starch based films

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Effect of cellulose fibers addition on the mechanical properties andwater vapor barrier of starch-based films

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, Brazilb 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 2 October 2007

Accepted 12 September 2008

Keywords:

Starch films

Cellulose

Fibers

Mechanical

Water

Permeability

a b s t r a c t

Starch-based films have promising application on food packaging, because of their environmental appeal,

low cost, flexibility 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 films with

cellulose fibers. Besides, the influences of both the solubility coefficient of water in the films (b) and the

diffusion coefficient of water vapor through the films (Dw) on the films’ water vapor permeability (K w)

were investigated. Films were prepared by the so-called casting technique, from film-forming suspen-

sions of cassava starch, cellulose fibers (1.2 mm long and 0.1 mm of diameter), glycerol and water. The

influence of fibers addition on K w was determined at three relative humidity gradient ranges, DRH

(2–33%, 33–64% and 64–90%). Films reinforced with cellulose fibers showed higher tensile strength and

lower deformation capacity, and presented lower K w than films without fibers. K w showed strong

dependency of b and Dw, presenting values up to 2–3 times greater at DRH¼ 64–90% than at

DRH¼ 33–64%, depending on the film formulation. Therefore, adding cellulose fibers to starch-based

films is a viable alternative to improve their mechanical and water barrier properties. Besides, this work

showed the importance of determining film’s water vapor permeability simulating the real environ-

mental conditions the film will be used.

2008 Elsevier 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 films

from carbohydrates and proteins adds value to low cost raw

materials and can play an important role in food preservation

(Averous, Fringant, & Moro, 2001; Gennadios, 2002; Krochta &

Miller, 1997, among others). Preparing these films involves the use

of at least one film-forming agent (macromolecule), a solvent anda 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 films and coatings with different properties, and

had indicated that these carbohydrates are promising materials in

this regard (Averous et al., 2001; Larotonda, Matsui, Sobral, &

Laurindo, 2005; Mali, Grossmann, Garcıa, Martino, & Zaritzky,

2005; Mali, Sakanaka, Yamashita, & Grossmann, 2005). However,

films formed from starch are brittle and difficult to handle. Plasti-

cizers are normally added to the film-forming solution before

casting and drying procedures, as a way to overcome films brit-

tleness. Unfortunately, plasticizers generally decrease the film

water vapor permeability (Gontard, Guilbet, & Cuq,1993; Krochta &

Mulder-Johnston, 1997; Muller, Yamashita, & Laurindo, 2007).

In order to improve starch-based film characteristics, many

researches reported results on the addition of natural fibers as

a suitable reinforcing component for thermoplastic materials. Most

of these works focused on films’ mechanical properties and haveshowed that fibers incorporation increases films’ tensile strength

and elasticity modulus and decreases their elongation capacity

(Averous et al., 2001; Curvelo, de Carvalho, & Agnelli, 2001;

Dufresne & Vignon, 1998; Follain, Joly, Dole, Roge, & Mathlouthi,

2006; Gaspar, Benko, Dogossy, Reczey, & Czigany, 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 fibers decreased the water vapor permeability ( K w)

of starch-based films. This behavior was attributed to the low

hygroscopicity of cellulose fibers. In fact, the behavior of K w

depends on the simultaneous effect of water diffusivity in the* Corresponding author. Tel.: þ55 48 37219448; fax: þ55 48 37219687.

E-mail address: [email protected] (J.B. Laurindo).

Contents lists available at ScienceDirect

Food Hydrocolloids

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f o o d h y d

0268-005X/$ – see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.foodhyd.2008.09.002

Food Hydrocolloids 23 (2009) 1328–1333

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polymeric matrix (Dw) and of the solubility coefficient of water in

the film (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 influence of this

impregnation on the K w, Dw and b values of these materials. The

b value was calculated through the first derivative of the film water

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

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

As film’s K w is proportional to the product of b and Dw, the values of

Dw were determined from K w and b values, at three relative

humidity gradient ranges.

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

the influence of b and Dw on the water vapor permeability of

keratin films plasticized with glycerol and reported that the K w

valueincreased almost six times when 0.09 g glycerol/g keratin was

added. This result was explained by the high increase of b, while Dw

did not change significantly. Variability in the film thickness and

density were also reported as influencing the valuesof K w of keratin

films. Muller et al. (2007) reported results for starch-based films

that are qualitatively similar to results reported by the authors

above. For high relative humidity range, the b values increased 6-

fold for films with glycerol and 7-fold for films with sorbitol, while

Dw values did not change significantly. These results showed thatK w values are dependent on the solubility coefficient (b) of water in

the film. Results about the influence of fibers addition on the water

vapor permeability of starch films are rare in the literature (Imam,

Cinelli, Gordon, & Chiellini, 2005).

The films 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 fibers improves films’ mechanical and

moisture barrier properties and to verify howfibers addition affects

the relative influences of the solubility coefficient (b) and the

diffusion coefficient (Dw) on the water vapor permeability (K w) of

starch films. These results are not available in the literature and

very important to evaluate possible applications of these films as

packaging material.

2. Materials and methods

2.1. Films preparation

An aqueous suspension of fibers (softwood short fibers 1.2 mm

long and with 0.1 mm of diameter- Klabin S.A-Brazil) was prepared

cutting 10 g fiber and mixing with 400 ml of distilled water, in

order to facilitate the incorporation of these fibers to the starch

suspension. The films were prepared according to the so-called

casting technique. Film-forming solutions were prepared with 3%

w/w of cassava starch (Yoki - Brazil), 0.30 g glycerol/g dry starch,

0.01 g guar gum/g dry starch (to avoid fibers sedimentation) and

three concentrations of cellulose fibers: 0.10 (P10), 0.30 (P30) and

0.50 (P50) g of fiber/g dry starch. Fibers suspension, guar gum andwater were stirred for 10 min at 14,000 rpm in a dispenser, before

adding starch and glycerol. Afterwards, under constant stirring

(90 rpm), the container with the mixture was heated up until 80 C,

and the film-forming suspension was poured homogeneously in

acrylic Petri dishes 14 cm in diameter.The dishes with film-forming

mixture were then put in a ventilated oven, at 40 C, for 16 h. Films

prepared without fibers served as control.

2.2. Moisture, thickness and density

Prior to films’ properties determination, samples were condi-

tioned at 25 C and 58% relative humidity (RH) for 48 h. Films

moisture were determined in triplicate, by the gravimetric method,

after drying at 105

C for 24 h, and expressed in g water/g dry mass.Films thicknesses were measured (exactness of 0.001 mm) using

a Digimatic digital external micrometer (Mitutoyo Co., Japan) at ten

different points of the film. For determining film density, samples of

2 cm 2 cm were maintained in a desiccator with phosphorus

pentoxide (0% RH) for 20 days and weighed. Thus, dry matter

densities were calculated by Eq. (1).

rs ¼ m

A d (1)

where A is the film area (4 cm2), d the film thickness (cm), m the

film dry mass (g) and rs the dry matter density of the film (g/cm3)

(Larotonda et al., 2005). The film density was expressed as the

average of ten determinations.

2.3. Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) of film samples was

obtained using a Philips XL-30 scanning electron microscope. The

samples were coated with a fine gold layer before obtaining the

micrographs. All samples were examined using an accelerating

voltage of 10 kV.

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), mo

is the monolayer water content, C is the Guggenheim constant,

which represents the sorption heat of the first 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 ¼ Ckmoaw

½ð1 kawÞð1 kaw þ CkawÞ (2)

2.5. Water vapor permeability (Kw)

Films’ water vapor permeabilities (K w) were determined in

appropriate diffusion cells (Sarantopoulos et al., 2002), using three

different ranges of relative humidity gradient (DRH¼ 2–33%,

DRH¼ 33–64% and DRH¼ 64–90%). The water vapor permeability

was determined using Eq. (3).

K w ¼ W d

Spsðaw1 aw2Þ (3)

where d is the average film thickness, S is the film permeation area

(0.005 m2), aw1 (RH1/100) is the water activity in the chamber, aw2

(RH2/100) is the water activity inside the cell, ps is the water vapor

pressure at the experimental system temperature (25 C) and

W ¼G/t (g of water/hour) wascalculated using the linear regression

of mass variation over time, under steady state permeation

condition.

2.6. Water solubility coefficient (b) and effective water

diffusion coefficient (Dw)

The solubility coefficient of water in the films, 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. Muller et al. / Food Hydrocolloids 23 (2009) 1328–1333 1329



b ¼ Ckmo

ps

" 1

ð1 kawÞð1 kaw þ CkawÞ

aw

½ð1 kawÞð1 kaw þ CkawÞ2½ kð1 kaw þ CkawÞ

þ ð1 kawÞðk þ CkÞ

# (4)

where C , mo 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 coefficients through the films were deter-

mined from water vapor permeability, water solubility coefficient

in the film and film density data. As b varies with aw, the value

correspondent to the aw median was used in Eq. (5).

K w ¼ rsbDw (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 film

samples used in tests were 25 mm 100 mm, cut with a sharp

scissor. Samples were clamped between grips and force and

deformation were recorded during extension at 50 mm/min, with

an initial distance between the grips of 50 mm. In this way, tensile

strength (MPa), elasticity modulus (MPa) and relative deformation

at break (%) were determined from ten replicates for each film

formulation.

2.8. Statistical analysis

Analysis of Variance (ANOVA) and Tukey mean comparisontest ( p 0.05) were performed (Statistica Software – Statsoft, OK –

USA).

3. Results and discussions

3.1. Moisture, thickness and density of the films

The incorporation of cellulose fibers reduced the films moisture

(Table 1), due to the lower water affinity of cellulose fibers

compared with starch (Curvelo et al., 2001; Dufresne & Vignon,

1998; Funke et al., 1998; Ma et al., 2005). There was no significant

difference among the thicknesses, which varied from 110 11 mm

to 12118 mm. On the other hand, fibers incorporation caused

a significant reduction of film density, e.g., the film P50 presenteddensity 1.8 times lower than the film without cellulose fibers,

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 films’ surfaces and fractures (Fig. 1) showed

homogeneous and random distributions of fibers within the

samples, without pores or cracks formation. It can also be observed,

from the fractured films micrographies, that cellulose fibers are

incrusted in the continuous starchy material.

3.3. Moisture sorption isotherms

The GAB model fitted well the films’ 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 aw ¼ 0.03)

were reduced as the fiber concentration in the films increased. The

water sorption isotherm of cellulose fibers shows clearly their

lower hygroscopicity. The P10 film presented equilibrium moisture

similar to the ones observed for the films without fibers. For the

last, the hygroscopicity increased due to the presence of guar gum,

which probably balanced the opposite effect provoked by the

cellulose fibers. 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 cellulosefibers (Averous et al., 2001; Curvelo et al., 2001;

Dufresne & Vignon, 1998; Ma et al., 2005). Averous et al. (2001)

reported that wheat starch films reinforced with cellulose fibers

had their equilibrium moisture reduced. They attributed this

behavior to the interactions between fibers and the hydrophilic

sites of starch chain, which substituted the starch–water interac-

tions that predominate in films without fibers. The effect of fibers

on films’ hygroscopicity can also be observed from the monolayer

water content data, mo, which was 0.094 g water/g solid for films

without fibers against 0.058 g water/g solid for films with 0.50 g of fiber/g of starch. On the other hand, the k value was not affected by

film composition (fiber addition).

3.4. Influence of relative humidity gradient (DRH) range and fibers

concentration on the water vapor permeability (Kw), solubilitycoefficient (b) and effective water diffusion coefficient (Dw)

For all samples K w values increased as the DRH range moved

from lower to higher values (Table 3). For the DRH ranges of 2–33%

and 64–90%, K w of P50 films presented values approximately 3.7

and 1.5 times lower if compared with starch films without fibers

(WF). However, at the intermediate gradient (33–64%), water vapor

permeabilities of WF films and P50 were of the same order

(3.43107

and 3.08 107

g/h m Pa). For the P30 formulation theK w value was about 10 times higher when the DRH range passed

from 2–33% to 64–90%. This result can be explained by the

increasing of 8.3 times observed for the solubility coefficient b,

Table 1

Moisture, thickness and density of starch films reinforced with different concentrations of cellulose fibers.

Samples Moisture (g water/g dry solid) Thickness (mm) Density 106 (g/m3)

WF 0.120 0.006a 111 11a 2.41 0.12a

P10 0.115 0.002b 111 8a 1.47 0.08b

P30 0.107 0.004c 111 13a 1.32 0.08bc

P50 0.090 0.002d 121 18a 1.31 0.03c

a,b,c,d Values with the same letter at the same column are not different statistically ( p< 0.05).

WF – without fibers.P50, P30 and P10 denote 0.50, 0.30 and 0.10 g fibers/g starch, respectively.

C.M.O. Muller et al. / Food Hydrocolloids 23 (2009) 1328–13331330



while the diffusion coefficient Dw did not change considerably (only

22% higher).

The b value of all formulations increased as the DRH range

moved from 2–33% to 64–92%. Fig. 3 shows that for water activities

lower than 0.10 the b value was very influenced by aw, i.e.,b decreased very quickly with the aw increase. From aw values

between 0.10 and 0.60, b was practically constant, but became very

responsive to aw when aw > 0.60.

Film densities decreased from 2.41 to approximately

1.3103 g/m3 due to cellulose fibers addition, which influenced

the values of K w

, accordingly to Eq. (5).The DRH rangedidnotaffect a lot the Dw values of films without

fibers, indicating that in this case the K w increasing was conse-

quence of the increment of b . The WF-film’ permeability was 4.5

times higher at the DRH ¼ 64–90% than at DRH¼ 2–33%, due to the

big increase of b (about 6 times), while Dw did not change

considerably.

The values of Dw for P10, P30 and P50 did not present appre-

ciable change, except at intermediate DRH range (33–64%). For P30

samples, when DRH passed from 2–33% to 33–64%, the K w

increased 5.6 times, due to the simultaneously increase of

b (2 times) and Dw (2.8 times). Under these DRH conditions, both

coefficients b and Dw had about the same influence on the K w

values.

For the DRH ranges of 2–33% and 64–90% the addition of cellulose fibers provoked decreasing of K w from approximately 10

Fig. 1. Scanning electron microscopy of cassava starch films with cellulose fibers. P50, P30 and P10 denote 0.50, 0.30 and 0.10 g fibers/g starch, respectively and the upper index s

and f denote surface and fracture, respectively.

Table 2

GAB model fitted parameters for sorption data from cassava starch films with

incorporation of cellulose fibers.

Sample GAB parameters R2

mo (g water/g solid) k C

WF 0.094 0.928 322.22 >0.99

P10 0.087 0.959 127.74 >0.99

P30 0.073 0.965 80.51 >0.99

P50 0.058 0.982 65.87 >0.99

Cellulose fibers 0.029 0.877 9.65 >0.99

WF – without fibers.P50, P30 and P10 denote 0.50, 0.30 and 0.10 fibers/g starch, respectively.




to 5.7 gm/m2 h Pa. It was due mainly to the film density reduction

from 2.410.12 to 1.31 0.03106 g/m3 and to a small reduction

of the b values. For the DRH¼ 33–64% the addition of fibers did not

affect K w appreciably, which could be explained by the small vari-

ation of b and Dw with the addition of cellulose fibers (Fig. 3, Table

3). The reduced density of films reinforced with cellulose fibers

could reduce K w, but it was not observed.

In a general manner, similar behavior of K w, b and Dw with RH

range was reported for keratin films (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

Dw. However, the mass transfer mechanisms controlling the barrier

properties of reinforced films can change with DRH range and

cellulose fiber concentration and cannot be evidenced from globalproperties 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-films with different formulations (Chen,

Lee, & Teoh, 2007; Straadt, Rasmussen, Andersen, & Bertram, 2007).

3.5. Mechanical properties

Films reinforced with fibers presented higher values of tensile

strength and elasticity modulus, and lower values of tensile

strength (deformation at break) (Fig. 4), if compared with WF films.

This behavior is in agreement with the results reported in the

literature about starch films reinforced with different kinds of

fibers (Averous & Boquillo, 2004; Curvelo et al., 2001; Funke et al.,

1998; Gaspar et al., 2005; Lu, Weng, & Cao, 2006).

The incorporation of 0.10 and 0.50 g fiber/g starch increased the

tensile strength of reinforced films in 6.7 and 18 times, respectively.

The elasticity modulus of P10 and P50 films were, respectively, 7.6

and 34 times greater than the elasticity modulus of WF films. This

significant increasing of films rigidity has been attributed to the

similarity between the chemical structures of cellulose and starch

(Ma et al., 2005).

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

aw

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

X w ( g

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

P10 P30 P50 Cellulose fibersWF

Fig. 2. Water sorption isotherms of cassava starch films with cellulose fibers fitted

with the GAB model. P50, P30 and P10 denote 0.50, 0.30 and 0.10 g fibers/g starch,

respectively.

Table 3

Water vapor permeability (K

w

), solubility coefficient (b) and diffusion coefficient(Dw) of cassava starch films with cellulose fibers incorporation as a function of the

relative humidity gradient (RH).

aw Gradients Samples K w 107 (gm/m2 h Pa) b 106 (g/gPa) Dw 108 (m2/h)

0.02–0.33 WF 2.33 0.13 1.78 5.30

0.33–0.64 3.43 0.24 2.91 4.65

0.64–0.90 10.3 0.18 10.6 3.93

0.02–0.33 P10 0.85 0.08 1.40 4.13

0.33–0.64 4.23 0.19 2.96 9.72

0.64–0.90 8.32 0.72 12.1 4.68

0.02–0.33 P30 0.67 0.01 1.27 4.00

0.33–0.64 3.78 0.13 2.53 11.3

0.64–0.90 6.87 0.09 10.5 4.96

0.02–0.33 P50 0.64 0.09 1.06 4.61

0.33–0.64 3.08 0.14 2.11 11.1

0.64–0.90 5.74 0.26 9.41 4.66

WF – without fibers.P50, P30 and P10 denote 0.50, 0.30 and 0.10 g fibers/g starch, respectively.

0,0 0,2 4,0 0,6 0,8 1,0

aw

0,00

2E-5

4E-5

6E-5

8E-5

1E-4

β ( g

w a t e r

/ g d r y s o l i d s . P a ) Cellulose fibers

P50

Fig. 3. Solubility coefficient values (b) for cassava starch films with cellulose fibers

(P50) and cellulose fibers as a function of water activity ( aw).

0

5

10

15

20

25

30

WF P10 P30 P50

Samples

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

0

20

40

60

80

100

120

P er c en t el on g a t i on

a t b r e ak ( % )

Tensile strength

Elongation

0

200

400

600

800

WF P10 P30 P50

Samples

Y o u n g ´ s M o d u l e ( M P a

)

a

b

Fig. 4. (a) Tensile strength and percent elongation at break of cassava starch films with

cellulose fibers incorporation. (b) Young’s modulus of cassava starch films with

cellulose fibers incorporation. WF denotes without fibers films, while P50, P30 and P10denote 0.50, 0.30 and 0.10 g fibers/g starch, respectively.

C.M.O. Muller et al. / Food Hydrocolloids 23 (2009) 1328–13331332



4. Conclusions

Films’ water vapor permeability have strong dependency on

solubility and water diffusion coefficients and, consequently, on

relative humidity gradient range, presenting values up to 2–3 times

greater at 33–64% than at 64–90%, depending on film formulation.

Therefore, it is important to determine this property based on the

environmental conditions the film will be used.

The incorporation of cellulose fibers reinforces mechanically

starch films, which have higher tensile strength and lower defor-

mation capacity. The reinforced films present lower water vapor

permeabilities if compared with starch films without fibers. As

cellulose fibers are biodegradable and available at low cost, their

use as reinforcing agent is a viable alternative to improve the

properties of starch-based films.

Acknowledgements

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

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