A simple method for improving the properties of the sago starch films prepared by using ultrasonication treatment

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Food Hydrocolloids

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A simple method for improving the properties of the sago starch fi lms prepared by using ultrasonication treatment

Hairul Abral

^a,∗

, Azmi Basri

^a

, Faris Muhammad

^a

, Yuzalmi Fernando

^a

, Fadli Ha fi zulhaq

^a

, Melbi Mahardika

^a

, Eni Sugiarti

^b

, S.M. Sapuan

^c

, R.A. Ilyas

^c

, Ilfa Stephane

^d

aDepartment of Mechanical Engineering, Andalas University, 25163, Padang, Sumatera Barat, Indonesia

bHigh-Temperature Coating Laboratory, Research Center for Physics, Indonesian Institute of Sciences (LIPI), Serpong, Indonesia

cDepartment of Mechanical and Manufacturing Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor, Malaysia

dDepartment of Information System, STMIK Indonesia Padang, 25136, Indonesia

A R T I C L E I N F O

Keywords:

Starchfilm Transparency Ultrasonication Ghost

A B S T R A C T

Starch granules containing amylopectin-rich fractions like sago starch may remain insoluble and undamaged decreasing properties of thefilm. The aim of this study is to characterize native sago starchfilms prepared using ultrasonication. An ultrasonication probe was used during gelatinization for 2.5, 5, and 10 min respectively.

Ultrasonication decreases the incomplete gelatinized granules resulting in afilm with a more compact structure, and lower moisture vapor permeability than non-treatedfilm. The longest duration resulted in afilm with the highest transparency, and the highest thermal resistance. The duration for 5 min increased tensile strength of the film by 227%, and its moisture absorption decreased by 29.83% compared to non-sonicatedfilm. After ultra- sonication for 10 min, melting temperature increased by 7% in comparison to non-sonicatedfilm. This work promotes a simple method to improve the tensile and physical properties of starch basedfilm.

1. Introduction

Development of starch-based bioplastics has been given consider- able attention as an environmentally friendly biodegradable alternative to hydrocarbon-based plastics (Abral et al., 2018). When starchfilms are used for food packaging it is required to have good transparency, sufficient strength and low moisture absorption so as be able to improve the shelf life of food. Solution casting is one of common method for preparing starch film, especially at the laboratory level. During this preparation, some defects resulting in inhomogeneous structures may occur. These inhomogeneous structure result from incompletely soluble starch granules often called ghosts (Abral et al., 2018). This can de- crease the transparency of starch film (Garcia-Hernandez, Vernon- Carter, & Alvarez-Ramirez, 2017). Ghost formation is a result of cross- linking of polysaccharide chains within swollen granules (Debet &

Gidley, 2007). Previous studies found that ultrasonication is effective in reducing insoluble and agglomerated starch granules (Abral, Putra, Asrofi, Park, & Kim, 2018; Lima & Andrade, 2010). This is because sound energy from ultrasonication produces acoustic cavitation: the formation, growth, and collapse of starch granules within the liquid matrix (Asrofiet al., 2018b;Mahardika, Abral, Kasim, Arief, & Asrofi, 2018). The violent collapse of the bubbles results in microjets and shock

waves which shatter the aggregations of the agglomerated granules suspended (Syafri, Kasim, Abral, & Asben, 2018).

Physical and mechanical properties of a maize starchfilm after ul- trasonication of the starch gel improve due to an increase in homo- geneity of the structural starchfilm resulting in increased transparency and tensile strength of thefilm along disappearance of the ghosts (Abral et al., 2018;Cheng, Chen, Liu, Ye, & Ke, 2010). In contrast, another previous study claimed that a high ghost phase fraction enhanced the tensile (elongation at break and tensile stress) properties of corn starch film (Garcia-Hernandez et al., 2017). These dissimilarities in results could be due to differences in the starch sources used for preparing starchfilms.

This present study used sago starch granules fromMetroxylon Sagu palm. This material is abundantly available in tropical country like Indonesia and has a very low cost compared with other common star- ches (Abral, Kasmianto, & Mastariyanto Perdana, 2012; Nouri &

Mohammadi Nafchi, 2014). Sago palm contains a large amount of starch in its trunk and its productivity was about 4 times that of paddy rice (Karim, Tie, Manan, & Zaidul, 2008). Sago starch has unique characteristics and some of its physicochemical properties similar to common starch like cassava and potato (Mohammadi Nafchi, Cheng, &

Karim, 2011). It is a potential source for bioplastics with high

https://doi.org/10.1016/j.foodhyd.2019.02.012

Received 14 August 2018; Received in revised form 24 January 2019; Accepted 6 February 2019

∗Corresponding author.

E-mail address:abral@ft.unand.ac.id(H. Abral).

Available online 10 February 2019

0268-005X/ © 2019 Elsevier Ltd. All rights reserved.

T

amylopectin content leading to better tensile properties (Ismail &

Zaaba, 2012). However, starch films from granules with high amylo- pectin may have incompletely soluble and agglomerated starch gran- ules decreasingfilm properties (Debet & Gidley, 2007).

Many studies have been reported on the properties of starch-based films made from various starches including high amylopectin corn, ji- cama, and wheat (Abral et al., 2018; Debet & Gidley, 2007; Garcia- Hernandez et al., 2017;Hafizulhaq, Abral, Kasim, Arief, & Affi, 2018).

However, as far as the authors are aware no work has been reported on sago starch film prepared using an ultrasonic probe. Therefore, the objective of the present study was to determine the properties of the sago starch prepared by a simple and environmentally friendly method using ultrasonication further. The main motivation was to obtain the optimum ultrasonic duration of the sago starch gel to maximize several properties of thefilm. Morphology of the fracture surface of the plas- ticized sagofilm was observed using afield-emission scanning electron microscopy (FESEM). X-ray diffraction (XRD) and Fourier transform infrared (FTIR) offilms were characterized. Other properties measured are viscosity, transparency, tensile strength and modulus, fracture strain, moisture absorption, water vapor permeability, and thermal resistance.

2. Materials and experiment

2.1. Material

Sago starch was obtained from local small-scale industry in Padang, Indonesia. For purification, the starch (1000 g) was mixed with distilled water (3000 mL). The solution was stirred manually for a 1 min. After 15 h, the liquid fraction containing purities was decanted offleaving purer sediment of starch. This was repeated three more times. The re- sulting wet pure starch was poured into a Teflon plate to be dried in a drying oven (Universal Oven Memmert UN-55) for 20 h at 50 °C. A small electrical blender was used to refine dried starch bundles for 5 min. Chemical analysis indicated that the purified sago starch con- tained 21% amylose, and 79% amylopectin indicating it was a high amylopectin content starch (Al-Hassan & Norziah, 2012).

2.2. Preparation of the bioplastic

The purified sago starch, distilled water, and glycerol were weighed for 15, 150, and 7.5 g respectively using a precision balance to the nearest 0.1 mg (Kenko). These materials were mixed and gelatinized using a hot plate magnetic stirrer (Daihan MSH-20D) at 500 rpm, 70 °C for 45 min. This gelatinization temperature of sago gel was determined by using a temperature probe integrated with the hot plate magnetic stirrer. When the solution started to gelatinize that was the temperature chosen for this process and subsequent ultrasonication. During gelati- nization, four gel samples were treated; one without and three with ultrasonication for 2.5, 5, and 10 min respectively using a 20 mm dia- meter SJIA 1200W ultrasonic probe at 600 W at 100% amplitude and 20 kHz. Ultrasonication times longer than 10 min were initially trailed but because the offilms from these proved too brittle for further testing so were not used in this study. Ultrasonic warming was limited so the gel temperature did not exceed 70 °C. Viscosity of four non- and soni- cated gel samples was measured using an NDJ-8S digital rotary visc- ometer at 50 °C with rotor #2 at 6 rpm about 10 s. The sample was cast in a petri dish (140 mm diameter). The petri dish was vibrated using an ultrasonic bath (PS-70AL, 420 W, 40 kHz, 100% amplitude) for 15 min to remove trapped air bubbles and ensure homogeneity of the gel. A film formed after drying in a drying oven (Universal Oven Memmert UN-55) at 50 °C for 21 h. All the tested samples were labelled with the letter S (starch film), and a number corresponding to the ultrasonic duration used. Thus the abbreviation S-2.5 represents starchfilm ul- trasonicated for 2.5 min.

2.3. Characterization 2.3.1. Opacity

The opacity of the film was measured using UV-vis spectro- photometer (Shimadzu UV 1800; Japan) between wavelengths of 400–800 nm according to ASTM D 1003-00 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics). Films of 0.34 mm thickness were cut into 9 × 35 mm rectangles. The opacity measurement was carried out in triplicate.

2.3.2. FESEM observation

The fracture surface of the sample after tensile testing was observed using FESEM. Thefilm samples were placed on the FESEM sample stub.

They were coated with carbon and gold to reduce the static charge. The surface morphology of the sample was observed using a model JFIB 4610 SEM from JEOL with 10 kV and 8 mA.

2.3.3. FTIR

FTIR spectra of the non- and sonicatedfilm (S-0, S-2.5, S-5, and S- 10) were recorded using an IR spectrometer (PerkinElmer Frontier equipment) to compare the functional groups and molecular bond structure of thefilm. Before the characterization, all samples were dried using a drying oven (Universal Oven Memmert UN-55 at 50 °C) until constant weight. The dried samples were cut (approximately 10 × 10 mm) with steel scissor, formed into a sheetfilm and scanned at a frequency range of 4000–600 cm⁻¹at 4 cm⁻¹resolution. Three dif- ferent locations for each sample per treatment were tested.

2.3.4. X-ray diffraction

PANalyticalXpert PRO at 25 °C, 40 kV and 30 mA was used to per- form X-ray diffraction testing. The samples were scanned from 2θ= 3°

to 40°. The crystallinity index (CI) percentage was measured using an Eq.(1)(Segal, Creely, Martin, & Conrad, 1958):

= −

CI I I

I x

(%) ( )

am 100

200

200 (1)

where I200is the intensity of the peak at 22.6° which is associated with the crystalline region of cellulose, and Iam is the intensity taken at 2θ= 18° in the diffraction pattern as characteristic of the amorphous regions in the cellulose.

2.3.5. Tensile properties

A testing machine (Com-Ten testing machine 95T) was used to characterize the tensile properties of films including tensile strength (TS), tensile modulus (TM) and fracture strain (FS). All samples were prepared with ASTMD882-12standard and conditioned for 48 h with 50 ± 5% relative humidity at 25 °C. Tensile tests were performed with tensile speed for 3 mm/min at room temperature and repeated five times for each sample.

2.3.6. Thermogravimetry analysis (TGA) and derivative (DTG), differential thermal analysis (DTA), and differential scanning calorimetry (DSC)

Thermal gravimetric analysis is a method to measure the thermal characteristics of materials such as the starch basedfilm (Abral et al., 2018). TGA, DTG and DTA of samples were performed by using a thermal analysis instrument DTG-60 from Shimadzu. The samples were Table 1

Viscosity of the starch gel before and after ultrasonication.

Sample Viscosity (MPa.s) Rotation speed (rpm)/T (^oC)

S-0 4999 6/50

S-2.5 125

S-5 164

S-10 465

H. Abral, et al. Food Hydrocolloids 93 (2019) 276–283

weighed between 4 and 5 mg and input into the instrument which was set up with a nitrogenflow rate of 50 ml/min and the heating rate of 20 °C/min. DSC was measured using TA Instrument (Model Q20) from room temperature up to 280 °C at 10 °C/min with nitrogenflow rate of 50 ml/min. The weight of each sample was 5–10 mg.

2.3.7. Moisture absorption and water vapor permeability

Moisture absorption (MA):Both non-sonicated and sonicatedfilms were dried in a drying oven (Universal Oven Memmert UN-55) until constant weight then stored in a closed chamber with 99% relative humidity (RH) for 9 h. The samples were taken out and weighed on a Fig. 1.FESEM fracture surface photographs of thefilm a) before and b) after ultrasonication. Crack (i) is observed in the boundary between homogeneous (ii) and inhomogeneous structures (iii).

Fig. 2.Opacity of the starchfilm after various treatments.

Table 2

Tensile properties, opacity, and crystallinity index (CI), T of O-H stretching in the peak at about 3300 cm⁻¹of thefilms before and after ultrasonication.

Sample TS (MPa)* TM (MPa)* FS (%)* CI (%) T (%)* Opacity (AUnm)*

S-0 0.59 ± 0.21^a 36.96 ± 5.91^a 8.07 ± 3.91^a,b 12.9 9.9 ± 0.64^a,b 385.9 ± 0.73^a

S-2.5 0.81 ± 0.27^a 60.46 ± 9.74^a 5.99 ± 2.49^a 15.6 9.7 ± 0.22^a 325.5 ± 0.72^b

S-5 1.93 ± 0.82^b 118.71 ± 50.55^b 13.36 ± 2.9^b 15.0 10.2 ± 0.61^a,b 283.3 ± 0.15^c

S-10 1.17 ± 0.17^a,b 80.97 ± 9.53^a,b 7.85 ± 2.05^a 15.4 13.0 ± 2.29^b 192.5 ± 0.06^d

*Values are mean ± SD. Different letters in each column explain significant difference at 5% level of probability among sago starchfilms.

Fig. 3.FTIR curves before and after ultrasound treatment.

precision balance to the nearest 0.1 mg (Kenko). Percentage of MA in the sample was calculated using Eq.(2)(Syafri, Kasim, Abral, & Asben, 2017):

= −

Moisture absorption w w

w x

(%) ( )

h o 100

o (2)

where:

whisfinal weight and woinitial weight of sample.

Water vapor permeability (WVP):Film with diameter of about 45 mm was stored in a desiccator at RH 50 ± 5% and 25 °C for seven days.

Thisfilm was sealed on top of a plastic bottle (45 mm diameter) con- taining distilled water of 30 mL (100% RH). Sealing the film uses Vaseline for preventing the leakage of moisture through seals. The bottle was weighed to get initial mass and stored in a closed desiccator (1000 g silica gel) at 25 °C. A Kenko precision balance to the nearest 0.1 mg was used to weigh the bottles for each 3 h during 24 h. Average WVP value was determined using the Eq.(3):

= dW x L dt x dp x A

WVP ( )

( ) (3)

where dW = weight loss of the plastic bottle (g), L =film thickness (m), A = the exposed area offilm sealing top of a plastic bottle (m²), dt = the time under the partial water vapor pressure gradient (dP = 2533 Pa). Three samples per treatment were tested.

2.3.8. Statistical analysis

IBM SPSS Statistics 25.0 (IBM Corporation, Chicago, USA) was used to conduct statistical analysis. The effect of non-sonicated and sonicated treatments on opacity, tensile properties, water vapor permeability, and moisture absorption of the films at the 5% significance level was identified using analysis of variance (ANOVA) andP-test. The mea- surements were replicated at least three times for eachfilm tested.

3. Results and discussions

3.1. Viscosity

Table 1displays viscosity of the starch gel before and after ultra- sonication. Two and a half minutes of ultrasonication reduces viscosity dramatically from 4999 to 125 MPa s. However longer ultrasonication increases viscosity that may be due to lower temperature of the gel. The drastic decrease in viscosity can be attributed to the high mobility of the

sonicated gel due to the kinetic energy of the jets of liquid from acoustic cavitation scissoring the sago starch long polymer chain into shorter lengths, thus increasing the fraction of free mobile starch molecules (Cheng et al., 2010). Ultrasonication also breaks cross-links between polymers, resulting in an increase in the number of mobile polymer molecules. It also destroys the granular starch agglomerations hin- dering mobility of the polymer chains in the gel which will result in lowering the viscosity. Similar decreases in viscosity of the starch gel have been observed in previous studies due to ultrasonic treatment (Cheng et al., 2010;Iida, Tuziuti, Yasui, Towata, & Kozuka, 2008).

3.2. FESEM morphology

Fig. 1shows the FESEM fracture surface of tensile samples before and after ultrasonication in cross-section. The non-sonicated film (Fig. 1a) displays a rougher surface than the sonicatedfilm (Fig. 1b,1c, 1d). The rough surface is attributed to the presence of partially gelati- nized starch that impedes crack propagation. Smoother and more homogeneous fracture surface are observed for longer ultrasonic duration. As shown in Fig. 1a, micro and nano cracks appear at boundaries between the complete and incomplete gelatinized sections (marked with i). These two sections had different molecular structures resulting in a weak bonding in the boundary. As a result of this weakness, initial crack may propagate along this boundary. Further- more, where clumps of starch agglomerations occur micro and nano- sized porosities will exist between the starch agglomerations. This may result in more spaces for water molecules to diffuse. Homogeneous and non-homogeneous gelatinized starch structures may experience slip- page at boundary surfaces due to poor interface bonding, thus reducing tensile properties of thefilm. Ultrasonication eliminates these defects resulting in better properties.

3.3. Opacity

Fig. 2displays the absorbance curve as function of wavelength for the plasticized film without and with ultrasonication for 2.5, 5 and 10 min. There is a significant increase of the transparency for longer duration (p≤0.05). The S-0 film had the highest opacity (385.9 ± 0.73 AUnm) as shown inTable 2. This opacity decreases to lowest value (192.5 ± 0.06 AUnm) after 10 min ultrasonication. This lower transparency of the S-0film is because some defects such as ag- glomerations of incompletely gelatinized starch hinder light transmis- sion (Li et al., 2015). When these are eliminated by ultrasonication, transmission increases. The more homogeneous and compact structures of the sonicated film are supported by FESEM results as shown in Fig. 1d. This result is in agreement with a last study (Fu, Wang, Li, Wei,

& Adhikari, 2011).

3.4. FTIR spectra

Fig. 3displays average FTIR curve of three different locations of each sago starchfilm for various ultrasonic treatment. Bands in range of 3500-3200, 2927, 1647 cm⁻¹ correspond to O-H stretching, C-H stretching, O-H of absorbed water, respectively (Al-Hassan & Norziah, 2017; Ismail & Zaaba, 2012). According to a previous study, FTIR spectra in the range of about 3500-3200 cm⁻¹can be used to measure the hydrophilicity of the biopolymer (Asrofi, Abral, Kasim, Pratoto, &

Mahardika, 2018; Matějková &Šimon, 2012). In this case, transmit- tance (T) values in the peak at about 3300 cm⁻¹ became higher for longer ultrasonic duration. For example, average T of O-H stretching for non-sonicatedfilm shifts from 9.9±0.64 to 13.0±2.29% after 10 min ultrasonication (Table 2). This increasing T value can be attributed to the higher hydrophobicity of thefilm after ultrasonication (Goyat, Ray,

& Ghosh, 2011). The increasing hydrophobicity is consistent with S-10 film having the highest moisture resistance as shown inFig. 6. Results of the FTIR study are in agreement with previous work which has Fig. 4.XRD patterns of the non-sonicated and sonicatedfilms.

H. Abral, et al. Food Hydrocolloids 93 (2019) 276–283

reported increasing hydrophobicity of biocomposites after ultrasonica- tion (Asrofi, Abral, Kurnia, Sapuan, & Kim, 2018).

3.5. X-ray diffraction

Fig. 4displays X-ray diffraction patterns for thefilm before and after ultrasonication. All samples show three main sharp peaks at about 18°, 19°, and 23°; a typical diffraction pattern of starch bioplastics as semi- crystalline materials (Abral et al., 2019, p. 1800224;Duy, Rashid, &

Ismail, 2012). After various ultrasonic treatments, the three main peaks change that can be related to polymer mobility and orientation of the polymer crystal lattice (Cheng et al., 2010; Kaith, Jindal, & Maiti, 2009).Table 2displays the crystallinity index (CI) of eachfilm. CI for S- 0 was lowest (13%), which indicated that S-0 had the largest number of amorphous structures. After 2.5 min ultrasonication, CI of S-2.5 in- creased slightly to 16%. However, further ultrasonication makes in- significant changes in CI of thefilm. The slight increase in CI may be due to the more homogeneous polymer structure after ultrasonication (Asrofiet al., 2018a). Disappearance of ghosts releasing amylopectin or amylose to the matrix modifies the diffraction pattern (Garcia-

Hernandez et al., 2017). This result is consistent with FESEM mor- phology (Fig. 1b) which showed sonicatedfilm was more compact and homogeneous. The fact that CI peaks after 2.5 min of sonication sug- gests this duration is sufficient to achieve a homogeneous polymer structure. The slight reduction in intensity and narrowing of the three main peaks in Fig. 4 indicates that longer sonication destroys the polymer structure (Abral, Lawrensius, Handayani, & Sugiarti, 2018).

3.6. Tensile properties

Average value of tensile strength (TS), tensile modulus (TM), and fracture strain (FS) were shown inTable 2. There was a slight increase in both TS and TM and decrease in FS with ultrasonication. For ex- ample, TS, TM, and FS increase to 227, 221, and 66% after 5 min ul- trasonication. These results are in contrast to the lastfindings (Garcia- Hernandez et al., 2017). The disappearance of defects, such as micro- and nano-sized cracks, ghosts (seeFig. 1a) and increased amylopectin in the sago starch matrix as amylopectin-rich ghosts were destroyed appeared to improve tensile properties (Cheng et al., 2010). However, film strength increases may also be a result of compaction of the Fig. 5.Curves of TG (a), DTG (b), DTA (c), and DSC (d) for all samplefilms. The inset offigure for glass transition (T_g) and melting temperature (T_m).

polymer structure resulting in a more effective resistance against ex- ternal load (Asrofiet al., 2018b). This compactness is a result of the restructuring of the polymers in the matrix tofill micro- and nano-sized porosities as more active short chains are created by the ultrasonic ki- netic energy (Abral et al., 2018). Similar results have also been reported in a previous study (Cheng et al., 2010).

3.7. Thermal properties

Fig. 5shows effect of ultrasonication duration on thermal properties

of thefilm. Thermal resistance of thefilm increases with a higher ul- trasonic duration.Fig. 5a shows the type of three steps of weight loss as temperature increases is observed for all films. The first step of the weight loss from 60 to 180 °C corresponds to evaporation of absorbed water (Ilyas, Sapuan, & Ishak, 2018). As temperature increases to 180–360 °C, a second large weight loss appeared due to the volatile decomposition of the starch film (Abral et al., 2018). As shown in Fig. 5b, the maximum decomposition rate and the temperature of the maximum decomposition for S-0 are −0.327 mg/°C, and 342.76 °C respectively, but−0.275 mg/°C, and 357.4 °C for S-10. This result is Fig. 6.MA (a) and WVP (b) of non-sonicated and sonicatedfilm. The mean difference is significant at p≤0.05. Different letters indicate significant difference among treatment.

H. Abral, et al. Food Hydrocolloids 93 (2019) 276–283

also in agreement with DTA curve (Fig. 5c) which displays higher ac- tivation energy for the decomposition of S-10 in comparison to S-0, as the second endothermic DTA peak shifts from−192.25μV at 317.89 °C for S-0 to −210.34μV at 333.80 °C for S-10. These results confirm thermal resistance of thefilm increased after ultrasonication. The more homogeneous structure and lower number of defects (Fig. 1d) result in a more compact structure which restricts the movement of the bio- plastic molecular chains which increases resistance to decomposition (Liu & Zhang, 2006). This result is also in agreement with a previous study (El-Shekeil, Sapuan, Jawaid, & Al-Shuja'a, 2014). In the tem- perature range 360–570 °C, a third weight loss represents a final de- composition to ash (Abral et al., 2018). DSC curve also confirms the enhancement of the thermal resistance as shown inFig. 5d. The glass transition (Tg) and melting temperature (Tm) of sample without ulra- sonication was lower than that with ultasonication. For example, Tmfor S-0 is 187.02 °C that is lower (7%) than Tmfor S-10 (200.06 °C).

3.8. Moisture absorption and water vapor permeability

Fig. 6a and6b shows average MA and WVP before and after ultra- sonication. Non-sonicated sago starchfilm displayed higher MA than sonicated, and required a longer time (greater than 6 h) to reach steady state than sonicatedfilm. The sample that reached steady state fastest was S-10, with MA of about 29.24% (a 27.39% decrease compared to S- 0). This is because the S-0film contains more nano and/or micro-sized porosities for water to diffuse into (Maneerung, Tokura, & Rujiravanit, 2008). Sonication reduced the viscosity of the sago starch gel resulting in the more active depolymerized chains filling the porosities. Both dimension and number of porosities decreased causing a decrease in diffusion of water molecules. The decrease in MA of thefilm is also confirmed by the FTIR curve which shows a weaker intensity of the hydrophilic functional group band at 3500-3200 cm⁻¹(corresponding to O-H stretching) and 1647 cm⁻¹(O-H of absorbed water). The en- hancement of the moisture resistance was also confirmed in WVP curve as shown inFig. 6b. WVP value of the sample after ultrasonication is lower than that before ultrasonication. This is because the greater compactness in the polymer structure of the sonicated film leads to greater resistance to the diffusion of the water molecules (Fu et al., 2011;Ilyas, Sapuan, Ishak, & Zainudin, 2018). Consequently, it results in lower WVP values.

4. Conclusions

Sago starch gel was sonicated for 0, 2.5, 5 or 10 min using an ul- trasonic probe sonicator, then the non- and sonicated gel were treated using an ultrasonic bath. Ultrasonication duration for 10 min at 600 W resulted in the best properties of sago starch film including highest thermal resistance, the lowest water vapor permeability, the highest transparency, and the lowest moisture absorption. The sonicatedfilm showed a more compact and homogeneous structure, and had higher thermal resistance, and tensile strength than the non-sonicated film.

Permeability to water vapor, opacity, and water absorption capacity of the films significantly decreased after ultrasonication. This present study provides further evidence that ultrasonication could become one simple, environmentally friendly process in the fabrication of sago starchfilm to improve its properties.

Acknowledgment

Acknowledgment is addressed to Directorate General of Higher Education for supporting research funding with project name Fundamental Research (grant number 06/UN.16.17/PP.PKLN.MM/

LPPM/2019).

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