- Email: [email protected]
Starch behaviors and mechanical properties of starch blend films with different plasticizers
Accepted Manuscript Title: Starch behaviors and mechanical properties of starch blend films with different plasticizers Author: Hoang Phuong Nguyen Vu Namfone Lumdubwong PII: DOI: Reference:
S0144-8617(16)30969-9 http://dx.doi.org/doi:10.1016/j.carbpol.2016.08.034 CARP 11459
To appear in: Received date: Revised date: Accepted date:
13-3-2016 10-8-2016 10-8-2016
Please cite this article as: Nguyen Vu, Hoang Phuong., & Lumdubwong, Namfone., Starch behaviors and mechanical properties of starch blend films with different plasticizers.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.08.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Starch behaviors and mechanical properties of starch blend films with different plasticizers
Hoang Phuong Nguyen Vu, Namfone Lumdubwong*
a
Department of Packaging and Materials Technology, Faculty of Agro-Industry, Kasetsart
University, Bangkok 10900, Thailand
*
Corresponding author. Tel.: +66 97 279 8563. E-mail address: [email protected] (N. Lumdubwong).
Highlights
Mechanical properties of plasticized starch blend (CSMB) films were non-additive.
The non-additive effect was more pronounced in the glycerol-plasticized system.
CSMB films had most of thermal properties close to one of individual (MB) films.
Plasticized CSMB and MB films had a similar crystallinity type.
Mw of amylose of CSMB films was similar to that of MB films but not the content.
Page 1 of 36
ABSTRACT The main objective of the study was to gain insight into structural and mechanical starch behaviors of the plasticized starch blend films. Mechanical properties and starch behaviors of cassava (CS)/ and mungbean (MB) (50/50, w/w) starch blend films containing glycerol (Gly) or sorbitol (Sor) at 33% weight content were investigated. It was found that TS and %E of the GlyCSMB films were similar to those of MB films; but %E of all Sor-films was identical. TS of plasticized films increased when AM content and crystallinity increased. When Sor was substituted for Gly, crystallinity of starch films and their TS increased. The CSMB and MB films had somewhat a similar molecular profile and comparable mechanical properties. Therefore, it was proposed the starch molecular profile containing amylopectin with high Mw, low Mw of amylose, and the small size of intermediates may impart the high TS and %E of starch films.
Keywords: Starch blend film, Plasticizer, Amylose content, Starch film properties
Page 2 of 36
1. Introduction
Production and functional properties of edible starch films and coatings have been extensively investigated during the past decade, due to their vast potential uses in the food and pharmaceutical industries (Pavlath & Orts, 2009). Among their various attributes, the mechanical properties of starch films are of interest to researchers since they have important functions for film applications (Alves, Mali, Beléia, & Grossmann, 2007; Bae, Cha, Whiteside, & Park, 2008; Mali, Sakanaka, Yamashita, & Grossmann, 2005; Parra, Tadini, Ponce, & Lugão, 2004; Zullo & Iannace, 2009). Amylose (AM) content, crystallinity, glass transition temperature (Tg), types and contents of plasticizers, and sources of starch are major factors governing the mechanical properties (Gupta, Brennan, & Tiwari, 2012). Starches containing high AM display excellent film-forming ability and good mechanical properties (Bae et al., 2008; Chaudhary, Torley, Halley, McCaffery, & Chaudhary, 2009; Li et al., 2011). The content of B-type crystallinity also increased film stiffness as well as TS at break (Chaléat, Halley, & Truss, 2014). In addition, brittleness of starch films increased when the films possessed high glass transition temperature (Tg) (Myllärinen et al., 2002). Types and contents of plasticizers also affect the mechanical properties of starch films. Glycerol (Gly) and sorbitol (Sor) are the most widely used polyol plasticizers for starch-based films. Their excellent ability to prevent film cracking during handling and storage was reported; and their effectiveness is possibly due to the close similarity between their chemical structures and the structure of starch polymer (Mali et al. 2005). Gly exhibited a superior plasticization effect to Sor at equivalent weight content (Laohakunjit & Noomhorm, 2004; Mali et al. 2005;
Page 3 of 36
Talja et al. 2007). Nonetheless, Cuq, Gontard, Cuq & Guillbert (1997) disputed that the effect was mainly due to higher molecular content of glycerol. And yet, Zhang & Han (2006) demonstrated the reverse effect which was the Gly- plasticized pea starch films had lower %E than Sor-plasiticized films at the same molar content. Both plasticized films also exhibited similar TS. Mungbean (MB) starch, containing high AM (30%), is available in tropical countries. The costly starch produces clear, colorless, and high-strength gel. Thus, it is considered as the best raw material for cellophane noodle production. MB starch films displayed the highest TS among other starch film samples, and the starch imparted an excellent ability to form pharmaceutical hard capsules (Bae et al., 2008; Prachayawarakorn, Hommannee, Phosee, & Chairapaksatien, 2010). Cassava (CS) starch, one of the major industrial starches, is abundant and inexpensive. CS starch films were odorless, tasteless, and colorless. The edible films also provided good flexibility. The drawback of CS films was their rather low tensile strength (TS) compared to that of films from other starches (Mali et al. 2005; Alves et al., 2007; Thirathumthavorn, D., & Charoenrein, S., 2007). Blending of starches can alter functional properties of starch products such as gelatinization, pasting, rheological, gelation, and mechanical properties (Hongsprabhas, 2007; Liu & Lelièvre, 1992; Waterschoot, Gomand, Fierens, & Delcour, 2015; López, Lecot, Zaritzky, & García, 2011; Rachtanapun, Pankan, & Srisawat, 2012). The properties of starch blends can be classified into additive behavior, i.e., the property of the blends can be predicted from each starch component, and non-additive behavior, i.e., the property of the blends cannot be predicted,
Page 4 of 36
indicating any interactions occurred between starches (Puncha-arnon, Pathipanawat, Puttanlek, Rungsardthong, & Uttapap, 2008). Based on the available information, it is beneficial to develop edible films from CS starch partly substituted for MB starch, yet maintaining their mechanical properties comparable to those of MB films. Besides, little information is known about the molecular profile, microstructure, and morphology of starch blend films. In this study, the CS/MB (50/50, w/w) starch blend films containing glycerol or sorbitol at the same weight content were prepared. Their mechanical properties, starch molecular weight profile, and polymer morphology were investigated, and compared with individual starch films. The objectives of the study were to gain insight into structural, thermal, and mechanical starch behaviors of the plasticized starch blend films, and to produce the plasticized CS/MB blend films possessing mechanical properties closed to those of plasticized MB films.
2. Materials
Mungbean (MB) starch (Sitthinan, Bangkok, Thailand) and cassava (CS) starch (E.T.C. International Trading, Bangkok, Thailand) were purchased locally. Glycerol and sorbitol were supplied by Ajax Finechem (New South Wales, Australia).
3. Chemical composition analyses
Page 5 of 36
Protein, ash, moisture and crude fat content of starch were determined using standard official methods of AOAC (2000). Apparent amylose content was measured by the colorimetric method of Chrastil (1987).
4. Film-casting experimental design
The study was a 3 × 2 factorial design using two starches – cassava (CS) and mungbean (MB) – a CSMB starch blend, and two types of plasticizers. Native starches were mixed with distilled water at a concentration of 5% w/w (dry basis, db). Each suspension was heated at 80 C while magnetically stirring at a speed of ~350 rpm. The CS and MB starch suspensions were stirred for 1 h and 3 h, respectively, to obtain clear solutions. A blend of CS and MB solutions was prepared by mixing the clear solutions of CS and MB starches, prepared separately as described, at a weight ratio of 50:50. The MB starch solution was prepared 2 h prior to the CS starch solution. The CSMB blend was then further stirred for 20 min at 80 C in order to obtain a homogeneous solution. Then, two types of plasticizer (glycerol and sorbitol) were individually added to samples of each solution at 33% of starch weight. The molar ratio of glycerol and sorbitol per 100 g of starch dispersion was 0.018 mol and 0.009 mol, respectively. The filmforming solutions were continuously stirred using the same speed at 80 C for 15 min. Each film solution (35 g) was poured into a plastic Petri dish (140 mm diameter). The dishes containing solutions were dried in a hot-air oven at 60 C for 6 h. The films were peeled off and kept in a desiccator at 52% relative humidity (RH) and ambient temperature for not more than 4 weeks, to be used for further analyses. Unplasticized films (CS, MB and a 50:50 blend of CS and MB starches) were also prepared and stored in the same manner, to be used as controls.
Page 6 of 36
5. Characterization of starch-based films
5.1. Mechanical and thermal properties
Tensile strength, elongation at break and Young’s modulus of the films were determined using a universal testing machine (model 5965; Instron, Norwood, MA, USA), according to ASTM standard method D882–02 (ASTM, 2002). Thermal properties of unplasticized and plasticized starch films were determined by differential scanning calorimetry (DSC) (model DSC 1, Stare System; Mettler-Toledo, Greifensee, Switzerland). Each sample (10 mg, db) was weighed in an aluminum pan, and the analysis was performed under nitrogen atmosphere. The samples were heated from −70 °C to 250 °C at 5 °C/min, then cooled to 25 °C at 20 °C/min, and reheated again from 0 °C to 250 °C at 5 °C/min. Glass transition temperatures of the films (10 mm width 50 mm length) were determined in the tension mode using a dynamic mechanical analyzer (DMA) (Eplexor®; Gabo Qualimeter, Ahlden, Germany). The static load and dynamic load were 1 N and 0.3 N, respectively. The maximum static strain and dynamic strain were set at 0.3%. Measurements were performed at a frequency of 1 Hz using a temperature range from −100 C to 100 C at 2 C/min.
5.2. Crystallinity and microstructure of native starches and starch films
Page 7 of 36
Crystallinity of native starches and unplasticized and plasticized starch films was measured by an X-ray diffractometer (X’Pert; Philips, Amsterdam, Netherlands). Each sample was scanned over a diffraction angle range of 2θ = 5−35 °C using a scan rate of 0.03°/sec. Percent of relative crystallinity was calculated as the ratio of the area above a smooth curve connected peak baselines to the total area over the diffraction angle 5-35. The data was analyzed by Jade software. Plasticized starch films (20 mm × 20 mm) were iodine vapor stained using Lugol’s solution (1% iodine (I2) and 2% potassium iodide (KI) in water) in a closed chamber for 2 h. The microstructure of starch films was investigated under a light microscope (LSM 5 PASCAL; Zeiss, Oberkochen, Germany) with and without a polarized filter.
5.3. Molecular weight profiles of native starches and starch films
The molecular weight profiles of native starches, a dry blend of both starches (50:50 w/w), and three unplasticized starch films (CS, MB and CSMB blend) were determined using the a high-performance size-exclusion chromatography (HPSEC) system, according to the method of Israkarn, Na Nakornpanom, and Hongsprabhas (2014). Six pullulan standards (1 mg/mL) (Showa Denko, Tokyo, Japan) were used to construct a standard curve. All samples were defatted by the AOAC method (AOAC, 2000) prior to the analyses. The amylose content of all samples was analyzed using the ratio of the peak area of amylose and amylopectin.
5.4. FTIR spectroscopy
Page 8 of 36
Fourier transform infrared (FT-IR) spectra of unplasticized and plasticized films were recorded in attenuated total reflectance (ATR) mode using a Bruker Tensor 27 FTIR spectrometer (Bruker, Billerica, MA, USA). The spectra were recorded from 500 cm-1 to 4000 cm-1.
5.5. Statistical analyses
Chemical compositions, thermal properties, molecular weight measurements and microstructure analyses were done at least in duplicate. The various films were prepared at least in duplicate and their mechanical properties were determined from at least five specimens of each film treatment. Analysis of variance (ANOVA) and Duncan’s multiple comparison (p<0.05) were performed using SPSS 11.0 software.
6. Results
6.1. Chemical compositions and gelatinization of native starches
The protein, fat, and ash contents of native CS and MB starches were 0.16%, 0.04% and less than 0.05%, respectively, indicating that the starches were pure (Table S1). The amylose (AM) content of CS and MB obtained from the HPSEC method and the iodine method was 21.54% and 29.84% vs 18.11% and 33.79%, respectively. The long-chain amylopectin (AP) of MB starch is possibly the cause of the higher value of AM content obtained from the iodine
Page 9 of 36
binding method. The peak gelatinization temperature (Tp) of MB and CS starch was 75.9 C and 72.7 C, respectively.
6.2. Film thickness and mechanical properties of plasticized starch films
All plasticized starch films were homogeneous, flexible and easy to peel, except the glycerol-plasticized cassava starch (Gly-CS) films which slightly stuck to the casting surface. The average thickness of plasticized films was 0.101 mm; the plasticized starch blend films were 6% thinner than plasticized CS and MB starch films (Table 1). Although containing 11% lower moisture content, Sor-plasticized films were 8% thicker than Gly-plasticized films. Thus, glycerol is likely more thoroughly incorporated in the starch films than sorbitol when used at 33% weight content. This is possibly due to its lower molecular weight compared with sorbitol (Zhang & Han, 2006). Tensile strength at peak (TSp) and tensile strength at break (TSb) of plasticized starch films ranged from 2.85 MPa to 20.64 MPa (Table 1). Regardless of plasticizer type, the low-AM CS films always displayed lower TS than the MB films (6.45 vs 14.99 MPa). Moreover, the Glyplasticized films displayed 2 to 4 times lower TS values than the Sor-plasticized films. The data show that TS of plasticized CSMB starch films was a non-additive behavior (Table 1). The Gly-CSMB films had TSp and TSb similar to the Gly-MB films. The Sor-CSMB films also displayed similar TSp to the Sor-MB films; however, their TSb was a little lower compared with the Sor-MB samples. The %EP of Gly-plasticized blended starch films was found to be a non-additive behavior, since the %EP value of Gly-CSMB was comparable to that of
Page 10 of 36
Gly-MB. In contrast, there was no difference in %EP of Sor-plasticized individual and blended starch films. (Table 1).
6.3. Microstructure of plasticized starch films
The microstructure of all plasticized starch films except the Gly-CS films displayed a rich network of leached-out AM clusters surrounding starch granules and remnants, as reported by Zhang and Han (2006) (Fig. 1). No birefringence in any samples was observed under a polarized filter, indicating that the native crystallinity of starches was destroyed (Fig. S1). The plasticized MB films had the most compacted and aggregated starch granule remnants, with dense AM clusters. Surprisingly, no granule remnants or aggregates were observed in the Gly-CS films. This result suggested that not only is glycerol a more effective plasticizer than sorbitol (Vieira, da Silva, dos Santos, & Beppu, 2011), but CS starch granules were also less resistant to heat and shear than MB starch granules (Hongsprabhas, 2007). The intensity of the blue color of Gly-plasticized films was significantly stronger than Sor-plasticized films. The weaker color intensity of Sor-plasticized films was probably due to a lower amount of sorbitol incorporated into starch materials, resulting in higher phase separation between sorbitol and starch. In fact, a few sorbitol crystals were observed in Sor-CS films, as shown in Fig. 2. When comparing incorporation of the same plasticizer, the blue color intensity order was CS < CSMB < MB films. This result was in agreement with the AM content of starch films (Table 4). The microstructure of plasticized CSMB blended starch films displayed a rich network of finer AM clusters surrounding granule remnants (Fig. 1C,D) compared with plasticized CS and
Page 11 of 36
MB starch films. Due to the size and shape of the remnants of the blended films, and also because no remnants existed in Gly-CS films, we suspect that the remnants in the plasticized CSMB films were MB. Granule remnants appeared to be more distorted and fragmented in the Gly-plasticized samples, supporting the high plasticization effect of glycerol. The crystallinity of the samples is shown in Fig. 3. All native starches displayed an Atype pattern, since strong peaks appeared at 2θ = ~15.1, 17.0, 18.1 and 22.9 (Fig. 3A). Unplasticized films kept for four weeks displayed a slight B-type pattern with diffractions at 2θ = ~16.9, 19.4, and 22.4, suggesting the complete destruction of native crystallinity and subsequent retrogradation (van Soest, Hullemna, de Wit, & Vliegenthart, 1996). The unplasticized CS films had low peak intensities at 2θ = ~16.9 and 19.4, but the unplasticized CSMB and MB films had stronger diffraction peaks at 2θ = ~16.9, 19.4 and 22.4. Depending on starches and plasticizers, the plasticized samples kept for similar storage times displayed different XRD patterns. The Gly-CS films had a VH-type pattern with a diffraction at 2θ = ~19.9, whereas the Sor-CS films displayed a VH+B-type pattern with diffractions at 2θ = ~17.2, 19.9 and 22.4 (Mali, Grossmann, García, Martino, & Zaritzky, 2002; Thiré, Andrade & Simão, 2005; van Soest et al., 1996). Thus, phase separation between CS starch and sorbitol likely occurred more rapidly than between CS starch and glycerol, although the molar ratio of glycerol to starch was twice that of sorbitol. All plasticized MB and CSMB films displayed a VH+B-type pattern, suggesting phase separation between starch and plasticizers.
6.4. Thermal properties of plasticized starch films
Page 12 of 36
Two tan peaks of all plasticized films determined by a dynamic mechanical analyzer (DMA) indicated the existence of amorphous regions in the plasticized films. The first transition temperature (Tg1), corresponding to the plasticizer-rich phase, was ~ −58.5 C and −17.7 C for Gly-plasticized films and Sor-plasticized films, respectively (Gaudin, Lourdin, Forssell, & Colonna, 2000; Lourdin, Bizot, & Colonna, 1997). The second transition of Gly-plasticized starch films, which was the glass transition temperature (Tg2) of the starch-rich phase, ranged from −0.40 C to 13.60 C. Differences in Tg2 depended on the type of starch, type of plasticizer, and the interaction between starches and plasticizers. In contrast, Tg2 of all Sor-plasticized starch films was not different (~15 C). Only the secondary transitions of Sor-plasticized films were clearly distinguished using DSC, at temperatures of ~ −30.31 C and 56.41 C. The DSC endotherm of unplasticized starch films associated with B-type crystals was observed (Table 2), and the peak temperature (Tp) was ~155.90 C. The endotherm attributed to the B+V-type crystals of plasticized starch films was also reported, but their Tp, ranging from 159.09 to 185.17 C, was significantly higher than that of unplasticized films. Tp of plasticized starch and starch blend films was a non-additive behavior, but the trend depended on the type of plasticizer. Gly-plasticized CSMB films had a higher Tp than Gly-plasticized CS and MB films (185 C vs 167 C). However, the opposite was found for Sor-plasticized films: Tp of Sor-CSMB was 17 C lower than that of Sorplasticized CS and MB films.
Page 13 of 36
6.5. FTIR spectra of starch films
The ATR-FTIR spectra of all starch films are shown in Fig. 4. Three peaks at ~928, 992 and 1150 cm-1 represent C–O bonding stretch in the anhydroglucose unit (AGU) ring. The peak at ~1637 cm-1 corresponds to strongly bound water in starch, whereas the peak at ~2927 cm-1 is attributed to the C–H stretching vibration (Ghosh Dastidar & Netravali, 2013; Zhang & Han, 2006). The latter peak also was proposed by Zullo et al. (2009) to represent C–H stretching in the AGU ring, which plays no part in thermo-plasticization. The broad band containing a peak around ~3293 cm-1 is due to vibrations of free, inter-, and intramolecular bound hydroxyl groups (Zhang & Han, 2006). It appears that the peak at ~1637 cm-1 of all plasticized starch films significantly shifted to a higher wavenumber ~1639 cm-1 (Fig. 4B), indicating a weaker interaction between bound water and starch compared with unplasticized starch films. Also, the peak at ~2932 cm-1 of plasticized CS films shifted to a lower wavenumber (2929, 2928 cm-1), but that of plasticized CSMB films shifted to a higher wavenumber (2925 cm-1 vs 2929 and 2927 cm1
) (Fig. 4). The data implied that the asymmetrical C–H stretching in plasticized CS samples was
slightly altered to a less asymmetric vibration, but the trend was the opposite for plasticized CSMB samples. The shift of a peak at 3293 cm -1 to 3283 cm-1 of sorbitol-plasticized starch films suggested an increase of intermolecular hydrogen bonding compared with unplasticized films. The peak height ratios of bound water (peak at ~1637 cm-1) and hydroxyl groups (3293– 3283 cm-1) to that of the skeleton C–O stretching (~928 cm-1) were used as relative values expressing the amount of bound water in starch samples and the degree of H-bonding (Table 3). In fact, the amount of bound water was not related to the moisture content of the films. It seems that the amount of bound water in glycerol-plasticized films was significantly lower than in Page 14 of 36
unplasticized films, especially in the blended starch films. A similar trend was observed in sorbitol-plasticized samples, except for the Sor-CSMB samples in which the amount of bound water was unchanged. The order of the amount of H-bonding was: Sor-plasticized films > Glyplasticized films > unplasticized films; the effect was the most pronounced in Sor-CSMB samples. The results indicated that sorbitol-plasticized films contained higher H-bonding than glycerol-plasticized films, even though the molar ratio of glycerol added was higher than that of sorbitol.
6.6. Molecular weight (Mw) of native starch and starch films
Native CS and MB starches displayed three molecular weight fractions when analyzed by HPSEC. It is noted here that the molecular weight of the samples reported here were relative to pullulans and not absolute (Vilaplana and Gilbert, 2010). The first fraction of MB and CS, representing amylopectin (AP), had a similar weight-average molecular weight (Mw) ( 1.0× 108 g/mol g/mol). However, Mw of the second and third fractions, intermediate materials (IM) and amylose (AM) of MB were lower than that of CS (Table 4). The AM content of MB was 8% higher than that of CS, whereas the fraction of the IM of MB was two times lower than that of CS. The unplasticized starch films also displayed three molecular weight fractions. The values of Mw and the percentage of each fraction were comparable to those of native starches, except for the AM fraction. AP fragmentation of MB likely occurred during the film casting procedure, resulting in an increase of AM (Table 4). The results also show that both Mw and % AP of the 50:50 CSMB starch blend were the highest, compared with those of CS and MB starch samples. In contrast, the Mw and % of
Page 15 of 36
their IM materials were the lowest. Interestingly, the average molecular size of AM in the CSMB starch blend as well as the molecular weight distribution of the CSMB blend was similar to that of MB, but its content was closer to the AM content of CS. Similar results were also observed for the starch blend films. The chromatograms of CS, MB, and CSMB films were normalized using the peak height (intensity) of AP and superimposed as shown in Fig. S2.
7. Discussion The starch molecular profile, microstructure, and polymer morphology of starch blend films with different plasticizers as well as their mechanical properties, compared to those of native starches are discussed as follows. The miscibility of unplasticized CS/MB blends was confirmed by the FTIR spectra (Fig. 4). The shift of a peak at 3296 cm-1 to 3293 cm-1 (p <0.05) exhibited an increase of intermolecular hydrogen bonding. The strength of intermolecular H-bonding in the Gly-CS, GlyMB, and Gly-CSMB films was not different from that of unplasticized starch films. In contrast, the shift of the peak to 3283 cm-1 of the Sor-CS and Sor-MB films indicated the shorter length of H-bonds (Fig. 4). The data suggested the Sor-starch films had the close alignment of any molecules which can form intermolecular H-bonding . The microstructure of all plasticized starch films displayed phase separation as darkdropetlike leached-out AM domains in the brighter AP matrix as reported by Rindlav-Westling, Stading, and Gatenholm (2002) (Fig.1). The figure also suggested that there was less contact between AM and AP of MB than that of CS since the apparent diameter of their AM domains was 4.18 and 1.40 m, respectively. The high granule rigidity of MB may hinder the inherent AM to form a continuous network. The large difference in molecular size of AP and AM of MB Page 16 of 36
as 1.01 x 108 vs 2.84-3.73 x 105 g/mol, possibly increased immiscibility between the two starch polymers (Table 4). A decrease in size of the AM domains ( 0.8-1.02 m) in the plasticized CSMB films indicated the miscible blends as well as less phase separation between AM and AP (Fig. 1). The amorphous regions in the semicrystalline plasticized starch films exhibited two Tgs of the plasticizer-rich phase and the starch-rich phase. The lower Tg2 of Gly-MB films, compared to that of Gly-CS films (Table 2) suggested that the free volume in the amorphous MB starch-rich phase was higher than that of CS. The single Tg2 of the Gly-CSMB films located between Tg2 of the Gly-CS and Gly-MB films indicated miscibility of the blends. The Sor-MB films had a higher Tg2 than the Gly-MB films. It insinuated that chain mobility of MB starch was greatly affected by type of plasticizer, and the cause was possibly due to the large discrepancy between AP, IM, and AM of MB. An increase of Tg2 of Sor-CS and Sor-MB films appeared once sorbitol was substituted for glycerol, indicating a decrease in free volume in the amorphous regions of their starch-rich phases. As mentioned, the only Gly-CS films displayed a distinctive VH-pattern (Fig 3). The better plasticizing effect of glycerol, the lower AM content, and/or larger molecular size of CS AM (Table 4) , resulting in slower recrystallization probably was the reason for the absence of the B-type pattern of the Gly-CS films (Kitamura, Hakozaki, and Kuge, 1993). Sor-plasticized films exhibited higher %RC than Gly-plasticized and unplasticized films (p < 0.05). This is probably due to the formation of V-type complex and further starch retrogradation. The order of %RC was Sor-CS < Sor-CSMB < Sor-MB (p < 0.05), which was agreeable with the order of TS of the samples (Fig. 3 and Table 1).
Page 17 of 36
The impact of type of plasticizer and starch/starch blend on mechanical properties of the films was complicated. In the glycerol-plasticized system, the unblended-native starch films containing high AM content and %RC displayed high TS. In the sorbitol-plasticized system, TS was rather agreeable with AM content and %RC. TS, %RC, and Tp of the Sor-plasticized films was significantly higher than the Gly-plasticized films from the same kind of starch (Table 1, Fig. 3, and Table 4). The study showed that AM content and the crystallinity increased TS. And TS of the same kind of starch film was further increased when the crystallinity was magnified by substituting sorbitol for glycerol. The significance of starch recrystallization on TS as it caused a stronger network from leached-out AM in the continuous phase was previously reported by Li, Xie, Hasjim, Witt, Halley, & Gilbert (2015). The higher phase separation between sorbitol and starch, suggesting by FTIR spectra, and the weaker color intensity of the sor-plasticized films was probably the cause of an additional increase in crystallinity of the sorbitol plasticized films (Fig. 1, and Fig. 4). The %E of the plasticized starch films might be influenced by the amorphous phases, as the %E was quite agreeable with Tg2 of starch-rich phase, except those of the GlyCSMB (Table 1, Table 2, and Table 4). Both Gly-CSMB and Gly-MB films had similar TS and %E, although the AM content of the CSMB films was 6-9% significant lower than that of the MB films. TS of Sor-CSMB and Sor-MB films were also comparable. Further, there is somewhat similarity between the molecular profile of the CSMB and MB (Table 1, Table 4 and Fig. S2). Thus, we suspected that the large difference in Mw of AP and AM, and the small size of IM, and AM might impart the equivalent mechanical properties of Gly- and Sor-plasticized films.
Page 18 of 36
8 . Conclusion
Blending CS and MB starches at a 50:50 weight ratio resulted in a non-additive behavior of mechanical properties of the starch films, and the effect was more pronounced in the glycerolplasticized system. TS and %E of the Gly-CSMB films were similar to those of MB films, whereas there was no difference in %E of all Sor-plasticized starch films. Compared with plasticized CS and MB starch films, the microstructure of plasticized CSMB starch films had a rich network of finer AM clusters surrounding granule remnants. The plasticized CSMB films were semi-crystalline, with a crystal pattern closer to that of plasticized MB films. TS of plasticized films increased when AM content and %RC increased. When sorbitol was substituted for glycerol, crystallinity of the same kind of starch films increased as well as their TS. Since the plasticized CSMB and MB films had both a similar molecular profile and comparable mechanical properties, it was proposed the starch molecular profile containing AP with high Mw of AP, low Mw of AM, and the small size of IM may impart the high TS and %E of starch films.
Acknowledgements
The authors thank Kasetsart University and the Faculty of Agro-Industry, Kasetsart University, Thailand (through the Scholarships for International Graduate Students 2013) for financial support.
Page 19 of 36
References
AOAC. (2000). Official methods of analysis (17th ed.). Arlington, VA: Association of Official Analytical Chemists. Alves, V. D., Mali, S., Beléia, A., & Grossmann, M. V. E. (2007). Effect of glycerol and amylose enrichment on cassava starch film properties. Journal of Food Engineering, 78(3), 941–946. ASTM. (2002). ASTM D 882-02: Standard test method for tensile properties of thin plastic sheeting. West Conshohocken, PA: ASTM International. Bae, H. J., Cha, D. S., Whiteside, W. S., & Park, H. J. (2008). Film and pharmaceutical hard capsule formation properties of mungbean, waterchestnut, and sweet potato starches. Food Chemistry, 106(1), 96–105. Chaléat, C., Halley, P. J., & Truss, R. W. (2014). Mechanical properties of starch-based plastics. In P. J. Halley, & L. Avérous (Eds.), Starch polymers (pp. 187–209). Burlington, MA: Elsevier. Chaudhary, A. L., Torley, P. J., Halley, P. J., McCaffery, N., & Chaudhary, D. S. (2009). Amylose content and chemical modification effects on thermoplastic starch from maize – processing and characterisation using conventional polymer equipment. Carbohydrate Polymers, 78(4), 917–925. Chrastil, J. (1987). Improved colorimetric determination of amylose in starches or flours. Carbohydrate Research, 159(1), 154–158.
Page 20 of 36
Cuq, B., Gontard, N., Cuq, J-L. & Guilbert, S. 1997. Selected functional properties of fish myofibrillar protein-based films as affected by hydrophilic plasticizers. Journal of Agricultural and Food Chemistry, 45(3):622-626. Famá, L., Rojas, A. M., Goyanes, S., & Gerschenson, L. (2005). Mechanical properties of tapioca-starch edible films containing sorbates. LWT – Food Science and Technology, 38(6), 631–639. Farahnaky, A., Saberi, B., & Majzoobi, M. (2013). Effect of glycerol on physical and mechanical properties of wheat starch edible films. Journal of Texture Studies, 44(3), 176–186. Gaudin, S., Lourdin, D., Forssell, P. M., & Colonna, P. (2000). Antiplasticization and oxygen permeability of starch–sorbitol films. Carbohydrate Polymers, 43(1), 33-37. Ghosh Dastidar, T., & Netravali, A. (2013). Cross-linked waxy maize starch-based “green” composites. ACS Sustainable Chemistry & Engineering, 1(12), 1537–1544. Gupta, M., Brennan, C., & Tiwari, B. K. (2012). Starch-based edible films and coatings. In J. Ahmed, B. K. Tiwari, S. H. Imam, & M. A. Rao (Eds.), Starch-based polymeric materials and nanocomposites: chemistry, processing, and applications (pp. 239–254). Boca Raton, FL: CRC Press. Hongsprabhas, P. (2007). On the gelation of mungbean starch and its composites. International Journal of Food Science & Technology, 42(6), 658–668. Israkarn, K., Na Nakornpanom, N., & Hongsprabhas, P. (2014). Physicochemical properties of starches and proteins in alkali-treated mungbean and cassava starch granules. Carbohydrate Polymers, 105, 34–40.
Page 21 of 36
Kitamura, S., Hakozaki, K., & Kuge, T. (1993). Effects of molecular weight on the retrogradation of amylose. In K. Nishinari, & E. Doi (Eds.), Food hydrocolloids (pp. 179–182). New York: Springer. Laohakunjit, N., & Noomhorm, A. (2004). Effect of plasticizers on mechanical and barrier properties of rice starch film. Starch - Stärke, 56(8), 348–356. Li, M., Xie, F., Hasjim, J., Witt, T., Halley, P. J., & Gilbert, R. G. (2015). Establishing whether the structural feature controlling the mechanical properties of starch films is molecular or crystalline. Carbohydrate Polymers, 117(3), 262-270. Li, M., Liu, P., Zou, W., Yu, L., Xie, F., Pu, H., Liu, H., & Chen, L. (2011). Extrusion processing and characterization of edible starch films with different amylose contents. Journal of Food Engineering, 106(1), 95–101. Liu, H., & Lelièvre, J. (1992). A differential scanning calorimetry study of melting transitions in aqueous suspensions containing blends of wheat and rice starch. Carbohydrate Polymers, 17(2), 145–149. López, O. V., Lecot, C. J., Zaritzky, N. E., & García, M. A. (2011). Biodegradable packages development from starch based heat sealable films. Journal of Food Engineering, 105(2), 254–263. Lourdin, D., Bizot, H., & Colonna, P. (1997). “Antiplasticization” in starch-glycerol films? Journal of Applied Polymer Science, 63(8), 1047–1053. Mali, S., Grossmann, M. V. E., García, M. A., Martino, M. N., & Zaritzky, N. E. (2002). Microstructural characterization of yam starch films. Carbohydrate Polymers, 50(4), 379–386.
Page 22 of 36
Mali, S., Grossmann, M. V. E., García, M. A., Martino, M. N., & Zaritzky, N. E. (2008). Antiplasticizing effect of glycerol and sorbitol on the properties of cassava starch films. Brazilian Journal of Food Technology, 11, 194–200. Mali, S., Sakanaka, L. S., Yamashita, F., & Grossmann, M. V. E. (2005). Water sorption and mechanical properties of cassava starch films and their relation to plasticizing effect. Carbohydrate Polymers, 60(3), 283–289. Myllärinen, P., Partanen, R., Seppälä, J., & Forssell, P. (2002). Effect of glycerol on behaviour of amylose and amylopectin films. Carbohydrate Polymers, 50(4), 355–361. Obanni, M., & BeMiller, J. N. (1997). Properties of some starch blends. Cereal Chemistry, 74, 431–436. Parra, D. F., Tadini, C. C., Ponce, P., & Lugão, A. B. (2004). Mechanical properties and water vapor transmission in some blends of cassava starch edible films. Carbohydrate Polymers, 58(4), 475–481. Pavlath, A. E., & Orts, W. (2009). Edible films and coatings: Why, what, and how? In M. E. Embuscado, & K. C. Huber (Eds.), Edible films and coatings for food applications (pp. 1–23). New York: Springer. Prachayawarakorn, J., Hommannee, L., Phosee, D., and Chairapaksatien, P. 2010. Property improvement of thermoplastic mung bean starch using cotton fiber and low-density polyethylene. Starch/Starke 62 (435-443). Puncha-arnon, S., Pathipanawat, W., Puttanlek, C., Rungsardthong, V., & Uttapap, D. (2008). Effects of relative granule size and gelatinization temperature on paste and gel properties of starch blends. Food Research International, 41(5), 552–561.
Page 23 of 36
Rachtanapun, P., Pankan, D., & Srisawat, D. (2012). Edible films of blended cassava starch and rice flour with sorbital and their mechanical properties. Journal of Agricultural Science and Technology, 2(2), 252–258. Rindlav-Westling, A., Stading, M., & Gatenholm, P. (2002). Crystallinity and morphology in films of starch, amylose, and amylopectin blends. Biomacromolecules, 3, 84-91. Talja, A. R., Helen, H., Roos, Y. H., & Jouppila, K. 2007. Effect of various polyols and polyol contents on physical and mechanical properties of potato starch-based films. Carbohydrate Polymers, 67(3), 288-295. Thirathumthavorn, D., & Charoenrein, S. (2007). Aging effects on sorbitol- and non-crystallizing sorbitol-plasticized tapioca starch films. Starch - Stärke, 59(10), 493–497. Thiré, R. M. S. M., Andrade, C. T., & Simão, R. A. (2005). Effect of aging on the microstructure of plasticized cornstarch films. Polímeros, 15, 130–133. van Soest, J. J. G., Hulleman, S. H. D., de Wit, D., & Vliegenthart, J. F. G. (1996). Crystallinity in starch bioplastics. Industrial Crops and Products, 5(1), 11–22. Vilaplana, F., & Gilbert, R. G. (2010). Characterization of branched polysaccharides using multiple-detection size separation techniques. Review Article. Journal of Seperation Science, 33, 3537-3554. Vieira, M. G. A., da Silva, M. A., dos Santos, L. O., & Beppu, M. M. (2011). Natural-based plasticizers and biopolymer films: a review. European Polymer Journal, 47(3), 254–263. Waterschoot, J., Gomand, S. V., Fierens, E., & Delcour, J. A. (2015). Starch blends and their physicochemical properties. Starch - Stärke, 67(1–2), 1–13. Zhang, Y., & Han, J. H. (2006). Plasticization of pea starch films with monosaccharides and polyols. Journal of Food Science, 71(6), E253–E261.
Page 24 of 36
Zhong, Y., & Li, Y. (2014). Effects of glycerol and storage relative humidity on the properties of kudzu starch-based edible films. Starch - Stärke, 66(5-6), 524–532. Zullo, R., & Iannace, S. (2009). The effects of different starch sources and plasticizers on film blowing of thermoplastic starch: correlation among process, elongational properties and macromolecular structure. Carbohydrate Polymers, 77(2), 376–383.
Page 25 of 36
A
B
C
D
0.8 m
E
F
4 m
Fig. 1. Morphology of plasticized starch films observed under light microscope (40×, bar = 20 µm): (A) Gly-CS; (B) Sor-CS; (C) Gly-CSMB; (D) Sor-CSMB; (E) Gly-MB; and (F) Sor-MB. Page 26 of 36
A
B
C
Fig. 2. Sorbitol crystals observed under light microscope at various magnifications: (A) 10×, bar = 100 µm; (B) 20×, bar = 50 µm; and (C) 40×, bar = 100 µm. Page 27 of 36
Page 28 of 36
Fig. 3. X-ray diffraction patterns of: (A) native starches; (B) unplasticized starch films; and (C) plasticized starch films. The value in the parenthesis represents % relative crystallinity (%RC).
Page 29 of 36
Page 30 of 36
Fig. 4. FTIR spectra of: (A) unplasticized starch films; (B) glycerol-plasticized starch films; and (C) sorbitol-plasticized starch films. Page 31 of 36
Page 32 of 36
Table 1: Mechanical properties of unplasticized and plasticized starch films. Films
Moisture content (%)
Thickness (mm)
Tensile strength (MPa) At peak At break
Elongation (%) At peak At break
* 8.58ab (± 0.5) 19.22d 0.103b 2.86a 2.85a 14.06b 18.82bc (± 1.1) (± 0.0) (± 0.0) (± 0.0) (± 1.9) (± 3.5) Sor-CS 9.43b 0.101b 8.25b 6.77b 2.74a 14.86ab (± 0.1) (± 0.0) (± 1.6) (± 1.1) (± 0.2) (± 1.7) Unplast7.73a CSMB (± 0.2) Gly22.11e 0.090a 7.93b 7.93b 19.86c 21.32c CSMB (± 1.8) (± 0.0) (± 0.6) (± 0.6) (± 2.6) (± 2.0) ab b c c a Sor8.84 0.105 19.56 15.87 2.66 10.84a CSMB (± 0.5) (± 0.0) (± 2.0) (± 2.6) (± 0.6) (± 2.0) ab Unplast8.21 MB (± 0.5) Gly-MB 19.66d 0.098b 9.34b 9.34b 20.43c 21.37c (± 1.4) (± 0.0) (± 1.3) (± 1.3) (± 1.1) (± 1.1) b c c d a Sor-MB 9.16 0.113 20.64 19.20 3.05 12.89a (± 0.1) (± 0.0) (± 0.9) (± 0.8) (± 0.8) (± 5.3) Means in the same column with different letters are significantly different (p<0.05).
UnplastCS Gly-CS
Unplast = unplasticized; Gly = glycerol; Sor = sorbitol; CS = cassava starch; MB = mungbean starch; CSMB = 50:50 blend of cassava and mungbean starches. *
Not available. The films were too brittle to peel off.
Page 33 of 36
Table 2: Thermal properties of unplasticized and plasticized starch films. Treatment
Moisture content (%)
DMA Tg1 (°C)
Tg2 (°C)
Transition 1st 2nd (°C) (°C)
To (°C)
8.58ab ND* ND 154.00ab (± 0.5) (± 1.4) Gly-CS 19.22d -56.95a 13.60c 160.50bc (± 1.1) (± 5.1) (± 2.8) (± 0.7) Sor-CS 9.43b -14.40b 17.05c -36.19a 55.12ns 176.34d (± 0.1) (± 3.8) (± 6.1) (± 2.1) (± 0.0) (± 0.4) Unplast7.73a ND ND 146.17a CSMB (± 0.2) (± 4.0) Gly-CSMB 22.11e -56.10a 6.15b 169.00cd (± 1.8) (± 4.1) (± 1.6) (± 1.4) ab b c b ns Sor-CSMB 8.84 -20.50 14.05 -28.17 56.58 153.42ab (± 0.5) (± 2.7) (± 0.7) (± 0.8) (± 0.1) (± 12.1) ab Unplast-MB 8.21 ND ND 153.42ab (± 0.5) (± 1.3) d a a Gly-MB 19.66 -61.10 -0.40 168.34cd (± 1.4) (± 2.1) (± 0.7) (± 0.9) b b c b ns Sor-MB 9.16 -18.10 16.25 -26.59 57.54 167.34cd (± 0.1) (± 3.5) (± 4.6) (± 0.1) (± 3.6) (± 3.7) Means in the same column with different letters are significantly different (p<0.05). Unplast-CS
ns *
DSC Endotherm Tp (°C) 157.88a (± 1.8) 164.61ab (± 0.1) 180.63cd (± 0.2) 154.09a (± 4.3) 185.17d (± 0.9) 159.09ab (± 9.3) 155.75a (± 0.3) 169.83bc (± 1.4) 176.65cd (± 9.7)
Te (°C)
H (J/g)
187.67b (± 8.0) 186.67ab (± 2.1) 205.59c (± 0.6) 177.59ab (± 5.5) 209.50c (± 0.7) 186.75ab (± 5.7) 176.75a (± 0.3) 184.50ab (± 3.5) 199.67c (± 6.1)
172.36ab (± 39.3) 157.24a (± 12.4) 138.24a (± 14.2) 165.34ab (± 1.8) 165.40ab (± 4.2) 144.36a (± 3.4) 203.03b (± 27.2) 162.53a (± 3.4) 133.20a (± 13.9)
No significant difference (p>0.05).
Not determined. The films were too brittle for dynamic mechanical analyzer (DMA) measurement.
Page 34 of 36
Table 3: Ratio of wavenumbers in the FTIR spectrum of unplasticized and plasticized starch films. Treatment
Moisture content (%)
Bound water (ratio)
Free, inter-, intramolecular OH groups (ratio) ab bc Unplast-CS 8.58 0.41 0.98a (± 0.5) (± 0.1) (± 0.2) d ab Gly-CS 19.22 0.30 2.06bcd (± 1.1) (± 0.0) (± 0.1) Sor-CS 9.43b 0.31ab 2.69cd (± 0.1) (± 0.0) (± 0.2) Unplast-CSMB 7.73a 0.38bc 0.88a (± 0.2) (± 0.0) (± 0.1) Gly-CSMB 22.11e 0.25a 2.00bc (± 1.8) (± 0.0) (± 0.2) Sor-CSMB 8.84ab 0.44c 3.13d (± 0.5) (± 0.0) (± 0.1) Unplast-MB 8.21ab 0.43c 1.23ab (± 0.5) (± 0.0) (± 0.3) Gly-MB 19.66d 0.31abc 2.10bcd (± 1.4) (± 0.0) (± 0.1) Sor-MB 9.16b 0.34abc 2.20bcd (± 0.1) (± 0.2) (± 1.6) Means in the same column with different letters are significantly different (p<0.05).
Page 35 of 36
Table 4: Molecular weight distribution and AM / IM /AP ratio of native starches and starch films. Fraction I (AP) Fraction II (IM) Fraction III (AM) * 8 * 6 * (× 10 ) (%) ( × 10 ) (%) (× 105) (%) a b c b Native CS 0.98 62.09 19.00 16.37 22.30b 21.54a starch (± 0.0) (± 1.3) (± 0.4) (± 1.3) (± 6.6) (± 0.1) CSMB 1.21b 69.20c 5.03a 7.37a 3.87a 23.43ab (± 0.0) (± 1.7) (± 0.0) (± 0.4) (± 0.0) (± 1.3) MB 1.01ab 61.69ab 10.00b 8.47a 3.73a 29.84c (± 0.0) (± 1.3) (± 0.0) (± 0.5) (± 0.1) (± 0.8) Starch CS 0.96a 59.32ab 21.5c 19.46b 16.80b 21.28a film (± 0.0) (± 1.7) (± 0.7) (± 2.8) (± 5.9) (± 1.2) CSMB 1.33c 68.60c 4.64a 6.91a 3.64a 24.50b (± 0.0) (± 0.8) (± 0.5) (± 0.1) (± 0.0) (± 0.9) MB 1.04ab 58.42a 10.00b 8.49a 2.84a 33.09d (± 0.0) (± 2.0) (± 0.0) (± 0.7) (± 0.2) (± 1.2) Means in the same column with different letters are significantly different (p<0.05). Sample
AP = amylopectin; IM = intermediate materials; AM = amylose. * Mw (g/mol)
Page 36 of 36
S0144-8617(16)30969-9 http://dx.doi.org/doi:10.1016/j.carbpol.2016.08.034 CARP 11459
To appear in: Received date: Revised date: Accepted date:
13-3-2016 10-8-2016 10-8-2016
Please cite this article as: Nguyen Vu, Hoang Phuong., & Lumdubwong, Namfone., Starch behaviors and mechanical properties of starch blend films with different plasticizers.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.08.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Starch behaviors and mechanical properties of starch blend films with different plasticizers
Hoang Phuong Nguyen Vu, Namfone Lumdubwong*
a
Department of Packaging and Materials Technology, Faculty of Agro-Industry, Kasetsart
University, Bangkok 10900, Thailand
*
Corresponding author. Tel.: +66 97 279 8563. E-mail address: [email protected] (N. Lumdubwong).
Highlights
Mechanical properties of plasticized starch blend (CSMB) films were non-additive.
The non-additive effect was more pronounced in the glycerol-plasticized system.
CSMB films had most of thermal properties close to one of individual (MB) films.
Plasticized CSMB and MB films had a similar crystallinity type.
Mw of amylose of CSMB films was similar to that of MB films but not the content.
Page 1 of 36
ABSTRACT The main objective of the study was to gain insight into structural and mechanical starch behaviors of the plasticized starch blend films. Mechanical properties and starch behaviors of cassava (CS)/ and mungbean (MB) (50/50, w/w) starch blend films containing glycerol (Gly) or sorbitol (Sor) at 33% weight content were investigated. It was found that TS and %E of the GlyCSMB films were similar to those of MB films; but %E of all Sor-films was identical. TS of plasticized films increased when AM content and crystallinity increased. When Sor was substituted for Gly, crystallinity of starch films and their TS increased. The CSMB and MB films had somewhat a similar molecular profile and comparable mechanical properties. Therefore, it was proposed the starch molecular profile containing amylopectin with high Mw, low Mw of amylose, and the small size of intermediates may impart the high TS and %E of starch films.
Keywords: Starch blend film, Plasticizer, Amylose content, Starch film properties
Page 2 of 36
1. Introduction
Production and functional properties of edible starch films and coatings have been extensively investigated during the past decade, due to their vast potential uses in the food and pharmaceutical industries (Pavlath & Orts, 2009). Among their various attributes, the mechanical properties of starch films are of interest to researchers since they have important functions for film applications (Alves, Mali, Beléia, & Grossmann, 2007; Bae, Cha, Whiteside, & Park, 2008; Mali, Sakanaka, Yamashita, & Grossmann, 2005; Parra, Tadini, Ponce, & Lugão, 2004; Zullo & Iannace, 2009). Amylose (AM) content, crystallinity, glass transition temperature (Tg), types and contents of plasticizers, and sources of starch are major factors governing the mechanical properties (Gupta, Brennan, & Tiwari, 2012). Starches containing high AM display excellent film-forming ability and good mechanical properties (Bae et al., 2008; Chaudhary, Torley, Halley, McCaffery, & Chaudhary, 2009; Li et al., 2011). The content of B-type crystallinity also increased film stiffness as well as TS at break (Chaléat, Halley, & Truss, 2014). In addition, brittleness of starch films increased when the films possessed high glass transition temperature (Tg) (Myllärinen et al., 2002). Types and contents of plasticizers also affect the mechanical properties of starch films. Glycerol (Gly) and sorbitol (Sor) are the most widely used polyol plasticizers for starch-based films. Their excellent ability to prevent film cracking during handling and storage was reported; and their effectiveness is possibly due to the close similarity between their chemical structures and the structure of starch polymer (Mali et al. 2005). Gly exhibited a superior plasticization effect to Sor at equivalent weight content (Laohakunjit & Noomhorm, 2004; Mali et al. 2005;
Page 3 of 36
Talja et al. 2007). Nonetheless, Cuq, Gontard, Cuq & Guillbert (1997) disputed that the effect was mainly due to higher molecular content of glycerol. And yet, Zhang & Han (2006) demonstrated the reverse effect which was the Gly- plasticized pea starch films had lower %E than Sor-plasiticized films at the same molar content. Both plasticized films also exhibited similar TS. Mungbean (MB) starch, containing high AM (30%), is available in tropical countries. The costly starch produces clear, colorless, and high-strength gel. Thus, it is considered as the best raw material for cellophane noodle production. MB starch films displayed the highest TS among other starch film samples, and the starch imparted an excellent ability to form pharmaceutical hard capsules (Bae et al., 2008; Prachayawarakorn, Hommannee, Phosee, & Chairapaksatien, 2010). Cassava (CS) starch, one of the major industrial starches, is abundant and inexpensive. CS starch films were odorless, tasteless, and colorless. The edible films also provided good flexibility. The drawback of CS films was their rather low tensile strength (TS) compared to that of films from other starches (Mali et al. 2005; Alves et al., 2007; Thirathumthavorn, D., & Charoenrein, S., 2007). Blending of starches can alter functional properties of starch products such as gelatinization, pasting, rheological, gelation, and mechanical properties (Hongsprabhas, 2007; Liu & Lelièvre, 1992; Waterschoot, Gomand, Fierens, & Delcour, 2015; López, Lecot, Zaritzky, & García, 2011; Rachtanapun, Pankan, & Srisawat, 2012). The properties of starch blends can be classified into additive behavior, i.e., the property of the blends can be predicted from each starch component, and non-additive behavior, i.e., the property of the blends cannot be predicted,
Page 4 of 36
indicating any interactions occurred between starches (Puncha-arnon, Pathipanawat, Puttanlek, Rungsardthong, & Uttapap, 2008). Based on the available information, it is beneficial to develop edible films from CS starch partly substituted for MB starch, yet maintaining their mechanical properties comparable to those of MB films. Besides, little information is known about the molecular profile, microstructure, and morphology of starch blend films. In this study, the CS/MB (50/50, w/w) starch blend films containing glycerol or sorbitol at the same weight content were prepared. Their mechanical properties, starch molecular weight profile, and polymer morphology were investigated, and compared with individual starch films. The objectives of the study were to gain insight into structural, thermal, and mechanical starch behaviors of the plasticized starch blend films, and to produce the plasticized CS/MB blend films possessing mechanical properties closed to those of plasticized MB films.
2. Materials
Mungbean (MB) starch (Sitthinan, Bangkok, Thailand) and cassava (CS) starch (E.T.C. International Trading, Bangkok, Thailand) were purchased locally. Glycerol and sorbitol were supplied by Ajax Finechem (New South Wales, Australia).
3. Chemical composition analyses
Page 5 of 36
Protein, ash, moisture and crude fat content of starch were determined using standard official methods of AOAC (2000). Apparent amylose content was measured by the colorimetric method of Chrastil (1987).
4. Film-casting experimental design
The study was a 3 × 2 factorial design using two starches – cassava (CS) and mungbean (MB) – a CSMB starch blend, and two types of plasticizers. Native starches were mixed with distilled water at a concentration of 5% w/w (dry basis, db). Each suspension was heated at 80 C while magnetically stirring at a speed of ~350 rpm. The CS and MB starch suspensions were stirred for 1 h and 3 h, respectively, to obtain clear solutions. A blend of CS and MB solutions was prepared by mixing the clear solutions of CS and MB starches, prepared separately as described, at a weight ratio of 50:50. The MB starch solution was prepared 2 h prior to the CS starch solution. The CSMB blend was then further stirred for 20 min at 80 C in order to obtain a homogeneous solution. Then, two types of plasticizer (glycerol and sorbitol) were individually added to samples of each solution at 33% of starch weight. The molar ratio of glycerol and sorbitol per 100 g of starch dispersion was 0.018 mol and 0.009 mol, respectively. The filmforming solutions were continuously stirred using the same speed at 80 C for 15 min. Each film solution (35 g) was poured into a plastic Petri dish (140 mm diameter). The dishes containing solutions were dried in a hot-air oven at 60 C for 6 h. The films were peeled off and kept in a desiccator at 52% relative humidity (RH) and ambient temperature for not more than 4 weeks, to be used for further analyses. Unplasticized films (CS, MB and a 50:50 blend of CS and MB starches) were also prepared and stored in the same manner, to be used as controls.
Page 6 of 36
5. Characterization of starch-based films
5.1. Mechanical and thermal properties
Tensile strength, elongation at break and Young’s modulus of the films were determined using a universal testing machine (model 5965; Instron, Norwood, MA, USA), according to ASTM standard method D882–02 (ASTM, 2002). Thermal properties of unplasticized and plasticized starch films were determined by differential scanning calorimetry (DSC) (model DSC 1, Stare System; Mettler-Toledo, Greifensee, Switzerland). Each sample (10 mg, db) was weighed in an aluminum pan, and the analysis was performed under nitrogen atmosphere. The samples were heated from −70 °C to 250 °C at 5 °C/min, then cooled to 25 °C at 20 °C/min, and reheated again from 0 °C to 250 °C at 5 °C/min. Glass transition temperatures of the films (10 mm width 50 mm length) were determined in the tension mode using a dynamic mechanical analyzer (DMA) (Eplexor®; Gabo Qualimeter, Ahlden, Germany). The static load and dynamic load were 1 N and 0.3 N, respectively. The maximum static strain and dynamic strain were set at 0.3%. Measurements were performed at a frequency of 1 Hz using a temperature range from −100 C to 100 C at 2 C/min.
5.2. Crystallinity and microstructure of native starches and starch films
Page 7 of 36
Crystallinity of native starches and unplasticized and plasticized starch films was measured by an X-ray diffractometer (X’Pert; Philips, Amsterdam, Netherlands). Each sample was scanned over a diffraction angle range of 2θ = 5−35 °C using a scan rate of 0.03°/sec. Percent of relative crystallinity was calculated as the ratio of the area above a smooth curve connected peak baselines to the total area over the diffraction angle 5-35. The data was analyzed by Jade software. Plasticized starch films (20 mm × 20 mm) were iodine vapor stained using Lugol’s solution (1% iodine (I2) and 2% potassium iodide (KI) in water) in a closed chamber for 2 h. The microstructure of starch films was investigated under a light microscope (LSM 5 PASCAL; Zeiss, Oberkochen, Germany) with and without a polarized filter.
5.3. Molecular weight profiles of native starches and starch films
The molecular weight profiles of native starches, a dry blend of both starches (50:50 w/w), and three unplasticized starch films (CS, MB and CSMB blend) were determined using the a high-performance size-exclusion chromatography (HPSEC) system, according to the method of Israkarn, Na Nakornpanom, and Hongsprabhas (2014). Six pullulan standards (1 mg/mL) (Showa Denko, Tokyo, Japan) were used to construct a standard curve. All samples were defatted by the AOAC method (AOAC, 2000) prior to the analyses. The amylose content of all samples was analyzed using the ratio of the peak area of amylose and amylopectin.
5.4. FTIR spectroscopy
Page 8 of 36
Fourier transform infrared (FT-IR) spectra of unplasticized and plasticized films were recorded in attenuated total reflectance (ATR) mode using a Bruker Tensor 27 FTIR spectrometer (Bruker, Billerica, MA, USA). The spectra were recorded from 500 cm-1 to 4000 cm-1.
5.5. Statistical analyses
Chemical compositions, thermal properties, molecular weight measurements and microstructure analyses were done at least in duplicate. The various films were prepared at least in duplicate and their mechanical properties were determined from at least five specimens of each film treatment. Analysis of variance (ANOVA) and Duncan’s multiple comparison (p<0.05) were performed using SPSS 11.0 software.
6. Results
6.1. Chemical compositions and gelatinization of native starches
The protein, fat, and ash contents of native CS and MB starches were 0.16%, 0.04% and less than 0.05%, respectively, indicating that the starches were pure (Table S1). The amylose (AM) content of CS and MB obtained from the HPSEC method and the iodine method was 21.54% and 29.84% vs 18.11% and 33.79%, respectively. The long-chain amylopectin (AP) of MB starch is possibly the cause of the higher value of AM content obtained from the iodine
Page 9 of 36
binding method. The peak gelatinization temperature (Tp) of MB and CS starch was 75.9 C and 72.7 C, respectively.
6.2. Film thickness and mechanical properties of plasticized starch films
All plasticized starch films were homogeneous, flexible and easy to peel, except the glycerol-plasticized cassava starch (Gly-CS) films which slightly stuck to the casting surface. The average thickness of plasticized films was 0.101 mm; the plasticized starch blend films were 6% thinner than plasticized CS and MB starch films (Table 1). Although containing 11% lower moisture content, Sor-plasticized films were 8% thicker than Gly-plasticized films. Thus, glycerol is likely more thoroughly incorporated in the starch films than sorbitol when used at 33% weight content. This is possibly due to its lower molecular weight compared with sorbitol (Zhang & Han, 2006). Tensile strength at peak (TSp) and tensile strength at break (TSb) of plasticized starch films ranged from 2.85 MPa to 20.64 MPa (Table 1). Regardless of plasticizer type, the low-AM CS films always displayed lower TS than the MB films (6.45 vs 14.99 MPa). Moreover, the Glyplasticized films displayed 2 to 4 times lower TS values than the Sor-plasticized films. The data show that TS of plasticized CSMB starch films was a non-additive behavior (Table 1). The Gly-CSMB films had TSp and TSb similar to the Gly-MB films. The Sor-CSMB films also displayed similar TSp to the Sor-MB films; however, their TSb was a little lower compared with the Sor-MB samples. The %EP of Gly-plasticized blended starch films was found to be a non-additive behavior, since the %EP value of Gly-CSMB was comparable to that of
Page 10 of 36
Gly-MB. In contrast, there was no difference in %EP of Sor-plasticized individual and blended starch films. (Table 1).
6.3. Microstructure of plasticized starch films
The microstructure of all plasticized starch films except the Gly-CS films displayed a rich network of leached-out AM clusters surrounding starch granules and remnants, as reported by Zhang and Han (2006) (Fig. 1). No birefringence in any samples was observed under a polarized filter, indicating that the native crystallinity of starches was destroyed (Fig. S1). The plasticized MB films had the most compacted and aggregated starch granule remnants, with dense AM clusters. Surprisingly, no granule remnants or aggregates were observed in the Gly-CS films. This result suggested that not only is glycerol a more effective plasticizer than sorbitol (Vieira, da Silva, dos Santos, & Beppu, 2011), but CS starch granules were also less resistant to heat and shear than MB starch granules (Hongsprabhas, 2007). The intensity of the blue color of Gly-plasticized films was significantly stronger than Sor-plasticized films. The weaker color intensity of Sor-plasticized films was probably due to a lower amount of sorbitol incorporated into starch materials, resulting in higher phase separation between sorbitol and starch. In fact, a few sorbitol crystals were observed in Sor-CS films, as shown in Fig. 2. When comparing incorporation of the same plasticizer, the blue color intensity order was CS < CSMB < MB films. This result was in agreement with the AM content of starch films (Table 4). The microstructure of plasticized CSMB blended starch films displayed a rich network of finer AM clusters surrounding granule remnants (Fig. 1C,D) compared with plasticized CS and
Page 11 of 36
MB starch films. Due to the size and shape of the remnants of the blended films, and also because no remnants existed in Gly-CS films, we suspect that the remnants in the plasticized CSMB films were MB. Granule remnants appeared to be more distorted and fragmented in the Gly-plasticized samples, supporting the high plasticization effect of glycerol. The crystallinity of the samples is shown in Fig. 3. All native starches displayed an Atype pattern, since strong peaks appeared at 2θ = ~15.1, 17.0, 18.1 and 22.9 (Fig. 3A). Unplasticized films kept for four weeks displayed a slight B-type pattern with diffractions at 2θ = ~16.9, 19.4, and 22.4, suggesting the complete destruction of native crystallinity and subsequent retrogradation (van Soest, Hullemna, de Wit, & Vliegenthart, 1996). The unplasticized CS films had low peak intensities at 2θ = ~16.9 and 19.4, but the unplasticized CSMB and MB films had stronger diffraction peaks at 2θ = ~16.9, 19.4 and 22.4. Depending on starches and plasticizers, the plasticized samples kept for similar storage times displayed different XRD patterns. The Gly-CS films had a VH-type pattern with a diffraction at 2θ = ~19.9, whereas the Sor-CS films displayed a VH+B-type pattern with diffractions at 2θ = ~17.2, 19.9 and 22.4 (Mali, Grossmann, García, Martino, & Zaritzky, 2002; Thiré, Andrade & Simão, 2005; van Soest et al., 1996). Thus, phase separation between CS starch and sorbitol likely occurred more rapidly than between CS starch and glycerol, although the molar ratio of glycerol to starch was twice that of sorbitol. All plasticized MB and CSMB films displayed a VH+B-type pattern, suggesting phase separation between starch and plasticizers.
6.4. Thermal properties of plasticized starch films
Page 12 of 36
Two tan peaks of all plasticized films determined by a dynamic mechanical analyzer (DMA) indicated the existence of amorphous regions in the plasticized films. The first transition temperature (Tg1), corresponding to the plasticizer-rich phase, was ~ −58.5 C and −17.7 C for Gly-plasticized films and Sor-plasticized films, respectively (Gaudin, Lourdin, Forssell, & Colonna, 2000; Lourdin, Bizot, & Colonna, 1997). The second transition of Gly-plasticized starch films, which was the glass transition temperature (Tg2) of the starch-rich phase, ranged from −0.40 C to 13.60 C. Differences in Tg2 depended on the type of starch, type of plasticizer, and the interaction between starches and plasticizers. In contrast, Tg2 of all Sor-plasticized starch films was not different (~15 C). Only the secondary transitions of Sor-plasticized films were clearly distinguished using DSC, at temperatures of ~ −30.31 C and 56.41 C. The DSC endotherm of unplasticized starch films associated with B-type crystals was observed (Table 2), and the peak temperature (Tp) was ~155.90 C. The endotherm attributed to the B+V-type crystals of plasticized starch films was also reported, but their Tp, ranging from 159.09 to 185.17 C, was significantly higher than that of unplasticized films. Tp of plasticized starch and starch blend films was a non-additive behavior, but the trend depended on the type of plasticizer. Gly-plasticized CSMB films had a higher Tp than Gly-plasticized CS and MB films (185 C vs 167 C). However, the opposite was found for Sor-plasticized films: Tp of Sor-CSMB was 17 C lower than that of Sorplasticized CS and MB films.
Page 13 of 36
6.5. FTIR spectra of starch films
The ATR-FTIR spectra of all starch films are shown in Fig. 4. Three peaks at ~928, 992 and 1150 cm-1 represent C–O bonding stretch in the anhydroglucose unit (AGU) ring. The peak at ~1637 cm-1 corresponds to strongly bound water in starch, whereas the peak at ~2927 cm-1 is attributed to the C–H stretching vibration (Ghosh Dastidar & Netravali, 2013; Zhang & Han, 2006). The latter peak also was proposed by Zullo et al. (2009) to represent C–H stretching in the AGU ring, which plays no part in thermo-plasticization. The broad band containing a peak around ~3293 cm-1 is due to vibrations of free, inter-, and intramolecular bound hydroxyl groups (Zhang & Han, 2006). It appears that the peak at ~1637 cm-1 of all plasticized starch films significantly shifted to a higher wavenumber ~1639 cm-1 (Fig. 4B), indicating a weaker interaction between bound water and starch compared with unplasticized starch films. Also, the peak at ~2932 cm-1 of plasticized CS films shifted to a lower wavenumber (2929, 2928 cm-1), but that of plasticized CSMB films shifted to a higher wavenumber (2925 cm-1 vs 2929 and 2927 cm1
) (Fig. 4). The data implied that the asymmetrical C–H stretching in plasticized CS samples was
slightly altered to a less asymmetric vibration, but the trend was the opposite for plasticized CSMB samples. The shift of a peak at 3293 cm -1 to 3283 cm-1 of sorbitol-plasticized starch films suggested an increase of intermolecular hydrogen bonding compared with unplasticized films. The peak height ratios of bound water (peak at ~1637 cm-1) and hydroxyl groups (3293– 3283 cm-1) to that of the skeleton C–O stretching (~928 cm-1) were used as relative values expressing the amount of bound water in starch samples and the degree of H-bonding (Table 3). In fact, the amount of bound water was not related to the moisture content of the films. It seems that the amount of bound water in glycerol-plasticized films was significantly lower than in Page 14 of 36
unplasticized films, especially in the blended starch films. A similar trend was observed in sorbitol-plasticized samples, except for the Sor-CSMB samples in which the amount of bound water was unchanged. The order of the amount of H-bonding was: Sor-plasticized films > Glyplasticized films > unplasticized films; the effect was the most pronounced in Sor-CSMB samples. The results indicated that sorbitol-plasticized films contained higher H-bonding than glycerol-plasticized films, even though the molar ratio of glycerol added was higher than that of sorbitol.
6.6. Molecular weight (Mw) of native starch and starch films
Native CS and MB starches displayed three molecular weight fractions when analyzed by HPSEC. It is noted here that the molecular weight of the samples reported here were relative to pullulans and not absolute (Vilaplana and Gilbert, 2010). The first fraction of MB and CS, representing amylopectin (AP), had a similar weight-average molecular weight (Mw) ( 1.0× 108 g/mol g/mol). However, Mw of the second and third fractions, intermediate materials (IM) and amylose (AM) of MB were lower than that of CS (Table 4). The AM content of MB was 8% higher than that of CS, whereas the fraction of the IM of MB was two times lower than that of CS. The unplasticized starch films also displayed three molecular weight fractions. The values of Mw and the percentage of each fraction were comparable to those of native starches, except for the AM fraction. AP fragmentation of MB likely occurred during the film casting procedure, resulting in an increase of AM (Table 4). The results also show that both Mw and % AP of the 50:50 CSMB starch blend were the highest, compared with those of CS and MB starch samples. In contrast, the Mw and % of
Page 15 of 36
their IM materials were the lowest. Interestingly, the average molecular size of AM in the CSMB starch blend as well as the molecular weight distribution of the CSMB blend was similar to that of MB, but its content was closer to the AM content of CS. Similar results were also observed for the starch blend films. The chromatograms of CS, MB, and CSMB films were normalized using the peak height (intensity) of AP and superimposed as shown in Fig. S2.
7. Discussion The starch molecular profile, microstructure, and polymer morphology of starch blend films with different plasticizers as well as their mechanical properties, compared to those of native starches are discussed as follows. The miscibility of unplasticized CS/MB blends was confirmed by the FTIR spectra (Fig. 4). The shift of a peak at 3296 cm-1 to 3293 cm-1 (p <0.05) exhibited an increase of intermolecular hydrogen bonding. The strength of intermolecular H-bonding in the Gly-CS, GlyMB, and Gly-CSMB films was not different from that of unplasticized starch films. In contrast, the shift of the peak to 3283 cm-1 of the Sor-CS and Sor-MB films indicated the shorter length of H-bonds (Fig. 4). The data suggested the Sor-starch films had the close alignment of any molecules which can form intermolecular H-bonding . The microstructure of all plasticized starch films displayed phase separation as darkdropetlike leached-out AM domains in the brighter AP matrix as reported by Rindlav-Westling, Stading, and Gatenholm (2002) (Fig.1). The figure also suggested that there was less contact between AM and AP of MB than that of CS since the apparent diameter of their AM domains was 4.18 and 1.40 m, respectively. The high granule rigidity of MB may hinder the inherent AM to form a continuous network. The large difference in molecular size of AP and AM of MB Page 16 of 36
as 1.01 x 108 vs 2.84-3.73 x 105 g/mol, possibly increased immiscibility between the two starch polymers (Table 4). A decrease in size of the AM domains ( 0.8-1.02 m) in the plasticized CSMB films indicated the miscible blends as well as less phase separation between AM and AP (Fig. 1). The amorphous regions in the semicrystalline plasticized starch films exhibited two Tgs of the plasticizer-rich phase and the starch-rich phase. The lower Tg2 of Gly-MB films, compared to that of Gly-CS films (Table 2) suggested that the free volume in the amorphous MB starch-rich phase was higher than that of CS. The single Tg2 of the Gly-CSMB films located between Tg2 of the Gly-CS and Gly-MB films indicated miscibility of the blends. The Sor-MB films had a higher Tg2 than the Gly-MB films. It insinuated that chain mobility of MB starch was greatly affected by type of plasticizer, and the cause was possibly due to the large discrepancy between AP, IM, and AM of MB. An increase of Tg2 of Sor-CS and Sor-MB films appeared once sorbitol was substituted for glycerol, indicating a decrease in free volume in the amorphous regions of their starch-rich phases. As mentioned, the only Gly-CS films displayed a distinctive VH-pattern (Fig 3). The better plasticizing effect of glycerol, the lower AM content, and/or larger molecular size of CS AM (Table 4) , resulting in slower recrystallization probably was the reason for the absence of the B-type pattern of the Gly-CS films (Kitamura, Hakozaki, and Kuge, 1993). Sor-plasticized films exhibited higher %RC than Gly-plasticized and unplasticized films (p < 0.05). This is probably due to the formation of V-type complex and further starch retrogradation. The order of %RC was Sor-CS < Sor-CSMB < Sor-MB (p < 0.05), which was agreeable with the order of TS of the samples (Fig. 3 and Table 1).
Page 17 of 36
The impact of type of plasticizer and starch/starch blend on mechanical properties of the films was complicated. In the glycerol-plasticized system, the unblended-native starch films containing high AM content and %RC displayed high TS. In the sorbitol-plasticized system, TS was rather agreeable with AM content and %RC. TS, %RC, and Tp of the Sor-plasticized films was significantly higher than the Gly-plasticized films from the same kind of starch (Table 1, Fig. 3, and Table 4). The study showed that AM content and the crystallinity increased TS. And TS of the same kind of starch film was further increased when the crystallinity was magnified by substituting sorbitol for glycerol. The significance of starch recrystallization on TS as it caused a stronger network from leached-out AM in the continuous phase was previously reported by Li, Xie, Hasjim, Witt, Halley, & Gilbert (2015). The higher phase separation between sorbitol and starch, suggesting by FTIR spectra, and the weaker color intensity of the sor-plasticized films was probably the cause of an additional increase in crystallinity of the sorbitol plasticized films (Fig. 1, and Fig. 4). The %E of the plasticized starch films might be influenced by the amorphous phases, as the %E was quite agreeable with Tg2 of starch-rich phase, except those of the GlyCSMB (Table 1, Table 2, and Table 4). Both Gly-CSMB and Gly-MB films had similar TS and %E, although the AM content of the CSMB films was 6-9% significant lower than that of the MB films. TS of Sor-CSMB and Sor-MB films were also comparable. Further, there is somewhat similarity between the molecular profile of the CSMB and MB (Table 1, Table 4 and Fig. S2). Thus, we suspected that the large difference in Mw of AP and AM, and the small size of IM, and AM might impart the equivalent mechanical properties of Gly- and Sor-plasticized films.
Page 18 of 36
8 . Conclusion
Blending CS and MB starches at a 50:50 weight ratio resulted in a non-additive behavior of mechanical properties of the starch films, and the effect was more pronounced in the glycerolplasticized system. TS and %E of the Gly-CSMB films were similar to those of MB films, whereas there was no difference in %E of all Sor-plasticized starch films. Compared with plasticized CS and MB starch films, the microstructure of plasticized CSMB starch films had a rich network of finer AM clusters surrounding granule remnants. The plasticized CSMB films were semi-crystalline, with a crystal pattern closer to that of plasticized MB films. TS of plasticized films increased when AM content and %RC increased. When sorbitol was substituted for glycerol, crystallinity of the same kind of starch films increased as well as their TS. Since the plasticized CSMB and MB films had both a similar molecular profile and comparable mechanical properties, it was proposed the starch molecular profile containing AP with high Mw of AP, low Mw of AM, and the small size of IM may impart the high TS and %E of starch films.
Acknowledgements
The authors thank Kasetsart University and the Faculty of Agro-Industry, Kasetsart University, Thailand (through the Scholarships for International Graduate Students 2013) for financial support.
Page 19 of 36
References
AOAC. (2000). Official methods of analysis (17th ed.). Arlington, VA: Association of Official Analytical Chemists. Alves, V. D., Mali, S., Beléia, A., & Grossmann, M. V. E. (2007). Effect of glycerol and amylose enrichment on cassava starch film properties. Journal of Food Engineering, 78(3), 941–946. ASTM. (2002). ASTM D 882-02: Standard test method for tensile properties of thin plastic sheeting. West Conshohocken, PA: ASTM International. Bae, H. J., Cha, D. S., Whiteside, W. S., & Park, H. J. (2008). Film and pharmaceutical hard capsule formation properties of mungbean, waterchestnut, and sweet potato starches. Food Chemistry, 106(1), 96–105. Chaléat, C., Halley, P. J., & Truss, R. W. (2014). Mechanical properties of starch-based plastics. In P. J. Halley, & L. Avérous (Eds.), Starch polymers (pp. 187–209). Burlington, MA: Elsevier. Chaudhary, A. L., Torley, P. J., Halley, P. J., McCaffery, N., & Chaudhary, D. S. (2009). Amylose content and chemical modification effects on thermoplastic starch from maize – processing and characterisation using conventional polymer equipment. Carbohydrate Polymers, 78(4), 917–925. Chrastil, J. (1987). Improved colorimetric determination of amylose in starches or flours. Carbohydrate Research, 159(1), 154–158.
Page 20 of 36
Cuq, B., Gontard, N., Cuq, J-L. & Guilbert, S. 1997. Selected functional properties of fish myofibrillar protein-based films as affected by hydrophilic plasticizers. Journal of Agricultural and Food Chemistry, 45(3):622-626. Famá, L., Rojas, A. M., Goyanes, S., & Gerschenson, L. (2005). Mechanical properties of tapioca-starch edible films containing sorbates. LWT – Food Science and Technology, 38(6), 631–639. Farahnaky, A., Saberi, B., & Majzoobi, M. (2013). Effect of glycerol on physical and mechanical properties of wheat starch edible films. Journal of Texture Studies, 44(3), 176–186. Gaudin, S., Lourdin, D., Forssell, P. M., & Colonna, P. (2000). Antiplasticization and oxygen permeability of starch–sorbitol films. Carbohydrate Polymers, 43(1), 33-37. Ghosh Dastidar, T., & Netravali, A. (2013). Cross-linked waxy maize starch-based “green” composites. ACS Sustainable Chemistry & Engineering, 1(12), 1537–1544. Gupta, M., Brennan, C., & Tiwari, B. K. (2012). Starch-based edible films and coatings. In J. Ahmed, B. K. Tiwari, S. H. Imam, & M. A. Rao (Eds.), Starch-based polymeric materials and nanocomposites: chemistry, processing, and applications (pp. 239–254). Boca Raton, FL: CRC Press. Hongsprabhas, P. (2007). On the gelation of mungbean starch and its composites. International Journal of Food Science & Technology, 42(6), 658–668. Israkarn, K., Na Nakornpanom, N., & Hongsprabhas, P. (2014). Physicochemical properties of starches and proteins in alkali-treated mungbean and cassava starch granules. Carbohydrate Polymers, 105, 34–40.
Page 21 of 36
Kitamura, S., Hakozaki, K., & Kuge, T. (1993). Effects of molecular weight on the retrogradation of amylose. In K. Nishinari, & E. Doi (Eds.), Food hydrocolloids (pp. 179–182). New York: Springer. Laohakunjit, N., & Noomhorm, A. (2004). Effect of plasticizers on mechanical and barrier properties of rice starch film. Starch - Stärke, 56(8), 348–356. Li, M., Xie, F., Hasjim, J., Witt, T., Halley, P. J., & Gilbert, R. G. (2015). Establishing whether the structural feature controlling the mechanical properties of starch films is molecular or crystalline. Carbohydrate Polymers, 117(3), 262-270. Li, M., Liu, P., Zou, W., Yu, L., Xie, F., Pu, H., Liu, H., & Chen, L. (2011). Extrusion processing and characterization of edible starch films with different amylose contents. Journal of Food Engineering, 106(1), 95–101. Liu, H., & Lelièvre, J. (1992). A differential scanning calorimetry study of melting transitions in aqueous suspensions containing blends of wheat and rice starch. Carbohydrate Polymers, 17(2), 145–149. López, O. V., Lecot, C. J., Zaritzky, N. E., & García, M. A. (2011). Biodegradable packages development from starch based heat sealable films. Journal of Food Engineering, 105(2), 254–263. Lourdin, D., Bizot, H., & Colonna, P. (1997). “Antiplasticization” in starch-glycerol films? Journal of Applied Polymer Science, 63(8), 1047–1053. Mali, S., Grossmann, M. V. E., García, M. A., Martino, M. N., & Zaritzky, N. E. (2002). Microstructural characterization of yam starch films. Carbohydrate Polymers, 50(4), 379–386.
Page 22 of 36
Mali, S., Grossmann, M. V. E., García, M. A., Martino, M. N., & Zaritzky, N. E. (2008). Antiplasticizing effect of glycerol and sorbitol on the properties of cassava starch films. Brazilian Journal of Food Technology, 11, 194–200. Mali, S., Sakanaka, L. S., Yamashita, F., & Grossmann, M. V. E. (2005). Water sorption and mechanical properties of cassava starch films and their relation to plasticizing effect. Carbohydrate Polymers, 60(3), 283–289. Myllärinen, P., Partanen, R., Seppälä, J., & Forssell, P. (2002). Effect of glycerol on behaviour of amylose and amylopectin films. Carbohydrate Polymers, 50(4), 355–361. Obanni, M., & BeMiller, J. N. (1997). Properties of some starch blends. Cereal Chemistry, 74, 431–436. Parra, D. F., Tadini, C. C., Ponce, P., & Lugão, A. B. (2004). Mechanical properties and water vapor transmission in some blends of cassava starch edible films. Carbohydrate Polymers, 58(4), 475–481. Pavlath, A. E., & Orts, W. (2009). Edible films and coatings: Why, what, and how? In M. E. Embuscado, & K. C. Huber (Eds.), Edible films and coatings for food applications (pp. 1–23). New York: Springer. Prachayawarakorn, J., Hommannee, L., Phosee, D., and Chairapaksatien, P. 2010. Property improvement of thermoplastic mung bean starch using cotton fiber and low-density polyethylene. Starch/Starke 62 (435-443). Puncha-arnon, S., Pathipanawat, W., Puttanlek, C., Rungsardthong, V., & Uttapap, D. (2008). Effects of relative granule size and gelatinization temperature on paste and gel properties of starch blends. Food Research International, 41(5), 552–561.
Page 23 of 36
Rachtanapun, P., Pankan, D., & Srisawat, D. (2012). Edible films of blended cassava starch and rice flour with sorbital and their mechanical properties. Journal of Agricultural Science and Technology, 2(2), 252–258. Rindlav-Westling, A., Stading, M., & Gatenholm, P. (2002). Crystallinity and morphology in films of starch, amylose, and amylopectin blends. Biomacromolecules, 3, 84-91. Talja, A. R., Helen, H., Roos, Y. H., & Jouppila, K. 2007. Effect of various polyols and polyol contents on physical and mechanical properties of potato starch-based films. Carbohydrate Polymers, 67(3), 288-295. Thirathumthavorn, D., & Charoenrein, S. (2007). Aging effects on sorbitol- and non-crystallizing sorbitol-plasticized tapioca starch films. Starch - Stärke, 59(10), 493–497. Thiré, R. M. S. M., Andrade, C. T., & Simão, R. A. (2005). Effect of aging on the microstructure of plasticized cornstarch films. Polímeros, 15, 130–133. van Soest, J. J. G., Hulleman, S. H. D., de Wit, D., & Vliegenthart, J. F. G. (1996). Crystallinity in starch bioplastics. Industrial Crops and Products, 5(1), 11–22. Vilaplana, F., & Gilbert, R. G. (2010). Characterization of branched polysaccharides using multiple-detection size separation techniques. Review Article. Journal of Seperation Science, 33, 3537-3554. Vieira, M. G. A., da Silva, M. A., dos Santos, L. O., & Beppu, M. M. (2011). Natural-based plasticizers and biopolymer films: a review. European Polymer Journal, 47(3), 254–263. Waterschoot, J., Gomand, S. V., Fierens, E., & Delcour, J. A. (2015). Starch blends and their physicochemical properties. Starch - Stärke, 67(1–2), 1–13. Zhang, Y., & Han, J. H. (2006). Plasticization of pea starch films with monosaccharides and polyols. Journal of Food Science, 71(6), E253–E261.
Page 24 of 36
Zhong, Y., & Li, Y. (2014). Effects of glycerol and storage relative humidity on the properties of kudzu starch-based edible films. Starch - Stärke, 66(5-6), 524–532. Zullo, R., & Iannace, S. (2009). The effects of different starch sources and plasticizers on film blowing of thermoplastic starch: correlation among process, elongational properties and macromolecular structure. Carbohydrate Polymers, 77(2), 376–383.
Page 25 of 36
A
B
C
D
0.8 m
E
F
4 m
Fig. 1. Morphology of plasticized starch films observed under light microscope (40×, bar = 20 µm): (A) Gly-CS; (B) Sor-CS; (C) Gly-CSMB; (D) Sor-CSMB; (E) Gly-MB; and (F) Sor-MB. Page 26 of 36
A
B
C
Fig. 2. Sorbitol crystals observed under light microscope at various magnifications: (A) 10×, bar = 100 µm; (B) 20×, bar = 50 µm; and (C) 40×, bar = 100 µm. Page 27 of 36
Page 28 of 36
Fig. 3. X-ray diffraction patterns of: (A) native starches; (B) unplasticized starch films; and (C) plasticized starch films. The value in the parenthesis represents % relative crystallinity (%RC).
Page 29 of 36
Page 30 of 36
Fig. 4. FTIR spectra of: (A) unplasticized starch films; (B) glycerol-plasticized starch films; and (C) sorbitol-plasticized starch films. Page 31 of 36
Page 32 of 36
Table 1: Mechanical properties of unplasticized and plasticized starch films. Films
Moisture content (%)
Thickness (mm)
Tensile strength (MPa) At peak At break
Elongation (%) At peak At break
* 8.58ab (± 0.5) 19.22d 0.103b 2.86a 2.85a 14.06b 18.82bc (± 1.1) (± 0.0) (± 0.0) (± 0.0) (± 1.9) (± 3.5) Sor-CS 9.43b 0.101b 8.25b 6.77b 2.74a 14.86ab (± 0.1) (± 0.0) (± 1.6) (± 1.1) (± 0.2) (± 1.7) Unplast7.73a CSMB (± 0.2) Gly22.11e 0.090a 7.93b 7.93b 19.86c 21.32c CSMB (± 1.8) (± 0.0) (± 0.6) (± 0.6) (± 2.6) (± 2.0) ab b c c a Sor8.84 0.105 19.56 15.87 2.66 10.84a CSMB (± 0.5) (± 0.0) (± 2.0) (± 2.6) (± 0.6) (± 2.0) ab Unplast8.21 MB (± 0.5) Gly-MB 19.66d 0.098b 9.34b 9.34b 20.43c 21.37c (± 1.4) (± 0.0) (± 1.3) (± 1.3) (± 1.1) (± 1.1) b c c d a Sor-MB 9.16 0.113 20.64 19.20 3.05 12.89a (± 0.1) (± 0.0) (± 0.9) (± 0.8) (± 0.8) (± 5.3) Means in the same column with different letters are significantly different (p<0.05).
UnplastCS Gly-CS
Unplast = unplasticized; Gly = glycerol; Sor = sorbitol; CS = cassava starch; MB = mungbean starch; CSMB = 50:50 blend of cassava and mungbean starches. *
Not available. The films were too brittle to peel off.
Page 33 of 36
Table 2: Thermal properties of unplasticized and plasticized starch films. Treatment
Moisture content (%)
DMA Tg1 (°C)
Tg2 (°C)
Transition 1st 2nd (°C) (°C)
To (°C)
8.58ab ND* ND 154.00ab (± 0.5) (± 1.4) Gly-CS 19.22d -56.95a 13.60c 160.50bc (± 1.1) (± 5.1) (± 2.8) (± 0.7) Sor-CS 9.43b -14.40b 17.05c -36.19a 55.12ns 176.34d (± 0.1) (± 3.8) (± 6.1) (± 2.1) (± 0.0) (± 0.4) Unplast7.73a ND ND 146.17a CSMB (± 0.2) (± 4.0) Gly-CSMB 22.11e -56.10a 6.15b 169.00cd (± 1.8) (± 4.1) (± 1.6) (± 1.4) ab b c b ns Sor-CSMB 8.84 -20.50 14.05 -28.17 56.58 153.42ab (± 0.5) (± 2.7) (± 0.7) (± 0.8) (± 0.1) (± 12.1) ab Unplast-MB 8.21 ND ND 153.42ab (± 0.5) (± 1.3) d a a Gly-MB 19.66 -61.10 -0.40 168.34cd (± 1.4) (± 2.1) (± 0.7) (± 0.9) b b c b ns Sor-MB 9.16 -18.10 16.25 -26.59 57.54 167.34cd (± 0.1) (± 3.5) (± 4.6) (± 0.1) (± 3.6) (± 3.7) Means in the same column with different letters are significantly different (p<0.05). Unplast-CS
ns *
DSC Endotherm Tp (°C) 157.88a (± 1.8) 164.61ab (± 0.1) 180.63cd (± 0.2) 154.09a (± 4.3) 185.17d (± 0.9) 159.09ab (± 9.3) 155.75a (± 0.3) 169.83bc (± 1.4) 176.65cd (± 9.7)
Te (°C)
H (J/g)
187.67b (± 8.0) 186.67ab (± 2.1) 205.59c (± 0.6) 177.59ab (± 5.5) 209.50c (± 0.7) 186.75ab (± 5.7) 176.75a (± 0.3) 184.50ab (± 3.5) 199.67c (± 6.1)
172.36ab (± 39.3) 157.24a (± 12.4) 138.24a (± 14.2) 165.34ab (± 1.8) 165.40ab (± 4.2) 144.36a (± 3.4) 203.03b (± 27.2) 162.53a (± 3.4) 133.20a (± 13.9)
No significant difference (p>0.05).
Not determined. The films were too brittle for dynamic mechanical analyzer (DMA) measurement.
Page 34 of 36
Table 3: Ratio of wavenumbers in the FTIR spectrum of unplasticized and plasticized starch films. Treatment
Moisture content (%)
Bound water (ratio)
Free, inter-, intramolecular OH groups (ratio) ab bc Unplast-CS 8.58 0.41 0.98a (± 0.5) (± 0.1) (± 0.2) d ab Gly-CS 19.22 0.30 2.06bcd (± 1.1) (± 0.0) (± 0.1) Sor-CS 9.43b 0.31ab 2.69cd (± 0.1) (± 0.0) (± 0.2) Unplast-CSMB 7.73a 0.38bc 0.88a (± 0.2) (± 0.0) (± 0.1) Gly-CSMB 22.11e 0.25a 2.00bc (± 1.8) (± 0.0) (± 0.2) Sor-CSMB 8.84ab 0.44c 3.13d (± 0.5) (± 0.0) (± 0.1) Unplast-MB 8.21ab 0.43c 1.23ab (± 0.5) (± 0.0) (± 0.3) Gly-MB 19.66d 0.31abc 2.10bcd (± 1.4) (± 0.0) (± 0.1) Sor-MB 9.16b 0.34abc 2.20bcd (± 0.1) (± 0.2) (± 1.6) Means in the same column with different letters are significantly different (p<0.05).
Page 35 of 36
Table 4: Molecular weight distribution and AM / IM /AP ratio of native starches and starch films. Fraction I (AP) Fraction II (IM) Fraction III (AM) * 8 * 6 * (× 10 ) (%) ( × 10 ) (%) (× 105) (%) a b c b Native CS 0.98 62.09 19.00 16.37 22.30b 21.54a starch (± 0.0) (± 1.3) (± 0.4) (± 1.3) (± 6.6) (± 0.1) CSMB 1.21b 69.20c 5.03a 7.37a 3.87a 23.43ab (± 0.0) (± 1.7) (± 0.0) (± 0.4) (± 0.0) (± 1.3) MB 1.01ab 61.69ab 10.00b 8.47a 3.73a 29.84c (± 0.0) (± 1.3) (± 0.0) (± 0.5) (± 0.1) (± 0.8) Starch CS 0.96a 59.32ab 21.5c 19.46b 16.80b 21.28a film (± 0.0) (± 1.7) (± 0.7) (± 2.8) (± 5.9) (± 1.2) CSMB 1.33c 68.60c 4.64a 6.91a 3.64a 24.50b (± 0.0) (± 0.8) (± 0.5) (± 0.1) (± 0.0) (± 0.9) MB 1.04ab 58.42a 10.00b 8.49a 2.84a 33.09d (± 0.0) (± 2.0) (± 0.0) (± 0.7) (± 0.2) (± 1.2) Means in the same column with different letters are significantly different (p<0.05). Sample
AP = amylopectin; IM = intermediate materials; AM = amylose. * Mw (g/mol)
Page 36 of 36