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amylopectin content
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Rheological performance of film-forming solutions made from plasma-modified starches with different amylose/amylopectin content Pablo Hernandez-Perez a, Pamela C. Flores-Silva a, Gonzalo Velazquez a, ´ndez b, Ernesto Herna ´ndez-Herna ´ndez b, Eduardo Morales-Sanchez a, Oliverio Rodríguez-Ferna Guadalupe Mendez-Montealvo a, *, Israel Sifuentes-Nieves b, * a
Instituto Polit´ecnico Nacional, Centro de Investigaci´ on en Ciencia Aplicada y Tecnología Avanzada, Cerro Blanco No. 141, Col. Colinas del Cimatario, C.P. 76090, Santiago de Quer´etaro, Quer´etaro, Mexico Centro de Investigaci´ on en Química Aplicada, Blvd. Enrique Reyna No. 140, C.P. 25253, Saltillo, Coahuila, Mexico
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma Film-forming solutions Viscoelasticity Creep-recovery behavior HMDSO coating
Normal and high amylose corn starches were modified using HMDSO plasma at different time treatments. Changes in functional properties of starch granule, film-forming solutions (FFS) and films were investigated. SEM analysis revealed HMDSO coating deposition on the granule surface, which limited the amylopectin leach out from the granules to the continuous matrix, affecting the rheological properties of the FFS. The amylopectin restriction resulted in a low reinforcement of the network decreasing the viscosity as indicated by n and k values. Also, a gel-like behavior (G′ > G′′ ) was observed when the amylose and time treatment increased, suggesting that the matrix becomes less elastic with softer entanglement. This behavior was confirmed by creep test and Burger model parameters. The plasma treatments allowed obtaining FFS with low viscosity, suitable for developing soft and hydrophobic films with low flexibility, as indicated by the decrease of the maximum stress, Hencky strain and permeance values.
1. Introduction Rheological measurements have been employed to predict the quality and texture of starchy foods during their processing and storage. These measurements are also useful to explain the behaviour of starchbased materials in other applications such as films, paper, textile, pharmaceutical and cosmetic (Saha & Bhattacharya, 2010). The rheo logical behaviour of gelatinized starch is influenced by the granular size, swelling power and amylose-amylopectin entanglement (Kaur, Singh, McCarthy, & Singh, 2007). Xie et al. (2009) studied the rheological properties of various native starches with different amylose content (0–80 %) and reported high viscosity and less Newtonian behaviour in high amylose starches due to its higher gelatinization temperature and greater molecular entanglements between linear polymer chains. Native starch, regardless its amylose content, does not always have the appropriated functional properties and rheological performance for in dustrial applications. To overcome this drawback, chemical, enzymatic and physical modification have been employed to enhance the
functionality of starch molecule (Din, Xiong, & Fei, 2017). Among these, physical treatments like ultrasound, irradiation and cold plasma are widely used because they are considered as safe since no chemicals or enzymes are left in the final product; besides, they are cost-effective and environmentally friendly (Zavareze & Dias, 2011). Cold plasma is a green technology and a physical method used to obtain modified starches with no toxic residues. The starch modification depends on the plasma reactor employed and active species reactivity, being the corona discharge, dielectric barrier discharges, radio fre quency plasma and the gliding arc discharge the most employed (Bogaerts, Neyts, Gijbels, & Van der Mullen, 2002). In this regard, the active species of plasma are able to collapse hydroxyl groups of starch molecule and lead to chemical changes on its supramolecular structure, resulting in important modifications of the thermal and rheological properties; which have been attributed to crosslinking, depolymeriza tion and etching phenomena (Thirumdas, Kadam, & Annapure, 2017). Rheological properties of normal-native starch after plasma treatment have been reported. Bie et al. (2016) found that the plasma-modified
Abbreviations: HMDSO, Hexamethyldisiloxane. * Corresponding authors. E-mail addresses: [email protected] (G. Mendez-Montealvo), [email protected] (I. Sifuentes-Nieves). https://doi.org/10.1016/j.carbpol.2020.117349 Received 21 May 2020; Received in revised form 22 September 2020; Accepted 28 October 2020 Available online 2 November 2020 0144-8617/© 2020 Published by Elsevier Ltd.
Please cite this article as: Pablo Hernandez-Perez, Carbohydrate Polymers, https://doi.org/10.1016/j.carbpol.2020.117349
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starch paste tended to change from a pseudo-plastic (non-Newtonian) fluid to a Newtonian fluid, and the viscosity decreased as a result of the degradation of starch molecules. In addition, Banura, Thirumdas, Kaurc, Deshmukhb and Annapure (2018) reported that plasma treatment led to starch depolymerization which resulted in the formation of simple sugars (glucose and maltose) formation and in an increase of viscosity by the leaching of particles from the starch granules. Recently, we evaluated the effect of hexamethyldisiloxane (HMDSO) plasma on the structure of corn starches with different amylose content (Sifuentes-Nieves et al., 2020). The thermal (gelatinization and enthalpy) and structural (crystallinity) properties of treated starch was inspected. However, the rheological performance of film-forming solu tions (FFS) made from plasma-modified starches after gelatinization process due to the amylose/amylopectin content was not studied and it may play an important role on the final characteristics of the starch-based films. Our hypothesis was that amylose content and plasma treatment time determine the rheological properties of the FFS. Furthermore, the influence of FFS on the barrier and mechanical prop erties of the final films was inspected.
adhesive carbon tape and the images were obtained at a magnification of 4000£ (starch granules) and 2500× (starch films). 2.6. FTIR analysis The presence or absence of specific chemical groups of the samples was inspected by Fourier transform infrared (FTIR) spectra using a spectrophotometer (Bruker Tensor 37, USA) with an attenuated total reflectance (ATR) attachment. The spectra of each sample were recorded in the range from 4000 to 650 cm− 1 at a resolution of 1 cm− 1. 2.7. Rheological measurements The rheological properties of the FFS from normal and high amylose starches were carried out in an Anton Paar rheometer (Physica MCR 101) using a plate-plate geometry of 20 mm with a gap of 1 mm. 2.7.1. Determination of flow curves Rotational tests were performed to describe the flow curves. The FFS solution was placed in the rheometer and cooled from 60 to 25 ◦ C at 2.5 ◦ C/min and a shear rate of 50 s− 1. At the final temperature, two measurement cycles were performed, upward and downward from 0.01 to 100 s− 1. Mineral oil was placed around the plates to prevent water evaporation during the measurement. The flow behavior was deter mined in the second cycle. The power law model of Ostwald-de Waele (Eq. 1) was used to describe the flow curves.
2. Materials and methods 2.1. Materials Corn starch with 30 % (normal — S30-C), 50 % (Hylon V — S50-C) and 70 % (Hylon VII — S70-C) amylose content were acquired from Ingredion (formerly, Corn Products and National Starch at Bridgewater, NJ). Glycerol (G7757) and HMDSO (205389) was purchased from Sigma-Aldrich (Saint Louis, MO, USA).
τ = kγ˙n
(1)
Where: τ is the shear stress (Pa), k is the consistency index (Pa.sn), γ˙ is the shear rate (s− 1) and n is the flow behavior index (dimensionless) (Steffe, 1996; Tecante & Doublier, 1999). Determinations were carried out in triplicate.
2.2. HMDSO plasma treatment on starches Normal and high amylose corn starches were modified in a rotatory cylindrical glass reactor (0.5 L), which was connected to a vacuum pump and a radio frequency (RF) power generator operating at 13.56 MHz; The pressure inside the reactor was fixed at 0.4 mbar and after HMDSO injection, the pressure was maintained at 0.45 mbar, allowing a constant flow of HMDSO gas of 0.35 sccm into the reactor. The samples were treated with HMDSO monomer during 10 and 30 min, and a power level of 90 W was used to achieve the plasma formation. The rotatory reactor kept the samples in agitation (20 rpm) during all plasma treatment, which led to a homogeneous modification throughout all starch granules.
2.7.2. Dynamic rheological measurements For the oscillatory measurements, the FFS solutions were stored at room temperature (23 ± 2 ◦ C) for 48 h in a petri plate. After that time, the frequency sweeps were performed at 25 ◦ C), applying a constant stress of 5 MPa once the linear viscoelastic region (LVR) was allocated. The frequency sweeps were performed at an interval of 0.01–10 Hz. The storage modulus (G′ ), loss modulus (G′′ ), complex viscosity (η*, Pa.s) and tan delta (Tan δ) were calculated from each test. 2.7.3. Creep test After storing the samples at room temperature for 48 h, the cr(Kumar & Khatkar, 2017)eep-recovery test was performed at 25 ◦ C by measuring the deformation while applying a constant stress of 5 MPa for 100 s and the recovery was measured for another 100 s. The data ob tained were adjusted to the Burgers model (Equation 2) to calculate the limiting viscosity at zero shear rate, μ0 (Pa*s), instantaneous compli ance, J0 (Pa− 1), retarded compliance, J1 (Pa− 1), and relaxation time, λrel (s). ( ( )) − t t J = J0 + J1 1 − exp + (2) λret μ0
2.3. Preparation of film-forming solutions Film-forming solutions (FFS) were prepared using a 1 g-sample of starch and 30 % w/w of glycerol, which were added in 25 mL of distilled water. To achieve the gelatinization and a uniform aqueous dispersion, the normal starch was gelatinized using a stirred hot plate (Corning, Model PC-220, USA) at 90 ◦ C during 10 min and the high amylose starches were gelatinized using an autoclave (LS-B75 L, NANBEI, China) at 121 ◦ C for 30 min. 2.4. Films elaboration The gelatinized FFS were immediately poured on acrylic plates of 20 × 20 cm and dried at 60 ◦ C for 6 h in a conventional oven (IBTF050). Finally, the films were stored until further analysis.
2.8. Mechanical properties The Hencky strain and the maximum stress (MPa) at the breaking point were evaluated at room temperature. Film samples (10 × 100 mm) were cut and conditioned at 53 % RHeq for 5 days at 30 ◦ C employing a saturated solution of sodium bromide. A texture analyzer TA-XT (TA Plus, Lloyd Instruments) equipped with mechanical grips was used at a cross-head speed of 1 mm/s and an initial separation of the grips of 50 mm. Ten replicates were tested for each film. The maximum stress was calculated dividing the maximum force by the
2.5. Scanning electronic microscopy (SEM) Scanning electron micrographs of modified starch granules and of the films made from plasma-modified starches were obtained using the Phenom-World microscope (Eindhoven, Netherlands) operated at an accelerating voltage of 5 kV. Samples were adhered to a double-sided 2
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transversal area at the breaking point and the Hencky strain was calculated from the maximum deformation of the film.
granule surface were observed, such as fissures or cavities and superfi cial deposition (coatings) on them (starch granules - Fig. 1). These changes can be attributed to etching and deposition phenomena caused by the reactive species of the treatment, as a result of plasma molecules penetration at molecular level or coating deposition of HMDSO on the granule surface. A similar behavior was found by Thirumdas, Trimukhe, Deshmukh and Annapure (2017) in cold plasma-treated rice starch. The authors reported that the fissures have an important effect on the solu bility and rheological performance starch molecules.
2.9. Water vapor permeance (WVPe) The permeance of the films was determined using the gravimetric technique following the ASTM E96-00 method. Glass cell (50 mm height and 40 mm diameter) containing distilled water was sealed with the films using silicon gaskets allowing an effective transfer area of 0.0013 m2. The glass cell was then placed on a balance plate inside of a temperature-controlled (30 ◦ C) chamber containing silica gel. The weight was automatically recorded every 1 min for 6 h using an analytical balance with 0.0001 g precision. The permeance was calcu lated using the Eq. (3).
3.2. FTIR analysis The FTIR spectra of native and plasma-modified starches are shown in Fig. 2. No new signal corresponding to HMDSO was observed in the plasma-modified starches, suggesting that the treatment only promoted the interaction between active species of plasma and the functional groups of starch. However, some changes in specific bands were observed. In this regard, all samples showed the wide band at 3280 cm− 1 corresponding to –OH. After normalizing the spectra using the 1082 cm− 1 band, the band at 3280 cm− 1 showed lower intensity ratio in the plasma-modified starches after 30 min of treatment, suggesting that the active species of plasma changed the starch ability to maintain bound water, probably due to the barrier effect of the HMDSO coat formed on the surface of the granule during the treatment. Also, the vacuum used for the plasma treatment may have triggered the evapo ration of water molecules from the starch granules (Sarangapani et al., 2017). The intensity band at 1648 cm-1 assigned to tightly bound water in the starch, was also weakened by the treatment after 30 min indi cating that the vacuum used during treatment promoted removal of some water molecules from the starch. This behavior can be attributed to the morphological changes observed in the treated starch granules
(3)
Permeance = Q/AΔP = g /m2 s Pa − 1
Where Q is the weight loss of the cell per unit of time (g⋅day ), A is the transfer area (m2), t is the time (days), and ΔP is the water vapor pressure gradient (4245 Pa) created by the distilled water and the silica at 30 ◦ C. 3. Results and discussion 3.1. Scanning electronic microscopy Scanning electronic microscopy was used to inspect the differences between starch granules before and after HMDSO plasma treatment. All starches showed granules with different shape and size. According to Singh, Singh, Kaur, Sodhi and Gill (2003) normal (S30-C) corn starch is characterized by round granules from 5 to 25 μm and high amylose starches (S50-C & S70-C) present round and some elongated granules (2− 30 μm). After HMDSO plasma treatment some differences on the
Fig. 1. SEM images of plasma-modified corn starches and films with different amylose content. 3
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enthalpy values due to the crosslinking of the starch molecule and the coating formed on the granule surface, as well as a decrease on their relative crystallinity as a result of the amylose fragmentation (Sifuen tes-Nieves et al., 2020). Therefore, it is important to characterize the rheological properties of FFS to understand the features of these systems, since gelatinization and drying conditions during the making of starch-based films strongly affects the network structure, physical properties and film performance (Chen, Kuo, & Lai, 2009; Romero-Bastida et al., 2005). 3.3.1. Flow curves Characterizing the flow behavior of the FFS is an important step when the solutions are used for casting process of pre-formed films or when applying a liquid coating directly on the surface of food products by dipping, brushing or spraying. It has been reported that high viscosity of gel type structures in the film-forming solution would make it difficult to eliminate air bubbles and hinder casting in thin layers (Cuq, Aymard, Cuq, & Guilbert, 1995). In the rotational test applied to the FFS, the power law model showed a good fitting to describe the rheograms (Fig. 3). The flow index behavior (n) and consistency index (k) parameters were estimated with R2 > 0.9. The n values lower than one (n<1) indicated a non-Newtonian behavior, showing a shear-thinning behavior. In these cases, the vis cosity decreased when the shear rate increased, as it was observed in the flow curves (Fig. 3). The n values increased as the plasma treatment time increased (30 min), showing less dependence in the shear rate (Fig. 4a). This trend was more noticeable in starches with 50 % amylose. The results suggest that the reorganization of the film-forming solutions is influenced not only by the amylose content, but also by HMDSO coating deposition on the granule surface and the helical-water interruption promoted by the plasma treatment, which favored the fluidity of FFS. According to (Kumar & Khatkar, 2017)Kumar and Khatkar (2017), higher viscosity (low n values) suggests a reinforcement effect of the network by the branched amylopectin molecule. However, the increase of the n values of FFS could be related to less amylopectin interactions in the network, as the HMDSO coating deposited on the granule could limit the amylopectin molecules leaching out to the system (Fig. 5), reducing the interactions between amylose chains and the amylopectin mole cules, which resulted in a viscosity decrease. A similar restraining effect of the HMDSO coating on the starch components was observed in the gelatinization behavior of plasma-modified starches, as the temperature and enthalpy of gelatinization increased, suggesting a granule rein forcement by HMDSO coating (Sifuentes-Nieves et al., 2020).
Fig. 2. FTIR spectra of native and plasma-modified corn starches with different amylose content.
(Fig. 1), as the cavities or fissures on the granules allowed high reactivity between the species of plasma and starch molecules resulting in depo lymerization and interruption of the helical-water bonds (Zhang et al., 2014). 3.3. Rheology of film-forming solutions (FFS) Nowadays, the trend in industry is to formulate bio-based materials for packaging. One important point is that these materials must include at least one component that forms a cohesive and continuous matrix. Starches have been studied as a main ingredient in film-forming solu tions (FFS) and modified starches are a feasible alternative to improve the film properties. As previously described in SEM and FTIR analysis, plasma-modified corn starches with different amylose content (S30, S50 and S70) at different treatment times (0, 10 and 30 min) showed some cavities and/or coating deposition on the granules surfaces; which resulted in removal of some water molecules as the helical-water bonds of starch were broken by active species of plasma. Furthermore, no new functional groups of HMDSO plasma were incorporated in the starch molecule as indicated by FTIR analysis. In addition, these modified starches showed an increase in the gelatinization temperature and
Fig. 3. Flow curves of the film-forming solutions made from native and plasma-modified corn starches with different amylose content. 4
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Fig. 4. Parameters of the Ostwald-de Waele (power law) model for the film-forming solutions made from native and plasma-modified corn starches with different amylose content. a) flow index behavior (n) and b) consistency index (k).
than G′′ , indicating a gel-like behavior (Okechukwu & Rao, 1997). The dynamic moduli (G′ and G′′ ) showed slight dependence on frequency (data not shown), indicating a “true gel” behavior. G′ was practically independent on the frequency. Starch with 30 % amylose show slight difference for G′ values when the starch was plasma-treated as the gel-like behavior was more pronounced. This behavior could be due to amylopectin contributions and remnant starch granules as the swollen starch granules promoted a better structure that could result in a rein forcement in the matrix, allowing greater interaction and favoring the conformation of a more elastic system (more solid-like). The G′ values decreased when increasing the amylose content (Fig. 6a) suggesting that the matrix becomes less elastic or the entan glement was softer. Also, at higher treated times, the gels are well conformed, but they are less rigid or compact (as it is noted in Fig. 6b, where G′′ decreased). Moreover, the complex viscosity (η*) decreased considerably when the amylose content and the plasma-treated time increased (Fig. 6c). According to Muscat, Adhikari, Adhikari & Chaudhary (2012), the viscous nature of FFS increases as a function of the plasticizer content. However, an opposite behavior was found in the plasma modified FFS, as the water removal could limit the glycerol plasticization in the binding sites of starch, and as a result, the FFS viscosity decreased. Measuring changes in Tan δ is useful to evaluate the transition from liquid-like to solid-like behavior as Tan delta decreases with a more solid-like behavior (Steffe, 1996). This trend is observed in Fig. 6d. Starch with 30 % amylose showed a better entanglement than starches with 50 and 70 % amylose. These FFS samples with treatment of 30 min showed higher values in Tan δ, indicating that the matrix is softer at the highest frequency studied. As it has been mentioned, the plasma-treatment promoted a viscous-like, less solid structure due to effects of the water removal and the thin coating deposited on the granules, being more notorious for starch with 70 % amylose treated for 30 min where the difference between the dynamic moduli was narrow, indicating that the elastic component (G′ ) was slightly higher than loss modulus (G′′ ).
Fig. 5. Proposed system of native and plasma-modified starch FFS.
In addition, Fig. 4b shows that the k values in the control samples decreased when amylose content increased, indicating that the system flows more easily. In starch with 30 % amylose with no plasma treat ment (S30-C), a higher consistency index was observed, which suggest that the amylopectin released from starch granules reinforced the amylose network. After its modification, the k values decreased around 3 folds. The starch with 50 % amylose was the most affected solution as the plasma treatment for 30 min decreased 18 folds compared to its control, suggesting that the less amylopectin released from starch granules promoted an easier ordering of FFS and less resistance to rearrangement of the network. Moreover, the k values of starch with 70 % decreased two-folds after treatment for 30 min. Also, similar behavior was reported in corn starch using air plasma, where the coefficient k was reduced after the treatment, indicating its change from a pseudo-plastic fluid to a Newtonian fluid (Bie et al., 2016). 3.3.2. Frequency sweep As Chen et al. (2009) mentioned, to understand the network struc ture in the FFS, small deformation rheological properties should be measured using oscillatory testing. In this study, the FFS did not show high viscosity during the preparation facilitating the removing of air bubbles and casting in thin layers (Chen et al., 2009; Cuq et al., 1995). This behavior allowed the organization of the starch molecules after 48 h at room temperature, before measurements. Fig. 6 shows the behavior of the storage modulus (G′ – Fig. 6a) and loss modulus (G′′ – Fig. 6b) from the frequency sweeps of the FFS as a function of amylose content and plasma-treatment time. In most of the FFS, G′ was higher
3.3.3. Creep recovery In the creep test, an instantaneous stress was applied to samples of FFS stored previously for 48 h and the changes in strain was observed over time. When the stress is released, some recovery may be observed while the material returns to the original shape. This test is useful in studying the behavior under constant stress environments, such as those found in leveling or coating applications (Steffe, 1996). As FFS could be used for food packaging applications in form of films or coatings, creep 5
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Fig. 6. Behavior of dynamic moduli, a) storage modulus (G′ ), b) loss modulus (G′′ ), c) complex viscosity (η*) and d) Tan delta of film-forming solutions with different amylose content and plasma-treated at different times.
recovery behavior was assessed, and it is shown in Fig. 7. Almost all FFS samples showed a viscoelastic behavior exhibiting a nonlinear response to strain. This behavior indicates the ability to recover some structure by storing energy, displaying a permanent deformation but less than the total deformation applied (Steffe, 1996). The S50 sample had the greatest deformation in the native form. This could be attributed to the matrix of amylose which contained swollen remnant starch granules that favored the entanglement; however, this behavior decreased after plasma treatment. The highest deformation was observed in S70 indi cating that the matrix is softer, probably due to the low interaction between the branched amylopectin and lineal amylose chains to conform an entanglement, as a result of amylose chains fragmentation during the treatment (Sifuentes-Nieves et al., 2020), however, S70T30 showed a steady flow, similar to an ideal viscous material. This sample had a linear response to stress with no recovery at any of the applied deformation, as it was noted by the G′ and G′′ values. Table 1 shows the rheological parameters calculated from the Bur gers model for the FFS samples. The instantaneous elastic modulus (G0 = 1/J0) reflects the minimally disturbed sample with most struc tural bonds intact, while the μ0 represents the state of material in flow with structural bonds broken (Noosuk, Hill, Farhat, Mitchel, & Pradi pasena, 2005). The FFS presented values of limiting viscosity at zero shear rate (μ0) between 7528.86 and 36262.56. S30 solutions reached
the highest values, and the lower values were obtained for S70. A slight increase was observed after 10 min of plasma treatment for the sample with higher amylose content. That was not the case for S30, which reached the highest value after 30 min of plasma treatment (S30T30). In this gel, the amylose leaches out was limited by the HMDSO coating resulting in the increased the firmness within the remnant swollen granules and promoted the in teractions between close-packed granules. Also, this behavior promoted the increase of gelatinization temperature and enthalpy values reported elsewhere (Sifuentes-Nieves et al., 2020). Moreover, G0 decreased as the amylose content increased, maintaining the same trend as G’. The λrel retardation time is unique for any substance. If a material is a Hookean solid, the retardation time would be zero and the maximum strain would be obtained immediately when the stress is applied, so the time to achieve maximum strain in viscoelastic materials is delayed. As shown in Table 1, at higher amylose content, the mobility of starch chains decreased when an external stress was applied (Li, Li, Wang, & Adhikari, 2015). All FFS samples presented an instantaneous compliance greater than the retarded compliance (J0 > J1) indicating that the gels need to deform initially before flowing. S70 presented the higher values in J0, which is correlated to the less recovery (Jimenez-Avalos, Ramos-Ramırez, & Salazar-Montoya, 2005). 6
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3.4. Film analysis 3.4.1. FFS capacity to form films The FFS made from modified starches treated during 30 min showed a thin consistency in comparison with other processed FFS. Low vis cosity facilitated its spreading on the plate where the films were formed. Also, no air bubbles formation was observed during the film processing. All films were successfully obtained and easily removed from the acrylic plates. Furthermore, the plasma-modified films showed a wellformed continuous matrix, with adequate structural integrity and ho mogeneous surface appearance compared to native films, which showed few surface bubbles on their surface. 3.4.2. Mechanical and barrier properties The mechanical properties of native and plasma-modified starch films were investigated. The maximum stress (MS) and Hencky strain (HS) are presented in Fig. 8. Regardless the time treatment, all plasmamodified films showed a decrease of the maximum stress (MS) values compared to those made from native starch (Fig. 8a). This behavior could be attributed to the decreasing of interactions between amylose chains by hydrogen bonds, which are responsible to the film rigidity (Muscat et al., 2012). In this regard, the decrement of amylose in teractions in the continuous matrix is associated with low amounts of this component leach out from the granule due to the HMDSO coating restriction, which limited the capability of linear amylose chains to interact through hydrogen bonds to a higher extent, and as a result, softer films with less organized structure were obtained. Moreover, the plasma-modified films showed a decreased of the Hencky strain (HS) values (Fig. 8b), suggesting that the water removal of starch granules promoted a lower plasticization effect in the films, as the treatment decreased the active sites available for the glycerol plastici zation, which limited the polymeric chains extensibility and provided lower flexibility to the films. The water vapor permeance (WVPe) of native and plasma-modified starches are shown in Fig. 8c. All plasma-modified films showed low values of WVPe, indicating the obtention of hydrophobic materials, especially after high time treatments (30 min). This behavior may be related to the coating formed on the granules surface after the plasma treatment (starch granules - Fig. 1), which, after the gelatinization process, were dispersed around the entire film (starch films - Figs. 1 and 5). As a result, a continuous hydrophobic matrix was obtained, which reduced the available spaces in the films and limited the water mole cules migration. Also, the remnant starch granules contributed to this end, as their high amylopectin content is associated with hydrophobic properties (Garcia-Hernandez, Vernon-Carter, & Alvarez-Ramirez, 2017).
Fig. 7. Creep-recovery behavior of film-forming solutions with different amylose content and plasma-treated at different times, a) normal starch, b) 50 % and c) 70) % of amylose. Table 1 Rheological parameters of the Burgers model of film-forming solutions with different amylose content and plasma-treated at different times at 25 ◦ C analyzed by creep measurement. Sample
J0 (Pa− 1) x 10−
S30-C S30T10 S30T30 S50-C S50T10 S50T30 S70-C S70T10 S70T30
2.69 2.46 1.07 2.71 2.61 1.39 31.20 15.83 n.d.
2
J1 (Pa− 1) x 10− 6.97 32.0 5.47 26.1 17.0 11.9 428.9 27.2 n.d.
3
μ0 (Pa⋅s)
λrel (s)
G0 (Pa)
20534 11644 36262 20749 27569 22330 7528 13965 n.d.
13.37 4.93 9.55 5.20 6.08 9.26 3.32 6.82 n.d.
37.22 40.71 93.54 36.86 37.83 72.20 3.21 6.32 n.d.
4. Conclusions Normal and high amylose starch granules showed morphological and structural changes after HMDSO plasma treatment (30 min), such as coating deposition on the surface granule and helical-water interruption of starch molecule, which modified the rheological properties (flow curves, frequency sweep and creep behavior) of its film-forming solu tions (FFS). The presence of HMDSO coating on the granule surface limited the amylopectin from leaching out to the continuous matrix, which reduced its reinforcement effect on the network, thus the viscosity decreased. Also, a gel-like behavior was observed when the amylose and time treatment increased, suggesting that the matrix becomes less elastic with softer entanglement. Furthermore, the rheological measurements were successfully used to predict the mechanical (softer and flexible) and barrier (low hydrophilicity) properties of the films. HMDSO plasma treatment is a promising technology for starch modification allowing to obtain FFS with adequate rheological performance and films with hy drophobic behavior.
Noosuk et al. (2005) mentioned that amylose is more effective in producing a high storage modulus than the swollen granules, as it forms strong structures in which the remnant granules are embedded. How ever, the FFS showed that much of the amylose was not leach out of the granule due to the HMDSO coating, which reinforced the granule as the amylopectin self-associates. These results suggest that an arrangement of polymers took place in different way and favoring the interaction between the starch structures, which gave adequate rheological per formance for film formation.
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Fig. 8. Mechanical and barrier properties of films made from native and plasma-modified starch containing 30, 50, and 70 % amylose. (a) maximum stress and (b) Hencky strain and c) water vapor permeance.
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Bie, P., Pu, H., Zhang, B., Su, J., Chen, L., & Li, X. (2016). Structural characteristics and rheological properties of plasma-treated starch. Innovative Food Science and Emerging Technologies, 34, 196–204. https://doi.org/10.1016/j.ifset.2015.11.019 Bogaerts, A., Neyts, E., Gijbels, R., & Van der Mullen, J. (2002). Gas discharge plasmas and their applications. Spectrochimica Acta - Part B Atomic Spectroscopy, 57(4), 609–658. https://doi.org/10.1016/S0584-8547(01)00406-2 Chen, C. H., Kuo, W. S., & Lai, L. S. (2009). Rheological and physical characterization of film-forming solutions and edible films from tapioca starch/decolorized hsian-tsao leaf gum. Food Hydrocolloids, 23, 2132–2140. https://doi.org/10.1016/j. foodhyd.2009.05.015 Cuq, B., Aymard, C., Cuq, J. L., & Guilbert, S. (1995). Edible packaging films based on fish myofibrillar proteins: Formulation and functional properties. Journal of Food Science, 60, 1369–1374. https://doi.org/10.1111/j.1365-2621.1995.tb04593 Din, Z., Xiong, H., & Fei, P. (2017). Physical and chemical modification of starches: A review. Critical Reviews in Food Science and Nutrition, 57, 2691–2705. https://doi. org/10.1080/10408398.2015.1087379 Garcia-Hernandez, A., Vernon-Carter, E. J., & Alvarez-Ramirez, J. (2017). Impact of ghosts on the mechanical, optical, and barrier properties of corn starch films. Starch/ Starke, 1600308, 1–7. https://doi.org/10.1002/star.201600308 Jimenez-Avalos, H. A., Ramos-Ramırez, E. G., & Salazar-Montoya, J. A. (2005). Viscoelastic characterization of gum Arabic and maize starch mixture using the Maxwell model. Carbohydrate Polymers, 62, 11–18. https://doi.org/10.1016/j. carbpol.2005.07.007 Kaur, L., Singh, J., McCarthy, O. J., & Singh, H. (2007). Physico-chemical, rheological and structural properties of fractionated potato starches. Journal of Food Engineering, 82, 383–394. https://doi.org/10.1016/j.jfoodeng.2007.02.059 Kumar, R., & Khatkar, B. S. (2017). Thermal, pasting and morphological properties of starch granules of wheat (Triticum aestivum L.) varieties. Journal of Food Science and Technology, 54(8), 2403–2410. https://doi.org/10.1007/s13197-017-2681-x. In this issue.
Pablo Hernandez-Perez: Methodology. Pamela C. Flores-Silva: Writing - review & editing. Gonzalo Velazquez: Data curation. Eduardo Morales-Sanchez: Methodology. Oliverio Rodríguez´ndez: Validation. Ernesto Herna ´ ndez-Herna ´ndez: Investiga Ferna tion. Guadalupe Mendez-Montealvo: Conceptualization, WritingReviewing and Editing. Israel Sifuentes-Nieves: Original draft, Su pervision, Writing- Reviewing and Editing. Declaration of Competing Interest The authors have declared no conflicts of interest. Acknowledgements Author Sifuentes-Nieves Israel acknowledges a scholarship from CONACYT, Mexico. References Banura, S., Thirumdas, R., Kaurc, A., Deshmukhb, R. R., & Annapure, U. S. (2018). Modification of starch using low pressure radio frequency air plasma. LWT - Food Science and Technology, 89, 719–724. https://doi.org/10.1016/j.lwt.2017.11.056
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Carbohydrate Polymers xxx (xxxx) xxx alternative to modify structural properties of granular starch. International Journal of Biological Macromolecules, 144, 682–689. https://doi.org/10.1016/j. ijbiomac.2019.12.111 Singh, N., Singh, J., Kaur, L., Sodhi, N. S., & Gill, B. S. (2003). Morphological, thermal and rheological properties of starches from different botanical sources. Food Chemistry, 81, 219–231. https://doi.org/10.1016/S0308-8146(02)00416-8 Steffe, J. F. (1996). Rheological methods in the food process engineering. East Lancing: Freeman Press. Tecante, A., & Doublier, J. L. (1999). Steady flow and viscoelastic behaviour of crosslinked waxy corn starch-k-carrageenan pastes and gels. Carbohydrate Polymers, 40, 221–231. https://doi.org/10.1016/s0144-8617(99)00057-0 Thirumdas, R., Kadam, D., & Annapure, U. S. (2017). Cold plasma: An alternative technology for the starch modification. Food Biophysics, 12, 129–139. https://doi. org/10.1007/s11483-017-9468-5 Thirumdas, R., Trimukhe, A., Deshmukh, R. R., & Annapure, U. S. (2017). Functional and rheological properties of cold plasma treated rice starch. Carbohydrate Polymers, 157, 1723–1731. https://doi.org/10.1016/j.carbpol.2016.11.050 Xie, F., Long, Y., Bing, S., Peng, L., Jun, W., Hongshen, L., et al. (2009). Rheological properties of starches with different amylose/amylopectin ratios. Journal of Cereal Science, 49, 371–377. https://doi.org/10.1016/j.jcs.2009.01.002 Zavareze, R. E., & Dias, G. R. A. (2011). Impact of heat-moisture treatment and annealing in starches: A review. Carbohydrate Polymers, 83, 317–328. https://doi.org/10.1016/ j.carbpol.2010.08.064 Zhang, B., Xiong, S., Li, X., Li, L., Xie, F., & Chen, L. (2014). Effect of oxygen glow plasma on supramolecular and molecular structures of starch and related mechanism. Food Hydrocolloids, 37, 69–76. https://doi.org/10.1016/j.foodhyd.2013.10.034
Li, M., Li, D., Wang, L. J., & Adhikari, B. (2015). Creep behavior of starch-based nanocomposites films with cellulose nanofibrils. Carbohydrate Polymers, 117, 957–963. https://doi.org/10.1016/j.carbpol.2014.10.023 Muscat, D., Adhikari, B., Adhikari, R., & Chaudhary, D. S. (2012). Comparative study of film forming behaviour of low and high amylose starches using glycerol and xylitol as plasticizers. Journal of Food Engineering, 109, 189–201. https://doi.org/10.1016/j. jfoodeng.2011.10.019 Noosuk, P., Hill, S. E., Farhat, I. A., Mitchel, J. R., & Pradipasena, P. (2005). Relationship between viscoelastic properties and starch structure in rice from Thailand. Starch/ Starke, 57, 587–598. https://doi.org/10.1002/star.200500401 Okechukwu, P. E., & Rao, M. A. (1997). Calorimetric and rheological behavior of cowpea protein plus starch (cowpea and corn) gels. Food Hydrocolloids, 11, 339–345. https:// doi.org/10.1016/S0268-005X(97)80064-1 Romero-Bastida, C. A., Bello-Perez, L. A., Garcia, M. A., Martino, M. N., Solorza-Feria, J., & Zaritzky, N. E. (2005). Physicochemical and microstructural characterization of films prepared by thermal and cold gelatinization from non-conventional sources of starches. Carbohydrate Polymers, 60, 235–244. https://doi.org/10.1016/j. carbpol.2005.01.004 Saha, D., & Bhattacharya, S. (2010). Hydrocolloids as thickening and gelling agents in food: A critical review. Journal of Food Science and Technology-Mysore, 47, 587–597. https://doi.org/10.1007/s13197-010-0162-6 Sarangapani, C., Devi, R. Y., Thirumdas, R., Trimukhe, A. M., Deshmukh, R. R., & Annapure, U. S. (2017). Physico-chemical properties of low-pressure plasma treated black gram. LWT - Food Science and Technology, 79, 102–110. https://doi.org/ 10.1016/j.lwt.2017.01.017 Sifuentes-Nieves, I., Velazquez, G., Flores-Silva, P. C., Hern´ andez-Hern´ andez, E., NeiraVel´ azquez, G., Gallardo-Vega, C., et al. (2020). HMDSO plasma treatment as
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Contents lists available at ScienceDirect
Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Rheological performance of film-forming solutions made from plasma-modified starches with different amylose/amylopectin content Pablo Hernandez-Perez a, Pamela C. Flores-Silva a, Gonzalo Velazquez a, ´ndez b, Ernesto Herna ´ndez-Herna ´ndez b, Eduardo Morales-Sanchez a, Oliverio Rodríguez-Ferna Guadalupe Mendez-Montealvo a, *, Israel Sifuentes-Nieves b, * a
Instituto Polit´ecnico Nacional, Centro de Investigaci´ on en Ciencia Aplicada y Tecnología Avanzada, Cerro Blanco No. 141, Col. Colinas del Cimatario, C.P. 76090, Santiago de Quer´etaro, Quer´etaro, Mexico Centro de Investigaci´ on en Química Aplicada, Blvd. Enrique Reyna No. 140, C.P. 25253, Saltillo, Coahuila, Mexico
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma Film-forming solutions Viscoelasticity Creep-recovery behavior HMDSO coating
Normal and high amylose corn starches were modified using HMDSO plasma at different time treatments. Changes in functional properties of starch granule, film-forming solutions (FFS) and films were investigated. SEM analysis revealed HMDSO coating deposition on the granule surface, which limited the amylopectin leach out from the granules to the continuous matrix, affecting the rheological properties of the FFS. The amylopectin restriction resulted in a low reinforcement of the network decreasing the viscosity as indicated by n and k values. Also, a gel-like behavior (G′ > G′′ ) was observed when the amylose and time treatment increased, suggesting that the matrix becomes less elastic with softer entanglement. This behavior was confirmed by creep test and Burger model parameters. The plasma treatments allowed obtaining FFS with low viscosity, suitable for developing soft and hydrophobic films with low flexibility, as indicated by the decrease of the maximum stress, Hencky strain and permeance values.
1. Introduction Rheological measurements have been employed to predict the quality and texture of starchy foods during their processing and storage. These measurements are also useful to explain the behaviour of starchbased materials in other applications such as films, paper, textile, pharmaceutical and cosmetic (Saha & Bhattacharya, 2010). The rheo logical behaviour of gelatinized starch is influenced by the granular size, swelling power and amylose-amylopectin entanglement (Kaur, Singh, McCarthy, & Singh, 2007). Xie et al. (2009) studied the rheological properties of various native starches with different amylose content (0–80 %) and reported high viscosity and less Newtonian behaviour in high amylose starches due to its higher gelatinization temperature and greater molecular entanglements between linear polymer chains. Native starch, regardless its amylose content, does not always have the appropriated functional properties and rheological performance for in dustrial applications. To overcome this drawback, chemical, enzymatic and physical modification have been employed to enhance the
functionality of starch molecule (Din, Xiong, & Fei, 2017). Among these, physical treatments like ultrasound, irradiation and cold plasma are widely used because they are considered as safe since no chemicals or enzymes are left in the final product; besides, they are cost-effective and environmentally friendly (Zavareze & Dias, 2011). Cold plasma is a green technology and a physical method used to obtain modified starches with no toxic residues. The starch modification depends on the plasma reactor employed and active species reactivity, being the corona discharge, dielectric barrier discharges, radio fre quency plasma and the gliding arc discharge the most employed (Bogaerts, Neyts, Gijbels, & Van der Mullen, 2002). In this regard, the active species of plasma are able to collapse hydroxyl groups of starch molecule and lead to chemical changes on its supramolecular structure, resulting in important modifications of the thermal and rheological properties; which have been attributed to crosslinking, depolymeriza tion and etching phenomena (Thirumdas, Kadam, & Annapure, 2017). Rheological properties of normal-native starch after plasma treatment have been reported. Bie et al. (2016) found that the plasma-modified
Abbreviations: HMDSO, Hexamethyldisiloxane. * Corresponding authors. E-mail addresses: [email protected] (G. Mendez-Montealvo), [email protected] (I. Sifuentes-Nieves). https://doi.org/10.1016/j.carbpol.2020.117349 Received 21 May 2020; Received in revised form 22 September 2020; Accepted 28 October 2020 Available online 2 November 2020 0144-8617/© 2020 Published by Elsevier Ltd.
Please cite this article as: Pablo Hernandez-Perez, Carbohydrate Polymers, https://doi.org/10.1016/j.carbpol.2020.117349
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starch paste tended to change from a pseudo-plastic (non-Newtonian) fluid to a Newtonian fluid, and the viscosity decreased as a result of the degradation of starch molecules. In addition, Banura, Thirumdas, Kaurc, Deshmukhb and Annapure (2018) reported that plasma treatment led to starch depolymerization which resulted in the formation of simple sugars (glucose and maltose) formation and in an increase of viscosity by the leaching of particles from the starch granules. Recently, we evaluated the effect of hexamethyldisiloxane (HMDSO) plasma on the structure of corn starches with different amylose content (Sifuentes-Nieves et al., 2020). The thermal (gelatinization and enthalpy) and structural (crystallinity) properties of treated starch was inspected. However, the rheological performance of film-forming solu tions (FFS) made from plasma-modified starches after gelatinization process due to the amylose/amylopectin content was not studied and it may play an important role on the final characteristics of the starch-based films. Our hypothesis was that amylose content and plasma treatment time determine the rheological properties of the FFS. Furthermore, the influence of FFS on the barrier and mechanical prop erties of the final films was inspected.
adhesive carbon tape and the images were obtained at a magnification of 4000£ (starch granules) and 2500× (starch films). 2.6. FTIR analysis The presence or absence of specific chemical groups of the samples was inspected by Fourier transform infrared (FTIR) spectra using a spectrophotometer (Bruker Tensor 37, USA) with an attenuated total reflectance (ATR) attachment. The spectra of each sample were recorded in the range from 4000 to 650 cm− 1 at a resolution of 1 cm− 1. 2.7. Rheological measurements The rheological properties of the FFS from normal and high amylose starches were carried out in an Anton Paar rheometer (Physica MCR 101) using a plate-plate geometry of 20 mm with a gap of 1 mm. 2.7.1. Determination of flow curves Rotational tests were performed to describe the flow curves. The FFS solution was placed in the rheometer and cooled from 60 to 25 ◦ C at 2.5 ◦ C/min and a shear rate of 50 s− 1. At the final temperature, two measurement cycles were performed, upward and downward from 0.01 to 100 s− 1. Mineral oil was placed around the plates to prevent water evaporation during the measurement. The flow behavior was deter mined in the second cycle. The power law model of Ostwald-de Waele (Eq. 1) was used to describe the flow curves.
2. Materials and methods 2.1. Materials Corn starch with 30 % (normal — S30-C), 50 % (Hylon V — S50-C) and 70 % (Hylon VII — S70-C) amylose content were acquired from Ingredion (formerly, Corn Products and National Starch at Bridgewater, NJ). Glycerol (G7757) and HMDSO (205389) was purchased from Sigma-Aldrich (Saint Louis, MO, USA).
τ = kγ˙n
(1)
Where: τ is the shear stress (Pa), k is the consistency index (Pa.sn), γ˙ is the shear rate (s− 1) and n is the flow behavior index (dimensionless) (Steffe, 1996; Tecante & Doublier, 1999). Determinations were carried out in triplicate.
2.2. HMDSO plasma treatment on starches Normal and high amylose corn starches were modified in a rotatory cylindrical glass reactor (0.5 L), which was connected to a vacuum pump and a radio frequency (RF) power generator operating at 13.56 MHz; The pressure inside the reactor was fixed at 0.4 mbar and after HMDSO injection, the pressure was maintained at 0.45 mbar, allowing a constant flow of HMDSO gas of 0.35 sccm into the reactor. The samples were treated with HMDSO monomer during 10 and 30 min, and a power level of 90 W was used to achieve the plasma formation. The rotatory reactor kept the samples in agitation (20 rpm) during all plasma treatment, which led to a homogeneous modification throughout all starch granules.
2.7.2. Dynamic rheological measurements For the oscillatory measurements, the FFS solutions were stored at room temperature (23 ± 2 ◦ C) for 48 h in a petri plate. After that time, the frequency sweeps were performed at 25 ◦ C), applying a constant stress of 5 MPa once the linear viscoelastic region (LVR) was allocated. The frequency sweeps were performed at an interval of 0.01–10 Hz. The storage modulus (G′ ), loss modulus (G′′ ), complex viscosity (η*, Pa.s) and tan delta (Tan δ) were calculated from each test. 2.7.3. Creep test After storing the samples at room temperature for 48 h, the cr(Kumar & Khatkar, 2017)eep-recovery test was performed at 25 ◦ C by measuring the deformation while applying a constant stress of 5 MPa for 100 s and the recovery was measured for another 100 s. The data ob tained were adjusted to the Burgers model (Equation 2) to calculate the limiting viscosity at zero shear rate, μ0 (Pa*s), instantaneous compli ance, J0 (Pa− 1), retarded compliance, J1 (Pa− 1), and relaxation time, λrel (s). ( ( )) − t t J = J0 + J1 1 − exp + (2) λret μ0
2.3. Preparation of film-forming solutions Film-forming solutions (FFS) were prepared using a 1 g-sample of starch and 30 % w/w of glycerol, which were added in 25 mL of distilled water. To achieve the gelatinization and a uniform aqueous dispersion, the normal starch was gelatinized using a stirred hot plate (Corning, Model PC-220, USA) at 90 ◦ C during 10 min and the high amylose starches were gelatinized using an autoclave (LS-B75 L, NANBEI, China) at 121 ◦ C for 30 min. 2.4. Films elaboration The gelatinized FFS were immediately poured on acrylic plates of 20 × 20 cm and dried at 60 ◦ C for 6 h in a conventional oven (IBTF050). Finally, the films were stored until further analysis.
2.8. Mechanical properties The Hencky strain and the maximum stress (MPa) at the breaking point were evaluated at room temperature. Film samples (10 × 100 mm) were cut and conditioned at 53 % RHeq for 5 days at 30 ◦ C employing a saturated solution of sodium bromide. A texture analyzer TA-XT (TA Plus, Lloyd Instruments) equipped with mechanical grips was used at a cross-head speed of 1 mm/s and an initial separation of the grips of 50 mm. Ten replicates were tested for each film. The maximum stress was calculated dividing the maximum force by the
2.5. Scanning electronic microscopy (SEM) Scanning electron micrographs of modified starch granules and of the films made from plasma-modified starches were obtained using the Phenom-World microscope (Eindhoven, Netherlands) operated at an accelerating voltage of 5 kV. Samples were adhered to a double-sided 2
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transversal area at the breaking point and the Hencky strain was calculated from the maximum deformation of the film.
granule surface were observed, such as fissures or cavities and superfi cial deposition (coatings) on them (starch granules - Fig. 1). These changes can be attributed to etching and deposition phenomena caused by the reactive species of the treatment, as a result of plasma molecules penetration at molecular level or coating deposition of HMDSO on the granule surface. A similar behavior was found by Thirumdas, Trimukhe, Deshmukh and Annapure (2017) in cold plasma-treated rice starch. The authors reported that the fissures have an important effect on the solu bility and rheological performance starch molecules.
2.9. Water vapor permeance (WVPe) The permeance of the films was determined using the gravimetric technique following the ASTM E96-00 method. Glass cell (50 mm height and 40 mm diameter) containing distilled water was sealed with the films using silicon gaskets allowing an effective transfer area of 0.0013 m2. The glass cell was then placed on a balance plate inside of a temperature-controlled (30 ◦ C) chamber containing silica gel. The weight was automatically recorded every 1 min for 6 h using an analytical balance with 0.0001 g precision. The permeance was calcu lated using the Eq. (3).
3.2. FTIR analysis The FTIR spectra of native and plasma-modified starches are shown in Fig. 2. No new signal corresponding to HMDSO was observed in the plasma-modified starches, suggesting that the treatment only promoted the interaction between active species of plasma and the functional groups of starch. However, some changes in specific bands were observed. In this regard, all samples showed the wide band at 3280 cm− 1 corresponding to –OH. After normalizing the spectra using the 1082 cm− 1 band, the band at 3280 cm− 1 showed lower intensity ratio in the plasma-modified starches after 30 min of treatment, suggesting that the active species of plasma changed the starch ability to maintain bound water, probably due to the barrier effect of the HMDSO coat formed on the surface of the granule during the treatment. Also, the vacuum used for the plasma treatment may have triggered the evapo ration of water molecules from the starch granules (Sarangapani et al., 2017). The intensity band at 1648 cm-1 assigned to tightly bound water in the starch, was also weakened by the treatment after 30 min indi cating that the vacuum used during treatment promoted removal of some water molecules from the starch. This behavior can be attributed to the morphological changes observed in the treated starch granules
(3)
Permeance = Q/AΔP = g /m2 s Pa − 1
Where Q is the weight loss of the cell per unit of time (g⋅day ), A is the transfer area (m2), t is the time (days), and ΔP is the water vapor pressure gradient (4245 Pa) created by the distilled water and the silica at 30 ◦ C. 3. Results and discussion 3.1. Scanning electronic microscopy Scanning electronic microscopy was used to inspect the differences between starch granules before and after HMDSO plasma treatment. All starches showed granules with different shape and size. According to Singh, Singh, Kaur, Sodhi and Gill (2003) normal (S30-C) corn starch is characterized by round granules from 5 to 25 μm and high amylose starches (S50-C & S70-C) present round and some elongated granules (2− 30 μm). After HMDSO plasma treatment some differences on the
Fig. 1. SEM images of plasma-modified corn starches and films with different amylose content. 3
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enthalpy values due to the crosslinking of the starch molecule and the coating formed on the granule surface, as well as a decrease on their relative crystallinity as a result of the amylose fragmentation (Sifuen tes-Nieves et al., 2020). Therefore, it is important to characterize the rheological properties of FFS to understand the features of these systems, since gelatinization and drying conditions during the making of starch-based films strongly affects the network structure, physical properties and film performance (Chen, Kuo, & Lai, 2009; Romero-Bastida et al., 2005). 3.3.1. Flow curves Characterizing the flow behavior of the FFS is an important step when the solutions are used for casting process of pre-formed films or when applying a liquid coating directly on the surface of food products by dipping, brushing or spraying. It has been reported that high viscosity of gel type structures in the film-forming solution would make it difficult to eliminate air bubbles and hinder casting in thin layers (Cuq, Aymard, Cuq, & Guilbert, 1995). In the rotational test applied to the FFS, the power law model showed a good fitting to describe the rheograms (Fig. 3). The flow index behavior (n) and consistency index (k) parameters were estimated with R2 > 0.9. The n values lower than one (n<1) indicated a non-Newtonian behavior, showing a shear-thinning behavior. In these cases, the vis cosity decreased when the shear rate increased, as it was observed in the flow curves (Fig. 3). The n values increased as the plasma treatment time increased (30 min), showing less dependence in the shear rate (Fig. 4a). This trend was more noticeable in starches with 50 % amylose. The results suggest that the reorganization of the film-forming solutions is influenced not only by the amylose content, but also by HMDSO coating deposition on the granule surface and the helical-water interruption promoted by the plasma treatment, which favored the fluidity of FFS. According to (Kumar & Khatkar, 2017)Kumar and Khatkar (2017), higher viscosity (low n values) suggests a reinforcement effect of the network by the branched amylopectin molecule. However, the increase of the n values of FFS could be related to less amylopectin interactions in the network, as the HMDSO coating deposited on the granule could limit the amylopectin molecules leaching out to the system (Fig. 5), reducing the interactions between amylose chains and the amylopectin mole cules, which resulted in a viscosity decrease. A similar restraining effect of the HMDSO coating on the starch components was observed in the gelatinization behavior of plasma-modified starches, as the temperature and enthalpy of gelatinization increased, suggesting a granule rein forcement by HMDSO coating (Sifuentes-Nieves et al., 2020).
Fig. 2. FTIR spectra of native and plasma-modified corn starches with different amylose content.
(Fig. 1), as the cavities or fissures on the granules allowed high reactivity between the species of plasma and starch molecules resulting in depo lymerization and interruption of the helical-water bonds (Zhang et al., 2014). 3.3. Rheology of film-forming solutions (FFS) Nowadays, the trend in industry is to formulate bio-based materials for packaging. One important point is that these materials must include at least one component that forms a cohesive and continuous matrix. Starches have been studied as a main ingredient in film-forming solu tions (FFS) and modified starches are a feasible alternative to improve the film properties. As previously described in SEM and FTIR analysis, plasma-modified corn starches with different amylose content (S30, S50 and S70) at different treatment times (0, 10 and 30 min) showed some cavities and/or coating deposition on the granules surfaces; which resulted in removal of some water molecules as the helical-water bonds of starch were broken by active species of plasma. Furthermore, no new functional groups of HMDSO plasma were incorporated in the starch molecule as indicated by FTIR analysis. In addition, these modified starches showed an increase in the gelatinization temperature and
Fig. 3. Flow curves of the film-forming solutions made from native and plasma-modified corn starches with different amylose content. 4
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Fig. 4. Parameters of the Ostwald-de Waele (power law) model for the film-forming solutions made from native and plasma-modified corn starches with different amylose content. a) flow index behavior (n) and b) consistency index (k).
than G′′ , indicating a gel-like behavior (Okechukwu & Rao, 1997). The dynamic moduli (G′ and G′′ ) showed slight dependence on frequency (data not shown), indicating a “true gel” behavior. G′ was practically independent on the frequency. Starch with 30 % amylose show slight difference for G′ values when the starch was plasma-treated as the gel-like behavior was more pronounced. This behavior could be due to amylopectin contributions and remnant starch granules as the swollen starch granules promoted a better structure that could result in a rein forcement in the matrix, allowing greater interaction and favoring the conformation of a more elastic system (more solid-like). The G′ values decreased when increasing the amylose content (Fig. 6a) suggesting that the matrix becomes less elastic or the entan glement was softer. Also, at higher treated times, the gels are well conformed, but they are less rigid or compact (as it is noted in Fig. 6b, where G′′ decreased). Moreover, the complex viscosity (η*) decreased considerably when the amylose content and the plasma-treated time increased (Fig. 6c). According to Muscat, Adhikari, Adhikari & Chaudhary (2012), the viscous nature of FFS increases as a function of the plasticizer content. However, an opposite behavior was found in the plasma modified FFS, as the water removal could limit the glycerol plasticization in the binding sites of starch, and as a result, the FFS viscosity decreased. Measuring changes in Tan δ is useful to evaluate the transition from liquid-like to solid-like behavior as Tan delta decreases with a more solid-like behavior (Steffe, 1996). This trend is observed in Fig. 6d. Starch with 30 % amylose showed a better entanglement than starches with 50 and 70 % amylose. These FFS samples with treatment of 30 min showed higher values in Tan δ, indicating that the matrix is softer at the highest frequency studied. As it has been mentioned, the plasma-treatment promoted a viscous-like, less solid structure due to effects of the water removal and the thin coating deposited on the granules, being more notorious for starch with 70 % amylose treated for 30 min where the difference between the dynamic moduli was narrow, indicating that the elastic component (G′ ) was slightly higher than loss modulus (G′′ ).
Fig. 5. Proposed system of native and plasma-modified starch FFS.
In addition, Fig. 4b shows that the k values in the control samples decreased when amylose content increased, indicating that the system flows more easily. In starch with 30 % amylose with no plasma treat ment (S30-C), a higher consistency index was observed, which suggest that the amylopectin released from starch granules reinforced the amylose network. After its modification, the k values decreased around 3 folds. The starch with 50 % amylose was the most affected solution as the plasma treatment for 30 min decreased 18 folds compared to its control, suggesting that the less amylopectin released from starch granules promoted an easier ordering of FFS and less resistance to rearrangement of the network. Moreover, the k values of starch with 70 % decreased two-folds after treatment for 30 min. Also, similar behavior was reported in corn starch using air plasma, where the coefficient k was reduced after the treatment, indicating its change from a pseudo-plastic fluid to a Newtonian fluid (Bie et al., 2016). 3.3.2. Frequency sweep As Chen et al. (2009) mentioned, to understand the network struc ture in the FFS, small deformation rheological properties should be measured using oscillatory testing. In this study, the FFS did not show high viscosity during the preparation facilitating the removing of air bubbles and casting in thin layers (Chen et al., 2009; Cuq et al., 1995). This behavior allowed the organization of the starch molecules after 48 h at room temperature, before measurements. Fig. 6 shows the behavior of the storage modulus (G′ – Fig. 6a) and loss modulus (G′′ – Fig. 6b) from the frequency sweeps of the FFS as a function of amylose content and plasma-treatment time. In most of the FFS, G′ was higher
3.3.3. Creep recovery In the creep test, an instantaneous stress was applied to samples of FFS stored previously for 48 h and the changes in strain was observed over time. When the stress is released, some recovery may be observed while the material returns to the original shape. This test is useful in studying the behavior under constant stress environments, such as those found in leveling or coating applications (Steffe, 1996). As FFS could be used for food packaging applications in form of films or coatings, creep 5
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Fig. 6. Behavior of dynamic moduli, a) storage modulus (G′ ), b) loss modulus (G′′ ), c) complex viscosity (η*) and d) Tan delta of film-forming solutions with different amylose content and plasma-treated at different times.
recovery behavior was assessed, and it is shown in Fig. 7. Almost all FFS samples showed a viscoelastic behavior exhibiting a nonlinear response to strain. This behavior indicates the ability to recover some structure by storing energy, displaying a permanent deformation but less than the total deformation applied (Steffe, 1996). The S50 sample had the greatest deformation in the native form. This could be attributed to the matrix of amylose which contained swollen remnant starch granules that favored the entanglement; however, this behavior decreased after plasma treatment. The highest deformation was observed in S70 indi cating that the matrix is softer, probably due to the low interaction between the branched amylopectin and lineal amylose chains to conform an entanglement, as a result of amylose chains fragmentation during the treatment (Sifuentes-Nieves et al., 2020), however, S70T30 showed a steady flow, similar to an ideal viscous material. This sample had a linear response to stress with no recovery at any of the applied deformation, as it was noted by the G′ and G′′ values. Table 1 shows the rheological parameters calculated from the Bur gers model for the FFS samples. The instantaneous elastic modulus (G0 = 1/J0) reflects the minimally disturbed sample with most struc tural bonds intact, while the μ0 represents the state of material in flow with structural bonds broken (Noosuk, Hill, Farhat, Mitchel, & Pradi pasena, 2005). The FFS presented values of limiting viscosity at zero shear rate (μ0) between 7528.86 and 36262.56. S30 solutions reached
the highest values, and the lower values were obtained for S70. A slight increase was observed after 10 min of plasma treatment for the sample with higher amylose content. That was not the case for S30, which reached the highest value after 30 min of plasma treatment (S30T30). In this gel, the amylose leaches out was limited by the HMDSO coating resulting in the increased the firmness within the remnant swollen granules and promoted the in teractions between close-packed granules. Also, this behavior promoted the increase of gelatinization temperature and enthalpy values reported elsewhere (Sifuentes-Nieves et al., 2020). Moreover, G0 decreased as the amylose content increased, maintaining the same trend as G’. The λrel retardation time is unique for any substance. If a material is a Hookean solid, the retardation time would be zero and the maximum strain would be obtained immediately when the stress is applied, so the time to achieve maximum strain in viscoelastic materials is delayed. As shown in Table 1, at higher amylose content, the mobility of starch chains decreased when an external stress was applied (Li, Li, Wang, & Adhikari, 2015). All FFS samples presented an instantaneous compliance greater than the retarded compliance (J0 > J1) indicating that the gels need to deform initially before flowing. S70 presented the higher values in J0, which is correlated to the less recovery (Jimenez-Avalos, Ramos-Ramırez, & Salazar-Montoya, 2005). 6
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3.4. Film analysis 3.4.1. FFS capacity to form films The FFS made from modified starches treated during 30 min showed a thin consistency in comparison with other processed FFS. Low vis cosity facilitated its spreading on the plate where the films were formed. Also, no air bubbles formation was observed during the film processing. All films were successfully obtained and easily removed from the acrylic plates. Furthermore, the plasma-modified films showed a wellformed continuous matrix, with adequate structural integrity and ho mogeneous surface appearance compared to native films, which showed few surface bubbles on their surface. 3.4.2. Mechanical and barrier properties The mechanical properties of native and plasma-modified starch films were investigated. The maximum stress (MS) and Hencky strain (HS) are presented in Fig. 8. Regardless the time treatment, all plasmamodified films showed a decrease of the maximum stress (MS) values compared to those made from native starch (Fig. 8a). This behavior could be attributed to the decreasing of interactions between amylose chains by hydrogen bonds, which are responsible to the film rigidity (Muscat et al., 2012). In this regard, the decrement of amylose in teractions in the continuous matrix is associated with low amounts of this component leach out from the granule due to the HMDSO coating restriction, which limited the capability of linear amylose chains to interact through hydrogen bonds to a higher extent, and as a result, softer films with less organized structure were obtained. Moreover, the plasma-modified films showed a decreased of the Hencky strain (HS) values (Fig. 8b), suggesting that the water removal of starch granules promoted a lower plasticization effect in the films, as the treatment decreased the active sites available for the glycerol plastici zation, which limited the polymeric chains extensibility and provided lower flexibility to the films. The water vapor permeance (WVPe) of native and plasma-modified starches are shown in Fig. 8c. All plasma-modified films showed low values of WVPe, indicating the obtention of hydrophobic materials, especially after high time treatments (30 min). This behavior may be related to the coating formed on the granules surface after the plasma treatment (starch granules - Fig. 1), which, after the gelatinization process, were dispersed around the entire film (starch films - Figs. 1 and 5). As a result, a continuous hydrophobic matrix was obtained, which reduced the available spaces in the films and limited the water mole cules migration. Also, the remnant starch granules contributed to this end, as their high amylopectin content is associated with hydrophobic properties (Garcia-Hernandez, Vernon-Carter, & Alvarez-Ramirez, 2017).
Fig. 7. Creep-recovery behavior of film-forming solutions with different amylose content and plasma-treated at different times, a) normal starch, b) 50 % and c) 70) % of amylose. Table 1 Rheological parameters of the Burgers model of film-forming solutions with different amylose content and plasma-treated at different times at 25 ◦ C analyzed by creep measurement. Sample
J0 (Pa− 1) x 10−
S30-C S30T10 S30T30 S50-C S50T10 S50T30 S70-C S70T10 S70T30
2.69 2.46 1.07 2.71 2.61 1.39 31.20 15.83 n.d.
2
J1 (Pa− 1) x 10− 6.97 32.0 5.47 26.1 17.0 11.9 428.9 27.2 n.d.
3
μ0 (Pa⋅s)
λrel (s)
G0 (Pa)
20534 11644 36262 20749 27569 22330 7528 13965 n.d.
13.37 4.93 9.55 5.20 6.08 9.26 3.32 6.82 n.d.
37.22 40.71 93.54 36.86 37.83 72.20 3.21 6.32 n.d.
4. Conclusions Normal and high amylose starch granules showed morphological and structural changes after HMDSO plasma treatment (30 min), such as coating deposition on the surface granule and helical-water interruption of starch molecule, which modified the rheological properties (flow curves, frequency sweep and creep behavior) of its film-forming solu tions (FFS). The presence of HMDSO coating on the granule surface limited the amylopectin from leaching out to the continuous matrix, which reduced its reinforcement effect on the network, thus the viscosity decreased. Also, a gel-like behavior was observed when the amylose and time treatment increased, suggesting that the matrix becomes less elastic with softer entanglement. Furthermore, the rheological measurements were successfully used to predict the mechanical (softer and flexible) and barrier (low hydrophilicity) properties of the films. HMDSO plasma treatment is a promising technology for starch modification allowing to obtain FFS with adequate rheological performance and films with hy drophobic behavior.
Noosuk et al. (2005) mentioned that amylose is more effective in producing a high storage modulus than the swollen granules, as it forms strong structures in which the remnant granules are embedded. How ever, the FFS showed that much of the amylose was not leach out of the granule due to the HMDSO coating, which reinforced the granule as the amylopectin self-associates. These results suggest that an arrangement of polymers took place in different way and favoring the interaction between the starch structures, which gave adequate rheological per formance for film formation.
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Fig. 8. Mechanical and barrier properties of films made from native and plasma-modified starch containing 30, 50, and 70 % amylose. (a) maximum stress and (b) Hencky strain and c) water vapor permeance.
CRediT authorship contribution statement
Bie, P., Pu, H., Zhang, B., Su, J., Chen, L., & Li, X. (2016). Structural characteristics and rheological properties of plasma-treated starch. Innovative Food Science and Emerging Technologies, 34, 196–204. https://doi.org/10.1016/j.ifset.2015.11.019 Bogaerts, A., Neyts, E., Gijbels, R., & Van der Mullen, J. (2002). Gas discharge plasmas and their applications. Spectrochimica Acta - Part B Atomic Spectroscopy, 57(4), 609–658. https://doi.org/10.1016/S0584-8547(01)00406-2 Chen, C. H., Kuo, W. S., & Lai, L. S. (2009). Rheological and physical characterization of film-forming solutions and edible films from tapioca starch/decolorized hsian-tsao leaf gum. Food Hydrocolloids, 23, 2132–2140. https://doi.org/10.1016/j. foodhyd.2009.05.015 Cuq, B., Aymard, C., Cuq, J. L., & Guilbert, S. (1995). Edible packaging films based on fish myofibrillar proteins: Formulation and functional properties. Journal of Food Science, 60, 1369–1374. https://doi.org/10.1111/j.1365-2621.1995.tb04593 Din, Z., Xiong, H., & Fei, P. (2017). Physical and chemical modification of starches: A review. Critical Reviews in Food Science and Nutrition, 57, 2691–2705. https://doi. org/10.1080/10408398.2015.1087379 Garcia-Hernandez, A., Vernon-Carter, E. J., & Alvarez-Ramirez, J. (2017). Impact of ghosts on the mechanical, optical, and barrier properties of corn starch films. Starch/ Starke, 1600308, 1–7. https://doi.org/10.1002/star.201600308 Jimenez-Avalos, H. A., Ramos-Ramırez, E. G., & Salazar-Montoya, J. A. (2005). Viscoelastic characterization of gum Arabic and maize starch mixture using the Maxwell model. Carbohydrate Polymers, 62, 11–18. https://doi.org/10.1016/j. carbpol.2005.07.007 Kaur, L., Singh, J., McCarthy, O. J., & Singh, H. (2007). Physico-chemical, rheological and structural properties of fractionated potato starches. Journal of Food Engineering, 82, 383–394. https://doi.org/10.1016/j.jfoodeng.2007.02.059 Kumar, R., & Khatkar, B. S. (2017). Thermal, pasting and morphological properties of starch granules of wheat (Triticum aestivum L.) varieties. Journal of Food Science and Technology, 54(8), 2403–2410. https://doi.org/10.1007/s13197-017-2681-x. In this issue.
Pablo Hernandez-Perez: Methodology. Pamela C. Flores-Silva: Writing - review & editing. Gonzalo Velazquez: Data curation. Eduardo Morales-Sanchez: Methodology. Oliverio Rodríguez´ndez: Validation. Ernesto Herna ´ ndez-Herna ´ndez: Investiga Ferna tion. Guadalupe Mendez-Montealvo: Conceptualization, WritingReviewing and Editing. Israel Sifuentes-Nieves: Original draft, Su pervision, Writing- Reviewing and Editing. Declaration of Competing Interest The authors have declared no conflicts of interest. Acknowledgements Author Sifuentes-Nieves Israel acknowledges a scholarship from CONACYT, Mexico. References Banura, S., Thirumdas, R., Kaurc, A., Deshmukhb, R. R., & Annapure, U. S. (2018). Modification of starch using low pressure radio frequency air plasma. LWT - Food Science and Technology, 89, 719–724. https://doi.org/10.1016/j.lwt.2017.11.056
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