Valorization of potato peel waste into starch-based bioplastic films using glycerol and sorbitol plasticizers
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MGloryJasmineRani1✉Emailranijasmine19@gmail.com
Dr.
RMallika11
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Department of BiochemistryV.V.Vanniaperumal College for women626001VirudhunagarM Glory Jasmine Rani1*, Dr. R Mallika 2
1*Research scholar, Department of Biochemistry, V.V.Vanniaperumal College for women, Virudhunagar, 626001
2 Associate Professor, Department of Biochemistry, V.V.Vanniaperumal College for women, Virudhunagar, 626001
*Corresponding Author's Email: ranijasmine19@gmail.com,7708896875
Abstract
The valorization of potato peel waste into biodegradable materials offers a sustainable route to reduce plastic pollution while utilizing agro-industrial residues. In this study, starch was extracted from potato peel waste by wet sedimentation, yielding 17.10% with high purity (ash 0.47%, moisture 14.00%, amylose 21.20%, amylopectin 78.80%). Three bioplastic films were prepared using glycerol, sorbitol, and a glycerol–sorbitol blend as plasticizers and evaluated for physical, mechanical, optical, structural, and biodegradation properties. Glycerol-plasticized films exhibited superior flexibility, high swelling (47.0 ± 3.0%) and water absorption (48.6 ± 3.40%), and the fastest biodegradation, reaching 95.62 ± 9.78% weight loss within 20 days under soil burial conditions. Sorbitol films showed the highest tensile strength (38.11 ± 1.52 N/cm2) and lowest water absorption (28.8 ± 1.15%), while blend films displayed intermediate strength (25.05 ± 1.20 N/cm2) and slower biodegradation (64.16 ± 5.41%). Fourier transform infrared (FTIR) spectra confirmed preservation of the starch polysaccharide backbone with distinct hydrogen bonding patterns, where glycerol induced greater amorphous character while sorbitol enhanced molecular packing. X-ray diffraction (XRD) analysis of Glycerol-potato peel starch films indicated semi-crystalline structures with a crystallinity index of 91.6%. Application trials demonstrated that glycerol-based films adhered strongly as a leak-proof coating for paper cups and were mouldable into 3D products. Overall, glycerol-plasticized potato peel starch films were identified as the most promising formulation, combining desirable flexibility, rapid biodegradability, and practical applicability for sustainable packaging.
Keywords
Potato peel starch. Bioplastic. Plasticizer. Glycerol. Sorbitol. Waste valorization
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Introduction
The escalating issues of plastic pollution and depletion of petroleum-based resources have intensified efforts to develop sustainable, biodegradable alternatives [1]. Worldwide plastic production has reached nearly 390 million tonnes annually, yet only about 9% is recycled; the remainder is incinerated or accumulates in landfills and ecosystems [2]. These concerns have prompted stricter regulations, including the EU Single-Use Plastics Directive (2019) and extended producer responsibility (EPR) policies aimed at reducing conventional plastic use [3]. Market projections indicate strong growth in the bioplastics sector, from 2.22 million tonnes in 2023 to more than 7 million tonnes by 2033, driven by legislative pressure, consumer demand, and circular economy initiatives [4]. Starch-based bioplastics are promising sustainable materials due to their renewability, biodegradability, and low cost [1], though native starch films often require plasticizers or other additives to overcome poor mechanical strength and water sensitivity [5]. Potato peel, an abundant food-industry waste, offers a low-cost starch source, yet has been limitedly explored for film production. Imoisili and Jen showed that glycerol-plasticized films from potato peel starch improved in tensile strength and water absorption with higher glycerol levels [6], while Chemiru and Gonfa also reported meaningful mechanical stability in similar formulations [7].
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Plasticizer choice strongly affects bioplastic performance. Sorbitol generally provides higher tensile strength and lower moisture uptake, whereas glycerol produces more flexible, hydrophilic films [8]. However, limited work has compared glycerol, sorbitol, and their blend specifically in potato peel starch films, leaving a gap in tailoring material properties. Accordingly, this study aims to: (i) extract and characterize potato peel starch (yield, amylose content, gelatinization temperature). (ii) develop films using glycerol, sorbitol, and a glycerol-sorbitol blend. (iii) evaluate physical (thickness, moisture content, density), mechanical (tensile strength, swelling, water absorption), and structural (transparency, FTIR, XRD) properties to determine plasticizer effects. (iv) assess biodegradation through soil burial tests and (v) examine the suitability of potato peel starch films for sustainable packaging applications.Materials and methods
Samples
Fresh potato tubers (Solanum tuberosum) were procured from a local market in Madurai, Tamil Nadu, India. Tubers were washed, peeled, and the peels were rinsed with distilled water to eliminate dirt and surface starch, then stored at 4°C in zip-sealed polyethylene bags. Analytical-grade chemicals (sodium hydroxide, acetic acid, ethanol, potassium iodide, iodine) were obtained from HiMedia Laboratories Pvt. Ltd., while food-grade glycerol and sorbitol were sourced from Merck Life Sciences Pvt. Ltd. All solutions were prepared using distilled water.
Starch extraction from potato peel
Starch was extracted using a modified wet sedimentation method (Fig. 1) described by Noorfarahzilah et al. and Nakkala et al. [9, 10]. About 260 g of potato peels were cut into ~ 1 cm² pieces and homogenized with 100 mL distilled water at 15,000 rpm for 5 min to obtain a fine slurry. The slurry was filtered through double-layer muslin cloth (~ 60 µm), and the filtrate was allowed to settle at ~ 28°C for 2 h. After decanting the supernatant, the sediment was washed with 200 mL distilled water, stirred briefly, and re-settled for 30 min. This wash–settle cycle was repeated three times to remove impurities. The final sediment was oven-dried at 55°C for 24 h, ground, and sieved (100-mesh). The resulting starch powder was stored in airtight amber containers at 4°C. Starch yield (%) was calculated as:
Weight of fresh potato peel (g)
Characterization of extracted starch
Microscopy
The morphology of potato peel starch granules was examined using a compound light microscope (Olympus CX23, Olympus Corporation, Tokyo, Japan) at 40x magnification under bright-field illumination. A small amount of dried starch was dispersed in a drop of distilled water on a glass slide and covered with a coverslip. Granule shape, size, and surface texture were observed, and representative images were captured using a digital camera attachment (Olympus LC30, Olympus Corporation, Tokyo, Japan).
Organoleptic evaluation
The extracted starch was visually inspected for color and clarity, and odor was assessed directly.
pH
One gram of starch was dispersed in 100 mL distilled water, and pH was recorded using a calibrated digital pH meter (Eutech pH700, Thermo Fisher Scientific, Singapore).
Qualitative confirmation tests for starch
Starch was qualitatively confirmed using standard phytochemical tests [11].
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Molisch’s test for carbohydrates
Formation of a reddish-violet ring indicated positive result.
Fehling’s test for reducing sugars
Absence of brick-red precipitate indicated negative result.
Benedict’s test for reducing sugars
Absence of red precipitate indicated negative result.
Iodine test for non-reducing sugars
Development of blue color after cooling confirmed starch presence.
Physico-chemical properties
Ash
The ash content of the extracted potato peel starch was determined following AOAC guidelines [12]. Approximately 5 g of dried starch was weighed into a pre-weighed silica crucible and incinerated in a muffle furnace at 550°C until a constant weight of white ash was obtained (about 4 h). The crucible was then cooled in a desiccator and reweighed. Ash content was calculated as:
Ash content (%) = Weight of dry sample (g)x100
Weight of ash (g)
Moisture
Moisture content was measured according to AOAC [12]. Five grams of starch was weighed into a pre-weighed moisture dish and dried in a hot air oven at 105°C until a constant weight was achieved (approximately 4–6 h). The dried sample was cooled in a desiccator and weighed. Moisture content was calculated using:
Moisture content (%)=[Initial weight − Final weight ] ×100
Initial weight
Amylose and Amylopectin
Amylose content was determined using the iodine-binding method [13]. Briefly, 20 mg of starch was dispersed in 10 mL of 1 N NaOH and heated in a boiling water bath for 10 min for complete gelatinization. The mixture was cooled, neutralized with 1 N acetic acid, and treated with 2 mL of iodine–potassium iodide solution (0.2% I2 in 2% KI). The final volume was made up to 100 mL with distilled water, mixed thoroughly, and absorbance was measured at 620 nm using a UV–Vis spectrophotometer (Shimadzu UV-1800, Japan). Amylose content was obtained from a standard calibration curve prepared with potato amylose (Sigma-Aldrich, USA), and amylopectin (%) was calculated as 100 minus the amylose value.
Functional Characteristics
Functional characteristics were determined following AOAC methods in comparision with commercial starch (laboratory grade) [12].
Solubility
Solubility was determined by heating 1 g starch in 100 mL distilled water at 60°C for 30 min, centrifuging at 3000 rpm for 10 min (Remi C-24BL, India), and drying the supernatant to a constant weight. Solubility was expressed as grams of soluble starch per 100 g (%) dry starch.
Starch
Determined by acid hydrolysis followed by titration with Fehling’s solution.
Foam capacity
Measured by whipping 2 g starch in 50 mL distilled water for 5 min in a high-speed blender and recording volume increase.
Swelling power
Determined by heating 1 g starch in 50 mL water at its gelatinization temperature (72°C for commercial starch, 71.33°C for potato peel starch) for 30 min, centrifuging at 3000 rpm for 15 min, and calculating the weight of swollen sediment per gram of dry starch.
Gelatinization temperature
Measured following Kandekar et al. [14]. A 2% starch suspension (w/v) was prepared in distilled water in a 250 mL beaker and stirred continuously with a magnetic stirrer. The suspension was heated at a rate of 2°C/min, and the temperature at which the suspension first exhibited a translucent, viscous consistency was recorded using a calibrated mercury thermometer.
Bioplastic film formation
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Bioplastic films were prepared using the solution-casting method adapted from Marichelvam et al. [15]. Three formulations (Table 1) were heated to 70°C on a magnetic stirrer at 500 rpm for ~ 15 min until a translucent gel formed. The hot gel was poured into 14 cm glass Petri dishes, levelled for uniform thickness, and dried at ambient temperature (25 ± 2°C) for 48 h. Dried films were peeled and stored in desiccators (25% RH, silica gel) before testing.Physical property analysis
Thickness
Film thickness was measured using a digital micrometre screw gauge (Mitutoyo, Japan) at five randomly selected points across each film, and the mean value was recorded [15].
Moisture content
Film samples (2 × 2 cm2) were weighed (Wi) and dried in an oven at 105°C until constant weight (Wf). Moisture content (%) was calculated as:
Wi
Density
Density was calculated by dividing the mass of the film by its volume (area x average thickness) as per the formula,
𝜌 = 𝑚𝑣𝑔/𝑐𝑚³
Where: 𝜌 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦, 𝑚 = 𝑚𝑎𝑠𝑠,𝑎𝑛𝑑 𝑣 = 𝑣𝑜𝑙𝑢𝑚𝑒
Mechanical and water interaction properties
Tensile strength
Mechanical strength was evaluated according to ASTM D882 using a universal testing machine (Instron 3365, USA). Rectangular strips (10 mm x 50 mm) were clamped between grips and stretched at a crosshead speed of 10 mm/min until breakage. Tensile strength (N/cm²) was calculated from the maximum force divided by the initial cross-sectional area [16].
Cross-sectional area (cm2)
Swelling capacity
Swelling capacity was determined by immersing pre-weighed (W₁) film specimens (2 cm x 2 cm) in 25 Ml of distilled water at room temperature for 30 min. The swollen films were removed, gently blotted to remove surface water, and weighed again (W₂). Swelling capacity (%) was calculated as described by Sudha et al. using the formula [17]:
W₁
Water absorption
Water absorption was measured by immersing pre-weighed dried film specimens (W₁) in distilled water at room temperature for 24 h. The films were removed, surface water was blotted off, and the samples were weighed (W₂). Water absorption (%) was calculated using [18]:
W₁
Optical characterization
Transparency
Film transparency was measured using a UV–Visible spectrophotometer (Model: UV-1800, Shimadzu, Japan) according to ASTM D1746-15. Film strips (2 cm x 1 cm) were placed in the sample holder, and transmittance was recorded at 600 nm using air as the blank. Transparency (T600) was expressed as the percentage of light transmitted, where lower values indicated greater opacity [19].
Biodegradability test
The biodegradability of potato peel starch-based bioplastic films containing different plasticizers (glycerol, sorbitol, and their combination) was assessed using the soil burial method. Film samples (2 x 2 cm) of uniform thickness and known initial weight were buried in moist garden soil under controlled laboratory conditions for 20 days. Samples were retrieved every 5 days, rinsed to remove soil, air-dried, and reweighed. Biodegradation (%) was calculated from the weight loss between the initial and final measurements.
W0
FTIR spectroscopy
FTIR analysis was carried out to identify functional groups in potato peel starch and bioplastic films prepared with glycerol, sorbitol, and their blend, using a commercial starch bioplastic as reference. Spectra were obtained using a Shimadzu IRTracer-100 spectrometer with a DLATGS detector. Film samples (1 cm2) were dried at 40°C for 4 h and analyzed in ATR mode. Spectra were recorded from 4000–400 cm− 1 at 4 cm− 1 resolution with 32 scans per sample. Background spectra were collected before each run and automatically subtracted to minimize environmental interference [20]. Based on the FTIR result the plasticizer which demonstrated pronounced hydrogen-bond formation and higher chain mobility, was selected for subsequent structural (XRD) and application-based evaluations.
XRD analysis
XRD analysis of the glycerol-based starch bioplastic film was performed to assess its crystalline and amorphous phases. A PANalytical X’Pert PRO diffractometer with Cu-Kα radiation (λ = 1.5406 Å) operated at 40 Kv and 30 Ma was used. Film samples (2 x 2 cm2) were mounted on a flat holder and scanned from 5°–50° (2θ) at a 0.02° step size and 2°/min scan speed [21]. The instrument was calibrated using a silicon standard, and crystallinity (%) was determined through peak deconvolution separating crystalline and amorphous regions using OriginPro 2023 [22]. The influence of glycerol and sorbitol on crystallinity was evaluated by comparing peak intensities and positions with standard A- and B-type starch patterns.
Application studies
Replacement of wax coating in paper cups
The methodology was adapted from Butkinaree et al. with minor modifications [23]. A hydrophobic starch coating solution was prepared by dispersing 2.5 g of potato peel starch in 25 mL distilled water, followed by the addition of 3.4 mL vinegar and 2.1 mL glycerol. The mixture was heated to 75°C with stirring until homogeneous. Commercial paper tea cups were pre-perforated at the inner base and coated internally with ~ 10 mL of the solution using a manual film applicator. Coated cups were dried at room temperature (28 ± 2°C) for 24 h. Waterproofing was evaluated by adding 5 mL distilled water and monitoring for leakage or seepage over a fixed period.
Moulding test for bioplastic products
The moulding procedure followed Kadam and Datta with minor modifications [24]. A moulding solution was prepared by dispersing 2.5 g potato peel starch in 25 mL distilled water and adding 2.1 mL glycerol. The mixture was heated with continuous stirring until gelatinization, after which 3.4 mL vinegar was added to aid starch hydrolysis and gel network formation. The molten bioplastic was poured into a pre-cleaned silicone keychain mould and allowed to set at ambient temperature (28 ± 2°C) for 24 h. After demoulding, the products were examined for shape retention, dimensional accuracy, and surface smoothness.
Statistical analysis
All experiments were performed in triplicate, and results are reported as mean ± standard deviation. Physical, mechanical, and optical properties were analyzed using one-way ANOVA to assess the influence of plasticizer type. Biodegradation data were evaluated using two-way ANOVA with replication to examine the effects of plasticizer type, burial duration, and their interaction. Statistical significance was set at p < 0.05, and all analyses were carried out using SPSS Statistics v.26 (IBM Corp., USA).
RESULTS AND DISCUSSION
Starch yield
The wet sedimentation method yielded 48.0 g of starch from 280 g of potato peel, corresponding to 17.10%. This value aligns with earlier reports indicating peel-derived starch yields of 15–20%, depending on extraction conditions and peel composition [25]. Residual fibers and moisture likely reduced starch recovery [26], whereas studies using enzymatic or alkaline-assisted methods have reported slightly higher yields, highlighting the influence of extraction technique on efficiency [27].
Microscopy
Light microscopy showed predominantly oval, smooth-surfaced granules, with few irregular forms (Fig. 2). Similar morphologies have been reported for potato peel starch, indicating preservation of native granule structure [28]. In contrast, some tuber-waste starches exhibit irregular, rough granules due to harsher extraction processes [29]. The smooth, uniform granules observed here suggest predictable gelatinization and swelling behavior, as granule shape and surface texture influence water uptake and enzymatic accessibility [30]. Overall, the moderate yield and intact morphology indicate that wet sedimentation provides a simple, low-cost, and sufficiently gentle method for obtaining starch suitable for biopolymer applications.
Organoleptic evaluation
The extracted starch appeared fine-textured, white, odorless, tasteless, and free of fiber or pigment contamination. These characteristics are consistent with earlier reports on potato peel and other tuber waste starches, which emphasize the importance of visual purity and neutral sensory attributes [31]. The absence of coloration or odor suggests effective removal of peel-derived phenolics and pigments during extraction [27]. Unlike some reported starches that retain fibrous residues due to insufficient washing, the light and uniform appearance here indicates that wet sedimentation provided an efficient purification step. Ensuring such purity is essential for consistent functional behavior in subsequent applications, including gelatinization studies and biopolymer production.
pH
The potato peel starch dispersion showed a near-neutral Ph of 6.40 ± 0.32, consistent with values previously reported for tuber-waste starches [32]. Such neutrality is expected because most acidic or basic peel components are removed during wet sedimentation [33]. In contrast, insufficiently washed or phenolic-rich peels typically yield slightly acidic starch, which can negatively affect functional performance in food and industrial applications.
Test for starch
Qualitative tests confirmed the presence of potato peel starch (Table 2): Molisch’s test produced a reddish-violet ring, and the iodine test showed a deep blue color, indicating a notable amylose fraction. Similar outcomes were reported for tuber-derived starches, showing that gentle extraction preserves amylose integrity [34]. Fehling’s and Benedict’s tests were negative, confirming the absence of reducing sugars, consistent with findings for purified potato peel starch [35]. These results indicate that wet sedimentation effectively isolates starch while minimizing monosaccharide and peel-derived contaminants, maintaining functional integrity for further applications.
Ash
The extracted potato peel starch showed an ash content of 0.47 ± 0.02%, indicating minimal mineral residue. This aligns with Namir et al., who reported that adequate washing during starch isolation removes most inorganic impurities [36]. Such low ash content suggests that wet sedimentation provided effective purification, reducing the likelihood of interference in applications such as film formation or gelation. By contrast, higher ash levels reported in some agro-waste starches are often attributed to residual peel fibers or environmental contaminants [37]. Overall, the low ash values support the purity and suitability of the extracted starch for food and industrial uses.
Moisture
The potato peel starch had a moisture content of 14.00 ± 0.7%, which falls within the typical range for tuber starches [38]. Moderate moisture is desirable, as excessive water promotes microbial growth while very low moisture can make the starch brittle [39]. This value reflects effective drying after extraction and the starch granules’ natural water-holding capacity. It is comparable to commercial starch and indicates that the extracted material was sufficiently stable for subsequent physicochemical and functional analyses.
Amylose and amylopectin
The extracted potato peel starch contained 21.20 ± 1.27% amylose and 78.80 ± 3.94% amylopectin, reflecting a typical tuber starch profile. Similar amylose–amylopectin ratios were reported by Maurya et al., who noted that wet sedimentation helps retain native granule structure [40]. The higher amylopectin fraction contributes to gel clarity and swelling, as amylopectin-rich starches show greater water absorption and flexibility during gelatinization [41]. Reported variations in amylose content generally arise from botanical differences, tuber maturity, or extraction conditions [42]. The composition observed here supports predictable functional behavior for edible films, coatings, and other starch-based biopolymers.
A comparative summary of the key physico-chemical and functional properties of extracted potato peel starch and commercial starch is presented in Table 3.
Solubility
The extracted potato peel starch showed a solubility of 1.83 ± 0.007%, slightly higher than the 1.66 ± 0.009% recorded for commercial starch (Fig. 3). Similar low solubility values have been reported for tuber starches due to their compact, semi-crystalline granules that restrict water penetration [43]. The marginal increase observed here may stem from minor surface irregularities or slight crystalline disruption during wet sedimentation, which improves water accessibility [27]. Since highly ordered granules typically yield lower solubility, these findings further emphasize the role of granule integrity in hydration behavior. The solubility profile suggests that the extracted starch would perform reliably in applications requiring controlled swelling or gelation.
Starch content
The extracted potato peel starch showed a total starch content of 65.38 ± 2.62%, slightly lower than the 72.50 ± 2.9% measured for commercial starch. Similar reductions have been reported for peel-derived starches, attributed to residual fiber, protein, or pectin remaining in the peel matrix after extraction [44]. This difference may also reflect variations in tuber maturity or inherent compositional differences between peel starch and whole-tuber starch. Despite the modest reduction, the starch remained sufficiently pure for functional use, supporting the potential of potato peel as a viable starch source for industrial and biopolymer applications.
Foam capacity
Both the extracted potato peel starch and the commercial starch showed no foam formation. Miller et al. similarly reported an absence of foam in potato starch due to the lack of surface-active proteins or lipids needed for foam stabilization [28]. This is beneficial for applications such as films and coatings, where foam can create pores and disrupt texture. The absence of foaming also reflects the inherent physicochemical nature of potato starch, which favors gelation and swelling rather than aeration-related behavior.
Swelling power
The isolated potato peel starch exhibited a swelling power at 71.33°C, slightly lower than the 72°C observed for commercial starch. Swelling behavior depends largely on amylopectin content and granule integrity, as noted by Kumar et al., with higher amylopectin facilitating water uptake and expansion [45]. The comparable swelling temperature indicates that the extracted starch retains adequate hydration properties, supporting its suitability for applications requiring controlled swelling, such as film formation or hydrogel preparation.
Gelatinization temperature
The extracted potato peel starch began gelatinizing at ~ 70°C, forming a translucent gel. Similar onset temperatures have been reported for potato-derived starches, where gelatinization reflects disruption of crystalline regions and hydration of amylopectin chains [46]. The slight deviation from commercial starch may stem from differences in granule crystallinity or minor peel-derived components affecting thermal behavior [47]. The clear gel formation indicates preserved amylopectin structure, supporting its suitability for applications requiring consistent thermal and textural performance, such as edible films, coatings, or gels.
Bioplastic film formation
The conversion of extracted potato starch into bioplastic films is shown in Fig. 4, where the milky starch suspension (Fig. 4-A) gelatinized upon heating (Fig. 4-B), reflecting the loss of crystalline order reported previously [48, 49]. Addition of glycerol produced a homogeneous film-forming solution (Fig. 4-C) consistent with its role in enhancing chain mobility [50], and the dried films (Fig. 4-D, E) displayed smooth surfaces, flexibility, and transparency comparable to cassava- and pea-starch films [51]. All three formulations (Fig. 5) appeared white, odorless, and smooth, indicating effective removal of peel residues and uniform plasticizer distribution, consistent with earlier starch-film reports [52]. Bioplastics A (glycerol), B (sorbitol), and C (glycerol + sorbitol) were transparent with slight clarity differences attributable to plasticizer type, concentration, and film thickness influencing light transmission [53]. Overall, these attributes underscore the films’ suitability for biodegradable packaging, and the influence of each plasticizer on their physical, mechanical, water-interaction, and optical properties is summarized in Table 4.
Thickness
The film thickness varied by formulation: Bioplastic A measured 0.183 ± 0.028 mm, Bioplastic B 0.196 ± 0.05 mm, and Bioplastic C 0.143 ± 0.005 mm, compared with 0.1 mm for the polypropylene control. The greater thickness of Bioplastic B is likely due to the higher sorbitol content, which increases polymer chain spacing and promotes swelling [53, 54]. Despite being thinner, Bioplastic C showed good uniformity and structural stability, indicating efficient polymer–plasticizer interactions. All starch-based films were thicker than the control, reflecting the bulk of the polysaccharide matrix, yet remained suitable for applications requiring moderate film thickness for packaging or barrier functions.
Moisture content
The moisture content of the films differed by formulation: Bioplastic A contained 21.50 ± 2.20%, Bioplastic B 11.23 ± 1.35%, Bioplastic C 19.45 ± 8.39%, while the polypropylene control showed 20.58 ± 10.33%. The higher moisture in Bioplastic A may result from the hygroscopic nature of its plasticizer system, which promotes water retention within the starch matrix [55]. Bioplastic B showed the lowest moisture content, likely due to stronger starch–sorbitol interactions that limit free water incorporation [56]. Bioplastic C displayed intermediate moisture levels, indicating balanced hydration. These variations highlight the influence of plasticizer type and concentration on water uptake, which in turn affects mechanical stability and shelf life.
Density
The density of the films varied with formulation: Bioplastic A measured 33.3 ± 1.5 g/cm3, Bioplastic B 28.8 ± 1.2 g/cm3, Bioplastic C 48.6 ± 2.0 g/cm³, and the polypropylene control 25 ± 1.0 g/cm3. The higher density of Bioplastic C may result from more efficient starch-chain packing and its lower thickness, producing a denser matrix [57]. In contrast, the lower density of Bioplastic B suggests a more porous structure, likely due to reduced packing efficiency caused by higher sorbitol content [58]. These differences demonstrate how plasticizer type and formulation influence structural compactness, with direct implications for barrier behavior and mechanical performance.
Tensile strength
Mechanical strength varied markedly with plasticizer type. Sorbitol-plasticized films showed the highest tensile strength (38.11 ± 1.52 N/cm2), followed by the polypropylene control (27.55 ± 1.10 N/cm2). Glycerol-plasticized films exhibited the lowest strength (12.00 ± 0.72 N/cm2), while the glycerol–sorbitol blend produced intermediate strength (25.05 ± 1.20 N/cm2). Similar trends were reported by Harussani et al., who found sorbitol-based films stronger than glycerol films, with blends providing balanced performance [59].
Swelling capacity
The swelling capacity of the films varied by formulation: 26.0 ± 2.5% for the blend film (Bioplastic C), 47.0 ± 3.0% for the glycerol-only film (Bioplastic A), 34.0 ± 2.8% for the sorbitol-only film, and 40.0 ± 2.0% for the polypropylene control. The reduced swelling in Bioplastic C indicates a more compact matrix formed by synergistic glycerol–sorbitol interactions that restrict water penetration, while the higher swelling in glycerol films reflects greater water uptake due to increased polymer chain mobility [60]. Sorbitol-only films showed moderate swelling, consistent with their higher rigidity. These findings show that plasticizer choice and blending can modulate hydration behavior, enabling films to be tailored for either improved water resistance or greater flexibility.
Water absorption
Water absorption followed the same pattern as swelling. Glycerol-only films (Bioplastic A) showed the highest uptake (48.6 ± 3.40%), followed by blend films (Bioplastic C) at 33.3 ± 1.99%, and sorbitol-only films (Bioplastic B) at 28.8 ± 1.15%. The polypropylene control absorbed minimal water (1.5 ± 0.10%). The high absorption of glycerol films reflects greater polymer chain mobility that facilitates water penetration [61]. Blend films exhibited moderate uptake, suggesting a more cohesive matrix formed through synergistic glycerol–sorbitol interactions, while the rigidity of sorbitol-only films limited water incorporation. These findings highlight the strong influence of plasticizer type on hydration behavior, flexibility, water resistance, and overall structural integrity of biodegradable films.
Transparency
Optical analysis showed distinct differences among the films: transparency was highest in the blend film (Bioplastic C, 4.20 ± 0.21 mm⁻¹) and lowest in the sorbitol film (Bioplastic B, 0.08 ± 0.02 mm⁻¹), while glycerol films (2.34 ± 0.12 mm⁻¹) and the polypropylene control (0.93 ± 0.05 mm⁻¹) were intermediate. Clarity followed the opposite trend, highest in sorbitol films (99.2 ± 1.5%) and lowest in blend films (95.7 ± 1.8%). These patterns agree with Mohammed et al. [62], who highlighted strong plasticizer effects on optical properties. The high transparency of blend films likely reflects improved molecular packing that reduces light scattering [63], while sorbitol films show lower transparency due to higher crystallinity [64]. Glycerol films remain more transparent because their amorphous, flexible structure enhances light transmission [65]. Overall, the results reflect established links between plasticizer type, matrix organization, and starch-film optical behavior.
ANOVA
The influence of three plasticizer formulations—glycerol (A), sorbitol (B), and a glycerol–sorbitol blend (C)—on potato peel starch bioplastic films was evaluated using one-way ANOVA (Table 5). Thickness, density, tensile strength, swelling capacity, water absorption, and transparency showed significant differences (p < 0.05), whereas moisture content and clarity did not. The different polyols produced meaningful variations in physical and mechanical behavior, with density and tensile strength displaying particularly high F-values, indicating strong plasticizer effects. Sorbitol, due to its higher molecular weight and ability to promote tighter starch-chain packing, generally yielded denser, stiffer films, while glycerol increased chain mobility and flexibility [8, 66]. The blend formulation provided intermediate characteristics, balancing rigidity and flexibility.
Water-related properties such as swelling and water absorption varied significantly with plasticizer type, with glycerol increasing water uptake due to its hydrophilicity and sorbitol reducing moisture sensitivity because of its lower water affinity [67]. Transparency differences further indicated that plasticizer–starch interactions affected light scattering and microstructure [68], while moisture content and clarity showed no significant changes, suggesting lower sensitivity to plasticizer variation. Overall, plasticizer choice allowed tailoring of mechanical, barrier, and optical properties in the films [69]. Sorbitol films showed the highest strength and lowest moisture uptake, glycerol–sorbitol films exhibited greater density and transparency, and glycerol films maintained good clarity despite higher water affinity, supporting the need for subsequent biodegradability assessment across all formulations
Biodegradability test
The soil burial test showed distinct biodegradation patterns based on plasticizer type (Fig. 6): glycerol-plasticized films degraded the fastest, increasing from 62.14 ± 27.28% (day 5) to 95.62 ± 9.78% (day 20), while sorbitol films degraded more gradually (41.28 ± 6.07% to 83.12 ± 9.79%), and glycerol–sorbitol blend films degraded the slowest (38.60 ± 26.85% to 64.16 ± 5.41%). Two-way ANOVA confirmed significant effects of plasticizer type (F(2,24) = 566.84, p < 0.001), burial duration (F(3,24) = 507.87, p < 0.001), and their interaction (F(6,24) = 19.86, p < 0.001), indicating different degradation trajectories among formulations. The rapid breakdown of glycerol films reflects glycerol’s high hydrophilicity and lower molecular weight, which disrupt hydrogen bonding, increase free volume, and enhance microbial access, whereas sorbitol’s stronger bonding and higher molecular weight create a denser, less permeable matrix that slows degradation; blend films showed intermediate behavior. These outcomes are consistent with earlier reports of fast degradation in glycerol-plasticized films and greater stability in sorbitol-based systems [53, 54].
FTIR
FTIR spectroscopy (Fig. 7, Tables 7 and 8) showed characteristic starch bands—O–H (~ 3420 cm-1), C–H (~ 2920 cm-1), O–H bending (~ 1650 cm-1), C–O–C/C–O (~ 1160–985 cm-1), and skeletal vibrations (~ 930–763 cm-1)—confirming that the polysaccharide backbone remained intact. Plasticization caused clear shifts in O–H and C–H stretching regions, indicating hydrogen bonding between starch and the plasticizers. Glycerol films displayed the largest O–H shift (~ 3260 cm-1) and a distinctive C–H deformation (~ 859 cm-1), suggesting stronger interactions and a more amorphous structure. Sorbitol films showed O–H shifts (~ 3280 cm-1), while the blend films exhibited intermediate shifts (~ 3270 cm-1).These trends agree with Laohakunjit and Noomhorm [70], who noted that glycerol increases flexibility while sorbitol enhances strength. Ballesteros-Mártinez et al. reported that glycerol improves flexibility and solubility, whereas sorbitol enhances thermal stability [8]. Gonzalez-Torres et al. showed that combined plasticizers can improve film performance [66]. Overall, FTIR results indicate that glycerol forms stronger hydrogen-bond interactions and increases chain mobility more effectively than sorbitol or the blend. Together with XRD findings, glycerol-plasticized films showed superior preliminary structural and mechanical performance, marking them as the most suitable candidate for further analysis.
XRD
The XRD profile (Fig. 8A) of the potato peel starch–glycerol bioplastic showed a broad diffraction pattern with peaks at 2θ ≈ 17°–19° and minor shoulders at 22°–24°, characteristic of semi-crystalline starch materials [71]. This indicates the presence of both ordered and amorphous regions. The crystallinity index, calculated via the Segal method, was ~ 91.6% (Fig. 8B), reflecting a predominance of crystalline domains. Such high crystallinity corresponds to tighter molecular packing, improved mechanical strength, dimensional stability, and reduced permeability [72, 73]. The semi-crystalline structure is consistent with previous reports on tuber starch films, confirming that extraction and film formation maintained the native ordered arrangement while providing sufficient flexibility for biodegradable applications [43].
Application studies
The potato peel starch–glycerol bioplastic was evaluated as a sustainable coating for paper cups. The coating adhered uniformly, forming a continuous barrier without cracking or peeling. Cups filled with 100 mL of hot liquid (90 ± 2°C) remained leak-proof for at least 10 min, demonstrating effective waterproofing. The film’s semi-crystalline structure and density likely contributed to reduced water permeability [74]. Strong adhesion to the paper substrate can be attributed to interfacial compatibility between the hydrophilic starch matrix and the cellulose fibers, similar to other starch–polyol coatings reported in food packaging [75, 18]. The coated cups’ ability to resist leakage and withstand hot liquids suggests good water resistance and thermal stability, properties associated with partial retrogradation and the dense structure formed during drying [54].
The mouldability of the potato peel starch–glycerol bioplastic was evaluated by casting the molten material into silicone moulds. The bioplastic formed a keychain model with smooth surfaces and stable dimensions after cooling, indicating good thermal workability and shape retention. This suggests that the glycerol plasticizer provides sufficient flexibility and cohesiveness to prevent cracking or deformation during shaping. Similar observations have been reported for other starch-based bioplastics, where plasticizer choice is critical for balancing rigidity and mouldability in small-scale product development [76, 77]. Thermoplastic starch systems containing plasticizers also show enhanced chain mobility, enabling precise reproduction of mould details without structural collapse [78, 79]. These findings indicate the potential of potato peel–derived bioplastics for producing functional moulded products in addition to packaging applications.
CONCLUSION
The results of this study demonstrated that starch-rich potato peel agronomic waste has the potential to be valorized into biodegradable films by means of a simple wet sedimentation and solution casting process. The extracted starch exhibited favorable physicochemical characteristics, making it a suitable feedstock for film formation. Film properties varied with plasticizer type: glycerol produced flexible films with higher moisture uptake, sorbitol enhanced tensile strength and reduced water sensitivity, while a glycerol–sorbitol blend balanced flexibility and strength. Soil burial tests confirmed the biodegradability of all films, with glycerol-based films showing the fastest degradation, making them the most suitable for applications requiring rapid environmental breakdown. Future work should focus on improving functional attributes, such as thermal stability, barrier properties, and durability, through incorporation of nanofillers, crosslinkers, or bioactive agents. Exploring large-scale processing, cost analysis, and performance in real food-packaging systems will further advance the practical applications of potato peel starch-based bioplastics in sustainable packaging industries.
ACKNOWLEDGMENTS
I sincerely thank Dr. R. Mallika, Associate Professor, Department of Biochemistry, V.V. Vanniaperumal College for Women, Virudhunagar, for her support and invaluable guidance throughout this research.
A
CONFLICT OF INTERESTThe authors declare that they have no conflicts of interest related to this study.
AUTHORS' CONTRIBUTION
All authors have significantly contributed intellectually and directly to the work and have approved it for publication.
A
DATA AVAILABILITY
All datasets produced or examined in this study are contained within the manuscript.
FUNDING
None.
ETHICS STATEMENT
Not applicable.
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Formulation code | Starch (g) | Distilled water (mL) | Acetic acid (mL) | Plasticizer(s) | Amount of plasticizer |
|---|---|---|---|---|---|
A | 2.5 | 25 | 3.4 | Glycerol | 2.1 mL |
B | 2.5 | 25 | 3.4 | Sorbitol | 2.0 g |
C | 2.5 | 25 | 3.4 | Glycerol + Sorbitol | 2.1 mL + 2.0 g |
Phytochemical | Test | Observation | Inference | Images |
|---|---|---|---|---|
Carbohydrates | Molisch’s test | Formation of reddish-violet ring | Positive (+) |  |
Reducing sugar | Fehling’s test | No brick-red precipitate | Negative (–) |  |
Reducing sugar | Benedict’s test | No red precipitate | Negative (–) |  |
Non-reducing sugar | Iodine test | Blue colour developed on cooling | Positive (+) |  |
Property | Extracted potato peel starch | Commercial starch | Observations |
|---|---|---|---|
Yield (%) | 17.10 ± 0.0 | - | Within reported range for potato peel starch (15–20%) |
Ash content (%) | 0.47 ± 0.02 | - | Low mineral residue, indicating high purity |
Moisture content (%) | 14.00 ± 0.70 | Similar | Stable for storage |
Amylose (%) | 21.20 ± 1.27 | - | Typical for tuber starch; supports good gelation |
Amylopectin (%) | 78.80 ± 3.94 | - | Higher fraction aids swelling and flexibility |
Solubility (%) | 1.83 ± 0.007 | 1.66 ± 0.009 | Slightly higher in extracted starch due to minor surface irregularities |
Starch content (%) | 65.38 ± 2.62 | 72.50 ± 2.90 | Slightly lower in extracted starch, possibly due to residual peel components |
Foam capacity | Absent | Absent | Lack of surface-active proteins; prevents pore formation in films |
Swelling temperature (°C) | 71.33 | 72.00 | Comparable; suitable for controlled gelatinization |
Gelatinization temperature (°C) | ~ 70 | ~ 70 | Similar thermal behavior |
- : Parameter not determined for commercial starch
Property | Bioplastic A (Glycerol) | Bioplastic B (Sorbitol) | Bioplastic C (Glycerol + Sorbitol) | Polypropylene control |
|---|---|---|---|---|
Thickness (mm) | 0.183 ± 0.028 | 0.196 ± 0.050 | 0.143 ± 0.005 | 0.100 ± 0.002 |
Moisture content (%) | 21.50 ± 2.20 | 11.23 ± 1.35 | 19.45 ± 8.39 | 20.58 ± 10.33 |
Density (g/cm3) | 33.3 ± 1.5 | 28.8 ± 1.2 | 48.6 ± 2.0 | 25.0 ± 1.0 |
Tensile strength (N/cm2) | 12.00 ± 0.72 | 38.11 ± 1.52 | 25.05 ± 1.20 | 27.55 ± 1.10 |
Swelling capacity (%) | 47.0 ± 3.0 | 34.0 ± 2.8 | 26.0 ± 2.5 | 40.0 ± 2.0 |
Water absorption (%) | 48.6 ± 3.40 | 28.8 ± 1.15 | 33.3 ± 1.99 | 1.5 ± 0.10 |
Transparency (mm− 1) | 2.344 ± 0.12 | 0.080 ± 0.02 | 4.202 ± 0.15 | 0.930 ± 0.05 |
Clarity (%) | 97.7 ± 1.6 | 99.2 ± 1.5 | 95.7 ± 1.8 | 99.1 ± 1.5 |
Property | df (Between) | df (Within) | F-value | P-value | Significance* |
|---|---|---|---|---|---|
Thickness (mm) | 3 | 8 | 6.80 | 0.0136 | Yes |
Moisture content (%) | 3 | 8 | 1.45 | 0.2986 | No |
Density (g/cm3) | 3 | 8 | 148.06 | 2.39 x 10− 7 | Yes |
Tensile strength (N/cm2) | 3 | 8 | 252.28 | 2.93 x 10− 8 | Yes |
Swelling capacity (%) | 3 | 8 | 35.25 | 5.85 x 10− 5 | Yes |
Water absorption (%) | 3 | 8 | 274.22 | 2.11 x 10− 8 | Yes |
Transparency (mm− 1) | 3 | 8 | 979.81 | 1.33 x 10− 10 | Yes |
Clarity (%) | 3 | 8 | 3.11 | 0.0886 | No |
Df – degrees of freedom; *Significance considered at p < 0.05.
Source of Variation | df | SS | MS | F | P-value | Significance |
|---|---|---|---|---|---|---|
Group (Sample) | 2 | 4312.12 | 2156.06 | 566.84 | 6.3 × 10–21 | *** |
Time (Columns) | 3 | 5795.25 | 1931.75 | 507.87 | 7.4 × 10–22 | *** |
Interaction | 6 | 895.34 | 149.22 | 19.86 | 3.2 × 10− 8 | *** |
Within (Error) | 24 | 180.79 | 7.53 | – | – | – |
Total | 35 | 10651.93 | – | – | – | – |
Df – degrees of freedom; SS – Sum of squares; MS -Mean square; *Significance considered at p < 0.001.
Sample | Wavenumber (cm⁻¹) | Assignment | Functional Group / Source |
|---|---|---|---|
Potato Peel Starch | ~ 3420 | O–H stretching | Hydroxyl groups, hydrogen bonding |
~ 2920 | C–H stretching | Alkyl groups (CH₂, CH₃) | |
~ 1650 | O–H bending of water | Bound water in starch granules | |
1160–985 | C–O–C, C–O stretching | Glycosidic linkages, starch backbone | |
930–763 | Skeletal vibrations | Glucose ring structure | |
Glycerol Bioplastic | ~ 3260 | O–H stretching | Hydrogen-bonded hydroxyls (starch + glycerol) |
~ 2925 | C–H stretching | Aliphatic CH₂ groups | |
~ 1635 | O–H bending / C = O stretching | Bound water, glycerol interaction | |
1155–988 | C–O, C–O–C stretching | Glycosidic linkages, starch backbone | |
~ 859 | C–H deformation | Amorphous starch phase | |
Sorbitol Bioplastic | ~ 3280 | O–H stretching | Hydrogen bonding (starch + sorbitol) |
~ 2920 | C–H stretching | CH₂ groups | |
~ 1640 | O–H bending of water | Water + starch–sorbitol interaction | |
1150–990 | C–O–C stretching | Polysaccharide backbone | |
< 900 | Skeletal vibrations | Glucose ring | |
Glycerol + Sorbitol Bioplastic | ~ 3270 | O–H stretching | Hydrogen bonding (starch + both plasticizers) |
~ 2920 | C–H stretching | Aliphatic groups | |
~ 1635 | O–H bending / bound water | Plasticizer–starch interactions | |
1150–985 | C–O, C–O–C stretching | Glycosidic bonds | |
< 900 | Skeletal vibrations | Glucose ring | |
Commercial Starch Bioplastic | ~ 3400 | O–H stretching | Hydroxyl groups, hydrogen bonding |
~ 2920 | C–H stretching | CH₂, CH₃ groups | |
~ 1650 | O–H bending of water | Bound water | |
1150–985 | C–O, C–O–C stretching | Glycosidic linkages | |
< 900 | Skeletal vibrations | Glucose ring |
Wavenumber (cm⁻¹) | Assignment | Potato Peel Starch | Glycerol Bioplastic | Sorbitol Bioplastic | Glycerol + Sorbitol Bioplastic | Commercial Starch Bioplastic |
|---|---|---|---|---|---|---|
~ 3420–3400 | O–H stretching | ✓ | – | – | – | ✓ |
~ 3260–3280–3270 | O–H stretching (hydrogen-bonded, plasticizer interaction) | – | ✓ | ✓ | ✓ | – |
~ 2920–2925 | C–H stretching | ✓ | ✓ | ✓ | ✓ | ✓ |
~ 1650–1635–1640 | O–H bending / C = O stretching (bound water / plasticizer interaction) | ✓ | ✓ | ✓ | ✓ | ✓ |
1160–1150–1155–988 | C–O–C / C–O stretching | ✓ | ✓ | ✓ | ✓ | ✓ |
~ 930–763 / <900 | Skeletal vibrations (glucose ring) | ✓ | – | ✓ | ✓ | ✓ |
~ 859 | C–H deformation (amorphous starch) | – | ✓ | – | – | – |
✓ – Presence of functional group/absorption band; – – Absence of functional group/absorption band.
Fig. 1
Schematic representation of starch extraction from potato peel using the wet sedimentation method
Fig. 2
Light microscopy images (40x) of potato peel starch granules showing oval morphology and smooth surfaces
Fig. 3
Solubility comparison of extracted potato peel starch and commercial starch
Fig. 4
Sequential stages in the preparation of potato starch-based biofilm
(A) Dispersion of extracted potato starch in distilled water; (B) gelatinization of the starch suspension by heating; (C) homogenized film-forming solution after plasticizer incorporation; (D) dried film after casting; (E) final bioplastic film exhibiting flexibility and transparency
Fig. 5
Visual appearance of potato peel starch-based bioplastic films prepared with glycerol (A), sorbitol (B), and glycerol–sorbitol blend (C)
Fig. 6
Biodegradation profiles of bioplastic films plasticized with glycerol, sorbitol, and glycerol–sorbitol blend during soil burial over 20 days
Fig. 7
FTIR Spectra of potato peel starch and plasticized bioplastic films
(A)
Potato Peel Starch, (B) Glycerol Bioplastic, (C) Sorbitol Bioplastic, (D) Glycerol + Sorbitol Bioplastic, and (E) Commercial Starch Bioplastic
Fig. 8A
XRD diffraction pattern of glycerol-based potato peel starch bioplastic film
Fig. 8B
XRD intensity profile depicting crystalline structure of potato peel starch- glycerol based bioplastic film(Crystallinity Index: 91.6%)
