Abstract
The food packaging characteristics and environmental impact of paper coated with polysaccharide dispersions were analyzed. Colloidal dispersions of xylan and xylan derivatives, as well as their combinations with chitosan and nanocrystalline cellulose, were applied in thin layers on both sides of the paper surface (5 g/m²). The barrier properties to water, water vapor, gases, oil/grease, and the antimicrobial properties of the coated paper were evaluated. Generally, polysaccharide coatings improved the barrier and antimicrobial features of coated papers compared to uncoated paper. Significant improvements were obtained by combining xylan derivatives and chitosan, where the contact angle of the coated paper reached 92.8° and achieved 100% inhibition of Bacillus sp. Furthermore, food simulant tests indicated that all tested polysaccharide combinations are suitable for use in food packaging, especially for fatty products. After 28 and 42 days of soil degradation, all samples of xylan and xylan/xylan derivatives/chitosan/nanocellulose coatings reached similar degradation levels (70 to 80% and 14 to 16 mg CO2 production).
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Food Packaging Performance and Environmental Impact of Polysaccharide-Coated Papers
Mirela Roman,a Petronela Nechita,b,* Aida Mihaela Vasile,c and Maria Violeta Guiman d
The food packaging characteristics and environmental impact of paper coated with polysaccharide dispersions were analyzed. Colloidal dispersions of xylan and xylan derivatives, as well as their combinations with chitosan and nanocrystalline cellulose, were applied in thin layers on both sides of the paper surface (5 g/m²). The barrier properties to water, water vapor, gases, oil/grease, and the antimicrobial properties of the coated paper were evaluated. Generally, polysaccharide coatings improved the barrier and antimicrobial features of coated papers compared to uncoated paper. Significant improvements were obtained by combining xylan derivatives and chitosan, where the contact angle of the coated paper reached 92.8° and achieved 100% inhibition of Bacillus sp. Furthermore, food simulant tests indicated that all tested polysaccharide combinations are suitable for use in food packaging, especially for fatty products. After 28 and 42 days of soil degradation, all samples of xylan and xylan/xylan derivatives/chitosan/nanocellulose coatings reached similar degradation levels (70 to 80% and 14 to 16 mg CO2 production).
DOI: 10.15376/biores.19.4.6994-7018
Keywords: Xylan hemicellulose; Food packaging paper; Coated paper; Antimicrobial; Biodegradation; Chitosan; Polysaccharides
Contact information: a: Doctoral School of Fundamental and Engineering Sciences, Dunărea de Jos University of Galați, Brăila 817112, Romania; b: Department of Environmental, Applied Engineering and Agriculture, Faculty of Engineering and Agronomy, Dunărea de Jos University of Galați, Brăila 817112, Romania; c: Department of Food Science, Food Engineering, Biotechnologies and Aquaculture, Faculty of Food Science and Engineering, Dunărea de Jos University of Galați, Romania; d: Faculty of Mechanical Engineering, “Transilvania” University of Brasov, 500024 Brasov, Romania;
* Corresponding author: petronela.nechita@ugal.ro
INTRODUCTION
There is a global focus on exploring biodegradable materials for packaging purposes (Mitrano et al. 2021). Utilizing plant biomass as a source for packaging materials production is an appealing option. These materials have the potential to be biodegradable, renewable, abundant, and cost-effective, as they can be derived from agricultural residues or lignocellulosic biomass (Imre et al. 2019; Macedo et al. 2022).
Paper is inherently biodegradable, making it an eco-friendly option for packaging. However, its hygroscopicity and porosity pose significant challenges in food packaging, as they can lead to food contamination through the exchange of gases and liquids with the environment. To address these issues, it is crucial to use biodegradable materials for paper coatings. Polysaccharides, which are chemically compatible with paper, can offer the advantages of being less hygroscopic than paper and derived from renewable resources. The polysaccharides have garnered significant attention in recent decades within the food packaging industry. This interest is due to their advantages and substantial potential for creating environmentally friendly products. Studies have demonstrated that some polysaccharides can exhibit excellent barrier properties against moisture, gas, thermal, and grease when applied under optimal manufacturing conditions and concentrations. To meet the demands of environmentally friendly food packaging, it is important to enhance their physical and chemical properties. Improving aspects such as hydrophobicity, flexibility, and mechanical strength is vital for extending the industrial-scale application of polysaccharides in the food packaging sector. The diversity in their categories and sources leads to different monosaccharide compositions and glycosidic linkages in polysaccharides, resulting in unique biological activities and functional characteristics. For example, chitosan’s antibacterial properties are attributed to the presence of amino groups (Liu et al. 2001), while the antioxidant properties of sodium alginate are mainly due to its sulfate groups. These structural variations make polysaccharides highly versatile in their applications and functionalities (Jiao et al. 2011; Raposo et al. 2015).
The abundant active hydroxyl groups present in polysaccharides facilitate prompt and diverse chemical modifications. These modifications can tailor the properties of polysaccharides, enhancing their functionality for specific applications. By modifying these hydroxyl groups, the characteristics such as solubility, thermal stability, mechanical strength, barrier to gases, water and oil, as well as bioactivity, can be improved, thereby expanding the potential uses of polysaccharides in various industries, including pharmaceuticals, food packaging, and biotechnology.
When used in food packaging applications, the polysaccharides can be utilized as edible films or as dense coating layers on the surface of paper. This is achieved through the intertwining or chemical binding of polysaccharide molecular chains with paper fibers. These films and dense coating layers, along with the functional groups present on the polysaccharides, effectively impede the penetration of oxygen, water, or oil. This barrier property enhances the protective and functional capabilities of the paper, making it suitable for various industrial applications, such as packaging and preservation.
Common examples of polysaccharides include cellulose, hemicellulose, pectin, starch, chitosan, and sodium alginate. Hemicellulose is the second most abundant class of polysaccharide in nature and is made up of pentose and hexose monomers; these contribute multiple hydroxyl groups that give it a strong affinity for water (Farhat et al. 2018; Melati et al. 2019; Zhao et al. 2020). The structure of hemicellulose is heterogeneous, consisting of various molecules that differ among plant species and even within the same individual’s tissues (Braga and Poletto 2020; Felipuci et al. 2021).
In the majority of biomass resources, xylan is the most abundant type of hemicellulose. As raw material with high added value, xylan has been utilized in various products and it is abundant in residual biomass produced by agricultural activities, food processing, sugar, alcohol, and the pulp and paper industry (Alves et al. 2020).
As with many other polysaccharides, the high number of free hydroxyl groups in xylan hemicelluloses makes them sensitive to humidity, resulting in low barrier resistance in moist conditions. However, this characteristic is advantageous for chemical functionalization through a variety of reactions that make use of the hydrophobic groups attached to the hemicellulose chains. Such modifications result in xylan derivatives with improved barrier properties. Notable chemical reactions used for modifying xylan hemicellulose include esterification (e.g., acetylation) and etherification (e.g., carboxymethylation or alkoxylation). Additional strategies to enhance the functional properties of xylan hemicelluloses involve the use of plasticizers, modification with long-chain anhydrides, or combining them with other biopolymers (Roman et al. 2023).
Results reported in the authors’ previous study (Nechita et al. 2021) on the performance of xylan hemicellulose coatings for food packaging papers indicate that, due to its high hydrophilic character, there were no significant improvements in the barrier properties of the coated papers. To improve the hydrophobic features of xylan hemicellulose, in this study two xylan derivatives were synthetized by acetylation with acetic anhydride and reaction of alkyl ketene dimers, and their performances in coatings for food packaging papers were evaluated.
Chitosan is a unique natural cationic polysaccharide that is biodegradable, biocompatible, and non-toxic. It is obtained by deacetylation of natural chitin. Furthermore, chitosan has various beneficial properties, such as antibacterial and good film-forming capacity (Ifuku et al. 2013; Tang et al. 2020). It has been used in a broad range of applications, such as packaging, textile, fruits coating, and tissue engineering (Roy et al. 2020; Tanpichai et al. 2020; Du et al. 2021). The abundant hydroxyl and amino groups in chitosan enable it to form strong hydrogen bonds with the hydroxyl groups present in cellulose. This interaction has been leveraged to enhance the water resistance and mechanical properties of paper when chitosan is used in coatings for paper surface (Tanpichai et al. 2020; Du et al. 2021).
Other studies have reported that by the utilization of chitosan and its derivatives in paper coating, improved antimicrobial properties as well as barriers against oxygen, water vapors, oils, and grease are obtained (Hampichavant et al. 2005; Bordenave et al. 2010; Solier et al. 2022).
Nanocellulose offers several advantages when applied to paper (Lengowski et al. 2019, 2020, 2023). The use of nanocellulose, particularly NFC, in paper products can be regarded as potentially advantageous due to its chemical compatibility, maintenance of biodegradability, and safety for human health. This makes it a promising material for enhancing the performance and sustainability of paper-based products. When applied as a coating on paper, NFC can enhance the paper’s barrier properties, such as its permeability to air, gas, oil, and grease, and improves its thermal and mechanical properties, making it a more suitable material for food packaging (Fujisawa et al. 2011; Hubbe et al. 2017; Lengowski et al. 2023a). There are many studies that reported high performance for oxygen barrier and water vapor transmission performance, strength properties, and the susceptibility of nanocellulose-based films and coatings to the presence of humidity or moisture (Plackett et al. 2010; Chinga-Carrasco and Syverud 2012; Rodionova et al. 2012b; Shimizu et al. 2016).
The combination of polysaccharides can yield synergistic properties, resulting in packaging materials with enhanced functional characteristics. In this context, there is reported research regarding the utilization of xylan hemicellulose to reduce the swelling capacity and water absorption of nanocellulose films, that confirm that their combination with other polysaccharides are beneficial to improve the properties of packaging materials (Chonnipa et al. 2021).
Furthermore, films prepared using a combination of xylan and chitosan in various proportions exhibited enhanced mechanical strength and improved water and oxygen barrier properties compared to films made solely of chitosan (Cheng et al. 2024). Additionally, these films exhibited superior antimicrobial activity against E. coli and S. aureus bacteria (Schnell et al. 2017; Solier et al. 2022).
This research study evaluated the food packaging performance and environmental behavior of coated papers and homogeneous films created by combining xylan derivatives, chitosan with a molecular weight of 234,000 Da, and nanocrystalline cellulose. The xylan derivatives were synthesized in the laboratory, and colloidal dispersions with a mass ratio of 50:50 of xylan-NCC and xylan-chitosan, and respectively, 50:50 of xylan derivatives – NCC and xylan derivatives – chitosan, were prepared. These dispersions were used as single-layer coatings for packaging paper and to produce homogeneous films using the casting method.
Studies were conducted on the solubility of homogeneous films in food simulants. The functional properties of the coated papers were analyzing, including their barriers to water, water vapor, oil, and grease, their resistance to microbial attack, as well biodegradation rate through soil burial and CO₂ production quantification.
EXPERIMENTAL
Materials
Xylan hemicellulose (Xy), obtained from beech wood was purchased from Carl Roth Germany as powder. It was coloured light beige to brown with a molecular weight of 20.000 g/mol and loss on drying ≤ 10.0%.
Hydrophobized xylan that had been reacted with alkylketene dimer (XyAKD) and acetylated xylan (XyAc) were synthetized in the laboratory.
Chitosan (Ch), a white to beige powder and glacial acetic acid (≥99.85%) were supplied by Sigma-Aldrich (Taufkirchen, Germany). For improved film-forming properties, chitosan with medium molecular-weight (234,000 Da) and 88% degree of deacetylation was used in the experiments.
Nanocrystalline cellulose (NCC) purchased from Nanografi Nanotechnology Germany, as aqueous suspension of 6 wt% of 1 to 45 µm particles.
Commercial packaging paper from unbleached pulp, 50 g/m2, was used as base paper for different coatings.
Analytical purity chemical reagents as ethylic alcohol and acetic acid were used for food simulants preparation.
Homogenous films were obtained according to the scheme presented in Fig. 1 and the composition from Table 1.
Obtaining of Xylan Derivatives
Hydrophobized xylan (XyAKD) was obtained by esterification reaction of native xylan with long chain anhydrides as alkyl ketene dimers (AKD) at 20 °C and 24 h while using a magnetic stirrer at 1500 rpm (Nechita et al. 2022).
Acetylated xylan (XyAc) with degree of substitution of 0.48 was obtained by esterification reaction of native xylan with acetic anhydride at 50 °C for 1 h and molar ratio of acetic anhydride to functional hydroxyl groups in the structural unit of xylan about 8:1 (Roman et al. 2023).
Fig. 1. The stages of homogenous films obtaining
Table 1. The Codification and Composition of Homogenous Films
Preparation of Xylan/Xylan Derivatives/Chitosan/Nanocrystalline Cellulose Colloidal Dispersions
A dispersion of 2.5 g/L chitosan (Ch) in a 1% acetic acid solution was mechanically stirred at 950 rpm for 2 h. Dispersions of xylan and acetylated xylan (2.5% in distilled water) were obtained under magnetic stirring at 1500 rpm for 24 h.
Colloidal dispersions of xylan/xylan derivatives (Xy/XyAKD/XyAc), chitosan (Ch), and nanocrystalline cellulose (NCC) were prepared by the dropwise addition of the xylan/xylan derivatives solution to the chitosan/NCC dispersion at a rate of approximately 60 mL/h under magnetic stirring. After the complete addition of the chitosan/NCC, the mixture of colloidal dispersions was magnetically stirred for 24 h. To ensure adequate electrostatic interaction between xylan, xylan derivatives, chitosan, and NCC, the pH of the dispersions was adjusted to 4.5 to 5.0 prior to mixing, as all biopolymers are partially ionized under these conditions. Colloidal dispersions with a (Xy/XyAKD/XyAc)/Ch/NCC mass ratio of 50:50 were prepared and used as coatings for paper.
Obtaining of Polysaccharides Films
Homogeneous films were prepared following the stages presented in Fig. 1. The biopolymer mixtures, with the ratios presented in Table 1, were obtained by adding xylan/xylan derivative dispersions to chitosan and NCC solutions. The mixture was kept under magnetic stirring at 1500 rpm for 24 h. The resulting mixture was divided into samples of approximately 1 g dry material, cast into Petri dishes with an 11 cm diameter, and dried at 60 °C for 4 h.
Preparation of Polysaccharides Coated Papers
(Xy/XyAKD/XyAc)/Ch/NCC colloidal dispersions were applied on paper surface as homogenous coatings in a single layer of 5 g/m2 on both sides of paper.
An automatic film applicator (TQC SHEEN type, TQC B.V., Netherlands) was used for coating. In this system, the aqueous coating dispersion is applied in front of a rod of 6 mm diameter. The automatic rotation of the rod over the paper substrate in longitudinal direction at 100 mbar and 125 mm/s ensures a well-defined amount of coating dispersion is applied. The thickness of the coating layer is controlled by the diameter of the rod.
A total of 20 samples of coated papers, each measuring 20 × 25 cm, were obtained. These samples were tested for their functional properties. Uncoated paper (base paper) was used as references (Table 2).
Table 2. The Codification and Composition of Polysaccharides Coated Papers
Homogenous Film Analysis
The aspect of films as integrity, homogeneity, the presence of microcracks, flexibility or resistance to handling were evaluated by visual inspection.
Thickness, was measured using an INSIZE IP65 electronic micrometer, in a range of 0 to 25mm/+0,0001mm accuracy.
Solubility in food simulants, %, was determined according to a method stated by López De Dicastillo et al. 2011). The films, cut into strips of 2 cm x 2 cm, were totally immersed for 24 h at 25 °C in (30 mL) food simulants prepared according to Simoneau, 2009 and Directive 85/572/EEC (Table 3). After that, the strips were oven dried at 40 °C for 24 h. The solubility of the film was calculated as the percentage weight of the film that dissolved in the simulant after 24 h.
Surface Morphology
The effect of polysaccharides coating layers on the surface morphology of coated papers was investigated using a scanning electron microscope (SEM) FEI QUANTA 200 operating at an acceleration voltage of 20 kV. Prior to this, the paper samples were mounted on aluminum stub through the conductive carbon adhesive tape and sputtered with a thin gold layer, using an SPI Sputter Coater Module equipment (SPI Supplies, West Chester, PA 19380-4512, USA). The materials were evaluated at 1000x magnifications and a representative number of pictures were taken.
Table 3. Food Type and Food Simulants
Wettability
A contact angle meter (Ossila Contact Angle goniometer) equipped with a high-resolution CCD camera was used to measure the wettability of the coated and uncoated paper samples, according to TAPPI T-458 cm-04 (2004). Using a micro syringe, a single droplet of 1 μL distilled water was dropped on the surface paper sample, and the contact angle between the droplet and the tangent of the liquid surface was measured every 5 s of water-substrate contact. Five replicates were determined for each sample.
The water absorption capacity of paper, measured as the Cobb60 index, was determined according to the standard method described in ISO 535 (2014). This method measures the amount of water absorbed by the paper over a 60-second period, providing essential data for evaluating the paper’s suitability for applications where water resistance is a critical property.
Oil and Grease Resistance
The oil absorption capacity of paper, measured as the Unger-Cobb600 index, is determined following the TAPPI T 441 om-09 (2009) standard. This index measures the amount of oil absorbed by a paper sample over a specified period. The paper sample came into contact with a given amount of rapeseed oil for 600 s and weight differences were compared.
Grease resistance was tested according to the TAPPI T 559 cm-12 (2012) standard methodology to determine the KIT rating of paper samples. KIT solutions numbered 1 to 12 are prepared as mixtures with varying contents of castor oil, n-heptane, and toluene. Each solution corresponds to a specific KIT number, with higher numbers indicating more challenging conditions for grease resistance. Each paper sample is placed on a clean and flat surface with the test side facing up. A drop of a specific kit solution is released onto the surface of the paper sample. The solution is left on the paper surface for 15 seconds. After the 15-second period, any excess solution is removed from the paper surface. The back of the paper sample is inspected for any signs of staining. If no stain appears on the back, the paper sample is considered resistant to that specific kit solution. The highest KIT number of the solution that does not cause a stain on the back of the paper sample is recorded as the grease resistance value (KIT rating) for that sample.
Barrier to Air and Water Vapours
The air permeability of paper, determined using the Gurley method (ISO 5636-5:2013), involves measuring the time it takes for a specified volume of air to pass through a paper sample of a given area. This method provides a reliable way to quantify the air permeance of paper and board, which is essential for applications where breathability or barrier properties are important.
The water vapor transmission rate (WVTR) of paper samples, determined according to the ISO 2528:2018 standard, uses the gravimetric (dish) method to measure the rate at which water vapor passes through the paper. By using a controlled atmosphere and desiccant-filled dishes, this method accurately quantifies the rate at which water vapor permeates the paper over a set period, providing essential data for evaluating the paper’s barrier properties against moisture.
Before testing, all of the samples were kept for 24 to 48 h in conditioned atmosphere at 23 °C and 50% RH, according to methodology presented in ISO 187:2022. All tests were performed in triplicate, and for each tests the average values and standard deviations (SD) were calculated.
Antimicrobial Properties
To evaluate the antimicrobial effect of chitosan the papers coated with xylan derivatives and chitosan were tested against Gram positive bacteria, Bacillus sp. from MIUG collection of BioAliment research Platform – Dunărea de Jos University of Galaţi.
The antibacterial activity of the coated papers was tested using a modified and adapted method of the ISO 846 (2000) standard. According to this method, the paper samples coated with xylan derivatives and chitosan, previously 15 min UV sterilized were placed onto culture medium surface (Plate Count Agar, Merck Germany). The inoculation of coated paper samples was realized by spraying with 1 µL of bacterial suspension (18 h aged). The samples were incubated in a thermostat, at 37 °C and analyzed after 24 and 48 h. The inhibition percentage of bacterial growth on and around the coated paper samples serves as a quantifiable indicator of the antimicrobial properties imparted by the chitosan coating. The level of bacterial growth was evaluated after 24 h and 48 h of incubation at 37 °C.
Biodegradation Capacity
The soil burial degradation test was done to examine the biodegradability of coated samples according to ASTM D5988 (2012). The biodegradation rate was obtained from measurements of weight losses (%) and CO2 production (mg) after 7, 14, 28 and 42 days of soil burial of coated paper samples.
The dried paper samples were cut to the dimensions 100 mm × 100 mm, put into clean textile bag, and were buried in the soil to a depth of 5 cm from the surface for 7, 28, 42 days. Prior soil burial, the initial mass of each sample was weighed. After the specified time period, bags with samples were removed from the soil, rinsed with water and dried. The samples were weighed again and weight losses were calculated.
Weight loss (%) = (W0 – Wt) x 100 (1)
where Wt is the paper sample weight after t days of soil burial (g) and W0 is the paper sample weight before burial (g).
To measure CO2 production during the biodegradation of paper samples in soil, the assay was performed using hermetic containers. The CO2 evolved over time was monitored by titrating the amount of KOH that reacts with CO2.The containers (1 L volume) were filled with 200 g of soil with 50 to 70% humidity and samples of 3 × 3 cm2 (0.5 to 1 g) were buried approximately 2 cm from the surface. A container with soil and no paper sample was used as a control for further calculations. Inside of each container a beaker with 50 mL of distilled water and a beaker with 20 mL of 0.5 KOH 0.5N of were placed. The containers were sealed to ensure the air tight seal and introduced in a dark place for 7, 14, 21, and 28 days. After each incubation period, titration the content of beaker with 20 mL KOH 0.5 N with HCl 0.5 N (mL) using phenolphthalein was performed. The CO2 evolved was calculated using the following equation,
mg of CO2 = 50 x (VtC -VtPi) x 0.044 x fHCl (2)
where Vtc represents the volume of titrant (HCl) spent on the control, VtPi is the volume of titrant (HCl) spent on the sample, 50 is the conversion factor equivalent to μmol of CO2, 0.044 is the conversion factor of μmol of CO2 to mg of CO2, and fHCl is the titrant (HCl) correction factor.
Following the biodegradation period, the presence and activity of microorganisms in the soil were analyzed to further understand the biodegradation process of the polysaccharide-coated papers. This involved isolating and counting the colony-forming units (CFUs) of microorganisms from the soil. Soil samples containing microorganisms were suspended in distilled water and the suspension was autoclaved with 0.85% NaCl to prepare it for microbial inoculation. The microbial suspension was used to inoculate Petri plates containing Potato Dextrose Agar (PDA) medium. Two sets of plates were prepared: with antibiotics: to promote the growth of filamentous fungi and without antibiotics: to facilitate the counting of single-celled microorganisms. After incubation in thermostatic chamber at 25 to 30 °C, for 3 days, the CFUs were counted on both sets of plates.
RESULTS AND DISCUSSION
Homogenous Films Characteristics
Due to its insufficient chain length and poor solubility, native xylan hemicellulose faces challenges in forming stable, flexible films. This results in brittle films with low mechanical stability. To address these issues, besides the utilization of plasticizers such as sorbitol, xylitol, glycerol, and propylene glycol to improve film properties, compounding xylan with other biopolymers can further enhance the film features. These solutions help to improve the flexibility and durability of the films by reducing the intermolecular forces between the xylan chains, thus enhancing their mobility and interaction. By visual evaluation of films based on xylan derivatives and their combination with chitosan, flexible and uniform structure were observed with thickness values in the range from 0.11 to 0.22 mm and without cracks compared with xylan derivatives and nanocrystalline cellulose films (Figs. 2, 3).
Fig. 2. The aspect of polysaccharides films
Fig. 3. The thickness of polysaccharides films
The food simulants solubility of xylan-based films is presented in Fig. 4. It was found that xylan and its derivatives, when combined with nanocrystalline cellulose (NCC), exhibited high solubility in both aqueous and alcoholic food environments. Conversely, films made from the hydrophobic xylan derivatives and chitosan showed lower solubility in these food simulants. Based on these findings, it can be inferred that all the tested polysaccharide combinations are suitable for use as packaging or coatings on paper packaging, particularly for fatty products (Fig. 4).
The results are according to literature (Tanpichai et al. 2022) and the research suggests that these polysaccharide-based films offer versatile and effective options for food packaging applications, especially for products with high oil and grease content. 
Fig. 4. The solubility of polysaccharides films in different food simulants
Surface Morphology
The surface morphology of polysaccharide-coated papers, as illustrated in Fig. 5, was found to vary according to the coating combination applied. The uncoated paper (P0) exhibited a porous structure of cellulose fibers with large amounts of cavities (Zhang et al. 2014). The incorporation of polysaccharide coatings, particularly those combining xylan derivatives with chitosan or NCC, significantly enhanced the surface morphology of paper by reducing porosity and creating a smooth, dense, and homogeneous surface. This was mainly attributed to the electrostatic bonding between the chitosan and xylan components and the superior film-forming capabilities of chitosan and NCC (Wang and Jing 2017; Feiz et al. 2021).
In addition, the use of nanocrystalline cellulose (NCC) in paper for food packaging offers several advantages due to its unique properties. With a larger surface area and nanometric dimensions, NCC can more effectively fill the empty spaces between fibers, resulting in a more homogeneous surface. This enhancement in surface quality is crucial for food packaging because it directly influences the barrier characteristics of the material (Spagnuolo et al. 2022; Lengowski et al. 2023b).
Improving surface quality by reducing empty spaces in paper represents a significant advancement in food packaging applications. Lower porosity in the packaging material enhances its barrier properties, making it more resistant to the passage of gases, moisture, and other substances. This improvement helps in maintaining the freshness and extending the shelf life of food products, thus offering better protection and preserving the quality of the packaged food (Wang et al. 2021).
Fig. 5. Surface morphology of the polysaccharides coated papers
Wettability
The water contact angle test helps in understanding the wettability of the paper surface. A higher contact angle indicates a more hydrophobic surface, which is desirable in food packaging to prevent moisture penetration. Conversely, a lower contact angle suggests a hydrophilic surface, which might be less suitable for packaging applications where moisture resistance is required (Andze et al. 2017; Roman et al. 2023).
The water absorption test, conducted by immersing the samples in water for a specified duration, provides insight into how much water the paper can absorb. Lower water absorption indicates better barrier properties, enhancing the material’s suitability for food packaging by reducing the risk of moisture-related degradation of the food product.
The results of contact angle tests are presented in Fig. 6. By including a coating layer on the paper surface, the contact angle increased to over 90° compared to uncoated paper, which had a contact angle of 63.8°. This increase is influenced by the type of polysaccharides used in the coating formula (Habibie et al. 2016). For instance, the contact angle of paper coated with xylan derivatives ranged between 74.2° and 82.7°, while combining xylan derivatives with chitosan led to even higher contact angles, ranging from 81.63° to 92.79°.
Fig. 6. Water barrier properties of polysaccharides coated papers
The chemical modification of xylan and its combination with other biopolymers provides an effective strategy to reduce the water affinity. In this study, xylan derivatives synthesized through esterification and combined with a chitosan biopolymer demonstrated enhanced water barrier properties. The coated samples exhibited contact angles ranging from 82.4° to 92.8°, which was significantly higher than the 74.2° contact angle of samples coated with native xylan (Stepan 2013; Egüésa et al. 2014).
For the paper samples coated with xylan derivatives and chitosan, the improvement was significantly greater, due to the supplementary effect of hydrophobic nature of chitosan when it is applied to cellulose fibers or integrated within the cellulose network. Additionally, chitosan filled the empty pores within the paper, reducing the number of freely available hydroxyl groups on the cellulose that can interact with water molecules. This reduction in available hydroxyl groups significantly enhanced the hydrophobic properties of the paper, as reflected in the higher contact angles (Ehman et al. 2022).
On the other hand, no significant change in the water contact angle was observed for the paper coated with NCC combinations. This suggests that while NCC might improve other properties, such as mechanical strength or barrier properties against gases, its influence on the hydrophobicity of the paper is minimal (Abdul Khalil et al. 2012). The lack of significant change in the contact angle indicates that the NCC coating does not substantially alter the surface’s interaction with water, maintaining a similar level of hydrophilicity as the uncoated paper
Paper samples coated with xylan and its derivatives exhibited a decrease in water absorptiveness by 15% to 18% compared to uncoated paper samples. A more significant reduction in water absorption, approximately 25%, was achieved for paper samples coated with xylan derivatives combined with chitosan and nanocrystalline cellulose (NCC). This indicates that the addition of xylan derivatives, especially when combined with chitosan and NCC, effectively reduced the water absorption properties of paper, enhancing its water resistance (Fig. 6).
Gases Barrier Properties
The barrier properties to gases of polysaccharides coated paper, as evaluated by analyzing the air permeability and water vapor transmission rate, are presented in Fig. 7. All coating combinations resulted in a reduction of the WVTR compared to uncoated paper. This indicates that the coatings improved the paper’s resistance to water vapor transmission.
Fig. 7. The gases barrier properties of polysaccharides coated papers
The evolution of WVTR was found to be consistent with the changes in air permeability. This means that as the air permeability decreased, the WVTR also decreased, suggesting that the coatings effectively block the passage of gases and water vapor. Paper samples coated with a combination of xylan derivatives and chitosan showed the lowest air permeability and WVTR. This combination forms a denser structure without pinholes and voids, which was attributed to the excellent film-forming ability of chitosan.
When comparing coatings based on xylan or xylan derivatives alone with those that combine xylan/xylan derivatives and nanocrystalline cellulose (NCC), it has been observed that the combination with NCC results in better water vapor transmission rate (WVTR) and air permeability. This improvement is attributed to the synergistic effects between xylan/xylan derivatives and NCC, leading to a more compact and cohesive coating structure. The interaction between xylan/xylan derivatives and NCC creates a more compact and uniform coating matrix. This synergistic effect helps in filling the microvoids and cracks that might be present in xylan-based coatings alone, thus enhancing the overall barrier performance, as has been confirmed in other research (Aulin et al. 2010; Lavoine et al. 2012; Balan et al. 2015).
Oil Barrier Characteristics
In the context of food packaging, particularly for bakery products, fast food, and pet food storage, there is a high demand for materials that exhibit excellent resistance to grease and oil. This resistance is very important to prevent the migration and penetration of fats from the product into the packaging material, ensuring both the integrity of the packaging and the quality of the food. The detailed results, as shown in Fig. 8, highlight the effectiveness of different polysaccharide coatings in enhancing the grease and oil barrier properties of food packaging papers.
Fig. 8. Oil and grease barrier properties of polysaccharides coated papers
Xylan/xylan derivatives and their combinations with chitosan and nanocrystalline cellulose improved the grease and oil resistance of coated papers compared with uncoated paper. Therefore, the oil absorption and KIT rating of papers coated with xylan derivatives and NCC formula was about 21% lower and respectively two times higher compared with uncoated paper. The significantly improvements were obtained in case of paper coated with xylan derivatives and chitosan formula. In this case the KIT rating value was approximately three times higher compared to that of uncoated paper. For these samples, the oil absorption was 46% lower than that of uncoated paper.
The presence of cationic groups (NH3+) in the chemical structure of chitosan plays a crucial role in retaining fat and preventing its penetration into the substrate. This significantly enhances the oil and grease barrier properties (Pichavant et al. 2005). Both NCC and chitosan have excellent film-forming properties, contributing to their effectiveness as oil barriers. Their combination with xylan derivatives results in coatings that provide superior resistance to grease and oil penetration (Chen et al. 2023).
The obtained results demonstrate that polysaccharide-coated papers, particularly those using combinations of xylan derivatives with chitosan and NCC, can achieve grease and oil resistance properties similar to those of fluorochemical-coated papers. This finding is supported by existing literature and highlights the potential of these biopolymer-based coatings to serve as sustainable alternatives in food packaging (Kjellgren et al. 2005).
Antibacterial Properties
Bacillus sp. was selected to evaluate the antibacterial activity of the polysaccharides coated papers. Gram-positive bacteria, such as Bacillus sp., exhibit higher resistance to the bactericidal action especially of chitosan compared to Gram-negative bacteria. This characteristic makes them suitable for testing the efficacy of antibacterial coatings.
Bacillus subtilis is commonly used as a model organism in laboratory studies. It is often regarded as the Gram-positive counterpart to Escherichia coli (E. coli), which is a well-studied Gram-negative bacterium.
The results presented in Fig. 9 demonstrated that the xylan-/xylan derivative-/chitosan-coated papers exhibited significant antibacterial activity against Bacillus sp. The antibacterial properties of chitosan are attributed to its cationic nature, which allows it to interact with the negatively charged bacterial cell walls, leading to cell disruption and death. This effect was observed even with the increased resistance of Gram-positive bacteria. The results are well correlated with WVTR values, which for the samples P4, P8, P9, P10, and P11 were the lowest. This created a low moisture content, which was unfavorable for bacterial proliferation.
Fig. 9. The antibacterial activity of polysaccharides coated papers
Biodegradability capacity
The soil burial degradation test confirmed the biodegradability of the polysaccharide-coated papers. The combination of weight loss, CO2 production, and visual inspection provided comprehensive evidence that the polysaccharides coated papers break down effectively in a soil environment. The results are presented in Figs. 10, 11, and 12.
The different compositions of the coatings showed certain patterns in relation to the rate of biodegradability and CO2 production. This could have been because the natural polysaccharides are highly compatible with the environment and made the sample degrade faster in the soil. Compared with uncoated paper, the CO2 production was higher for all the paper samples coated with polysaccharides. This is related to the growing of microorganisms during the catabolic process of sugars. The growing microorganism may be due to of more enzymes that catalyze xylan hemicellulose, chitosan, or nanocellulose breakdown reactions, which degrade polysaccharides into oligosaccharides and monosaccharides, respectively (Carpinelli Macedo et al. 2022). At 28 days and 42 days of soil degradation, all the samples of xylan and xylan/chitosan/nanocellulose coatings reach similar degradation rate (70 to 80% and 14 to 16 mg CO2 production, as is presented in Figs. 10 and 11.