch or by quinone-mediated reac-tions [44]. There has been reports on the cross linking property of ferulicacid in the preparation of edible lms from soy protein isolate [44] andgelatin [45]. The TS increased with the increase in the level of cross-linking agent until the ratio of the ferulic acid/carbohydrate moietybecame too high. This may be attributed to the redundant hydroxylgroups which may interact with similar hydroxyl groups and reducethe attractive force.

Theexibility of thelm is indicated by the percentage elongation (E)value and it was found to be inuenced by the ferulic acid content. Theaverage E values of the lms decreased from 29.3% for the control lmto a minimum of 21.6% and 22.9% for the blend lms containing 75 and100 mg of ferulic acid (Fig. 9). The reduction in percentage elongationwith increase in ferulic acid content might be due to the increase in thenumber of intermolecular crosslinks and decrease in the intermoleculardistance.

Starch from two sources, tapioca and rice, has been used with chito-san to make blends with better qualities than from the individual poly-mers. Tapioca is a signicant crop in South America, [46]. Its edible

cerol. (A) Tensile stress atmaximum load and% elongation at break. (B) Tensilemodulus [40].lms exhibit appropriate physical characteristics, since they are odorless,tasteless, colorless and impermeable to oxygen. However, lms show

f coated lm containing 6 wt.%glycerol in MD and TD [40].


6 M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxbrittleness with inadequate mechanical properties. Chillo et al. [47] haveblended chitosan (CS) with tapioca starch and glycerol as a plasticizingagent. The apparent viscosity of the lm-forming solution, mechanicaland dynamic-mechanical properties, water vapor permeability (WVP)and color of the blend lms have been investigated. The mechanicalmeasurements and viscoelastic properties of the chitosan/tapiocastarch-based edible lms have been measured using a stress-controlledDynamic-Mechanical Analyzer equippedwith a tension clamp. Mechan-ical tests under static, transient and dynamic conditionswere performed,i.e. uniaxial tension, stress relaxation and oscillatory stress, respectively.The inuence of CS and glycerol concentrations on the properties of tap-ioca starch-based edible lm was analyzed. The obtained data inferredthat, the elasticmodulus (Ec) valueswere positively affected by the linearand quadratic terms of CS and glycerol contents, respectively, while neg-atively inuenced by glycerol and CSglycerol interaction. In addition, thetensile strength values were inuenced by the individual positive term ofthe CS and by the negative inuence of the CSglycerol interaction.

Previous studies have shown that the starchchitosan blend lmsto exhibit signicantly higher elongation values compared to lms

Fig. 7. Thermogravimetric curves of (a) blend lm; (bd) ferulic acid incorporatedlms (50, 75 and 100 mg); (e) ferulic acid and (f) potato starch [43].made from starch or chitosan alone [48].Xu et al. [31] observed a dependence of the elongation at break by

starch to CS ratio with a maximum value corresponding to a ratio of

Fig. 8. Derivative thermogravimetric curves of (a) blend lm; (bd) ferulic acid incor-porated lms (50, 75 and 100 mg); (e) ferulic acid and (f) potato starch [43].

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (21.5:1. Probably, in this study, the higher ratio starch to CSwas responsiblefor a negligible effect of the CS on the elongation at break.

A biodegradable or edible lm must withstand the normal stressencountered during its application and subsequent shipping and han-dling of the food to maintain its integrity and barrier properties [49].High tensile strength is generally required, but deformation valuesmust be adjusted according to the intended application of the lms.That is, whether it is undeformablematerial to provide structural integ-rity or reinforce structure of the food. The TS of biodegradable blendlms from rice starch/chitosan with different chitosan ratios is shownin Fig. 10A. The TS of biodegradable blend lms was affected by the chi-tosan ratios. The results demonstrated that the TS of biodegradableblend lms increased with the addition of chitosan, and the maximumoccurred at the rice starch and chitosan ratio of 1:1 and 0.5:1.0. The in-creasing TS values of the biodegradable blend lms, with the increase ofrice starch and chitosan ratios from 2:1 to 0.5:1, are attributable to ahigh formation of intermolecular hydrogen bonding between NH2 ofthe chitosan backbone and OH of the rice starch. The amino groups(NH2) of chitosan were protonated to NH3+ in the acetic acid solution,whereas the ordered crystalline structures of starch molecules weredestroyed with the gelatinization process, resulting in the OH groupsbeing exposed to readily form hydrogen bonds with NH3+ of thechitosan. However, the TS of biodegradable blend lm prepared at thestarch to chitosan ratio of 1:1 and 0.5:1 was not signicantly different.This phenomenon indicated the critical ratios of the greatest miscibility

Fig. 9. Effect of ferulic acid concentration on the tensile strength and percentage elon-gation of the blend lms [43].of the two main lm-forming components.Elongation at the break (E) is an indication of the lms exibility and

stretchability (extensibility), which is determined at the point when thelm breaks under tensile testing. The value of Ewas affected by the chito-san ratios (Fig. 10B). The average E values of the biodegradable blend lmbehaved inversely to the TS value, decreasing from 12.99% to a minimum8.06% when the rice starch and chitosan ratio was 0.5:1 (Fig. 10B).

E was reduced in the presence of chitosan, probably due to the in-creased crystallinity of starch in the blend lm.

Liu et al. [50] studied the mechanical properties of starch/chitosanblending membrane. Elongation-at-break (E), values of chitosan/starchblendmembranes with the different starchmasses are shown in Fig. 11.The E membranes' values were affected by the starch contents. Thisphenomenon has also been reported by Xu et al. [31]. With the massof starch increasing, the E value of obtained membrane increased inthe initial stage until reaching amaximum afterwhich the curve startedbending downwards. The data in Fig. 11 could be explained as follows:with the addition of starch, the E value of blend membrane increasesdue to the formation of hydrogen bonds between NH3+ of the protonat-ed chitosan and OH of the starch. However, when the addition ofstarch was too high, the exibility of obtained membrane was lowered


7M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxand the E value also decreased for the brittle nature of starchmembrane[51]. Thus, it is comparatively difcult to form homogeneous starch/chi-tosan membrane with higher content of starch.

Effects of potassium sorbate (KS) and chitosan incorporation onthe tensile strength (TS) and elongation at break (E) of sweet potatostarch lm was studied by Shen et al. [52] as seen in Fig. 12. It has

Fig. 10. Tensile strength (MPa) (A), and elongation at break (%) (B) as a function ofstarch:chitosan ratio [49].been observed that the E values of the lms signicantly decreasedwhen (KS) was added, and the higher the addition of potassium sor-bate, the lower the E of the lms.

Flores et al. [53] also veried the fact that (KS) could decrease the TSof tapioca-starch edible lms. This was attributed to the interaction be-tween potassium sorbate and the starch molecules resulting in a modi-cation of the starch network in the lms. It is a well-known fact thatthe mechanical behavior of starch lms is affected by the presence ofcrystalline phases. Fama, et al. showed that the control lms havehigher degree of crystallinity than those with potassium sorbate (KS),

Fig. 11. Elongation-at-break (E) values of chitosan/starch blend membranes with thedifferent chitosan masses [50].

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2which contributed to the better mechanical behavior of the controlstarch lm [54]. This was also proved by FT-IR spectra analysis.

The addition of chitosan signicantly improved both TS and E ofsweet potato starch lms. The TS of lms with 5% and 10% chitosanwas lower than those of lms with 15% chitosan, but no signicantdifference was observed between TS of lms with 5% and 10% chito-san. Films containing 10% and 15% chitosan had higher values of Ethan those with 5% chitosan.

Gallstedt and Hedenqvist [55] studied the mechanical and barrierproperties of pulpberchitosan sheets. They used ve methods toprepare pulpberchitosan sheets, optical micrographs representingthe produced sheets are shown in Fig. 13.

The results of their work show that, the fracture strain and the frac-ture stress decreased with increasing chitosan solution content. At chi-tosan solution contents above 50 wt.%, the shrinkage during the buffertreatment could be reduced effectively by the presence of pulp bers.The buffered sheets also had the highest Young's modulus.

Considering that chitosan is a disintegrating polysaccharide with asimilar structure to that of cellulose Phisalaphong and Jatupaiboon[56] studied the supplement of chitosan duringAcetobacter xylinum cul-tivation of cellulose. The aim of their study was to develop a new nano-structure lm composed of chitosan and cellulose. Microstructure andmechanical properties of the developed lms are then characterizedto provide indications for the modication of bacterial cellulosechito-san (BCC) lm. The blank sample of the bacterial cellulose (BC) lm isthe sample with 0% of chitosan content. In comparison to that of theBC lm, the tensile strength of BCC lms (samples containing chitosan)increased with an increase of chitosan content Fig. 14.

Fig. 12. Tensile strength (TS) and elongation at break (E) of sweet potato starch lmsas a function of potassium sorbate (K) and chitosan (C) addition. Different lowercaseletters in the same curve indicate signicant differences (pb0.05). Data shown inmeanstandard deviation (n=5) [52].The tendency of tensile strength of the re-swollen (wet) lms rathercorresponded to the dry lms but in a lower range. Fig. 15 shows that theYoung's modulus increased with the increase of chitosan supplementcorrespondingly to the effect on tensile strength.

Fig. 16 shows that in contrast to the effect on tensile strength, thepercentage of elongation at break decreased with increasing chitosanconcentration.

Rice is the most widely consumed basic food in the world [49]. Eachyear over 500 million tons of rice are harvested, providing sustenanceto many countries and people throughout the world. The unique proper-ties of rice starches are found in its many varieties. Due to different cli-mates, soil characteristics and cultures, over 240,000 registered varietiesof rice exist in the world. These varieties lead to a wide range of ricestarches with many different characteristics including: different startinggelatinization temperatures, textures, processing stabilities and viscosi-ties. Rice starch and its major components, amylose and amylopectin,are biopolymers, which are attractive raw materials for use as barriersin packaging materials. They have been used to produce biodegradablelms to partially or entirely replace plastic polymers because of its low


cost and renewability, aswell as goodmechanical properties [31]. Howev-er, wide application of starch lm is limited by its mechanical properties

Color of the packaging is also an important factor in terms of gen-eral appearance and consumer acceptance [59]. The results of themeasurements performed on the blend lm's color were expressedin accordance with CIELAB system and the rectangular coordinates;

Fig. 13. Optical micrographs representing sheets produced according to (1) Method 1 (10 wt.% chitosan solution), (2) Method 2 with 20 wt.% chitosan solution and pressure(3) Method 4 with 90% chitosan solution and pressure. The sides of the micrographs correspond to 16.5 cm (A) and (B) and 8.25 cm (C) [55].

8 M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxand efcient barrier against low polarity compounds [57]. This constrainthas led to the development of the improved properties of rice-based lmsby modifying its starch properties and/or incorporating other materials.Starch was blended with different proteins to decrease the water vaporpermeability of the lms and to increase their tensile strength [58].

BourtoomandChinnan [49] prepared biodegradablelms using chito-san with starch. A series of lms was prepared by mixing 100 ml of thestarch solutions (1, 2, 3 and 4 g/100 ml) with 100 ml of the chitosan so-lution (2 g/100 ml). Sorbitol was added as 40% (w/w) of the total solidweight in solution. The mixtures were cast onto at, leveled, non-sticktrays to set. Film solubility (FS) is a very important property wherewater resistance is an important property of biodegradable or ediblelms for applications as food protection where water activity is high, orwhen the lm must be in contact with water during processing of thecoated food (e.g. to avoid exudation of fresh or frozen products). Howev-er, a high solubility may be an advantage for some applications. Film sol-ubility is advantageous in situations when the lms will be consumedwith aproduct that is heatedprior to consumption andmayalso be an im-portant factor that determines biodegradability of lms when used aspackaging wrap. Biodegradable blend lm produced from rice and chito-san maintain their integrity (i.e., did not dissolve or break apart) evenafter 24 h of incubation with gentle motion. This indicates that the ricestarch and chitosan intra and/or intermolecular network remained intactand only the monomers, small peptides, and non-protein material weresoluble [49].Fig. 14. The tensile strength of BCC lms in re-swollen (wet) form: (a) BCC-MW 30,000and (b) BCC-MW 80,000 and in dry form (c) BCC-MW 30,000 and (d) BCC-MW 80,000as a function of chitosan content (% w/v) in culture medium [56].

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2the main difference observed was that biodegradable blend lmswith higher content of chitosan had lighter color [49].

2.1.2. Chitosan and gelatin based edible lmsComposite edible lms and coatings can be formulated to combine

the advantages of each component. Whereas biopolymers, such as pro-teins and polysaccharides, provide the supportingmatrix, lipids providea good barrier towater vapor [60]. Since gelatin and chitosan are hydro-philic biopolymers with good afnity and compatibility, they areexpected to form composite lms with good properties [61]. Gelatin/chitosan blends have been used extensively for the production of scaf-folds and bi-layers for biomedical applications [62,63]. Rivero et al.[64] developed composite, bi-layer and laminated biodegradable lmsbased on gelatin and chitosan, to determine lm barrier andmechanicalproperties and to characterize their microstructure. The gelatin blendswith chitosan and chitosan gel to form wound healing materials [65].The blend has a smooth and homogeneous surface as revealed by SEMand X-ray measurements [66].

The effect of chitosan molecular weight (MW) and degree ofdeacetylation (DD) on the physicochemical properties of gelatin-based lms were studied [67]. Determination of the dynamicFig. 15. The Young's modulus of BCC lms in re-swollen (wet) form: (a) BCC-MW30,000 and (b) BCC-MW 80,000 and in dry form (c) BCC-MW 30,000 and(d) BCC-MW 80,000 as a function of chitosan content (% w/v) in culture medium [56].


9M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxviscoelastic properties (elastic modulus G and viscous modulus G) ofthe lm-forming solutions revealed that the interactions between gela-tin and chitosan were stronger in the blends made with chitosan ofhigher molecular weights or higher degrees of deacetylation than theblendsmadewith lower molecular weights or degrees of deacetylation.Fish gelatin lmsmodiedwith chitosan of highermolecularweights orhigher degrees of deacetylation had higher tensile strengths.

Films of chitosan and gelatin were prepared by casting their aque-ous solutions (pHb4.0) at 60 C and evaporating at 22 or 60 C (low-and high-temperature methods, respectively). The physical (thermal,mechanical and gas/water permeation) properties of these compositelms, plasticized with water or polyols, were studied [68]. An in-crease in the total plasticizer content resulted in a considerable de-crease of elasticity modulus and tensile strength (up to 50% of theoriginal values when 30% plasticizer was added), whereas the elonga-tion percentage increased (up to 150% compared to the originalvalues). The low-temperature preparation method led to the devel-opment of a higher percentage crystallinity of gelatin which resultedin a decrease, by one or two orders of magnitude, of CO2 and O2 per-meability in the chitosan/gelatin blends. An increase in the total plas-ticizer content (water and polyols) of these blends was found to beproportional to an increase in their gas permeability. This blendshould be a promising candidate for good edible biodegradable lms.

It has been found that gelatin origin plays a role in improving thephysical characteristics of lms with chitosan [69]. The data revealedthat the interactions between gelatin and chitosan were stronger inthe blends made with tuna-skin gelatin than in the blends made withbovine-hide gelatin. As a result, the sh gelatin chitosan lms were

Fig. 16. The elongation at break of BCC lms in re-swollen (wet) form: (a) BCC-MW30,000 and (b) BCC-MW 80,000 and in dry form (c) BCC-MW 30,000 and(d) BCC-MW 80,000 as a function of chitosan content (% w/v) in culture medium [56].morewater resistant (w18%water solubility for tuna vs. 30% for bovine)and more deformable (w68% breaking deformation for tuna vs. 11% forbovine) than the bovine gelatin chitosan lms. The breaking strength ofgelatin chitosan lms, whatever the gelatin origin, was higher than thatof plain gelatin lms. Bovine gelatin chitosan lms showed a signicantlower water vapor permeability (WVP) than the corresponding plainlms, whereas tuna gelatin chitosan ones were only signicantly lesspermeable than plain chitosan lm. Complex gelatin chitosan lms be-haved at room temperature as rubbery semi-crystalline materials. Inspite of gelatin chitosan interactions, all the chitosan-containing lmsexhibited antimicrobial activity against Staphylococcus aureus, a rele-vant food poisoning. Mixing gelatin and chitosanmay be ameans to im-prove the physico-chemical performance of gelatin and chitosan plainlms, especiallywhen using sh gelatin,without altering the antimicro-bial properties.

The effect of glycerol on themechanical andwater barrier properties,as well as on thewater solubility, of sh gelatinchitosan lmswas stud-ied by Koodziejska and Piotrowska [70]. The nal goal of their studywasto design biodegradable material with good mechanical and barrier

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2properties, suitable for packages ofmany kinds of food productswith dif-ferent acidities and contents of moisture.

More research should be directed to the exploitation of theseblends with potential applications for food protection.

2.1.3. Chitosan with alginate and carageenanAlginate is a linear copolymer extracted frombrown seaweeds known

as Phaeophyceae and composed of (14)-linked--D-mannuronate (M)and (14)-linked--L-guluronate (G) units. These units are arranged inG-blocks, M-blocks and alternating sequences of GM-blocks forming thepolymeric structure, where the sequential arrangement depends on dif-ferent factors such as the specie, age or parts of the seaweeds fromwhich this material was obtained [71]. It has been well characterized inboth the liquid and gel state, making this biopolymer unique comparedto other gelling polysaccharides. Alginates possess good lm-formingproperty, producing uniform, transparent, and water soluble lms.Alginate-based lms are impervious to oils and fats but, as other hydro-philic polysaccharides, have high WVP. However, alginate gel coatingcan act as a sacricing agent, where moisture is lost from the coating be-fore the food signicantly dehydrates. The coating can also improve theadhesion of batter to the surface of fruits and vegetables [72]. Alginatecoatings are good oxygen barriers that can retard lipid oxidation in vari-ous fruits and vegetables, and have been found to reduce weight lossand natural micro ora counts in minimally processed carrots [73]. Calci-um alginate coatings were found to improve the quality of fruits and veg-etables, such as reducing shrinkage, oxidative rancidity, moisturemigration, oil absorption, and sealing-in volatile avors, improving ap-pearance and color, and reducingweight loss of freshmushrooms in com-parison with uncoated ones [74].

Carrageenans are anionic linear polysaccharides extracted from redseaweed (Rhodophyceae), consisting of alternating -1, 4 and -1, 3linked anhydrogalactose residues. There are three major fractions ( kappa, iota and lambda) with varying number and position ofsulfate groups on the galactose dimer. Carrageenan-based coatingshave been applied to fresh fruits and vegetables such as fresh applesfor reducing moisture loss, oxidation, or disintegration of the apples[72,75]. In combination with anti-browning agents such as ascorbicacid, carrageenan-based coatings resulted in positive sensory resultsand reduction of microbial levels on minimally processed apple slices.By acting as a sacricial moisture layer, carrageenan coating was ableto protect moisture loss of grapefruits. In addition, -carrageenan lmscan effectively carry food-grade antimicrobials such as lysozyme, nisin,grape fruit seed extract, and EDTA for a wide range of applications as afood package material [76].

Both the alginates and the carrageenans can interact with chitosanforming polyelectrolyte complexes [77] which were used to obtainmicrocapsules for cell encapsulation and devices for the controlled re-lease of drugs or other substances. It seems there is good potential toinvestigate this interaction to produce edible lms from these mate-rials which could be of great value, however not yet explored.

3. Chitosan/essential oil lms

Many spices and herbs and their extracts possess antimicrobial ac-tivity. The composition, structure as well as functional groups of theoils play an important role in determining their antimicrobial activity[78]. Usually compounds with phenolic groups are most effective [79].Among these, the oils of clove, thyme, cinnamon, rosemary, sage andvanillin have been found to be most consistently effective against mi-croorganisms. Besides antibacterial properties [80,81] essential oils,Eos, or their components have been shown to exhibit antimycotic[82], antitoxigenic [83] and antiparasitic [84] properties. These charac-teristics are possibly related to the function of these compounds inplants [85]. A comprehensive review dealing with the more commonsynthetic and natural antimicrobial agents derived from essential oils

incorporated into or coated onto synthetic packaging lms for


antimicrobial packaging applications has been published by Kuorwelet al. [86]. The focus is on the widely studied herb varieties includingbasil, oregano, and thyme and their essential oils (EOs).

Because of the effect of direct addition of essential oils to food incor-poration of essential oils to lms may have supplementary applicationsin food packaging [87,88]. Combining antimicrobial agents such as plantessential oils directly into a food packaging is a form of active packaging.

Rosemary, a good source of antioxidant compounds, is widely used inthe food industry to prevent oxidative degradation of foods [8991]. Theantioxidant activity of rosemary extracts is associated with the presenceof several phenolic diterpenes, which break free radical chain reactionsby hydrogen donation. Oxygen pretreatment and chitosan coating0.03% rosemary extracts have improved the quality of fresh-cut pearsand extend the shelf-life [92]. The chitosanlmwas prepared by dispers-ing chitosan in an aqueous solution of 1% glacial acetic acid (v/v) at 4 C,1.5% glycerol (w/v) and 0.2% Tween-80 (v/v) were added to themixtureand the mixture was homogenized at 1000 rpm for 10 min. The pHvalue was adjusted to 5.6 with 1 M NaOH. The solution was strainedthrough layers of cheesecloth and degassed under vacuum at room tem-perature 0.03% rosemary extractwas added to thenal chitosan solution.The chitosan lm solutions were stored in a refrigeration condition(4 C) for 12 days [92].

In spite of the extensive use of chitosan as edible lms for coating, itstill suffers from high water vapor permeation which lowers its protec-tive action, therefore trials were made to add oils to increase its hydro-phobicity and improve its water vapor permeation. Chitosan was mixedwith increasing concentration of olive oils to prepare homogeneous

lms. Incorporating CEO at level of 0.4%, 0.8%, 1.5% and 2% (v/v)into chitosan lms increased the tensile strength values signicantly.The authors claimed that a strong interaction between the polymerand the CEO produced a cross-linker effect, which decreases the freevolume and the molecular mobility of the polymer. This phenomenonled to a sheet like structure as seen in (Fig. 17C).

Arrangement of stacking layers of CEO added chitosan sheets(Fig. 17D) means that in these lms a compact structure has formedleading to a decrease in elongation at break and increasing of the ten-sile strength. There are also possibilities that such structure enhancesthe decrease in moisture content of the lms incorporated with CEO.

The active component of CEO is cinnamaldehyde (~60%) [95].Chitosan control lms did not show inhibitory zone in bacterial

strains tested. Despite antimicrobial activity of chitosan because of itsinnate characteristic, this effect of chitosan occurred without migrationof active agents. Chitosan does not diffuse through the adjacent agarmedia in agar diffusion test method; so that only organisms in directcontact with the active sites of chitosan are inhibited [13].

Cinnamon oil has also been used by the same authors to blend withchitosan for the preservation of trout sh let [87]. This treatmentcould maintain trout let shelf life till the end of the storage period(16 days) without any signicant loss of texture, odor, color or overallacceptability and without signicant microbial growth. Variations inthe value of total viable counts (TVC) on the sh surface during therefrigerated storage are presented in Fig. 18. The initial TVC (log CFU/g)in trout let ranged from 3.51 in Ch+C-coated samples to 3.86 in con-trols. In the meantime, the control samples had a shelf life of only

10 M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxlms with decreasing moisture sorption, lower water vapor permeationthrough the lms and smaller effective diffusion coefcients of the lmsas the oil concentration increases [93]. All the tensile properties (Youngmodulus, strength and maximum elongation) increased with olive oilconcentration and were explained considering the interactions devel-oped between lipid and carbohydrate phases in addition to the lubricantcharacteristics of the oil [93].

Ojagh et al. [94] studied the effect of adding cinnamon essential oil(CEO) into chitosan-based lms. It was found that CEO increased theantimicrobial activity, while decreased the moisture content, the sol-ubility in water, the WVP and elongation at break of the chitosanFig. 17. Scanning electronic microscopic images of chitosan control lm (A) and (B), and lm

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (212 days. Therefore, chitosan coating together with cinnamon oil provedto be an efcientmethodof protecting thesh under refrigerated storagefor a longer period f time.

Bergamot essential oil (BO) is citrus oil (from Citrus bergamia),whose major chemical compounds are limonene (3245%) and linalool(around 10.23%) [9698]. The antimicrobial efciency of BO, and itscomponents, linaool and citral, have been found to be effective againstCampylobacter jejuni, E. coliO157, Listeriamonocytogenes, Bacillus cereus,S. aureus, Arcobacter butzleri and Penicillium digitatum [99], amongothers, both when oil is applied directly and when in contact with theoil vapor. Chitosan-based lms containing BO (CSBO) at 0.5, 1.2 andcontaining CEO at level of 1.5% (C) and (D) surfaces and cross sections, respectively [95].


11M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxx3%w/wwere prepared to evaluate their physical and antifungal proper-ties. Film-forming dispersions (FFD)were also characterized in terms ofrheological properties, particle size distribution and -potential. Fur-thermore, the antifungal effectiveness of CSBO composite lms againstPenicillium italicum was studied. Results showed that incorporation ofBO provoked a decrease in the water vapor permeability, this reductionbeing around50%when using a BOCS ratio of 3:1. Concerningmechan-ical and optical properties, CSBO composite lmswere less resistant tobreak, less deformable and less glossy. The loadparameters (TS and EM)decreasedmore than 50% and the percentage of elongation at breakwasalso dramatically reduced from 22% to 5%, as compared with the purechitosan lms. CSBO composite lms showed a signicant inhibitoryeffect on the growth of P. italicum, which depended on the BO concen-tration. Chitosan lms with the maximum bergamot oil content (3:1BOCS ratio) led to a total inhibition of the fungus growth during therst 5 days at 20 C.

Although the antifungal effectiveness of the lms decreased through-out the storage time, a signicant reduction of 2 log units as comparedwith the control remained possible, after 12 days at 20 C, using thehighest BO content. The mechanisms by which essential oils bringabout their antimicrobial effect are not clear, Holley and Patel [78] hadsuggested that terpenes have the ability to disrupt and penetrate thelipid structure of the cell membrane, as well as the mitochondrial mem-brane, leading to the denaturation of proteins and the destruction of cellmembrane, cytoplasmatic leakage, cell lysis and eventually, cell death.

Fig. 18. Changes in total viable counts (TVC) of sh samples during refrigerated storage[87]. (For interpretation of the references to color in this gure legend, the reader is re-ferred to the web version of this article.)The essential oil of Melaleuca alternifolia, also named as tea tree oil(TTO), is a complexmixture of terpen hydrocarbons and tertiary alcohols[100]. Themain compounds responsible for the antimicrobial activity areterpinen-4-ol and 1, 8-cineole, TTO has been used successfully in themanagement of oral candidiasis in AIDS patients [101] and other oralfungal infections in patients suffering from advanced cancer [102].Sanchez-Gonzalez et al. [100] incorporated TTO into chitosan matrixand investigated the physical and antibacterial behavior of the obtainedcomposite. The CS lms were rough and a reduction of its gloss occurredafter the incorporation of TTO. Water vapor permeability was also re-duced by 40% when the CS/TTO ratio was 1:2. Likewise, the lms' resis-tance to break was notably reduced by TTO incorporation due to thepresence of discontinuities in the lm matrix that affect its mechanicalresponse. The poor mechanical properties obtained by the addition ofTTO may be related with the structural arrangement of the lipid phaseinto the chitosan matrix. Thus, the structural discontinuities provokedby the incorporation of the oil could explain the lowest resistance to frac-ture of the composite lms. Some of these results are in line with thosereported by other authors when adding oils to a chitosan matrix[103,29] but differ in some aspects due to the great inuence of several,widely studied factors related to CS preparation. Only the composite

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2lmswith TTO:CS ratios higher than 1 showed a limited antifungal effec-tiveness against Penicillium, which was notably reduced after 3 days ofstorage. Nevertheless, CS lms presented a signicant antimicrobial ac-tivity against L. monocytogenes and the incorporation of TTO in the CS/TTO ratio of 1:2 improved the antibacterial properties of these lms,showing a complete inhibition of the microbial growth during the fthday at 10 C.

Synergism and antagonism between components of EOs and foodconstituents require more study before these substances can be reli-ably used in commercial applications [104].

Biodegradable coatings based on chitosan (CS) with and withoutbergamot essential oil were applied to table grapes, cv. Muscatel in-vestigated by Snchez-Gonzlez et al. [105] in order to nd environ-mentally friendly, and healthy treatments with which to betterpreserve fresh fruit quality and safety during postharvest cold stor-age. Physicochemical properties (weight loss, Brix, total phenols, an-tioxidant activity, color and texture), respiration rates and microbialcounts of samples were determined throughout cold storage whenusing CS. CS coatings containing bergamot oil produced the most ef-fective antimicrobial activity, and showed the greatest inhibition ofthe respiration rates in terms of both O2 consumption and CO2 gener-ation. Although the coatings did not seem to reduce the rate of grapebrowning during storage, they inhibited color development, thus im-proving the product appearance. Taking into account the overall re-sults obtained, the most recommended coating for Muscatel tablegrape is the CS containing bergamot oil.

An intensive study has been conducted by Gmez-Estaca et al. [106]in which chitosangelatin lms, containing sorbitol and glycerol as plas-ticizers were incorporatedwith several different essential oils and testedfor their antibacterial behavior against 18 different bacterial strainswhich included some important food pathogen and spoilage bacteria.Clove essential oil showed the highest inhibitory effect, followed by rose-mary and lavender. Clove and thyme essential oils were the mosteffective food preservatives, when tested on an extract made of sh.The gelatin/chitosan-based ediblelms incorporatedwith clove essentialoil were tested against six selected microorganisms: Pseudomonasuorescens, Shewanella putrefaciens, Photobacterium phosphoreum,Listeria innocua, E. coli and Lactobacillus acidophilus. The clove-containing lms inhibited all these microorganisms irrespectively ofthe lm matrix or type of microorganism. When the complex gelatinchitosan lm incorporating clove essential oil was applied to sh duringchilled storage, the growth ofmicroorganismswas drastically reduced inGram-negative bacteria, especially enterobacteria, while lactic acid bac-teria remained practically constant for much of the storage period [106].

Mayachiew et al. [107] studied the effect of drying methods andconditions (i.e., ambient drying, hot air drying at 40 C, vacuum dry-ing and low-pressure superheated steam drying (LPSSD) within thetemperature range of 7090 C at an absolute pressure of 10 kPa) aswell as the concentration of galangal extract on the antimicrobial ac-tivity of edible chitosan lms against S. aureus. Galangal extract wasadded to the lm forming solution as a natural antimicrobial agentin the concentration range of 0.30.9 g/100 g.

Galangal similar to ginger and turmeric is a member of the rhi-zome family. Rhizomes are knobby underground stems that areknown for their pungent and avorful esh, it is a traditional spiceused extensively for avoring and medicinal purposes. Galangal ex-tract has also proved to be an effective natural antimicrobial agentagainst some food poisoning bacteria, e.g., S. aureus [108]. The maincompounds of galangal extract are the terpenes, which have potentialantimicrobial activity [78,106,109].

Chitosan lms containing galangal extract at 0.6% and 0.9% (w/w)were effective in inhibiting the growth of S. aureus. No inhibition zonewas observed when the extract concentration of 0.3% (w/w) wasused. This could be ascribed to a limited galangal extract release proba-bly due to interaction between the extract and chitosan. Another possi-

ble reason could be the limit of detection of antimicrobial activity when


using the disk diffusion method [13,110]. It was noted that dryingmethods and conditions had signicant effects on the antimicrobial ac-tivity of chitosan lms incorporated with galangal extract. The resultsshowed that ambient dried lm had the highest antimicrobial activity;this was followed by LPSSD lms and vacuum dried lms. This may bedue to the fact that the lm temperature increased more rapidly andstayed at higher levels in the case of vacuum drying than in the caseof LPSSD, thus inducing more thermal degradation of the antimicrobialcompound [104,111]. In addition, different intermolecular interactionsalso contributed to the observed results. The decrease in bacteria inhibi-tion might be due to lower diffusion of the active agent into the agarmedium as a result of higher interaction between chitosan and galangalextract. The antimicrobial lms prepared at higher drying temperatureshad lower antimicrobial activity, both in the cases of vacuumdrying andLPSSD. The antimicrobial lms prepared by LPSSD at 70 C had thehighest antimicrobial activity compared with lms prepared at otherconditions of vacuum drying.

The results showed that the antimicrobial activity of the lms in-creased with an increase in the extract concentration, as expected.Chitosan lm incorporated with 0.9% (w/w) galangal extract and pre-pared by ambient drying could reduce the number of S. aureus byabout 3.6 log cycle within the contact time of 24 h. On the otherhand, ambient dried lm incorporated with 0.3% (w/w) galangal ex-

clude the addition of plasticizers such as glycerol which increases theexibility of the nal product [103]. The addition of other biodegradablealiphatic polyesters, such as poly-caprolactone (PCL), poly(butylenesuccinate) (PBS), poly(lactic acid) (PLA), poly(butylene terephthalateadipate) (PBTA), and poly(butylene succinate adipate) (PBSA), hasalso been investigated to produce materials with properties intermedi-ate between the two components [118].

Othermethods included the addition of layered silicates nanoparticles(e.g. sodiummontmorillonite) to chitosan to improve its end-use proper-ties such as barrier and mechanical properties [119]. Montmorillonite(MMT) is the most studied nanoscale clays. It is hydrated alumina-silicate layered clay made up of two silica tetrahedral sheets fused to anedge-shared octahedral sheet of aluminum hydroxide. Its advantages ofhigh surface area and platelet thickness of 10 make it suitable for rein-forcement purposes.

When nanoclay is mixed with a polymer, three types of composites(tactoids, intercalation, and exfoliation) can be obtained. In the case oftactoids, complete clay particles are dispersedwithin the polymermatrixand the layers do not separate.Mixing a polymer and organoclay forms amicro-scale composite, with the clay serving only as a conventional ller.Intercalation and exfoliation are two ideal nano-scale composites. Inter-calation occurswhen a small amount of polymer is inserted between the

12 M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxtract exhibited lower cell reduction number of around 2.0 log cycle.Sanchez-Gonzalez et al. [112] prepared antimicrobial lms by

incorporating different concentrations of tea tree essential oil (TTO)into chitosan (CS) lms. These lms have been tested againstL. monocytogenes and P. italicum. The possible antifungal effect againstP. italicum at 20 C of CS and composite lms was determined onpotato dextrose agar (PDA) medium and shown in Figs. 19 and 20. Thiseffectiveness was evaluated through the analysis of the growth (or sur-vival) of a determined infection level of P. italicum (105 spores/ml), thegrowth of fungus was followed by counts immediately after the inocula-tion and periodically during the storage period of (PDA) plates.

CS lms did not show antifungal effect for the assayed times. Pre-vious studies have demonstrated that the antimicrobial effect of chi-tosan depends on the type of microorganism, being mainly effectiveagainst bacteria and also against some molds and yeast [113].

The TTO composite lms delayed the fungal growth of P. italicum (incomparison to the control), which was dependent on the TTO concen-tration. At low TTO levels, no antimicrobial effect was observed. Onlywhen TTOCS ratio was higher than 1, a moderate inhibition of fungus

Fig. 19. Effect of CS and CSTTO composite lms on the growth and survival of Penicilliumitalicum on PDA medium stored at 20 C. Mean values and 95% LSD intervals for each

sample time [112].

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2growth was detected. The level of reduction of the Penicillium popula-tion in CS2TTOlms observed during therst 3 days of storagewas es-pecially remarkable, reaching a fungal reduction of 3 logs in comparisonwith the control plates. Nevertheless, the inhibition level of the compos-ite lms decreased throughout storage time. Considering that the anti-microbial activity of TTO has been probedwith very low concentrationsin the liquid phase, the observed behavior could be explained by theavailability level of active antimicrobial compounds against the fungiagent. Numerous studies have demonstrated that these compoundsare more effective in reducing microbial growth when incorporatedinto a lm or gel and applied to the product surface than when appliedon the surface via spray solution or directly added to the product[114117] because of the active substances ability to evaporate or dif-fuse into the medium.

4. Chitosan and clay

Natural polymers suffer from lower mechanical strength comparedto synthetic polymers and high moister barrier because of their hydro-philic nature. Many strategies have been explored to improve theseproblems of chitosan based biodegradable packaging lms. These in-

Fig. 20. Effect of CS and CSTTO composite lms on the growth and survival of Listeriamonocytogenes on TSA NaCl medium stored at 10 C. Mean values and 95% LSD inter-vals for each sample time [112].layers of the clay, thus expanding the interlayer spacing and forming a


andMMT, hindering the occulation and leaves theMMT stacks random-ly orientated in space. A higher reduction of permeability (50%) isobtained in chitosan lms not containing glycerol due to the alignmentandocculation ofMMT stacks. This system could have potential applica-tions in the packaging eld.

5. Gas permeation properties of edible coatings

Permeability is a steady-state property that describes the extent towhich a permeating substance dissolves and then the rate atwhich it dif-fuses through a lm, with a driving force related to the difference in con-centration of that substance between the two sides of the lm [128].

Gas permeability of edible lms and coatings depend on severalfactors such as the integrity of the lm, the ratio between crystallineand amorphous zones, the hydrophilichydrophobic ratio and thepolymeric chain mobility; the interaction between the lm-formingpolymer and the presence of a plasticizer or other additives are alsoimportant factors in lm permeability [129].

The measurement of permeability of oxygen and carbon dioxide ofthe edible lms provides important information for their further devel-opment. Oxygen is the key factor that might cause oxidation, inducingseveral unwanted food changes such as odor, color and avor, as wellas nutrients deterioration. Therefore, lms providing a proper oxygenbarrier can help in improving food quality and extending food shelf life[130]. Carbon dioxide that is formed in some foods due to deteriorationand respiration reactions should be removed from the package to avoidfurther food deterioration and/or package destruction [131]. Such lmscan maintain food quality and improve stability and shelf life by

13M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxwell-orderedmultilayer structure. In exfoliation, the layers of the clay areseparated completely and the individual layers are distributed through-out the polymer matrix. The formation of intercalation or exfoliationdepends on the types and amounts of nanoclay used [119].

Several reports deal with the preparation characterization of MMT/chitosan composites and lms [120126]. It has been shown that chito-san interact with MMT and forms a homogenous lm. Chitosan/MMTnanocomposites were prepared by an ion exchange reaction betweenwater soluble oligomeric chitosan and Na MMT. Chitosan showedhigh afnity toMMT clay host. According to thermogravimetric analysis(TG) and powder X-ray diffraction analysis the thermal stability of chi-tosan was remarkably improved due to the strong electrostatic interac-tion of cationic chitosan molecules with anionic silicate layers. It wasalso found that the nanocomposites showed a synergistic effect in theantimicrobial activity against to E. coli and S. aureus [127]. DelaminatedMMT were found to be enriched on the surface of the nanocompositeswhen the amount of MMT was >103 ppm. This was accompanied bya decrease of the contact angle [121]. The proliferation of broblastson MMT/CS 103 ppmwas signicantly greater than on other materials.The antimicrobial activity was enhanced markedly with the increasedamount of MMT. The inammatory responses of MMT in vitro and insubcutaneous rats were not obvious until the concentration of MMTwas >103 ppm. The biocompatibility of MMT/CS at 103 ppm waseven better than that of CS. The biodegradation rate of CS of the MMT/CS nanocomposite was much faster than that of the pure CS polymer.These results suggested potential antimicrobial applications for MMT/CS nanocomposites, especially for those containing 103 ppm (0.1%) ofMMT [121].

Fig. 21. Inuence of medium pH on the effects of chitosan, without chitosan, 0.005% chitosan, and 0.01% chitosan. Open symbols are pH 4 and closed symbolsare pH 6 (T=7 C). The lines represent the tted Baranyi-model [154].Montmorillonite (MMT) nanoclay and rosemary essential oil (REO)were incorporated into chitosan lm [120]. TheMMTweight percent rel-ative to chitosan was varied from 1 to 5 and was activated by three REOlevels (0.5%, 1%, and 1.5% v/v), and their impact on physical, mechanical,and barrier properties of the chitosan was investigated. The results ofthese investigation showed that the combined effect of adding MMTand REO improved the tensile strength and the antibacterial propertiesof the chitosan composites (lms). It was also found that theMMT/chito-san biocomposite particles exhibited a higher thermal decompositiontemperature compared to pure chitosan particles. The dual effect ofadding glycerol and MMT to chitosan has been investigated by Lavorgnaet al. [124]. The authors found that the mechanical properties ofnanocomposite containing glycerol are improved as clay loading in-creases. This is due to a combined effect of clays and plasticizer. Glycerolmodies the hydrogen-bonding network within the material and allowsbetter interaction between ller and matrix, thus facilitating the stresstransfer to the reinforcement phase and improving its mechanicalproperties. The addition of glycerol lowers the water permeability by(30%). Glycerol reduces the hydrogen interactions between chitosan

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2retarding unwanted mass transfer (O2 and CO2) in food products migra-tion of moisture for dried and intermediate moisture foods, and migra-tion of solutes for frozen foods.

There are several possible edible coatings for fruits, such as cellulose,casein, zein, soy protein, and chitosan. These were chosen since theyhave the desirable characteristics of generally being odorless, tastelessand transparent. It is not easy tomeasure the gas permeation propertiesof the coatings after being placed on fruits. Therefore, separate at lmsneed to be prepared and tested.

Fig. 22. The inuence of soluble starch on the antimicrobial activity of chitosan. Lagphase (A) and growth rate (B) of Candida lambica with different chitosan and starch

concentrations (T=7 C and pH=5). , 0% starch; , 1% starch; , 30% starch [154].


to the higher degree of cross linking that results from the formation ofquinones and free radicals which tend to promote the cross linking[133]. However, the permeability of the ferulic acid incorporated lmat a concentration of 100 mg increased slightly in comparison to theblend lm containing 75 mg of ferulic acid, but was lower than that ofthe control lm indicating 75 mg of ferulic acid as the optimal concen-tration of the cross-linking agent. Blend lms with higher TS had lowerWVT rates probably owing to the better degree of organization of thepolysaccharide network up to the optimum concentration of ferulicacid. They also studied oxygen transmission rates of chitosan/starchlms with different amount of ferulic acid. In general, polysaccharidelms are expected to be good oxygen barriers, due to their tightlypacked and ordered hydrogen-bonded network structure and low solu-bility. The results showed that the oxygen transmission rate reducedwith the increase in the level of cross-linking agent. Kucuk and Caner[134] have reported better stability and quality for sunower oilsamples stored under packaging conditions free of air. High levels of ox-ygen in food packages have been reported to cause the development ofoff-avors, off-odors and nutritional loss in food stuffs [135].

Since amain function of a food packaging is often to avoid or at least

14 M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxxFormulations based on polysaccharides and proteins have provedtheir excellent selective permeability to O2 and CO2. However, becauseof their hydrophilic nature they exhibit poormoisture barrier properties,which can be improved by adding hydrophobicmaterials such as naturalwaxes, acetylated monoglycerides and surfactants through emulsion orlamination technology. That is why edible lms usually are heteroge-neous in nature.

Literature data indicate that O2, CO2 and water vapor permeabilityof edible coatings are lower than the conventional plastic lms. Starchlms ability for food protection is by controlling and reducing oxygentransport, thus extending the shelf life of the food.

However, the incorporation of antimicrobial agents to starch lms in-creases O2 gas permeability thus incorporation of potassium sorbate tosweet potato starch lms, led to higher oxygen permeability [52]. Themass transfer of oxygen in a semi-crystalline polymer is primarily a func-tion of the amorphous phase, because the crystalline phase is usuallyassumed to be impermeable. Fama et al. [53,55] showed that the incorpo-ration of potassium sorbate decreased the crystallinity of tapioca-starchedible lms causing an increase in oxygen permeation (OP) of the starchlms. The incorporation of potassium sorbate weakened inter-molecularforces between adjacent starch polymeric chains, facilitated chain mobil-ity and increased the free volume between starch molecules, whichpromoted oxygenpermeability [132]. However, a better oxygen transpor-tation barrier property was obtained with the starch lms when chitosan

Fig. 23. The change of antibacterial activities of starch/chitosan blend lms with radi-ation dose (80% starch, 20% chitosan) [155].was incorporated. With 15% of chitosan, sweet potato starch lm had asignicantly lower OP (1.940.95106 cm3m/(m2dkPa)) thanthat of the control starchlm. Chitosan could form inter-molecular hydro-gen bonds with starch, which limited the inter-molecular chain mobilityand decreased its free volume, contributing to the decrease of OP. More-over, Xu et al. [31] reported that chitosan lmpossessed lowpermeabilityto oxygen.

Mathew and Abraham [43] determined the water vapor transmis-sion (WVT) of lms gravimetrically. The lms were xed on to thecircular opening of permeation cell-containing anhydrous calciumchloride (0% RH) using melted parafn. The cups were then weighedand placed at 92% relative humidity and 37 C in a humidity chamber.The cups were weighed at 1-h intervals until the change in weight be-came constant. The water vapor transferred through the lms at dif-ferent time intervals were determined from the weight gain of thecups. Changes in the weight of the permeation cell were recordedand plotted as a function of time. The slope of each line was calculatedby linear regression and WVT rate was calculated from the slope ofthe straight line (g/h) divided by the transfer area (m2).

Oxidized ferulic acid incorporated lms were found to signicantlydecrease the WVT compared to the control blend lms, probably due

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2to decreasemoisture transfer between the food and the surrounding at-mosphere, or between two components of a heterogeneous food prod-uct, WVP should be as low as possible.

Shen et al. [52] showed that theWVP of sweet potato starch lms de-creased signicantly with the addition of over 10% chitosan. This wasalso the case for tapioca-starch-based edible lm examined by Chilloet al. [47]. They explained the decreasing WVP transmission rate athigher concentrations of chitosan as a result of reducing the available hy-drophilic group [31,47]. In addition the hydrogen bonding interactionbetween chitosan and starch decreased the free sorption sites forwater, which is responsible for decreasing the WVP of the lm [136].

WVP of chitosan lms (CS) contain different concentrations of teatree essential oil (TTO) had been studied by Sanchez-Gonzalez et al.[100]. The room temperature conditions used for measuring the WVP(100/54.4) of the lms were established to simulate the environmentalconditions when the lms are applied as a coating for vegetables. WVPvalues were in the range of those reported by other authors workingwith lms based on chitosan [29].

The WVP values showed a signicant decrease in line with the in-crease in TTO concentration, reaching a maximum WVP reduction ofabout 40% with incorporation of 2% TTO in the lm-forming disper-sions. This behavior is expected as an increase in the hydrophobiccompound fraction usually leads to an improvement in the water bar-rier properties of lms, as was previously reported for essential oilsaddition in CS lms [96].

Fig. 24. The change of antibacterial activities of starch/chitosan blend lms with the

content of chitosan (50 kGy) [155].


lm

15M.Z. Elsabee, E.S. Abdou / Materials Science and Engineering C xxx (2013) xxxxxx6. Effect of electric eld on lm formation

Preliminaryworkshave shown that thepresence of amoderate elec-tric eld during the preparation of chitosan coating solutionsmay inu-ence their transport properties. If such effect is conrmed, moderateelectric elds could be used to tailor edible lms and coatings for specif-ic applications. Ohmic heating is based on the passage of electrical cur-rent through a sample where the electrical energy is directly convertedto heat and instant heating occurs. The use of electric elds in the foodarea has gained a new interest [137,138]. The application of electricelds has also been an important instrument among researchers inthe area of edible lms and coatings, and there are works showingthat the application of electric elds promotes a signicant improve-ment of several properties. Garcia et al. [139] analyzed the effect of anelectrical eld applied during drying on the microstructure and macro-scopic properties of lms obtained with different mixtures of chitosan(CS) and methylcellulose (MC). The analysis indicated that CS electri-cally treated lm exhibited a more ordered structure lower WVP andhigher Young's modulus values leading to stronger lms. The authorsconcluded that electrical eld treatment would be a good alternativeto improve lm exibility and water vapor barrier properties.

Lei et al. [140] reported that using ohmic heating for the productionof proteinlipid lms, improves the yield, of the lm formation rate andthe rehydration capacity of the lms. Souza et al. [141] determined the

Fig. 25. Inhibition area of (A) controleffect of eld strength on the functional properties of chitosan coatings.Four different eld strengths were tested, for each electric eld treat-ment, the water vapor, oxygen and carbon dioxide permeability of thelms formed were determined, together with their color, opacity andsolubility in water. Chitosan lms formed from solutions subjected toelectric elds at 100 V cm1 or higher were found to have lower valuesof O2P and CO2P (oxygen and CO2 permeability). Atomic force micros-copy AFM observation of chitosan lms surface treated at 100 V cm1

or above (conrmed by the Ra values) showed smoother surface as op-posed to a rougher surface of untreated lms.

AFM imaging modes can potentially provide structural informationfor a sample in itsmore natural state (without dehydration or coatings).Nanoscale measurements by AFM allow the inuence of different fac-tors on the hardness, elasticity and permeability of the lm surface tobe quantied, which is extremely useful for the design of high perfor-mance edible food packaging systems [142].

7. Antibacterial activity of chitosan and chitosan blends

The greatest losses in food are due to microbiological alterations.Many chemical and physical processes have been developed to preserve

Please cite this article as: M.Z. Elsabee, E.S. Abdou, Mater. Sci. Eng., C (2food quality. Among such processes, adequate packaging is a funda-mental factor in the conservation and marketing phases. Thus, packag-ing plays a prominent role in maintaining food quality. Antimicrobiallms and coatings have vitalized the concept of active packaging andhave been developed to reduce, inhibit or delay the growth of microor-ganisms on the surface of foods in contact with the packaged product[143,144].

Inmost fresh or processed foods, microbial contamination occurs at ahigher intensity on the food surface, thus requiring an effectivemicrobialgrowth control. Traditionally, antimicrobial agents are added directly tothe foods, but their activity may be inhibited by many substances inthe food itself, diminishing their efciency. In such cases, the use of anti-microbial lms or coatings can be more efcient than adding antimicro-bial agents directly to the food since these may selectively and graduallymigrate from the package onto the surface of the food, thereby high con-centrations being maintained when most necessary [114].

L. monocytogenes, a Gram-positive rod, is a bacterium that cancause illness in a variety of food products [145]. One food product ofgreat concern is the refrigerated, ready-to-eat (RTE) foods contami-nated with L. monocytogenes [146]. Eating foods contaminated withL. monocytogenes normally causes the disease listeriosis which ismore serious for elderly adults and adults with compromised im-mune systems and can cause meningitis [147]. In pregnant women,the disease may cause spontaneous abortions or stillborn babies.

and (B) AM incorporated lm [167].Greenwood [147] has studied the antimicrobial effect of chitosan, asan edible lm, that was dissolved in lactic acid or acetic acid againstL. monocytogenes on RTE roast beef. Chitosan with low and high mo-lecular weight (MW) of (4.7105 g/mol) and (1.1106 g/mol)were used to test the molecular weight effect on the antimicrobial ca-pacity. This study showed that the acetic acid chitosan coating wasmore effective in reducing L. monocytogenes counts than the lacticacid chitosan coating. The study indicated also that chitosan coatingscould be used to control L. monocytogenes on the surface of RTE roastbeef. However, it has been found that L. monocytogenes was able togrow on the surface of the RTE roast beef regardless of the chitosantreatments used. Coma et al. [13] also observed the ability ofL. monocytogenes to grow on the surface of cheese regardless of chito-san treatments. This could be due to the decreased antimicrobial ac-tivity of chitosan lms over time as a consequence of the decreasedavailability of amino groups on chitosan [147]. The L. monocy

Chitosan Based Edible Films and Coatings

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