Effects of ultrasonic treatment on amylose-lipid complex formation and properties of sweet potato starch-based films

Accepted Manuscript Effects of ultrasonic treatment on amylose-lipid complex formation and properties of sweet potato starch-based films Pengfei Liu, Rui Wang, Xuemin Kang, Bo Cui, Bin Yu PII: DOI: Reference:

S1350-4177(18)30249-9 https://doi.org/10.1016/j.ultsonch.2018.02.029 ULTSON 4090

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

19 December 2017 6 February 2018 15 February 2018

Please cite this article as: P. Liu, R. Wang, X. Kang, B. Cui, B. Yu, Effects of ultrasonic treatment on amylose-lipid complex formation and properties of sweet potato starch-based films, Ultrasonics Sonochemistry (2018), doi: https:// doi.org/10.1016/j.ultsonch.2018.02.029

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Effects of ultrasonic treatment on amylose-lipid complex formation and properties of sweet potato starch-based films Pengfei Liu, Rui Wang , Xuemin Kang, Bo Cui , Bin Yu School of Food Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Ji’nan, Shandong 250353, China Abstract To investigate the effect of ultrasonic treatment on the properties of sweet potato starch and sweet potato starch-based films, the complexing index, thermograms and diffractograms of the sweet potato starch-lauric acid composite were tested, and light transmission, microstructure, and mechanical and moisture barrier properties of the films were measured. The results indicated that the low power density ultrasound was beneficial to the formation of an inclusion complex. In thermograms, the gelatinization enthalpies of the ultrasonically treated starches were lower than those of the untreated sample. With the ultrasonic amplitude increased from 40% to 70%, the melting enthalpy (△H) of the inclusion complex gradually decreased. X-ray diffraction revealed that the diffraction intensity of the untreated samples was weaker than that of the ultrasonically treated samples. When the ultrasonic amplitude was above 40%, the diffraction intensity and relative crystallinity of inclusion complex gradually decreased. The scanning electronic microscope showed that the surface of the composite films became smooth after being treated by

Abbreviations: SPS, sweet potato starch; LA, lauric acid; DSC, differential scanning calorimetry; SEM, scanning electron microscopy; AFM, atomic force microscopy; E, elongation at break ; TS, tensile strength; Ra, average roughness; Rmax, maximum roughness height.  

Corresponding author at: Qilu University of Technology (Shandong Academy of Sciences), Daxue Road, Changqing District, Ji’nan City, Shandong Province 250353, China. Tel.: +86 13964170512; fax: +86 531 89631195. E-mail address: [email protected] (P.F. Liu). 1

ultrasonication. Ultrasonication led to a reduction in film surface roughness under atomic force microscopy analysis. The films with ultrasonic treatment exhibited higher light transmission, lower elongation at break, higher tensile strength and better moisture barrier property than those without ultrasonic treatment. Keywords: sweet potato starch / ultrasonic amplitude / amylose-lipid complex / low power density ultrasound

1. Introduction Native starch, which is one of the most consumed polysaccharides in the human diet, is composed of two main polymers, i.e., amylose and amylopectin [1]. Amylose, the starch fraction comprised of relatively linear molecules, can interact with lipids and form an inclusion complex under suitable conditions [2]; i.e., similar to cyclodextrin, the amylose molecules have a hydrophobic cavity, and hydrophobic guest compounds can lie within the helical cavity, which is stabilized by hydrophobic interaction [3]. A considerable amount of researches has been devoted to the study of amylose-lipid complexes [3,4]. Previous studies reported that the amylose-lipid complex formation depended on the starches involved and the lipid types, amounts and chain lengths of lipids and amylose, degree of unsaturation of the lipids and thermal treatment conditions, etc [4-11]. These conditions applied in the process are very important factors that affect the amylose-lipid complex formation. The effects of these factors on the inclusion complex formation have been widely studied, whereas research on ultrasonic treatment is relatively scarce.

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Ultrasound technique is regarded as one of the most promising applications in the food industry for modifying foods [12]. As early as 1933, ultrasonic technology was used in starch processing [13]. Previous studies suggested that ultrasonication preferentially degraded amorphous regions of starch granules and invaded amylose molecules more easily than amylopectin molecules [14]. Recent papers have investigated the effect of ultrasonic technology on the behaviour of gelatinized starch suspensions [15,16]. These papers about ultrasonic treatment of starch suspensions were concentrated primarily on the differences in the swelling of the starch granule. Despite the widespread use of ultrasonic treatment to modify starch, the ultrasonication of starch with fatty acid to form an inclusion complex is a new field to explore. No systematic study has been reported about the effects of ultrasonic treatment on the inclusion complex formation and on the properties of starch-based films. Furthermore, there are some questions that remain unclear concerning the effects of ultrasonic treatment on the properties of starch and starch-based films. For instance, does the ultrasonic treatment promote or disrupt the formation of the inclusion complex? Do different ultrasonic amplitudes affect the mechanical and moisture barrier properties and other properties of the starch-based films? Sweet potato is widely cultivated in China, which has the highest yield (approximately 73 million tons in 2016) in the world. Because of inefficient processing and storage difficulties, a large number of (up to 15%) of sweet potatoes are wasted every year [17]. The preparation of sweet potato starch (SPS)-based film is a good way to reduce the waste. Furthermore, the biodegradable SPS-based films can

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alleviate the problem of environmental pollution caused by plastic food packaging. To form a homogeneous, continuous and compact polymer film, the starch granules should be dissolved in water or other solvents [18]. However, the starch is difficult to dissolve in water because of the strong intermolecular and intramolecular hydrogen bonding force. Previous studies reported that after being treated by ultrasonication, the starch molecules showed an increase in solubility [19]. Jenny et al. [20] suggested that the ultrasonication treatment favoured the formation of a homogeneous starch suspension. Compared with conventional dissolution methods for starch molecules, ultrasonication is relatively effective and inexpensive [21]. The aim of this paper was to further investigate the effect of ultrasonic treatment with different amplitudes on the structural and physicochemical characteristics of SPS-lauric acid (LA) composites and SPS-based films. 2. Materials and Methods 2.1. Materials The SPS was provided by Rushan Huamei Starch Products Co., Ltd. (Weihai, China). The LA (≥ 97.5% purity) was purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). The glycerol (≥ 99% purity) and ethanol (≥ 99.7% purity) were obtained from Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). 2.2. Ultrasonic treatment The LA (2% of starch), was first dissolved in absolute ethanol, then added to the SPS suspension (6 g of SPS, 60 ml of distilled water). The starch suspension was stirred at 60 ºC and 90 ºC for 60 min by continuous mixing (300 rpm). Then, the

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SPS-LA suspension was treated by ultrasound for 5 min, using a 20 kHz ultrasonic processor (Model VCX800, Sonics & Materials, Inc., Newtown, USA) with a tapered horn tip (13-mm end diameter). The ultrasonic amplitudes were set to 30% (power density was 240 W/cm2), 40% (320 W/cm2), 50% (400 W/cm2) and 70% (560 W/cm2), respectively and was pulsed on and off for 4 s each. The SPS-LA suspensions in the beaker were held in a cold-water bath to prevent the temperature from increasing. Then, the suspensions were cooled to room temperature and centrifuged using the TDZ5-WS Centrifuge (Xiangyi Laboratory Instrument Development Co., Ltd., Hunan, China) at 3000 rpm for 15 min, washed three times with a 50% ethanol/water mixture, and centrifuged at 3000 rpm for 15 min. Finally, the precipitates were dried in an oven at 50 ºC and were ground for subsequent tests. 2.3. Complexing index The complexing index (CI) value determines the degree of SPS-LA inclusion complex formation [22,23]. The composites (2 g) were weighed and dispersed into distilled water (20 ml). The mixture was heated at 95 ºC for 30 min with continuous mixing. The obtained SPS-LA composite pastes were used for analysing complexing index values. The SPS-LA pastes (5 g) were weighed in a centrifuge tube (50 ml) and 25 ml of distilled water was added. The centrifuge tube was vortexed with a XW-80A Vortex Mixer (Qilin Medical Instrument Factory, Jiangsu, China) for 2 min. Then, the samples were centrifuged with the TG1650-WS Centrifuge (Luxiangyi Centrifuge Instrument Co., Ltd., Shanghai, China) at 3000 rpm for 15 min. The supernatant (500 μl) obtained was mixed with distilled water (15 ml) and 2 ml of an iodine solution

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(2.0 g of KI, 1.3 g of I2 and 100 ml of distilled water) in a capped tube. The test tube was inverted approximately 10 times. The absorbance of the solutions was measured with the TU-1810 UV-Vis spectrophotometer (Beijing Pgeneral Instrument Co., Ltd., Beijing, China) at 690 nm (As). The absorbance value of the starch paste (without LA) was used as a reference (A0). The CI value was obtained from the equation: CI (%) = (A0 － As) × 100 / A0

(1)

2.4. Differential Scanning Calorimetry (DSC) The thermal properties of SPS-LA composites were measured with a DSC 214 (Netzsch Scientific Instruments, Selb, Germany). The composite (5 mg) and distilled water (15 μl) were directly weighed in aluminium pans and hermetically sealed. The samples in sealed pans were equilibrated at room temperature for 24 h before analysis. The samples were scanned from 20 to 140 ºC within 12 min. All the samples were run in triplicate. In the process of testing, an empty aluminium pan was used for comparison [24]. 2.5. X-ray Diffraction The diffractograms of the composites were carried out with a D8 Advance X-ray Diffractometer (Bruker-AXS, Karlsruhe, Germany). The parameters were performed in reflection mode at 40 kV, 40 mA with graphite-filtered Cu Kα radiation and a θ compensating slit. Measurement was taken between 5 and 40° (2θ) with a step size of 0.03° [4]. The relative crystallinity values of the SPS and SPS-LA composites were obtained from the equation: Relative crystallinity (%) = 100 Sc / (Sc + Sa)

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(2)

where Sc is the crystalline area on the diffractograms, and Sa is the amorphous area. 2.6. Film casting The composites (6 g SPS, 120 mg LA) were mixed with 200 ml of distilled water, and then glycerol (2 g) was added. The mixture was heated in a water bath at 90 ºC by continuously stirring (350 rpm) for 90 min. The solutions obtained were degassed in a vacuum desiccator. The film-forming suspension was poured on Teflon-coated glass plates (120 mm × 240 mm). The SPS-LA composite films were obtained by evaporating solvent in an oven at 40 ºC and peeling from the Teflon-coated glass plates. All of the tested films were conditioned for at least 48 h in the temperature humidity chamber before analysis. The temperature and relative humidity were 23±2 ºC and 53%, respectively [25]. 2.7. Scanning Electron Microscopy (SEM) Micro-structural analysis of the SPS-LA composite films was performed using a scanning electron microscope (ZEISS-Supra 55, Jena, Germany) with a magnification of 500. All of the SPS-LA composite films were bonded to a conductive carbon tape, coated with Au/Pd, and determined by using an accelerating voltage of 5 kV [24]. 2.8. Atomic Force Microscopy (AFM) The morphological surface of the SPS-LA composite films was determined using an atomic force microscope (Bruker Multimode8, Madison, USA) according to the instruction manual. The analysis was performed in a ScanAsyst in Air mode with a scanning speed of 0.977 Hz. The scan size was 10 μm × 10 μm. The morphological

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images and surface roughness were obtained using NanoScope Analysis software directly provided in the equipment. 2.9. Light transmission of the films The light transmission values of the SPS-LA composite films were measured at selected wavelengths (range from 400 to 800 nm), using a TU-1810 UV-Vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). The samples were cut into 5 cm × 1 cm strips prior to testing. The values of the samples were averaged from three replicates [25]. 2.10. Mechanical properties The mechanical properties of the SPS-LA composite films were measured with an XLW (PC) auto tensile tester (Labthink Instruments Co., Ltd., Jinan, China). The films were cut into rectangles (100 mm × 15 mm). The initial separation of the claws was 50 mm, and the test speed was 100 mm/min. The elongation at break (E, %) and tensile strength (TS, MPa) of the films were averaged from three replicates according to the following equations [26]: E = L/L0×100%

(3)

where L is the stretched length of the film (mm), and L0 is the original length (mm). TS = F/S×10-6 MPa

(4)

where F is the maximum load (N), and S is the sectional area of the sample (film width × film thickness) (m2) 2.11. Water vapor permeability The moisture barrier property of the films was measured by a Perme W3/030

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Water Vapor Transmittance Tester (Labthink Instruments Co., Ltd., Jinan, China). The sample was cut into a round shape, and the testing area was 33.00 cm2. The preheating time and test judgement were 4 h and 10%, respectively. The test temperature and relative humidity were 38 ºC and 90%, respectively. The thickness of the SPS-LA composite films was determined with a MDC-25PX Digimatic Micrometer (Mitutoyo Corporation, Kawasaki, Japan). Water vapor permeability of test samples was averaged from three replicates [25]. 2.12. Statistical analysis The statistical analyses of the results were performed by analysis of variance (ANOVA) and Duncan’s multiple range test at the 5% significance level, using SPSS 17.0. 3. Results and Discussion 3.1. Effects of ultrasonic treatment on the complexing index Changes in the complexing index values of SPS-LA composite before and after ultrasonic treatment with different amplitudes are presented in Fig. 1. Seen from Fig. 1, values for the complexing index for ultrasonically treated SPS-LA composites 30% (31.4%), 40% (41.1%), 50% (36.4%), and 70% (34.0%) were higher than for the untreated sample (27.9%). Previous research reported that the complexing index reflects the capacity of amylose in starch molecules to form an amylose-lipid complex with fatty acids [27]. Therefore, a larger complexing index value means a lower As (sample absorbance), and more amylose-lipid complexes would form between amylose and fatty acid. That is to say, the ultrasonic processing promoted the

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amylose-lipid complex formation. This is probably because of the fact that the ultrasonication process would disintegrate the starch granules to release amylose, which increased the probability of contact between the LA and amylose [28]. Another possible reason is that the ultrasonication process improves the LA dispersibility in SPS dispersion. Fig. 1 shows that the complexing index values increased progressively with the increase of ultrasonic amplitude (30-40%). The results indicated that the low power density ultrasound was favourable to the formation of the inclusion complex. The complexing index values of the composites gradually decreased as the ultrasonic amplitude continued to increase. After the ultrasonic amplitude increased to 70%, the complexing index value of the composite decreased to 34.0%. There was an approximately 17.3% decrement in complexing index values compared with the 40% of ultrasonic amplitude-treated sample, which could be attributed to the high power density ultrasound resulting in the breaking of the hydrophobic interaction between amylose molecules and LA molecules. Furthermore, the strong mechanical force and powerful shock waves in solution by high power density ultrasound accelerated the disruption of the amylose-lipid complex. 3.2. Thermal properties of the SPS-LA composites DSC is a usual technology for the measurement of the thermal properties of a polymer [25]. Effects of different ultrasonic amplitudes on thermal properties of SPS-LA composites are summarized in Table 1. To investigate the effect of different ultrasonic amplitudes on the gelatinization enthalpies of starch, the SPS-LA

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composites were prepared at 60 ºC. Two endothermic transitions were observed with each scanned sample in the DSC thermograms. For the control sample, the first peak appearing at 64.9 - 83.4 ºC was assigned to the gelatinization endothermic peak; the second peak emerging at approximately 87.7 to 103.5 ºC (data are not shown) was attributed to the melting endothermic peak of the inclusion complex [29]. As Table 1 shows, the gelatinization enthalpies (△H) of the ultrasonically treated samples were lower than the △H of the control samples, which required less energy for gelatinization. With the increase in the ultrasonic amplitude, the gelatinization enthalpies gradually decreased. Ultrasonic treatment of SPS distorted the crystalline region in the starch granule, which led to the destruction of the starch granular structure [30]. Therefore, the destruction of the starch granular structure under ultrasonication conditions resulted in decreasing gelatinization enthalpies. These values for gelatinization enthalpies agreed with those obtained by other researchers [31]. As shown in Table 1, broad endotherms showing two transition peaks were observed for the ultrasonically treated samples and the untreated sample prepared at 90 °C, whereas the SPS-LA composites prepared at 60 °C exhibited a single transition peak of the inclusion complex on the DSC thermograms. Amylose-lipid complexes were divided into two types, the less ordered type I (melting temperature between 90 105 °C) and the semi-crystalline type II (melting temperature between 115 - 120 °C) [32]. Therefore, SPS-LA composites formed at 60 °C likely corresponded with the less-ordered type I, but the SPS-LA composites formed at 90 °C might be a mixture of

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the less-ordered type I and semi-crystalline type II. These results were in line with those obtained by other authors [33]. Based on the DSC thermograms, a parameter of the inclusion complex was evaluated. The enthalpy (△H) (total △H values of type I and type II) of the inclusion complex was calculated from the area of the endothermic peaks, which corresponds to the relative amount of amylose-lipid complex in the composite [34,35]. The melting enthalpies (△H) of the ultrasonically treated samples were higher than those of the control, indicating the formation of more amylose-lipid complexes during ultrasonication. The SPS-LA composite with 40% of ultrasonic amplitude exhibited the highest melting enthalpy, which suggested that low power density ultrasound was beneficial to the formation of the inclusion complex. With the ultrasonic amplitude increased from 40% to 70%, the melting enthalpy (△H) of the complex gradually decreased, indicating the formation of a small amount of amylose-lipid complex during the violent ultrasound. This result can be credited to the high power density ultrasound impairing the structure of the inclusion complex. The △H results enhanced the accumulating evidence that low power density ultrasound contributed to inclusion complex formation, while the high power density ultrasound accelerated the disruption of the inclusion complexes. 3.3. Effects of ultrasonic treatment on the X-ray diffraction pattern of the composites The X-ray diffraction technology was applied to obtain qualitative evidence of the formation of the inclusion complex. The effects of ultrasonic treatment on the crystalline structure of SPS-LA composites were investigated. Fig. 2 shows the diffraction patterns and relative crystallinity of the ultrasonically treated samples and

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the untreated sample (30% and 50%, data are not shown). The native SPS granules had a typical Ca-type crystalline arrangement, showing the main diffraction peaks at approximately 14.7°, 16.9°, 17.7° and 22.6° (2θ), in agreement with the previous findings [36]. When the SPS interacted with LA molecules at 90 °C and was treated by ultrasonication, the crystal pattern was altered from Ca to the V-type structure. The V-type crystalline pattern was inferred from two main diffraction peaks at approximately 2θ of 13.3° and 20.15° and one minor diffraction peak at approximately 2θ of 7.5° [7]. These crystalline patterns exhibited the well-known V6-helical type diffraction which consists of six D-glucosyl residues per turn [32]. Seen from Fig. 2, the V-type pattern became more obvious after the dispersion treated by ultrasonication. In other words, the diffraction intensity of the untreated samples was weaker compared with that of the ultrasonically treated samples, indicating that the quantity of inclusion complex was increased after ultrasonic treatment. This change in peak intensity was attributed to the ultrasound increasing the amylose content and improving the dispersibility of the LA, which facilitated the formation of the amylose-lipid complex. The diffraction intensity of the inclusion complex increased when the ultrasonic amplitude ranged from 30 to 40%, which further indicated that the low power density ultrasound could promote the formation of the inclusion complex. This finding was consistent with the results of the complexing index tests. When the ultrasonic amplitude was above 40%, the diffraction intensity of the inclusion complex gradually decreased. It meant that high power density ultrasound would destroy the structure of the amylose-lipid complex. Similar results

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had been found for the formation of a complex between beta-carotene and starch dextrin [37]. Subsequently, the effect of ultrasonic treatment on the relative crystallinity of the SPS-LA composite was investigated. Apparently, the ultrasonically treated samples and the untreated samples exhibited lower relative crystallinity than native SPS, which was credited to the dissociation of double-helix amylopectin during heating. The relative crystallinity of SPS-LA composites was ascribed mainly to the formation of the amylose-lipid inclusion complex, i.e., the more inclusion complex formation, the higher the relative crystallinity was. The SPS-LA composite with 40% ultrasonic amplitude exhibited the highest relative crystallinity value among all of the samples (excluding the native SPS). With a further increase in ultrasonic amplitude (up to 70%), relative crystallinity decreased to 27.1%. These results suggested that some inclusion complexes already formed may have experienced disruption due to the intense mechanical effect and powerful shock waves from applying high power density ultrasound. This was in accordance with the results from the above DSC analysis. 3.4. Effects of ultrasonic treatment on light transmission of the films Fabra et al. [38] reported that the light transmission values are strongly connected with the function of the starch-based films, credited to their great influence on the appearance of the packaging product. The light transmission values of the films were related to their inner structure developed during the drying process. The higher values are connected to the greater film homogeneity, which represents more transparent

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films. Seen from the Table 2, the light transmissions of the ultrasonically treated samples were higher than the untreated sample, which suggested that ultrasonication made the film-forming compositions more compatible. In general, the better the film-forming composition compatibility of the film is, the higher the light transmission it is [25]. These results corresponded with those obtained by other authors [39]. A similar trend was observed for the control and ultrasonically treated films in the visible region (400-800nm). The values increased as the wavelength increased for both the untreated and the ultrasonically treated films. Fang et al. [40] reported that the wavelength of 600 nm is commonly used in transparency measurements of the polymer films. Therefore, the light transmission at 600 nm was selected to measure the transparency of the SPS-LA composite films in this study. As Table 2 shows, the light transmission values gradually increased with the increase of ultrasonic amplitude. The light transmission values of the films treated by low power density ultrasound were lower than those of the films prepared by high power density ultrasound. After 70% ultrasonic amplitude treatment, the value of the films showed an increase of 15.4% compared with the control film. This increase is likely caused by the thorough rupture of large starch granules and the elimination of air bubbles in SPS-LA dispersions after treatment by high power density ultrasound, which indicates high power density ultrasound is more beneficial to film-forming composition compatibility. In this sense, the film with high power density ultrasound shows higher light transmission values than those of the films with low power density ultrasound.

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3.5. Effects of ultrasonic treatment on mechanical property of the composite films The mechanical property of the starch-based film is an important parameter for predicting its behaviour in food packaging. The TS and E of the films are the useful indices for describing their mechanical properties [41]. The obtained results indicated that the mechanical force and shock waves in the ultrasonic process affected the mechanical property of the SPS-LA composite films. Seen from Fig. 3, the TS value of the ultrasonically treated films was higher than that of the control film. For the film with 40% ultrasonic amplitude, the TS value achieved 8.31 MPa, representing approximately 34.5% enhancement in TS compared to the control without ultrasonic treatment. The results can be ascribed to the greater inclusion complex formation for ultrasonically samples that may have given greater strength to the film matrix during the film forming process. Furthermore, it seemed to be favourable to form a more tight and compact polymer matrix in the ultrasonic process. This improved polymer structure is probably an important factor for the TS enhancement. Meanwhile, the E value of the ultrasonically treated films was lower than that of the control film. This finding could be due to the degradation of starch molecules by ultrasonication, which would result in more brittle ultrasonically treated films. Therefore, the E values of the films decreased after application of the ultrasonic treatment. This result was in line with those obtained by other researchers [39]. The TS and E values were

(

) by ultrasonic amplitude.

The films prepared by high power density ultrasound showed poor mechanical properties compared to the films made by 40% of the ultrasonic amplitude. By

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applying ultrasonic amplitude up to 70%, the TS and E values of the films decreased to 7.23 MPa and 5.33%, respectively. For the SPS-LA composites treated by high power density ultrasound, relatively more starch granules were destroyed during the ultrasonic process, which led to the intermolecular interaction being weakened, and the compact structure of the polymer matrix being impaired more severely. 3.6. Effect of ultrasonic treatment on the microstructure of SPS-LA composite films Scanning electron microscopy (SEM) micrographs of the SPS-LA composite films (ultrasonically treated and untreated) are shown in Fig. 4. It could be seen from [Fig. 4 (a)] that the untreated film exhibited a discontinuity in the polymer matrix, with an irregular and rough surface. Fig. 4(b-e) shows the microstructure of the SPS-LA composite films at different ultrasonic amplitudes for 30%, 40%, 50% and 70%. Obviously, the surface of the ultrasonically treated films became smooth and showed a continuous and uniform texture compared with the untreated films. The result suggested that the film-forming components became more compatible after being treated by ultrasonication. In addition, the formation of the inclusion complex during ultrasonication had an ability to level the film surface on account of reduced crystallite dimensions and lowered the friction coefficients in the film-forming process [23]. Therefore, the ultrasonically treated films exhibited a continuous polymer matrix. With the increase of the ultrasonic amplitude, the surface of the ultrasonically treated films became increasingly smooth. This result can be explained by the starch granules being physically broken up into small fragments and disintegrating gradually

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under the strong mechanical force and powerful shock waves induced by ultrasonication. A continuous structure was formed under intense ultrasonication conditions as a result. 3.7. Effect of ultrasonic treatment on surface morphology of SPS-LA composite films The qualitative (morphological images) and quantitative (average roughness, Ra; maximum roughness height, Rmax) parameters of SPS-LA composite films with and without ultrasonic treatment were determined by atomic force microscopy (AFM), as shown in Fig. 5, through a topographic study resulting from the atomic interaction between the components of the polymer films. Fig. 5(a) demonstrates the control film showed a rough structure with many peaks and valleys. Fig. 5(b-e) shows the AFM 3D-micrograph of the SPS-LA composite films at different ultrasonic amplitudes for 30%, 40%, 50% and 70%. Obviously, ultrasonication led to a reduction in peaks and valleys. The surface Ra and Rmax of the ultrasonically treated films were lower than those of the untreated films, indicating that the ultrasonication was favourable for forming a well-ordered structure with a relatively smooth surface [42]. This finding was in accordance with the SEM test results. The Ra and Rmax values of the ultrasonically treated films declined from 25.7 to 13.6 nm and 137.9 to 65.4 nm, respectively, when the ultrasonic amplitude increased from 30% to 70%. The results indicate that the high power density ultrasound tends to produce a more ordered and uniform polymer film structure with low Ra and Rmax. 3.8. Effects of ultrasonic treatment on water vapor permeability of films The moisture barrier property of the polymer films is a very crucial characteristic

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in wrapping foods with high humidity [43]. Fig. 6 shows the water vapor permeability of the ultrasonically treated and untreated films. Generally, the SPS-LA composite films exhibit poor moisture barrier properties because of their hydrophilic characterization [44]. Nevertheless, as seen from Fig. 6, the values of treated films were lower than those of the untreated films, indicating that the ultrasonic process improved the moisture barrier properties of the film. The values of water vapor permeability ranged from 1.82×10−12 to 2.14×10−12 (g cm cm-2 s-1 Pa-1) for the ultrasonically treated films compared to the untreated films, for which the water vapor permeability value was 2.26×10−12 (g cm cm-2 s-1 Pa-1). This result is likely caused by the fact that ultrasonication seems to be favourable to forming a more compact polymer matrix and thus improving its moisture barrier properties. These values for water vapor permeability are consistent with those observed by other researchers, who investigated maize starch-based films with ultrasonic treatment [39]. The films with 40% of ultrasonic amplitude showed the lowest water vapor permeability. When the ultrasonic amplitude exceeded 40%, the water vapor permeability values gradually increased. This result can be ascribed to the greater inclusion complex formation for low power density ultrasound, which led to construction of a more homogeneous and compact polymer matrix. Previous studies reported that more compact and homogeneous structures with less free volume can lower the water vapor permeability with regard to synthetic polymer materials [45]. Therefore, the films with 40% ultrasonic amplitude showed better moisture barrier properties.

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4. Conclusions In this research, ultrasonic treatment was used to treat SPS, and the effect of different ultrasonic amplitudes on the light transmission, microstructure, surface morphology, water vapor permeability and mechanical properties of the SPS-LA composite films was investigated. The low power density ultrasound could contribute to inclusion complex formation, and the high power density ultrasound would destroy the structure of the amylose-lipid complex. The film-forming components became more compatible after treatment by ultrasonication. Ultrasonic treatment prior to film-making could enhance the light transmission and TS, while the ultrasonic treatment could reduce the water vapor permeability and E values of the composite films. With the increase of ultrasonic amplitude, light transmission gradually increased, and E values gradually decreased. The films with 40% ultrasonic amplitude showed the highest TS values and lowest water vapor permeability. Therefore, a low power density ultrasound of 40% in this case was found to be effective in both promoting amylose-lipid complex formation and improving the properties of the films. Acknowledgements The authors thank Doctor Shenglin Sun for his kind advice in this work. References [1] S. Meng, Y. Ma, J. Cui, D.W. Sun, Preparation of corn starch–fatty acid complexes by high-pressure homogenization, Starch/ Stärke. 66 (2014) 1–9. [2] W.C. Obiro, S.S. Ray, M.N. Emmambux, V-amylose structural characteristics,

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Figure Captions Figure 1 The effects of ultrasonic treatment on the complexing index. Figure 2 X-ray diffraction patterns of sweet potato starch-lauric acid composites. Figure 3 Effects of ultrasonic treatment on mechanical properties of films. Figure 4 Effect of ultrasonic treatment on the microstructure of films. Figure 5 Effect of ultrasonic treatment on surface morphology of films Figure 6 Effects of ultrasonic treatment on water vapor permeability of films.

1

Table 1 The effect of ultrasonic treatment on thermal properties of sweet potato starch. Ultrasonic

Gelatinization peak (60 ºC)

amplitude

To

TP

Tc

△H

To

TP

Tc

To

TP

Tc

△H

(%)

(ºC)

(ºC)

(ºC)

(J/g)

(ºC)

(ºC)

(ºC)

(ºC)

(ºC)

(ºC)

(J/g)

Control

64.9

75.8

83.4

14.4

88.1

99.2

106.6

108.6

114.9

120.2

3.96

±

±

±

±

±

± 0.80 30

40

± b

0.42

± a

1.01

± a

1.15

a

0.76

a

0.68

ab

0.76

a

Total

1.21

a

0.29e

76.7

83.0

13.1

86.1

96.3

106.0

109.4

114.6

119.2

5.06

±

±

±

±

±

±

±

±

±

±

±

1.12a

0.99a

1.01a

0.36b

1.36b

1.02bc

0.85ab

0.60a

0.46a

0.97a

0.13d

66.0

76.3

82.7

12.4

84.6

94.7

103.3

104.5

113.0

117.6

7.83

±

±

±

±

±

0.96

± ab

65.4

1.07

0.66

0.87

0.93

0.56

± a

85.0

± a

0.74

0.85

1.10

a

96.5

± d

0.91

c

99.2

± c

9.8

± a

0.45

0.55

± b

88.3

± a

81.8

± ab

0.46

0.67

± b

11.3

± a

76.5

±

0.87

± a

82.3

± ab

65.6

0.60

0.70

± a

76.1

±

70

1.14

± a

Type II (90 ºC)

66.8

±

50

1.10

± a

Type I (90 ºC)

± b

0.61

b

1.27

c

1.08

c

0.96

b

0.57

b

0.34a

105.5

107.2

112.6

120.0

6.34

±

±

±

±

±

1.12

ab

0.35

b

1.05

b

0.70

a

0.22b

104.6

109.5

113.9

119.6

5.83

±

±

±

±

±

0.67

bc

0.95

a

0.70

ab

0.91

a

0.12c

The abbreviated parameters of To, TP, Tc and △H are onset temperature, peak temperature, conclusion temperature and enthalpy, respectively.

a – e: Mean values in the same column with different superscripts are significantly different (P＜ 0.05). Data shown in mean ± standard deviation (n = 3).

2

Table 2 The effect of ultrasonic treatment on light transmission of the films. Ultrasonic amplitude (%)

Light transmission (%) 400-nm

500-nm

600-nm

700-nm

800-nm

wavelength

wavelength

wavelength

wavelength

wavelength

Control

58.2 ± 0.86e

65.0 ± 0.85d

67.6 ± 0.45d

69.6 ± 0.66d

71.8 ± 0.57d

30

63.1 ± 0.91d

69.7 ± 1.29c

72.9 ± 0.81c

75.4 ± 1.12c

77.8 ± 0.47c

40

64.6 ± 0.71c

71.9 ± 0.62b

73.8 ± 0.38c

77.5 ± 0.47b

80.0 ± 0.83b

50

66.0 ± 0.65b

72.4 ± 0.87b

75.5 ± 0.78b

78.5 ± 0.42b

80.7 ± 0.64b

70

67.8 ± 0.71a

74.8 ± 0.74a

78.0 ± 1.24a

80.4 ± 0.81a

83.0 ± 0.67a

a – e: Mean values in the same column with different superscripts are significantly different (P＜ 0.05). Data shown in mean ± standard deviation (n = 3).

3

Complexing index/%

45

a

40

b c

35 30

d e

25 20 Control

30

40

50

70

Ultrasonic amplitude (%)

Fig.1. The effects of ultrasonic treatment on the complexing index. a – e : The different letters show significant difference (P＜0.05).

4

Fig.2. X-ray diffraction patterns of sweet potato starch-lauric acid composites.

5

10

TS B

a B

8

10 b c

C

d e

8 D

6

6

5

4

4

2 Control

30

40

50

E/%

TS/MPa

9

7

12

E

A

70

Ultrasonic amplitude (%)

Fig.3. Effects of ultrasonic treatment on mechanical properties of films. a – e, A-D: Different letters within the same indicator indicate significant differences among formulations (p < 0.05).

6

a. Control

b. 30%

d. 50% e. 70% Fig.4. Effect of ultrasonic treatment on the microstructure of films.

7

c. 40%

a. Control (Ra: 50.3 nm)

b. 30% (Ra: 25.7 nm)

c. 40% (Ra: 20.1 nm)

d. 50% (Ra: 18.9 nm) e. 70% (Ra: 13.6 nm) Fig.5. Effect of ultrasonic treatment on surface morphology of films.

8

WVP/10-12 g cm cm-2 s -1 Pa-1

2.5 2.3

a b

2.1

c

c

d

1.9 1.7 1.5 Control

30

40

50

70

Ultrasonic amplitude (%)

Fig.6. Effects of ultrasonic treatment on water vapor permeability of films. a – d : The different letters show significant difference (P＜0.05).

9

Highlights

-A low power density ultrasound could contribute to amylose-lipid complex formation. -The high power density ultrasound would destroy the structure of inclusion complex. -Ultrasonic treatment prior to film-making could improve the properties of the polymer films.

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