Characterization of bacterial cellulose films combined with chitosan and polyvinyl alcohol: Evaluation of mechanical and barrier properties

Highlights

• Bacterial cellulose is a raw material to develop biodegradable composites.

• The developed films may be used for applications in medicine, cosmetics, and food industry.

• Poly(vinyl alcohol) and chitosan improve bacterial cellulose films properties.

• Developed films showed optical barrier properties against UV-radiation.

• Transparent films with adequate visual appearance were obtained.

Abstract

Bacterial cellulose (BC) produced by Komagataeibacter xylinus is a biomaterial with a unique three-dimensional structure. To improve the mechanical properties and reinforce the BC films, they were immersed in polyvinyl alcohol (0–4%) and chitosan (0–1%) baths. Moisture content, mechanical properties and water vapour permeability were measured to assess the effect of polyvinyl alcohol and chitosan. The morphology, optical, structural and thermal properties were evaluated by scanning electron microscopy, spectral analysis, thermogravimetry and differential scanning calorimetry. Results showed that moisture content was significantly affected by the chitosan presence. Tensile strength values in the 20.76–41.65 MPa range were similar to those of synthetic polymer films. Percentage of elongation ranged from 2.28 to 21.82% and Young's modulus ranged from 1043.88 to 2247.82 MPa. The water vapour permeability (1.47 × 10⁻¹¹–3.40 × 10⁻¹¹ g/m s Pa) decreased with the addition of polyvinyl alcohol. The developed films own UV light barrier properties and optimal visual appearance.

Introduction

In the last decade, research in biopolymers has been widely carried out to design and develop renewable, biodegradable and biocompatible products with applications in medicine, pharmacy, cosmetics, food industry and biotechnology (Hu, Chen, Yang, Li, & Wang, 2014). Among biopolymers, cellulose is the most abundant natural compound on the earth. This low-cost biopolymer has renewability, non-toxicity, biocompatibility, biodegradability and chemical stability (Cazón, Velazquez, Ramírez, & Vázquez, 2017). Due to the properties of cellulose, it has become a very interesting material to develop new biodegradable polymers, like biofilms aimed to food packaging applications.

Cellulose is a highly crystalline polymer formed by a linear chain with two anhydroglucose rings ((C₆H₁₀O₅)_n), covalently linked through an oxygen in a β(1–4) glycosidic bond. Cellulose possesses abundant hydroxyl groups forming plenty inter- and intra-molecular hydrogen bonds (Cazón et al., 2017). It is found in plant cell wall and accordingly, it can be obtained from wood, cotton, hemp and plant-based materials. Furthermore, cellulose is also produced by tunicates, several species of algae, and by some species of bacteria including Acetobacter, Agrobacterium, Pseudomonas, Rhizobium, Sarcina and Komagataeibacter xylinus. The last one was renamed as Acetobacter xylinum and more widespread as Gluconoacetobacter xylinus (Szymańska-Chargot et al., 2017, Yamada et al., 2012). It produces high amounts of cellulose, being one of the most commonly studied sources of bacterial cellulose (BC) (Mohammadkazemi, Azin, & Ashori, 2015; Ruka, Simon, & Dean, 2012).

In a suitable culture medium and static conditions, G. xylinus synthesizes BC in the form of pellicle on the surface of the liquid medium. This microorganism produces glucose chains from the carbon source contained in the medium. The glucose chains are extruded out through tiny pores present on their cell wall. Combining the glucose chains, the microorganism forms microfibrils that aggregate to form cellulose ribbons. These ribbons form a three-dimensional structure consisting of an ultrafine network of cellulose nanofibers with an expanded surface area and high porosity. The three-dimensional structure determines its physical and mechanical properties (Jozala et al., 2016; Shah, Ul-Islam, Khattak, & Park, 2013; Shao et al., 2016). Unlike vegetable cellulose, BC is obtained with high purity, free of vegetable remains such as lignin and hemicellulose (Jozala et al., 2016).

Several studies have been focused on developing new films based on BC. Usually, BC films are combined with other polymers or plasticizers to improve or modify the physicochemical properties and expand its potential applications. There are two main strategies to combine BC with other polymers or plasticizers, keeping the film structure obtained from the fermentation process. One option is by supplementation of the culture medium with the desirable reinforcing agents, such as Aloe vera (Saibuatong & Phisalaphong, 2010), chitosan (Phisalaphong & Jatupaiboon, 2008), polyvinyl alcohol (PVOH) (Gea, Bilotti, Reynolds, Soykeabkeaw, & Peijs, 2010) or polyethylene oxide (Brown & Laborie, 2007). Other strategy is by immersion of the BC film in a bath with a solution of the desirable polymer or plasticizer, e.g. polyvinyl alcohol (PVOH) (Gea et al., 2010), polyethylene glycol or diacrylate (Cai & Kim, 2010; Numata, Sakata, Furukawa, & Tajima, 2015).

A widespread practice is to use nanofibers or nanowhiskers of BC as a reinforcing agent. BC pellicles are usually subjected to a hydrolysis process using strong acids, breaking down the structure of the material into nanofibers or nanocrystals (Martínez-Sanz, Lopez-Rubio, & Lagaron, 2013). These BC nanocomponents are incorporated into the matrix of a polymer to modify the properties of the composite film such as starch (Martins et al., 2009), PVOH (Jipa et al., 2012), arabinogalactan and xyloglucan (Lucyszyn et al., 2016), chitosan (Salari, Sowti Khiabani, Rezaei Mokarram, Ghanbarzadeh, & Samadi Kafil, 2018; Velásquez-Cock et al., 2014; Wang, Xie, et al. 2018), polylactic acid, polyethylene glycol (Martínez-Sanz et al., 2013), fish proteins (Shabanpour, Kazemi, Ojagh, & Pourashouri, 2018), gelatin (George & Siddaramaiah, 2012) or agar (Wang, Guo, et al. 2018) among other biopolymers.

From the strategies mentioned to obtain BC-polymers composite films, it is interesting to pay special attention to the immersion method. It is a simple technique that allows using the films formed from the culture medium. This method allows avoiding previous steps of dissolution, regeneration or homogeneous dispersion of the cellulose, simplifying the process. In addition, this technique allows taking advantage of the unique structure of the BC as the main component of the final matrix. On the other hand, unlike the supplementation culture media method, by immersion allows expanding the range and concentration of the possible combinations. Some supplementation components in the culture media can interfere with the cellulose production yield. Hence, the concentration of these components in the formulation film are limited (Phisalaphong and Jatupaiboon, 2008, Saibuatong and Phisalaphong, 2010).

Following the strategy of combining polymers to improve the properties of the final material, PVOH and chitosan can improve the potential applications of BC-based films. PVOH is a hydrophilic semi-crystalline polymer produced by polymerization of vinyl acetate to polyvinyl acetate, followed by a hydrolysis process. It is a synthetic polymer, water soluble, non-toxic, biodegradable, with film forming properties, optimal transparency and good elasticity properties (Carvalho et al., 2009; Cazón, Vázquez, & Velazquez, 2018a). These properties make PVOH an ideal polymer to combine with BC by immersion.

Chitosan is one of the most studied polysaccharides with potential applications in biomedical, food, and chemical industries. It is the second most abundant polysaccharide in nature. It can be obtained mainly from residues of the shellfish industry. This polymer is non-toxic, biodegradable, with film forming properties and soluble in dilute organic acids such as acetic acid. One of the most interesting properties of the chitosan is its antimicrobial activity against a wide range of foodborne filamentous fungi, yeast, and bacteria, being more active against yeasts (Helander, Nurmiaho-Lassila, Ahvenainen, Rhoades, & Roller, 2001; No, Meyers, Prinyawiwatkul, & Xu, 2007). Among these properties, its solubility and antimicrobial activity make it an ideal polymer to combine with BC by immersion. Incorporating antimicrobial agents from natural source into BC composite films addresses the current consumer demand of a natural alternative to chemically synthesized antimicrobial polymers. In food packaging, a strategy to increase the shelf life is to develop active films with antimicrobial properties. The direct contact with the active films inhibit the growth of microorganisms on the surface of the food (Broek Van Den, Knoop, Kappen, & Boeriu, 2015; Moreira, del, Roura, & Ponce, 2011).

In previous studies of our group, films based on regenerated vegetable cellulose-PVOH-chitosan were characterized. The results obtained suggested an adequate interaction of cellulose-chitosan-PVOH to combine both polymers by immersion, improving the film properties (Cazón et al., 2018a; Cazón, Vázquez, & Velazquez, 2018b). Nevertheless, as mentioned, there are important structural differences between vegetable cellulose and BC. These differences could affect the interactions of cellulose-PVOH-chitosan, improving the properties of the cellulose-based films and expanding its potential applications. Besides, it was observed the low transmittance of regenerated cellulose and chitosan films developed, manifesting UV protect properties of cellulose-based films (Cazón et al., 2018a). Packaging materials against UV light has received a great deal of attention since UV-light is one of those responsible factors of the oxidative process in vitamins, lipids and proteins. These oxidative process produce undesirable off-flavours that decrease the shelf-life of the products (Olarte, Sanz, Federico Echávarri, & Ayala, 2009). Hence, the UV-barrier properties observed increase the interest in the development of BC-based films for food applications.

On the other hand, the application of BC-PVOH-chitosan components in the field of controlled drug administration has been also studied. The results suggested that BC-PVOH-chitosan composites could be used as a biopolymeric carrier for drug delivery applications (Pavaloiu, Dobre, & Hlevca, 2013). Despite the good interaction among these biodegradable polymers, the characterization of these films has not been carried out to aim applications like biopolymers for active food packaging.

The aim of this work was to develop biodegradable films based on BC with chitosan and PVOH to enhance the mechanical and optical properties to obtain cellulose films more manageable and transparent with antimicrobial and UV-barrier properties. The effect of the ratio of these polymers incorporated to the BC matrix by immersion, maintaining intact the characteristic BC structure, was studied and compared to regenerated cellulose, previously analyzed. Polynomial models were used to evaluate the effect of the composition of the blend on the moisture content, mechanical properties (tensile strength, percentage of elongation to break and Young's Modulus) and water vapour permeability. Besides, microstructure, optical properties, structure and thermal analyses of the BC-PVOH-chitosan films were also evaluated.