A Biodegradable Bamboo-Based Foam as a Cleaner Alternative to Petroleum-Based Cushioning Materials for Sustainable Fruit Packaging

Abstract

The proliferation of single-use petroleum-based foams in protective packaging has become a major source of persistent plastic waste, posing significant challenges to environmental sustainability. To address this issue, we developed a fully biodegradable cushioning foam from bamboo, a rapidly renewable biomass, using an environmentally benign deep eutectic solvent (DES) process that avoids harsh chemical bleaching. The resulting lignin-containing cellulose nanofibril (LCNF)/sodium alginate (SA) foam exhibits low density (0.23 g/cm³), high compressive strength (0.24 MPa at 70% strain), and excellent elasticity (90% recovery at 50% strain), enabled by a dual-network structure of Ca²⁺-crosslinked SA and entangled LCNFs. Critically, the material is fully compostable and leaves no microplastic residues, offering a circular end-of-life pathway. In real-world banana drop tests, it matched the performance of commercial expanded polyethylene (EPE) while outperforming polyethylene bubble wrap. This work demonstrates a practical, scalable route to replace fossil-derived cushioning materials with a bio-based alternative that aligns with the principles of cleaner production and circular economy.

1. Introduction

The rapid expansion of e-commerce and global logistics has generated unprecedented demand for protective packaging materials, predominantly composed of petroleum-based plastic foams [1]. These foams are derived from non-renewable resources, and their recalcitrance to degradation poses severe environmental challenges despite their advantages, such as low production costs, lightweight properties, excellent cushioning performance, and superior thermal insulation [2]. Therefore, developing sustainable, high-performance, and biodegradable alternatives from renewable biomass is imperative.

Cellulose nanofibers (CNF), known for their outstanding mechanical properties and renewable nature, have emerged as promising green materials for packaging [3,4]. Among numerous cellulose sources, bamboo is an excellent choice due to its rapid growth, abundant resources, and excellent mechanical properties [5]. As a fast-growing, non-food industrial crop, bamboo offers a sustainable, underutilized feedstock for the production of high-value materials. The primary components of bamboo are cellulose, hemicellulose, and lignin [6]. Conventional CNF production often involves chemical-intensive bleaching/delignification steps to remove lignin [7]. In contrast, retaining lignin to produce lignin-containing cellulose nanofibers (LCNFs) simplifies the process, reduces chemical consumption, especially aggressive acids or alkalis, and leverages lignin’s intrinsic benefits, such as hydrophobicity and interfacial adhesion, for enhanced material performance [8,9]. The DES method is also an effective route for producing lignocellulosic nanocellulose, enabling partial delignification and the preparation of lignin-containing nanocellulose. Simultaneously, due to its low cost, simplicity of preparation, and biodegradability, DES is gaining increasing attention as a green solvent for biological and chemical applications [10]. From a cleaner-production perspective, the sustainability advantages of DES-enabled processing depend not only on solvent selection but also on the feasibility of solvent recovery and recycling; accordingly, recent reviews have summarized separation-based routes that enable DES regeneration and potential reuse within production processes [11]. This aligns highly with the principles of green chemistry and the circular bioeconomy.

Foam fabrication often requires careful selection of methods. While techniques like freeze-drying can produce excellent foams, freeze-drying is widely reported to require substantially higher energy input than conventional drying [12]. Accordingly, mechanical foaming followed by oven/ambient drying may offer a more scalable route and potentially lower energy demand compared with freeze-drying, as suggested in prior studies. A key challenge in oven-drying cellulose foams is mitigating pore collapse caused by capillary forces during water evaporation [13]. Incorporating reinforcing polymers or cross-linkers is an effective strategy to enhance mechanical strength and structural stability [14,15,16,17].

Sodium alginate (SA), a natural polysaccharide biomass derived from seaweed, is an ideal candidate for this purpose. Pure sodium alginate foam suffers from drawbacks, including high hygroscopicity, poor mechanical properties, limited thermal stability, and insufficient flame-retardant and durable performance, restricting its application across diverse environments. However, it is biodegradable, non-toxic, and can rapidly form hydrogels through ionic cross-linking in the presence of divalent or trivalent metal ions, such as Ca²⁺ [18], or by blending with other polymers, such as cellulose. This property has been exploited to enhance the mechanical integrity of various bio-composites. For instance, SA has been used as a binder in bagasse fiber foams [19] and as a matrix for composite foams [20], significantly improving their mechanical properties.

Despite progress in bio-based packaging, most alternatives still rely on energy-intensive fabrication or fail to match the performance of conventional plastics, limiting their real-world adoption. Moreover, the environmental burden of single-use protective foams, particularly their persistence in landfills and contribution to microplastic pollution, demands urgent innovation toward truly biodegradable solutions. Bamboo, as a fast-growing, non-food lignocellulosic resource, offers a sustainable feedstock that can be processed via green chemistry to yield functional materials without competing with food security. Recent studies have confirmed the potential of bamboo-based components in composite materials [21,22].

In this study, we present a cleaner production strategy for protective packaging: a fully biodegradable LCNF/SA foam fabricated from bamboo using a choline chloride/lactic acid DES system, followed by mechanical foaming and oven-drying. This approach eliminates the need for aggressive chemicals, offers a scalable fabrication route, and retains lignin to enhance material functionality. The foam’s performance is validated through standardized mechanical tests and, more importantly, real-world drop trials using bananas as a model perishable commodity. By providing a high-performance, compostable alternative to EPE, this work contributes to reducing plastic waste in the logistics and food packaging sectors, aligning with global efforts toward sustainable consumption and production.

2. Materials and Methods

2.1. Materials

Bamboo material, originating from Zhuzhou, Hunan Province, was powdered into a 120-mesh size. Sodium alginate (SA, AR, 90%) and lactic acid (AR, 85–90%) were purchased from Shanghai Yien Chemical Technology Co., Ltd. (Shanghai, China). Choline chloride and calcium carbonate were purchased from Shanghai Aladdin Bio-technology Co., Ltd. (Shanghai, China). Glycerin was purchased from Xilong Chemical Co., Ltd. (Shantou, China). Glucono-delta-lactone (GDL) was purchased from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China). Sodium dodecyl sulfate (SDS) (classified as readily biodegradable) was purchased from Fuchen (Tianjin) Chemical Reagent Co., Ltd. (Tianjin, China). All chemicals were of analytical grade and could be used without further purification. Deionized water was used throughout the experiments.

2.2. Preparation of LCNF

LCNFs were prepared from bamboo using the method described by Long Li et al. [23], with some modifications. Briefly, the dry bamboo material was ground and sieved through a 120-mesh screen to obtain a uniform powder. Choline chloride and lactic acid were mixed at a 1:10 molar ratio in a conical flask and stirred at 80 °C until a clear liquid formed. Then, bamboo powder (4 wt%) was added to the solvent, and the mixture was heated to 120 °C with stirring at 300 rpm for 12 h. The resulting slurry was cooled, washed with ethanol and deionized water until neutral, then passed through a high-pressure homogenizer at 1200 bar for 8 cycles to obtain the LCNF suspension with partial lignin removal.

2.3. Fabrication of LCNF/SA Foams

The LCNF suspension was mixed with SA at different mass ratios and stirred at 60 °C for 2 h. SDS (0.1% w/v) was added as a foaming agent, and the mixture was stirred at 2000 rpm for 10 min. Subsequently, GDL, CaCO₃ (mass ratio 1:1), and glycerol (7% wt%) were added, and stirring was continued for 1 min. As a result, the wet foam was formed. Finally, the wet foam was cast into a mold and dried at 50 °C for 48 h. The resulting foams were designated as LCS-1, LCS-2, and LCS-3, corresponding to different LCNF/SA ratios (Table S1).

2.4. Characterization

Fourier-transform infrared (FTIR) spectroscopy was performed on LCNF and LCNF/SA samples at different ratios using a spectrometer (TENSOR II, Bruker Optics, Ettlingen, Germany) in ATR mode. The chemical components of the aerogel samples were analyzed by XPS (Thermo Nexsa, Thermo Fisher Scientific, Waltham, MA, USA). The morphology was observed using a scanning electron microscope (SEM, Regulus 8230, Hitachi High-Tech Corporation, Tokyo, Japan) at a voltage of 3 kV. The density of the foam was determined by measuring its weight using an analytical balance and its dimensions using an electronic caliper. Measurements were carried out using a precision balance with a resolution of 0.1 mg, and each sample was weighed in triplicate. The morphology of the LCNF was examined with a JEOL-2100F transmission electron microscope (TEM, JEOL-2100F, JEOL Ltd., Tokyo, Japan). Before observation, the suspensions were diluted to about 0.01% and ultrasonicated for 10 min to ensure a homogeneous dispersion. Before observation, the fibers were negatively dyed with 2 wt% phosphotungstic acid solution to enhance image contrast. Observations were conducted under an accelerated voltage of 120 kV. LCNF data were statistically analyzed using ImageJ software (v1.54g).

2.5. Measurement of Thermal Stability, Water Contact Angle, and Moisture Absorption

Thermogravimetric analysis (TGA) was conducted on an instrument (TG 209 F3, NETZSCH-Gerätebau GmbH, Selb, Germany) from 30 to 700 °C at a heating rate of 10 °C/min under nitrogen.

Tests were conducted using a contact angle analyzer (OCA-20, DataPhysics Instruments GmbH, Filderstadt, Germany). To minimize the influence of sample texture, the following method was employed to prepare films for measuring water contact angles. A mixed solution of LCNF/SA and 2 wt% SA was prepared following the preliminary steps. Equal masses of the samples were applied uniformly onto glass slides. The solutions were dried in a 50 °C oven and were then sprayed with 0.05 M calcium chloride solution to simulate Ca²⁺ crosslinking. Finally, the films were dried again in a 50 °C oven.

The moisture absorption assessment employed a modified version of the method described by Soni et al. [24]. Four foam materials prepared using the same method were tested. The specimens were first pre-conditioned at 0% RH for 24 h until mass stability to standardize the initial moisture content and obtain a reproducible dry reference mass. Following this pretreatment step, the initial weight (W₀) of each specimen was recorded. Specimens were then transferred to an environment maintained at 85% relative humidity and room temperature using a constant temperature and humidity chamber for an additional 24 h. After this period, specimens were reweighed to obtain the W₈₅ value. Moisture absorption rate was calculated using Equation (1):

$$Moisture absorption (\%)={\displaystyle \frac{W_{85}-W_0}{W_0}}\times 100$$

(1)

where W₈₅ denotes the mass recorded after the sample has remained at 85% relative humidity for 24 h and 48 h, whereas W₀ corresponds to the baseline weight obtained following its equilibration in an environment with 0% relative humidity.

2.6. Mechanical and Cushioning Properties

The as-prepared foams (cut to cylindrical specimens of Φ18 mm × 15 mm) were conditioned for 24 h at a constant ambient temperature of 25 ± 2 °C and 65% relative humidity to minimize the influence of moisture uptake and prior environmental exposure on the mechanical response of the porous structure. Their static compression properties were tested using a universal testing machine at 12 mm/min up to 70% strain. The compressive modulus was derived from the initial linear region (0–5% strain). Strain energy (e) and static cushioning coefficient (C) were determined using the following equations:

$$e={\int }_0^{\epsilon }\sigma d\epsilon$$

(2)

$$C=\sigma /e$$

(3)

where ε is the compressive strain and σ is the compressive stress.

The rebound rate was determined according to the method of Xue et al. [25] at 50% strain, holding for 3 min. After releasing the pressure, the recovered height (H₂) was measured relative to the original (H₁). Then, the rebound rate (D) was calculated using Equation (4).

$$D\left(\%\right)={\displaystyle \frac{H_2}{H_1}}\times 100$$

(4)

2.7. Packaging Cushioning Capacity: Banana Drop Test

The packaging cushioning capacity test employed the method developed by Pontree, Itkor et al. [26]. All bananas were sourced from the same supplier. Prior to each test, two laboratory technicians selected bananas with uniform ripeness (primarily yellow in color, with minimal green areas, and no visible bruising or browning), consistent size (approximately 3.8 cm diameter at the midpoint and 16 cm length), and uniform weight (138 ± 7 g). Bananas of uniform ripeness were packaged in polyester (PET) trays (220 mm × 160 mm × 60 mm) with different cushioning liners. Three bananas were assigned randomly to each package: no cushion (Control), EPE (5 mm thick), 5 mm diameter polyethylene bubble wrap (PABB), and LCS (5 mm thick LCS-1). Each group underwent 10 drops from 760 mm onto a concrete surface according to ISTA 1A standards. After drop testing, bananas were stored for 48 h at 25 °C and 85–90% RH for better evaluation of the protection of different liners according to the mechanical damage, color, pH, firmness, and total soluble solid content.

Mechanical damage of bananas was quantified by bruise area (BA), volume (BV), and susceptibility (BS), and a Mechanical Damage Index (MDI) according to the references [27,28,29]. BA, BV and BS were calculated as follows:

$$BA\left({\mathrm{mm}}^2\right)={\displaystyle \frac{\pi }{4}}\times w_1{w}_2$$

(5)

$$BV\left({\mathrm{mm}}^2\right)=\pi d/24\times (3w_1{w}_2+4d^2)$$

(6)

$$BS\left({\mathrm{mm}}^2/\mathrm{J}\right)=BV/IE$$

(7)

$$IE\left(\mathrm{J}\right)=m_i\times g\times h_d$$

(8)

$$SBS\left({\mathrm{mm}}^3/\mathrm{J}/\mathrm{g}\right)=BS\times m_f$$

(9)

where the major diameter (w₁), minor diameter (w₂), and depth (d) of the elliptical bruise shape were measured to assess bruising damage in bananas; IE is the impact energy (J); m_i is the mass of the falling object (g) and g is the gravitational acceleration (m/s²); h_d is the drop height (m), and m_f is the mass of the banana (g).

The Mechanical Damage Index (MDI) is obtained from Equation (9) [29], in which the value is determined by evaluating the extent of the damaged region.

$$\begin{array}{c}MDI\left(\%\right)=0.1\times Tracedamages\left(\%\right)+0.2\times Slightdamages\left(\%\right)\\ 0.7\times Moderatedamages\left(\%\right)+1.0\times Severedamages(\%)\end{array}$$

(10)

where the damaged area (A) is classified as follows: A ≥ 3 cm², Severe; 2 cm² ≤ A < 3 cm², Moderate; 1 cm² ≤ A < 2 cm², Slight; and 0.5 cm² ≤ A < 1 cm², Trace.

Color parameters were measured using a spectrophotometer. L* (lightness), b* (coordinate on the yellow–blue axis), and chroma (C*) were recorded. Weight loss of the bananas was calculated using the following formula:

$$Weightloss(\%)=(W_0-W_t)/W_0\times 100$$

(11)

where W₀ is the initial mass of the sample and W_t is the mass of the sample after 48 h.

The firmness was tested using a fruit firmness tester. The tester recorded the maximum force exerted (in Newtons). Blended banana pulp was passed through cheesecloth to yield juice. The juice was analyzed for pH and total soluble solids: pH with a pH meter and TSS with a portable refractometer, reported in °Brix.

2.8. Statistical Analysis

All experiments were conducted in triplicate. Data are presented as mean ± standard deviation and were analyzed by one-way analysis of variance (ANOVA) with Tukey’s test (p < 0.05) using open-source R software (v4.5.0).

3. Results and Discussion

3.1. Chemical and Structural Characterization

FTIR analysis confirmed the successful integration of LCNF and SA (Figure 1a). The LCNF spectrum showed characteristic cellulose Iβ peaks (1429, 1161, and 898 cm⁻¹) [30] and lignin-associated bands (1457 cm⁻¹, aromatic skeleton vibration; 1257 cm⁻¹, C=O in guaiacyl unit) [31,32]. The SA spectrum showed asymmetric and symmetric COO⁻ stretches at 1618 and 1421 cm⁻¹, respectively. In the LCNF/SA, these COO⁻ peaks shifted to lower wavenumbers (e.g., 1602 cm⁻¹), indicating successful ionic cross-linking between SA and Ca²⁺ ions [33,34]. A broad O-H stretching band around 3347 cm⁻¹ suggested intermolecular hydrogen bonding between LCNF and SA, contributing to the integrated network [35].

XPS spectroscopy (Figure 1b) confirmed the successful introduction of Ca²⁺ ions into the crosslinking network. A distinct Ca2p peak appeared, contrasting with the full-scan spectra of purely physically blended LCNF and SA (LCNF+SA). The XPS C1s spectra (Figure 1c) of LCS showed four prominent peaks at 284.8, 286.5, 288.1, and 288.9 eV, corresponding to the chemical states C-C/C-H, C-O, C=O/O-C-O, and O-C=O [36,37]. Notably, no carbonate ion signal (289–290 eV) was detected. This absence confirms that the Ca2p peak indicates successful introduction of Ca²⁺ ions, rather than residual undissolved calcium carbonate.

3.2. Thermal Stability, Water Contact Angle, and Moisture Absorption

TGA demonstrated that the incorporation of LCNF significantly enhanced the thermal stability of the foams (Figure 1d). The major degradation step occurred between 200 and 400 °C, corresponding to the decomposition of SA and polysaccharide components in LCNF [38,39]. The pure SA foam had the lowest onset (T_onset = 158.4 °C) and maximum degradation (T_max = 185.1 °C) temperatures. All LCNF/SA foams exhibited higher thermal stability, with LCS-2 (having the lowest SA proportion) showing the highest T_onset and T_max. This is primarily related to its lower SA content (LCNF is identical in LCS-2 and LCS-3), a characteristic that reduces early volatile release and delays the main degradation phase. The contribution of lignin-containing LCNF is more pronounced in high-temperature behavior and residue evolution. Lignin, due to its aromatic structure, decomposes over a broad temperature range and promotes condensed-phase carbonization, consistent with the more gradual high-temperature mass loss behavior observed in LCS foams. When SA content was fixed at 1 wt%, increasing LCNF content from 1 wt% (LCS-1) to 2 wt% (LCS-3) resulted in higher residue levels at 700 °C (from 17.73% to 19.23%), indicating that the synergistic effect of a more rigid nanocellulose network with lignin-related coking enhances phase stability. Additionally, Ca²⁺-related effects may promote residue increase. At the same LCNF content (2 wt%), LCS-3 exhibited higher residue than LCS-2 (15.71%), partly attributed to increased calcium-containing inorganic ash due to elevated calcium alginate content after heating [40]. Simultaneously, studies indicate that alkaline earth metal ions (including Ca²⁺) can alter cellulose pyrolysis pathways, promoting dehydration and carbonization-related processes (e.g., polycondensation reactions) [41]. Furthermore, the interfacial contact between calcium-related species and cellulose may enhance secondary reactions of volatile intermediates. Based on the above two points, we speculate that the increased retention rate resulting from elevated sodium alginate content primarily stems from its contribution to the inorganic phase and may also originate from the potential influence of Ca²⁺/calcium-related species on cellulose pyrolysis and carbonization behavior.

To evaluate the moisture-absorption properties of foam materials under identical humid conditions, the water-absorption rate was first assessed. After 24 h, the water absorption of LCS-1 and LCS-3 was 53.88% and 44.16%, respectively, whereas LCS-2 reached 73.45%. In contrast, the neat SA foam exhibited a markedly higher uptake of 94.78% at 24 h, indicating that the moisture absorption of the SA foam approached saturation within the first day. After 48 h of testing, the water absorption rates were 76.38% (LCS-1), 81.55% (LCS-2), and 74.08% (LCS-3), while SA foam stabilized at nearly 96%, further confirming the near-saturated behavior of pure SA foam at 24 h. It should be noted that the water absorption behavior of foam materials depends not only on the inherent hydrophilicity of the polymer matrix but also on the pore structure. SEM observations indicate that localized pore collapse may enhance capillary-driven permeation, thereby promoting deeper water penetration into the internal pore network. This likely explains the relatively high water absorption exhibited by the LCS-2 material. Simultaneously, measurements under static high-humidity conditions provide a controlled reference benchmark for evaluating the inherent moisture absorption capacity of foam materials. In practical transportation and storage scenarios, when relative humidity fluctuates, these results serve as a comparative baseline for understanding material behavior in variable humidity environments.

To further validate whether LCNF reduces the inherent hydrophilicity of the substrate, wettability was characterized using glycerol-free compact films. This approach avoids interference from porous, rough foam surfaces [42] and mitigates potential interference from glycerol migration to the surface [43]. As shown in Figure S1, the incorporation of LCNF increases the contact angle. The water contact angle of the Ca²⁺-crosslinked SA film was 42°, while that of the composite film containing lignin/nanocellulose increased to 60–70°, indicating reduced surface hydrophilicity after LCNF incorporation. In summary, an increase in contact angle indicates a reduction in the material’s inherent wettability and a weakening of the interfacial affinity between water and the pore wall surface. Simultaneously, it reduces capillary-driven liquid penetration and the formation of liquid bridges/films within pores [44,45]. The contact angle of the film reflects the wettability at the substrate level, while the actual moisture absorption behavior of the foam is also jointly regulated by its porous structure. However, the reduction in foam water absorption and the increase in contact angle both confirm that LCNF enhances moisture resistance at the substrate level. This offers significant advantages for packaging materials used in humid environments.

These findings demonstrate that composite structures can effectively enhance hydrophobicity and thermal stability. This indicates that the corresponding foam materials exhibit greater stability in environmental conditions, expanding their potential applications in the packaging sector.

3.3. Morphology and Physical Properties

TEM images reveal that the sample formed nanoscale cellulose structures after high-pressure homogenization (Figure 2a), with a diameter distribution as shown in Figure 2b. The distinct lignin particles can be seen in the image. Additionally, Figure 2c–e shows the porous structure of the oven-dried LCS foams [46,47]. LCS-1 (Figure 2c) and LCS-3 (Figure 2e) exhibited uniform, closed-cell structures with pore sizes of 150–300 μm, characteristic of well-stabilized foams. In contrast, LCS-2 (Figure 2d) showed irregular, collapsed pores, attributable to its lower SA content, which provided insufficient cross-linking to withstand capillary stress during drying [48,49].

Although oven-drying involves liquid-vapor evaporation and the associated capillary pressure that typically leads to structural shrinkage, pore stability in this system is maintained by the intrinsic strength of the polymer network.

Specifically, Ca²⁺-induced ionic crosslinking of sodium alginate forms a mechanically robust hydrogel skeleton prior to drying, while the entangled LCNFs further reinforce pore walls through physical interlocking and load transfer, as evidenced by the continuous and thickened cell boundaries observed in Figure 2d,e. In addition, the high viscosity of the LCNF/SA wet foam retards water migration during evaporation, resulting in a more uniform stress distribution across the porous framework.

In contrast to freeze-dried systems, where pore morphology is primarily governed by ice crystal templating, the pore preservation observed here arises from network reinforcement during evaporative drying, demonstrating that oven-drying can serve as an energy-efficient yet structurally reliable processing route for bio-based foams.

Increasing the LCNF content led to thicker pore walls (LCS-1 compared to LCS-3) due to the increased viscosity and enhanced stability of the wet foam, which reduced shrinkage. This morphological evolution directly influenced foam density, which increased from 0.12 g/cm³ (LCS-1) to 0.17 g/cm³ (LCS-2) and 0.23 g/cm³ (LCS-3).

3.4. Mechanical and Cushioning Performance

The compressive stress–strain curves (Figure 3a) displayed a typical elastic region followed by a densification plateau. LCS-3 absorbed the most energy (the largest area under the curve), while LCS-2 performed the worst. This underscores the synergistic role of SA and LCNF: SA provides cross-linking sites with Ca²⁺ to enhance strength [19], while a higher LCNF content reinforces the foam via a denser fibrillar network, leading to thicker cell walls (as seen in SEM).

This mechanistic interpretation is supported by the Young’s modulus data (Figure 3b). LCS-2 had the lowest modulus (30.75 kPa), which increased significantly with higher LCNF and SA content. The rebound rate (Figure 3d) followed a similar trend, with LCS-1 and LCS-3 showing excellent elasticity (~90%). Glycerol plasticization improves chain mobility, while the rigid dual-crosslinked network enables elastic recovery [50,51]. The poor performance of LCS-2 is due to its compromised structure, leading to irreversible pore collapse.

The cushioning coefficient C, a key indicator for packaging design, is plotted in Figure 3c. All samples exhibit a rapid decrease in C at low stress levels, corresponding to the initial elastic deformation and activation of the cellular structure. With further increase in stress, the curves gradually flatten, indicating that the materials enter a more stable energy absorption regime dominated by cell collapse and frictional dissipation. In this region, differences among the samples become more pronounced. LCS-1 consistently shows the lowest cushioning coefficient, suggesting that it transmits less stress for a given deformation and thus provides superior cushioning efficiency under practically relevant load conditions. At higher stress levels (>0.10 MPa), the cushioning coefficient plateaus for all samples, indicating that the cellular structures are largely densified and the energy-absorption mechanism is saturated. Notably, LCS-3 exhibits a slightly higher C value, which may be related to its stiffer structure and reduced capacity for further deformation, thereby increasing stress transmission. Overall, LCS-1 exhibits the lowest C value at stresses above 0.025 MPa, indicating its superior shock-absorbing efficiency under practically relevant loads. Cyclic compression testing (Figure 3e,f) confirmed good mechanical stability, with LCS-1 (Figure 3e) exhibiting approximately 50% stress reduction after 10 cycles and LCS-3 (Figure 3f) showing about 25% stress reduction after 10 cycles. This further demonstrates the influence of LCNF on the mechanical properties of foam materials. However, it also indicates that significant improvements are still required regarding long-term durability.

3.5. Practical Packaging Application: Banana Drop Test

The mechanical properties of LCS foam make it an ideal choice for cushioning fresh produce. LCS-1 was selected as the representative formulation by balancing cushioning performance, practical manufacturability, and cost. LCS-1 (containing 1% LCNF by weight) achieves uniform dispersion directly via high-pressure homogenization (HPH). In contrast, formulations with 2% LCNF by weight (LCS-2/3) exhibit increased viscosity and reinforced fiber networks, typically requiring higher operating pressures to achieve equivalent dispersion [52]. Thus, achieving 2 wt% LCNF typically requires rotary evaporation to remove approximately 50% moisture, significantly increasing processing time and cost. Preliminary mechanical testing indicates that LCS-1 exhibits cushioning properties closer to LCS-3 (Figure 3). Therefore, LCS-1 was examined using a banana drop-test protocol with two commercial references (PABB and EPE) under identical PET-tray packaging. Bananas were used as a sensitive impact indicator due to their bruise-prone soft tissues, and quality was tracked for 2 days under ambient conditions to capture early, damage-driven changes while minimizing longer-term, ripening-related variability.

In the drop test, LCS-1 lowered the mechanical damage indices (MDI, BA, BV, and BS) relative to the uncushioned control and PABB, while approaching the overall level of EPE (Figure 4 and Figure 5), indicating effective impact attenuation by the LCS foams [53,54,55].

Post-impact quality changes were consistent with the injury mitigation effect. External/structural protection indicators—including brighter color coordinates (Figure 6a–c), suppressed weight loss (Figure 6d, associated with reduced osmotically driven water transport) [27], and enhanced firmness retention capacity (related to wound respiration) [53]—were significantly superior to the control group in both LCS and EPE-treated groups. Internal indicators related to ripening showed a similar trend: although total soluble solids (TSS) increased post-impact due to sugar accumulation during ripening (Figure 6g) [56], impact-induced cell wall disruption and water loss exacerbate this increase [57]. The smaller increase in LCS and EPE-treated groups indicates diminished promotion of ripening by injury. pH increased with organic acid respiration (Figure 6f) [58], peaking on day 2 in the control group while showing smaller fluctuations in buffered groups. Collectively, these convergent responses confirm that LCS-1 performs comparably to EPE and outperforms PABB as a buffering material protecting fragile fruits during transport.

Across the drop-test evaluations, the cushioning systems either absorbed or dissipated impact energy effectively. Notably, the LCS material delivered performance comparable to that of standard commercial expanded EPE, indicating that it can serve as a practical substitute for safeguarding fresh produce during transportation. These findings highlight a promising path toward sustainable packaging solutions and, in turn, may reduce dependence on conventional plastic foams.

4. Conclusions

In conclusion, we have successfully developed a fully biodegradable cushioning foam from bamboo, an abundant and renewable industrial crop, through a green, scalable process based on mechanical foaming and low-temperature oven-drying. By retaining lignin during DES-assisted fibrillation, we simplified the manufacturing workflow while simultaneously enhancing the foam’s hydrophobicity and thermal stability. The synergistic dual-network structure of LCNF/SA enabled exceptional mechanical performance, including high compressive strength and near-complete shape recovery. Critically, the foam demonstrated practical efficacy in protecting bananas, performing on par with commercial EPE. From a cleaner production perspective, this bamboo-based foam not only utilizes a renewable resource but also avoids hazardous chemicals during processing and ensures complete biodegradability at end-of-life, thereby closing the material loop and preventing long-term environmental accumulation. Its successful demonstration in protecting perishable goods highlights its potential to displace millions of tons of non-degradable plastic foams used annually in global supply chains. Future work will focus on optimizing performance and exploring practical production applications such as industrial-scale manufacturing and scalability.