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JFS E: Food Engineering and Physical Properties

Effects of Water-Glycerol and Water-Sorbitol Interactions on the PhysicalProperties of Konjac Glucomannan FilmsLLLLLAIAIAIAIAI H H H H HOONGOONGOONGOONGOONG C C C C CHENGHENGHENGHENGHENG, A, A, A, A, ALIASLIASLIASLIASLIAS A A A A ABDBDBDBDBD K K K K KARIMARIMARIMARIMARIM, , , , , ANDANDANDANDAND C C C C CHEEHEEHEEHEEHEE C C C C CHOONHOONHOONHOONHOON S S S S SEOWEOWEOWEOWEOW

ABSTRAABSTRAABSTRAABSTRAABSTRACTCTCTCTCT: K: K: K: K: Konjac glucomannan (Konjac glucomannan (Konjac glucomannan (Konjac glucomannan (Konjac glucomannan (KGM)-edible films wGM)-edible films wGM)-edible films wGM)-edible films wGM)-edible films wererererere pre pre pre pre prepareparepareparepared with differed with differed with differed with differed with different amounts of glyent amounts of glyent amounts of glyent amounts of glyent amounts of glycercercercercerol or sorol or sorol or sorol or sorol or sor-----bitol as a plasticizbitol as a plasticizbitol as a plasticizbitol as a plasticizbitol as a plasticizererererer. F. F. F. F. Films wilms wilms wilms wilms wererererere chare chare chare chare characteracteracteracteracterizizizizized bed bed bed bed by moistury moistury moistury moistury moisture sorption isothere sorption isothere sorption isothere sorption isothere sorption isotherm, and follom, and follom, and follom, and follom, and following conditioning atwing conditioning atwing conditioning atwing conditioning atwing conditioning atdifferent relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbi-different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbi-different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbi-different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbi-different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbi-tol and glytol and glytol and glytol and glytol and glycercercercercerol) wol) wol) wol) wol) wererererere found to plasticize found to plasticize found to plasticize found to plasticize found to plasticize Ke Ke Ke Ke KGM-based films with rGM-based films with rGM-based films with rGM-based films with rGM-based films with respect to their tensile prespect to their tensile prespect to their tensile prespect to their tensile prespect to their tensile properoperoperoperopertiestiestiestiesties. H. H. H. H. Hooooowwwwwevevevevevererererer,,,,,thertherthertherthermal prmal prmal prmal prmal properoperoperoperoperties and water sorption capacity (ties and water sorption capacity (ties and water sorption capacity (ties and water sorption capacity (ties and water sorption capacity (WSC) of polyWSC) of polyWSC) of polyWSC) of polyWSC) of polyol-plasticizol-plasticizol-plasticizol-plasticizol-plasticized Ked Ked Ked Ked KGM films wGM films wGM films wGM films wGM films wererererere found to ve found to ve found to ve found to ve found to vararararary withy withy withy withy withwater activity (water activity (water activity (water activity (water activity (aaaaawwwww), namely at lo), namely at lo), namely at lo), namely at lo), namely at low w w w w aaaaawwwww (<0.6), (<0.6), (<0.6), (<0.6), (<0.6), WSC and melting enthalpWSC and melting enthalpWSC and melting enthalpWSC and melting enthalpWSC and melting enthalpy wy wy wy wy wererererere decre decre decre decre decreased with increased with increased with increased with increased with increasing in polyeasing in polyeasing in polyeasing in polyeasing in polyolololololcontent and the opposite was true at higher content and the opposite was true at higher content and the opposite was true at higher content and the opposite was true at higher content and the opposite was true at higher aaaaawwwww (>0.6). This was attributed to extensive interactions between (>0.6). This was attributed to extensive interactions between (>0.6). This was attributed to extensive interactions between (>0.6). This was attributed to extensive interactions between (>0.6). This was attributed to extensive interactions betweenplasticizplasticizplasticizplasticizplasticizer and Ker and Ker and Ker and Ker and KGM that rGM that rGM that rGM that rGM that reduced the aveduced the aveduced the aveduced the aveduced the available activailable activailable activailable activailable active site (-OH gre site (-OH gre site (-OH gre site (-OH gre site (-OH groups) for water adsorption. oups) for water adsorption. oups) for water adsorption. oups) for water adsorption. oups) for water adsorption. The prThe prThe prThe prThe presence ofesence ofesence ofesence ofesence ofpolypolypolypolypolyols at lools at lools at lools at lools at low w w w w aaaaawwwww appear appear appear appear appeared to suppred to suppred to suppred to suppred to suppress cress cress cress cress crystalline strystalline strystalline strystalline strystalline structuructuructuructuructures due pres due pres due pres due pres due probably to robably to robably to robably to robably to restrestrestrestrestricted molecular mobilityicted molecular mobilityicted molecular mobilityicted molecular mobilityicted molecular mobility.....These effects were diminished when the moisture content was >20%.These effects were diminished when the moisture content was >20%.These effects were diminished when the moisture content was >20%.These effects were diminished when the moisture content was >20%.These effects were diminished when the moisture content was >20%.

Keywords: konjac glucomannan, edible films, equilibrium moisture content, sorption isotherm, plasticizers,Keywords: konjac glucomannan, edible films, equilibrium moisture content, sorption isotherm, plasticizers,Keywords: konjac glucomannan, edible films, equilibrium moisture content, sorption isotherm, plasticizers,Keywords: konjac glucomannan, edible films, equilibrium moisture content, sorption isotherm, plasticizers,Keywords: konjac glucomannan, edible films, equilibrium moisture content, sorption isotherm, plasticizers,tensile propertiestensile propertiestensile propertiestensile propertiestensile properties

Introduction

For certain biopolymer-based edible films, such as wheat glu-ten and hydrolyzed whey protein films, plasticizers play an im-

portant role in conferring workability and flexibility (Gontard andothers 1993; Sorthonvit and Krochta 2000). Plasticizers are capableof reducing film brittleness and modifying the molecular organiza-tion of the polymer network because they are able to decrease thecumulative intermolecular forces, increase intermolecular spacingand mobility of polymeric chains, and reduce polymer chain cohe-siveness upon incorporation (Banker 1966; Lieberman and Gilbert1973; Cuq and others 1997).

Apart from water, the most usual plasticizers are polyols, mono-,di-, and oligosaccharides (Cuq and others 1997). The effects of com-position, shape, polarity, chain length and number of active function-al groups on the ability of plasticizers to plasticize a polymer networkhas been intensively studied (Lieberman and Gilbert 1973; Donhoweand Fennema 1993; Cuq and others 1997; Gueguen and others 1998;Sorthornvit and Krochta 2001). Plasticizers with characteristics suchas small size, high polarity, more polar groups per molecule, andgreater distance between polar groups within a molecule generallyimpart greater plasticizing effects on a polymeric system. However,the selection of a plasticizer for a specified system is normally basedon the compatibility and permanence of the plasticizer and the de-sired physical properties of the films (Banker 1966; Donhowe andFennema 1993; Sorthornvit and Krochta 2001). When a plasticizer isadded to a film, it will generally induce a large decrease in filmstrength, elasticity, and barrier and thermal properties (such as glasstransition temperature) but increase film flexibility and extensibil-ity (Donhowe and Fennema 1993; McHugh and Krochta 1994; Cuq

and others 1997; Garcia and others 2000; Yang and Paulson 2000;Sorthornvit and Krochta 2001).

Water is a ubiquitous natural diluent, which plasticizes and/orantiplasticizes some films depending on the amount sorbed ontothe films matrix (Kumar and others 1991; Gontard and others 1993;Seow and others 1999; Chang and others 2000; Cheng and others2002). During initial hydration from the dry state, a biopolymer filmexperiences improved film elasticity as a result of extensive connec-tions established between water and polymer. With increasing hy-dration, film elasticity and cohesiveness decrease progressively be-cause water-polymer interactions develop to the detriment ofpolymer-polymer bonds. In a 3-phase system (water-plasticizer-polymer), the presence of the 2nd plasticizer normally has a decid-ing influence on the plasticizing action of the primary one (Lourdinand others 1997). This is supported by the work of Gaudin and oth-ers (2000), in which they reported that the classic plasticizing effectof water (an increase in oxygen permeability) was less at a low con-centration of sorbitol. Kumar and others (1991) demonstrated thesimilar effect, where the thermal properties of starch-polyethyleneglycol and starch-polyethylene glycol stearate were reported to bemoisture dependent. The nature of this effect was not entirelyunderstood because water content at a given relative humidityresults from the combined affinities of the macromolecular networkand the characteristics of the plasticizers (Lourdin and others 1997).

In this study, an attempt has been made to explore in muchgreater detail the effect of equilibrium moisture content on konjacglucomannan (KGM) film properties in a tertiary system. Attentionhas been focused on the sorption isotherm, thermal, and tensileproperties of the plasticized KGM films.

Materials and Methods

MaterialsMaterialsMaterialsMaterialsMaterialsPurified KGM (PROPOL A) was obtained from Shimizu Chemical

MS 20050370 Submitted 6/21/05, Revised 8/10/05, Accepted 11/16/05. AuthorsCheng and Karim are with Food Biopolymer Science Research Group, FoodTechnology Div., School of Industrial Technology, Univ. Sains Malaysia,Penang, Malaysia. Author Seow is a food technology consultant, Penang,Malaysia. Direct inquiries to author Karim (E-mail: nadhaz@streamyx.com).

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Corp., Mihara, Hiroshima, Japan. Glycerol-anhydrous (86% to 88%purity) from R&M Chemicals and D-Sorbitol (98% purity) from Flu-ka were used in this study.

Film preparationFilm preparationFilm preparationFilm preparationFilm preparationFilms were obtained by the casting method described by Cheng

and others (2002), where KGM (7 g on dry basis) and glycerol or sorbitol(0, 0.7, 1.4, 2.1, 2.8, or 3.5 g) was added to 1 L of distilled water in aNational® home blender (Model MX-595N). Blending was carried outfor 15 min. The mixture was then left to stand for 1 h at 30 °C, afterwhich 90 g portions were poured and spread onto a level, square per-spex plate fitted with rims around the edge to give a 16 × 16-cm film-forming area. The solutions were allowed to dry at room temperaturefor approximately 20 h. Films that formed were peeled off and kept ina desiccator over phosphorus pentoxide before being subjected toanalysis. The thickness of each film was measured at 9 different posi-tions and to the nearest 0.0001 mm using a micrometer model nr 49-61 (Testing Machines Inc, Amityville, N.Y., U.S.A.) with 16-mm-diacontact faces before mechanical tests was carried out. The thicknessof the films produced ranged from 0.03 to 0.04 mm.

Film characterizationFilm characterizationFilm characterizationFilm characterizationFilm characterizationFilms were characterized in terms of sorption, tensile, and ther-

mal properties. Preparation of samples for sorption isotherms andtensile measurements was carried out as described previously(Cheng and others 2002).

Differential scanning calorimetry (DSC) measurements were car-ried out using a DuPont 2910 differential scanning calorimeter (E.I.DuPont de Nemours & Co., Inc., Wilmington, Del., U.S.A.). Films werecut into small pieces (approximately 3 × 3 mm). The cut sample piec-es were equilibrated under vacuum for approximately 7 d over spec-ified saturated salt solution of known relative humidities (32%, 43%,

56%, 69%, 75%, 84%) at 30 °C (Greenspan 1977). Samples (8 to 10 mgeach) were hermetically sealed in aluminum sample pans using aninverted lid configuration. Samples were heated from room temper-ature (~25 °C) to 270 °C at a heating rate of 20 °C/min and an oxy-gen-free nitrogen flow rate of 30 mL/min. The transition tempera-tures (onset, To; and peak, Tp) and enthalpy of melting (�H) weredetermined from the thermograms obtained. Sample means werecalculated with 2 individually prepared films as the replicated exper-imental units and 2 subsamples tested from each film. Statisticalanalysis on the data was carried out as appropriate using Minitab forWindows, Release 10.1 (Minitab Inc., State College, Pa., U.S.A.).

Results and Discussion

Sorption isothermsSorption isothermsSorption isothermsSorption isothermsSorption isothermsFigure 1a and 1b show the moisture sorption isotherms of the

glycerol- and sorbitol-plasticized konjac glucomannan (KGM) films,respectively. The sigmoidal-shaped moisture sorption isotherms(type II according to classification scheme introduced by Brunauer[1945]) associated with complex or polymeric foods (Coupland andothers 2000) were evident for all film types studied. Moisture con-tent of the film increased slowly with progressive increase in aw upto ~0.5, after which it increased rapidly with a small increase in aw.

The effect of polyol concentration on the water sorption capacity(WSC) of polyol-plasticized KGM films was aw dependent. Over therange of aw from 0 to 0.6, incorporation of polyol significantly (P <0.05) reduced WSC of the films. Within this range, the effect (trend)of glycerol concentration on water sorption was not directly evidentin glycerol-plasticized films (Figure 1a). In contrast, in the sorbitol-plasticized KGM films, water sorption decreased when the sorbitolcontent was increased from 0% to 30%, while the difference between40% and 50% sorbitol was marginal (Figure 1b). At aw >0.6, WSC in-

Figure 1—Moisture sorptionisotherms (at30 °C) of konjacglucomannanfilms plasticizedwith (a) glyceroland (b) sorbitol atdifferent concen-trations (0% =control). (Typicalcoefficient ofvariation [C.V.]for quadruplicatemeasurementsdid not exceed12%.)

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creased rapidly when the plasticizer concentration was progressivelyincreased. Similar observations have been reported for methylcellu-lose-based films (Debeaufort and Voilley 1995), pullulan-based films(Diab and others 2001), gluten-based films (Gontard and others1993), and starch-based films (Lourdin and others 1997).

The reason for such a phenomenon might lie in the polar natureof polyols. At the 0 to 0.6 aw range, it is likely that polyols competewith water for active sites on the biopolymer chains, thus reducingthe adsorption surface for water sorption of the KGM films (Debeau-fort and others 1973). At higher aw levels, WSC was enhanced withincreasing plasticizer contents. This may be attributed to the cre-ation of hydrogen bonds between the plasticizer and water as wassuggested by Lourdin and others (1997). It is also known from theliterature that low-molecular-mass polyols with hygroscopic naturehave a relatively greater ability to associate with water comparedwith biopolymers (Chinachoti and Steinberg 1984, 1988; Baik andChinachoti 2001). Consequently, the plasticizer-polymer interac-tions may be diminished at high moisture content (Gontard and oth-ers 1993) and greater free volume was created, allowing the matrix tosorb more water and promoting water clustering at successively high-er hydration levels. This suggests that glycerol- and sorbitol-plasti-cized KGM films could serve as a reasonably good protective layeragainst moisture sorption for food products only if aw is less than 0.6.

On the other hand, the effect of plasticizer types (glycerol versussorbitol) on the WSC of KGM films was aw and concentration depen-dent. At low polyol concentration (10% and 20%) and aw (0 to 0.6) level,glycerol-plasticized KGM films (Figure 1a) sorbed significantly lower (P< 0.05) moisture content as compared with sorbitol-plasticized KGMfilms (Figure 1b). This could be attributed to the smaller molecular sizeof glycerol compared with sorbitol, which facilitated the penetration ofglycerols into the amorphous regions of the KGM network, as was sug-gested by Donhowe and Fennema (1993) and McHugh and Krochta(1994). This suggests that glycerol may reduce a larger number ofavailable -OH groups of the KGM for water adsorption, thus resultingin a relatively lower WSC compared with the sorbitol-plasticized films.In addition, according to Lieberman and Gilbert (1973), an increase inthe molecular dimensions of the plasticizer between polar groupswould increase the free volume between polymer chains. In this re-gard, to explain the relatively enhanced WSC of sorbitol-plasticizedKGM films at low moisture levels, one could attribute to sorbitol largermolecular dimensions between polar groups compared with glycerol,which may create greater free volume in the KGM network and en-hance water sorption of the films.

The effect of plasticizer types (glycerol versus sorbitol) on theWSC of KGM films was, however, diminished at a higher concentra-tion (30% to 50%) and/or aw level (>0.6). The glycerol-plasticizedKGM films (Figure 1a) at any particular level of incorporation and aw

exhibited a relatively higher WSC (P < 0.05) than the correspondingsorbitol-plasticized KGM films (Figure 1b). This could be attributedto the highly hygroscopic nature of glycerol as compared with sor-bitol, which draws relatively more water into the KGM network. Inaddition, on the same mass basis in both film types (glycerol- andsorbitol-plasticized), those films with the lower-molecular-weightplasticizer possessed a greater molar content of the plasticizer (Cuqand others 1997). Therefore, glycerol-plasticized KGM films shouldsorb relatively more moisture than sorbitol-plasticized ones, at anequivalent concentration and aw level.

In summary, the underlying mechanisms governing the WSC ofKGM films depend on many factors, such as the water activity of thesystem and degree of interactions between water molecules, plasticiz-ers, and polymers. The cumulative effect of these interactions on theWSC of KGM films is too complicated to be elucidated due to the com-plex interactions between polymer, polyol, and/or water molecule.

Thermal propertiesThermal propertiesThermal propertiesThermal propertiesThermal propertiesFigure 2 shows the typical DSC thermograms for the control KGM

films equilibrated at 84% relative humidity. Statistical analysis ofthe thermal data obtained revealed that transition temperatures(onset and peak) did not appear to be significantly (P > 0.05) affect-ed by moisture content and amount of plasticizer added to the sys-tem. However, melting enthalpy was significantly (P < 0.01) affectedby moisture, type of plasticizer, and plasticizer contents.

Figure 3a and 3b illustrates the variations in melting enthalpy as afunction of moisture content for glycerol- and sorbitol-plasticizedfilms, respectively. The moisture content of KGM films equilibrated atdifferent relative humidity was determined from the sorption curvesobtained previously. Figure 3a shows that the melting enthalpy of thecontrol and glycerol-plasticized films increased with increasing mois-ture content up to a critical point before decreasing. The critical pointat which a maximum (peak) was observed appeared to shift to high-er moisture content as glycerol content was increased, that is, shiftedfrom 18% to 26% moisture content as glycerol concentration increasedfrom 0% to 30%. In the case of sorbitol-plasticized films (Figure 3b) andthose containing 40% and 50% glycerol (Figure 3a), no such peak inmelting enthalpy was evident over the range of moisture content stud-ied, but melting enthalpy was increased with a progressive increase inmoisture content for all those films.

For both film types (except those with a peak evident), meltingenthalpy increased with successive hydration. This could be relatedto the enhanced mobility of molecular chain segments broughtabout by the plasticizing effect of water molecules. This effect mayhave reduced the activation energy for the formation of a stablecrystalline structure as was suggested by Arvanitoyannis and others(1996) and Garcia and others (2000). Thus, the increase in meltingenthalpy with progressive hydration may have been the result of theenhancement in long-range order crystalline structure formation.

On the other hand, for those film types where a peak was evi-dent (Figure 3a), a drop in melting enthalpy was observed withfurther humidification level (>20% moisture content). The sameexplanation mentioned earlier probably holds for the phenomenonobserved, namely a gradual increase in melting enthalpy with aprogressive increase in moisture content. With successive hydra-tion from the dry state, mobility of polymer chains may be en-hanced and better chain alignment promoted, and subsequentlycontributed toward higher crystallinity. The maximal enthalpyobserved may be reflective of a critical level, at which local osmoticdehydration of the biopolymer molecules could have taken place

Figure 2—Typical differential scanning calorimetry (DSC)thermogram of the control konjac glucomannan (KGM)films equilibrated at 84% relative humidity

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(Baik and Chinachoti 2001). This may be attributed to the highhygroscopic nature of glycerols compared with biopolymer, whichshows higher affinity to water molecules (Chinachoti and Stein-berg 1984, 1988; Baik and Chinachoti 2001). According to Baik andChinachoti (2001), this could subsequently reduce the ability ofpolymer molecules to form a more ordered structure. In addition,at this maximum melting enthalpy, polyols may be saturated withwater molecules. In the further hydration range, the later-sorbedwater fraction in either control or glycerol-plasticized films mayexhibit “liquid-like” properties, a result which is in agreement withthe findings of Arvanitoyannis and others (1996). The subsequentwater-polymer interactions probably may develop to the detrimentof polymer-polymer bonds as was suggested by Barie (1968). Thus,a reduction in melting enthalpy was observed.

Effect of plasticizer content on the melting enthalpy of glycerol-and sorbitol-plasticized films was found to be moisture contentdependent. In general, at any particular moisture content withinthe range of 0% to 20%, a decrease in melting enthalpy was ob-served with a progressive increase in plasticizer concentration. Theopposite trend was observed for those films contained more than20% moisture content. This suggests that interactions may haveoccurred between water and polyol molecules.

At any particular moisture content within the range of 0% to20%, the presence of glycerol or sorbitol may interfere with the for-mation of crystalline structures. This is supported with the work ofBaik and Chinachoti (2001), who reported that the ability of a poly-ol to inhibit crystallization might be explained by molecular hin-drance, or penetration of polyols into the amorphous regions of thepolymer network. In other words, local mobility of polymer chainmay be lowered and reorientation into a crystalline structure maybe inhibited. Both or the combined effects of the 2 may be respon-sible for the decrease in melting enthalpy with increasing plasticiz-

er content at low moisture content. On the other hand, at a highermoisture content (>20%), successive addition of plasticizer wasfound to increase the melting enthalpy of polyol-plasticized KGMfilms. This may be attributed to the plasticizing effect of polyolmolecules that enhanced molecular mobility and formation of astable crystalline structure. Thus, it appears that the melting en-thalpy of glycerol-plasticized KGM films were relatively more mois-ture sensitive than sorbitol-plasticized ones.

TTTTTensile prensile prensile prensile prensile properoperoperoperopertiestiestiestiestiesFigure 4 shows some sample tensile curves of the control and

polyol-containing KGM films that were equilibrated at 84% relativehumidity. It is interesting to note that moisture and plasticizer con-tents over the respective range studied had no significant (P > 0.05)effects on tensile strength, which fell within a range of 3.5 × 107 to5.5 × 107 Pa. This is rather unusual because plasticizer such as glyc-erol is known to reduce tensile strength at moderate concentration(for example, 30% [d.b.] for methylcellulose films [Donhowe andFennema 1993] and 17% [d.b.] for wheat gluten films [Gontard andothers 1993]). Nevertheless, there were some reported examples onthe lack of effect of plasticizers on mechanical properties of films.Yang and Paulson (2000) reported that a relatively higher effectiveglycerol concentration (about 60% d.b.) was needed to plasticizegellan film. Coffin and Fishman (1994) reported similar observationthat there was little change in tensile strength of a pectin/starchfilm (90:10 ratio) at glycerine levels up to 45% (d.b.); beyond 45%,modulus and tensile strength fell off rapidly.

Changes in elastic modulus (EM) and tensile elongation (TE) ofpolyol-plasticized KGM films as a function of moisture or plasticizercontents are shown in Figure 5 and 6. Overall, a decrease in EM (Fig-ure 5) was observed with increase in humidification. An exceptionwas observed at the lower range of humidification, in which EM was

Figure 3—Variations inmelting enthalpyas a function ofmoisture contentfor konjacglucomannanfilms plasticizedwith (a) glyceroland (b) sorbitol atdifferent concen-trations. (Typicalcoefficient ofvariation [C.V.] forquadruplicatemeasurementsdid not exceed10%.)

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raised with initial hydration. This is a common phenomenon asso-ciated with antiplasticization by water in biopolymer films previous-ly reported by many other researchers (Kumar and others 1991;Gontard and others 1993; Chang and others 2000). As describedpreviously (Cheng and others 2002), the basic mechanism behindthe antiplasticization is still not clear.

According to Coupland and others (2000), a polyol such as glycerol,has 2 plasticizing effects; 1st as a result of its presence in the film and2nd because its intensively hygroscopic character tends to draw addi-tional water into the matrix. Coupland and others (2000) suggestedthat the latter effect might be uniquely responsible for the plasticiz-

ing properties of glycerol. However, the water sorption and tensile dataobtained in this study goes against their suggestion. At low aw range (0to 0.6) the amount of water sorbed (ranged from 0% to 15%) was re-duced with increasing plasticizer content (Figure 1a and 1b). However,within the same moisture content, the plasticizing effect of polyol onEM and TE was not counteracted (Figure 3a, 3b, 4, and 5), eventhough a smaller amount of water was sorbed. On the other hand, athigher aw, WSC of polyol-plasticized KGM films increased drasticallywhen the plasticizer concentration was progressively increased. At thishigh moisture level, EM and TE were drastically reduced and in-creased, respectively. This was probably due to the cumulative and/or synergistic effect caused by glycerol and water. However, accordingto Mueller-Plathe (1991a, 1991b, 1992), such a cumulative effect istoo complicated to be elucidated due to the complex interactions be-tween polymer, polyol, and/or water molecule.

At particular moisture content, EM decreased with increasingplasticizer contents and the reverse was true for TE. Such effects arean indirect indication of the degree of plasticization exerted byglycerol and sorbitol. When plasticized KGM films were understress, water as well as plasticizers can act as a lubricant and im-prove flow between polymer chains.

Furthermore, at given moisture content, a higher sorbitol thanglycerol concentration was required to achieve a similar EM and TE.In other words, at any moisture and plasticizer content, glycerolimparted a greater plasticizing effect (relatively lower EM and high-er TE) on KGM films than that of sorbitol. This is because glycerolhas greater molar content per mass unit, as well as a smaller molec-ular size that would facilitate its penetration into the film matrix(Donhowe and Fennema 1993; McHugh and Krochta, 1994). Inconclusion, sorbitol-plasticized KGM films were stiffer and strongerthan glycerol-plasticized ones, thus more stress will be required toproduce a given amount of deformation (Banker 1966).

Figure 4—Sample tensile curves of the control and polyolcontaining konjac glucomannan (KGM) films equilibratedat 84% relative humidity. The sample length was 10 cm.

Figure 5—Elasticmodulus of konjacglucomannan filmscontaining differ-ent concentrationsof (a) glycerol and(b) sorbitol as afunction ofmoisture content.(Vertical barrepresents plus 1standard deviationfrom the mean.)

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Conclusions

The present study underlines the significant influence of “equi-librium” moisture content in altering the plasticizing effect of

polyols on KGM films. Incorporation of glycerol or sorbitol to KGMfilms did not significantly reduce film tensile strength but en-hanced their flexibility and extensibility. This resulted in stretch-able films suitable for use at moderately high relative humidity.

AcknowledgmentsLHC is grateful to Universiti Sains Malaysia for the award of a fellow-ship under the Academic Staff Training Scheme. This work wassupported by the 8th Malaysia Plan R&D grant under the Intensi-fication of Research in Priority Areas (IRPA) Programme of the Min-istry of Science, Technology and Environment, Malaysia.

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Figure 6—Tensileelongation of konjacglucomannan filmscontaining differentconcentrations of(a) glycerol and (b)sorbitol as afunction of moisturecontent. (Verticalbar represents plus1 standard deviationfrom the mean.)