Food Research International 42 (2009) 976–982

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Food Research International

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A comparison between the physico-chemical properties of tuber and cereal starches

Nancy L. García a,b, Lucia Famá a, Alain Dufresne c, Mirta Aranguren d, Silvia Goyanes a,*

a Laboratorio de Polímeros y Materiales Compuestos, Depto. de Física, FCEN – UBA, Ciudad Universitaria, C1428EGA, Ciudad Autónoma de Buenos Aires, Argentinab Universidad Nacional de San Martín (UNSAM), Buenos Aires, Argentinac Grenoble Institute of Technology (Grenoble INP), The International School of Paper, Print Media and Biomaterials (PAGORA), BP 65, F-38402 Saint Martin d’Hères Cedex, Franced INTEMA, Universidad Nacional de Mar del Plata, Av. Juan B. Justo 4302 7608FDQ, Mar del Plata, Argentina

a r t i c l e i n f o

Article history:Received 9 February 2009Accepted 10 May 2009

Keywords:BiofilmsStarchMechanical propertiesTGAX-rayFTIRSEM

0963-9969/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.foodres.2009.05.004

* Corresponding author. Tel.: +54 11 45763300; faxE-mail address: goyanes@df.uba.ar (S. Goyanes).

a b s t r a c t

Biofilms based on waxy maize and cassava starches (cereal and tuber starch, respectively), plasticizedwith glycerol were characterized through their crystallinity, dynamic-mechanical behavior (DMA), ther-mal degradation (TGA), moisture content and water vapor permeability (WVP). X-ray diffraction exper-iments show that both materials were mainly amorphous, with the waxy starch presenting a discreetlyA-type X-ray pattern. Microscopic investigation of the cryo-fractured surfaces supported this observation.The glass transition of the glycerol-rich phase (measured by DMA) occurs at higher temperatures for cas-sava than for waxy maize starch, suggesting that the dispersion level of glycerol is higher in the former.TGA showed that maize starch has a slightly higher thermal stability than cassava starch, while glycerolinteracts more strongly with the last one. The WVP was 18% higher in the case of the cassava starch film.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The interest in using natural and renewable polymers isincreasing in different areas of application, among them, the sub-stitution of synthetic packaging films. In this context, starch ap-pears as a well-known material having green connotation: it isbiodegradable, edible, not depending on fossil sources and avail-able in large quantities. The growing demand for biodegradabilityallows foreseeing larger volume productions, as starch is used inplastic films and sheets, as well as in natural fibre composites thatcould eventually replace plastics foams (Morton, 2007).

Worldwide, corn represents the major commercial source ofstarch, while cassava is another important starch source in somecountries like Brazil, which is the largest cassava-producing coun-try (Leonel & Cereda, 2002), and also in Thailand, Malaysia, Indone-sia and some parts of Africa (International Starch Institute, 2001).Cassava harvest can take place most of the year with comparativelylower manufacturing and storage costs than corn. Additionally,tapioca starch (from cassava plant) gives a competitive advantageover the use of other starches, like potato-starch, in many applica-tions. On the other hand, waxy maize starch, which has built-intextural stability due to its molecular composition, has an annual

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: +54 11 45763357.

production in the European Union of about 220,000 tons (Mason,2007).

According to their origin, starches differ by their amylose andamylopectin content, two macromolecular polymers of a-D-gluco-pyranosyl. Amylose is a linear glucan with a-1,4 glycosidic link-ages, composed of long chains that take mainly a helicalconformation, whereas amylopectin is a branched polymer ofhigher molecular weight than amylose (Ophardt, 2003; Thomas& Atwell, 1997). This difference in the conformation of moleculesgrants them different properties. Several studies have been pub-lished on the analysis of the properties of starch-based filmsand have demonstrated that variations in the amylose/amylopec-tin ratio lead to starch-films with different mechanical properties(Alves, Mali, Beléia, & Grossmann, 2007; Arvanitoyannis, Biliade-ris, Ogawa, & Kawasaki, 1998; Lourdin, Valle, & Colonna, 1995;Mali, Grossmann, Garcia, Martino, & Zaritzky, 2002; Tharanathan,2003).

Characterizations of different forms of thermoplastic starchesbased on cassava starch were carried out by De Morais Texeira,Da Roz, De Carvalho, and Da Silva Curvelo (2005); they showedthat the presence of sugars and different molecular conformationsconfers interesting results in terms of crystallinity, water absorp-tion and mechanical behavior. However, Guinesi et al. (2006), pre-sented not significant differences between values of the kinetictriplet of the starches, indicating a little influence on the thermaldegradation process of the starches from different botanicals

N.L. García et al. / Food Research International 42 (2009) 976–982 977

origins. In addition, multiple factors appear in the bibliographymodifying the properties of the starches (Da Roz, Carvalho, Gandi-ni, & Curvelo, 2006; Flores, Famá, Rojas, Goyanes, & Gerschenson,2007).

Following the above observations, the objective of the presentwork was to analyze the relation between the physico-mechanicalresponse and the structure (composition) of two starches with po-tential applications in the manufacture of food packaging, namelywaxy maize and cassava, a cereal and a tuber starch, respectively.

2. Materials and characterizations

2.1. Materials

The cassava starch (72 wt.% amylopectin and 28 wt.% amylose)was provided by Bernesa S.A., Buenos Aires, Argentina. The nativemaize starch (99 wt.% amylopectin) was provided by Roquette S.A.,Lestrem, France. The used plasticizer was glycerol (Baker, purity99.9 wt.%).

2.2. Film processing

Thermoplastic starch was processed by casting solutions pre-pared with native waxy maize or cassava starch granules, glyceroland distilled water. A quantity of 10 g of starch granules togetherwith 5 g of glycerol were dissolved in 200 g of distilled water.The glycerol content (33.3 wt.%) was calculated on a dry basis.Then, the mixture was heated from room temperature at a heatingrate of 1.59 �C/min under mechanical stirring during 28 min untilgelatinization, which occurred at �70 �C. After gelatinization, thegel was degassed for 1 h, under mechanical vacuum pumping.Then, the mixture was cast in a plastic mold and evaporated inan air-convection oven at 50 �C for 24 h. Solid films having a thick-ness of about 300–400 lm (according to optical microscopy) wereobtained. Finally, the films were stored in controlled relativehumidity boxes (43% RH atmosphere, by means of K2CO3 saturatedsolution) for 2 weeks.

2.3. Attenuated total reflectance fourier transform infraredspectroscopy (ATR/FTIR)

Infrared spectra of the produced films were recorded on a Gen-esis II Fourier transform infrared spectrometer using the attenu-ated total reflectance (ATR) accessory. The spectra were obtainedwith a resolution of 2 cm�1, averaging over 32 scans.

2.4. X-ray diffraction

Films were submitted to X-ray radiation using a diffractometerPhilips; model PW 1830 operating at Co Ka radiation wavelength(k = 1.79018 Å), 40 kV, 30 mA and sampling interval of 0.02�. Scat-tered radiation was detected in the angular range of 10–35� (2h).

2.5. Scanning electron microscopy (SEM)

Cryo-fractured surfaces of samples cooled in liquid nitrogen andthen broken were used to examine the morphology of the filmsusing a SEM with Field Emission Gun (FEG) Zeiss DSM982 GEMINI.The samples were coated with a thin sputtered gold layer beforethe analysis.

2.6. Thermogravimetric analysis (TGA)

Thermogravimetric analysis was carried out using a Perkin–El-mer TGA-50H instrument under nitrogen flow of 30 mL min�1

and at a heating rate of 10 �C min�1. The temperature scans wereperformed from room temperature to 500 �C.

2.7. Dynamic mechanical analysis (DMA)

Dynamic mechanical measurements were performed using Dy-namic Mechanical Thermal Analyzer (DMTA IV) Rheometric Scien-tific equipment in the Rectangular Tension mode at 1 Hz, in thetemperature range between �120 and 70 �C, at a heating rate of2 �C min�1. The dimensions of the samples were25 � 9 � 0.4 mm3, measured at ±0.01 mm. The samples were sub-jected to a cyclic strain lower than 0.04%. This strain value was suf-ficiently small to ensure that the mechanical response of thespecimen was within the linear viscoelastic range. The results ofthe tests were acquired in the form of the storage modulus E0,the loss modulus E00 and the ratio between the two components,tan d = E00/E0.

2.8. Moisture determination

Moisture content of the films was determined using standardmethods of analysis of the international Association of Official Ana-lytical Chemistry (AOAC, 1990). Samples (�0.5 g) were dried in avacuum oven at 70 �C, over calcium chloride, until reaching con-stant weight. The reported results represent the average of threesamples and the standard deviations are included.

2.9. Water vapor permeability (WVP)

WVP tests were conducted using ASTM E96-00 (1996). Filmspecimens were conditioned for 2 weeks in desiccators at 25 �Cand 43% relative humidity (equilibrium with a K2CO3 saturatedsolution) before being analyzed. After that, each film sample wassealed over a circular opening of 21.85 mm in a permeation cellthat was stored at 25 �C in a dessicators. To maintain a 58% RH gra-dient across the film, silica gel (0% RH), activated at 200 �C, wasplaced inside the cell and a sodium bromide (NaBr) saturated solu-tion (58% RH) was placed in the desiccators, outside the cell. Watervapor transport was determined from the weight gain of the per-meation cell. After steady state conditions were reached (about2 h), eight weight measurements were made over 24 h. Changesin the weight of the cell were recorded to the nearest 0.0001 gand plotted as a function of time for 10 days. Cells were carefullyshaken horizontally after every weighing. The slope of each linewas calculated by linear regression (r2 > 0.99) and the rate of watervapor transfer (WVT, g h�1 m�2) was determined from the slope ofthe straight line (g h�1) divided by the cell area (m2). Thickness ofeach film was measured with a microscope at six randomly se-lected points after the permeation test. Three replicate sampleswere analyzed.

The WVP was calculated as:

WVP ¼ ðWVT=SðR1� R2ÞÞ � T ð1Þ

where S is the saturation vapor pressure of water at the test temper-ature (25 �C), R1 is the relative humidity in the desiccators, R2 is therelative humidity inside the permeation cell, and T is the filmthickness.

3. Results and discussion

3.1. Attenuated total reflectance fourier transform infraredspectroscopy (ATR/FTIR)

Fig. 1 shows a comparison between the FTIR spectrum obtainedfor the waxy maize and the one obtained for cassava starch. Both

I

II

Fig. 1. (a) FTIR-ATR spectra of cassava and maize starch. (b) Detail of the zone around 1000 cm�1. Curves have been shifted for comparison, y-axis in arbitrary units.

978 N.L. García et al. / Food Research International 42 (2009) 976–982

spectra are dominated by a broad band assigned to the stretchingvibration modes of OH-groups from the absorbed water and fromthe polymer itself, at ca. 3300 cm�1.

Van Soest, Tournois, De Wit, and Vliegenthart (1995), studyingthe influence of water content and crystallinity on starch samples,related the 1000 cm�1 infrared band to hydrogen bonding of thehydroxyl group at C6 and showed, for amorphous starch withwater content above 10%, that the position of the band at1022 cm�1 shifts to ca. 1015 cm�1, and that the band at994 cm�1 shifts to 991 cm�1, as the water content increases. Theseresults, which are mainly due COH bending vibrations, are spe-cially sensitive to the water content and are caused by changesin hydrogen bonding. Changes of this band have been ascribed tovariations in the molecular environment of the primary hydroxylgroup in amylose, resulting from changes in intramolecular hydro-gen bonding. In addition, the intensity of the band at 1022 cm�1 in-creases with decreasing crystallinity. Comparing the resultsreported by Van Soest with those presented in this work, the bandat 994 cm�1 (see Fig. 1b) does not present different intensities inboth starches, which would suggest similar water contents. Theband reported by Van Soest et al. (1995) at 1022 cm�1 appears at1014 cm�1, approximately, for both starches. The band shape at1022 cm�1 shows differences between both spectra and the peakis more intense in the cassava starch. This last observation suggeststhat waxy starch possesses a larger fraction of regions of local or-der than cassava starch. Other authors, Capron, Robert, Colonna,

0 5 10 15 20 25 30 35 40

4Å type B Cassava Starch

+++ Waxy maize Starch

Inte

nsity

(u. a

.)

2θ

5.9Å type A5.1Å type A

4.5Å type B

3.9Å type A

5.2Å type B

4.5Å type B 4Å type B

Fig. 2. X-ray diffraction patterns of cassava and waxy maize starch.

Brogly, and Planchot (2007) have revealed that the structural orga-nization of amylopectin within the semi-crystalline part of thegranules involves double helix structures linked to each other bychains to form clusters. All hydrogen atoms are located insidethe double helix, whereas hydroxyl groups are found outside. Uponaddition of a plasticizer or moisture absorption, new hydrogenbonds are formed. These links are located between the double heli-ces and could facilitate the formation of links between them, lead-ing to greater local order.

3.2. X-ray diffraction

The X-ray diffraction patterns obtained for both films are shownin Fig. 2. The two films show an important contribution of theamorphous phase, containing a small crystalline fraction, in agree-ment with previous published results (Angellier, Molina-Boisseau,Dole, & Dufresne, 2006; Famá, Flores, Gerschenson, & Goyanes,2006; Famá, Goyanes, & Gerschenson, 2007; Famá, Rojas, Goyanes,& Gerschenson, 2005; Flores et al., 2007; Kristo & Biliaderis, 2007).

The crystalline peaks were analyzed in the interval from 10� to30� (2h), identifying the most intense peaks and calculating the dis-tances between the planes of the crystals d (Å) from the diffractionangles (�) according to Bragg’s law: nk = 2d sin h; where k is thewavelength of the X-ray beam and n is the order of reflection.

In both starch films, the diffraction of the B-crystalline structureappears, with peaks at 2h = 19.8�, 23.2�, 25.6� and 26.7�, corre-sponding to interplanar spacing d = 5.2 Å, 4.5 Å, 4.0 Å and 3.9 Å,respectively. Angellier et al., 2006; Mathew & Dufresne, 2002, havepreviously reported that waxy maize granules recrystallize in Bstructure after being completely dissolved in water. Nevertheless,it would seem that waxy starch film develops a smoothly A-typeX-ray pattern with peaks that appears discreetly at interplanarspacing, d = 5.9 Å, 5.1 Å and 3.9 Å (see Fig. 2) (Primo-Martın, VanNieuwenhuijzen, Hamer, & Van Vliet, 2007; Thielemans, Belgacem,& Dufresne, 2006).

This A-type X-ray diffraction patterns could be due to a local or-der generated by associated amylopectin chains, whose presence issuggested by SEM observation like a radially oriented fibrillar net-work (see morphological characterization by SEM, next section).This local order is in agreement with observations from infraredspectroscopy FTIR. On the other hand, according to data reportedby Osella, Sanchez, Carrara, de la Torre, and Buera (2005), thedevelopment of A or B-type diffraction patterns during aging de-pends on the water content. In our case the water content in thesamples is in the limit of the results reported by Osella et al.(2005), which could lead waxy maize starch to develop a pattern

Fig. 3. SEM images of cryo-fractured surfaces of films made from cassava starch (a) and waxy maize starch (b).

N.L. García et al. / Food Research International 42 (2009) 976–982 979

type A. In the case of cassava, the presence of amylose and thefavorable interaction with glycerol would avoid the formation ofan order of short-range as it was suggested by FTIR, generating aB-type pattern.

3.3. Morphological characterization by SEM

Fig. 3a and b shows the micrographs of the cryo-fractured sur-faces for cassava and waxy maize starch films, respectively. Inthis figure, it can be seen that the cassava starch film showsextensive plastic deformation, while in the case of waxy starchfilm the fracture surface shows zones similar to the cassavastarch, but also spherical zones, with regions darker than theperipheral zone.

Higher magnification of these spherical zones (Fig. 4) showsthat they present essentially no plastic deformation (relativelysmooth surfaces) and, that they contain smaller spherical spots in-side, which could be associated to the circular space filling patcheswith clusters observed by Fishman, Cooke, White, and Damert(1995) in images of amylopectin solubilized from waxy maizestarch granules. Amylopectin, the major, branched component instarch is built up from clusters of short chains of 6–35 (1–4) linkedalpha-D-glucosyl residues (Manners, 1989). Li, Vasanthan, Hoover,and Rossnagel (2004), observed in gelatinized starch of waxyhull-less barley (HB) the development of a radially oriented fibril-

Fig. 4. SEM images of the cryo-fractured surface of a film made from waxy maize

lar network in granules heated at 65 �C. It is very probable that thisfibrillar network is observed as a spot inside the region, due to thecryogenic fracture of the surfaces.

A similar structure of amylopectin is also observed in heatedwaxy potato starches (Hermansson & Svegmark, 1996) and heatedwaxy maize starch (Barron, Bouchet, Della Valle, Gallant, & Plan-chot, 2001).

3.4. Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) was used to analyze the dif-ferences in the thermal stability of cassava and waxy starch filmsand the effect of the glycerol added to plasticize the films.

Fig. 5 show the TGA curves for both unplasticized and glycerol-plasticized starch-films above 100 �C, so that the mass losses arecompared on essentially a dry basis (after water loss).

The thermal decomposition of the biofilms occurs in three mainsteps, which agrees with the reports of Wilhelm, Sierakowski, Sou-za, and Wypych (2003), and other authors (Jiang, Qiao, & Sun,2006; Rajan, Prasad, & Abraham, 2006; Rath & Singh, 1998). Thefirst stage does not appear in Fig. 5. since it corresponds to the lossof water. The second stage is due to the decomposition of the glyc-erol-rich phase, which also contains starch, and the third stage cor-responds to the oxidation of the partially decomposed starch(Wilhelm et al., 2003).

starch (left) and higher magnification of the zone of ‘‘spherical spots” (right).

Fig. 5. TGA curves for unplasticized waxy maize and cassava starch films, and forglycerol-plasticized waxy maize and cassava starch films.

-100 -75 -50 -25 0 5025

1E-3

0.01

0.1

1

Tan

δ

E'/E

'g

Temperature °C

Cassava Starch+++ Waxy maize Starch

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Fig. 6. Isochronal evolution of the logarithm of the relative storage modulus andtangent of the loss angle as a function of temperature for glycerol-plasticized waxymaize and cassava starch films (E0g is the value of the storage modulus in the glassystate).

980 N.L. García et al. / Food Research International 42 (2009) 976–982

As it can be seen in Fig. 5 the degradation behaviour of glycerol-plasticized films is very different from the one of the unplasticizedsamples. The glycerol addition in the starch formulations reducesits thermal stability (in Fig. 5: bubble ‘‘o” and solid lines) and, asexpected, it modifies the degradation in the temperature range be-tween 120 �C and 300 �C, where the decomposition of the glycerol-rich phase takes place. It should be noted that this effect is morenotorious in the case of cassava starch, suggesting that, in this sys-tem more hydrogen bonding interactions (between hydroxylgroups of starch chain and glycerol molecules) are possible. Thedegradation occurring around 320 �C, associated with starch deg-radation, is similar in all the samples, although a slight shift to-wards lower temperatures can be seen in the samples containingglycerol. It is clear that the glycerol–starch interaction facilitatesthe thermal degradation of the starches, also leading to lower charvalues than those obtained for the unplasticized samples.

These results can be attributed to the following effect: the addi-tion of glycerol decreases the interaction (both intra and intermo-lecular links) of the starch–starch chains, because of the stronghydrogen bonding formed between the hydroxyl groups of starchchains and the glycerol molecules. As a consequence, it becomeseasier to degrade the polymer chains in the presence of the plasti-cizer (Cyras, Tolosa Zenklusen, & Vazquez, 2006).

Nevertheless the glycerol–starch interaction in plasticized waxystarch appears different from the cassava counterpart, according tothe TGA results. In the case of the film obtained from the plasti-cized waxy maize starch an abrupt degradation starting at260 �C, associated to the degradation of the starch, is observed,and this behavior was not much affected by the presence of glyc-erol in the film.

Starch occurs in the form of granules, consisting of highlybranched amylopectin and linear amylose molecules. It is sug-gested that the highly branched amylopectin (the major compo-nent in waxy maize starch) does not interact with glycerol asstrongly as amylose, which may reduce the cohesive strength ofthe glycerol–waxy starch (Cyras et al., 2006).

The difference observed is in agreement with the difference incrystallinity showed by the films (FTIR, X-ray, and SEM) and thelower interaction of the waxy maize starch with glycerol as com-pared to cassava starch.

3.5. Dynamic mechanical analysis (DMA)

Fig. 6 shows the isochronal evolution (1 Hz) of the logarithm ofthe relative storage modulus (log E0/Eg), defined as the ratio be-tween the actual modulus and the modulus in the glassy state, as

a function of temperature for biofilms formulated from bothsources of starch.

As it has been previously reported in the open literature,starch–glycerol is a partially miscible system, giving rise tostarch-rich and glycerol-rich phases (Curvelo, Carvalho, & Agnelli,2001; Da Roz et al., 2006 Mathew & Dufresne, 2002). Accordingly,the dynamic-mechanical response of the films shows two relaxa-tion peaks. The low temperature peak is related to the main relax-ation of the plasticizer-rich phase, and the high temperaturerelaxation peak is attributed to the glass transition temperatureof the corresponding thermoplastic starch, as already suggestedby other authors (Famá et al., 2006). Fig. 6 shows that the tan dpeak, located around �60 �C and associated to the relaxation ofthe glycerol-rich phase, appears at higher temperatures for the cas-sava starch film compared to the waxy maize starch film. Consid-ering this and the fact that the glass transition temperature, Tg,of pure glycerol is located at �78 �C (Standing, Rindlav-Westling,& Gatenholm, 2001); this suggests that glycerol interacts morestrongly with the cassava starch. The higher interaction of glycerolwith the cassava starch compared to waxy maize can be explainedby the formation of hydrogen bonding between glycerol and amy-lose and this observation is in good agreement with the resultsfrom other techniques as already discussed (in particular, TGA).

According to Mathew and Dufresne (2002), the very broad, lowintensity relaxation observed at �10 �C may be associated to therearrangement of amorphous amylopectin chains in the presenceof moisture. This process should appear more intensely in the caseof waxy maize starch because it contains around 99 wt.%amylopectin.

The relative value of the rubbery storage modulus (E0r=E0g) ishigher for cassava starch compared to waxy maize starch film.For instance, the absolute value of the storage modulus at 50 �Cis 7.34 MPa for the waxy maize starch film, whereas it is 38 MPafor the cassava starch film. It is well known, that the storage mod-ulus value for this type of materials is highly dependent on theglycerol content, the film processing method, the antimicrobialcontent, etc. However, there is also wide agreement in the litera-ture with respect to the positive effect of an increase of the amy-lose content on the elastic modulus of starch films (Alves et al.,2007). According to Rindlav-Westling, Stading, Hermansson, andGatenholm (1998), the structure of the amylose network is verystable, with strong molecular orientation, forming denser filmsthan amylopectin.

Table 1Equilibrium moisture content and water vapor permeability for cassava and maizestarch films.

Starch film Moisture content WVP (gm�1 s1 Pa�1) � 1010 (0/58%) RH

Cassava 30 ± 1.13 4.5 ± 0.6Waxy maize 27 ± 0.21 3.8 ± 0.3

N.L. García et al. / Food Research International 42 (2009) 976–982 981

The larger glycerol–amylose interaction may have an effect onthis molecule mobility, facilitating the necessary polymer chainrearrangements that could lead to the mentioned characteristicsof the films.

3.6. Equilibrium moisture content and water vapor permeability

Thermogravimetric analysis of both plasticized films did notshow important differences with respect to water loss (not shown).Similar results were obtained from determining the equilibriummoisture content (AOAC International norm, 1990).

The values of moisture content are reported in Table 1 and aresimilar to previous reports in the literature (Famá et al., 2006; Maliet al., 2002). As it can be seen, the equilibrium moisture content inthe case of the cassava starch film is similar to that of the waxystarch film.

The values of the water vapor permeability are reported in Ta-ble 1 and are of the same order of magnitude that those reportedby other authors for different types of starch (Kester & Fennema,1986; Talja, Helén, Roos, & Jouppila, 2007). Additionally, the per-meability is slightly lower for the film obtained from waxy maizestarch, although the dispersion of the results is rather high andthe difference may be within the experimental error. It is wellknown that WVP of films is dependent on both, the solubility coef-ficient and the diffusion rate of water in the film.

Considering the morphologies observed in the micrographs ofthe cryo-fractured samples (Figs. 3 and 4), the ill-defined differ-ences observed in the WVP values between both starch-films couldbe attributed to the presence of the spherical regions in waxymaize starch films (SEM micrographs), associated with regions ofhigher crystallinity (clusters of amylopectin). These crystalline re-gions could reduce the water diffusion rate.

4. Conclusions

The physico-chemical properties of two glycerol-plasticizedstarch films based on cassava and waxy maize (a tuber and a cerealstarch, respectively) were measured and compared.

Although both types of starch films are mostly amorphous, thewaxy maize starch film presents A-type crystallinity in addition tothe B-type structure. The higher contribution of amorphous struc-tures in the tuber starch was confirmed by FTIR, while the regionscontaining higher crystallinity (clusters of amylopectin) were ob-served in the cereal starch micrographs.

The cassava starch presents an important interaction with glyc-erol, indicating that more H-bonds are formed with amylose mol-ecules than with amylopectin chains. This observation wassupported by the large effect of the plasticization on the TGA ofcassava as compared to waxy maize starch films and by the posi-tion of the relaxation peaks in the dynamic mechanical analysis.These different interactions also result in a higher temperaturefor the beginning of thermal degradation of the waxy maize starchplasticized film. The stiffness of the glycerol-plasticized cassavastarch film in the rubbery state is also found to be higher comparedto its waxy maize starch counterpart.

The behavior of the two types of films subjected to humid envi-ronments were rather similar, with slightly lower values of diffu-

sivity in the film made from the cereal starch, which was relatedto the presence of associated amylopectin chains.

Acknowledgments

We acknowledge the financial support from the Universidad deBuenos Aires, Consejo Nacional de Investigaciones Científicas yTécnicas de la República Argentina, Agencia Nacional de Investi-gaciones Científicas y Tecnológicas de la República Argentina andScientific program of Cooperation between the Ministry of Science,Technology and Productive Innovation of the Argentine Republic(MINCYT) and the ECOS-Sud of France.

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