Edible Film Based on Rambutan Seed (Nephelium lappaceum) Starch: An Alternative Biodegradable Food Packaging

Abstract

This study evaluated the quality and characteristics of edible films derived from rambutan seed (Nephelium lappaceum) starch (3% and 4%) using glycerol and sorbitol as plasticizers at concentrations of 1.0%, 1.5%, and 2.0% (w/v). The optimal formulation was produced using 4% starch combined with 1% (w/v) glycerol and 1% (w/v) sorbitol, exhibiting favorable properties, including thicknesses of 0.24 and 0.22 mm, tensile strengths of 6.90×10⁻³ and 12.06×10⁻³ N/mm², elongations of 38.97% and 4.44%, elasticities of 1.770×10⁻³ and 6.98×10⁻³ kgf/mm², and water absorption rates of 33.3% and 50.0%. Fourier transform infrared spectroscopy analysis confirmed that the film formation involved physical blending without the formation of new functional groups. Scanning electron microscopy revealed that sorbitol provided better compatibility with rambutan seed starch than glycerol. The results of biodegradability tests demonstrated complete degradation of the edible films within seven days, underscoring their environmental sustainability. Rambutan seed starch can be a promising precursor for the development of low-cost and eco-friendly film for packaging applications.

INTRODUCTION

Generally, foodstuffs are very susceptible to quality decline because of the presence of oxygen, water, light, and temperature. Proper packaging is an efficient method to prevent or slow down this decline (Wang et al., 2015). Food packaging refers to the use of appropriate materials that can maintain and protect the quality of food. Plastic is commonly used as a food packaging material. However, it contains hazardous chemicals, is difficult to decompose, and is not environmentally friendly because of its tendency to cause pollution (Wang et al., 2015).

To address these problems, environmentally friendly plastic packaging derived from biodegradable materials is needed. In this context, edible films can be used because they do not pollute the environment, can be decomposed, and are inexpensive (Kumar et al., 2022). Edible films protect products from transporting oxygen, water, heat, carbohydrates, carbon, and lipids, thereby maintaining their original appearance and ensuring environmental safety (Hassan et al., 2018; Zibaei et al., 2021). Edible films are thin sheets of food-grade material made from a biodegradable polymer/biomass. Starch is one example of an abundant and potential biomass. It is commonly used as a biodegradable film that can replace polymers in the food industry because of its excellent physical qualities, affordability, and renewable nature (Yassaroh et al., 2019).

Many studies on edible films sourced from starch, including taro tubers, corn, durian seeds, and jackfruit seeds, have been performed (Rahmawati et al., 2021, 2022). The results showed that the edible film from durian seed starch totally degraded within seven days, was environmentally friendly, and had good mechanical strength and elasticity and a water absorption capacity of 60%. Sliced potatoes treated with edible films had corresponding five- and seven-day shelf lives at room and low temperatures, respectively (Rahmawati et al., 2022). Furthermore, the edible film from jackfruit seed starch was completely degraded within five days, had a water absorption capacity of 75% and good mechanical strength and elasticity, and was environmentally friendly. In addition, sliced potatoes covered with jackfruit seed starch film had four and six-day shelf lives at room and low temperatures, respectively (Rahmawati et al., 2024). Meanwhile, edible films from taro tubers completely degraded within 14 days, were environmentally friendly, had a shelf life of five days at room temperature, and had good mechanical strength and elasticity (Oliveira et al., 2022). Generally, starch from durian seeds, jackfruit seeds, and taro tubers has good mechanical strength and elasticity but tends to absorb more water. This underscores the need for an alternative product (namely, rambutan seed starch, which has relatively good water absorption).

Other researchers investigated the potency of other agrifood by-products and wastes, including potato peel and pulp and pumpkin and orange peel (Jorge et al., 2023). Typically, food by-products comprise polysaccharides, proteins, and lipids, which can be used to create biodegradable packaging materials. These materials are primarily easily degradable and decompose into carbon dioxide and water. Consequently, their application in the production of biodegradable packaging presents a promising alternative, offering more sustainable solutions for food preservation. Arquelau et al. (2019) reported the characteristics of edible films derived from banana peel flour. Meanwhile, Andrade et al. (2016) developed edible films from various fruit and vegetable residues, including passion fruit, watermelon, spinach, cucumber, lettuce, and others.

Rambutan is a seasonal tropical fruit native to tropical Southeast Asian regions. Its name comes from the word “rambut,” which means “hair,” reflecting its distinctive appearance. Thailand, Indonesia, and Malaysia are the three largest producers of rambutan (Nilmat et al., 2023). In Indonesia, the annual rambutan production reaches 9×10⁸ kg in 107,119 ha of land (Wahini et al., 2018). Meanwhile, in Thailand, the annual rambutan production is approximately 3×10⁸ to 4×10⁸ kg (Nilmat et al., 2023). Rambutan is commonly consumed fresh or processed and canned before consumption, resulting in a rapid and high production of rambutan seed and peel waste. These by-products are often immediately disposed of in the environment, without being processed into more valuable materials or products.

Some studies have reported the nutritional value and bioactive compounds contained in rambutan seeds (Olaniyi and Mehdizadeh, 2013; Akhtar et al., 2018; Wahini et al., 2018; Jahurul et al., 2020; Nguyen and Nguyen, 2025). The local rambutan seed contains 64.19% carbohydrates, 15.20% moisture, 11.38% proteins, 6.01% lipids, and 2.51% fiber (Wahini et al., 2018). The high carbohydrate content in rambutan seed makes it a good matrix base for edible film formation. Moreover, rambutan seeds are rich in phenolic compounds, including geraniin, gallic acid, ellagic acid, and corilagin (Akhtar et al., 2018; Jahurul et al., 2020). These phenolic compounds contribute to the antioxidant and antibacterial ability of rambutan seeds, suggesting their potential for film production as an active packaging material (Thitilertdecha et al., 2008). Furthermore, rambutan seeds have been reported to have antinociceptive and central nervous system effects (Rajasekaran et al., 2013). In addition, they contain some essential minerals, including iron, magnesium, zinc, calcium, and manganese (Olaniyi and Mehdizadeh, 2013). These superior constituents and the potential of rambutan seed waste are promising for further processing into edible films.

Although rambutan seed waste has numerous benefits, it lacks thermoplastic properties, making it unsuitable for direct application as a packaging material. Native starch products have poor dimensional stability and low mechanical properties (Sanyang et al., 2015). Many studies have highlighted the brittleness of native starch-based materials, which often exhibit surface cracks that hinder their handling. However, these limitations can be addressed by incorporating plasticizers into pure starch to enhance its workability and mitigate film brittleness (Razavi et al., 2015; Zhang et al., 2016; Farhan and Hani, 2017; Syafiq et al., 2022). The primary function of plasticizers is to improve the flexibility and processability of starch by weakening the strong intermolecular bonds among starch molecules. This addition of plasticizers might increase the mobility of the polymer chains, thereby enhancing the flexibility, extensibility, and ductility of starch-based films. Among the various plasticizers used in starch-based films, polyols (e.g., glycerol and sorbitol) are the most common (Razavi et al., 2015; Sanyang et al., 2015; Zhang et al., 2016; Farhan and Hani, 2017; Syafiq et al., 2022). However, to the best of our knowledge, no study has reported the effects of plasticizer type and concentration in rambutan seed film. Therefore, the present study aims to investigate the effects of plasticizer type (glycerol and sorbitol) and concentrations and to determine the appropriate plasticizer for rambutan seed film production.

MATERIALS AND METHODS

Materials

The materials used included rambutan seeds from the local region of Palu, Indonesia; 2.3 g/L of sodium bisulfite (Na₂SO₃), distilled water, carboxymethylcellulose (CMC), sorbitol, and glycerol obtained from the Advanced Chemistry Laboratory of Fakultas Keguruan dan Ilmu Pendidikan (FKIP), Tadulako University, Indonesia, and Hidayah Chemical Company, Palu City, Central Sulawesi.

Preparation of rambutan seed (Nephelium lappaceum) starch

Approximately 2 kg of rambutan seeds was peeled from the skin and cleaned. After being divided into tiny parts, the rambutan seeds were cleaned using distilled water. Subsequently, the seeds were blended slowly and added with 0.0230% sodium bisulfite solution (comprising 2.3 g of sodium bisulfite and 2 L of distilled water) until the material was smoothed out. Moreover, a small permeable towel was used to squeeze the seeds. The resulting residue was disposed, and the juice was allowed to stand for one day. After being cleaned with distilled water, the precipitate was dried in an oven set at 50°C for 24 h. The dried starch was mashed or mixed and then sieved through a 100-mesh sieve.

Preparation of edible film

Approximately 3 and 4 g of rambutan seed starch were weighed and placed in different beakers. Subsequently, 80 mL of distilled water and plasticizer (glycerol and sorbitol in different containers) with concentrations of 1.0%, 1.5%, and 2.0% (w/v) were mixed. The solution was heated on a hot plate at 80°C for 15 min while stirring. Thereafter, the solution was added with 1% (w/v) of CMC in 20 mL of distilled water. The mixture was heated for another 5 min at 80°C. Next, the solution was poured on a 25×20×3 cm mold and placed in an oven at 50°C for 24 h to dry out. The mold was then removed from the oven and allowed to cool for 5 min at room temperature. The thin film that was created was scraped using a spatula and placed in a desiccator for storage (Rahmawati et al., 2024).

Physical and chemical characterization of the edible film

Edible film thickness test: The thickness of the edible film sample was measured with a micrometer at five different areas, with an accuracy of 0.01 mm. The measurement was performed in triplicate, and the results were presented as the average±standard deviation.

Tensile strength, percent elongation, and elasticity tests: The tensile strength is the highest force that the edible film can withstand and percent elongation describes the greatest length change that occurs when a film is pulled or stretched until it breaks. After cutting the edible film to a size of 10×5 mm, it was gradually loaded onto the point at which the film broke. The tensile strength and percent elongation of the edible film were determined using the following formulas (Liyanapathiranage et al., 2023):

$$\text{Tensile strength }\left({\text{N/mm}}^{\text{2}}\right)\text{=}\frac{\text{Fmax}}{\text{A}}$$ (1)

where F max is the sample tensile value (N) and A is the area of the sample (mm²).

$$\text{\% Elongation=}\frac{\text{ΔL}}{{\text{L}}_{\text{o}}}\text{×100\%}$$ (2)

where ΔL is the difference in the film extension and L_o is the initial length of the film.

The relationship between tensile strength and elongation yield elasticity (Rahmawati et al., 2022). The measurements were performed in triplicate, and the results were presented as the average±standard deviation.

Functional group analysis with fourier transform infrared spectroscopy (FTIR): FTIR functional group analysis aimed to identify the functional groups and compounds in the edible film. The samples were kept in a set holder before being looked up in the spectrum. A spectrophotometer operating at room temperature was used to record the FTIR spectra. The information was acquired as a spectrum image showing the correlation between the transmittance and the wave number (Ningsih et al., 2021).

Biodegradability test: The biodegradability test was performed using effective microorganism 4 (EM4) bacteria. These microbes are frequently used to ferment organic materials found in soil. Actinomycetes (a type of photosynthetic bacteria), yeast, phosphate-solubilizing bacteria, and fermentation fungi are all found in EM4 (Rahmawati et al., 2022).

Water absorbance test: After measuring the starting weight (D) of the 3×3 cm edible film sample, it was placed in a container with 10 mL of distilled water for 10 s, and its final weight (B) was determined. Furthermore, the sample was immersed and weighed again until its final weight remained constant (Rahmawati et al., 2022). The measurement was performed three times and the results were presented as the average±standard deviation.

$$\text{\% Water absorption capacity=}\frac{\text{B-D}}{\text{B}}\text{×100\%}$$ (3)

where B is the final weight and D is the initial weight.

Scanning electron microscopy (SEM): The film’s morphology was evaluated using the Thermo Scientific Quattro S Scanning Electron Microscope at an accelerating voltage of 10 kV.

Quality parameters of the edible film: Some factors affect quality properties, including the strength in manufacturing the edible film. Table 1 shows the standards for making edible films (Santoso and Atma, 2020).

Table 1.

Standards for making edible films

Parameter	Value
Thickness (mm)	<0.25
Tensile strength (MPa)	Min 0.39
Elongation	<10% bad
	10%－50% good
	>50% very good

Data from the article of Santoso and Atma (2020).

Statistics analysis: The experimental data are expressed as mean±standard deviation.

RESULTS

Edible film thickness test

Rambutan seed starch is powdery and yellowish in color and has a unique aroma. The edible film was created from rambutan seed starch by adding glycerol and sorbitol in various ratios (1.0%, 1.5%, and 2.0%). The thickness of edible film samples was measured using a micrometer at five different areas, with an accuracy of 0.01 mm. Twelve samples were tested and the average results are shown in Table 2. The edible films produced had thicknesses between 0.20 mm and 0.24 mm, meeting the Japanese Industrial Standard (JIS) 1975, which specifies a maximum thickness of 0.25 mm (Ramos et al., 2012). The thickness of the films slightly decreased with increasing plasticizer concentration. The maximum thickness of 0.24 mm was achieved with 4% starch and 1% glycerol, whereas a thickness of 0.23 mm was obtained with 3% starch and 1% sorbitol. In a previous study, increased plasticizer concentration significantly resulted in higher thickness (Farhan and Hani, 2017). Furthermore, the type of plasticizers also plays an important role in improving the film thickness (Syafiq et al., 2022).

Table 2.

Physicochemical characteristics of rambutan seed starch-based edible film at various concentrations of glycerol (1.0%, 1.5%, and 2.0%) and starch (3% and 4%) and sorbitol (1.0%, 1.5%, and 2.0%) and starch (3% and 4%)

Parameters	Sample composition
	Starch 3%				Starch 4%				Starch 3%				Starch 4%
	G1 (1.0%)	G2 (1.5%)	G3 (2.0%)		G4 (1.0%)	G5 (1.5%)	G6 (2.0%)		S1 (1.0%)	S2 (1.5%)	S3 (2.0%)		S4 (1.0%)	S5 (1.5%)	S6 (2.0%)
Thickness (mm)	0.21±0.004	0.20±0.000	0.20±0.000		0.24±0.017	0.21±0.002	0.22±0.000		0.23±0.005	0.21±0.004	0.21±0.005		0.22±0.000	0.21±0.005	0.22±0.001
Tensile strength(10—3 N/mm2)	3.71±0.005	3.75±0.005	3.37±0.009		6.90±0.005	4.38±0.005	3.14±0.005		9.61±0.005	4.16±0.005	6.26±0.005		12.06±0.004	5.38±0.005	5.02±0.004
Elongation (%)	14.37±0.000	116.61±0.009	118.75±0.005		38.97±0.005	93.67±0.001	82.67±0.001		13.7±0.004	51.57±0.004	109.23±0.005		4.44±0.005	75.89±0.005	74.05±0.004
Elasticity(10—3 kgf/mm2)	25.81±0.005	3.21±0.005	2.84±0.009		1.77±0.094	4.68±0.005	3.80±0.005		70.14±0.005	8.06±0.004	5.73±0.005		6.98±0.050	7.08±0.047	6.78±0.047
Water absorbance (%)	25.0±0.047	50.0±0.005	40.0±0.005		33.3±0.004	33.3±0.005	25.0±0.047		40.0±0.047	25.0±0.050	25.0±0.047		50.0±0.816	33.3±0.471	40.0±0.820

Values are presented as mean±SD.

G1－G3 and G4－G6 contain 3% and 4%, with 1.0%, 1.5%, and 2.0% glycerol, respectively.

S1－S3 and S4－S6 contain 3% and 4%, with 1.0%, 1.5%, and 2.0% sorbitol, respectively.

Tensile strength, elongation, and elasticity

The tensile strength values of films are shown in Table 2. Among the films containing glycerol, the highest tensile strength (6.90×10⁻³ N/mm²) was observed in the film with 4% starch, 1% glycerol, and 1% CMC (sample G4). For the films containing sorbitol, the highest tensile strength was 12.06×10⁻³ N/mm² in sample S4 containing 4% starch and 1% sorbitol. For glycerol and sorbitol, the lowest plasticizer concentration resulted in the highest tensile strength. However, the tensile strength of the sorbitol-plasticized film was about two times higher than that of the glycerol-plasticized film. Adding plasticizers at higher concentrations (1.5% and 2.0%) decreased the film’s strength. The tensile strength largely decreased from 6.90×10⁻³ N/mm² to 3.14×10⁻³ N/mm² for glycerol-plasticized film and from 12.06×10⁻³ N/mm² to 5.02×10⁻³ N/mm² for sorbitol-plasticized film. Several studies have reported the same trend (Razavi et al., 2015; Sanyang et al., 2015; Syafiq et al., 2022). The elongation at break, shown in Table 2, increased with higher glycerol and sorbitol concentrations and was inversely related to the tensile strength. Several studies have also reported similar results (Suppakul et al., 2013; Kurt and Kahyaoglu, 2014; Razavi et al., 2015; Syafiq et al., 2022). The highest elongation was 118.75% in a film made with 3% of starch and 2% glycerol, whereas an elongation of 109.23% was achieved in a film made with 3% of starch and 2% sorbitol. Elasticity, which was calculated as the ratio of tensile strength to elongation, decreased with increasing plasticizer concentration, indicating a reduction in the film’s ability to return to its original shape.

FTIR functional group analysis

The results of FTIR analysis of edible films (Fig. 1) revealed that the films contained the same functional groups as the starch from rambutan seeds. The presence of plasticizers increased the hydroxyl (O-H) group intensity at different wave numbers. The films displayed functional groups, including carbon-hydrogen alkane at around 2,920 to 2,922 cm⁻¹, O-H at 3,356 to 3,568 cm⁻¹, and carbon-oxygen at 1,016 to 1,716 cm⁻¹ for sorbitol- and glycerol-plasticized films, indicating their biodegradability and ecofriendliness (Table 3). However, a major alteration was observed for peaks at 3,300 to 3,500 cm⁻¹. The peaks were broader in films with sorbitol than those with glycerol because of the higher O-H group content in sorbitol.

Fig. 1.

FTIR spectra of rambutan seed starch-based edible film supplemented with glycerol (A) and sorbitol (B). FTIR, fourier transform infrared spectroscopy. The sample compositions are the same as those in Table 2.

Table 3.

Analysis of FTIR results for edible film added with glycerol and edible film added with sorbitol

Functional group	Wave number (cm—1)
	Starch	G1	G2	G3	G4	G5	G6	S1	S2	S3	S4	S5	S6
C-H alkane	2,920	2,922	2,922	2,922	2,922	2,922	2,922	2,922	2,922	2,922	2,920	2,922	2,920
	1,463	1,463	1,463	1,463	1,463	1,463	1,463	1,463	1,462	1,463	1,462	1,462	1,462
O-H	3,356	3,446	3,452	3,568	3,435	3,554	3,464	3,477	3,500	3,442	3,523	3,500	3,477
C-O ester, alcohol, ether, carboxylic acid	1,016	1,018	1,047	1,047	1,020	1,024	1,024	1,049	1,020	1,020	1,018	1,020	1,020
C=O aldehydes, ketones, acid carboxylate, ester	1,708	1,714	1,714	1,716	1,714	1,714	1,714	1,712	1,710	1,712	1,710	1,710	1,710

FTIR, fourier transform infrared spectroscopy; C-H, carbon-hydrogen; O-H, hydroxyl; C-O, carbon-oxygen single bond; C=O, carbon-oxygen double bond.

The sample compositions are the same as those in Table 2.

Biodegradability test

The biodegradability of the films was evaluated by immersing them in an EM4 bacteria solution, a brownish liquid containing a consortium of microorganisms (e.g., bacteria, fungi, and yeast) (Rahmawati et al., 2024). The EM4 solution comprises naturally growing microorganisms that are widely used in agriculture, decomposing organic waste, and environmental remediation. The biodegradability was measured based on the physical appearance observed daily until the films were completely degraded. Film degradation was assessed based on the physical alterations resulting from the chemical reactions, including the breaking of the macromolecular bonds. The biodegradability test showed that glycerol- and sorbitol-based edible films fully disintegrated in seven days, as shown in Fig. 2A and 2B. This finding suggested that the films containing glycerol and sorbitol are environmentally friendly and break down faster than conventional plastics.

Fig. 2.

Biodegradation of film containing different glycerol (A) and sorbitol (B) concentrations. The sample compositions are the same as those in Table 2.

Water absorption test

The water absorption test results are presented in Table 2. The water absorption values varied depending on the concentrations of glycerol and sorbitol. Films with glycerol and sorbitol exhibited water absorption values ranging from 25% to 50%. The water absorption decreased with increasing glycerol and sorbitol concentrations. Increasing the plasticizer concentrations decreased water absorption because of the more compact network formed by the interaction of starch and plasticizers. However, other studies found that increasing the glycerol concentration significantly enhanced water absorption, but not the sorbitol concentration (Razavi et al., 2015; Syafiq et al., 2022).

Morphology of the film

SEM analysis at 500× and 1,000× magnifications revealed that the glycerol-based films had a donut-like structure with small hollows in the starch granules (Fig. 3). This finding might be referred to as an incomplete dissolution of the starch granules and less compatibility with glycerol. However, the starch granules were distributed evenly on the films. In the sorbitol films, the starch granules were less noticeable, and the film appeared smoother and more homogeneous. This result indicated that more granules were dissolved well in the sorbitol-water system and that rambutan seed starch had better compatibility with sorbitol than with glycerol.

Fig. 3.

Surface morphology of films containing 4% starch and 1% glycerol (A) and sorbitol (B) at different magnifications (500×, 1,000×, 2,500×, and 5,000×).

DISCUSSION

The results demonstrate that rambutan seed starch can be used to effectively create edible films with desirable mechanical and physical properties. The physical characteristics of rambutan seed starch, including its powdery texture and unique color, align with the expectations for starch-based films. The addition of glycerol and sorbitol at varying concentrations helped create edible films, confirming the suitability of these plasticizers for film formation. The thickness values of the edible films also met the JIS 1975 requirement of ≤0.25 mm, highlighting the feasibility of using rambutan seed starch for food packaging applications. In our study, the thickness of the films slightly decreased with increasing plasticizer concentration. This could be explained by the effect of plasticizers on the structural arrangement and packing density of polymer chains. Plasticizers act by interfering with the strong intermolecular interactions among starch polymer molecules, including hydrogen bonds. This disruption enhances the mobility and flexibility of the polymer chains, allowing them to rearrange and pack more closely. Consequently, the overall thickness of the film tends to decrease because the material becomes less rigid and more conformable. Moreover, during the casting process, the increased mobility of the polymer chains because of the higher plasticizer concentrations may lead to better spreading and leveling of the film on the glass plate, reducing the overall bulk of the film and further contributing to the reduced thickness. Nguyen and Nguyen (2025) reported that increasing glycerol concentrations in starch-based films decreased the film thickness because of enhanced chain mobility and improved packing of the polymer network (Nguyen and Nguyen, 2025). However, other studies found that the types and concentrations of plasticizers significantly increased the thickness of the prepared films (Farhan and Hani, 2017; Syafiq et al., 2022). The differences in the film thickness when using various plasticizers can be explained by their respective molar masses. The films plasticized with sorbitol (molar mass 182.17 g/mol) typically exhibited a thicker structure compared with those plasticized with glycerol (molar mass 92.09 g/mol), which is attributed to the higher molar mass of sorbitol (Syafiq et al., 2022).

The results of FTIR analysis confirmed the compatibility of rambutan seed starch with glycerol and sorbitol. Furthermore, the results verified that the addition of glycerol and sorbitol did not alter the fundamental chemical structure of the starch but did influence the interaction between the starch and the plasticizers. The increased presence of O-H groups, especially in the sorbitol-based films, suggested that the plasticizers are well integrated into the starch matrix. The broader peaks in the sorbitol films indicated better miscibility and hydrogen bonding, supporting the superior mechanical performance observed. The results of SEM morphology analysis further supported the superior compatibility of sorbitol with rambutan seed starch. The smoother surface and more homogeneous structure of the sorbitol films suggested enhanced hydrogen bonding, corroborating the improved mechanical and water-resistant properties. This finding suggests that sorbitol might enhance the formation of a more compact and stable film structure compared with glycerol.

The tensile strength results suggested that sorbitol contributes better mechanical properties than glycerol, likely because of the stronger hydrogen bonding within the polymer matrix. The decrease in tensile strength with increasing plasticizer concentration aligns with the theory that plasticizers reduce hydrogen bonding, enhancing flexibility but reducing strength (Ballesteros-Mártinez et al., 2020). The increased tensile strength observed at low plasticizer concentrations can be attributed to the predominance of robust hydrogen bonding among starch molecules, which outweighs the interactions between the starch and the plasticizer (Syafiq et al., 2022). Starch-based bioplastics have a dense hydrogen bonding network because of O-H groups on the glucose units of amylose and amylopectin (Yassaroh et al., 2021, 2022). Glycerol and sorbitol, which are polyhydric alcohols, interact with the O-H groups of starch through hydrogen bonding. This interaction replaces some hydrogen bonds among starch molecules, thereby reducing the strong inter- and intramolecular forces within starch that cause rigidity and facilitating hydrogen bond formation between the starch and the plasticizer. These plasticizers decrease the glass transition temperature by reducing the hydrogen bonding density within the starch network and enhance the flexibility and ductility of the films by increasing the chain mobility (Sanyang et al., 2015). Consequently, the hydrogen bonds between the starch chains are weakened, reducing the tensile strength of the films with increasing plasticizer concentrations (Sanyang et al., 2015). Glycerol resulted in a more significant reduction in tensile strength compared with other polyols. This effect can be attributed to its lower molar mass of 92.09 g/mol compared with sorbitol’s molar mass of 182 g/mol, which allows for easier interactions between glycerol and starch molecular chains (Syafiq et al., 2022). The tensile strength was high at low plasticizer concentrations. The high tensile strength at lower glycerol or sorbitol concentrations can be explained by the preferable strong hydrogen bonding between starch-starch molecules over starch-glycerol and starch-sorbitol interaction (Sanyang et al., 2015; Syafiq et al., 2022). High plasticizer concentrations reduce the starch–starch interactions and the tendency to form hydrogen bonds between the starch and the plasticizer. At the same time, this occurrence increased the elongation of films at higher plasticizer concentrations. The elongation results indicate improved flexibility at higher glycerol and sorbitol concentrations, as confirmed by the higher elongation percentages and elasticity values (Farhan and Hani, 2017).

Our result meets the JIS standard for high-quality films, with sorbitol producing slightly better flexibility than glycerol, likely because of differences in the molecular interactions within the polymer matrix. These findings align with the standard for flexible edible films, showcasing their potential for various applications. The increase in film elongation can be attributed to the plasticizers weakening the intermolecular interactions among the starch molecules. This process involves the substitution of starch-starch hydrogen bonds with plasticizer-starch hydrogen bonds (Syafiq et al., 2022). The disruption and reorganization of the starch molecular chains likely reduced the rigidity of the matrix, thereby enhancing the films’ flexibility by increasing the molecular chain mobility. The decrease in elasticity with higher plasticizer content aligns with the expectation that adding glycerol or sorbitol reduces the intermolecular forces in the film, thereby increasing the flexibility but reducing its overall rigidity. The decrease in the bond distance between the molecules resulted in a decrease in the elasticity value. Consequently, more plasticizer molecules are in a different phase, decreasing the tension that holds the molecules together and allowing for more flexible chain movement (Jost et al., 2014; Yadav et al., 2021).

The results of the water absorption tests showed that higher plasticizer concentrations resulted in lower water absorption, which is attributed to a more compact network formation, thereby reducing the film’s ability to absorb water. This is an important characteristic of food packaging as it suggests that the films could provide better protection against moisture. However, Syafiq et al. (2022) reported that the glycerol concentration increased the moisture of the films. They explained that the O-H groups in glycerol exhibited a strong attraction to water molecules, enabling glycerol-plasticized films to readily retain water by forming hydrogen bonds within their structure. Sorbitol did not significantly change the moisture content of the films (Razavi et al., 2015; Syafiq et al., 2022). The close resemblance in the molecular structure between the glucose units and sorbitol led to stronger interactions between the intermolecular polymer chains and sorbitol. Consequently, sorbitol’s ability to interact with water molecules was reduced (Syafiq et al., 2022).

The rambutan seed starch-based biofilm was completely degraded within seven days in an EM4 bacterial solution. Similar biodegradability was also observed in jackfruit seed-based and durian seed-based biofilms, which degraded in seven days (Ningsih et al., 2021; Rahmawati et al., 2022). Compared with the pure starch from taro, which required 14 days to degrade completely, the fruit seed waste-based biofilms degraded faster. The shorter degradation time of the rambutan seed waste-based film can be attributed to its lower carbohydrate content (64.19%) than that of taro starch (80%). The higher amount of starch content may lead to more and stronger interactions of starch molecules in the films. Hence, the microorganisms had more difficulty attacking the films. The rapid biodegradation of the films within seven days is a key finding, highlighting the environmental benefits of using rambutan seed starch-based edible films. This finding demonstrates that the films are a viable alternative to traditional plastics, which are slower to decompose (Silva et al., 2020). Moreover, this finding is significant for reducing environmental pollution. In conclusion, the study highlights the potential of rambutan seed starch as a sustainable and efficient raw material for biodegradable edible films, with sorbitol as a preferred plasticizer for optimal performance.