Development and characterization of Nile tilapia (Oreochromis niloticus) protein isolate-based biopolymer films incorporated with essential oils and nanoclay

Highlights

• This work aimed to produce and characterise biofilms based on recovered fish protein.

• The increase of the protein isolate (HSPI) decreased the solubility in water of the films.

• The increase of the HSPI increased the solubility in acid and tensile strength of the films.

• Higher opacity was observed with higher values HSPI and clove essential oil (CEO).

• The film with the best characteristics had 25 % glycerol, 2.5 g HSPI and 0.5 g CEO.

Abstract

Fish residues are a natural resource of valuable components, e.g. proteins. The aim of this work was to develop biopolymer films from Nile tilapia (Oreochromis niloticus) protein isolate (NTPI), incorporated with glycerol, montmorillonite nanoclay, and essential oils. The extracted protein was characterized for its isoelectric potential, proximate composition, solubility, water retention capacity (WRC), oil retention capacity (ORC), and color. The casting technique was utilized to obtain the films. Experiments were conducted using a 2^(5–1) factorial design with central point. The films were characterized for thickness, solubility in water (Sw), solubility in acid (Sa), water vapor permeability (WVA), and mechanical, structural, and thermal properties. The protein isolate concentered 85 % of the total protein from the raw material. Glycerol, nanoclay, and clove and oregano essential oils were successfully incorporated to the films. The films with the best characteristics were obtained with T6 (1.5 g NTPI, 0.2 g glycerol, 0.4 g oregano essential oil, 0.3 of nanoclay), and T12 (1.5 g NTPI, 0.4 g glycerol, 0.4 g clove essential oil, 0.1 g nanoclay) because they were more stabilized within the cross-linking protein structure. The increase of the protein isolate decreased the Sw of the films but increased the Sa and tensile strength. The Sw ranged from 30.62 to 85.45 %, the Sa between 13.19 and 45.92 %, the WVP between 2.38 and 4.02 g mm/kPa day m². Tensile strength ranged from 0.65 to 2.76 MPa and elongation from 0.29 to 2.07 %. The essential oils did not show antimicrobial activity when incorporated to the polymer matrix of the films.

Introduction

The indiscriminate disposal and the accumulation of non-biodegradable packaging have been causing serious environmental problems (Pimentel, Pizzuti, Fakhouri, Innocentini-Mei, & Fonseca, 2018). One of the alternatives is the use of biodegradable films. Proteins are among the natural biopolymers most used to produce biofilms (Silva, Santos, Fonseca, Prentice, & Cortez-Vega, 2020).

Fish proteins discarted during industrial fish processing can be used e.g. to prepare biodegradable films (Batista, Araújo, Joele, Silva, & Lourenço, 2019; Munir, Hu, Liu, & Xiong, 2019; Pereira et al., 2019). The properties of protein films are known as better than other macromolecules, mainly in terms of gases barrier, water vapor permeability and mechanical properties (Romani et al., 2019).

Glycerol is an organic compound belonging to the class of polyols. It acts as a plasticizer, due to its characteristics, as stability and compatibility with the polymeric chains of films (Chillo et al., 2008). It is present in all oils and fats of animal and vegetable origin in their combined form, i.e., bound to fatty acids, e.g. stearic, oleic, palmitic, lauric acid forming the triacylglycerol molecule (Knothe, van Gerpen, Krahl, & Ramos, 2006).

Addition of antioxidant and antimicrobial substance in the preparation of renewable films has been done in the last few years. Essential oils are aromatic and volatile liquids extracted from plant components e.g. roots, flowers, stems, leaves, seeds, fruits and the whole plant, and are one of the compounds with these characteristics (Menezes et al., 2019; Monica & Ioan, 2019). The essential clove oil, e.g., has eugenol in its composition substance responsible for analgesic, anti-inflammatory and antioxidant activity (Oliveira, Cruz, & Almeida, 2012).

The effects of nanoclays in composite films have also attracted considerable attention due to their high surface area, low cost and ease of availability (Celik & Baştürk, 2019). Nanoclay consists of clay minerals, which are hydrated filosilicates belonging to the family of smectites, where montmorillonite, hectorites, and saponites are the most frequently used. The introduction of nanoclay into biodegradable films can be used to improve their mechanical properties (Slavutsky, Bertuzzi, & Armada, 2012). Montmorillonite nanoclay, in special, can improve also the optical and barrier properties when included at low concentrations. This advantage of adding montmorillonite clay, compared to the traditional composites, means the production of lighter components, a desirable factor in many applications, beyond higher microbial stability (Menezes, Cortez-Vega, & Prentice, 2017). Edible covers with nanoclays can extend the shelf life and improve the quality of fruits by reducing mass transfer, increasing the integrity/handling and/or the functional properties e.g. antimicrobial and antioxidant properties (Neves, Hashemi, & Prentice, 2015).

Considering that the properties of polymers are strongly dependent on the film composition, it is believed that constitute tailor made polymers can be obtained from the combination of different renewable materials for different applications, including films for packaging, mulch films to agriculture, coating films with antimicrobial and water-resistant properties, osteosynthetic materials, bone plates, surgical sutures and other materials in medicine, e.g. matrix to controlled release of drugs, source for enantiomerically pure chemicals, binder polymer in the formulation of environmentally friendly paints and pressure sensitive adhesives (Fonseca, Silva, & Gomez, 2008). The understanding of their physical and chemical behavior and their response to external stimuli is the bottleneck to improve their functionality (Da Silva, Cortez-Vega, Prentice, & Fonseca, 2019).

Thus, the aim of this work was to produce and characterize films obtained from Nile tilapia filleting residues, added of glycerol, clove and oregano essential oils, and nanoclay.