Thermomechanical Behavior of Wheat Gluten Films: Effect of Sucrose, Glycerin, and Sorbitoll

Transcription

1 PROTENS Thermomechnicl Behvior of Whet Gluten Films: Effect of Sucrose, Glycerin, nd Sorbitoll GEORGE CHERAN, 2 ARSTPPOS GENNADOS, 3 CURTS WELLER, 3 nd PAVNEE CHNACHOT 2 ABSTRACT Cerel Chem. 72(1):1-6 Glss trnsition temperture (Tg) nd mechnicl nd wter vpor However, the Tg filed to chnge with solute composition nd exhibited brrier properties of whet gluten films contining glycerin, sucrose, low correltion with brrier nd mechnicl properties. nitil ddition glycerin-sucrose, nd glycerin-sorbitol were studied. Glycerin nd sucrose of glycerin incresed the WVP drmticlly. Sucrose decresed the WVP were immiscible; the whet gluten film contining both solutes showed (only slightly), but resulted in rigid nd frgile film. The gluten-glycerintwo seprte therml trnsitions (-58"C nd -51C, respectively). The sorbitol film t rtio of 15:3:3 (w/w), 16.7% moisture (wb), showed low temperture (low-7) trnsition (-58 C) ws due to glycerin-rich single effective Tg (-421C) nd intermedite vlues for tensile strength, region. The low-t tn 8 pek height influenced the tensile strength nd percent elongtion, nd WVP, which were between those of the 15:6: elongtion linerly nd the wter vpor permebility (WVP) curvilinerly. nd 15:4:2 gluten-glycerin-sucrose films. Reserch interest in the development of edible films nd cotings for use in food pckging nd preservtion hs recently incresed. The obvious ppeling chrcteristics of edible films include the renewble nture of their ingredients; the films' bility to function s crriers of food dditives (e.g., ntioxidnts, flvors); nd the potentil use of such films in the interior of heterogeneous food systems s selective brriers to the trnsport of vpors, gses, nd solutes. An dditionl nticipted benefit from the wide commerciliztion of edible films is the further utiliztion of severl commodities employed s sources of film-forming mterils. Comprehensive reviews on the film-forming properties of protein, polyscchride, nd lipid mterils hve been published (Guilbert 1986, Kester nd Fennem 1986, Krocht 1992). Protein films in prticulr hve been discussed in detil by Genndios et l (1994). Some commerciliztion of protein films hs been relized in collgei susge csings (Hood 1987); geltin phrmceuticl cpsules (Rose 1987); nd corn zein-bsed protective cotings for nutmets nd cndy (Alikonis 1979, Andres 1984). The formtion nd property evlution of whet gluten films hs been delt with in severl studies (Wll nd Beckwith 1969; Anker et l 1972; Okmoto 1978; Prk nd Chinnn 199; Aydt et l 1991; Gontrd et l 1992, 1993; Genndios et l 1993-d). n ll these studies, films were produced by drying cst queous ethnol solutions of whet gluten. A plsticizer, usully glycerin, ws dded to reduce film brittleness nd ensure the formtion of free-stnding films. Whet gluten films in dry stte were very effective oxygen brriers (Genndios et l 1993b). However, whet gluten films, nd protein films in generl, re poor wter vpor brriers becuse of the inherent hydrophilicity of the proteins (Krocht 1992). The incorportion of wxes, ftty cids, surfctnts, sugrs, etc., to crbohydrte nd protein films, either by mixing or s seprte lyer, my help in limiting the moisture migrtion (Kester nd Fennem 1986, Kester nd Fennem 1989, Rico-Pen nd Torres 199). Use of glycerin s plsticizer not only increses flexibility, but lso gives very high wter permebility. Modifiction of the mount of glycerin nd the ph hs decresed permebility, but not to desirble level (Gontrd et l 1993). Becuse plsticiztion increses diffusion through the film, it is possible tht replcing glycerin with lrger moleculr weight solute such s sucrose my help retrd diffusion (given 'Journl series 161, Agriculturl Reserch Division, nstitute of Agriculture nd Nturl Resources, University of Nebrsk-Lincoln. University of Msschusetts Agriculturl Experiment Sttion, Project MAS Deprtment of Food Science, University of Msschusetts, Amherst. 3 Deprtment of Biologicl Sciences Engineering, University of Nebrsk. o 1995 Americn Assocition of Cerel Chemists, nc. tht the rheologicl properties re cceptble). One of the objectives of this study ws to investigte this possibility. Glss trnsition temperture (Td) hs been n importnt prmeter in the study of synthetic polymers. Above the Tg, polymeric mterils exist in soft, rubbery stte, wheres below the Tg, polymers ssume glssy stte (Ferry 198). The vlue of Tg is governed primrily by chemicl composition nd the presence of dded plsticizers nd, secondrily, by structurl fetures, such s chin brnching, cross-linking, nd crystllinity (Rogers 1985). n the cse of hydrophilic synthetic nd nturl polymers, wter functions s plsticizer depressing the Tg (Slde nd Levine 1988, Slde et l 1989). Therml nlysis techniques such s differentil scnning clorimetry (DSC) nd dynmic mechnicl nlysis (DMA), re employed for mesurement of Tg. The mechnicl nd brrier properties of polymers t ny prticulr temperture re ffected by the vlue of Tg (Odin 1991). For exmple, it is expected tht the permetion of gs nd vpor molecules through film is higher bove Tg where polymer chins re more mobile (Rogers 1985). The ppliction of the glss trnsition concept cn be vluble id in edible film reserch. n the only relevnt study vilble, Koelsch nd Lbuz (1992), using DSC, determined the Tg of edible emulsion films mde from methyl cellulose nd ftty cids. They reported tht their films, nlyzed t mbient condition, were in the rubbery stte since recorded Tg vlues were in the pproximte rnge of-l 1 to 53 C. Thus, they resoned the higher permence of the film my be due to the rubbery stte. Glss trnsition behvior of whet gluten nd its glutenin nd glidin frctions hs been reported (Hoseney et l 1986, Cocero nd Kokini 1991, de Grf et l 1993). Klichevsky et l (1992) determined the effect of sucrose nd glycerin on the Tg of whet gluten. There hs been no work done, however, to relte these effects to the physicl properties of edible gluten brriers, such s wter vpor permebility nd mechnicl properties. The Tg of glycerin-plsticized whet gluten films is expected to increse when glycerin is prtilly replced with sucrose (Klichevsky et l 1992). Most likely, such n increse in Tg will be ccompnied by n improvement of the wter vpor brrier chrcteristics of the film. Evidence for this ssertion comes from study of strch-sucrose system where wter diffusivity decresed with the ddition of sucrose (Chinchoti nd Stengle 199). Diffusion is very importnt (often governing) step in the whole permetion process, nd substituting sucrose is expected to ffect it. The objectives of the present study were to exmine the effect of sucrose on the Tg, wter vpor permebility, nd selected mechnicl properties of whet gluten films nd to investigte the effects of using sorbitol in the film formultion. Vol. 72, No. 1,1995 1

2 MATERALS AND METHODS Regents Vitl whet gluten (Whetpro-8) with -82% (db) protein content (N X 6.25; vlue provided by the mnufcturer) ws donted by Ogilvie Mills Ltd., Quebec, Cnd. Glycerin nd sucrose, both of ACS grde, were purchsed from Fisher Scientific, Pittsburgh, PA. Ammonium hydroxide (4.96N solution in wter) nd D-sorbitol (>99% purity) were purchsed from Aldrich Chemicl Co., Milwukee, W. Preprtion of Film-Forming Solutions Whet gluten (15 g) ws mixed nd stirred with 95% ethnol (72 ml) nd glycerin (6, 4, 3, or g) or sorbitol (3 g) (Tble ) before the obtined mixtures were homogenized (Virtisher, Virtis Co., Grdiner, NY) for 5 min t 5, rpm. Subsequently, mixtures were heted nd stirred on mgnetic stirrer hot plte (model PC-32, Corning, nc., New York, NY) while slowly dding 14 ml of mmonium hydroxide (4.96N) to provide n lkline condition to disperse the proteins, nd wrm distilled wter (48 ml) in which sucrose (, 2, 3, or 6 g) ws previously dissolved. The dispersion of protein ws noted by n pprent decrese in solution viscosity, t which point the solution hd temperture of C. After removl from the hot plte, the solutions were kept t mbient conditions (25C) for 2-3 min to llow bubbling to stop before csting. Csting nd Drying Film-forming solutions were cst onto flt glss pltes using thin-lyer chromtogrphy spreder br (Desg, Brinkmnn Co., New York, NY) nd plced in n ir-circulting oven TABLE Weight Rtios of Glycerin, Sucrose, nd Sorbitol Added per 15 g of Whet Gluten in Gluten-Bsed Films Rtio Glycerol Sucrose Sorbitol Film (w/w) (g) (g) (g) Gluten-glycerin-sucrose 15:6: 6 NA Gluten-glycerin-sucrose 15:4:2 4 2 NA Gluten-glycerin-sucrose 15:3:3 3 3 NA Gluten-glycerin-sucrose 15::6 6 NA Gluten-glycerin-sorbitol 15:3:3 3 NA 3 'Not pplicble. -1 l T gluten:glycerin:sucrose 15:6: (sotemp, model 738F, Fisher Scientific, Pittsburgh, PA) set t 45C. Films were peeled off the pltes fter drying for 12 hr. Tg Mesurements A differentil scnning clorimeter (DSC model 1, Seiko nstruments, Torrnce, CA) nd dynmic mechnicl nlyzer (DMA model 11, Seiko) were used to determine the Tg of the conditioned films (25C, 5% rh, two dys). For DSC nlysis, -1-2 mg of the conditioned smples ws plced in n hermeticlly seled smple pn. The reference ws n empty pn. Both were heted from -12 to 15 C t 5 C/ min. The instrument ws clibrted using mercury nd indium s stndrds. Tg for ech type of film ws determined in duplicte nd ws identified s the midpoint temperture of shift in the bseline due to chnge in het cpcity upon glss trnsition (Fig. 1). The film for DMA nlysis ws cut into 12- X 5-mm strips. Three strips were pressed together mnully nd then clmped into the DMA furnce t both ends. The smple ws heted from -12 to 2C t 2C/min. Simultneously, sinusoidl force ws pplied to the center of the strip (three-point bending or flexure mode) t 1 Hz frequency with strin in the 1-,um rnge. Storge modulus (E'), loss modulus (E") nd loss ngle (tn 8) were recorded nd plotted ginst temperture for further evlution of therml trnsitions. Typicl plots re presented in Figure 2. The mesurements were done in triplicte with the Tg vlues mesured s the pek temperture of tn 6, nd were within 3% experimentl error. Tensile Testing Rectngulr strips 1 mm long nd 25.4 mm wide were cut for the tensile test. The film thickness mesurements of the tensile test specimens were tken to the nerest 2.54,um (.1 mil) with hnd-held micrometer (B.C. Ames, Wlthm, MA). Thickness of the individul specimens ws determined s mens of five mesurements tken over the length of the strip. t ws then used to estimte the cross-sectionl re. Before testing, film specimens were conditioned for two dys in chmber set t 25 C nd 5% rh. Tensile testing ws performed with universl testing instrument (UTM) (model TM, nstron Engineering Corp., Cnton, MA). The initil grip seprtion ws set t 5 mm. Crosshed speed nd chrt recorder were both set t 5 mm/ min. Tensile strength (TS) ws clculted by dividing the mximum (pek) lod developed during the test with the cross-sectionl re of the specimen. Percentge elongtion t brek (E) ws clculted by dividing film extension t the moment of rupture by 5 mm (initil guge length of specimen) nd multiplying by 1. TS nd E for ech type of film were determined in triplicte with individully prepred nd cst films s the replicted experimentl units nd eight subsmples (strips) tested from ech film replicte. C.) TE: MPERATURE [ CJ Fig. 1. Differentil scnning clo)rimetry (DSC) thermogrm of whet gluten-glycerin-sucrose film t rrtio of 15:6: (w/w) showing series of decreses in the bseline upon heting nd indicting chnge in het cpcity. 2 CEREAL CHEMSTRY Wter Vpor Permebility Mesurement Films specimens (7 X 7 cm) were cut from oven-dried films. Five thickness mesurements were tken on ech specimen, one t the center nd four round the perimeter. The men ws used s the specimen thickness in wter vpor permebility (WVP) clcultions. Before testing, ll film specimens were conditioned for two dys in chmber mintined t 25 C nd 5% rh. WVP of films ws determined grvimetriclly using modifiction of the ASTM stndrd method E96-8, known s the "cup method". Cups used for testing were similr to those described by Genndios et l (1994). Conditions of testing were 25C nd 5% rh grdient (1% rh inside cups; 5% rh outside cups). Wter vpor trnsmission dt were corrected to ccount for the resistnce of the ir gp (1 cm) between the underside of the film nd the wter surfce in the cup (Genndios et l 1994). WVP for ech type of film ws determined in triplicte with individully prepred nd cst films s the replicted experimentl units nd three subsmples (specimens) tested from ech film replicte.

3 Sttisticl Anlyses Clcultion of men TS, E, WVP, Tg, nd pek heights (tn 6) of trnsition vlues for ech type of film, nd comprisons mong them, were mde using the generl liner models procedure (SAS nstitute nc., Cry, NC). Within ech property, significnt clculted men vlues were compred using Duncn's new multiple-rnge test (Steel nd Torrie 198) t the =.5 level of significnce. RESULTS AND DSCUSSON The DSC thermogrm for gluten-glycerin-sucrose film t 15:6: (18.7% moisture) is shown in Figure 1. A shift in the bseline 3 x cx 1 x 11 iv L 13 x 1-1 i1 x lo-' - x 1-2 upon heting cn be observed, indicting tht chnge in het cpcity is likely to be cused by glss trnsitions. All other smples showed multiple trnsitions tht could not be ccurtely mesured by DSC becuse of very smll chnges in het cpcity. Thus, only the T. vlues obtined by DMA re reported here. From the DSC dt, none of the films showed ny ice melting endotherms. Tg for Gluten-Glycerin Films DMA thermogrms of vrious films re shown in Figure 2. The plots of tn 6, E', nd E" ginst temperture showed chnges in these prmeters s the films underwent therml trnsitions. Upon trnsition, there ws n exponentil drop in E', nd peking in E" nd tn 8. n gluten-glycerin films (Fig. 2), min low temperture (low-i) trnsition occurred t -62 C (initil nd finl tempertures, T, nd Tf, respectively, re presented in Tble ) nd slight shoulder, possibly indicting seprte trnsition, clled high temperture (high- i) trnsition. The Tg temperture t -62 C ws higher thn the reported vlue of -93 C for pure glycerin (Dvies nd Jones 1953). The higher vlues were probbly cused by gluten domins tht were miscible with glycerin, resulting in n incresed Tg due to n increse in the verge moleculr weight (Aklonis nd McKnight 1983). This vlue is supported by other studies of glycerin-contining systems (Hllberg nd Chinchoti 1992, Klichevsky et l 1992).. l 3 x "nn l b ; A 3 x 1[V- 73X;19 b x i1 i c ' tn x x 18 -l Tg for Gluten-Glycerin-Sucrose nd Gluten-Sucrose Films Films in which portion of the glycerin ws replced with 3 sucrose (gluten-glycerin-sucrose rtios of 15:4:2, 15:3:3, nd 15::6) were studied using DMA (Figs. 2b, 2c, nd 3, respectively). The low- T nd high- T trnsitions observed in Figure 2 lso pper in Figures 2b nd 2c. The low- T trnsition occurred 3 xlo- t pproximtely -58 C s observed from the tn 8 pek (temperture rnge -55 to -62 C), nd decresed in mplitude with decresing glycerin nd incresing sucrose content. The high- 1 1 x1 T trnsition occurred t pproximtely -5 to -12 C (Tj nd 1 Tf re listed in Tble ). This high-t tn 8 pek height incresed CO with sucrose content (Figs. 2 nd 3). n Figure 3 (gluten-glycerinsucrose 15::6), substitution of ll of the glycerin with sucrose resulted in the disppernce of the low-t pek completely, indi o cting tht the low- T trnsition ws ssocited with the presence of glycerin. The effect of glucose, fructose, nd sucrose on gluten ws reported by Klichevsky et l (1992). They noted tht broder trnsitions were observed in the presence of these sugrs, indi- 3 x 19 _. -1; 2 3 x 1 1 r- 3 x c ' 'r n7 _i., LV C x 19 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -t1 - ~~~~~~~~~E' ~~~~~ ~~~~tn 8 1 x 1 9 co 1 xlo 1X 18 t.1.,n7 A U 1 x x 1-' 1 x 1-1 i x 1 x 1-3 Fig. 2. Dynmic mechnicl nlysis thermogrms for whet glutenglycerin-sucrose films t vrious rtios (w/w) nd moisture contents (%, wb)., 15:6: (18.7%). b, 15:4:2 (14.4%). c, 15:3:3 (9.6%) conditioned t 5% rh nd 25C for two dys. Storge modulus (E'), loss modulus (E"), nd loss ngle (tn 6) were recorded nd plotted ginst temperture. El TABLE Glss Trnsition Temperture (Tl) of Whet Gluten Film with Glycerin, Sucrose, Glycerin-Sucrose, nd Glycerin-Sorbitol' Film Low-T High-T (w/w) [% wb] Tempertureb ( C) ( C) Gluten-glycerin-sucrose 15:6: [18.7%] Ti ± ± 5. Tg Tf ± ± :4:2 [14.4%] Ti ± ±.6 Tg ± ± 2.3 Tf ± ± :3:3 [9.6%] Ti Tg ± ± 1.5 Tf ± ± ::6[9.3%] Ti NOC Tg -5.. Tf Gluten-glycerin-sorbitol 15:3:3 [16.7%] Ti ± 3. Tg Tf -2.5 ± 2.4 Determintion mde by dynmic mechnicl nlysis. b-t = initil temperture, Tf = finl temperture, Tg = pek tn 6 temperture. 'Not observed. Vol. 72, No. 1,

4 cting possible phse seprtion in gluten. n our study, dt in Tble shows tht this could hppen; however the results were not consistent. t is interesting to note tht for gluten-glycerin-sucrose film t 15::6, the DMA thermogrm (Fig. 3) not only shows single tn 6 pek, but lso n unusul chnge in E'. After the glss trnsition t -5"C, E' incresed quite significntly. This my indicte hrdening effect, nd it occurred specificlly for smples within the 5-13% (wb) moisture rnge. n this rnge, it ws speculted tht sucrose crystllized s the mobility of the system incresed upon pssing through the glss trnsition region, thereby hrdening the smple. However, the DSC dt (Fig. 3b) showed little or no exothermic crystlliztion pek upon heting t 2C/ min. Additionlly, heting the smple to 12C (2C/min), quench cooling, nd then reheting (2 C/min) filed to show ny exothermic pek. So this possibility ws ruled out. Another possibility would be tht prticulr smple might hve lost some smll mount of moisture s the mteril ws undergoing glssy-rubbery trnsition (i.e., increse in diffusion through the polymer network). This loss of moisture would result in more glssy mteril, consequently incresing E'. t is not quite certin wht other mechnism could hve cused such n increse in E', but it is lso possible tht some cross-linking or other chnges occurred in the smple promoted by sucrose in this moisture rnge. Tg for Gluten-Glycerin-Sorbitol Film The replcement of some glycerin by sorbitol (mintining the gluten-glycerin rtio t 15:3) resulted in single trnsition t pproximtely -42C (Fig. 4). Considering tht the T7 reported for sorbitol is C (Oxford et l 199) nd for glycerin is -93C (Dvies nd Jones 1953), the gluten-glycerin-sorbitol film gve n intermedite vlue of -42C. Figure 4b shows only one tn 6 pek in the DMA thermogrm for gluten-glycerin-sorbitol film t 15:3:3, s compred to two for the gluten-glycerin-sucrose film t 15:6: in Figure 2. Compring sorbitol nd sucrose, both t the 15:3:3 combintion, Figures 4 nd 4b show tht sorbitol ws more comptible with glycerin nd gluten (one Tg) thn it ws with sucrose, which resulted in phse seprtion (two Tg). Thus, in this regrd, the contribution of sorbitol is more dvntgeous thn sucrose, since the mteril hs only one tn 6 pek. All Tg, including the initil nd finl tempertures for films conditioned t 5% rh nd 25C for two dys re presented in Tble. The dt from DSC showed results similr to tht from DMA (dt not shown). Moisture content vried gretly mong the different films, depending on the types of solute present. Even though the moisture content of the film decresed with dded sucrose, the low-t Tg did not chnge significntly. The high- T trnsition did vry only slightly with sucrose content; Tg ws x 1 i -13 x x x i C 9LO tn 6 E11 3 x 1 9 i3 x 1-1x19 lxl1-1 x 11-1 x 1-' Jlxl C - - E' tn6 1 x lo-2 \ El -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~- 1 x io8 1 x 1,, x lo-, 1 ri El' i 1 xi 1n9 i i -X x i 11 1 x x x TEMPERATURE [OC] 5 - o i b 1 x 111 3xlO 9 13x x 199,31 x 1-1 _x 1-1, x1 8 ji x 1-, lx x x1-12 Fig. 3. Thermogrms for whet gluten-glycerin-sucrose film t rtio of 15::6 (w/w), conditioned t 5% rh nd 25 C, moisture content 9.3% (wb). Dynmic mechnicl nlysis () records storge modulus (E'), loss modulus (E"), nd loss ngle (tn 6) plotted ginst temperture. Differentil scnning clorimetry (b) shows no ice melting pek. Fig. 4. Dynmic mechnicl nlysis thermogrms compring 15:3:3 (w/w) whet gluten-glycerin films contining sucrose () or sorbitol (b). Moisture contents (wb): 9.6 nd 16.7%, respectively. Storge modulus (E'), loss modulus (E"), nd loss ngle (tn 6) were recorded nd plotted ginst temperture. 4 CEREAL CHEMSTRY

5 TABLE Tensile Strength (TS), Percent Elongtion t Brek (E), Wter Vpor Permebility (WVP), Glss Trnsition Temperture (T.), nd tn 8 Pek Heights of Whet Gluten Films' tn 6 tn 6 Filmb TS E WVPC Tg ( C) Tg ( C) Pek Height Pek Height (w/w) (MP) (%) (X1-9 g/m.s.p) Low-T High-T Low-T (X 1-2) High-T (X 1-2) Gluten-glycerine-sucrose 15:6: 4.2 ± ± ± ± ± ±.3 1. ±. 15:4:2 5.6±.2b 39.±5.b 1.15 ±.3c -55.7±.1 b -11.5±2.3b 1.2±.b.9 ±.b 15:3: b 11. ± 1.8 c.95 ±.3 d ± 2.3 b -1.5 ± 1.5 b.7 ±.1 c.7 ±. c 15:: d 3.2 ±.6 c.18 ±.4 e NOe -5. ±. NO 1.5 ±.4 d Gluten-glycerine-sorbitol 15:3:3 5.7 ±.2 b 57.2 ± 4.4 b 1.22 ±.3 b ± ±.1 Reported property vlues for ech film re mens of three replictes ± stndrd devition. Mens followed by the sme letter re not significntly (P <.5) different ccording to Duncn's multiple rnge test. bmen thickness = 87 ± 9 tim. cmesured fter conditioning t 25 C nd 5% rh grdient (1/5%) for two dys. dsmple ws frgile nd crcked. enot observed. within the -5 C to -12 C rnge. For the gluten-glycerin-sucrose (15::6) film, single trnsition ws observed t -5 C tht corresponded to the sme rnge s the high-t trnsition. - 7 R 2 =1. Correltion of Mechnicl nd Brrier Properties with tn 6 Pek Height Tble compres the chnge in TS, E, WVP, Tg, nd tn 6 pek heights for the whet gluten films. Chnging the film composition by decresing glycerin content (or correspondingly incresing the sucrose content) of the films resulted in n increse in the TS, nd decrese in E, WVP, nd the low-t tn 6 pek height, except for the low TS observed for the gluten-glycerinsucrose (15::6) film, which ws frgile nd crcked. However, for the high- Ttrnsition, within experimentl error, the mplitude did not chnge pprecibly with chnging solute compositions (Figs. 2 nd 3). Visul correltions of the low-t tn 6 pek heights with TS, E, nd WVP re shown in Figure 5. TS nd E (Figs. 5 nd 5b) chnged linerly with pek height nd showed higher liner correltion (R 2 = 1. nd.98, respectively). This my indicte tht the low-ttrnsition (ssocited with glycerin plsticiztion) ws directly relted to chnges in the films' mechnicl properties. Figure 5c shows WVP s correlted with low-t tn 6 height; the reltionship ws curviliner. Thus, glycerin plsticiztion (directly proportionl to tn 6 pek height) hd significnt effect on the WVP of the film. t is interesting to note tht WVP is highly sensitive to the initil increse in low- Tpek height (Fig. 5c). This corresponded to smples with little or no glycerin dded initilly. Becuse this low-t trnsition is ssocited with glycerin, possible hypothesis is tht the gluten mtrix plsticized by glycerin resulted in chnges in its network (creting highly mobile chnnels) tht llowed moisture to penetrte esily t room temperture. Therefore, we conclude tht the chnges in mechnicl nd wter brrier properties re relted to the low- Ttrnsition. The low-t trnsition promoted by glycerin nd wter is most likely ssocited with the chnges in the structure of hydrophilic regions of film tht influence the wter movement through the film. n contrst to its excellent plsticizing property, glycerin is poor cndidte for improving WVP. Substitution of glycerin with sucrose decresed the wter mobility through the film (WVP decresed from 1.41 X 1-9 g/m.s.p to.18 X 1-9 g/m.s.p, for gluten-glycerin-sucrose films of 15:6: nd 15::6). Unfortuntely, this improvement in the WVP due to sucrose cnnot be fully utilized becuse of the poor mechnicl properties of sucrose-plsticized films. n ddition, sucrose redily tkes up wter when reltive humidity is high, cusing loss of film integrity. On the other hnd, sorbitol shows profound effect on the mechnicl nd brrier properties of the film. Sorbitol-contining films, with lower Tg, hve reduced WVP while mintining good mechnicl properties. However, from prcticl stnd- - r-4._ c6w CA *= cl u m P [ /.2 b. 1 t Low-T trnsition pek height Fig. 5. Plots of tensile strength, percent elongtion, nd wter vpor permebility (-c) with low-temperture trnsition pek height (pproximtely -58 C). 1 Vol. 72, No. 1,1995 5

6 point, the sorbitol-contining films still exhibited high WVP. Sorbitol hs higher plsticizing bility thn sucrose nd thus depresses the Tg of the system significntly more thn sucrose. This lowering of the Tg improves mechnicl properties of sorbitolcontining films s compred to those of sucrose. CONCLUSON The plsticizing effect of sucrose, glycerin, nd sorbitol on whet gluten films ws studied. Sorbitol showed good miscibility with glycerin nd gluten, resulting in single trnsition pek, intermedite to the trnsition of glycerin nd sorbitol. However, sucrose ws incomptible with the film components, resulting in phse seprtion. Two trnsitions were observed: low-t trnsition t pproximtely -58 C, nd high-t trnsition t -5 C. ncresing glycerin incresed the low-t pek mplitude, which indicted strong ssocition with glycerin or glycerinplsticizing mtrix. Mechnicl properties nd WVP correlted very well with the low-t trnsition tn 6 pek height (but not with Tg). The low-t temperture ssocited with glycerin ws relted to the opening up of the hydrophilic protein mtrix, llowing wter to diffuse, nd t the sme time plsticizing the film. Becuse of its high miscibility with glycerin nd gluten, sorbitol reduced WVP somewht. However, the resulting Tg of sorbitol ws very low, nd WVP ws still high from prcticl stndpoint. ACKNOWLEDGMENTS This work ws supported by the The Sugr Assocition, nc., Wshington, DC. We cknowledge support of uthor Genndios through n AACC-dministered Ntionl Strch nd Chemicl Compny Grdute Fellowship. LTERATURE CTED AKLONS, J. J Cndy Technology. Avi: Westport, CT. AKLONS, J. J., nd McKNGHT, W. J Trnsitions nd relxtion in morphous polymers. Pge 41 in: ntroduction to Polymer Viscoelsticity. J. J. Aklonis nd W. J. McKnight, eds. John Wiley & Sons: New York. ANDRES, C Nturl edible coting hs excellent moisture nd grese brrier properties. Food Process. 45(13):48. ANKER, C. A., FOSTER, G. A., nd LOADER, M. 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