Peach Gum Polysaccharide as an Additive for Thermoplastic Starch to Produce Water-Soluble Films

Abstract

Thermoplastic starch (TPS) has gained considerable attention during the last few years in developing starch-based biodegradable food packaging materials or edible coatings due to its high availability and low cost. TPS is manufactured from starch plasticized with food-grade plasticizers, making it suitable for food contact applications. In addition, TPS is bio-based and biodegradable, which, from an environmental perspective, closes the circle of the circular economy. However, the industrial application of TPS is somewhat limited due to its poor mechanical performance and low water resistance. However, the low water resistance could increase the water sensitivity of TPS, which could be advantageous for coating application or food encapsulation. The present work aims to tailor the water sensitivity of TPS by adding peach gum polysaccharide to obtain water-soluble films. With this aim, peach gum polysaccharide (PGP) was extracted from peach gum (PG) using the thermal hydrolysis method. Films of TPS-PG and TPS-PGP were prepared and characterized by their water sensitivity and mechanical, microstructural, and thermal properties. The results show that PGP allows the obtaining of films with water sensitivities higher than 70% but also improves TPS elongation at break, making the material more suitable for application as film.

1. Introduction

The increasing interest in minimizing the environmental impact of packaging plastics and their indiscriminate use has focused scientists’ attention on using bio-based polymers to reduce petroleum-based plastic residue. Bio-based polymers can be directly extracted from sustainable biomass or synthesized from biomass resources by biological, physical, or chemical means [1]. Bio-based polymers have the advantages of being biobased, i.e., obtained from renewable resources, and biodegradable, i.e., able to break down in controlled compost soil because of their potentially hydrolyzable ester linkages [2,3]. Bio-based polymers extracted directly from renewable sources that are abundant in nature, such as proteins, lipids, and polysaccharides, have been positioned as an actual alternative to synthetic polymers and are characterized by their biodegradability, wide availability, and low cost [1]. Researchers are looking into polysaccharides, including starches, cellulose derivatives, and plant gums, regarding food packaging and preservation as edible films and coatings [4].

Starch is one of the natural polymers most researched for packaging applications because of its availability, biodegradability, and affordability. Starch is an abundant polymer stored in plants for energy consumption as granules and can be extracted from corn, rice, wheat, and potato. Thermoplastic starch (TPS) is produced by applying shear forces at high temperatures in starch while a plasticizer is present, causing granular disruption [5]. However, TPS films have two main disadvantages compared to synthetic polymers: moisture absorption susceptibility and poor mechanical performance [6]. Starch-based films require water insolubility to enhance product integrity and water resistance in biodegradable packaging materials. However, dissolving in water or other aqueous phases could be advantageous for applying coatings or encapsulating dry goods like food [4,7].

Natural materials have become more and more popular as viable, sustainable, and eco-friendly additives for polymeric matrices to lessen the environmental impact of synthetic products. For instance, Ref. [8] created wood plastic composites with high-density polyethylene derived from sugarcane (bio-HDPE) and a lignocellulosic filler (micronized argan shell). The plastic wood displayed high ductile properties, comparatively high stiffness, and enhanced thermal stability. Moreover, the material’s visual appearance presented a striking resemblance to reddish-colored wood. Ref. [9] used functionalized hemp microparticles (HMPs) to create a hydrophobic epoxy-based composite. Adding functionalized hemp particles to the epoxy resin raised the glass transition temperature and improved thermo-oxidative stability due to the polymer matrix and the functionalized fiber particles’ strong chemical attraction.

Adding naturally occurring compounds to biopolymers, such as natural fillers, fibers, and stabilizing molecules, benefits the formulation of fully bio-based systems [10]. For example, Ref. [11] developed film composites based on PLA and maleinized linseed oil using bacterial cellulose obtained from kombucha fermentation in spent coffee grounds as fillers. The addition of MLO demonstrated an effective plasticizing effect, whereas the kombucha filler increased strength and elongation at break because it presented a good interaction with the cellulose particles. Ref. [12] used thermoplastic extrusion to combine starch and leftovers from the passion fruit juice industry to make films. It was found that a low content of the residue increased the mechanical strength and Young’s modulus while lowering the permeability of water vapor. Ref. [13] used five pine resin derivatives as additives in TPS to produce films by compression molding. The resulting films show an increased glass transition temperature, allowing them to obtain more resistant materials than neat TPS. Moreover, the authors found that the final properties can be tubes in TPS according to the pine resin derivative used.

Peach gum (PG) is a natural gum produced by peach trees (Prunus persica, family Rosaceae) because of physiological processes or mechanical damage [14]. Generally, PG consists of polysaccharides (80–85 wt%), moisture (2–12 wt%), ash (0.3–4 wt%), protein (0.2–2 wt%), and trace amounts of polyphenols and inorganic elements such as K, Mg, P, and Ca [8,9]. Raw PG is insoluble in an aqueous solution because of its extremely high molecular weight and high degree of branching, which limits its applications [14]. Nonetheless, peach gum contains water-soluble polysaccharides made of arabinogalactans (arabinose (36–37%) and galactose (42%) and small amounts of xylose, mannose, and trace rhamnose) [15]. Therefore, raw PG must be hydrolyzed to obtain water-soluble peach gum polysaccharide (PGP) [16]. One of the most-used hydrolysis techniques in the industry is thermal hydrolysis. Since this is a straightforward technology, it does not require adding other substances in the process [14,16].

In a prior study, Juan-Polo et al. used raw peach gum as an additive to formulate thermoplastic starch. The samples with 10 and 15 phr of PG in TPS had an increase in water solubility of 24%. However, because PG is not water-soluble, the increase in the water solubility was attributed to the presence of PGP and to PG swelling capacity, which favored water absorption, enhancing the ability of the water to dissolve the remaining soluble starch [17].

The present work aims to produce water-soluble thermoplastic starch films using water-soluble peach gum polysaccharide (PGP) as an additive to boost TPS solubility without affecting its mechanical performance. To our knowledge, PGP has not been used to fabricate water-soluble TPS films. Different concentrations of PGP were added to TPS and processed using the solution casting method. Moreover, this study uses raw PG to produce TPS films as a control to determine the differences between PG and PGP.

2. Materials and Methods

2.1. Materials

Native rice starch (CAS: 9005-25-8) was provided by Manuel Riesgo S.A. (Madrid, Spain). The raw peach (PG) was supplied by Plant Gift (Guangdong, China). Plasticizers, distilled water, and glycerol 99% from Sigma Aldrich (Schnelldorf, Germany) were used to produce the TPS films. The raw PG comprised Ara, Xyl, Man, Gal, Rha, and Glucuronic acids in a 35:6:4:40:13:2 molar ratio [18].

2.2. Peach Gum Polysaccharide Extraction

Peach gum polysaccharide was extracted using thermal hydrolysis [9,10,13]. In summary, distilled water was used to submerge raw peach gum at a solid-to-liquid ratio of 1:50 (w/v) and heated to 80 °C while stirring for 1 h. The mixture was cooled to room temperature and homogenized with a blender. Then, the mixture was heated again to 90 °C for 1 h. The mixture was cooled and separated by centrifugation at 14,981× g for 30 min. The supernatant was collected by decanting. The residue was extracted twice using the same conditions. All supernatants were combined and precipitated by adding 4 volumes of ethanol. The precipitate was re-dissolved in H₂O and freeze-dried to obtain a water-soluble peach gum polysaccharide called PGP.

2.3. Film Preparation

Solution casting was used to create films from a suspension containing 3 wt% starch. Glycerol was added as a plasticizer at a 0.30 g glycerol/g dry starch ratio. The solution was heated at constant stirring until the starch began to gelatinize. Then, 5, 10, and 15 phr of PG or PGP were added to the present dry starch. The solution was stirred until complete dissolution.

Table 1. Composition of TPS-PG suspension formulations in 100 g of distilled water.

Material	Rice Starch (g)	Glycerol (g)	Content PG\PGP (phr)	PG\PGP Content (g)
TPS	3	0.9	0	0
TPS-5PG	3	0.9	5	0.15
TPS-10PG	3	0.9	10	0.30
TPS-15PG	3	0.9	15	0.45
TPS-5PGP	3	0.9	5	0.15
TPS-10PGP	3	0.9	10	0.30
TPS-15PGP	3	0.9	15	0.45

shows the composition of the prepared samples.

Then, 20 g of each film-forming mixture was poured onto a polystyrene Petri dish and dried at 40 °C in a JP Selecta S.A oven (Barcelona, Spain) with circulating air for 24 h. All the formed films were peeled off and conditioned at 25 °C in dissectors at 52% relative humidity (RH) (saturated solution of KBr) for 48 h before testing. The conditioning allows the material to reach a uniform moisture balance, ensuring the stability of its mechanical properties and processing qualities. All the films were prepared in triplicate.

2.4. Optical Characterization

Color characterization was performed using the Commission Internationale de l’Eclairage (CIELAB system). The values of $L^{*}$ (lightness), $a^{*}$ (component red–green), and $b^{*}$ (yellow–blue component) were detected by a Konica CM-3600d Colorflex-DIFF2 colorimeter spectrophotometer, from Hunter Associates Laboratory, Inc. (Reston, VA, USA). Five distinct film samples yielded the $L^{*}{a}^{*}{b}^{*}$ coordinate values, from which the color difference, ${∆\mathrm{E}}_{a,b}^{*}$, was computed using Equation (1).

$${∆\mathrm{E}}_{a,b}^{*}=\sqrt{{\left({∆L}^{*}\right)}^2+{\left({∆a}^{*}\right)}^2+{\left({∆b}^{*}\right)}^2},$$

(1)

where ${∆L}^{*}$, ${∆a}^{*}$, and ${∆b}^{*}$ correspond to the difference between the neat TPS film’s values and the color parameters of the tested films.

The following criteria were used to assess color change: The samples exhibit a clear noticeable difference (3.5 ≤ ${∆\mathrm{E}}_{a,b}^{*}$ < 5), the observer notices different colors (${∆\mathrm{E}}_{a,b}^{*}$ ≥ 5), the difference is unnoticeable (${∆\mathrm{E}}_{a,b}^{*}$ < 1), only an expert observer can notice the difference (1 ≤ ${∆\mathrm{E}}_{a,b}^{*}$ < 2), and an inexperienced observer notices the difference (2 ≤ ${∆\mathrm{E}}_{a,b}^{*}$ < 3.5) [19].

The morphology of raw peach gum powder and peach gum polysaccharides was observed using field emission scanning microscopy.

2.5. Field Emission Scanning Microscopy (FESEM)

Field emission scanning microscopy (FESEM) was carried out at 1 kV using a Zeiss Ultra 55 microscope from Oxford Instruments (Abingdon, UK). PG, PGP, TPS film, TPS-15PG film, and TPS-15 PGP film were analyzed to observe the effects of the particles inside the TPS matrix. Before analysis, the film samples underwent cryofracture. The samples were then coated with a layer of gold–palladium alloy using a Sputter Mod Coater Emitech SC7620 from Quorum Technologies (East Sussex, UK) to enable electrical conduction.

2.6. Mechanical Characterization

The mechanical characterization of the films was carried out by tensile test according to Ref. [20] with a universal test machine Duotrac-10/1200 from Iberest (Madrid, Spain). The testing parameters were 100 N load cell at a 100 mm/min crosshead rate. Five samples of each formulation were examined. Young’s modulus, elongation at break, and tensile strength average values and standard deviations are reported. An analysis of variance (ANOVA) was performed using OriginPro2018 software. Tukey’s test was used to statistically evaluate significant differences in mechanical parameters at a 95% confidence level.

The film thickness was also measured in ten randomly selected locations around each film sample using a manual micrometer Mitutoyo No. 2109S-10 (Tokyo, Japan) with a 0.001 mm sensitivity.

2.7. Water Sensitivity

The samples’ water sensitivity was determined gravimetrically; first, samples of the films with a 1 cm side were dried for 24 h at 40 °C in a JP Selecta S.A oven (Barcelona, Spain). Then, the dried films were immersed in a flask with 50 milliliters of distilled water. The flask was agitated at room temperature (20 °C) for 24 h. The remaining films were left to dry for 24 h at 40 °C in a JP Selecta S.A oven (Barcelona, Spain) with circulating air. Then, the films were weighted. The water sensitivity $S (\%)$ was calculated using Equation (2).

$$S (\%)=\frac{w_1-w_2}{w_1}\times 100,$$

(2)

where w₁ represents the dry weight before testing and w₂ represents the dry weight following testing. Three samples of each formulation were used for the analysis. Both the standard deviation and the average value were given.

2.8. Moisture Content

The films’ moisture content (MC) was determined using an electronic moisture analyzer Kern DAB 100-3 (Balingen, Germany) at 110 °C, using 3 mg of sample.

2.9. Wettability

The use of contact angle measurements to study surface wettability and other surface properties is well established. Typically, the surface wettability behavior of polymers is expressed using a contact angle formed between the solvent droplet and the polymer that comes in contact. The water contact angle was determined using an EasyDrop-FM140 optic goniometer from Kruss Equipment (Hamburg, Germany). For every formulation, three specimens underwent five WCA measurements. An ANOVA was performed using OriginPro2018 to ascertain the 95% confidence interval for the statistical differences between the samples based on the Tukey test.

2.10. Thermal Characterization

Differential scanning calorimetry (DSC) was performed in a DSC25 Discovery Series differential scanning calorimeter from TA Instruments (New Castle DE, USA). Prior to the test, film samples were dried, and the test was conducted in hermetic capsules using a nitrogen environment with a 50 mL/min flow rate to avoid moisture interference. The thermal cycle consisted of a first heating scan from −80 to 150 °C, a cooling, and a second heating to 200 °C to determine thermal transitions in TPS. The tests were conducted at a heating rate of 10 °C/min.

Thermogravimetric analyses (TGA) were performed with a TGA 1000 thermal analyzer from Linseis (Selb, Germany) to study the thermal stability of the films. Samples (10–15 mg) were heated from 30 °C to 700 °C at a heating rate of 10 °C/min. A flow of 50 mL/min of nitrogen was used. The degradation onset temperature (T5%) was determined at 5% weight loss, while the maximum degradation temperature (Tmax) was determined from the maximum at the first derivative of the TGA curve.

2.11. Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (FTIR–ATR)

The chemical interactions between TPS and PGP and between TPS and PG were investigated using FTIR–ATR. An FTIR–ATR Perkin Elmer Spectrum BX was used for the analysis. With a resolution of 4 cm⁻¹, a range of 2 cm⁻¹, and 16 scans, all formulations were assessed over a 4000–450 cm⁻¹ range.

2.12. Disintegration under Composting Conditions

The test for disintegration under composting conditions was conducted following the guidelines provided by Ref. [21]. Film squares with a side length of 25 mm were prepared for the test. The test film samples were first dried for 48 h at 40 °C. Subsequently, the samples were weighed and placed in a mesh structure that permitted samples to be easily removed and allowed moisture and microbial access [22]. Samples were collected from the container on different days to monitor the disintegration process (1, 4, 7, and 9). All the samples were visually inspected after being taken out of the composting reactor. The disintegration degree was determined gravimetrically by comparing the dry sample pre- and post-composting.

3. Results and Discussion

3.1. Peach Gum Polysaccharide Extraction

Figure 1 displays photographs of raw peach gum (PG), dried PG powder, and the peach gum polysaccharide (PGP) obtained after the thermal hydrolysis extraction. The pictures show that peach gum has a yellowish-red coloration, while PG powder and PGP present a much lighter pale-yellow coloration. The FESEM micrographs show that PG power particles present a polyhedral shape with variable sizes, from 3 to 200 µm, with the most abundant between 5 and 15 µm [17,23]. In contrast, PGP presents layer-like particles that are almost transparent under a magnifying glass.

The obtained extraction yield was 71.37%, close to the value reported in the literature for a thermal hydrolysis extraction [16].

3.2. Optical Characterization of the Films

The appearance of TPS films formed via the solution casting technique is shown in Figure 2. Based only on visual inspection, it can be observed that the inclusion of PG and PGP does not affect the transparency of the TPS film. Nevertheless, the film turns yellow due to both PGP and PG.

Table 2. Color parameters (L*, a*, b*) and color difference (${∆\mathrm{E}}_{a,b}^{*}$) of the TPS-PG and TPS-PGP films.

Film	${\mathit{L}}^{\mathit{*}}$	${\mathit{a}}^{\mathit{*}}$	${\mathit{b}}^{\mathit{*}}$	${∆\mathbf{E}}_{\mathit{a},\mathit{b}}^{\mathit{*}}$
TPS	32.70 ± 0.14 a,b	−0.08 ± 0.01 a	−0.29 ± 0.07 a	-
TPS-5PG	33.81 ± 0.52 c	−0.11 ± 0.01 a,b	1.25 ± 0.11 b	1.94 ± 0.33 a,c
TPS-10PG	33.03 ± 0.38 a,c	−0.13 ± 0.01 b	1.46 ± 0.14 c	1.81 ± 0.12 a
TPS-15PG	32.24 ± 0.17 b	−0.03 ± 0.01 c	1.66 ± 0.09 d	2.01 ± 0.07 a,c
TPS-5PGP	33.04 ± 0.22 a,c	−0.11 ± 0.01 a,b	−0.21 ± 0.02 a	0.40 ± 0.15 b
TPS-10PGP	32.31 ± 0.66 b	−0.10 ± 0.02 a,b	0.31 ± 0.03 e	0.85 ± 0.43 b
TPS-15PGP	34.48 ± 0.31 d	−0.00 ± 0.04 c	1.21 ± 0.14 d	2.33 ± 0.32 c

^a–e Different letters in the same column indicate a significant difference among the samples (p < 0.05).

gathers the CIELAB color coordinates of the TPS-PG and TPS-PGP films. The color coordinate $a^{*}$ (green to red) varies in values near zero, showing no dominant red or green coloration in the films. However, significant differences were observed between 10 PG, 15 PG, and 15 PGP contents and neat TPS. The color coordinate $b^{*}$ (blue to yellow) indicates a change in hue in all the films, from blue to yellow, that increases with the increase in PG or PGP content. Although all the films have a slightly yellow coloration, attributed to the intrinsic coloration of PG and PGP, the color variation of each film is different. TPS-5PG and TPS-10PG color variations can be noticed only by an experienced observer (${∆\mathrm{E}}_{a,b}^{*}$ ≥ 1 and <2), whereas TPS-5PGP and TPS-10PGP color variations remain unnoticeable (${∆\mathrm{E}}_{a,b}^{*}$ < 1). TPS-15PG and TPS-15PGP color variations are noticeable even for an inexperienced observer (${∆\mathrm{E}}_{a,b}^{*}$ ≥ 2 and <3.5).

3.3. Field Emission Scanning Microscopy (FESEM)

Figure 3 shows a comparative study performed by FESEM on the cryofracture cross-sections of TPS, TPS-15PG, and TPS-15PGP films. First, the TPS film (Figure 3a) has a ductile fracture with a homogeneous and smooth surface, confirming the plasticization of starch with glycerol and water. The cracks observed in the surface could be attributed to the cryofracture of the films. PG and PGP are expected to be compatible with starch due to their hydroxyl groups and structural similarity [11,15]. The addition of 15PG in TPS changes the film’s microstructure, making more pores and microcracks appear all over the surface. The size of the pores matches the observed size for PG particles in optical microscopy, which suggests that PG is not miscible with TPS and presents a lack of cohesion in contents of 15 phr. The pores and microcracks produced in the film with the addition of PG may adversely impact the mechanical and barrier properties of the films. On the other hand, adding PGP in 15 phr does not produce more pores or microcracks in TPS, pointing to good miscibility of the materials. This difference in the miscibility of PG and PGP observed in FESEM shows the change in the properties of PGP concerning PG, suggesting an improvement in film manufacturing.

3.4. Mechanical Characterization of the Films

Table 3. Mechanical properties of the TPS-PG and TPS-PG films in terms of elastic modulus (E_t), strength at yield (σ_y), and elongation at break (ε_b).

Film	Thickness (mm)	σy (MPa)	εb (%)	Et (MPa)
TPS	0.192 ± 0.005	1.28 ± 0.23 a,b	24.11 ± 3.45 a	11.67 ± 2.89 a
TPS-5PG	0.223 ± 0.006	0.92 ± 0.19 a	25.03 ± 1.79 a	8.06 ± 1.63 a,c
TPS-10PG	0.226 ± 0.002	1.34 ± 0.21 a,b	34.79 ± 0.58 b,c	10.62 ± 1.97 a,c
TPS-15PG	0.225 ± 0.003	1.72 ± 0.20 b	34.53 ± 2.20 b	27.42 ± 4.46 b
TPS-5PGP	0.182 ± 0.002	0.86 ± 0.19 a	35.77 ± 2.17 a	5.43 ± 0.56 c
TPS-10PGP	0.222 ± 0.003	0.89 ± 0.02 a	40.68 ± 3.09 b,c	4.97 ± 0.82 c
TPS-15PGP	0.187 ± 0.005	0.97 ± 0.10 a	41.33 ± 2.51 c	5.06 ± 0.21 c

^a–c Different letters in the same column indicate a significant difference among the samples (p < 0.05).

summarizes the results of the tensile tests of the films of neat TPS, TPS-PG, and TPS-PGP. Neat TPS film displayed the mechanical characteristics of a ductile material, with values of 24.11% for elongation at break (ε_b), 1.28 MPa for tensile strength at yield (σ_y), and 11.67 MPa for tensile elastic modulus (E_t). One can observe that the σ_y of TPS did not significantly change (p > 0.05) with the addition of PG or PGP in any of the studied contents. ε_b significatively increases (p < 0.05) with the addition of PG and PGP at 10 and 15 phr; however, it is observed that ε_b at 15 phr of PGP is higher than at 15 phr of PG, which may be explained by the reduction in PGP miscibility in TPS in the indicated content. Additionally, TPS E_t does not change significantly with the addition of PG. In contrast, E_t decreases with the addition of PGP, turning TPS into a more ductile material, which suggests a plasticizing effect of this additive. A plasticization effect in TPS due to adding natural resins has been previously reported [24]. This effect could be beneficial for the fabrication of films.

3.5. Water Sensitivity, Moisture Content, and Water Contact Angle

The water sensitivity of TPS, TPS-PG, and TPS-PGP films at 20 °C after 24 h (

Table 4. Water sensitivity, moisture content, and water contact angles (WCA) of raw peach gum (PG), peach gum polysaccharide (PGP), thermoplastic starch (TPS), and formulations with 5, 10, and 15 phr of peach gum (PG) and peach gum polysaccharide (PGP).

Material	Water Sensitivity (%)	WCA (°)	Moisture Content (%)
PG	28.1 ± 0.2 a	-	7.85
PGP	100.0 ± 0.0 b	-	-
TPS	38.3 ± 1.5 c	44.62 ± 1.64 a	9.65 ± 0.10 a
TPS-5PG	46.8 ± 1.5 d	43.52 ± 1.14 a	9.93 ± 0.19 a
TPS-10PG	66.9 ± 1.4 e	40.41 ± 0.85 b	10.60 ± 0.08 b
TPS-15PG	75.4 ± 1.2 f	38.91 ± 1.42 c	11.02 ± 0.08 c
TPS-5PGP	72.8 ± 1.0 f	36.83 ± 0.75 c	11.08 ± 0.04 c
TPS-10PGP	81.7 ± 1.7 g	34.32 ± 1.21 d	11.37 ± 0.05 d
TPS-15PGP	100.0 ± 0.0 b	31.67 ± 0.82 e	11.86 ± 0.11 e

^a–g Different letters in the same column indicate a significant difference among the samples (p < 0.05).

) shows that TPS is only partially soluble in water (38%) near the values reported in the literature [7]. Adding either PG or PGP to the film-forming solutions significantly increased (p < 0.05) the films’ water sensitivity. Moreover, the water sensitivity significantly increases directly with PG and PGP content. PGP was most effective in increasing the water sensitivity (up to 100% with 15 phr of PGP). Additionally, the sensitivity of TPS-15PG is similar to that of TPS-5PGP (75.4% and 72.8%, respectively). Although PG and PGP increase the sensitivity of TPS to water, the mechanisms by which this is produced are different. According to Juan-Polo et al., the ability of TPS to absorb water is enhanced by PG swelling capacity, which also raises the area of contact between TPS and water. This swelling improves the water’s ability to dissolve any remnant soluble starch in the matrix [17]. On the other hand, PGP increases TPS water sensitivity due to its hydrophilic nature; the fraction extracted from the PG is the water-soluble fraction and corresponds to arabinogalactans, which are water-soluble [25]. Therefore, when blended with starch, they can increase the overall hydrophilicity of the material. Also, arabinogalactans can form hydrogen bonds due to their hydroxyl group [26] with water molecules; these bonds could increase TPS’s water absorption and water sensitivity.

Table 4 presents the surface measurements of TPS, TPS-PG, and TPS-PGP films. Neat rice TPS presents a water contact angle of 44.62°, presenting a high hydrophilic character (a substance is hydrophilic if its WCA is less than 65° [27]), which agrees with the literature [28]. It is observed that the WCA gradually decreases in formulations with increasing content of either PG or PGP. The WCA values follow the solubility of the film, showing that as the material turns more hydrophilic (low WCA), the interaction with water facilitates its solubility.

The moisture content increases with PG and PGP in TPS. The highest moisture content was determined for TPS-15PGP film, which is 11.86%. The results agree with the WCA as the films with higher moisture content have higher WCA, meaning they are more hydrophilic, resulting in improved water sensitivity.

3.6. Thermal Characterization

Figure 4 shows the thermogravimetric analysis curves (TG) and first derivative thermo-gravimetry (DTG) curves for TPS films and their formulations with PG and PGP.

Table 5. Main thermal parameters of TPS, TPS-PG, and TPS-PGP in terms of glass transition temperature (T_g), melting temperature (T_m), melting enthalpy (ΔH_m), onset degradation temperature (T_5%), and maximum rate decomposition temperature (T_max1, T_max2).

	TGA				DSC
Material	T5% (°C)	Tmax1 (°C)	Tmax2 (°C)	Residual Mass (%)	Tg (°C)
PG	94.78	242.33	286.33	4.77	89.48
PGP	92.21	266.83	272.83	3.56	67.15
TPS	74.31	201.34	291.81	5.01	−59.30
TPS-15PG	109.06	203.56	289.56	6.89	−58.59
TPS-15PGP	92.07	217.50	289.00	2.79	−58.14

presents the main thermal parameters obtained from the thermogravimetric curves, which correspond to the onset degradation temperature, which was taken into consideration for the temperature with a loss of 5% (T_5%); to the maximum degradation rate temperature (T_max1, T_max2); and to the residual mass at 700 °C. Three distinct mass losses were observed during the degrading process, attributed to water evaporation, polysaccharide decomposition, and charring. The weight loss peaked at 70 °C and was associated with the evaporation of absorbed water. The polysaccharide decomposition was noted between 150 and 350 °C. The loss peaks at about 240–250 °C, probably due to the degradation of the branches which were easily separated from the main chain and degraded to volatiles evolving out at relatively lower temperatures. The chain’s backbone was pyrolyzed at much higher temperatures [29]. The maximum weight loss rate increase was obtained at about 280 °C for TPS films and 270 °C for PGP. The DTG curves indicated that no obvious pyrolysis occurred at temperatures above 350 °C. Above 400 °C, the degradation is related to the thermo-oxidative degradation of the char produced during the first degradation step. The DTG curve of PG and PGP showed a low mass loss rate, suggesting a higher thermal stability than neat TPS. It was mainly attributed to the mixtures of polysaccharides in their composition. Moreover, it is observed that PG provides thermal stability to TPS when added in 15 phr. At the same time, TPS-15PGP presents a similar thermal stability to neat TPS that could be related to water solubility, as PG is a non-water-soluble PG bond that is hard to decompose.

Figure 5 shows the differential scanning calorimetry (DSC) curves for TPS, TPS-15PG, TPS-15PGP, raw PG, and PGP, whereas Table 5 shows the differential scanning calorimetry (DSC) parameters. The first heating curve shows the glass transition temperature (T_g) of PG and PGP at 89.48 °C and 67.15 °C, respectively. The second heating curve shows the T_g of the TPS films. It is observed that PG and PGP produce a negligible change in the TPS glass transition temperature, which is around −59 °C. The lack of change in T_g may be due to the small content of additives and the fact that all the materials are amorphous. As TPS is mainly an amorphous material, it does not present a melting point [30].

3.7. Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (FTIR–ATR)

Figure 6 presents the Fourier infrared spectra of raw PG, PGP, net TPS, and the formulations of TPS with PG and PGP, TPS-15PG, and TPS-15PGP. PG and PGP display similar spectra, presenting the characteristic groups of polysaccharides.

Within TPS, O-H stretching vibration was identified at the board peak at 3298 cm⁻¹, while C-H stretching vibration was linked to the peak at 2924 cm⁻¹ [31,32]. The C-O stretching of the C-O-C bond is associated with the bands at 1010 cm⁻¹ and 1140 cm⁻¹. The band located at 921 cm⁻¹ is indicative of the pyranose rings’ C–O stretching. At 1647 cm⁻¹, the bound water band was found. The typical bands of the glycerol were displayed at 2883 cm⁻¹ (C-H), associated with the hydroxyl groups, as well as at 1420 cm⁻¹ and at 1140 cm⁻¹ (C-O stretching), associated with carbon–oxygen (C–O) absorption peaks characteristic of primary and secondary alcohols [33,34].

PG and PGP present similar spectra as TPS as polysaccharides from all the materials. However, the bands at 1640 cm⁻¹ in PG and PGP spectra were assigned to both the intramolecularly absorbed water and the presence of the carboxyl group [29] The band at 1370 cm⁻¹, according to Ref. [35], was attributed to uronic acids present in PG and PGP A band at 1032 cm⁻¹ was attributed to the C-O-C stretching vibration. These bands are also present in their respective films, confirming these additives’ presence in the TPS matrix.

3.8. Disintegration under Composting Conditions

Figure 7a depicts the visual appearance and the disintegration degree under composting conditions as a function of the time of neat TPS, TPS-PG, and TPS-PGP films.

The picture shows that the color of the film changes and becomes opaque with the composting time, starting from day 1, in TPS and TPS-PG and TPS-PG films. This color’s changes show that the polymeric matrixes underwent a hydrolytic disintegration, changing the sample’s refractive index due to water absorption and hydrolytic process byproducts [36]. Additionally, as shown in Figure 7b, the films exhibit cracks and disintegration from day one, with a disintegration degree of 50% in TPS and TPS-PG and 70% in TPS-PGP. One possible explanation for this could be that the films’ hydrophilic properties rendered them sensitive to water and increased the hydrolysis of the polymer chain, leading to a chain break and the formation of short-chain molecules available for the microorganism’s attack [22]. It should be noted that under composting conditions, all the films decomposed in less than 10 days, indicating their potential benefit as biodegradable, water-soluble films for encapsulating dry food products.

4. Conclusions

The present study evaluates the potential use of peach gum polysaccharide as an additive to improve the water sensitivity of thermoplastic starch films. First, peach gum polysaccharide was extracted using thermal hydrolysis, and a yield of 71.37% was attained. The films were prepared using the casting solution method. FESEM images show that PG and PGP were anticipated to be compatible with starch due to their hydroxyl groups and structural similarity. However, introducing 15 phr PG altered the microstructure, increasing pores and microcracks throughout the TPS film.

In contrast, the film with 15 phr of PGP showed a uniform surface, suggesting good miscibility of PGP in TPS. This disparity in miscibility observed in FESEM implies improved film manufacturing properties for PGP compared to PG. The mechanical characterization results exhibited that PGP significantly increased TPS elongation at break and reduced its elastic modulus, keeping its tensile strength constant, making TPS a more ductile material that may be useful for film applications. Adding PG or PGP to film-forming solutions led to a significant increase (p < 0.05) in the water sensitivity of the resulting films. Furthermore, water sensitivity showed a direct and significant correlation with the content of PG and PGP, with PGP demonstrating the highest effectiveness (up to 100% with 15 phr PGP). The difference between the solubility degrees could be attributed to the water-soluble fraction of PGP, identified as arabinogalactans, which increases the overall hydrophilicity of the material when blended with starch. These molecules form hydrogen bonds with water molecules, leading to enhanced water absorption by TPS, which increases its water sensitivity.

Moreover, the films show a high biodegradability rate, reaching total biodegradation in 10 days under composting conditions. Therefore, peach gum polysaccharide can be used as a biodegradable film additive for coatings or dry food encapsulation. Therefore, peach gum polysaccharide can be used as a biodegradable film additive for coatings or dry food encapsulation.