Mechanical_properties_and_water_vapor_permeability_of_thin_film_from_corn_hull_arabinoxylan1

Mechanical Properties and Water Vapor Permeability of Thin Film from Corn Hull Arabinoxylan

Pingyi Zhang,Roy L.Whistler

Whistler Center for Carbohydrate Research and Department of Food Science,Purdue University,

West Lafayette,Indiana47907-2009

Received2March2004;accepted20April2004

DOI10.1002/app.20910

Published online in Wiley InterScience(19a802ef988fcc22bcd126fff705cc1755275fde).

ABSTRACT:Isolated corn hull arabinoxylan was dis-solved in water and provided a clear solution.Plasticizer (glycerol,propylene glycol,or sorbitol)was added to the arabinoxylan solution at0–20wt%(?lm dry weight),which was cast into stable?lms.Film thickness ranged from22to 32?m.Mechanical properties,moisture content,and water vapor permeability(WVP)were studied for the arabinoxy-lan-based?lms as a function of plasticizer concentration. Measured data for the corn hull arabinoxylan–based?lms were13–18wt%moisture content,10–61MPa tensile strength,365–1320MPa modulus,6–12%elongation,and 0.23–0.43?10?10g m?1Pa?1s?1water vapor permeability. Plasticized arabinoxylan?lms produced in this study had lower WVPs than those of unplasticized?lms,which is likely attributable to the phenomenon known as antiplasti-cization.Scanning electron micrographs showed a homoge-neous structure on?lm surfaces.Films containing sorbitol had the best moisture barrier properties.When grapes were coated with arabinoxylan and arabinoxylan/sorbitol?lms, weight loss rates of the fruit decreased by18and41%, respectively,after7days.?2004Wiley Periodicals,Inc.J Appl Polym Sci93:2896–2902,2004

Key words:corn hull arabinoxylan;thin?lms;mechanical properties;plasticizers;coatings

INTRODUCTION

Published studies on carbohydrate?lms and coatings have successfully demonstrated their potential useful-ness in control of moisture,oxygen,lipid,aroma,and ?avor in food systems where they improve quality, safety,and product shelf life.1–8One useful polysac-charide group for?lm formation is arabinoxylans, which can be made into?lms of high strength and lubricity with the addition of plasticizers.9Arabinoxy-lans,a class of hemicellulose,occur in a wide variety of cereal crops.10Arabinoxylans constitute50–60%of corn seed coat and are available in isolated30–40% yield through cost-effective extraction with dilute al-kali.11Corn hulls are byproducts from the corn-mill-ing industry,where over4million tons are produced per year in the United States.12Currently,corn hulls have little recognized economic value(?0.01$/lb) and sometimes present waste-disposal problems.13 This large quantity of residue represents a low-cost resource that could be turned into an industrial op-portunity through processing into value-added prod-ucts.Arabinoxylans may contribute a signi?cant por-tion of human dietary?ber intake.Further advantages of this hemicellulose,as cited in the literature,are the potential health bene?ts of improving lipid metabo-lism and mineral balance,14improving colon func-tion,15protecting against colon cancer,16reducing the risk of heart disease,and improving general body health.17Design of thin edible?lms and coatings with corn hull arabinoxylan may provide alternative ways for consumers to intake this potential dietary nutrient and hence would be of interest to the food industry. Objectives of this work are to develop and character-ize thin edible?lms from isolated arabinoxylan,to examine the effect of type and concentration of plas-ticizers(glycerol,propylene glycol,and sorbitol)on the functional properties of the?lms,and to determine the ability of the arabinoxylan?lms to reduce mois-ture loss from grapes.

EXPERIMENTAL

Materials

Studies of the isolation and characterization of corn hull arabinoxylan were previously reported.11In brief, dried corn hulls were extracted with4%dilute alkali and supernatants were combined,neutralized,and centrifuged.Arabinoxylan in the supernatant was pre-cipitated,washed with ethanol,and dried.Arabinoxy-lan samples were then fully hydrolyzed and derivat-ized to alditol acetates for analysis of carbohydrate composition using gas chromatography with a mass

Correspondence to:P.Zhang(zhangp@foodsci.purdue. edu).

Journal of Applied Polymer Science,Vol.93,2896–2902(2004)?2004Wiley Periodicals,Inc.

selective detector.Size-exclusion chromatography with multiangle laser light scattering was used to de-termine molecular weight and distribution of arabi-noxylan.Corn amylopectin(9037-22-3;Sigma–Al-drich,Milwaukee,WI)and carboxymethyl cellulose (CMC,9004-32-4;Sigma–Aldrich)were used as?lm-forming polysaccharides to compare performance with that of arabinoxylan.Polysaccharides were main-tained in desiccators after drying under vacuum at 60°C to a constant weight before use in?lm-forming studies.Propylene glycol,glycerol,and sorbitol were purchased from Sigma–Aldrich(purity?98%). Arabinoxylan film formation

Corn hull arabinoxylan and corn amylopectin were each dissolved in deionized water overnight with con-stant stirring to give clear solutions of7.5wt%(?lm dry weight).Propylene glycol,glycerol,or sorbitol concentration of0–22%was then added for plastici-zation.The solution was degassed under vacuum be-fore casting onto glass plates(30?40cm)to a thick-ness of0.8mm.CMC?lm was made in parallel from a3.25wt%solution with1.6mm coating thickness. Casting plates were placed on a level surface at am-bient conditions until the dried?lm could be peeled. Dried?lms were then cut into test specimens and conditioned at22°C and54%relative humidity(RH)at least2days before measurements.Amylopectin?lms thus formed were too brittle to be peeled off the glass plates.For each formulation a minimum of two?lms were made and characterized by each of the following methods:scanning electron microscopy(SEM),thick-ness,tensile test,moisture content,water vapor per-meability(WVP),and grape preservation as described in the following sections.

Scanning electron micrographs

Scanning electron micrographs were taken with a JSM-840scanning electron microscope(JEOL,Tokyo, Japan).Film samples were mounted on12-mm alumi-num stubs and sputter-coated with gold–palladium. Magni?cation was?10,000and voltage was5kV. Tensile test

Tensile strength and breaking elongation of?lms were measured on a Universal testing machine(MTS?Sin-Teck10;DeFelsko Corp.,Ogdensburg,NY)according to ASTM standard D882-97.18At least?ve repeated measurements of each?lm were used to determine mechanical properties.

Thickness and moisture content

Film thickness was measured(PosiTector?6000Coat-ing Thickness Electronic Gage,DeFelsko Corp.)to the nearest1?m around the?lm testing area at?ve random positions.An average of?ve values of?lm thickness was used in calculations.Water content of the?lms was determined by

Moisture content?%??

W0?W1

?100(1)

where W0and W1are the weight of?lms maintained at controlled cabinet(22°C,54%RH)for48h,and in an oven(80°C)to a constant weight,respectively.All samples were analyzed in duplicate.

Water vapor permeability

The cup method(ASTM E96-95)19was used to deter-mine water vapor permeability(WVP)with a54%RH gradient at22°C.Cups were?lled with32g anhy-drous calcium chloride(0%RH)as desiccant and?lm samples were mounted over the cups.The desiccant was dried at240°C for3–5h before use.Cups were then placed in a54%RH cabinet containing a satu-rated magnesium nitrate solution.WVP was deter-mined by

WVP?

?dm/dt?L

A?P

(2)

where dm/dt,L,A,and?P are the slope of the weight loss versus time(g/s),?lm thickness(m),the?lm area exposed to moisture(3.167?10?3m2),and the differ-ence of the vapor pressure between the two sides of ?lms(1.47?103Pa),respectively.For each WVP value three samples were tested.

Film-coating preservation of grapes

Commercial grapes were washed,dried in the open air,and then randomly divided into?ve groups with 20fruits per group.Fruits were sprayed with water (control),2wt%arabinoxylan,and2wt%arabinoxy-lan/0.22wt%sorbitol,2wt%corn amylopectin,and 2wt%CMC solution for30s,respectively.Corn amylopectin and CMC?lms were used as compari-sons for?lm coating preservation.Fruits were dried in open air for3h.Thin?lms that formed on the fruits displayed shiny surfaces.Both control and experimen-tal samples were placed at ambient conditions(22°C and RH40?10%)for7days.Weight loss of the fruits was measured at24-h intervals and was expressed by

Weight loss?%??

W0?W1

W0

?100(3)

where W0and W1are the original weight and measur-ing weight of the whole group after preservation,

THIN FILM FROM CORN HULL ARABINOXYLAN2897

respectively.Experiments were performed in dupli-cate.Statistics

Differences between means values were analyzed us-ing one-way ANOVA and followed by Tukey’s mul-tiple-range tests.A value of p ?0.05was considered to be statistically signi?cant.

RESULTS AND DISCUSSION

Structure of corn hull arabinoxylan

Corn hull arabinoxylan consists of l -arabinose (30.9mol %),d -xylose (48.6mol %),d -galactose (6.0mol %),and d -glucose/glucuronic acids (11.4mol %).Weight-average molecular weight and intrinsic viscosity of arabinoxylan,as determined by light scattering and viscometry in water,were 506,000and 174cm 3/g,respectively.11Corn hull arabinoxylan has a substi-tuted backbone of ?-(134)-d -xylopyranosyl units with side chains of ?-l -arabinofuranosyl units,at-tached mainly at O-3positions,and also at O-2posi-tions of the d -xylose moieties (Fig.1).There are other attached groups such as ?-d -glucuronic acid residues at O-2and d -galactose residues.20,21Although the backbone xylan structure is similar to that in cellulose,arabinoxylan has less driving force to produce crys-talline-type structures than cellulose because of its irregularities,where the presence of side chains re-duces chain interactions.However,in relatively un-substituted regions of the xylan,the chain should be able to associate and introduce inter-and intramolec-ular interactions.The polymer features of corn hull arabinoxylan are responsible for its water solubility and relatively strong ?lm-forming property.Microstructure

Arabinoxylan ?lms were smooth,transparent,and 22–32?m in thickness.Figure 2shows micrographs of the evaporation surface of arabinoxylan-based ?lms.Both the arabinoxylan ?lms and the plasticized arabi-

noxylan ?lms display a homogeneous structure on the surface.Uniform deposition of arabinoxylan-based ?lms may explain their surface gloss and transpar-ency.

Mechanical properties

Mechanical property data [elastic modulus (EM),ten-sile strength (TS),and percentage (%)elongation]of arabinoxylan-based ?lms are shown in Table I.Pro-pylene glycol–plasticized ?lms were more brittle,and mechanical properties were almost independent of plasticizer content.Increasing propylene glycol con-tent did not produce a signi?cant change in mechan-ical properties.Films containing glycerol and sorbitol exhibited negative dependency on plasticizer concen-tration for TS and EM,although increasing plasticizer concentration increased elongation.Glycerol-and pro-pylene glycol–plasticized ?lms exhibited positive de-pendency on plasticizer concentration for moisture content,whereas sorbitol content did not signi?cantly modify moisture content (Fig.3).Propylene glycol–plasticized ?lms showed a sharp break with higher breaking force (EM ?1290–1314MPa and TS ?52.6–60.7MPa,respectively)and low deformation (5.9–7.9%elongation).Similar results were reported in pro-pylene glycol–plasticized cellulose 22and methylcellu-lose ?lms 23where propylene glycol had less effect than glycerol on ?lm tensile strength and elongation.Glycerol and sorbitol have similar chain structures;however,glycerol is a lower molecular weight plasti-cizer and is more hygroscopic.It was reported that lower molecular weight plasticizers produce more ?lm plasticization than do those of higher molecular weight.24This is consistent with results obtained in this study.

Water vapor permeability

Plasticizers such as glycerol,propylene glycol,and sorbitol are often used to modify the mechanical prop-erties of a ?lm,although these additives may cause signi?cant changes in barrier properties,as

observed

Figure 1Typical structure of corn hull arabinoxylan.

2898ZHANG AND WHISTLER

in this study (Fig.4).Sorbitol-plasticized arabinoxylan ?lms had the best moisture barrier properties (WVP ?0.23?10?10g m ?1Pa ?1s ?1at C P ?0.128).Glyc-erol-and propylene glycol–plasticized ?lms had re-duced WVP values (0.31?10?10g m ?1Pa ?1s ?1at C P ?0.051and 0.36?10?10g m ?1Pa ?1s ?1at C P ?0.163)compared with those of unplasticized ?lms (0.47?10?10g m ?1Pa ?1s ?1);however,both had higher WVP than that of sorbitol-plasticized ?lms;whereas arabinoxylan and high concentration glycerol-plasti-cized ?lms were poor moisture barriers (WVP ?0.43?10?10g m ?1Pa ?1s ?1at C P ?0.200).As shown in Figure 4,the WVP curve goes up with glycerol con-centration above 10%,indicating that excess glycerol and the greater amount of af?nitive water improve its effectiveness to break chain-to-chain interactions,and thus introduce more free volume in the arabinoxylan ?lm matrix.

Propylene glycol and glycerol have size and hygro-scopic similarities,although propylene glycol has less effect than glycerol on mechanical properties and WVP,which may be attributable to the lower number of hydroxyl groups available to interact with arabi-noxylan.Sorbitol-plasticized ?lms have lower WVP values than those of glycerol-plasticized ?lms (Fig.4),as expected from the higher tensile strength and elas-tic modulus values for sorbitol–arabinoxylan ?lms compared to those of glycerol–arabinoxylan ?lm with similar plasticizer concentration.Both results in me-chanical property and WVP suggest that the bulky and poorly hygroscopic sorbitol is less able than glyc-erol to affect hydrogen bonding between polysaccha-ride chains.Results showed that glycerol-plasticized ?lms are more ?exible,and therefore likely have more free volume in the arabinoxylan matrix compared to that in the propylene glycol and sorbitol-plasticized ?lms.Sorbitol also gives lower WVP than glycerol in protein ?lms.25,26

It is quite common that increasing the level of the plasticizers would increase WVP of the ?lms;how-ever,all plasticized arabinoxylan ?lms produced in this study have lower WVPs than those of

unplasti-

Figure 2SEM images of the arabinoxylan (AX)?lms.AX (top left);AX ?PG (C P ?0.200,top right);AX ?Gly (C P ?0.194,bottom left);AX ?Sor (C P ?0.198,bottom right).C P ?g plasticizer/(wt plasticizer ?wt arabinoxylan).

THIN FILM FROM CORN HULL ARABINOXYLAN 2899

cized ?lms.Chitosan ?lm was reported to have low-ered WVPs as the level of plasticizers increased.27Also,plasticizers could enhance or retard WVP,de-pending on their concentrations in cellulose-based ?lms.22,28,29This behavior is likely attributable to the

phenomenon known as antiplasticization.30–34Anti-plasticization involves intermolecular bonding be-tween plasticizers and the polymers,and thus reduc-tion of molecular mobility in polymer matrix by small amounts of plasticizers would result in decreased WVPs.Addition of glycerol 30or sorbitol 31,32was re-

TABLE I

Thickness,Mechanical Properties of Arabinoxylan Films with Different Plasticizer Concentrations (C P ),

and Data of Edible Films From the Literature

Film a

C P b Thickness (?m)c

Tensile Strength (MPa)c

Modulus (MPa)c

Elongation (%)c

AX

026.0?0.853.8?0.4A 1316?90A 6.2?1.6A AX ?PG

0.06826.4?1.553.2?2.6A 1290?126A 5.9?0.7A 0.11523.4?2.355.4?4.9A 1320?136A 6.5?0.4A 0.16326.8?2.660.7?4.4A 1287?266A 7.9?1.1A 0.19426.3?1.555.0?5.9A 1225?115A 6.6?1.4A 0.21927.6?3.752.6?6.7A 1314?197A 6.0?0.5A AX ?Gly

0.02722.3?1.246.5?3.5B 1148?163A 5.6?0.5A 0.05126.3?1.935.3?5.8B 1093?55A 5.9?0.6A 0.10024.3?3.925.4?8.1C 884?112B 7.4?2.6A 0.14629.8?2.614.1?1.3C 477?60C 10.0?2.5B 0.20028.4?0.99.7?2.9D 365?73D 12.1?1.5B AX ?Sor

0.05125.3?1.547.5?10.9A 1161?133A 5.5?1.4A 0.09128.0?1.043.7?3.8B 1094?35A 6.5?1.0A 0.12828.0?0.832.1?8.6C 969?131B 6.7?1.3A 0.16432.3?1.329.7?4.1C 837?100B 7.3?1.1A 0.19829.5?1.5

19.7?4.3C

463?90C

8.9?1.4A

Corn starch 39—2546— 2.5Soluble starch 40—5050.5?5.4— 4.2?0.5Pullulan 41

—81.3?2.534.2?0.5

— 2.9?0.2Methylcellulose 42—25?220— 1.2–1.8Zein 43

—40–927–9270–3203–12Cellophane 44

—

—

50–120

3000

10–50

a At 22°C,54%RH,arabinoxylan (AX),propylene glycol (PG),glycerol (Gly),and sorbitol (Sor).b

C P ?wt plasticizer/(wt plasticizer ?wt arabinoxylan).c

Mean ?standard deviation.Values within table columns with different superscripts (A,B,C,D)are statistically signi?cant in Tukey tests at p ?

0.05.

Figure 3Moisture content of arabinoxylan ?lms at 22°C and 54%RH and comparison with starch ?lm,39,40zein ?lm,43and cellophane.44C P ?g plasticizer/(wt plasticizer ?wt

arabinoxylan).

Figure 4Water vapor permeability of arabinoxylan ?lms at 22°C,and 54%RH.C P ?g plasticizer/(wt plasticizer ?wt arabinoxylan).

2900ZHANG AND WHISTLER

ported to avoid antiplasticization when their concen-trations reached 20–25and 21–27%in starch ?lms,respectively.

Additionally,water vapor permeability and me-chanical property data of arabinoxylan are inconsis-tent with the results from Peroval et al.6and Phan The et al.7,8The reason may be attributed to different ?lm thicknesses (22–32versus 58–91?m),humidity gradi-ents (0/54versus 22/84%),draw rates at tensile test-ing (10versus 100mm/min),and different origins of corn hull arabinoxylans.It appeared that ?lm thick-ness and humidity gradient signi?cantly affect WVP values of hydrophilic ?lms.35,36These differentiations make the results less comparable.Film-coating preservation of grapes

Weight loss rates of grapes during storage (shown in Fig.5)suggest that the weight loss rate decreased after ?lm-coating preservation,especially for grapes coated with arabinoxylan–sorbitol (AX/Sor)?lms.The weight loss rate for grapes coated with AX/Sor ?lms was signi?cantly different from that of the control group after the third day (p ?0.05).Weight losses were found to be 82,80,93,and 59%of the control group for grapes coated with AX,corn amylopectin,CMC,and AX/Sor after 7days of preservation,re-spectively.These ?ndings demonstrate the good mois-ture barrier ability of sorbitol-plasticized arabinoxylan ?lms.The ?lm coatings on grapes could be easily washed off before serving,or the arabinoxylan-based ?lm coatings could be eaten with the grapes as a source of dietary ?ber.

Edible ?lm coatings with moisture barrier proper-ties can prevent deterioration and extend the shelf life

of food products.37Application of edible ?lms on fruits (such as citrus products,apples,and pears)can help to prevent moisture loss and improve gloss.5Because 25to 80%of harvested fresh fruits and vege-tables are lost because of spoilage,4one method of extending postharvest shelf life could be the use of an edible ?lm coating.Such coatings can be made of edible materials that provide a barrier to gases and water vapor.Modi?ed internal atmosphere was re-ported to reduce respiration and thereby delay ripen-ing and prolong shelf life of fruits.38Polysaccharide ?lms,because of their hydrophilic nature,are gener-ally poor moisture barriers.However,the arabinoxy-lan–sorbitol ?lms used in this study signi?cantly de-layed moisture loss from grapes.Advantages of many polysaccharide ?lms may be more in the area of gas exchange rather than retardation of water loss.1In the future,a coating may reduce the need for energy-intensive refrigeration and costly controlled atmo-sphere storage.4

CONCLUSIONS

Thin edible ?lms were developed using corn hull ar-abinoxylan.These ?lms were stable,strong,smooth,and transparent and had mechanical properties,mois-ture content,and water vapor permeability controlled by the plasticizer.Sorbitol-plasticized arabinoxylan ?lms were good moisture barriers.All plasticized ar-abinoxylan ?lms produced in this study had lower WVPs than those of unplasticized ?lms,likely attrib-utable to the antiplasticization effect of these plasticiz-ers at low concentrations.

Microstructure images con?rmed the ?ne micro-structure of ?lms and the good miscibility between arabinoxylan and the plasticizer.The ?lm-coating preservation experiment to grapes showed that arabi-noxylan-based ?lm coatings have moisture barrier ability.

The authors thank Drs.L.J.Mauer and L.F.Chen of De-partment of Food Science for reading through the manu-script and for their suggestions.

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