Introductory Chapter: Alginate – A Brief Presentation

1. Introduction

Alginate is a natural biopolymer synthesized mainly by brown seaweed but also by microorganisms, also called bacterial alginate [1]. Alginate, also known as alginic acid, has a linear structure constituted by unbranched chains of polysaccharides, consisting of blocks of two monomeric uronic acids, β-(1–4)-D-mannuromic acid (D or M), and in its epimere, the α-L-guluronic acid(G) [2, 3]. Due to its composition, the chemical structure of alginate can be considered by combining three types of blocks: GG, MM, and MG, where in the GG blocks there are only α-L-guluronic acid units, in the MM blocks there are only β-(1–4)-D-mannuromic acid units, and in the MG blocks there are alternating units of both acids [3].

The origin of the alginate as well as the age of the seaweed and the harvesting season influence the ratio and the arrangement of the MG blocks, resulting in different biological and physicochemical properties [4, 5]. In an overview, alginate gels with a low ratio of MG blocks have the characteristics of being stiff and brittle, but if the ratio of MG blocks is high, there is the formation of flexible and elastics gels [5, 6]. For the formation of alginate gels by the addition of calcium ions, a high concentration of the GG blocks guarantees an improvement in gel strength [3]. However, for the formation of polymeric nanoparticles, the more significant presence of MM blocks contributes to the stability of the aggregation of nanoparticles [7].

Alginate is a polysaccharide that has similar characteristics to pectin present in plants; however, it is present in the cell walls of brown algae, of the Phaeophyceae class, in a similar format to cellulose microfibrils, arranged in a crystalline arrangement, in addition to being derived from the intercellular matrix of these seaweed [8]. The most common and easy-to-find species commercially are Ascophyllum, Durvillaea, Ecklonia, Laminaria, Lessonia, Macrocystis, Sargassum, and Turbinaria [9]. Furthermore, alginate can be extracted from bacteria such as Azetobacter Vineland and Pseudomonas species [10].

The most commonly used in industry is sodium alginate, which is extracted through steps involving physicochemical procedures, starting with a treatment of the dry material using formaldehyde, followed by an acid process, after collecting the algae in its marine habitat. After these steps, an alkaline extraction is performed, followed by bleaching, precipitation, and drying [11].

The applications of alginate currently depend on its characteristics such as low toxicity, anti-inflammation, high absorption, and thickener in food mixtures, in addition to having the ability to accelerate healing in pharmaceutical processes [12]. For these reasons, the polysaccharide is exploited in industries involving food, beverages, fabrics, printing, and pharmaceuticals [13]. An example of its use is in materials from the pharmaceutical and cosmetics industries, which guarantee improved stability and damage control due to external conditions such as temperature and UV light, or even protection in gastric environments, if they are for oral use [14]. In the food industry, alginate applications are based on three properties: thickening, gelling, and film forming [15].

Alginate can also be seen as an additive in packaging, in order to improve the quality and prevent damage to the coated product and promote good conservation because of the added attributes such as antimicrobial and antioxidant action [16]. These packages are edible coatings used as a biopolymer that sits on the surface of the food and is included by methods of molding, coating, extrusion, and dipping, among others [17]. Another area where alginate is highly used is in agriculture where it functions in improving productivity, treating water, and improving the quality of the crop through uses in seed coatings, fruits, and vegetables that help with growth, in addition to being used in formulations that control the use of agrochemicals [18].

Due to the versatility of uses of alginate, several new technologies have been developed in order to create sustainable solutions with lower environmental impact, including self-healing asphalt [19], bio-ink for 3D printers [20], flame-retardant materials [21], and alginate-based bio-composite materials for wastewater treatment [22]. By way of exemplification, microorganisms, such as microalgae and cyanobacteria, can be immobilized in sodium or calcium alginate beads, forming small spheres, and used several times without significant loss of cell activity, ensuring the stability of the processes in which they are applied. This technological route has received increasing interest from researchers and scientists in the area of microalgae-based processes for industrial effluent treatment. The microalgae cell immobilization in polymeric alginate matrices can be performed by several methods and can be exploited for the bioremediation of different types of wastewater and contaminants [23].

In short, alginates can be modified into multiple products, such as hydrogels, fibers, films, sponges, capsules, and light foams [22, 24]. These characteristics make this biomaterial interesting to be studied and applied in a vast number of different situations.

In this sense, the chapters presented in this book are intended to provide an in-depth understanding of the applications and future perspectives of alginate, contributing to the consolidation of information about its characterization, properties, synthesis, current uses, and trends.