Notes
Enhancing Functional Properties of Low Amylose Bengkoang (Pachyrhizus erosus) Starch Film by Ultrasonication
2020 Volume 26 Issue 1 Pages 159-166
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2020 Volume 26 Issue 1 Pages 159-166
Complete plasticization of starch granules plays an important role in maximizing the properties of a starch film. Ultrasonication treatment is one way to achieve this by breaking up insoluble starch grains. This study reports on the effects of ultrasonication on the characterization of bengkoang (Pachyrhizus erosus) starch films. Starch gel was sonicated using an ultrasonic probe at 66.9 W/cm2 for 4 min. SEM of the fracture surface of the sonicated film showed a more compact structure than for the non-sonicated or control film. The sonicated films displayed a lower opacity than the control film. Opacity of the sonicated film was 6.82% lower than control film. The sonicated film also had lower hydrophilicity; moisture absorption of film was decreased about 10.28% after sonication. Furthermore, ultrasonication increased the tensile strength and decreased the fracture strain of films.
Development of biodegradable films is a rapidly growing field of research. Of the natural polymers that can be used to fabricate biodegradable films, one of the most promising candidates is starch because of its abundance, low price, thermoplastic behavior and good film formability (Wilhelm et al., 2003). Starches studied to date include those from corn (Bertuzzi et al., 2007; Biliaderis et al., 1980; Garcia-Hernandez et al., 2017; Isotton et al., 2015; Jambrak et al., 2010), rice (Lii et al., 1996; Zuo et al., 2009), oat (Galdeano et al., 2009), cassava, taro, sweet potato, yam, ginger (Moorthy, 2002; Sukhija et al., 2016).
Starch film generally has poor mechanical properties and low thermal and moisture resistance (Shah et al., 2016). Previous studies have shown that the higher the level of incomplete plasticized starch granules (often called ghosts), the lower the properties of the starch film (Cheng et al., 2010; Garcia-Hernandez et al., 2017; Iida et al., 2008). Cheng et al. (2010) have found high fractions of starch ghosts decreased tensile strength and solubility of the starch film. They used ultrasonication treatment to destroy the ghost, thus the increasing the tensile properties of the film. Similar results have reported the ultrasonic treatment during film preparation improved the properties of the starch film (Jambrak et al., 2010; Luo et al., 2008; Manchun et al., 2012). However, they did not report the FTIR and mois ture absorption characterization of starch film prepared with high power ultrasonication. Furthermore, amylose content of the starch plays important role to the functional properties of starch films. Lourdin et al. (1995) reported the increasing of tensile properties of starch film as the increasing of amylose content. Thus, special treatment was needed to enhance the properties of the film with low amylose starch.
Relatively little information is available regarding the effect of ultrasonic on the characterization of the starch film. The objective of this present study is to enhance the functional properties of starch film from low amylose starch by ultrasonication with power density. Bengkoang (Pachyrhizus erosus) tuber starch was used as a starch-based film material since it is an agricultural product that is easy to find in tropical and subtropical regions including Indonesia, especially in Padang. Furthermore, reports about the properties of bengkoang starch film are still limited. In our previous work, we have used bengkoang starch as matrix of biocomposite (Hafizulhaq et al., 2018). Gelatinized suspensions were sonicated with 47.78 W/cm2 to complete gelatinization of insoluble starch granules. Bengkoang starch and cellulose fiber showed good compatibility in biocomposite structure. The addition of cellulose decreased moisture absorption and increased opacity of films. However, the ultrasonication effect was not discussed in the study.
The uniqueness of this study is to sonicate the starch gel from low amylose content starch using high ultrasonic power densities. Viscosity, opacity, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), moisture absorption, thermogravimetric analysis (TGA), derivative thermogravimetric (DTG), tensile strength (TS) and fracture strain (FS) are measured for control and sonicated film.
Materials Fresh locally grown bengkoang tuber was sourced from a farm at Padang, West Sumatera, Indonesia and the starch (8% amylose content) extracted. Glycerol purchased from Brataco (Jakarta, Indonesia) was used as a plasticizer. All experiments were conducted with distilled water.
Extraction of bengkoang starch Peeled tubers were pressed using a slow speed juicer (SKG-J-1001 Slow Juicer, SKG, Jakarta, Indonesia) at 45 rpm and a suspension of bengkoang juice and starch was obtained. The suspension was left stationary for 12 h at room temperature until the starch formed a sediment at the bottom of the container then the liquid separated from this sediment. The wet starch sediment was dried using a ventilated oven for 20 h at 50 °C. Dried starch was ground with the grinder attachment of a blender for 5 min to break down agglomerations of starch granules.
Film preparation About 10 g starch, 4 g glycerol, and 100 g of distilled water were mixed in a 250 mL beaker (diameter 70 mm, IWAKI). The solution was heated using a hot plate with a magnetic stirrer (MSH-20D, Daihan Scientific Co., Ltd, Seoul, Korea) at 65 °C and 500 rpm for 45 min. The starch gel was then sonicated for 4 min using a 20 kHz ultrasonic cell crusher (Model SJIA-1200 W, Ningbo Yinzhou Sjia Lab Equipment Co., Ltd., Ningbo, China) at 66.9 W/cm2 (840 W power), respectively. This power was chosen due to good relation between high-power ultrasonication and viscosity of starch paste (Kang et al., 2016). After that, the suspensions were then poured into petri dishes (15 cm diameter) and dried in a ventilated oven for 22 h at 50 °C. The resulting films were labeled control (for non-sonicated film) and sonicated (for ultrasonic treated film). Other suspensions were prepared for viscosity measurement. The viscosity of the starch gel for each suspension was measured using an NDJ-8S Digital Rotary Viscometer (Graigar, Guangdong, China) at 50 °C ± 1 °C with rotor #2 and rotor speed of 6 rpm.
Surface morphology Morphology of bengkoang starch films fracture surface was analyzed using a Scanning Electron Microscope (HITACHI SU-3500, Hitachi High-Technologies Corporation, Tokyo, Japan) at acceleration voltage 5 kV in a vacuum of 5 × 10−4 Pa. All films were coated with gold (Au).
Film opacity The opacity of the films was measured with a UV-VIS spectrophotometer (Shimadzu UV 1800, Shimadzu Corporation, Kyoto, Japan) and scanned between 400 and 800 nm. About 1 × 2.5 cm rectangles were cut from films and fixed onto the inner side of a 1 cm spectrophotometer cell and the absorbance spectrum recorded. The opacity of film was determined as the area under the absorbance spectrum between 400 and 800 nm according to ASTM D 1003-00 (Standard test method for haze and luminous transmittance of transparent plastics). The opacity determinations were repeated three times.
X-ray diffraction X-ray diffractograms were recorded using a PANalytical Xpert Pro diffractometer with Cu Kα radiation at 40 kV and 30 mA. All film samples were scanned between 2θ = 5–40°. Crystallinity index of starch films was calculated by the ratio between the area under the crystalline and the amorphous region (González et al., 2015).
Fourier transform infrared spectroscopy FTIR spectra of films were recorded with an FTIR spectrometer (Frontier, PerkinElmer, Waltham, USA) within the wavenumber range of 4 000–600 cm−1 under resolution 4 cm−1.
Moisture absorption Moisture absorption of all films was determined by the method described by (Abral et al., 2018a) with the following modifications. Films were cut to 2 × 1 cm rectangles and dried in a ventilated oven at 50 °C for 24 h. Dried pieces were weighed to determine initial mass then placed in a covered box containing a saturated NaCl solution (RH 75%) at 25 °C ± 2 °C. Every 30 min all pieces were weighed.
Thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG) The thermal degradation process was investigated using DTG-60 DTA-TG Apparatus (Corporation, Kyoto, Japan) using 1 mg samples under a nitrogen flow of 50 mL/min and heating rate of 20 °C/min from room temperature until 600 °C.
Tensile strength and elongation test Tensile strength (TS) and fracture strain (FS) were measured using a Com-Ten testing machine 95T Series with a load cell of 25 kg and tensile speed of 7.6 mm/min. Films were cut into rectangular strips (10 cm × 1.5 cm) according to ASTM D882-00 method. Films specimens were conditioned in a desiccator with 50 ± 5% (RH) for 48 h prior to testing. The testing was repeated three times for each sample.
Statistical analysis IBM SPSS Statistics 25 (IBM Corporation, Chicago, USA) was used to analyze the data. One-way variance analysis was carried out followed by Tukey's test in order to determine the effect of the different ultrasonication power densities with confidence level as p ≤ 0.05.
Viscosity of the starch gel Viscosity is one of important characteristic in the development of starch film. The viscosity of starch gel can affect the stability, thickness, and surface of resulting film. According to the Iida et al. (2008), we chose to use 10% starch (w/w of distilled water) to provide high viscosity starch gel. The result of the viscosity measurement of starch gels before and after ultrasonication was shown in Table 1. Suspension of sonicated decreased viscosity by 93% compared to the control film (3 270 mPa·s to 285 mPa·s). This indicates that the ultrasonication produced a number of shorter macromolecular chains so improving the mobility of the chains significantly (Kang et al., 2016; Shah et al., 2016). The decreasing in viscosity also occurred because the ghost particles or partially soluble starch granules were removed from the starch gel. The collapsing bubbles resulting from ultrasonication create microjet and shockwave on the ghost particles. Along with the increasing temperature by cavitation energy, the ghost particles were solubilized and the viscosity was increased. A similar result was also reported in maize starch (Cheng et al., 2010; Jambrak et al., 2010), potato, tapioca and sweet potato starch (Iida et al., 2008).
| Sample | Viscosity (mPa·s) | Opacity (AUnm) | CI (%) | TS (MPa) | FS (%) |
|---|---|---|---|---|---|
| Control | 3 270 | 374.77 ± 0.236a | 43.59 | 1.43 ± 0.20a | 9.03 ± 1.92a |
| Sonicated | 215 | 349.23 ± 0.48b | 41.67 | 1.82 ± 0.08b | 6.28 ± 0.49b |
Values with the same superscript letters within a column are not significantly different (p < 0.05)
Scanning electron microscopy Fig. 1 shows SEM photographs of the fractured surface of the control and sonicated films. The surface of the control film (Fig. 1a) was rougher than the sonicated film surfaces (Fig. 1b). It also displays more cracks and insoluble starch particles compared to sonicated film (Fig. 1b). After ultrasonication, the structure was more compact; microcracks and insoluble starch grains (symbolized with yellow arrow) decreased. Sonicated film had smoother fractured surface than control film. It can be occurred due to the removal of ghost particles. Those particles interacted with starch gel with weak interfacial bonding that induced the microcracks in the microstructure of film. The microcracks were the weak point that can affect the functional properties of resulting film. This result is in good agreement with previous studies (Abral et al., 2018b; Asrofi et al., 2018a; Cheng et al., 2010; Marcuzzo et al., 2010).
SEM images of starch films, (a) control, and (b) sonicated.
Film opacity The opacity of the film was measured as the area under the absorbance spectrum curve (Table 1). Ultrasonic treatment decreased the opacity of the film about 6.82% from control (374.77 AUnm) to sonicated (349.23 AUnm). This more transparent sonicated film is a result of more transmitted light passing through the sonicated film. This is supported with SEM fracture surface (Fig. 1) showing the sonicated film is homogeneous, lacking the agglomerated insoluble starch grains that scatter light, thus reducing the transparency of the control film. This is also probably because of the alignment of the crystalline lamellae in the film. Lamellar fractions consisting of aligned macromolecular chains and the size and orientation of these influence the amount of light scattered. The decrease in opacity is in good agreement with previous studies (Cheng et al., 2010; Garcia-Hernandez et al., 2017; Kang et al., 2016).
X-ray diffraction Fig. 2a shows the X-ray diffraction pattern of control and sonicated films. Three main peaks were observed at 2θ= 17°, 19°, and 22°. As shown in the figure, ultrasonication did not change the crystallites type of films, a similar phenomenon also reported by a previous study (Cheng et al., 2010). All films show a B-type X-ray pattern. As reported by Sit et al. (2014), starch with B-type crystallinity exhibited good paste clarity. Meanwhile, the ultrasonic treatment results in decreasing in the peak intensity. The height of the peak intensity corresponds to the crystallinity index (CI) of the film (Asrofi et al., 2018b; Syafri et al., 2017). CI value for control and sonicated film can be seen at Table 1. CI of control (43.59%) was 4.61% higher than sonicated film (41.67%). The decreasing of CI is probably due to the disruption of the crystalline lamellae resulting from ultrasonication. A similar phenomenon also reported by a previous study (Li et al., 2018). The CI did not change significantly, it is may be due to the tendency of amylose to form a film with stable crystallinity (Myllärinen et al., 2002).
The properties of control and sonicated starch films, (a) XRD patterns and (b) FTIR spectra.
Fourier transform infrared FTIR spectra of control and sonicated films can be seen in Fig. 2b. All curves show similar patterns on the appearance of new functional groups. The strong peak at around wavenumber 3 290 cm−1 corresponds to hydrogen-bonded hydroxyl groups (-OH stretching). Other bands were visible at 2 930 cm−1 (-CH stretching), 1 645 cm−1 (H-O-H bending) and 1 353 cm−1 (CH2 bending). As shown in Fig. 2b, ultrasonication did not change the functional groups of films but changed the position of bands slightly. For example, the band of hydroxyl groups, the control appeared wavenumber at approximately 3 297 cm−1 and slightly shifted to 3 300 cm−1 for sonicated. The shifting of peak position in FTIR spectra indicated the change of hydrogen bonding (Prachayawarakorn et al., 2013). According to the previous report, physical treatment can induce substantial changes in the chemical and physical nature of starch due to the changes of intra- and intermolecular hydrogen bonding (Freitas et al., 2004). Similar result also reported by previous studies (Abral et al., 2019; Hafizulhaq et al., 2018).
Moisture absorption Fig. 3 shows the moisture absorption of the control and sonicated films. Ultrasonication decreased the hydrophilicity of the film. Moisture absorption of sonicated for 360 mins in a closed humid chamber was 26.26%; it was 10.28% lower than that of control (29.27%). The lower moisture absorption of the sonicated film was a result of the decreasing the insoluble starch granules and micro cracks in the microstructure of films. The use of ultrasonication produced a more compact structure of films, it made water molecules difficult to diffuse into films and thus increased moisture resistance. This result was in a good agreement with previous studies (Cheng et al., 2010; Garcia-Hernandez et al., 2017). Microstructural changes of films can be seen in Fig. 1.
Moisture absorption of control and sonicated starch films.
TGA and DTG Fig. 4 shows the thermal stability of the control and sonicated films. Both films in Fig. 4a show weight loss in three steps. A slight weight loss in the range about 60–150 °C corresponds to the evaporation of absorbed water (Asrofi et al., 2018a; Ma and Yu, 2004). A second larger weight loss at 150–360 °C was attributed to the decomposition of the starch film (Colussi et al., 2017). In the temperature range 360–570 °C a third weight loss was observed due to a final decomposition to ash. As can be seen at the figure, the sonicated film had lower weight loss in all steps compared to the control film. Fig. 4b displays the DTG curves of the films. The maximum decomposition rate for control was −1.05%/°C at 330.36 °Chigher than that for the sonicated film (−0.96%/°C at 328.9 °C). This result indicated that ultrasonication improved the thermal stability of the starch film. The more compact polymer structure after ultrasonication increased restriction in the movement of the macromolecular chains, thus improving the thermal stability (Garcia-Hernandez et al., 2017). This is in agreement with the SEM fracture surface in Fig. 1 showing the more homogeneous structure of the sonicated film than the control film. This result is in line with other studies that found ultrasonication increased the thermal properties of starch-based biocomposites (Abral et al., 2018b).
Thermal properties of control and sonicated starch films, (a) TGA and (b) DTG.
Tensile strength and fracture strain Tensile strength (TS) and fracture strain (FS) of films from control and sonicated starch films. TS for sonicated film was increased compared to the control film. TS of sonicated was 1.82 MPa; it was 27.27% higher than that of control. The increase in TS was because of the removal of micro cracks and insoluble starch grains that allow the initial crack when the load was applied. The compact structure of films as the result of ultrasonication treatment improved the tensile strength. It is in a good agreement with previous studies (Abral et al., 2018b; Cheng et al., 2010). Meanwhile, FS of sonicated (6.28%) was decreased by 30.45% than that of control (9.03%). The decrease of FS indicated the ultrasonicated film was more brittle than control film.
This study investigated the influence of ultrasonication on the properties of bengkoang starch films. Ultrasonication resulted in a decrease in the viscosity of the starch gel and improved the moisture resistance of the film significantly. The use of ultrasonication also increased the thermal properties by decreasing the rate of maximum decomposition of film. Furthermore, the tensile strength was increased and the fracture strain was decreased. The improvement in the functional properties of the sonicated film was mainly due to the increase in the compactness of the film. The results were indicated that ultrasonication was a good physical method to enhance the functional properties of low amylose starch film.
Acknowledgments This research was funded by Directorate General of Higher Education, Ministry of Research, Technology, and Higher Education, Indonesia for supporting research funding with project name “Skim PMDSU”, number 13/H.16/PMDSU/LPPM/2016.
