Cellulose nanofibers produced from banana peel by chemical and mechanical treatments: Characterization and cytotoxicity assessment

Highlights

• CNFs were isolated from banana peel using acid and mechanical treatments.

• CNFs showed no cytotoxic effect against Caco-2 at concentrations lower than 500 μg/mL.

• Banana peel CNFs have potential application as reinforcing agent in polymeric matrices.

Abstract

Cellulose nanoparticles from a vegetable source (cellulose fiber) have been evaluated for future use as reinforcement of polymeric matrixes (e.g., biodegradable films). Cellulose nanoparticles have numerous advantages: they are inexpensive and biodegradable, and they originate from renewable sources. Here, cellulose nanofibers (CNFs) were isolated from banana peel by chemical (alkaline treatment and bleaching followed by acid hydrolysis with 0.1, 1, or 10% (v/v) H₂SO₄) and mechanical (high pressure homogenizer) treatments. Atomic Force Microscopy (AFM) analysis showed all treatments effectively isolated banana fibers at the nanometer scale (average diameter of 3.72 nm). CNFs displayed ζ-potential values ranging from −37.60 to −67.37 mV, which prevented their aggregation. CNFs had high crystallinity values, from 63.1 to 66.4%, which indicated they could be good reinforcing agents. FTIR results confirmed that the chemical and mechanical treatments removed the amorphous fractions. Regarding cytotoxicity, low CNF concentrations (50–500 μg/mL) did not cause cell death, but CNFs at concentrations above 1000 μg/mL significantly decreased cell viability. The use of different sulfuric acid concentrations provided more detailed knowledge of the treatment methods and CNF features, which could help to improve the CNF production process. The combination of chemical and mechanical treatments proved to be an efficient strategy to prepare CNFs from banana peels as a potential reinforcing agent of polymeric matrixes (e.g., food packaging).

Introduction

In recent decades, growing environmental concern has motivated the development of sustainable processes and products. The need to develop biodegradable materials to replace synthetic products has led researchers to explore natural and renewable resources (Jiménez, Fabra, Talens, & Chiralt, 2012). Biodegradable films employed as packaging materials can prevent moisture migration and protect food products during transport, avoiding mechanical damage and maintaining their physical integrity. Furthermore, they act as modified-atmosphere packaging that maintains the sensory characteristics of the product and prolongs its shelf life (Mali, Grossmann, & Yamashita, 2010).

Nanometric natural fibers (cellulose source) have been explored as an alternative material to reinforce biodegradable films. Incorporation of these nanofibers into composites offers numerous advantages, including the fact that they are biodegradable and originate from renewable sources (Pelissari, Sobral, & Menegalli, 2014). Agricultural waste has become an attractive nanofiber source: this waste consists mainly of plant fibers that are rich in cellulose, the main component of plant cell walls (Lavoine, Desloges, Dufresne, & Bras, 2012). Researchers have isolated cellulose nanofibers (CNFs) from the peels of fruits and vegetables such as cassava (Leite, Zanon, & Menegalli, 2017), tomato (Jiang & Hsieh, 2015), and potato (Chen, Lawton, Thompson, & Liu, 2012).

Banana is one the most popular fruits worldwide, and its cultivation is widespread in tropical and subtropical regions. A wide variety of fruits, including banana, grow in Brazil because this country has the largest area of cultivable land in the world. The edible part of banana constitutes only 12% (w/w) of the plant, and the banana peel becomes a banana processing byproduct during the production of foods like banana chips, baby foods, and unripe banana biomass. In turn, banana cultivation and industrialization generates a considerable amount of waste that is rich in cellulose and starch, which makes this waste a promising source of ingredients for the production of biodegradable materials (Elanthikkal et al., 2010, Pelissari et al., 2014). Pelissari, Andrade-Mahecha, Sobral, and Menegalli (2017) suggested the integral use of banana fruit: these authors used the banana pulp and peel to obtain starch and CNFs that served as film matrix and reinforcing agent, respectively.

CNF isolation from lignocellulosic materials usually involves three steps: (1) pretreatment, (2) partial hydrolysis, and (3) mechanical disintegration of the raw material (Sun, Sun, Zhao, & Sun, 2004). Acid hydrolysis is a technology that has long industrial history and which is traditionally used to break cellulose microfibers into nanofibers (Yu, Lou, & Wu, 2008). This process commonly employs sulfuric or hydrochloric acid because these highly aggressive acids are essential to cleave the glycosidic bonds between the cellulose chains (Pirani & Hashaikeh, 2013). According to Chen et al. (2011), it is preferable to use sulfuric acid (H₂SO₄) because it generates a more stable nanofiber aqueous suspension: the sulfate groups provide the nanofiber with a negatively charged surface (Saito et al., 2009), thereby preventing CNF agglomeration.

The wide application of nanotechnology has improved the characteristics of materials and generated new commercial products, but it has also increased human exposure to nanomaterials. Although nanoparticles have beneficial effects, they may endanger the human health, so possible risks should be evaluated (Jones & Grainger, 2009). The small size, the chemical and physical properties, and the high reactivity of nanoparticles may cause them to be cytotoxic (Pereira et al., 2013). Because nanoparticles incorporated into films used as food packaging may migrate and consequently be ingested by humans, toxicological analysis is an essential part of CNF characterization. In vitro methods are currently employed for toxicological evaluation of nanomaterials. Human epithelial cells are commonly used in in vitro cytotoxicity experiments because they resemble the epithelial phenotype in vivo, including the presence of mucin coating the cell membrane, microvilli, desmosomes, and lamellar body. Caco-2 cells, which originate from human colon adenocarcinoma and are similar to the small intestinal epithelial layer, is gaining prominence in in vitro assays. In vitro permeability studies have employed Caco-2 cells to assess the cytotoxicity and the potential effect of delivery systems in terms of enhanced drug permeation (Antunes et al., 2013, Araújo and Sarmento, 2013, Jones and Grainger, 2009). Ma et al. (2013) studied lactoferrin nanoliposome anticancer efficiency by assessing Caco-2 cell viability, to find out that such nanoliposomes are a potential therapeutic modality in tumor management. Dong, Hirani, Colacino, Lee, and Roman (2012) tested the cytotoxicity of cellulose nanocrystals prepared from wood pulp fibers exposed to different cell lines representing the epithelia cells of different human organs. After 48 h of exposure, the cellulose nanocrystals at concentrations of up to 50 μg/mL were not cytotoxic to any of the investigated cell lines.

Previous studies reported by Pelissari et al. (2014) have shown that CNFs obtained by acid hydrolysis have potential application as reinforcing agents in composites. However, acid hydrolysis efficiency depends on factors like the acid concentrations. This study aimed to characterize CNFs isolated from an agro-industrial waste (unripe banana peel) by chemical treatment at different H₂SO₄ concentrations, followed by mechanical treatment (high-pressure homogenizer). Moreover, an evaluation on how the CNFs affected Caco-2 cell viability was conducted, which constituted the innovative character of this scientific study.