Active TPS/PBAT Blown Films Incorporating Sodium Lactate for Improved Oxygen Barrier, Antimicrobial Activity, and Cheese Preservation

Abstract

Biodegradable active packaging that incorporates food-grade additives offers a promising solution for extending shelf life and minimizing food waste. This study investigates the development of functional packaging films for cheese applications by blending thermoplastic starch (TPS) and poly (butylene adipate-co-terephthalate) (PBAT) in a 60/40 (w/w) ratio with various concentrations of sodium lactate (SL; 1–7% w/w) using blown-film extrusion. Spectroscopic analyses, including ¹H NMR and FTIR, confirmed the presence of hydrogen-bonding and ionic interactions between the hydroxyl (–OH) groups of thermoplastic starch (TPS) and the carboxylate (–COO⁻) groups of sodium lactate, which enhanced interfacial compatibility and produced smoother, more compact film morphologies. SL acted as a multifunctional plasticizer and compatibilizer, improving film flexibility while slightly reducing tensile strength. Notably, SL incorporation increased water vapor permeability and surface wettability but significantly decreased oxygen permeability to below 1 cc·mm/m²·day·atm. At moderate concentrations (≥ 3% w/w), SL also exhibited antimicrobial activity against Staphylococcus aureus. When applied to cheese packaging, SL-modified films effectively maintained color stability for up to 9 days under refrigerated storage. Notably, cheeses packaged with films containing 3–7% (w/w) SL exhibited significantly lower hardness values than the control on day 3, indicating improved moisture retention and texture preservation, although these differences were no longer significant by day 9. These findings demonstrate that sodium lactate can simultaneously enhance interfacial miscibility, oxygen barrier performance, and antimicrobial functionality in sustainable, biodegradable active packaging systems.

1. Introduction

Food waste remains one of the most pressing challenges confronting the global food industry, with microbial spoilage and contamination by foodborne pathogens accounting for a substantial proportion of post-processing losses. Dairy products, particularly cheese, are especially susceptible to surface microbial growth during storage due to their relatively high moisture content, near-neutral pH, and nutrient-rich composition. These intrinsic characteristics create favorable conditions for the proliferation of spoilage microorganisms and foodborne pathogens, ultimately limiting shelf life and compromising product safety. As such, the development of effective preservation strategies that ensure microbial safety while maintaining product quality has become a critical priority for the dairy sector. Among food-grade antimicrobial agents, sodium lactate (SL)—a lactate salt classified as Generally Recognized as Safe (GRAS) by the U.S. Food and Drug Administration—has been extensively applied in food systems due to its broad antimicrobial spectrum, buffering capacity, and high compatibility with diverse food matrices [1,2]. In dairy applications, SL is widely used as an antimicrobial agent because it inhibits the growth of spoilage and pathogenic microorganisms, while also helping to stabilize pH and, at appropriate concentrations, enhance flavor. These properties make SL particularly suitable for cheese preservation. Its inhibitory effects against pathogens such as Staphylococcus aureus, Escherichia coli, Listeria monocytogenes, and Clostridium botulinum have been well documented in various food systems. However, most previous applications have relied on the direct incorporation of SL into food formulations [1,2], which may negatively affect sensory attributes at higher concentrations and may also raise concerns regarding regulatory limits and consumer acceptance.

In recent years, the incorporation of antimicrobial agents into packaging materials has emerged as an effective post-processing strategy to enhance food safety and extend shelf life without the direct addition of preservatives to the food product. Antimicrobial packaging films can suppress surface microbial contamination at the food–package interface, thereby reducing microbial risks while meeting the increasing consumer demand for minimally processed and “clean-label” foods [3,4,5]. Despite these advantages, the incorporation of sodium lactate into biodegradable packaging materials—particularly for cheese applications—remains relatively underexplored, and its multifunctional role within polymeric packaging systems has not been fully elucidated. Growing concerns regarding plastic waste accumulation and environmental sustainability have accelerated the development of biodegradable packaging systems incorporating active agents capable of extending shelf life and improving food safety. Active biodegradable packaging offers a dual advantage: reducing dependence on petroleum-based plastics while mitigating microbial contamination during storage and distribution [2]. Thermoplastic starch (TPS), derived from renewable agricultural resources, is a low-cost and biodegradable polymer widely recognized as a promising candidate for sustainable food-contact materials [6]. Nevertheless, TPS exhibits inherent limitations, including high hydrophilicity, poor moisture resistance, and insufficient mechanical strength, which restrict its practical application in food packaging [7]. To address these limitations, TPS is often blended with poly (butylene adipate-co-terephthalate) (PBAT), a flexible aliphatic-aromatic copolyester known for its superior toughness, ductility, and biodegradability. Although TPS/PBAT blends can exhibit improved mechanical performance, strong intermolecular hydrogen bonding within starch and pronounced polarity mismatch between TPS and PBAT often lead to phase separation and weak interfacial adhesion, ultimately compromising barrier and mechanical properties [8]. Therefore, the development of high-performance TPS/PBAT packaging materials requires a multifunctional additive capable of enhancing interfacial compatibility while simultaneously introducing active functionality. Sodium lactate represents a promising additive in this system. Its low molecular weight facilitates diffusion throughout the polymer matrix, while its hydroxyl and carboxylate functional groups promote hydrogen bonding and ionic interactions with both starch and polyester chains. Specifically, ionic interactions primarily occur between the hydroxyl (–OH) groups of thermoplastic starch (TPS) and the carboxylate (–COO⁻) groups of sodium lactate, whereas sodium ions (Na⁺) may additionally form electrostatic interactions with the polar carbonyl (C=O) groups of PBAT. Collectively, these interactions enhance intermolecular associations, improve interfacial adhesion, and promote overall compatibility within the blend matrix. These interactions may improve polymer miscibility, enhance chain mobility, and contribute to improved mechanical and barrier performance [6]. Moreover, unlike many antimicrobial agents, SL is non-volatile, food-grade, and highly compatible with dairy products, making it particularly attractive for cheese packaging applications. Most previous studies on antimicrobial biopolymer films have relied on solution-casting techniques, which suffer from limitations such as high solvent consumption, long drying times, and poor industrial scalability. In contrast, melt-processing methods, including extrusion and blown-film processing, offer environmentally favorable and industrially relevant alternatives, enabling continuous production, uniform dispersion of functional additives, and direct translation to commercial manufacturing [9]. However, the incorporation of sodium lactate into extrusion-processed TPS/PBAT blown films and its combined effects on interfacial compatibility, oxygen barrier performance, antimicrobial efficacy, and cheese preservation have not yet been systematically investigated. Accordingly, this study focuses on the development and characterization of biodegradable TPS/PBAT blown films incorporating sodium lactate as a multifunctional compatibilizer and antimicrobial agent. The work aims to clarify the interactions between SL and the TPS/PBAT matrix, determine its influence on mechanical and oxygen barrier performance, evaluate antimicrobial activity against Staphylococcus aureus, and assess the effectiveness of the resulting films in cheese packaging during storage.

2. Materials and Methods

2.1. Materials

The polymeric matrices used to produce the active film included native cassava starch (Food-grade, SMS Corporation Co., Ltd., Pathum Thani, Thailand) and poly (butylene adipate-co-terephthalate) (PBAT, Ecoflex^® F Blend C1200, BASF, Ludwigshafen, Germany). Sodium lactate (SL; 60% aqueous solution, Chanjao Longevity Co., Ltd., Bangkok, Thailand) was used as the active compound. Glycerol (vegetable grade, EMSURE^® ACS, Merck KGaA, Darmstadt, Germany) was employed as the plasticizer. For antimicrobial evaluation, Staphylococcus aureus ATCC 25,923 was obtained from the Department of Microbiology, Faculty of Science, Kasetsart University, Thailand.

2.2. Sample Preparation via Cast Sheet Extrusion

In the first extrusion step, sodium lactate (SL) at concentrations of 1, 2, 3, 5, and 7% (w/w) was dissolved in glycerol under continuous magnetic stirring until a homogeneous solution was obtained, thereby forming the plasticized thermoplastic starch (TPS) matrix. The resulting plasticizer solution was mixed with native cassava starch using a laboratory-scale dough mixer (SC-236A, Stelang Electric Appliance Co., Ltd., Foshan, China) for 10 min. The mixture was subsequently fed into a co-rotating twin-screw extruder (L/D = 40, screw diameter = 20 mm) (LTE-20-48, Labtech Engineering Co., Ltd., Samut Prakarn, Thailand) operated at 180 rpm with a temperature profile ranging from 110 to 150 °C. The extruded pellets (TPS/SL) were conditioned at ambient temperature overnight. In the compounding step, poly (butylene adipate-co-terephthalate) (PBAT) was melt-blended with the TPS/SL pellets at a weight ratio of 60:40 using the same twin-screw extruder operated at 180 rpm and a temperature gradient of 100–155 °C. The resulting compounded pellets (TPS/PBAT/SL) were then dried at 50 °C overnight prior to film blowing. SL-modified films were produced using a single-screw blown film extruder (L/D = 30, screw diameter = 25 mm; CF-W400, Chareon TUT Co., Ltd., Samut Prakarn, Thailand) operated at barrel temperatures of 145–155 °C and a screw speed of 30–40 rpm. The films were collected using a nip roll rotating at 2.6–3.0 rpm and subsequently stored in aluminum pouches containing desiccants at room temperature (25 ± 2 °C) for further analyses.

2.3. Scanning Electron Microscopy (SEM)

The surface morphology and cryo-fractured cross-sections of the films were examined using a JEOL JSM-IT300 scanning electron microscope (JEOL Ltd., Tokyo, Japan). Film samples were fractured in liquid nitrogen, desiccated over silica gel, mounted on aluminum stubs, and sputter-coated with gold using a Quorum Polaron coater (SC7620, Quorum Technologies Co., Ltd., Lewes, UK). Morphological uniformity was further evaluated at 1000× magnification using optical microscopy (LSM5 PASCAL, Zeiss, Oberkochen, Germany).

2.4. Nuclear Magnetic Resonance (¹H NMR)

¹H NMR spectra were recorded using an Ascend™ 600/Avance III HD spectrometer (Bruker, Fällanden, Switzerland). Deuterated dimethyl sulfoxide (DMSO-d₆) was used as the solvent to dissolve selected TPS/SL blends containing 2, 5, and 7% (w/w) SL. All analyses were conducted at a resonance frequency of 600 MHz, and chemical shifts were reported in parts per million (ppm) relative to tetramethylsilane (TMS). Signal assignments were employed to confirm molecular interactions between the hydroxyl groups of starch and sodium lactate molecules.

2.5. Fourier Transform Infrared Spectroscopy (FTIR)

Infrared spectra were recorded using a Bruker Tensor 27 FT-IR spectrometer (Bruker Tensor 27, Bruker Optics GmbH & Co. KG, Ettlingen, Germany) equipped with a diamond attenuated total reflectance (ATR) accessory. Film samples (1 cm × 1 cm) were analyzed at a spectral resolution of 4 cm⁻¹ with 64 accumulated scans according to the method of Klinmalai et al. (2026) [10]. Spectra were collected over the range of 500–4000 cm⁻¹. Baseline correction and normalization were applied to enable comparison of relative band intensities.

2.6. Dynamic Mechanical Thermal Analysis (DMTA)

The thermomechanical behavior of the films was analyzed using a dynamic mechanical analyzer (DMA-1 analyzer, Mettler-Toledo (Switzerland) GmbH, Greifensee, Switzerland). Rectangular film specimens were scanned over a temperature range of −100 to 100 °C at a heating rate of 2 °C/min under oscillation frequencies of 0.5, 1.0, and 5.0 Hz. The glass transition temperature (Tg) was determined from the maximum of the tan δ peak.

2.7. Mechanical Properties

Tensile strength (TS), elongation at break (EB), and Young’s modulus (YM) were measured in accordance with ASTM D882-09 using an Instron 5965 universal testing machine (Instron Universal Testing Machine model 1193, Instron Corp., Canton, USA). Film strips (10 × 2.5 cm²) were conditioned at 25 °C and 50% relative humidity for 48 h prior to testing. Measurements were conducted in both the machine direction (MD) and cross direction (CD), with 15 replicates per direction, at a crosshead speed of 500 mm/min.

2.8. Light Transmission

Optical transparency was evaluated using a UV–Vis spectrophotometer (Evolution 300, Thermo Fisher Scientific Co., Ltd., Massachusetts, United States). Film samples (3 × 4 cm²) were scanned over a wavelength range of 200–800 nm using air as the reference according to the method of Klinmalai et al. (2026) [10]. The percent transmittance (%T) at 600 nm was reported as an indicator of light-barrier efficiency.

2.9. Surface Hydrophobicity

Contact angle measurements were conducted using a goniometer (OCA15EC, DataPhysics Instruments GmbH, Filderstadt, Germany) equipped with SCA20 software (SCA20 version 6.1, Informer Technologies, Inc., Los Angeles, CA, USA). Distilled water droplets (3 µL) were gently deposited onto the film surface, and static contact angles were recorded within 2 s. Ten measurements were averaged for each film to ensure reproducibility.

2.10. Water Vapor Permeability

Water vapor permeability was determined according to the method of Klinmalai et al. (2026) [10] by using ASTM E96/E96M-12 (Desiccant method, ASTM International, West Conshohocken, PA, USA). Circular specimens (7 cm diameter) were sealed on aluminum test cups containing 25 g of silica gel and incubated at 25 °C and 50% RH for 7 days. Weight gain was recorded daily to calculate the water vapor transmission rate (WVTR). WVP was calculated using Equation (1):

$$\mathrm{WVP}={\displaystyle \frac{\mathrm{WVTR}\times L}{\mathsf{\Delta }P}},$$

(1)

where L is the film thickness (mm) and ΔP is the water vapor partial pressure difference (kPa) across the film.

2.11. Oxygen Permeability

Oxygen transmission rate (OTR) was measured using an oxygen permeability analyzer (OX-TRAN 8500, Illinois Instruments, Johnsburg, IL, USA) following ASTM D3985-05 according to the method of Klinmalai et al. (2026) [10]. Film specimens (10 × 10 cm²) were preconditioned at 25 °C and 50% RH for 48 h and tested at 23 °C and 50% RH. Oxygen permeability was calculated using Equation (2):

$$\mathrm{OP}={\displaystyle \frac{\mathrm{OTR}\times L}{\mathsf{\Delta }P}}$$

(2)

where L is the film thickness (µm) and ΔP is the oxygen partial pressure gradient (atm).

2.12. Antimicrobial Activity

Antimicrobial activity of the films against Staphylococcus aureus ATCC 25,923 was assessed using the broth dilution assay. The bacterial inoculum (~5.3 log CFU/mL) was prepared in nutrient broth (HiMedia M096; HiMedia Laboratories Pvt. Ltd., Mumbai, India). UV-sterilized film strips (1 g) were immersed in 9 mL of 0.1% peptone solution containing 1 mL of inoculum and incubated at 37 °C for 24 h. After incubation, serial dilutions (10⁻¹–10⁻⁶) were plated on nutrient agar and incubated for 24 h at 37 °C. Antimicrobial efficacy was expressed as logarithmic reduction relative to the control, with colony counts reported as log CFU/mL.

2.13. Cheese Packaging Preparation

Commercial cheese (TOPS Market, Bangkok, Thailand) was cut into 3 × 4 cm² portions and packaged with TPS/PBAT and SL-active films (film size: 10 × 12 cm², heat-sealed on three sides). Packages were stored at 4 °C for 9 days.

2.14. Texture Analysis

The texture of the cheese was measured using a texture analyzer (TA-XT Plus C Texture Analyzer, Stable Micro Systems Ltd., Godalming, Surrey, UK). A P/6 probe (6 mm diameter cylindrical probe) was used to penetrate the cheese samples to a depth of 5 mm. A 50 kg load cell was applied. The penetration speeds were set to 10, 15, and 10 mm/s for the pre-test, test, and post-test phases, respectively. The results were expressed as the average force in newtons (N).

2.15. Color Analysis

The color change in the cheese during storage was analyzed using a 3nh colorimeter version NR-200 (3nh Co., Ltd., Shenzhen, China), equipped with a D65 light source and a CIE 10° standard observer at 8/d illuminating geometry, LED blue light excitation, and a 4 mm aperture. The b* value, indicating yellowness, was measured at five points on each sample across three replicates, and the results were reported accordingly.

2.16. Statistical Analysis

The experimental data were analyzed using IBM SPSS Statistics version 22 (IBM Corp., Armonk, NY, USA). One-way analysis of variance (ANOVA), followed by Duncan’s multiple range test, was used to determine significant differences among mean values at p ≤ 0.05.

3. Results and Discussion

3.1. Microstructure

The microstructural evolution of TPS/PBAT films incorporating sodium lactate (SL) at various concentrations is illustrated in Figure 1. The fractured surface of the neat TPS/PBAT film exhibited a heterogeneous and irregular morphology, characterized by distinct starch-rich domains and pronounced phase discontinuities, indicative of limited interfacial compatibility between TPS and PBAT. Upon incorporation of SL, the fracture morphology progressively transitioned toward smoother and more compact structures. Films containing 1 % (w/w) SL still displayed rough surface features comparable to the control, suggesting that low SL levels were insufficient to markedly modify interphase interactions. In contrast, films incorporating 3 % (w/w) SL or higher exhibited a substantial reduction in domain boundaries and interfacial gaps, reflecting enhanced interfacial adhesion and improved phase continuity between the TPS and PBAT phases.

Sodium lactate, a low–molecular-weight ionic plasticizer with hygroscopic characteristics, is capable of forming extensive hydrogen-bonding and ionic–dipole interactions with the hydroxyl groups of starch chains. At elevated SL contents, the granular remnants of starch became indistinct, indicating more extensive disruption of native starch granules and the formation of a more homogeneous thermoplastic starch phase during melt processing. This morphological refinement suggests an ion-mediated reduction in starch–starch hydrogen-bond density, which likely increases chain mobility within TPS and facilitates the penetration and entanglement of PBAT chains into starch-rich regions. Consequently, the polarity mismatch between the two polymers is effectively mitigated, leading to improved miscibility and interphase cohesion.

To date, systematic studies addressing the role of simple lactate salts, such as sodium lactate, in TPS/PBAT blend systems remain limited. Therefore, the present findings extend previous reports demonstrating that lactate-based ionic liquids and deep eutectic solvents can disrupt the hydrogen-bonding network of starch, thereby promoting thermoplasticization and enhanced phase mixing in starch-based materials [11,12,13]. During melt blending, the strong water-binding capacity of SL likely induced localized swelling of starch chains, reducing granule rigidity and enabling more uniform dispersion within the polymer matrix. Upon cooling, partial re-association of hydroxyl groups may have occurred, resulting in a denser and more cohesive microstructure.

This microstructural evolution closely resembles that observed in ionic-liquid-plasticized starch systems, where ionic species disrupt native hydrogen bonding, increase amorphous-phase mobility, and promote structural homogeneity [14,15]. Overall, SEM observations confirm that SL functioned as an effective compatibilizing additive, markedly improving interphase continuity and dispersion of TPS within the PBAT matrix. The transition from rough, discontinuous fracture surfaces to smooth and compact morphologies provides clear microstructural evidence of strengthened interfacial coupling, which underpins the enhancements in mechanical performance and oxygen barrier properties discussed in subsequent sections.

3.2. Film Chemical Structure

The ¹H NMR spectra clearly confirmed the successful incorporation of sodium lactate (SL) into the TPS/PBAT matrix (Figure 2A). A distinct resonance corresponding to the methyl group (–CH₃) of lactate was observed at approximately 1.1 ppm, in agreement with the characteristic sodium lactate signal reported by Ellis et al. (2003) [16]. At low SL loadings, slight line broadening was evident, suggesting localized heterogeneity within the polymer matrix, likely arising from non-uniform hydration or restricted chain mobility associated with partial interaction between SL and the surrounding polymer chains. In contrast, films containing higher SL concentrations (≥5% (w/w)) exhibited sharper and better-resolved resonances, indicative of enhanced proton mobility. This behavior is consistent with the plasticizing effect of SL, which increases free volume and facilitates molecular motion within the polymer phase. Resonances in the 3.2–3.8 ppm region, predominantly assigned to hydroxyl and ring protons of starch [17], showed small but discernible chemical-shift changes upon SL incorporation. These shifts reflect subtle alterations in the local hydrogen-bonding environment and are attributed to weak ion–dipole interactions between lactate carboxylate groups (–COO⁻) and starch hydroxyl groups (–OH). Additionally, minor shifts in the α-(1→6) and α-(1→4) glycosidic proton signals at approximately 5.1 and 5.4–5.6 ppm suggest that SL partially perturbs the native starch hydrogen-bond network without disrupting the fundamental glycosidic backbone. This observation indicates a non-destructive plasticization mechanism, in which molecular mobility is enhanced while the structural integrity of the polysaccharide framework is preserved.

The FTIR spectra further supported these molecular interactions (Figure 2B). With increasing SL content, an intensified absorption band near 1588 cm⁻¹, corresponding to asymmetric stretching of the carboxylate C=O group of the lactate salt, became apparent. Concurrently, the broad O–H stretching band in the 3100–3600 cm⁻¹ region became more asymmetric and slightly broadened [18], indicating a redistribution and partial weakening of hydrogen bonding within the TPS phase. Notably, a modest increase in the intensity of the PBAT aromatic C–H bending band at approximately 728 cm⁻¹ was also observed, which may be associated with improved chain packing or enhanced interfacial interactions between TPS and PBAT in the presence of SL. Importantly, the absence of new absorption bands or drastic peak shifts suggests that SL does not induce chemical modification of either polymer component but instead interacts through physical and ionic interactions.

Overall, ¹H NMR and FTIR analyses confirm the role of sodium lactate as an ionic plasticizer and interfacial modifier in the TPS/PBAT system. By disrupting inter- and intramolecular hydrogen bonding in starch and promoting more favorable interactions at the TPS–PBAT interface, SL enhances chain mobility and compatibility without altering the primary polymer structures. This molecular-level mechanism is consistent with previous reports on starch systems modified with ionic liquids or low-molecular-weight salts [14,15,19] and provides a robust mechanistic foundation for the improved mechanical performance, morphology, and barrier properties discussed in subsequent sections.

3.3. Dynamic Mechanical Thermal Analysis (DMTA)

Dynamic mechanical thermal analysis (DMTA) results (Figure 3) revealed three distinct α-relaxation transitions in the control TPS/PBAT film at approximately −50, −26, and 60 °C, corresponding to the glycerol-rich phase, PBAT phase, and starch-rich phase, respectively [17]. The presence of these well-separated relaxation events confirms the multiphase nature of the blend and highlights the limited miscibility and weak interfacial coupling between TPS and PBAT in the absence of compatibilizing agents. Such phase-resolved dynamics are characteristic of starch–polyester blends dominated by strong starch–starch hydrogen bonding and polarity mismatch. Upon incorporation of sodium lactate, the low-temperature relaxation associated with the glycerol-rich phase shifted slightly toward lower temperatures, indicating enhanced segmental mobility induced by the plasticizing action of SL. This behavior can be attributed to the hygroscopic and ionic nature of lactate salts, which increase free volume and weaken intermolecular hydrogen bonding, thereby facilitating localized molecular motion. With increasing SL concentration, the α-relaxation associated with the starch-rich phase progressively shifted toward that of PBAT and became partially overlapped, suggesting strengthened interfacial interactions and more cooperative molecular dynamics across the TPS–PBAT interface.

This convergence of relaxation temperatures reflects improved phase compatibility and reduced dynamic heterogeneity within the blend. At higher SL loadings (≈7 % (w/w)), a slight upward shift in the starch-dominated relaxation was observed. This behavior suggests that beyond a critical plasticization threshold, partial ionic association and reorganization of hydrogen-bonded clusters may occur, leading to the formation of localized regions with constrained chain mobility. Similar non-monotonic viscoelastic responses have been reported in starch systems plasticized with ionic additives and deep eutectic solvents, where competing effects of hydrogen-bond disruption and secondary association govern the final dynamic mechanical behavior [12,13]. Concurrently, a progressive decrease in tan δ magnitude with increasing SL content was observed, indicating reduced viscoelastic damping and a shift toward more elastic energy storage. The narrowing of the α-relaxation peak and the diminished distinction between TPS- and PBAT-related transitions imply enhanced interfacial adhesion and more efficient stress transfer between the two phases [20,21]. These dynamic features are characteristic of blends with improved interphase coupling and corroborate the spectroscopic evidence of hydrogen-bond reorganization discussed earlier. Overall, the DMTA results demonstrate that sodium lactate primarily functions as an ionic plasticizer and interfacial modifier at moderate concentrations, promoting chain flexibility, phase interaction, and cooperative segmental motion. At higher concentrations, SL additionally induces partial reinforcement through secondary ionic and hydrogen-bond associations. These dynamic mechanical responses correlate well with the tensile property trends discussed in Section 3.4, where enhanced extensibility and reduced stiffness accompany the increased molecular mobility and improved interfacial compatibility observed in the SL-modified TPS/PBAT films.

3.4. Mechanical Properties

The mechanical properties of the TPS/PBAT films, including tensile strength (TS), elongation at break (EB), and Young’s modulus (YM), are summarized in

Table 1. Mechanical properties, namely (A) Tensile strength, (B) Elongation at break, and (C) Young’s modulus of TPS/PBAT and TPS/PBAT containing SL at concentrations of 1, 2, 3, 5, and 7%.

TPS/PBAT/SL Films	Tensile Strength (MPa)		Elongation at Break (%)		Young’s Modulus (MPa)
	MD	CD	MD	CD	MD	CD
TPS/PBAT	5.847 ± 0.247 d	5.839 ± 0.319 e	294.532 ± 33.9 a	307.652 ± 36.3 c	51.090 ± 4.174 e	50.838 ± 3.2 d
TPS/PBAT SL 1%	5.559 ± 0.325 c	3.484 ± 0.261 d	547.804 ± 51.5 c	317.193 ± 47.3 c	39.437 ± 4.084 d	30.273 ± 5.109 c
TPS/PBAT SL 2%	5.595 ± 0.425 cd	3.010 ± 0.157 c	590.143 ± 36.4 de	172.525 ± 15.0 ab	34.543 ± 2.331 c	24.306 ± 1.554 b
TPS/PBAT/SL 3%	5.710 ± 0.313 d	3.366 ± 0.188 d	607.464 ± 37.6 e	181.197 ± 32.9 ab	32.163 ± 2.220 c	29.956 ± 3.765 c
TPS/PBAT/SL 5%	4.194 ± 0.142 b	2.312 ± 0.159 b	552.913 ± 14.7 cd	209.379 ± 77.0 b	23.162 ± 2.220 b	14.481 ± 1.880 a
TPS/PBAT/SL 7%	2.441 ± 0.171 a	1.999 ± 0.082 a	466.531 ± 68.1 b	146.076 ± 15.7 a	17.459 ± 2.408 a	11.719 ± 0.830 a

Values shown are mean ± SD. ^a–e Different small letters indicate significant difference within the same column between different treatments (p < 0.05). MD and CD refer to the machine direction and cross direction, respectively.

. The control TPS/PBAT film exhibited TS values in the range of 2–5 MPa in both the machine direction (MD) and cross direction (CD), which are in good agreement with previously reported values for starch–polyester blend systems processed by extrusion [17]. The incorporation of sodium lactate (SL) at low to moderate levels (≤3% (w/w)) did not result in a significant change in TS, indicating that SL was homogeneously distributed within the matrix and that its presence did not compromise load-bearing interactions at these concentrations. This behavior suggests that, at optimal levels, SL promotes molecular-level interactions without severely disrupting the polymer network. In contrast, further increasing the SL content to 5–7% (w/w) led to a pronounced reduction in TS, which can be attributed to excessive disruption of intermolecular hydrogen bonding and polymer chain packing caused by surplus SL molecules. At higher concentrations, SL likely increases free volume and weakens cohesive forces within the matrix, resulting in diminished resistance to applied stress. This trade-off between flexibility and strength is characteristic of over-plasticized polymer systems. Elongation at break exhibited a progressive increase with rising SL content, particularly in MD specimens, confirming the plasticizing role of SL through enhanced segmental mobility and improved ductility. The maximum EB was observed at 3% (w/w) SL, beyond which a decline was evident, reflecting a transition from effective plasticization to over-plasticization and loss of structural coherence. Correspondingly, YM decreased steadily with increasing SL concentration, indicating reduced stiffness and increased flexibility of the films. This inverse relationship between YM and EB is typical of plasticized biopolymer matrices and further supports the role of SL in modulating polymer chain dynamics. Similar trends were reported by Kristo et al. (2008) [22], who demonstrated that sodium lactate reduces intermolecular cohesion and induces a brittle-to-ductile transition in starch-based systems below the glass transition temperature. Film orientation exerted a significant influence on mechanical behavior. Films tested in the MD consistently exhibited higher tensile strength and lower elongation compared to those tested in the CD. This anisotropic behavior arises from preferential molecular alignment and chain orientation during extrusion and blown-film processing, which enhance stress transfer and mechanical continuity along the drawing direction. Considering the combined effects of strength, extensibility, stiffness, and interfacial compatibility, a sodium lactate concentration of 3% (w/w) was identified as the optimal level. At this level, SL provides sufficient chain mobility to improve flexibility while maintaining mechanical integrity, rendering the films suitable for practical food packaging applications.

3.5. Transparency, Surface Hydrophobicity and Barrier Properties

The UV–visible light transmission spectra (Figure 4A) revealed that all TPS/PBAT-based films, regardless of SL incorporation, exhibited excellent shielding against UV-C and UV-B radiation (<320 nm). This strong UV-blocking performance can be attributed to the intrinsic light-absorbing characteristics of starch chromophores and the aromatic moieties of PBAT. Partial transmission was observed in the UV-A region (approximately 340–400 nm), which is likely associated with microvoids and semi-amorphous transport pathways formed within the plasticized TPS phase. Notably, increasing the SL content from 5 to 7% (w/w) significantly enhanced visible-light transmittance, resulting in optically clearer films. This improvement in transparency is attributed to increased free volume and improved refractive-index uniformity induced by SL, which minimizes interfacial light scattering between TPS- and PBAT-rich domains. Such optical clarity is advantageous for retail packaging applications that require product visibility while still providing effective protection against short-wavelength UV-induced oxidation and quality deterioration.

Water contact angle (WCA) measurements (Figure 4B) yielded values of approximately 80° for all films, indicating moderately hydrophilic surface characteristics. Incorporation of SL at low to moderate levels (1–3% (w/w)) did not result in significant changes in WCA compared with the neat TPS/PBAT film, suggesting that the film surfaces remain dominated by polar constituents, including starch, glycerol, and lactate salts. In contrast, films containing 5% (w/w) SL exhibited a slight decrease in WCA, which may be attributed to enhanced exposure of surface hydroxyl groups arising from partial disruption of starch–starch interactions. This consistently polar surface promotes good wettability and supports biodegradability; however, it also implies increased susceptibility to moisture uptake. From an application perspective, this limitation suggests that SL-containing TPS/PBAT films are most suitable for use as functional inner layers in multilayer biodegradable packaging systems, where an external hydrophobic layer can provide improved moisture resistance without compromising sustainability.

As shown in Figure 4C, water vapor permeability (WVP) increased progressively with increasing SL content. At 2% (w/w) SL, WVP increased by approximately 87% compared with the control. This behavior can be attributed to plasticization-induced expansion of free volume and the hygroscopic nature of SL, whose hydroxyl and carboxylate groups promote water sorption and diffusion through the polymer matrix [22]. At higher SL loadings (≥3% (w/w)), this effect became more pronounced, consistent with disruption of the hydrogen-bonding network and reduced polymer packing density. Nevertheless, WVP values of SL-containing films remained higher than those of TPS/PBAT films without SL. These results indicate that SL primarily affects the diffusivity component of water vapor transport rather than selectively altering solubility. Although the increased WVP may limit the applicability of these films for high-moisture food packaging, they remain suitable for dry or moderately humid food systems, particularly when used as inner sealing or active antimicrobial layers.

In contrast to water vapor permeability, oxygen permeability (OP) decreased markedly with the incorporation of sodium lactate (SL) (Figure 4D). Within the SL concentration range of 1–5% (w/w), OP values dropped sharply from 364 to below 1 cc·mm/m²·day·atm, indicating a substantial enhancement in oxygen barrier performance. This improvement is attributed to increased polymer chain packing density and the formation of a more cohesive polar network that restricts the diffusion of non-polar oxygen molecules. At a higher SL content (7% (w/w)), a slight increase in OP (approximately 3.7 cc·mm/m²·day·atm) was observed, likely resulting from over-plasticization and the generation of excess free volume that facilitates gas transport. Nevertheless, the overall oxygen barrier performance of SL-modified TPS/PBAT films remains excellent, underscoring their strong potential for packaging oxygen-sensitive, lipid-rich foods such as cheese, cured meats, and nuts, where oxidative stability is a critical quality determinant.

3.6. Antimicrobial Activity

The antimicrobial performance of TPS/PBAT and TPS/PBAT/SL films against Staphylococcus aureus is presented in Figure 5A,B. Both the control films and those containing 1% (w/w) sodium lactate (SL) showed no statistically significant reduction in bacterial counts, indicating that this concentration was insufficient to produce an effective antimicrobial effect under the tested conditions. The control film exhibited S. aureus counts of 8.02 ± 0.09 CFU/g, while the film containing 1% (w/w) SL showed 7.72 ± 0.16 CFU/g. In contrast, films incorporating 3–7% (w/w) SL achieved a notable reduction in S. aureus populations, with bacterial counts decreasing by approximately 15–20% relative to the control. Notably, no significant differences were observed among the 3% (6.77 ± 0.31 cfu/g), 5% (6.44 ± 0.31 cfu/g), and 7% (w/w) SL (6.79 ± 0.37 cfu/g) formulations, suggesting that increasing SL content beyond 3% did not confer additional antimicrobial benefits. This plateau behavior indicates that a threshold concentration was reached, at which antimicrobial efficacy was maximized while further increases in SL primarily influenced physicochemical rather than biological performance. Consequently, a loading level of 3% (w/w) SL represents an optimal balance between antimicrobial activity and the preservation of mechanical integrity.

The observed antimicrobial action of sodium lactate can be attributed to multiple complementary mechanisms. As a weak acid salt, SL disrupts the proton motive force across bacterial cell membranes, resulting in intracellular acidification and depletion of cellular energy reserves [23]. In addition, lactate ions can chelate essential divalent cations and perturb osmotic balance, further impairing enzymatic activity and metabolic homeostasis. These mechanisms are particularly effective against Gram-positive bacteria such as S. aureus, which lack an outer membrane barrier and are therefore more susceptible to membrane-active compounds. Similar antimicrobial trends have been reported by Sierra et al. (2025) [1], who demonstrated that chicken gelatin–nanocellulose films containing 1–3% (w/w) SL effectively suppressed Listeria monocytogenes growth in beef bolognese during five weeks of refrigerated storage. It is also important to note that the antimicrobial efficacy of SL is strongly influenced by environmental moisture and food matrix characteristics, as water facilitates SL diffusion from the polymer matrix to the food surface. Therefore, the moderate antimicrobial performance observed in this study may be further enhanced under real food packaging conditions, particularly for high-moisture, high-protein foods such as cheese. Collectively, these results demonstrate that the incorporation of sodium lactate at low to moderate concentrations can impart measurable antimicrobial functionality without compromising film integrity, supporting its potential application in active biodegradable packaging systems designed to control surface microbial contamination in protein-rich foods.

3.7. Packaged Cheese Quality

The visual appearance of cheese packaged with TPS/PBAT and TPS/PBAT/SL films is shown in Figure 6A. Cheeses wrapped in the control film and in films containing 1% (w/w) SL exhibited a noticeable fading of their characteristic yellow coloration toward the end of the storage period. This discoloration is consistent with oxidative degradation of annatto, the primary pigment responsible for cheese color, which is highly susceptible to oxygen and light exposure [24]. In contrast, cheeses packaged with films containing higher SL loadings generally retained a more uniform and intense yellow appearance. Sporadic brown or pink discolorations observed in some samples may be associated with localized microbial activity, potentially involving spoilage microorganisms such as Pseudomonas spp. or Lactobacillus helveticus, which are known to produce chromogenic metabolites during growth and spoilage [25]. These visual features suggest that, while SL-modified films effectively mitigate oxidative discoloration, microbial succession may still occur under certain conditions, highlighting the need for complementary microbiological analyses.

Quantitative color measurements, expressed as the yellowness index (b*) (Figure 6B), corroborated the visual observations. Cheeses packaged in films containing 2–7% (w/w) SL exhibited significantly higher b* values throughout storage, with a pronounced increase observed by day 6, followed by a slight decline by day 9. The enhanced color retention can be attributed, at least in part, to the exceptionally low oxygen permeability of SL-modified films (<1 cc·mm/m²·day·atm), which likely reduced oxygen ingress and slowed the oxidative bleaching of annatto pigments. In addition, the partial migration of sodium lactate from the films into the cheese matrix may have influenced cheese microstructure and optical properties. Localized pH modulation can alter casein–casein interactions and light scattering behavior, thereby increasing perceived yellowness. Sodium lactate is also recognized for its buffering, preservative, and flavor-enhancing roles in dairy systems, all of which may contribute indirectly to improved pigment stability during storage [26].

Changes in cheese texture, assessed by hardness measurements (Figure 6C), revealed a progressive decrease in firmness during refrigerated storage across all treatments. This trend is characteristic of cheese ripening and storage, where proteolysis, moisture redistribution, and gradual weakening of the casein network led to softening over time. On day 3, cheeses packaged in films containing 3–7% (w/w) SL exhibited significantly lower hardness values compared with the control, although no significant differences among treatments were detected by day 9. The initially softer texture observed in SL-containing treatments contrasts with the findings of Xu et al. (2025) [27], who reported increased hardness during storage due to moisture loss. In the present study, the hygroscopic nature of sodium lactate likely reduced moisture migration from the cheese into the surrounding environment or packaging material, thereby preserving internal moisture content and contributing to a softer texture during early storage. Furthermore, the moderate water-vapor transmission rates of SL-modified films may have mitigated dehydration, maintaining a more plastic and less brittle cheese matrix, particularly during the initial stages of refrigerated storage.

4. Conclusions

Sodium lactate was effectively incorporated into TPS/PBAT blend films, resulting in the development of multifunctional biodegradable packaging suitable for cheese applications. At moderate concentrations (≤3% w/w), SL acted as an ionic plasticizer, enhancing chain mobility and flexibility without compromising mechanical integrity, as supported by FTIR and ¹H NMR analyses. While SL increased water vapor permeability due to its hygroscopic nature, it significantly reduced oxygen permeability by nearly two orders of magnitude, demonstrating the formation of an effective polar barrier against non-polar gas diffusion. Films containing 3% w/w or higher SL exhibited antimicrobial activity against Staphylococcus aureus, contributing to improved cheese quality during storage by preserving color and texture. Overall, the multifunctional plasticizing, oxygen barrier, and antimicrobial properties imparted by sodium lactate underscore the potential of TPS/PBAT–SL films as active, food-grade, and sustainable packaging solutions for dairy applications. Future studies should incorporate additional cheese quality assessments, including pH, moisture loss, lipid oxidation, as well as comprehensive evaluations of microbiological and sensory attributes, to better correlate microbial status with the antimicrobial performance of the films.