A new approach in the hydrophobic modification of polysaccharide-based edible films using structured oil nanoparticles (original) (raw)

A new approach in the hydrophobic modification of polysaccharide-based edible films using structured oil nanoparticles

Fatemeh Ghiasi, Mohammad-Taghi Golmakani*, Mohammad Hadi Eskandari, Seyed Mohammad Hashem Hosseini
Department of Food Science and Technology, School of Agriculture, Shiraz University, Shiraz, Iran

A R T I C L E I N F O

Keywords:
Farsi gum
Hydrophobic modification
Structured oil nanoparticles
Ethyl cellulose
Monoglyceride

ABSTRACT

This work aimed to study the effect of structured oil nanoparticles (SONs) for improving the hydrophobicity of polysaccharide-based edible films as a novel modification approach. SONs were fabricated from sunflower oil (SFO) using ethyl cellulose or monoglyceride as gelator. Farsi gum (FG) was selected as a model polysaccharide to develop edible films. The physicochemical, mechanical, thermal, and morphological properties of FG-based films incorporated with different concentrations ( 0%,0.5%0 \%, 0.5 \%, and 1%(w/w)1 \%(\mathrm{w} / \mathrm{w}) ) of SONs were compared with those of SFO-free (control) and SFO-incorporated FG films. Irrespective of the physical nature of added oil, the water vapor permeability, solubility, swelling ability, and surface hydrophilicity of FG films incorporated with SONs or SFO were significantly decreased. The SFO was more effective than the SONs in improving some hydrophobic properties; however, the flocculation of SFO droplets decreased the water vapor barrier properties. Incorporating SONs resulted in a compact and more uniform surface morphology, while a higher roughness and surface irregularity were observed after modification with SFO. The modified films prepared by incorporating SONs showed appropriate tensile strength and higher flexibility than control film, while those incorporated with liquid SFO droplets were too sticky and stretchy to be subjected to the tensile forces. The thermal stability of modified films was enhanced after incorporating SONs. The results indicated that the key properties of FG films were better improved by incorporating SONs than liquid oil droplets. Therefore, the SONs can be considered as an appropriate candidate for developing novel edible food packaging materials with high moisture barrier characteristics.

1. Introduction

Packaging is an integral part of food industry which can effectively extend the shelf life and improve the safety and quality of food products. In the recent decades, considerable eff ;orts have been devoted to reducing petroleum-based polymers with bio-based ones (e.g., polysaccharides and proteins) for developing food packaging materials ( Li and Ye, 2017). Despite outstanding achievements regarding the development of biopolymer-based packaging materials with appropriate mechanical and optical properties, their applications are still challenging mainly due to their hydrophilic nature and poor water vaporbarrier characteristics (Li and Ye, 2017). To date, several routes have been recommended to improve the water resistance and moisture barrier properties of bio-based food packaging materials such as adding
hydrophobic ingredients (e.g., waxes, oils, and fatty acids) (Galus and Kadzińska, 2015), crosslinking (Giz et al., 2020), irradiation (Soliman et al., 2009), incorporating nanoscale fillers (Li and Ye, 2017) and the development of biopolymer composite films (Otoni et al., 2017). Among the above-mentioned strategies, incorporating hydrophobic ingredients is the common method for reducing the surface hydrophilicity of bio-based edible films for food packaging. However, lipid-incorporated films suffer from opaque appearance and poor mechanical properties (Galus and Kadzińska, 2016). Therefore, there is a challenge to find appropriate novel alternative hydrophobic ingredients for incorporating into bio-based networks to develop new and more efficient edible food packaging materials with high water vapor barrier and mechanical properties.

Structured oils (also known as oleogels) are relatively novel classes

[1]

1. Abbreviations: DDW, double distilled water; DSA, drop shape analyzer; DSC, differential scanning calorimetry; EAB, elongation at break; EC, ethyl cellulose; FG, Farsi gum; FTIR, Fourier transform infrared; MG, monoglyceride; SFO, sunflower oil; SONs, structured oil nanoparticles; TGA, thermogravimetric analysis; TS, tensile strength; WVP, water vapor permeability; YM, Young’s modulus
   • Corresponding author.
     E-mail address: golmakani@shirazu.ac.ir (M.-T. Golmakani). ↩︎

of lipid-based matrices and can be considered as a subclass of organogels (Calligaris et al., 2020; Co and Marangoni, 2012). Structured oils are composed of large amounts of edible oils in 3D entangled gel networks (Ghiasi et al., 2019). Structured oils are directly obtained by low molecular weight organogelators or polymeric ones (Patel and Dewettinck, 2016). Among the former group, monoglycerides (MGs) are considered as one of the most promising lipid-based gelling agents mainly due to their emulsifying properties and ability to develop a wide range of structural properties (Lupi et al., 2017). Among the latter group, the only approved food-grade polymer for direct oleogelation is ethyl cellulose (EC) (Zetzl et al., 2012). Generally, depending on the organogelator type, the required concentration and processing conditions for the structuring liquid oils are different. In this regard, MG and EC are usually dissolved in oil at 10%−15%10 \%-15 \% concentration and 75∘C75^{\circ} \mathrm{C} and 6%−10%6 \%-10 \% concentration and 130−160∘C130-160^{\circ} \mathrm{C}, respectively. At these critical points (above glass transition temperature), the gelator molecules unfold and then after cooling to room temperature, the structured oil is formed within a gel network mainly due to the intermolecular hydrogen bonds (Gravelle et al., 2013). Nanoparticles of structured oil can be considered as novel alternatives for solid lipid nanoparticles, nanostructured lipid carriers, polymeric nanoparticles, and inorganic nanoparticles to overcome their practical challenges in the food sector including physical instability, utilization of organic solvents and nonfood grade ingredients, limited loading capacity, sensitivity to changes in particle characteristics, lipid polymorphism, and affordability (Ghiasi et al., 2019; Katouzian et al., 2017; Zambrano-Zaragoza et al., 2018).

Today, food scientists are looking for the new sources of natural hydrocolloids which are safe, cheap, biodegradable, biocompatible, environmental-friendly, widely available, and sustainable to investigate their potentials compared to existing natural gums and mucilages. In this regard, Farsi gum (FG) is a relatively new and abundant hydrocolloid which can be considered as a promising source of natural polysaccharides with more practical applications to develop new edible films and coatings for food packaging in comparison with current gums such as expensive tragacanth, unsustainable and unavailable gum Arabic and biotechnological ones like gellan, xanthan, and curdlan due to health concerns (Li and Ye, 2017). FG, also known as Persian, Angum, and Zedo gum is the exudate of wild almond trees. FG, as an anionic hydrocolloid, is commercially available in bulky granules, sugar crystals, or fine powder forms that varies in color from white, yellow, and brown to red. Since it contains arabinose and galactose in its chemical structure, it is known as an arabinogalactan hydrocolloid (Hadian et al., 2016; Dabestani et al., 2018). There are limited research works regarding edible films and coatings based on FG (Azarikia and Abbasi, 2016; Khorram et al., 2017; Joukar et al., 2017; Dehghani et al., 2018; Khodaei et al., 2020). To our knowledge, the hydrophobic modification of FG-based edible films has not yet been reported. Since nanoparticles are promising tools to improve the properties of edible films for food packaging applications, the focus of this work was given to develop a novel approach in the hydrophobic modification of FG edible films using SONs and then evaluating their barrier, optical, mechanical, thermal, and morphological properties compared with the properties of oil-free and liquid oil-incorporated FG-based films. For this purpose, FG was incorporated with different amounts of two types of SONs prepared by MG and EC as gelator.

2. Materials and methods

2.1. Materials

Finely powdered white FG containing 89%89 \% carbohydrate, 6.70%6.70 \% moisture, 3.63%3.63 \% ash, and 0.5%0.5 \% protein was purchased from Dena Emulsion Company (Shiraz, Iran). Distilled MG and EC were kindly provided by ZTCC Company (Xingyang, Henan, China) and SigmaAldrich (St. Louis, MO), respectively. Anhydrous glycerol ( 98%98 \% ), CaCl2\mathrm{CaCl}_{2} and NaCl were purchased from Kimia Mavad Company (Tehran, Iran).

Refined sunflower oil was supplied by Narges Shiraz Oil Company (Shiraz, Iran). All dispersions were prepared by double distilled water (DDW).

2.2. Preparation of structured oil

MG and EC gelators were dissolved in SFO at concentrations of 15%15 \% and 6%(w/w)6 \%(\mathrm{w} / \mathrm{w}), respectively. These different concentrations were selected to have similar contact angle values as determined in our preliminary experiments. The mixtures were heated on a magnetic hot plate at 75∘C75^{\circ} \mathrm{C} and 160∘C160^{\circ} \mathrm{C} for MG and EC, respectively. Heating was continued for 10−15 min10-15 \mathrm{~min} to ensure complete dissolution of gelator molecules. To prepare the structured SFO, the mixtures were cooled to 20∘C20^{\circ} \mathrm{C} under static conditions and then kept at 4∘C4^{\circ} \mathrm{C} for 12 h . After that, structured oil samples were sheared by an Ultra-Turrax high speed homogenizer (T25, IKA, Germany) at 13,500 rpm for 4 min .

2.3. Preparation of film-forming solutions

FG powder ( 2%,w/w2 \%, \mathrm{w} / \mathrm{w} ) was dissolved in DDW and then hydrated completely for 24 h on a roller mixer. Glycerol ( 0.6%,w/w0.6 \%, \mathrm{w} / \mathrm{w} ) was used as a plasticizer and the final mixture was mixed at 300 rpm for 30 min . Glycerol expands interstitial spacing and enhances the mobility of FG chains in the film matrix without any involvement in the reactions between the film components, except for the formation of H-bonding with water molecules. Liquid SFO and sheared structured oils (section 2.2) were incorporated into film-forming solutions at different concentrations ( 0.5%0.5 \% and 1%,w/w1 \%, \mathrm{w} / \mathrm{w} ) under stirring at 600 rpm for 10 min . The stable dispersions of liquid SFO and SONs with a uniform distribution within the film’s network were prepared using Ultra-Turrax high speed homogenizer at 13,500 rpm for 5 min . For removing air bubbles, the final mixture was subjected to vacuum generation (0.02MPa)(0.02 \mathrm{MPa}) for 25 min and then cast (25 g)(25 \mathrm{~g}) onto polystyrene plates (50.24 cm2)\left(50.24 \mathrm{~cm}^{2}\right). Drying was conducted at room conditions for approximately 48 h . After peeling off, dried film conditioned at room temperature and 48%48 \% relative humidity (RH) using a saturated solution of potassium acetate. In this study, different FG-based films were named based on the concentration and type of SFO and SON in the film matrices: 0.5%SFO,1%SFO,0.5%MG,1%MG,0.5%EC0.5 \% \mathrm{SFO}, 1 \% \mathrm{SFO}, 0.5 \% \mathrm{MG}, 1 \% \mathrm{MG}, 0.5 \% \mathrm{EC} and 1%1 \% EC. FG control film without any added oil was also prepared.

2.4. Physical properties

2.4.1. Thickness and density

The films thickness (mm) was evaluated by a Mitutoyo digital micrometer (No. 293-766, Tokyo, Japan) in at least six random locations of the films, and the average values were reported. The density was expressed as the ratio of film mass to its volume, in which the volume was equal to film thickness ×\times area (Gahruie et al., 2019).

2.4.2. Moisture content

The moisture content measurements were carried out according to Esteghlal et al. (2016). Films cuts ( 1 cm×3 cm1 \mathrm{~cm} \times 3 \mathrm{~cm} ) were oven dried at 90∘C90^{\circ} \mathrm{C} for 24 h . The moisture content was calculated using the following equation:

Moisture content (%)=[(W0⋅ Wt)/W0]×100(\%)=\left[\left(\mathrm{W}_{0} \cdot \mathrm{~W}_{t}\right) / \mathrm{W}_{0}\right] \times 100
where, W0\mathrm{W}_{0} and Wt\mathrm{W}_{t} are the weight of the sample before and after drying, respectively.

2.4.3. Solubility

The water solubility of samples was determined in accordance with the method of Gahruie et al. (2017). Samples were dried at 60∘C60^{\circ} \mathrm{C} to a constant dry weight (Wt)\left(\mathrm{W}_{t}\right). Afterwards, the samples were transferred into 50 ml . Falcons containing 25 mL DDW and stirred at 60 ppm for 24 h at

room temperature. Films cuts were separated from water using Whatman circular filter papers, re-dried at 60∘C60^{\circ} \mathrm{C} and then weighed (Wt)\left(\mathrm{W}_{\mathrm{t}}\right). The percentages of the total soluble matters were calculated according to eq. 2 :

Total soluble matter (%)=[(Wt−Wt)/Wt]×100(\%)=\left[\left(\mathrm{W}_{\mathrm{t}}-\mathrm{W}_{\mathrm{t}}\right) / \mathrm{W}_{\mathrm{t}}\right] \times 100

2.4.4. Swelling

Dried films cuts (Wt)\left(\mathrm{W}_{\mathrm{t}}\right) were transferred into the 50 mL Falcons containing 25 mL DDW under constant agitation using a roller shaker for 1 h . Wet films cuts were taken out of Falcon, the excess water wiped between filter papers, and then weighed (W) (Kavoosi et al., 2017). The swelling of the films was calculated using eq. 3 :

Swelling (%)=[(Wt−W)/Wt]×100(\%)=\left[\left(\mathrm{W}_{\mathrm{t}}-\mathrm{W}\right) / \mathrm{W}_{\mathrm{t}}\right] \times 100

2.4.5. Water vapor permeability

The Water vapor permeability (WVP) measurements were performed according to the standard method of ASTM E96-95 modified by Spotti et al. (2016). Circular films cuts were prepared from the preconditioned films and sealed with an O-ring on aluminum permeation cups containing 50 g anhydrous CaCl2(0%\mathrm{CaCl}_{2}\left(0 \%\right. RH) and then transferred into a desiccator containing NaCl saturated solution ( 75%RH75 \% \mathrm{RH} ). The weight changes of the cups were recorded and then weight measurements were taken for each cup during 24 h . The weight gain with respect to the initial weight was plotted over time. WVP (g.mm/ h.mm2⋅kPa\mathrm{h} . \mathrm{mm}^{2} \cdot \mathrm{kPa} ) was measured as:
WVP=(S/A)×(X/P0)×(RH1⋅RH2)\mathrm{WVP}=\left(S / A\right) \times\left(X / \mathrm{P}_{0}\right) \times\left(\mathrm{RH}_{1} \cdot \mathrm{RH}_{2}\right)
where, S is the slope of the weight changes against time ( g/s\mathrm{g} / \mathrm{s} ), A is the film area (m2),X\left(\mathrm{m}^{2}\right), \mathrm{X} is the film thickness ( mm ), P0\mathrm{P}_{0} is the water vapor pressure difference across two sides of the film (1753.55 Pa ), RH1\mathrm{RH}_{1} is relative humidity within the desiccator ( 0.75%0.75 \% ), and RH2\mathrm{RH}_{2} is relative humidity in the cup (0%)(0 \%).

2.4.6. Surface hydrophobicity

Drop shape analyzer (DSA 100, Hamburg, Germany) equipped with a high-speed CCD camera was used for contact angle measurement. Droplets of DDW were deposited on the film surfaces at different positions using a Hamilton syringe, and then the images were analyzed by the DSA software (Gahruie et al., 2019).

2.4.7. Opacity

The visible light barrier properties of FG films were determined at a wavelength of 600 nm using a UV-vis spectrophotometer (UV1601PC, Shimadzu, Kyoto, Japan), according to the method of Tongnuanchan et al. (2015). The FG films were cut into rectangular strips ( 3×1 cm23 \times 1 \mathrm{~cm}^{2} ) and placed on the interior side of the test cell. The films opacity was determined as:

Film opacity =A600/=\mathrm{A}_{600} / film thickness (mm)(\mathrm{mm})

2.4.8. Color

Color parameters of films including lightness (L∗)\left(L^{*}\right), redness/greenness (a∗)\left(a^{*}\right) and yellowness/blueness (b∗)\left(b^{*}\right) values were measured by digital photography using a Canon digital camera (14 M P, Tokyo, Japan) from the samples placed at a specific condition in a sealed wooden box. Total color difference (ΔE)(\Delta \mathrm{E}) relative to the white standard plate (LS∗=93.49\left(L_{S}^{*}=93.49\right., aS∗=0.25,bS∗=0.09)\left.a_{S}^{*}=0.25, b_{S}^{*}=0.09\right) and whiteness index were also calculated (Hosseini et al., 2019).

2.5. Mechanical properties

The tensile strength (TS), elongation at break (EAB), and Young’s modulus (YM) were determined in accordance with the standard
method of ASTM D882-12 with some modifications (Li et al., 2018). Rectangular (1×6 cm2)\left(1 \times 6 \mathrm{~cm}^{2}\right) strips of pre-conditioned films were clamped between two grips of texture analyzer (TA-XT2, Stable Microsystems, Surrey, UK) at an initial length of 40 mm . The cross-head speed was set to 1 mm/s1 \mathrm{~mm} / \mathrm{s}. Mechanical parameters were calculated using the following equations:
TS=Fm/A\mathrm{TS}=\mathrm{F}_{\mathrm{m}} / \mathrm{A}
EBA(%)=(ΔH/H0)×100\mathrm{EBA}(\%)=\left(\Delta \mathrm{H} / \mathrm{H}_{0}\right) \times 100
YM=Fm/AΔH/H0\mathrm{YM}=\frac{\mathrm{Fm} / \mathrm{A}}{\Delta \mathrm{H} / \mathrm{H} 0}
where, Fm is the maximum force at the breakage point (N), A is the film cross-sectional area (m2),ΔH\left(\mathrm{m}^{2}\right), \Delta \mathrm{H} is the film elongation at the breakage point, and H0\mathrm{H}_{0} is the film length between the grips.

2.6. Fourier transform infrared spectroscopy

Powder samples of dried films were mixed with KBr and compressed into tablets. Fourier transform infrared (FTIR) spectra were recorded in the range of 4000−400 cm−14000-400 \mathrm{~cm}^{-1} (Thermo Nicolet Avatar 370, Madison, WI) (Gahruie et al., 2019).

2.7. Differential scanning calorimetry

The thermal properties of films were evaluated by a differential scanning calorimeter (Perkin-Elmer, Beaconsfield, UK). Five mg of preconditioned samples were weighed by a microbalance with high accuracy inside pans and sealed hermetically. The samples were heated from 30∘C30^{\circ} \mathrm{C} to 250∘C250^{\circ} \mathrm{C} at a heating rate of 10∘C/min10^{\circ} \mathrm{C} / \mathrm{min} under a nitrogen atmosphere (20 mL/min)(20 \mathrm{~mL} / \mathrm{min}). A second scan was also conducted in the same method, followed by quench-cooling of the sample after completing the first scan (Tongnuanchan et al., 2015).

2.8. Thermogravimetric analysis

Thermogravimetric analysis (TGA) (Perkin-Elmer, Beaconsfield, UK) was performed in a nitrogen atmosphere as a purge gas at a flow rate of 50 mL⋅ min−150 \mathrm{~mL} \cdot \mathrm{~min}^{-1}. Films were scanned at a temperature range of 25−300∘C25-300^{\circ} \mathrm{C} and a rate of 10∘C/min10^{\circ} \mathrm{C} / \mathrm{min} (Tongnuanchan et al., 2015).

2.9. Microscopic structure

The surface microstructures of film samples were evaluated using a Vega3 scanning electron microscopy (SEM) (TESCAN, Czech Republic). The samples were cut into small pieces under liquid nitrogen, mounted on aluminum stubs, and then coated with a thin layer of gold using a DSR1 Desk Sputter Coater (Nanostructured Company, Tehran, Iran). Observations were carried out at an accelerating voltage of 15 kV (Khodaei et al., 2020).

2.10. Statistical analysis

All measurements were conducted at least in triplicate. Statistics were carried out using analysis of variance (ANOVA) at the significance level of 0.05 . The differences among mean values were determined through Duncan’s multiple range test with SAS* software (version. 9.1, USA).

3. Results and discussion

3.1. Physical properties

The visual appearance of FG films incorporated with liquid SFO and SONs is shown in Fig. 1. Control film (Fig. 1a) and those modified with SNOs at different levels (Fig. 1d-g) were visually homogeneous with

Fig. 1. Visual appearance of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).
smooth surfaces without any pores visible to the naked eye. Also, they had appropriate flexibility which made the peeling off process easy except for the sample incorporating 1%1 \% SONs prepared by EC. Liquid SFO-incorporated films were also homogenous in visual appearance but they were too sticky and stretchy and hence difficult to handle and peel off from the casting surfaces due to shrinkage. Moreover, the visual transparency of films was affected by the modification, which was more
noticeable for the FG films incorporated with liquid SFO droplets.
The SEM micrograph of the surface (air-side) of the control FG film (Fig. 2a) was smooth and homogeneous. After modification with SONs, the smooth and compacts surfaces without pores, cracks, and insoluble particles were also observed (Fig. 2d-g), suggesting the good compatibility between SONs and FG in the film matrix. However, a relatively less homogenous microstructure was observed in the modified FG-

Fig. 2. Scanning electron micrographs of the surface of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).

Table 1
Physical properties of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).

a{ }^{a} Data represent the mean ±\pm standard deviation of three independent repeats; different superscript letters in each column indicate significant differences (P<0.05)(P<0.05).
based film with incorporating 1%1 \% SONs prepared by EC (Fig. 2g). On the contrary, incorporating liquid SFO droplets to the FG matrix resulted in a surface morphology with a higher roughness (Fig. 2b and c) than control sample, indicating the accumulation of SFO droplets at the film surface. This might be due to the physical instability of casted film forming emulsified solution via different mechanisms such as flocculation and coalescence during drying (Ma et al., 2012). Thus, structural discontinuity and surface irregularity were observed at the top surface of dried films. Such a trend was reported for sodium caseinate-ginger oil films (Atarés et al., 2010) and kefiran-oleic acid films (Ghasemlou et al., 2011). Our previous study showed that, in an O/W\mathrm{O} / \mathrm{W} emulsion, the presence of oil phase in a structured form resulted in a higher physical stability and therefore, the surface morphology was smoother in the films incorporated with SONs (Ghiasi et al., 2019).

The FG film density was not changed significantly by incorporating different types and concentrations of SONs and liquid SFO ( P<0.05P<0.05 ), as shown in Table 1. Also, the moisture content of films was not affected by this hydrophobic modification ( P<0.05P<0.05 ) (Table 1). The highest solubility ( 41.35%41.35 \% ) was obtained for the control film. The presence of oil in both liquid and structured state significantly ( P<0.05P<0.05 ) decreased the water solubility which can be related to the hydrophobic nature of modifier ingredients. Increasing the concentration of liquid SFO and SONs led to the further decrease in water solubility. This property is valuable when the integrity and water resistivity of food packaging materials are of concern. The obtained results supported that the inclusion of liquid SFO in FG films had more effect on reduction of their solubility. This behavior may be attributed to the fact that some of SFO droplets are exposed to the film surface. Therefore, they may contribute to the more resistance of FG films to water absorption. At the same concentration, SONs prepared by MG were less effective than those structured with EC in reducing the water solubility of films which can be related to the amphiphilic properties of MG molecules and hence their relative affinity to water molecules.

In agreement with the results of solubility, incorporating both liquid SFO and SNOs limited the exposure of FG polar groups for the interaction with water molecules and hence reduced the water swelling. This behavior was mostly ascribed to the intermolecular interactions between the functional groups of FG chains and SFO, leading to a decrease in the availability of hydroxyl groups and limiting FG-water interactions by hydrogen bonding and produced films with low swelling ability. These findings are consistent with the results obtained by FTIR. Resistance of modified FG film to water is desirable if the film is supposed to be used for the packaging of intermediate- or high-moisture foods. As expected, control films had the highest WVP value mainly due to the hydrophilic functional groups along with the FG backbone which
absorb a large number of water molecules by hydrogen bonds (Dabestani et al., 2018). The presence of SFO, either in liquid or structured state, led to a considerable decrease in WVP of the FG-based films (Table 1). The incorporation of dispersed hydrophobic droplets within the network increased the tortuosity factor for mass transfer, and hence reduced the WVP (Rodrigues et al., 2018). As presented in Table 1, the lowest WVP was measured in sample 0.5%EC0.5 \% \mathrm{EC}, while the WVP measurement of modified FG film incorporated with 1%1 \% SONs prepared by EC was not possible as its brittleness led to the formation of tiny cracks during the sealing step by O-rings and flanges in this test. There were no significant differences in WVP values of SONs-incorporated films as affected by different concentrations of MG. At the same concentration, SONs-incorporated films were more effective in reducing WVP than those incorporated with liquid SFO. The uniform distribution of SONs within the film matrix as well as their appropriate compatibility with FG resulted in lower adsorption and diffusion of water vapor through the film as evidenced by their lower WVP. The accumulation of SFO droplets at the film surface led to a reduction in the tortuosity factor, and hence reduced the water vapor barrier ability. Therefore, modified FG films incorporated with SONs have a good ability to lengthen the shelf life of food products and to protect them from moisture loss.

According to Table 1, the control sample showed the lowest contact angle value mainly due to the backbone of unmodified FG was rich in -OH groups. A noticeable increase ( P<0.05P<0.05 ) in the contact angle was observed after incorporating liquid SFO and SONs. However, modification with 0.5%0.5 \% SONs of MG had no significant ( P>0.05P>0.05 ) effect on the contact angle value. The incorporation of liquid SFO droplets into FG network induces higher hydrophobicity than SONs which can be related to the exposure of SFO droplets at the film surface, leading to a noticeable improvement of surface hydrophobicity (The et al., 2009), while SONs completely enclosed in the FG network (Fig. 1d-g). Similar findings have been previously reported by Pereda et al. (2012) for chitosan film modified by olive oil. In addition, SONs prepared by EC was more effective than those structured with MG in increasing hydrophobicity which may be due to the positioning of polar groups of MG at the interface, leading to the higher wettability by water molecules. However, the increasing concentration of both liquid SFO and SONs significantly increased the contact angel of films.

The lowest opacity was measured in the control FG film (Table 1). Incorporating liquid SFO and SONs significantly ( P<0.05P<0.05 ) increased the film opacity due to the light-scattering effects of oil droplets within the film matrix and consequently reducing the transparency (Sahuae et al., 2017). However, the modification of FG films by adding liquid SFO droplets led to higher opacity than SONs. This observation can be

Table 2
Color parameters of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).

a{ }^{a} Data represent the mean ±\pm standard deviation of three independent repeats; in each column means with different superscript letters are significantly different (P<0.05)(P<0.05).
attributed to the homogeneous distribution of SONs and their higher stability during drying process as compared to liquid SFO droplets. Similar results were also reported in olive oil-incorporated gelatin-(Ma et al., 2012) and fatty acids-incorporated basil seed gum-(Gahruie et al., 2020) edible films. Color properties are important in visual appearance and consumer acceptability of food wrappings. As shown in Table 2, incorporating liquid SFO and SONs decreased (P<0.05)La(P<0.05) L^{\mathrm{a}} and whiteness index values compared to control FG film which was more noticeable in the presence of liquid SFO droplets. Regardless of the physical nature of added ingredient, the bab^{\mathrm{a}} and ΔE\Delta \mathrm{E} values increased significantly after FG modification. The SONs resulted in higher bab^{\mathrm{a}} values compared to liquid SFO, suggesting that the yellow color of films appear more dominant. Moreover, the incorporation of liquid SFO droplets led to a lower aaa^{\mathrm{a}} value, while FG films modified with SONs had a higher aaa^{\mathrm{a}} value which denoted that these films were tended towards redness. Generally, the color changes of modified FG films can be related to the various interactions between FG chains and hydrophobic ingredients in the film structure which incline to change the color parameters. Moreover, the differences between the original color parameters of liquid SFO and SONs as well as their distribution within the film matrix are responsible for the color differences of modified FG films (Pak et al., 2020; Pereda et al., 2010).

3.2. Mechanical properties

The mechanical characteristic of the edible films considered as crucial parameters for food packaging during transportation and storage. As reported in Table 3, the maximum TS ( 4 MPa ) and YM ( 1870 MPa ) and also the minimum elasticity ( 2.5%2.5 \% ) were measured in control film. Modification with 0.5%0.5 \% SONs caused an insignificant decrease of TS ( P>0.05P>0.05 ), while the EAB increased and YM decreased significantly compared with control film (Table 3). These results can be related to the role of dispersed SONs in increasing the interstitial spaces between the biopolymer chains and hence increasing the sliding effects

Table 3

Mechanical properties of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).

a{ }^{a} Data represent the mean ±\pm standard deviation of three independent repeats; in each column means with different superscript letters are significantly different (P<0.05)(P<0.05).

Fig. 3. Stress vs. strain curves of control Farsi gum (FG)-based film and those incorporated with different concentrations of structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).
of chains against each other, suggesting films became more flexible and stretchable (Benavides et al., 2012). On the other hand, the incorporating SONs increased the discontinuities in the overall FG matrix, leading to a weaker structure as shown by their lower YM (Sahraee et al., 2017). Regardless of the SONs type, increasing the level of SONs decreased (P<0.05)(P<0.05) the EBA and YM of modified FG films (Fig. 3), while the TS remained constant at the current concentrations. Therefore, these results demonstrated that the incorporation of 0.5%0.5 \% SONs made FG-based films more resistant to stress and more flexible than 1%1 \% SONs for food packaging.

According to Fig. 3, incorporating SONs prepared by MG led to higher flexibility and lower stiffness compared to those structured with EC, whereas TS results revealed no significant changes (Table 3) as a function of SONs type. Our mechanical results were comparable to those of Cassava starch-(Farias et al., 2012), sodium alginate-(RojasGraü et al., 2007), gum Cordia-(Haq et al., 2014), and sago starch/guar gum-(Dhumal et al., 2019) edible films in which the reported tensile strength, elongation at break and Young modulus values were considered desirable and sufficient for edible food packaging materials applications. The mechanical properties of too sticky SFO-incorporated FG films were not measurable due to the shrinkage of films during the tensile test.

3.3. Fourier transform infrared spectroscopy

Fig. 4 represents the FTIR spectra of control FG-based film and those incorporated with different concentrations of liquid SFO and SONs. Specific bands of carbohydrates were in the fingerprint region from 1500 to 500 cm−1500 \mathrm{~cm}^{-1}. The peaks appeared at 1441−1428 cm−11441-1428 \mathrm{~cm}^{-1} corresponded to the molecular vibration mode of the CH3\mathrm{CH}_{3} group or the presence of carboxylate groups (COOH) of uronic acid residues in FG as well as free fatty acids in SFO (Pak et al., 2020). The moderate strong

Fig. 4. FT-IR spectra of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).

Table 4
FTIR peak assignments of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).

Table 5
Thermal properties of control Farsi gum (FG)-based film and those incorporated with different concentrations of liquid sunflower oil (SFO) and structured oil nanoparticles prepared by monoglyceride (MG) and ethyl cellulose (EC).

peaks at around 1620−1613 cm−11620-1613 \mathrm{~cm}^{-1} may be attributed to the presence of anhydride components produced via carboxyl groups or intramolecular bound water (Fadavi et al., 2014). Other distinguishing peaks were observed in the range of 2927−2926 cm−12927-2926 \mathrm{~cm}^{-1} due to the CH2\mathrm{CH}_{2} - asymmetric and CH - symmetric and asymmetric stretching vibrations. The CH2\mathrm{CH}_{2} symmetric stretching vibrations were also appeared at around 2890−2855 cm−12890-2855 \mathrm{~cm}^{-1} (Abbasi, 2017; Pavli et al., 2018). The broad
intensity peak at 3421−3316 cm−13421-3316 \mathrm{~cm}^{-1} (Table 4) were attributed to the free hydroxyl groups and the formation of inter- or intra-molecular -OH stretching vibration between the carboxylic acid along FG backbone and the water molecules present in the film matrix (Dabestani et al., 2018; Gahruie et al., 2019). Also, among different interactions between the functional groups of FG and SONs, hydrogen bonding might have a dominant role. Table 4 shows that the modified films incorporating SFO

and SONs had two distinctive sharp peaks at wavenumbers of 1747−1740 cm−11747-1740 \mathrm{~cm}^{-1} and 3013−3009 cm−13013-3009 \mathrm{~cm}^{-1} which were attributed to the ester C=O\mathrm{C}=\mathrm{O} stretching and =C−H=\mathrm{C}-\mathrm{H} stretching vibrations of carbon-carbon double bonds along unsaturated alkyl chain of triacylglycerols, respectively. The presence of oil characteristic peaks in the spectra of modified FG films confirmed that SFO and SONs were introduced into the FG structure. Moreover, incorporating 1%1 \% SFO resulted in stronger bands in the wavenumbers of 1744 cm−11744 \mathrm{~cm}^{-1} and 2924 cm−12924 \mathrm{~cm}^{-1} compared with 0.5%0.5 \% SFO (Fig. 4) due to the increases in C=O\mathrm{C}=\mathrm{O} and =C−H=\mathrm{C}-\mathrm{H} bonds in the film formulation with higher concentration of SFO. Also, PérezMateos et al. (2009) reported similar observations when incorporating SFO blends into cod gelatin edible film. Moreover, the peak at 1041−1039 cm−11041-1039 \mathrm{~cm}^{-1} confirmed the presence of C−O\mathrm{C}-\mathrm{O} stretching vibrations of C−O−C\mathrm{C}-\mathrm{O}-\mathrm{C} and C−O−H\mathrm{C}-\mathrm{O}-\mathrm{H} bonds, which corresponded to the both oil and FG.

3.4. Differential scanning calorimetry

The results of the first and second heating DSC scans for different FG-based films are summarized in Table 5. Control film experienced an irreversible structural change at the onset temperature of approximately 213∘C213^{\circ} \mathrm{C}, which was not observed again after cooling and reheating. This event suggested that FG chains could not realign themselves in an ordered network during the cooling scan. This irreversible structural change was similarly reported in the DSC thermograms of frost grape polysaccharide at approximately 157∘C157^{\circ} \mathrm{C} (Hay et al., 2017). In SONs-incorporated films, this phase transition was gradually shifted to higher temperatures (215−228∘C)\left(215-228^{\circ} \mathrm{C}\right). Moreover, the DSC results of the first heating scan for FG films incorporated with 1%1 \% SONs exhibited an additional transition peak before the onset temperature of FG, but it was not found in the control sample and those incorporated with 0.5%0.5 \% SONs (Table 5 and Fig. S1 in supplementary data). This transition peak was observed at the onset temperatures of about 65∘C65^{\circ} \mathrm{C} and 117∘C117^{\circ} \mathrm{C} for the films incorporated with 1%1 \% SONs prepared by MG and EC, respectively. This event was related to the melting transition of dispersed SONs in the FG film matrix. The incorporation of SONs prepared by EC shifted this transition temperature to the higher temperature due to its higher melting point and more hydrophobic nature in comparison with SONs jellified with MG molecules (Óğüncü and Yılmaz, 2014). The absence of the first transition peak in the FG films incorporated with 0.5 %\% SONs could likely be attributed to the limited sensitivity of DSC detector for the low levels of SONs. As demonstrated in Table 5, the onset temperature of FG films decreased to 196∘C196^{\circ} \mathrm{C} and 204∘C204^{\circ} \mathrm{C} in the films incorporated with 0.5%0.5 \% and 1%1 \% liquid SFO, respectively. This result suggested that incorporating liquid SFO more likely impeded intermolecular interactions in FG chains, thereby increasing mobility of polymer matrix. Niknam et al. (2019) also reported that the incorporation of olive, maize, and canola oils into the Plantago major seed gum-based films decreased thermal stability.

According to Table 5a, the lowest ΔH\Delta \mathrm{H} was obtained in the film containing liquid SFO. It was reported that the presence of liquid oil droplets in the biopolymer film matrix weakens the film network, leading to the lower enthalpy changes for destroying the intermolecular interactions (Tongnuanchan et al., 2015). Interestingly, it seems that the development of the strong gel network in SONs via extensive intermolecular hydrogen bonding could improve the thermal stability of modified FG films, particularly in those incorporated with 1%1 \% SONs prepared by EC as evidenced by their higher ΔH\Delta \mathrm{H} values. In contrast, incorporating liquid SFO in FG films resulted in lower ΔH\Delta \mathrm{H} than control film, as shown in Table 5. This result suggests that the presence of dispersed liquid SFO droplets reduced the cohesive structure integrity, therefore resulted in a weaker film network. According to the results of the second heating scan (Table 5), the transition temperatures were observed at 97∘C97^{\circ} \mathrm{C} and 181∘C181^{\circ} \mathrm{C} for films incorporated with 1%1 \% SONs prepared by MG and EC, respectively. Whereas, it was not observed for the control sample. As discussed before, this phase transition with a
lower magnitude was associated with the melting transition of SONs which re-structured during cooling scan.

3.5. Thermogravimetric analysis

TGA thermograms (Fig. S1 in supplementary data) revealed the thermal degradation behavior of the control film and those incorporated with liquid SFO and SONs. Their corresponding degradation temperatures and weight loss are presented in Table 5. All films showed two distinct regions of weight loss. The first stage ( 1.56−8.72%1.56-8.72 \% ) was appeared below 100∘C100^{\circ} \mathrm{C} due to the free and bound water molecules evaporation in the films. The second stage of degradation (8.98-30.86 %\% ) was observed approximately at 206−252∘C206-252^{\circ} \mathrm{C}, which was related to the thermal decomposition of FG molecular structure. The obtained results showed that at the same concentration, modified films with liquid SFO had higher heat susceptibility than control samples, whereas incorporating SONs decreased weight loss values, which showed greater reduction by increasing SONs concentration. This can be attributed to the variations in WVP of films as SONs incorporated films had lower WVP values compared with control films which may reflect more thermal stability. The FG films incorporated with SONs prepared by MG underwent easier thermal degradation and higher weight loss compared to those incorporating SONs prepared by EC. Moreover, degradation temperatures of films containing SONs increased with the concomitant decrease in weight loss as the concentration of SONs increased, confirming the increase of film heat resistance.

4. Conclusion

The hydrophobic modification of FG-based edible film using structured oil (i.e., as oleogel) nanoparticles was successfully reported. SEM micrographs of FG films revealed the presence of smooth surface after modification with SONs; whereas, higher roughness as a result of the flocculated oil droplets was observed for the modified films incorporated with liquid SFO. The results showed that the hydrophobic properties of the films were improved after modification especially at higher concentration of hydrophobic modifiers. Moreover, SONs prepared by EC was more effective than those structured with MG in improving water vapor barrier properties, surface hydrophobicity and the reduction of solubility. Nevertheless, the incorporation of SFO directly affected color and transparency of films, particularly after modification with liquid oil. The FTIR spectra of films confirmed that the SONs were introduced into the FG structure. The incorporation of 0.5%0.5 \% SONs made FG-based films more resistant to stress and more flexible and extensible for most practical applications (like food wrapping). The incorporation of SONs had marked influence on the thermal properties of FG films, which was more substantial for the films modified with SONs prepared by EC. More work will be needed to demonstrate the successful applications of biopolymer-based edible films modified with SONs. In particular, investigating the utilization of such nanoparticles in edible films and coatings as a carrier for the hydrophobic bioactive substances (including antioxidants, antimicrobial agents, flavors, and colorants) to develop active or intelligent food packaging materials is suggested.

CRediT authorship contribution statement

Fatemeh Ghiasi: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization. Mohammad-Taghi Golmakani: Conceptualization, Methodology, Validation, Resources, Data curation, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Mohammad Hadi Eskandari: Conceptualization, Validation, Resources, Data curation, Visualization, Supervision, Project administration, Funding acquisition. Seyed Mohammad Hashem Hosseini:

Conceptualization, Methodology, Validation, Data curation, Writing review & editing, Visualization, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The financial support of this work was provided by Shiraz University (Grant number 96GCU6M2036).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.indcrop.2020.112679.

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