International Journal of Biological Macromolecules 67 (2014) 490–502

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Properties and characteristics of dual-modified rice starch based biodegradable films Thewika Woggum a,∗ , Piyarat Sirivongpaisal b , Thawien Wittaya a a Department of Material Product Technology, Faculty of Agro-Industry, Prince of Songkla University, 15 Kanjanawanich, Khor Hong, Hat Yai 90112, Songkla, Thailand b Department of Food Technology, Faculty of Agro-Industry, Prince of Songkla University, Hat Yai 90112, Songkhla, Thailand

a r t i c l e

i n f o

Article history: Received 6 December 2013 Received in revised form 18 March 2014 Accepted 19 March 2014 Available online 28 March 2014 Keywords: Rice starch Dual-modified rice starch Biodegradable film Mechanical properties

a b s t r a c t In this study, the dual-modified rice starch was hydroxypropylated with 6–12% of propylene oxide followed by crosslinking with 2% sodium trimetaphosphate (STMP) and a mixture of 2% STMP and 5% sodium tripolyphosphate (STPP). Increasing the propylene oxide concentrations in the DMRS yielded an increase in the molar substitution (MS) and degree of substitution (DS). However, the gelatinization parameters, paste properties, gel strength and paste clarity showed an inverse trend. The biodegradable films from the DMRS showed an increase the tensile strength, elongation at break and film solubility, while the transparency value decreased when the concentration of propylene oxide increased. However the water vapor permeability of the films did not significantly change with an increase in the concentration of propylene oxide. In addition, it was found that DMRS films crosslinked with 2% STMP demonstrated higher tensile strength, transparency value and lower water vapor permeability than the DMRS films crosslinked with a mixture of 2% STMP and 5% STPP. The XRD analysis of the DMRS films showed a decrease in crystallinity when the propylene oxide concentrations increased and the crystallinity of DMRS films with 2% STMP were higher than the DMRS films with a mixture of 2% STMP and 5% STPP. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Biodegradable packaging, such as edible film, biodegradable film and coatings, is increasing because it uses natural materials and renewable resources, which do not contribute to environmental pollution [1,2]. Among all biopolymers, starch is being investigated as a potential material for biodegradable films [3]. Starch is a natural carbohydrate based polymer that is globally available from various natural sources including wheat, rice, corn and potato. Starch consists of two molecules: the essentially linear polysaccharide amylose; and the highly branched polysaccharide amylopectin. The amylose content in starch contributes to film strength and the branched structure of amylopectin generally leads to film with low mechanical properties [4]. Biodegradable films from starches with high-amylose content exhibit excellent oxygen barrier properties, lower water solubility, lower retrogradation temperature, and more stable mechanical properties at high RHs compared with those of native starch [5–7]. In addition, Muscat et al. [8] found that, the films with high amylose content showed higher tensile

∗ Corresponding author. Tel.: +66 74286359; fax: +66 74212889. E-mail addresses: [email protected], proteinfi[email protected] (T. Woggum). http://dx.doi.org/10.1016/j.ijbiomac.2014.03.029 0141-8130/© 2014 Elsevier B.V. All rights reserved.

strength and modulus of elasticity values but lower elongation values than those of low amylose starch films. Based on this, the rice starch (Chiang rice) has a higher amylose content of 30.40% in its composition [9]. This is assumed to be suitable for use as the basis for good biodegradable films which exhibit high oxygen barriers and good mechanical strength [10]. However, the strength, flexibility and film transparency of high amylose film are still poor and these undesirable properties limited its application in packaging. Modified starch has to be used to improve the properties of the film. Chemical modification is usually undertaken to improve film properties. Hydroxypropylation and crosslinking are methods widely used to produce modified starch. Starches that are etherified with propylene oxide (hydroxypropylated starches) exhibit lower gelatinization temperature, increased granule swelling, higher paste viscosity and greater paste clarity than native starches. This is because the incorporation of a hydroxypropyl group is capable of disrupting inter- and intra-molecular hydrogen bonds in the starch chains, thus weakening the granular structure of starch and increasing the accessibility of the starch granules to water [10]. Crosslinking reinforce the hydrogen bonds in the granules with chemical bonds that act as a bridge between the starch molecules [11]. Crosslinking provides paste viscosity and temperatures, and

T. Woggum et al. / International Journal of Biological Macromolecules 67 (2014) 490–502

acid and shear stability [12]. Reagents such as phosphorus oxychloride, sodium trimetaphosphate, and epichlorohydrin were reported to be generally used as crosslinking reagents [13]. Lim and Seib [14] investigated the preparation of starch phosphates and found that a mixture of phosphate salts (sodium trimetaphosphate (STMP) and sodium tripolyphosphate (STPP)) gave better results than using STMP alone to prepare distarch phosphate (crosslinked starch). This combination of STMP and STPP showed that the starch paste was more stable with heating and shearing and gave a high consistency after cooling [14]. In a previous study, the film from hydroxypropylation modified starches was formed and provided a higher flexibility and film transparency obtained, while the tensile strength value was low [10,15]. Meanwhile Kim and Lee [16] reported that the mechanical properties of starch films prepared with crosslinked corn starch show higher values than native corn starch. Detduangchan et al. [17] also reported that the tensile strength and water barrier of the rice starch films were improved by using crosslinking agents while the flexibility and film transparency were poor. When starch is modified by both hydroxypropylation and crosslinking or dual modification, the functional benefits from each individual modification are realized. The obtained films from dual modification would promise and has good potential in many food applications. Hence the objective of this study was to prepare and investigate the properties of dual-modified rice starch and dual-modified rice starch films from Chiang rice starch with various levels of propylene oxide and different crosslinking reagents compared with native starch.

2.3. Determination of hydroxypropyl group and molar substitution (MS) The hydroxypropyl group in the modified starches was determined according to the procedure described by the Joint FAO/WHO Expert [24]. A sample (50–100 mg) was weighed into a 100-ml volumetric flask and 25 ml of 0.5 M sulfuric acid was added. The flasks were placed in a boiling water bath and heated until the solution became clear. The samples were cooled and diluted to 100 ml with distilled water. One milliliter of the solution was placed by pipette into 25 ml test tubes with glass stoppers. The test tubes were immersed in an ice bath and then 8 ml of concentrated sulfuric acid were dropped into each tube. The solution was mixed well and the tubes were placed in a boiling water bath for exactly 3 min. The tubes were immediately transferred to an ice bath until the solution was chilled. An aliquot (0.6 ml) of ninhydrin reagent was added, and the reagent was carefully allowed to run down the walls of the test tubes. The test tubes were immediately shaken well and placed in room temperature for 100 min. The volume in each tube was adjusted to 25 ml with the concentrated sulfuric acid and mixed by inverting the tubes several times. Portions of the solution were immediately transferred to 1-cm cells, and, after exactly 5 min, the absorbance was measured at 590 nm, using the starch blank as a reference. A calibration curve was prepared with an aliquot (1 ml) of standard aqueous solution, containing 10, 20, 30, 40 and 50 mg of propylene glycol per ml. The hydroxypropyl groups (by %) were calculated by the following equation: Hydroxypropyl groups (%) =

2. Materials and methods 2.1. Raw material Chiang rice grain was purchased from a local rice mill in Phattalung, Thailand. The Chiang rice starch was isolated in our laboratory by the alkaline method following the procedures of Sawai and Morita [18], Sugimoto et al. [19] and Ju et al. [20].

2.2. Preparation of dual-modified rice starch (DMRS) The rice starch was modified by its reacting with propylene oxide following modified procedures from Hung and Morita [21], Suwanliwong [22], and Wattanachant et al. [13]. 15 g of sodium sulphate (15% based on dry wt. of starch) were added to 300 ml of distilled water and stirred. When the salt was dissolved, 100 g of rice starch (dbs, equivalent to 30% starch solid in suspension) were added and stirred to make up a uniform suspension. Then a 5% sodium hydroxide solution was added with vigorous stirring to prevent starch gelatinization and to adjust the suspension to pH 11.5. Then 6–12% propylene oxide (vol. by weight of starch solid) was added and stirred at room temperature for 30 min. The suspension was then transferred to centrifugal bottles and placed in a 40 ◦ C water bath with continuous shaking for 24 h. The starch suspension was transferred into a mixing container at room temperature. Then it was cross-linked by using 2% STMP (w/wt. of dry starch, HPC1) and a mixture of 2% STMP and 5% STPP (w/wt. of dry starch, HPC2) based on the methods of Hung and Morita [21], Suwanliwong [22], Wattanachant et al. [13] and Woo and Seib [23]. The suspension was held in a 40 ◦ C water bath for 3 h with continuous stirring. Then the suspension was adjusted to pH 5.5–6.0 with 10% hydrochloric acid solution to terminate the reaction. The starch suspension was washed with 1 time the volume of distilled water four times and isolated by centrifuging (3000 × g, 5 min); thereafter the starch was dried at 50 ◦ C for 24 h.

491

C × 0.7763 × 10F W

where C is amount of propylene glycol in the sample solution read from the calibration curve (mg/ml), F is the dilution factor and W is the weight of the sample (mg). The molar substitution (MS) of the modified starch was calculated in the normal fashion [12,22]. MS (hydroxypropylated starch) =

MS =

moles of substituent mole of anhydro − glucose unit

%HP × 162 59.08 × (100 − %HP)

2.4. Determination of phosphorus content and degree of substitution (DS) The content of the phosphorus in modified starches was determined according to a modified procedure as described by the A.O.A.C. [25]. A sample (2 g) was burned in a muffle furnace at 600 ◦ C for 4 h. This was then cooled to room temperature and 10 ml of 5 M hydrochloric acid was added in the ash. NRS was prepared in the same manner. The contents were diluted to 100 ml with distilled water and 50 ml of the solution were then diluted again to 100 ml with distilled water. The vanadate molybdate (25 ml) was added to the solution and then the absorbance of the sample was measured at 470 nm. A calibration curve was prepared from 0, 2.5, 5, 10, 20, 30, 40 and 50 ml of potassium dihydrogen phosphate (KH2 PO4 ), using the 0 ml of KH2 PO4 as a reference. The phosphorous content was calculated from the absorbance curve which was obtained by using a standard phosphorous solution. Phosphorous (%) =

P × dilution volume in ml × 100 (aliquot volume in ml × sample weight in g × 1000)

where P is the phosphorous content (mg/100 ml) from the calibration curve.

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The degree of substitution (DS) of modified starch was calculated in the normal manner [12,22,26]. DS (Disodium salt) =

162 × %P 3100 − (124 × %P)

2.5. Functional properties of dual-modified rice starch (DMRS) 2.5.1. Gelatinization properties (DSC) Gelatinization and dissociation parameters were measured using differential scanning calorimetry (DSC-7, Perkin Elmer Inc., Norwalk, CT, USA). A starch: distilled water suspension of 1:4 weight ratio was hermetically sealed in a pre-weighed aluminum pan and reweighed on a microbalance. Calibration was based on pure indium and sapphire. An empty Aluminum pan was used as reference. The scanning temperature and the heating rates were 30–120 ◦ C and 10 ◦ C/min, respectively. An empty pan was used as a reference for all measurements. The onset (To ), peak (Tp ) and conclusion (Tc ) temperatures of the gelatinization were determined. The enthalpy of gelatinization (H) was calculated in terms of joules per unit weight of the dry starch (J/g). 2.5.2. Pasting properties The pasting properties of the starches were determined using a Rapid Visco Analyser (RVA) model 3D (Newport Scientific, Warriewood, Australia). Distilled water (21.5 g) was added to the starch (3.5 g, db) in the RVA canister to obtain a total constant sample weight of 25 g (14% starch concentration). The suspension was then manually homogenized using a plastic paddle to avoid lump formation before the RVA run. A programmed heating and cooling cycle was set for 12 min. It was first held at 50 ◦ C for 1.0 min, heated to 95 ◦ C in 4 min, further held at 95 ◦ C for 2 min, cooled to 50 ◦ C within 4 min, and held at 50 ◦ C for 1 min. 2.5.3. Paste clarity A 1% starch paste was heated in a boiling water bath for 30 min and cooled to 25 ◦ C for 1 h. Its clarity (%T650 ) was evaluated using percent transmittance at 650 nm against a water blank in a Spectrophotometer (LIBRA S21–S22, Biochrom Ltd, Cambridge, UK) [27]. 2.5.4. Gel strength The gel strength was measured using a Texture Analyzer (TAXTplus, Stable Micro System, Ltd.). Starch pastes (15% db) were prepared by boiling in a water bath for 30 min and then the hot pastes were poured into cylindrical plastic tubes (20 mm diameter and 20 mm height). These were covered, cooled at room temperature for 1 h, and kept at 4 ◦ C f or 24 h. The starch gels were placed in room temperature for at least 1 h before measuring their gel strength. A cylindrical probe with a diameter of 35 mm was used to penetrate into the gel at a speed of 2 mm/s for distances of 20% strain. The maximum force of the penetration was recorded as the gel strength (g) and the measurements were repeated five times. 2.6. Film casting and drying Native rice starch (NRS) and DMRS (3.5% db) were dissolved in distilled water and gelatinized by boiling in a water bath with continuous stirring (85 ◦ C for 15 min) and then cooled to 45 ◦ C. Sorbitol was added as 35% (w/w) of the total solid weight in solution [28,29]. The mixtures were cast onto flat leveled, non-stick trays to set. Once set, the trays were held overnight at 50 ◦ C for 10 h undisturbed, and then cooled to room temperature before the films were peeled off the plates. The film samples were stored at 25 ◦ C and 55 ± 5% RH until used for further testing. All the treatments were made in triplicate.

2.7. Dual-modified rice starch films (DMRS films) testing 2.7.1. Conditioning All films were conditioned prior to subjecting them to permeability and mechanical tests according to the Standard method, D618-61 [30]. Films used for testing water vapor permeability (WVP), tensile strength (TS), and elongation (E) were conditioned at 60% RH and 27 ± 2 ◦ C by placing them in desiccators over a saturated solution of Mg(NO3 )2 ·6H2 O for 72 h or more. For other tests, film samples were transferred to plastic bags after peeling and placed in desiccators. 2.7.2. Film thickness Thickness of the films was measured with a precision digital micrometer (Digimatic Indicator, Mitutoyo Corporation, Japan) to the nearest 0.0001 (±5%) at five random locations on the film. Mean thickness values for each sample were calculated and used in water vapor permeability (WVP) and tensile strength (TS) calculations. 2.7.3. Mechanical properties Prior to the measurement of the mechanical properties, the films were conditioned for 72 h in a ventilated oven at 25 ◦ C and 55 ± 5% RH. The tensile strength (TS) and elongation at break (%E) of the films were determined as described by Bourtoom and Chinan [28] with a slight modification. This was done using a Universal Testing Machine (Lloyd Instruments, Hampshire, UK) equipped with tensile load cell of 100 N [30]. Ten samples (2.54 cm × 12 cm) with an initial grip length of 10 cm were used for testing. The samples were clamped and deformed under a tensile load with a cross-head speed of 50 mm/min until the samples were broken. The TS was calculated by dividing the maximum force by the initial specimen cross-sectional area, and the %E was calculated as follows: %E =

100 × (dafter − dbefore ) dbefore

where d was the distance between grips holding the specimen before or after the breaking of the specimen. 2.7.4. Water vapor permeability (WVP) and film solubility (FS) The gravimetric modified cup method based on ASTME96-92 [31] was used to determine the WVP of the films. The test cups were filled with 20 g of Silica gel (the desiccant) to produce a 0% RH below the film. A sample was placed between the cup and the ring cover of each cup coated with silicone sealant (high vacuum grease, Lithelin, Hanau, Germany) and held with four screws around the cup’s circumference. The air gap was at approximately 1.5 cm between the film surface and the desiccant. The water vapor transmission rates (WVTR) of each film were measured at 55 ± 5% RH and 25 ± 2 ◦ C. After taking the initial weight of the test cup, it was placed in a growth chamber with an air velocity rate of 135 m/min (Model KBF115, Contherm Scientific, Lower Hutt, New Zealand). The weight gain measurements were taken by weighing the test cup to the nearest 0.0001 g with an electronic scale (Sartorious Corp.) every 3 h for 18 h. A plot of the weight gained versus time was used to determine the WVTR. The slope of the linear portion of this plot represented the steady state amount of water vapor diffusing through the film per unit of time (g/h). The WVTR was expressed in gram units per square meter per day (g/m2 day). The steady state over time (slope) yielded a regression coefficient of 0.99 or greater. Six samples per treatment were tested. The WVP of the film was calculated by multiplying the steady WVTR by the film thickness and dividing that by the water vapor pressure difference across the film. The WVP of the film was expressed in g mm/day m2 kPa unit. A modified method from Jangchud and Chinnan [32] was used to measure film solubility. Film pieces, 20 mm × 20 mm, were dried

T. Woggum et al. / International Journal of Biological Macromolecules 67 (2014) 490–502

at 70 ◦ C in a vacuum oven for 24 h and then weighed to the nearest 0.0001 g for the initial dry mass. Films were immersed in 20 ml of distilled water in 50 ml screw cap tubes containing 0.01 g/100 g sodium benzoate. The tubes were capped and placed in a shaking water bath for 24 h at 25 ± 2 ◦ C. A portion of the solution was removed and set aside for later use in protein solubility tests as described below. The remaining solution and film pieces were poured onto quality filter paper (Whatman No. 1). These were rinsed with 10 ml distilled water, and dried at 70 ◦ C in a vacuum oven for 24 h to determine the dry mass of the film. Five measurements were taken for each treatment. The total soluble matter was calculated from the initial gross mass and the final dry mass using the following equation: %FS (db) =

(film mass before test − film mass after test) × 100 film mass before test

2.7.5. Film transparency The transparency of films was determined using a Spectrophotometer (LIBRA S21–S22, Biochrom Ltd, Cambridge, UK). Film pieces, 10 mm × 30 mm and placed on the internal side of the 1-cm spectrophotometer cells. The transparency of films was determined at 600 nm against an empty cuvette blank as described by Han and Floros [33]. Five samples per treatment were tested and calculated as follows: Transparency value =

−log T600 x

where T600 is the fractional transmittance at 600 nm and x is the film thickness (mm). The greater value represents lower transparency of the film. 2.7.6. FTIR spectroscopy Prior to the analysis, the films were conditioned in a desiccator containing dried silica gel for 7 days at room temperature to obtain the most dehydrated films. The films were scanned with a Bruker Model Equinox 55 FTIR spectrometer (Bruker Co., Ettlingen, Germany). The samples were measured in a horizontal ATR Trough plate crystal cell (45◦ ZnSe; 80 mm long, 10 mm wide and 4 mm thick) (PIKE Technology Inc., Madison, WI). The spectra were performed in the 4000–400 cm−1 regions to resolve overlapping bands. 2.7.7. X-ray diffraction The X-ray patterns of the rice starch films were analyzed using an X-ray diffractometer (X’Pert MPD, PHILIPS, Netherlands) with Cu K␣ radiation at a voltage of 40 kV and 30 mA. The samples were scanned between 2 = 3–40◦ with a scanning speed of 2◦ /min. Prior to testing, the samples were dried and stored in a desiccator. The relative crystallinity index was calculated following equation [34,35]. %Xc =

Ac Ac = At Ac + Aa

where Ac is the crystalline area, Aa is the non-crystalline area and At is total area. 2.8. Statistical analysis A completely randomized experimental design was used to characterize the blend films. Analysis of variance (ANOVA) was used to compare mean differences of the samples. If the differences in mean existed, multiple comparisons were performed using Duncan’s Multiple Range Test (DMRT).

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3. Results and discussion 3.1. Effect of propylene oxide concentrations and crosslinking reagents on properties of rice starch 3.1.1. Hydroxypropyl groups, molar substitution, phosphorus content and degree of substitution The reaction efficiencies of hydroxypropylation and crosslinking were reported through molar substitution (MS) of the hydroxypropyl group and the degree of substitution (DS) of the phosphate group into granular starches. The MS and DS of all dual-modified rice starches are shown in Table 1. The MS and DS of DMRS were in the range 0.019–0.039 and 0.0026–0.0106, respectively, and increased with the increase of the content of propylene oxide. The phosphorus content of all treatments was in the range 0.0037–0.2015% which is lower than the maximum level (0.4%) allowed by the U. S. regulations [36]. These results showed different DS values in their treatment although the DMRS samples were crosslinked with the same amount of crosslinking reagents (HPC1 and HPC2). Thus the different DS values obtained were due to the different amounts of propylene oxide. In addition, these results also show that the level of hydroxypropylation during the first stage of the dualmodification process enhanced the subsequent crosslinking. This was shown by the increase in the phosphorus content and DS values. Phosphate salts from the crosslinking reagent (STMP) enabled phosphate groups to produce the distarch phosphate derivative which has significantly higher phosphorus content than the NRS [36]. Gunaratne and Corke [37] described how the hydroxypropyl groups in the neighboring starch chains prevent amylose association and results in weaker granules, and they had higher MS values. Thus allowing the crosslinking reagent to react more with the starch chains results in higher DS values. Wattanachant et al. [36] also reported that hydroxypropylation caused weaker granules by disrupting the hydrogen bonds between starch chains and these results in the swelling of granules. In this way the crosslinking reagents reacted more easily with the starch molecules. In addition, Morikawa and Nishinari [38] also found that the DS values of dualmodified potato starch increased when there was an increase in the amount of propylene oxide. In consideration, the phosphorus content and DS values, those of the HPC2 were higher than HPC1. These results might suggest that both the STMP and STPP reagents in HPC2 were phosphate salts which gave higher phosphorus content than by only using 2% STMP. However, the MS and DS values are difficult to determine directly so the physical properties were measured for confirming the characterization of crosslinked starch [12,39].

3.1.2. Gelatinization properties The gelatinization characteristics of NRS and DMRS as determined by DSC are shown in Fig. 1, and their corresponding thermal parameters are summarized in Table 2. The NRS and DMRS exhibited similar profiles on the DSC thermograms. The transition temperatures (To , Tp and Tc ) of all DMRS samples were lower than those of the NRS, and gradually decreased with an increase the amount of propylene oxide. The lowest values were observed when 12% of propylene oxide was applied. These results are consistent with some previous studies [21,37,40]. The DMRS which had higher MS values (Table 1) loosened the granular structure and thus lowered the transition temperatures. The Tp values from the DSC results and the paste temperature from RVA results of all DMRS samples decreased by 2–6 ◦ C and 1–3 ◦ C, respectively, compared to the NRS. These results might have been due to the fact that substitution occurred primarily in the amorphous region [21]. Thus the substitution reflected swelling in that region and disrupted the crystalline

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Table 1 Hydroxypropyl groups, molar substitution (MS), phosphorus content and degree of substitution (DS) of native rice starch (NRS) and dual-modified rice starches (DMRS) with 6–8% of propylene oxide and crosslinked with 2% of STMP and a mixture of 2% of STMP and 5% of STPP. Starch

Hydroxypropyl groups (%)

Molar substitution (MS)

Phosphorus content (%)

NRS HP6C1 HP8C1 HP10C1 HP12C1 HP6C2 HP8C2 HP10C2 HP12C2

– 0.679 0.922 1.132 1.396 0.498 0.853 0.918 1.295

– 0.019 0.026 0.031 0.039 0.014 0.024 0.025 0.036

0.0037 0.0504 0.0821 0.1021 0.1189 0.1296 0.1794 0.1832 0.2015

± ± ± ± ± ± ± ±

0.021f 0.024d 0.043c 0.007a 0.014g 0.026e 0.008d 0.060b

± ± ± ± ± ± ± ±

0.001f 0.001d 0.001c 0.000a 0.000g 0.001e 0.000d 0.002b

± ± ± ± ± ± ± ± ±

Degree of Substitution (DS)

0.0000i 0.0005h 0.0016g 0.0006f 0.0012e 0.0014d 0.0018c 0.0045b 0.0018a

– 0.0026 0.0043 0.0054 0.0062 0.0068 0.0095 0.0097 0.0106

± ± ± ± ± ± ± ±

0.0000g 0.0001f 0.0000e 0.0001d 0.0001c 0.0001b 0.0002b 0.0001a

Mean values in the same column with different letters are significantly different (p < 0.05). NRS = native rice starch, HP6C1–HP12C1 and HP6C2–HP12C2 = dual-modified rice starches at various levels of propylene oxide (6–12%) and crosslinking reagents using 2% of STMP and a mixture of 2% of STMP and 5% of STPP for HPC1 and HPC2, respectively.

Table 2 Thermal properties of native rice starch (NRS) and dual-modified rice starches (DMRS) with 6–8% of propylene oxide and crosslinked with 2% of STMP and a mixture of 2% of STMP and 5% of STPP. Temperature (◦ C)

Starch To NRS HP6C1 HP8C1 HP10C1 HP12C1 HP6C2 HP8C2 HP10C2 HP12C2

Tp

71.23 69.38 67.29 67.35 65.84 67.41 67.03 66.24 65.16

± ± ± ± ± ± ± ± ±

1.24a 0.57b 0.38c 0.35c 0.08d 0.59c 0.57c 0.83cd 0.08d

H (J/g) Tc

76.13 74.44 73.12 73.46 72.16 72.66 72.00 71.53 70.71

± ± ± ± ± ± ± ± ±

0.25a 0.57b 0.34cd 0.51bc 0.31def 0.36cde 0.75ef 0.78fg 0.95g

79.79 78.70 77.25 78.11 76.92 76.86 75.96 75.64 75.26

± ± ± ± ± ± ± ± ±

0.83a 0.50b 0.31cd 0.25bc 0.30d 0.36d 0.52e 0.78e 0.51e

10.62 8.91 9.93 10.60 10.99 7.63 7.75 7.83 8.23

± ± ± ± ± ± ± ± ±

0.14ab 0.92c 0.44b 0.78ab 0.23a 0.27d 0.11d 0.02d 0.30cd

Mean values in the same column with different letters are significantly different (p < 0.05). NRS = native rice starch, HP6C1–HP12C1 and HP6C2–HP12C2 = dual-modified rice starches at various levels of propylene oxide (6–12%) and crosslinking reagents using 2% of STMP and a mixture of 2% of STMP and 5% of STPP for HPC1 and HPC2, respectively.

Endothermic Heat Flow

(A) NRS HP6C1 HP8C1 HP10C1 HP12C1 30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Temperature (°C)

Endothermic Heat Flow

(B) NRS

HP6C2 HP8C2 HP10C2 HP12C2

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Temperature (°C) Fig. 1. Thermograms of native rice starch (NRS) and dual-modified rice starches (DMRS) with 6–8% of propylene oxide and crosslinked with 2% of STMP (A) and a mixture of 2% of STMP and 5% of STPP (B).

phase, resulting in a lower temperature and H in melting when compared with those of the NRS [41]. However, the transition temperatures with different crosslinking reagents (HPC1 and HPC2) did not show significant differences. The H of almost all DMRS samples was lower than those of the

NRS. This could be because all modifications of the rice starches yielded a loss of structural order resulting in lower H. In addition, it was found that the H of DMRS gradually increases with an increase in the amount of propylene oxide. Hydroxypropylation causes the bonding in the starch molecules to weaken and increased the swelling powder thus allowing more crosslinking reagents to react in the starch molecules [13,37,42]. Gunaratne and Corke [37] reported that the crosslinking starches have strong internal bonding forces with behavior that is evidenced by low swelling, high pasting temperatures and high H values in gelatinization. Basically, gelatinization enthalpy (H) reflects the energy required for disrupting the granule structure [15]. In addition, Gunaratne and Corke [37] reported that hydroxypropylation caused a decrease in gelatinization temperature and enthalpy. This occurred because extensively hydrated granules require less driving force and energy to achieve gelatinization. Kaur et al. [43] also reported that the addition of propylene oxide into the starch polymer backbone allowed higher flexibility and the effect was to lower the melting temperature. Perera et al. [44] also showed that the double helices in starch structure were disrupted by the rotation of the flexible hydroxypropyl groups within the amorphous region and the result was that fewer helices were left to melt during gelatinization. The results show that the H of HPC1 (using STMP alone) was higher than HPC2. This was because the STMP gave distarch phosphate. Distarch phosphates have the phosphate esterified with two hydroxyl groups, often from two neighboring starch molecules. This leads to the formation of a covalent bridge or crosslinking [11,45]. The STTP in a mixture of phosphate salts (HPC2) produced monostarch phosphates which have an ionic character and hydrates readily in water. These results also show that the crosslinking reaction was reduced by adding STPP [14]. Generally, the reduction of swelling resulting from crosslinking should delay gelatinization. However, the crosslinking had very little effect on the gelatinization parameters, in contrast to its marked

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495

Fig. 2. Pasting profiles of native rice starch (NRS) and dual-modified rice starches (DMRS) with 6–8% of propylene oxide and crosslinked with 2% of STMP (A) and a mixture of 2% of STMP and 5% of STPP (B).

effect on the pasting properties [37]. Furthermore, crosslinking retains the integrity of starch granules so they need more heat to gelatinize [12]. 3.1.3. Pasting properties Pasting curve of NRS and DMRS with different levels of hydroxypropylation and crosslinking reagents determined by RVA is shown in Fig. 2. After dual-modification, the DMRS showed the pasting temperature and viscosity (peak, breakdown and setback viscosity) to be lower than the NRS. During the period of heating up and holding at 95 ◦ C, there was similarity in the pasting profile of all DMRS samples. The effects of the different levels of propylene oxide and crosslinking reagents on the reduction of pasting temperatures of the DMRS are presented in Table 3. At higher levels of propylene oxide the effect on the pasting temperature and viscosity (peak, breakdown and setback viscosity) tended to decrease. These results showed thick to thin viscosity with increasing amounts of propylene oxide. This might be because the levels of propylene oxide induced high crosslinking in starch granules [36]. Rutenberg and Solarek [12] reported that the breakdown viscosity can be reduced by crosslinking. Thus the decrease in breakdown viscosity as seen in the HPC1 and HPC2 confirms that crosslinking has taken place. The setbacks in almost all DMRS samples were lower than in the native starch and showed a decreases when the amount of propylene oxide increased. After hydroxypropylation

the structure of the starch granules was weaker compared with unmodified granules [37], thus the addition of crosslinking reagents caused more reaction in the starch chains. Yook et al. [46] also reported that the setbacks in the DMRS were decreased when the starches were dual-modified. Hydroxypropylation caused the bonding in the starch molecules to weaken thus allowing more crosslinking reagents to react in the starch molecules [13]. This indicated that after the dualmodification of rice starch, the starch granules were stronger and more difficult to breakdown. Thus the amylose chains inside the granules slightly leached out and they were then less likely to reassociate in an ordered structure, resulting in lower breakdown and setbacks. The effects of different crosslinking reagents on the pasting temperature and viscosity of DMRS are showed in Table 3. The pasting temperature did not show any significant difference (p > 0.05) with different crosslinking reagents. The peak viscosity, breakdown and setback in the DMRS with the mixture of 2% STMP and 5% STPP were higher than when only using STMP. This might be because the STMP produced a distarch phosphate which is reported to be a strongly effective crosslinking reagent [23]. This means that in the DMRS with only STMP these are employed to strengthen the structure of swollen granules upon gelatinization and enhance the resistance to the breakdown of the viscosity [47]. While the STPP in DMRS with the mixture of phosphate 2% STMP and 5% STPP give monostarch

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Table 3 Pasting properties of native rice starch (NRS) and dual-modified rice starch (DMRS) with 6–8% of propylene oxide and crosslinked with 2% of STMP and a mixture of 2% of STMP and 5% of STPP. Pasting temperature (◦ C)

Starch

Viscosity (mPa s) Peak

NRS HP6C1 HP8C1 HP10C1 HP12C1 HP6C2 HP8C2 HP10C2 HP12C2

82.43 81.62 81.55 81.67 79.43 82.70 81.62 81.60 79.20

± ± ± ± ± ± ± ± ±

Breakdown

2454.67 ± 28.22 1472.33 ± 9.07c 1445.67 ± 11.15d 1086.00 ± 6.24h 972.00 ± 10.58i 2329.33 ± 15.82b 1391.67 ± 13.61e 1346.33 ± 13.58f 1152.00 ± 4.58g

a

a

0.03 0.08b 0.05b 0.06b 0.40c 0.52a 0.08b 0.05b 0.00c

Setback

329.67 ± 8.08 50.67 ± 13.65c 45.33 ± 5.86c 18.67 ± 6.35d 7.33 ± 1.53d 86.33 ± 7.93b 45.67 ± 8.96c 51.00 ± 4.36c 20.00 ± 4.11d

a

507.67 203.67 212.67 139.00 176.33 642.67 322.00 365.00 318.33

± ± ± ± ± ± ± ± ±

14.01b 5.03e 5.51e 4.58g 2.08f 16.86a 11.79d 13.23c 14.57d

Mean values in the same column with different letters are significantly different (p < 0.05). NRS = native rice starch, HP6C1–HP12C1 and HP6C2–HP12C2 = dual-modified rice starches at various levels of propylene oxide (6–12%) and crosslinking reagents using 2% of STMP and a mixture of 2% of STMP and 5% of STPP for HPC1 and HPC2, respectively.

phosphates [14]. Then the starch structure will weaken and lead to rapid hydration and swelling [48]. Lim and Seib [14] reported that monostarch phosphates from wheat and corn starches modified by STPP had lower pasting temperature but higher pasting peaks than starch modified with STMP. In addition, the monostarch phosphates from STPP are produced with a higher level of phosphate substitution in the starch. The introduction of phosphate substitution on amylose chains prevents the inter-molecular or intra-molecular association within starch granules. This leads to better paste viscosity and paste clarity [11]. Xie et al. [49] also reported that the starch phosphates (STPP/STMP) are strongly anionic polymers. They yield higher viscosity and more clear and stable dispersions with a long cohesive texture and resistance to retrogradation. 3.1.4. Gel strength and paste clarity (%T650 ) The gel property and paste clarity of NRS and DMRS are shown in Table 4. The NRS shows long cohesive gels with high strengths which are undesirable for application in canned foods [13]. However, these undesirable properties could be improved through hydroxypropylation and crosslinking [50,51]. Cold storage is suitable for presenting retrogradation or to encourage the close alignment of starch chains to form a three-dimensional network, resulting in higher gel strength [52]. The gel strengths of all DMRS samples were lower than those of NRS. Generally hydroxypropylation causes a decrease in the gel strength because the loosening of the starch granule structure permits more granules to breakdown, leaving weaker cohesive swollen starch particles in the gel matrix, leading to a weaker starch gel. In addition, the hydroxypropyl

Table 4 Gel strength and paste clarity of native rice starch (NRS) and dual-modified rice starch (DMRS) with 6–8% of propylene oxide and crosslinked with 2% of STMP and a mixture of 2% of STMP and 5% of STPP. Starch

Propylene oxide concentration (%w/w)

Gel strength (g)

NRS HP6C1 HP8C1 HP10C1 HP12C1 HP6C2 HP8C2 HP10C2 HP12C2

– 6 8 10 12 6 8 10 12

179.94 140.80 123.41 115.54 104.75 132.08 119.53 103.02 96.64

± ± ± ± ± ± ± ± ±

5.97a 1.56b 1.87d 1.98f 2.95g 3.05c 1.30e 2.97g 3.70h

Paste clarity (%T650 ) 5.57 ± 4.33 ± 4.23 ± 3.27 ± 3.40 ± 4.20 ± 3.67 ± 3.53 ± 0.06cde

0.06a 0.15b 0.15b 0.21e 0.20de 0.10b 0.15c 0.15cd

Mean values in the same column with different letters are significantly different (p < 0.05). NRS = native rice starch, HP6C1–HP12C1 and HP6C2–HP12C2 = dualmodified rice starches at various levels of propylene oxide (6–12%) and crosslinking reagents using 2% of STMP and a mixture of 2% of STMP and 5% of STPP for HPC1 and HPC2, respectively.

groups in the neighboring starch chains prevent inter-chain association and interrupt the formation of junction zones. Crosslinking causes an increase in the gel strength because its starch granule structure will allow stronger and more rigid granules to remain in the gel matrix. Furthermore, the possible creation of more junction zones by covalent crosslinking bonding could increase the gel strength [37]. However, a significant reduction of amylose leaching upon crosslinking would decrease the amylose concentration in the continuous network. This effect may thus result in a weaker gel structure after crosslinking [37]. Hence, the DMRS of all samples showed a decrease in the gel strength. This is because the crosslinking would decrease the amylose leaching. Furthermore, it would also reduce the deformability of gelatinized swollen granules in the gel matrix [37]. As was seen, the gel strength of the DMRS was reduced when the amount of propylene oxide increased. This indicates that the propylene oxide levels induce high crosslinking in starch granules. This result may occur from weakening in the starch granules through hydroxypropylation, as the crosslinking reagents react more with the starch molecules [13]. This caused a substantial lessening of the breakdown in the starch granules and decreased the amylose leaching [37]. It was observed that the DMRS, using only STMP, has a higher gel strength than the DMRS with a mixture of phosphate salts. This indicated that in the STMP there was more crosslinking occurring in the starch granules because the STMP produced distarch phosphates (diesters). Furthermore, the STMP has been reported to be an efficient crosslinking reagent [30], whereas the STPP with a mixture of phosphate salts produced monostarch phosphate (monoesters). This monostarch phosphate is introduced at a much higher level of substitution on starch than distarch phosphate and this prevents linearity in the molecular chains [53]. The crosslinking of starch can be confirmed by the reduction of paste clarity [14,27]. In this study, therefore, paste clarity was used to compare the efficiencies of the crosslinking reagents. The high clarity of NRS paste at 5.57% T650 decreased significantly (p < 0.05) after dual-modification, as shown in Table 4. It was observed that the paste clarity of DMRS tended to decrease when the amount of propylene oxide increased. This result might occur from the dualmodification that the hydroxypropylation brings about to increase the swelling factor compared with their NRS, while crosslinking caused a decrease in the swelling factor. Hydroxypropyl groups in the neighboring starch chains prevent association and this leads to an increase in granule swelling. Thus the crosslinking reacts more in the hydroxypropylated starch; the crosslinking will reinforce the structure of the starch granules and limits water absorption. Therefore the dual-modification resulted in less water absorption in the starch granule, resulting in less transmission of light into the starch granule [37]. However, the differences in the effects of

T. Woggum et al. / International Journal of Biological Macromolecules 67 (2014) 490–502

Fig. 3. Tensile strength (A) and elongation at break (B) of native rice starch (NRS) and dual-modified rice starch (DMRS) films with 6–8% of propylene oxide and crosslinked with 2% of STMP and a mixture of 2% of STMP and 5% of STPP.

the crosslinking reagents when using STMP alone and a mixture of phosphate salts were not significant. This showed that the paste clarity of DMRS with a mixture of phosphate salts was like that when using only STMP. 3.2. Effect of propylene oxide concentration and crosslinking reagents on properties of rice starch film 3.2.1. Mechanical properties Tensile strength (TS) and Elongation at break (%E) of the NRS and DMRS films are shown in Fig. 3A and B. It was observed that the TS values and %E values of the NRS and DMRS films were in the range 6.18–8.71 MPa and 68.7–126.11%, respectively. The NRS film presented the lowest of both TS value and %E value. The %E values of DMRS films tended to increase with addition the propylene oxide. The TS values of dual-modified rice starch tended to increase with addition the 6–8% propylene oxide and it’s gradually decrease with addition the 10–12% propylene oxide. These results reflect that the DMRS films were stronger and more flexible than the NRS films. This might be because the hydroxypropylation caused a weakening of the hydrogen bonding between starch chains and this leads to more crosslinking reagents reacting with the starch molecules [13,37]. Previous results have pointed out that the adding of propylene oxide provided higher MS and DS values (Table 1) and crosslinking in starch granules. Wittaya [54] reported that starch crosslinking is normally performed by treating starches (semi-dry or suspension) with reagents capable of forming either ether or ester linkages between hydroxyl ( OH) groups in starch molecules. In addition, the increase in mechanical properties is due to the increase of crosslinking density. The crosslinking reagents react with the OH groups present in starch and make ether linkages with the available hydroxyl groups. This helps to increase the mechanical properties [55]. This is

497

because crosslinking reinforces the structure of starch granules and limits the water absorption of starch, thereby restricting the mobility of the starch chain in the amorphous region [56]. In addition, Singh et al. [53] reported that crosslinking reagents were involved in the reduced mobility of amorphous chains in the starch granule from the intermolecular bridges. These effects showed the stronger chemical bond in the amylose chains as the matrix in the drying process for film formation [57]. Kim and Lee [16] reported that the mechanical properties of starch films prepared with crosslinked corn starch show higher values than those from native corn starch. The optimum concentration of propylene oxide and crosslinking reagents is an important factor for improving the mechanical properties of DMRS films. It was observed that the DMRS with HP8C1 films and HP10C2 films showed the highest TS values compared with other treatments. The TS values of DMRS films with STMP and a mixture of 2% STMP and 5% STPP showed a gradual improvement when increasing the propylene oxide. The HPC1 and HPC2 began to drop when the amount of propylene oxide was 10% and 12%, respectively. This may be because the high concentrations of propylene oxide brought about the loosening of the starch granules. This would allow more granule breakdown and leave weaker cohesive swollen starch particles in the gel matrix, and the hydroxypropyl groups would be bulky in the amylose chains [37]. Thus the crosslinking reagents are not sufficient for crosslinking the hydroxypropylated starch. Furthermore, the monostarch phosphates in the STPP produce the phosphate substitution in the starch chain, like the substitution with propylene oxide which brought about the decreased TS values in the DMRS films. In addition Yang et al. [58,59] also reported that with a low concentration of crosslinking agent, there is not enough crosslinking between the starch molecules to improve the tensile strength of the films. Furthermore, with high concentrations, there is excess crosslinking and this limits the mobility of the starch molecules, leading to lower tensile strength. Elongation at break (%E) indicates the flexibility and stretch of the film, which is determined at the point when the film breaks under tensile testing. It is expressed as the percentage of change of the original length of the specimen to stretch (extend) between grips holding a film [28]. The %E values of all DMRS samples tended to increase when adding the propylene oxide. This might be due to the bulky hydroxypropyl groups in the DMRS inhibiting the reassociation of the amylose chains. In addition, the propylene oxide may act like a plasticizer in the films and this induces the formation of starch-plasticizer interactions resulting in higher %E [54]. In our results, the effect of crosslinking reagents are expressed in higher both TS and %E values. These results show that the dualmodification of rice starch was more efficient than crosslinking alone. However, Detduangchan and Wittaya [60] reported that only photo-crosslinked rice starch showed an increase in the tensile strength but there was a decrease in the elongation at break. Our results show that the %E of DMRS films prepared from HPC2 demonstrated higher values than those of HPC1 films with the same content of propylene oxide. These might be because the mixture of 2% STMP and 5% STPP in HPC2 gave a combination of monostarch phosphates (STPP) and distarch phosphates (STMP). The monostarch phosphate introduced higher substitution in the starch chains [53]. In addition, the bulky molecules of STMP/STPP coagulated and interrupted the crosslinking reaction in the starch granules resulting in increased %E values of the films. 3.2.2. Water vapor permeability and film solubility The WVP and FS of the NRS and DMRS films with various concentrations of propylene oxide and different crosslinking reagents are shown in Table 5. The results show that the WVP of DMRS films were in the range 1.23–1.89 g mm/day m2 kPa which is lower than the NRS film. These results suggest that the DMRS films showed

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Table 5 Water vapor permeability and film solubility of native rice starch (NRS) and dualmodified rice starch (DMRS) films with 6–8% of propylene oxide and crosslinked with 2% of STMP and a mixture of 2% of STMP and 5% of STPP. Film

Propylene oxide concentration (%w/w)

WVP g mm/day m2 kPa

NRS HP6C1 HP8C1 HP10C1 HP12C1 HP6C2 HP8C2 HP10C2 HP12C2

– 6 8 10 12 6 8 10 12

2.98 1.23 1.26 1.32 1.29 1.89 1.77 1.82 1.62

± ± ± ± ± ± ± ± ±

0.14d 0.04a 0.03a 0.15a 0.11a 0.10c 0.16bc 0.13c 0.08b

Film solubility (%) 4.45 6.53 6.83 6.79 7.09 6.29 7.17 7.07 7.86

± ± ± ± ± ± ± ± ±

0.14a 0.33bc 0.31bc 0.26bc 0.19c 0.18b 0.47c 0.47c 0.53d

Mean values in the same column with different letters are significantly different (p < 0.05). NRS = native rice starch, HP6C1–HP12C1 and HP6C2–HP12C2 = dualmodified rice starches at various levels of propylene oxide (6–12%) and crosslinking reagents using 2% of STMP and a mixture of 2% of STMP and 5% of STPP for HPC1 and HPC2, respectively.

superior water barrier properties than did the NRS films However, it was observed that the propylene oxide content did not affect the WVP of DMRS films. The crosslinking of hydroxypropylated rice starch film brought about a decrease in the WVP value. This is because the crosslinking reinforces the structure of starch granules and limits water absorption by restricting the mobility of the starch chains in the amorphous region [37]. Thus, the crosslinking expresses to limit the interaction of starch with water and provides a structural integrity of starch-based film during exposure to pressure and moisture [61]. Furthermore, the crosslinking agents react with the OH groups present in starch and make ester linkages with the available hydroxyl groups. This helps to increase the mechanical properties of the films as well as to reduce their water absorption behavior [55]. Das et al. [55] reported that moisture absorption was considerably decreased in the a crosslinked starch/poly(vinyl alco˜ hol) blend films. In addition, Munoz et al. [62] reported that the WVP of glutenin-rich film values decreased by around 30% when formaldehyde, glutaraldehyde or glyoxal, were used as crosslinking reagents. Theoretically, the maximum WVP could only be as high as would be achieved in the NRS film. In comparing the effect of crosslinking reagent on DMRS films it was found that the WVP of HPC2 films was higher than that of the HPC1 films. This might due to the bulky and strongly anionic nature of monostarch phosphates (STPP) in the HPC2 which leads to cold water swelling when the DS increased [49]. The HPC2 had a higher amount of phosphorus content (Table 1) than with 2% STMP only. It is believed that the high level of phosphate groups in the HPC2 induced the ionic character and ready hydration in water Thus the HPC2 films showed higher WVP values than did the HPC1 [14]. The FS of the DMRS with different propylene oxide levels and crosslinking reagents were in the range 6.29–7.86% which was higher than the NRS films (4.45%), as shown in Table 5. This might due to the hydroxypropyl groups after hydroxypropylation induced the increase the hydrophilicity in the nature of the starch, thus giving higher MS values [42]. In addition, the crosslinking reagents, STMP and STPP, produced starch phosphates which disrupted the aggregation of the adjacent starch chains. This resulted from the negative charge in the phosphate groups apparently reducing the inter-chain associations and gave increased levels of hydrated molecules [14]. Thus, the solubility of the NRS and DMRS films most likely reflected the leaching of plasticizer, starch chains and a hydrophilic modification reagent from the film matrix after 24 h immersion in water. The results demonstrate that FS of DMRS film from HPC2 (2% STMP and 5% STPP) showed higher values than the HPC1 (2% STMP). This was because the amount of phosphate groups

Fig. 4. Transparency value of native rice starch (NRS) and dual-modified rice starch (DMRS) films with 6–8% of propylene oxide and crosslinked with 2% of STMP and a mixture of 2% of STMP and 5% of STPP.

in the HPC2 was higher than in the HPC1, which was noted in the higher DS values in the resulting starch (Table 1). 3.2.3. Film transparency The transparency may be affected by various factors including the film thickness [63] and it is important to consider this when it is used as packaging materials [64]. The influence of propylene oxide and crosslinking agents on film transparency is shown in Fig. 4. Technically, greater transparency values show lower transparency in the film. The NRS film showed a higher transparency value (they were more opaque) than the DMRS. It was observed that increasing the propylene oxide tended to decrease the transparency values (less opaque). This was because the bulky hydroxypropyl groups in the neighboring starch chains prevented inter-chain association [37] and this resulted in a high transmittance of the light. The results show that the HPC1 films demonstrated higher transparency values (they were more opaque) than the HPC2 films. These results might be because the bulk of monostarch phosphate in the HPC2 led to the higher transmittance of light than in the HPC1 films. 3.2.4. FTIR spectroscopy FTIR spectroscopy was used to confirm the formation of the dual-modified rice starch. The qualitative interaction of the rice starch and modification reagents is shown in Fig. 5. The broad band of the NRS and DMRS films demonstrated generally similar intensities in the FTIR frequencies. The characteristic absorption ranges of starch include absorption bands of O H stretching (3200–3900 cm−1 ), C H (2927 cm−1 ), O H bending of absorbed water (1641 cm−1 ), and C H stretching (1250–900 cm−1 ) [65]. The broad band of all films at 3257 cm−1 in the HPC1 and 3264 cm−1 in HPC2 corresponded to the O H stretching of water and glucose. The peaks at 2917 cm−1 of the HPC1 and 2917 cm−1 of HPC2 were from C H stretching, while the peaks at 1626 cm−1 of HPC1 and 1646 cm−1 of HPC2 corresponded to the O H bending of water. The peaks at 1414 cm−1 of HPC1 and 1368 cm−1 were from C H bending and the peaks at 1150 cm−1 of both HPC1 and HPC2 were from C O and C C stretching. In addition, the peaks at 1077 cm−1 of HPC1 and 1076 cm−1 of HPC2 were from C H bending [16,28]. These results show that the functional groups were newly formed by hydroxypropylation which have a C O C ether linkage (Fig. 7). The anhydroglucose unit of a NRS is linked by the glycosidic linkage and contains hemiacetal group within itself [16]. In addition, it was found that the peaks at 990 and 1024 cm−1 of HPC1 and the peaks at 994 and 1014 cm−1 of the HPC2 related to an ether group (hydroxypropylation) and an ester group (crosslinking) (C O). This is because both ether group and ester group exhibited

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499

Fig. 5. FTIR spectra of native (NRS) and dual-modified rice starches (DMRS) films with 6–8% of propylene oxide and crosslinked with 2% of STMP (A) and a mixture of 2% of STMP and 5% of STPP (B).

strong intensity absorptions near 1000–1300 cm−1 . Moreover, Smits et al. [66] also reported that the peak at 991 cm−1 was sensitive to the moisture. The addition propylene oxide introduced to higher the hydrophilicity in DMRS films so that the FS were higher than the NRS and increased with increasing the propylene oxide levels. In STMP found the peak at 2850.75 and 1415.37 cm−1 correspond to the C H stretch attached to O and O H bending band and the peak at 1012.79 and 1261.38 cm−1 that correspond to phosphate stretching (P O C) and vibration (P O) [67], respectively, in 3% STMP rice starch film. Moreover, in STMP/STPP, it was found that the peak at 1035.75 correspond to phosphate stretching (P O C). Aziz et al. [68] reported that the stretching vibrations of P O or P O bonds were in the range 1100–1200 cm−1 . This was used to confirm the peak at 1150 cm−1 and that all the DMRS films sampled were stronger in intensity than the NRS film.

3.2.5. X-ray scattering The X-ray diffractograms of rice starch powder, NRS, and DMRS films are shown in Fig. 6. As can be seen, the rice starch powder was a typical A-type and the dried regular rice starch films, after gelatinization, had different structures. However, all the films had a similar structure which showed one peak at 20.5◦ . The amount of crystallinity and amorphous was observed in the NRS and DMRS as shown in Table 6. These results show that the NRS film had the highest crystallinity while in the DMRS films it gradually decreased with an increase in the propylene oxide. This might be because the hydroxypropyl groups limit the ability of the amylose molecules to coil and align with one another [69,70]. In addition, the hydroxypropyl groups on the amylose chain may prevent amylose aggregation and interrupts formation after gelatinization. On the

Table 6 Amount of crystallinity and amorphous of native rice starch powder (NRS powder) NRS film and dual-modified rice starch films (DMRS films). Starch

Crystallinity (%)

Amorphous (%)

NRS powder NRS film HP6C1 HP8C1 HP10C1 HP12C1 HP6C2 HP8C2 HP10C2 HP12C2

29.92 16.83 16.46 15.35 14.92 14.27 15.46 14.62 14.13 13.89

70.08 83.17 83.54 84.65 85.08 85.73 84.54 85.38 85.87 86.11

NRS = native rice starch, HP6C1–HP12C1 and HP6C2–HP12C2 = dual-modified rice starches at various levels of propylene oxide (6–12%) and crosslinking reagents using 2% of STMP and a mixture of 2% of STMP and 5% of STPP for HPC1 and HPC2, respectively.

other hand, it seems the covalent bonds established by crosslinking, which can be thought of as adding to the hydrogen bonds in native starch, did not increase the re-association of amylose chains [37]. The results show that the crystallinity of the HPC1 films was higher than the HPC2 films (a mixture of phosphate salts). This might be because the bulky monostarch phosphate (STPP) in the HPC2 films prevents amylose aggregation and this decreases the crystallinity. Basically, chemical reactions generally occur randomly with the primary hydroxyls (C-6), the secondary hydroxyls (C-2, C-3, and C-4), the aldehydic reducing end groups, and the glycol groups (the cleavage of C-2 C-3 bonds). The properties of starches may be mainly affected by modifications at the C-2, C-3,

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(A) 15.29 ° 17.29 °

23.12 ° Rice starch powder

Intensity

20.5 °

NRS HP6C1 HP8C1 HP10C1 HP12C1 3

5

7

9

11

13

15

17

19

21

23

25

27

29

31

33

35

37

39

Position (2 )

(B) 15.29 ° 17.29 °

23.12 ° Rice starch powder

20.5 °

Intensity

NRS HP6C2 HP8C2 HP10C2 HP12C2 3

5

7

9

11

13

15

17

19

21

23

Position (2

25

27

29

31

33

35

37

39

)

Fig. 6. XRD scattering of native rice starch powder (NRS powder), native (NRS) and dual-modified rice starch (DMRS) films with 6–8% of propylene oxide and crosslinked with 2% of STMP (A) and a mixture of 2% of STMP and 5% of STPP (B).

Fig. 7. Mechanism of starch with propylene oxide, STMP and STPP which (A) propylene oxide; (B) STPP; (C) STMP.

T. Woggum et al. / International Journal of Biological Macromolecules 67 (2014) 490–502

501

Table 7 Tensile strength (TS), elongation at break (%E) and water vapor permeability (WVP) of native rice starch film (NRS), dual-modified rice starch (DMRS) films and various films. Films type

Tensile strength (MPa)

Elongation at break (%)

Water vapor permeability (g mm/m2 day kPa)

Reference

Native rice starch (Chiang rice) Dual-modified rice starch (DMRS) Photo crosslinked rice starch Cassava starch Corn starch Rice starch-chitosan Whey protein Soy protein Low density polyethylene High density polyethylene

6.18 6.45–8.17 5.5–8.0 9.0–17.0 3.8–4.3 27.5–38.1 3.3 3.22 7.6–17.3 17.3–34.6

68.7 96.98–126.11 60–90 9.0–28.0 4.0–10.0 8.1–13.0 4.4 22.53 500 300

2.98 1.62–1.83 4.12–5.5 – – 4.11–7.08 3.5 20.82 0.08 0.02

Current study Current study Detduangchan and Wittaya [60] Mali et al. [77] Mali et al. [77] Bourtoom and Chinan [28] Soazo et al. [78] Gonzalez et al. [79] Briston [80] Briston [80]

and C-6 positions because of the large number of hydroxyls at these positions (Fig. 7) [49,71]. Kavitha and Bemiller [72] reported that the mono substitution of potato starch occurred the most by substitution at the C-2 position. Xu and Seib [73] reported that 67–78% of the substitution took place at the C-2 position. While 15–29% and 2–17% took place at the C3 and C-6 positions, respectively. Moreover, Wootton and Haryadi [74] also reported that 94% of hydroxypropylation took place at the C2 position. An X-ray diffraction study was undertaken by Muhrbeck et al. [75] of phosphorylated potato starches with varying DS values, They found that phosphorylation (the reaction between starch and phosphate salts) of the starch at C-3 position had no effect on crystallinity, and only phosphorylation at the C-6 position interfered with starch crystallinity and gelatinization enthalpy. The C-3 carbons face toward the interior of the starch amylose helix or amylopectin double helix thus the C-3 phosphates are deprived of interaction possibilities with other large molecules. Muhrbeck et al. [75] also reported an almost linear relationship between increasing starch phosphorylation (monostarch phosphate) and the reduction of film crystallinity. These outcomes indicated that the higher DS values resembled an amorphous film.

4. Comparison of modified rice starch films with selected biopolymer and synthetic polymer films The mechanical properties (TS and %E) and WVP of the rice starch film, modified rice starch films and various films were compared (Table 7). The DMRS films had greater both mechanical properties and water vapor barrier than NRS films. This indicated that the DMRS films from this study were successful for improvement the properties of native rice starch film. Therefore the characteristics of the modified rice starch films were compared with those of other biopolymers and synthetic polymer films in order to evaluate their quality for further applications. By comparing with other starch films found that the tensile strength and elongation at break of DMRS films were better (films were more mechanically resistant) than corn starch film, whey protein film and soy protein film. However, the tensile strength of DMRS films were poor than cassava starch film and rice starch–chitosan film but they were higher elongation at break and water resistance. It was observed that the DMRS films had higher elongation at break and water resistance than photo crosslinked rice starch. This indicated that the dual-modification rice starch had efficiency than only crosslinking on the flexibility and water resistance of rice starch films. Furthermore, the DMRS films had similarly the minimum tensile strength but lower elongation at break and water resistance than some synthetic polymers (low density polyethylene and high density polyethylene) (Table 15). Resistance of hydrocolloid-based biodegradable films to water vapor transmission is limited due to the inherent hydrophilicity

of polymers. Transmission of water vapor through hydrocolloidbased biodegradable film is also facilitated by the presence of a hydrophilic plasticizer, which favors absorption of water molecules [76]. 5. Conclusion The properties of dual-modified rice starch (DMRS) studied were significantly different to those of the NRS and were dependent on the concentration of propylene oxide and crosslinking reagents. Increasing the propylene oxide concentration in the DMRS resulted in an increase in MS and DS. However, an inverse trend was shown in the gelatinization parameters, pasting temperature, paste consistency, breakdown, setback, gel strength and paste clarity. The TS, %E and FS in the films tended to increase when the propylene oxide increased, whereas the transparency value decreased. However the water vapor permeability of the films did not significantly change with an increase in the content of the propylene oxide. The DMRS films crosslinked with STMP provided higher TS, transparency value and lower WVP than did the DMRS crosslinked with STMP and STPP. The XRD analysis of the DMRS films showed a decrease in crystallinity when the propylene oxide content increased. Acknowledgments The author is grateful for the financial support provided from the Higher Education Research Promotion and National Research University Project of Thailand, Office of Higher Education Commission. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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