Characterization of biodegradable films based on extracellular polymeric substances extracted from the thermophilic microalga Graesiella sp.

Highlights

• The thermophilic microalgae Graesiella sp., Extracellular Polymeric Substances “EPS” film is of adequate elasticity.

• “Graesiella sp., EPSs” film has a homogeneous structure.

• “Graesiella sp., EPSs” film presents a high thermal stability, and low permeability against water and oxygen.

• EPS film displays a potent antioxidant activity via scavenging free radicals and chelating iron.

• EPS film can extend the shelf life of meat and preserve its quality during cold storage.

Abstract

In this research, a new type of biodegradable film based on the extracellular polymeric substances (EPS) and isolated from the thermophilic microalga Graesiella sp., was formulated and characterized. The EPS film was 0.221 mm thick. Atomic force microscopy and scanning electron microscopy images revealed a homogeneous character with a lamellar microstructure. The EPS film displayed yellowish color, high transparency, high ultraviolet barrier properties, and low oxygen (0.008 SI), and water-vapor permeability (0.037 SI). Film tensile strength (16.24 MPa) and elongation at break (4.76%) were in the range of common biofilms and the thermal analyses showed high transition temperature (126 °C) and high thermal stability (up to 800 °C). Compared to ascorbic acid, results indicated that the EPS film shows a higher antioxidant activity, mainly as β-carotene anti-bleaching (84%), DPPH- free radical scavenging ability (80%), and ferrous iron-chelating (55%).

Graesiella sp., EPS film effects on beef meat packaging were studied during nine days of cold storage. Compared to polyvinylchloride-packed meat, EPS-packed meat samples showed higher stability of color (redness = 13.6) and pH (5.85) during storage and low proliferation of total viable counts (4.04 CFU·g⁻¹) and Pseudomonas bacteria (4.09 CFU·g⁻¹). They also exhibit lower drip loss (9%) and less metmyoglobin (32%), heme iron (4.87 μg·g⁻¹) total volatile basic nitrogen (TVB-N = 22.96 mg·kg⁻¹), and lipid oxidation (MDA = 0.025 mg·kg⁻¹). The obtained results highlight the potential for use of microalgae EPS as a new film-forming material that could be applied in beef meat preservation.

Introduction

Recently, plastics have become a source of worrying contamination due to their abundance, their persistence in nature, as well as their harmful effects on biodiversity and the environment [1]. Plastics are mainly used as packaging materials, up to 38%, and most are released into the environment after a single-use. Plastic waste, more than 170 million tons in 2017 [2], [3], and their additives, including butylated hydroxytoluene (BHT) or butylated hydroxyanisole (BHA), cause serious pollution problems, particularly in soil and water [4], [5]. Therefore, extensive research has been conducted to replace common synthetic materials with natural ones [6]. The exploitation of natural materials in the preparation of eco-friendly packaging films has been a new promising issue for meat products safety [7].

Biodegradable films are being successfully used as active packaging in several processed foods, including seafood/meat products and fruits, as oxygen scavengers, barriers to water and light, antibacterial, and anti-browning agents [9]. The bio-based films made from natural polymers have recently attracted considerable interest because of their ecological properties, their safety, and their biodegradability in nature [10].

Natural polysaccharides, proteins, and lipids biopolymers are used for bio-based film preparation. Besides their role as a barrier against the external environment, they provide active functions by prolonging the shelf life and improving the safety and the sensory properties of different products [8]. In recent years, the exploitation of extracellular polymeric substances (EPS) has been of particular interest. However, EPS have not yet been tested regarding the production of bioactive films nor as a packaging material [11].

EPS obtained from microalgae are versatile biopolymers produced at a high amount by several microalgae species, particularly thermophilic ones [12], [13].

In a previous work [13], results showed that Graesiella sp., is found amongst the most efficient EPSs producers reaching values close to 1.62 g·L⁻¹ in laboratory cultures. The native Graesiella sp., EPS are hetero-sulphated polysaccharides of anionic nature composed mainly with polysaccharides (80%) and proteins (14%), presenting a high crystalline nature. They exhibited a high viscosity, emulsifying and flocculating proprieties. EPS have been utilized as additives, thickeners, emulsifiers, gelling agents, and stabilizers in several industrial sectors such as food products, pharmacy, petroleum industry, and bioremediation agents [14]. Furthermore, EPS provides high intrinsic bioactive properties such as antioxidant, anti-inflammatory, antiviral, antifungal, and antibacterial [13], [14] and therefore have a high potential for use as biodegradable active packaging, especially for meat products.

The present study aims to characterize an eco-friendly bioactive film using EPS extracted from the thermophilic microalga Graesiella sp., the EPS film was characterized according to its structure, optical and mechanical properties, thermal stability, water and oxygen barrier ability, and antioxidant potential.

Furthermore, we evaluated the shelf life of meat enrobed with the EPS film in terms of pH, water activity, drip losses, color degradation, hemic iron, metmyoglobin, lipid and protein oxidation, and microbial stability, during nine days of preservation at 4 ± 1 °C.