Effect of cellulose fibers addition on the mechanical properties and

Effect of cellulose ?bers addition on the mechanical properties and water vapor barrier of starch-based ?lms

Carmen M.O.Mu

¨ller a ,b ,Joa ?o Borges Laurindo b ,*,Fabio Yamashita a a Department of Food Science and Technology,State University of Londrina,Londrina,PR,Brazil

b

Department of Chemical and Food Engineering,Federal University of Santa Catarina,Florianopolis,SC 88040-900,Brazil

a r t i c l e i n f o

Article history:

Received 2October 2007

Accepted 12September 2008Keywords:Starch ?lms Cellulose Fibers

Mechanical Water

Permeability

a b s t r a c t

Starch-based ?lms have promising application on food packaging,because of their environmental appeal,low cost,?exibility and transparency.Nevertheless,their mechanical and moisture barrier properties should be improved.The aim of this work was to enhance these properties by reinforcing the ?lms with cellulose ?bers.Besides,the in?uences of both the solubility coef?cient of water in the ?lms (b )and the diffusion coef?cient of water vapor through the ?lms (D w )on the ?lms’water vapor permeability (K w )were investigated.Films were prepared by the so-called casting technique,from ?lm-forming suspen-sions of cassava starch,cellulose ?bers (1.2mm long and 0.1mm of diameter),glycerol and water.The in?uence of ?bers addition on K w was determined at three relative humidity gradient ranges,D RH (2–33%,33–64%and 64–90%).Films reinforced with cellulose ?bers showed higher tensile strength and lower deformation capacity,and presented lower K w than ?lms without ?bers.K w showed strong dependency of b and D w ,presenting values up to 2–3times greater at D RH ?64–90%than at D RH ?33–64%,depending on the ?lm formulation.Therefore,adding cellulose ?bers to starch-based ?lms is a viable alternative to improve their mechanical and water barrier properties.Besides,this work showed the importance of determining ?lm’s water vapor permeability simulating the real environ-mental conditions the ?lm will be used.

ó2008Elsevier Ltd.All rights reserved.

1.Introduction

Starch production and industrialization represent a good alter-native for developing countries (FAO,2007).Developing new products from this raw material can add value and expand its industrial use.The production of biodegradable and edible ?lms from carbohydrates and proteins adds value to low cost raw materials and can play an important role in food preservation

(Ave

′rous,Fringant,&Moro,2001;Gennadios,2002;Krochta &Miller,1997,among others).Preparing these ?lms involves the use of at least one ?lm-forming agent (macromolecule),a solvent and a plasticizer.The most used macromolecules are polysaccharides and their derivatives,proteins and lipids (Cuq,Gontard,&Guilbert,1998;Krochta,2002;Krochta &Mulder-Johnston,1997).

Several studies reported the use of starches from different sources to prepare ?lms and coatings with different properties,and had indicated that these carbohydrates are promising materials in

this regard (Ave

′rous et al.,2001;Larotonda,Matsui,Sobral,&Laurindo,2005;Mali,Grossmann,Garc?

′a,Martino,&Zaritzky,2005;Mali,Sakanaka,Yamashita,&Grossmann,2005).However,?lms formed from starch are brittle and dif?cult to handle.Plasti-cizers are normally added to the ?lm-forming solution before casting and drying procedures,as a way to overcome ?lms brit-tleness.Unfortunately,plasticizers generally decrease the ?lm water vapor permeability (Gontard,Guilbet,&Cuq,1993;Krochta &Mulder-Johnston,1997;Muller,Yamashita,&Laurindo,2007).

In order to improve starch-based ?lm characteristics,many researches reported results on the addition of natural ?bers as a suitable reinforcing component for thermoplastic materials.Most of these works focused on ?lms’mechanical properties and have showed that ?bers incorporation increases ?lms’tensile strength and elasticity modulus and decreases their elongation capacity

(Ave

′rous et al.,2001;Curvelo,de Carvalho,&Agnelli,2001;Dufresne &Vignon,1998;Follain,Joly,Dole,Roge,&Mathlouthi,

2006;Ga

′spar,Benko ′,Dogossy,Re ′czey,&Cziga ′ny,2005;Ma,Yu,&Kennedy,2005).

Concerning to the barrier properties,Dufresne and Vignon (1998)and Funke,Bergthaller,and Lindhauer (1998)reported that the addition of ?bers decreased the water vapor permeability (K w )of starch-based ?lms.This behavior was attributed to the low hygroscopicity of cellulose ?bers.In fact,the behavior of K w depends on the simultaneous effect of water diffusivity in the

*Corresponding author.Tel.:t554837219448;fax:t554837219687.

E-mail address:joao@enq.ufsc.br (J.B.

Laurindo).Contents lists available at ScienceDirect

Food Hydrocolloids

journal homepage:

https://www.360docs.net/doc/0512164059.html,/locate/foodhyd

0268-005X/$–see front matter ó2008Elsevier Ltd.All rights reserved.doi:10.1016/j.foodhyd.2008.09.002

Food Hydrocolloids 23(2009)1328–1333

polymeric matrix(D w)and of the solubility coef?cient of water in

the?lm(b)(Krochta&Miller,1997;Larotonda et al.,2005;Muller et al.,2007).Larotonda et al.(2005),working with Kraft paper

impregnated with starch acetate,investigated the in?uence of this

impregnation on the K w,D w and b values of these materials.The b value was calculated through the?rst derivative of the?lm water

sorption isotherm in relation to the water activity(a w),divided by

the water vapor pressure(at the sorption isotherm temperature).

As?lm’s K w is proportional to the product of b and D w,the values of D w were determined from K w and b values,at three relative humidity gradient ranges.

Moore,Martelli,Gandolfo,Sobral,and Laurindo(2006)studied

the in?uence of b and D w on the water vapor permeability of keratin?lms plasticized with glycerol and reported that the K w value increased almost six times when0.09g glycerol/g keratin was added.This result was explained by the high increase of b,while D w did not change signi?cantly.Variability in the?lm thickness and density were also reported as in?uencing the values of K w of keratin ?lms.Muller et al.(2007)reported results for starch-based?lms that are qualitatively similar to results reported by the authors above.For high relative humidity range,the b values increased6-fold for?lms with glycerol and7-fold for?lms with sorbitol,while D w values did not change signi?cantly.These results showed that K w values are dependent on the solubility coef?cient(b)of water in the?lm.Results about the in?uence of?bers addition on the water vapor permeability of starch?lms are rare in the literature(Imam, Cinelli,Gordon,&Chiellini,2005).

The?lms made in this work were supposed to be used as

packaging material.So,the aim of this work was to investigate how

the addition of cellulose?bers improves?lms’mechanical and

moisture barrier properties and to verify how?bers addition affects

the relative in?uences of the solubility coef?cient(b)and the diffusion coef?cient(D w)on the water vapor permeability(K w)of starch?lms.These results are not available in the literature and very important to evaluate possible applications of these?lms as packaging material.

2.Materials and methods

2.1.Films preparation

An aqueous suspension of?bers(softwood short?bers1.2mm long and with0.1mm of diameter-Klabin S.A-Brazil)was prepared cutting10g?ber and mixing with400ml of distilled water,in order to facilitate the incorporation of these?bers to the starch suspension.The?lms were prepared according to the so-called casting technique.Film-forming solutions were prepared with3% w/w of cassava starch(Yoki-Brazil),0.30g glycerol/g dry starch, 0.01g guar gum/g dry starch(to avoid?bers sedimentation)and three concentrations of cellulose?bers:0.10(P10),0.30(P30)and 0.50(P50)g of?ber/g dry starch.Fibers suspension,guar gum and water were stirred for10min at14,000rpm in a dispenser,before adding starch and glycerol.Afterwards,under constant stirring (90rpm),the container with the mixture was heated up until80 C, and the?lm-forming suspension was poured homogeneously in acrylic Petri dishes14cm in diameter.The dishes with?lm-forming mixture were then put in a ventilated oven,at40 C,for16h.Films prepared without?bers served as control.

2.2.Moisture,thickness and density

Prior to?lms’properties determination,samples were condi-tioned at25 C and58%relative humidity(RH)for48h.Films moisture were determined in triplicate,by the gravimetric method, after drying at105 C for24h,and expressed in g water/g dry mass. Films thicknesses were measured(exactness of?0.001mm)using a Digimatic digital external micrometer(Mitutoyo Co.,Japan)at ten different points of the?lm.For determining?lm density,samples of 2cm?2cm were maintained in a desiccator with phosphorus pentoxide(0%RH)for20days and weighed.Thus,dry matter densities were calculated by Eq.(1).

r s?m

A?d

(1) where A is the?lm area(4cm2),d the?lm thickness(cm),m the ?lm dry mass(g)and r s the dry matter density of the?lm(g/cm3) (Larotonda et al.,2005).The?lm density was expressed as the average of ten determinations.

2.3.Scanning electron microscopy(SEM)

Scanning electron microscopy(SEM)of?lm samples was obtained using a Philips XL-30scanning electron microscope.The samples were coated with a?ne gold layer before obtaining the micrographs.All samples were examined using an accelerating voltage of10kV.

2.4.Moisture sorption isotherms

Films’moisture sorption isotherms were determined through the static method,using saturated saline solutions to obtain different relative humidities(Labuza&Ball,2000).The Guggen-heim–Anderson–de Boer(GAB)model(Eq.(2))was used to represent the experimental equilibrium data.In this equation the parameter X w is the equilibrium moisture(g water/g dry mass),m o is the monolayer water content,C is the Guggenheim constant, which represents the sorption heat of the?rst layer and k is the sorption heat of the multilayers.The GAB model parameters were determined by non-linear regression,using the Statistica Software 6.0(Statsoft,USA).

X w?

Ckm o a w

?e1àka wTe1àka wtCka wT

(2)

2.5.Water vapor permeability(K w)

Films’water vapor permeabilities(K w)were determined in appropriate diffusion cells(Saranto′poulos et al.,2002),using three different ranges of relative humidity gradient(D RH?2–33%, D RH?33–64%and D RH?64–90%).The water vapor permeability was determined using Eq.(3).

K w?

W d

s w1w2

(3)

where d is the average?lm thickness,S is the?lm permeation area (0.005m2),a w1(RH1/100)is the water activity in the chamber,a w2 (RH2/100)is the water activity inside the cell,p s is the water vapor pressure at the experimental system temperature(25 C)and W?G/t(g of water/hour)was calculated using the linear regression of mass variation over time,under steady state permeation condition.

2.6.Water solubility coef?cient(b)and effective water

diffusion coef?cient(D w)

The solubility coef?cient of water in the?lms,b(g of water/g of dry mass?Pa),was determined according to Larotonda et al. (2005),based on the experimental moisture sorption isotherms, GAB model and Eq.(4).

C.M.O.Mu¨ller et al./Food Hydrocolloids23(2009)1328–13331329

b ?

Ckm o s "

1

w w w à

a w

?e1àka w Te1àka w tCka w T 2

?àk e1àka w tCka w T

te1àka w Teàk tCk T

#

(4)

where C ,m o and k are the adjust parameters of GAB model and b is given in g of water/g of dry solid ?Pa.

The water diffusion coef?cients through the ?lms were deter-mined from water vapor permeability,water solubility coef?cient in the ?lm and ?lm density data.As b varies with a w ,the value correspondent to the a w median was used in Eq.(5).

K w ?r s b D w

(5)

2.7.Mechanical properties

Films’mechanical properties were determined from tension tests,using the TA-XT2i texture analyzer (Surrey –England),in accordance with ASTM-882-00(2000).The dimensions of ?lm samples used in tests were 25mm ?100mm,cut with a sharp scissor.Samples were clamped between grips and force and deformation were recorded during extension at 50mm/min,with an initial distance between the grips of 50mm.In this way,tensile strength (MPa),elasticity modulus (MPa)and relative deformation at break (%)were determined from ten replicates for each ?lm formulation.

2.8.Statistical analysis

Analysis of Variance (ANOVA)and Tukey mean comparison test (p 0.05)were performed (Statistica Software –Statsoft,OK –USA).

3.Results and discussions

3.1.Moisture,thickness and density of the ?lms

The incorporation of cellulose ?bers reduced the ?lms moisture (Table 1),due to the lower water af?nity of cellulose ?bers compared with starch (Curvelo et al.,2001;Dufresne &Vignon,1998;Funke et al.,1998;Ma et al.,2005).There was no signi?cant difference among the thicknesses,which varied from 110?11m m to 121?18m m.On the other hand,?bers incorporation caused a signi?cant reduction of ?lm density,e.g.,the ?lm P50presented density 1.8times lower than the ?lm without cellulose ?bers,which can be explained by the low density of the cellulosic material (Dufresne &Vignon,1998;Wollerdorfer &Bader,1998).

3.2.SEM –scanning electron micrographies

Micrographies of ?lms’surfaces and fractures (Fig.1)showed homogeneous and random distributions of ?bers within the samples,without pores or cracks formation.It can also be observed,from the fractured ?lms micrographies,that cellulose ?bers are incrusted in the continuous starchy material.3.3.Moisture sorption isotherms

The GAB model ?tted well the ?lms’sorption data (Table 2),as

previously reported by other authors (Godbillot,Dole,Joly,Roge

′,&Mathlouthi,2006;Mali,Sakanaka,et al.,2005;Martelli,Moore,Paes,Gandolfo,&Laurindo,2006,among others).All isotherms presented sigmoidal shape (Fig.2),which is a characteristic of starchy materials,but the curves’‘‘shoulders’’(close to a w ?0.03)were reduced as the ?ber concentration in the ?lms increased.The water sorption isotherm of cellulose ?bers shows clearly their lower hygroscopicity.The P10?lm presented equilibrium moisture similar to the ones observed for the ?lms without ?bers.For the last,the hygroscopicity increased due to the presence of guar gum,which probably balanced the opposite effect provoked by the cellulose ?bers.According to Chaisawang and Suphantharika (2005,2006)the addition of guar gum (1%)to cassava starch causes a larger absorption of water,due to hydrocolloids interaction with the amylose chain.These results are in agreement with results reported by other researchers who worked with composites of

starch and cellulose ?bers (Ave

′rous et al.,2001;Curvelo et al.,2001;Dufresne &Vignon,1998;Ma et al.,2005).Ave

′rous et al.(2001)reported that wheat starch ?lms reinforced with cellulose ?bers had their equilibrium moisture reduced.They attributed this behavior to the interactions between ?bers and the hydrophilic sites of starch chain,which substituted the starch–water interac-tions that predominate in ?lms without ?bers.The effect of ?bers on ?lms’hygroscopicity can also be observed from the monolayer water content data,m o ,which was 0.094g water/g solid for ?lms without ?bers against 0.058g water/g solid for ?lms with 0.50g of ?ber/g of starch.On the other hand,the k value was not affected by ?lm composition (?ber addition).

3.4.In?uence of relative humidity gradient (D RH)range and ?bers concentration on the water vapor permeability (K w ),solubility coef?cient (b )and effective water diffusion coef?cient (D w )

For all samples K w values increased as the D RH range moved from lower to higher values (Table 3).For the D RH ranges of 2–33%and 64–90%,K w of P50?lms presented values approximately 3.7and 1.5times lower if compared with starch ?lms without ?bers (WF).However,at the intermediate gradient (33–64%),water vapor permeabilities of WF ?lms and P50were of the same order (3.43?10à7and 3.08?10à7g/h m Pa).For the P30formulation the K w value was about 10times higher when the D RH range passed from 2–33%to 64–90%.This result can be explained by the increasing of 8.3times observed for the solubility coef?cient b

,

Table 1

Values with the same letter at the same column are not different statistically (p <0.05).WF –without ?bers.

P50,P30and P10denote 0.50,0.30and 0.10g ?bers/g starch,respectively.

C.M.O.Mu

¨ller et al./Food Hydrocolloids 23(2009)1328–13331330

while the diffusion coef?cient D w did not change considerably (only 22%higher).

The b value of all formulations increased as the D RH range moved from 2–33%to 64–92%.Fig.3shows that for water activities lower than 0.10the b value was very in?uenced by a w ,i.e.,b decreased very quickly with the a w increase.From a w values

between 0.10and 0.60,b was practically constant,but became very responsive to a w when a w >0.60.

Film densities decreased from 2.41to approximately 1.3?10à3g/m 3due to cellulose ?bers addition,which in?uenced the values of K w ,accordingly to Eq.(5).

The D RH range did not affect a lot the D w values of ?lms without ?bers,indicating that in this case the K w increasing was conse-quence of the increment of b .The WF-?lm’permeability was 4.5times higher at the D RH ?64–90%than at D RH ?2–33%,due to the big increase of b (about 6times),while D w did not change considerably.

The values of D w for P10,P30and P50did not present appre-ciable change,except at intermediate D RH range (33–64%).For P30samples,when D RH passed from 2–33%to 33–64%,the K w increased 5.6times,due to the simultaneously increase of b (2times)and D w (2.8times).Under these D RH conditions,both coef?cients b and D w had about the same in?uence on the K w values.

For the D RH ranges of 2–33%and 64–90%the addition of cellulose ?bers provoked decreasing of K w from approximately

10

Fig.1.Scanning electron microscopy of cassava starch ?lms with cellulose ?bers.P50,P30and P10denote 0.50,0.30and 0.10g ?bers/g starch,respectively and the upper index s

and f denote surface and fracture,

respectively.

Table 2

GAB model ?tted parameters for sorption data from cassava starch ?lms with WF –without ?bers.

P50,P30and P10denote 0.50,0.30and 0.10?bers/g starch,respectively.

C.M.O.Mu ¨ller et al./Food Hydrocolloids 23(2009)1328–13331331

to 5.7g m/m 2h Pa.It was due mainly to the ?lm density reduction from 2.41?0.12to 1.31?0.03?106g/m 3and to a small reduction of the b values.For the D RH ?33–64%the addition of ?bers did not affect K w appreciably,which could be explained by the small vari-ation of b and D w with the addition of cellulose ?bers (Fig.3,Table 3).The reduced density of ?lms reinforced with cellulose ?bers could reduce K w ,but it was not observed.

In a general manner,similar behavior of K w ,b and D w with RH range was reported for keratin ?lms (Moore et al.,2006)and Kraft paper impregnated with starch acetate (Larotonda et al.,2005)and were explained by the effect of RH range on the parameters b and D w .However,the mass transfer mechanisms controlling the barrier properties of reinforced ?lms can change with D RH range and cellulose ?ber concentration and cannot be evidenced from global properties as sorption isotherms and water vapor permeability.For this purpose,techniques as confocal laser scanning microscopy could be very useful to investigate microstructure and water distribution in composite-?lms with different formulations (Chen,Lee,&Teoh,2007;Straadt,Rasmussen,Andersen,&Bertram,2007).3.5.Mechanical properties

Films reinforced with ?bers presented higher values of tensile strength and elasticity modulus,and lower values of tensile

strength (deformation at break)(Fig.4),if compared with WF ?lms.This behavior is in agreement with the results reported in the literature about starch ?lms reinforced with different kinds of

?bers (Ave

′rous &Boquillo,2004;Curvelo et al.,2001;Funke et al.,1998;Ga

′spar et al.,2005;Lu,Weng,&Cao,2006).The incorporation of 0.10and 0.50g ?ber/g starch increased the tensile strength of reinforced ?lms in 6.7and 18times,respectively.The elasticity modulus of P10and P50?lms were,respectively,7.6and 34times greater than the elasticity modulus of WF ?lms.This signi?cant increasing of ?lms rigidity has been attributed to the similarity between the chemical structures of cellulose and starch (Ma et al.,2005).

a w

X w (g w a t e r /g d r y s o l i d )

Fig.2.Water sorption isotherms of cassava starch ?lms with cellulose ?bers ?tted with the GAB model.P50,P30and P10denote 0.50,0.30and 0.10g ?bers/g starch,

respectively.

Table 3

Water vapor permeability (K w ),solubility coef?cient (b )and diffusion coef?cient (D w )of cassava starch ?lms with cellulose ?bers incorporation as a function of the WF –without ?bers.

P50,P30and P10denote 0.50,0.30and 0.10g ?bers/g starch,

respectively.

a w

0,00

2E-54E-56E-58E-51E-4β (g w a t e r /g d r y s o l i d s .P a )

Fig.3.Solubility coef?cient values (b )for cassava starch ?lms with cellulose ?bers (P50)and cellulose ?bers as a function of water activity (a w

).

51015202530

Samples

T e n s i l e s t r e n g t h (M P a )

20406080100120

Percent elongation at break (%)

200

400

600

800

Samples

Y o u n g ′s M o d u l e (M P a )

Fig.4.(a)Tensile strength and percent elongation at break of cassava starch ?lms with cellulose ?bers incorporation.(b)Young’s modulus of cassava starch ?lms with cellulose ?bers incorporation.WF denotes without ?bers ?lms,while P50,P30and P10denote 0.50,0.30and 0.10g ?bers/g starch,respectively.

C.M.O.Mu

¨ller et al./Food Hydrocolloids 23(2009)1328–13331332

4.Conclusions

Films’water vapor permeability have strong dependency on solubility and water diffusion coef?cients and,consequently,on relative humidity gradient range,presenting values up to2–3times greater at33–64%than at64–90%,depending on?lm formulation. Therefore,it is important to determine this property based on the environmental conditions the?lm will be used.

The incorporation of cellulose?bers reinforces mechanically starch?lms,which have higher tensile strength and lower defor-mation capacity.The reinforced?lms present lower water vapor permeabilities if compared with starch?lms without?bers.As cellulose?bers are biodegradable and available at low cost,their use as reinforcing agent is a viable alternative to improve the properties of starch-based?lms.

Acknowledgements

This research was supported by CAPES and MCT/CNPq-Brazil. References

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