Alginates

Applications in the Biomedical and Food Industries

Shakeel Ahmed

• Publisher: Wiley-Scrivener
• Year: 2019
• Print ISBN:
  9781119487913
• eBook ISBN:
  9781119487975
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Chemie- & Biochemietechnik

Alginate is a hydrophilic, biocompatible, biodegradable, and relatively economical polymer generally found in marine brown algae. The modification in the alginate molecule after polymerization has shown strong potential in biomedical, pharmaceutical and biotechnology applications such as wound dressing, drug delivery, dental treatment, in cell culture and tissue engineering. Besides this, alginates have industrial applications too in the paper and food industries as plasticizers and additives.

The few books that have been published on alginates focus more on their biology. This current book focuses on the exploration of alginates and their modification, characterization, derivatives, composites, hydrogels as well as the new and emerging applications.

Part 1
ALGINATES—INTRODUCTION, CHARACTERIZATION AND PROPERTIES

Chapter 1
Alginates: General Introduction and Properties

Rutika Sehgal¹, Akshita Mehta¹ and Reena Gupta^1*

¹Department of Biotechnology, Himachal Pradesh University, Summerhill, Shimla, India

*Corresponding author: [email protected]

Abstract

Alginates (ALGs) are a group of naturally occurring anionic polysaccharides derived from brown seaweeds. They are linear biopolymers of 1,4-linked β-D-mannuronic acid (M) and 1,4 α-L-guluronic acid (G) residues that are arranged in homogenous (poly-G, poly-M) or heterogenous (MG) block-like patterns. The physiological and chemical characteristics of ALGs depend on this arrangement of residues. Alginates are primarily used as thermally stable cold-setting gelling agents, which are formed in presence of divalent cations. They are more efficient gelling agents than gelatin and can gel at far lower concentrations as compared to other agents. This ability to create a chemically set, irreversible gel has proved to be useful in many food applications. Among various ALGs, sodium ALG is most widely studied in the pharmaceutical and biomedical field. Its various properties favor its use for viscosity enhancement, encapsulation polymer, matrixing agent, stabilizer, bioadhesive, and film former in transdermal and transmucosal drug delivery. With well-established uses in dentistry, the ALGs also offer interesting possibilities in the field of medicine and cosmetics as a skin care ingredient. This chapter will include general introduction, understanding of structure and properties of ALGs, and different forms of ALGs used in industries.

Keywords: Alginates, biopolymer, polysaccharide, medicines, cosmetics

1.1 Introduction

Alginates (ALGs) are naturally occurring anionic polysaccharides that are present as a structural component in cell walls of brown algae, mainly from Macrocystis pyrifera, Ascophyllum nodosum, and Laminaria hyperborea and as a capsular polysaccharide in bacterial strains like Azotobacter and Pseudomonas. It is present in the cell wall of brown algae as the calcium, magnesium, and sodium salts; therefore, it is usually referred to as “alginic acid and its salts.” Alginates are available commercially as sodium, potassium, or ammonium salts in filamentous, granular, or powdered forms. Their color ranges from white to yellowish-brown. The molecular weight of ALG generally ranges from 60,000 to 700,000 Da depending on the application [1]. The size (diameter) of ALG gel particles can be macro (greater than 1 mm), micro (from 0.2 to 1,000 mm), or nano (less than 0.2 mm). These gel particles have high water holding capacity to form a viscous gum and have adjustable chemical and mechanical properties that are dependent on the type of cross-linking agent used. As a natural ingredient, ALG gel particles are attractive for various biological applications because they are biocompatible, nontoxic, biodegradable, and relatively cheap [2, 3]. Alginate is also a significant component of the biofilms produced by the bacterium Pseudomonas aeruginosa, the major pathogen in cystic fibrosis, that confers it a high resistance to antibiotics and killing by macrophages.

1.2 History

Alginate was discovered in the late 19^th century by a British Pharmacist, E.C.C. Stanford, who called it “algin,” which was a viscous solution obtained initially from Laminariaceae. Since its discovery in 1883, it has become an important industrial product that is commercially obtained from coastal brown seaweeds. Later its extracts were termed as “alginic acid.” Its commercial production started in 1929. It has been estimated that algal ALGs are produced nearly 38,000 tons worldwide annually, and their major part contributes to food and pharmaceutical industries because of their increased demand [4].

1.3 Structure

Alginates are linear biopolymers of 1,4-linked β-D-mannuronic acid (M) and 1,4 α-L-guluronic acid (G) residues (Figure 1.1) organized in homogenous (poly-G, poly-M) or heterogenous (MG) block patterns. The G and M block pattern and sequence may be different in commercial ALG depending on the source of seaweed used, harvesting season, and geographical location of the seaweed source [5]. The random sequence of M and G block chains (Figure 1.2) are composed of regions of alternating MG blocks whose monad, diad, and triad frequencies are determined. Rigid six-membered sugar rings and restricted rotation around the glycosidic linkage make ALG molecules stiff. The rigidity of the chains further is due to electrostatic repulsion between the charged groups on the polymer chain and on ALG composition. It increases in the order MG < MM < GG; therefore, G-rich ALGs generally form hard and brittle gels, while soft and elastic gels are produced by M-rich samples. Hence, the physicochemical properties and degree of polymerization of the ALG depend on the arrangement of these blocks [5].

Figure 1.1 Structure of ALG monomers (L-guluronic acid and D-mannuronic acid).

Figure 1.2 (a) Homopolymeric blocks of poly-β-1,4-D-mannuronic acid (MM blocks); (b) homopolymeric blocks of poly-α-1,4-L-guluronic acid (GG blocks); (c) heteropolymeric blocks of MG monomers in random pattern [6].

1.4 Alginates and Their Properties

1.4.1 Gel Formation

Alginate can form gel independent of temperature as compared to other polysaccharides such as gelatin or agar. The ALG gels can either be ionic gels (formed by cationic cross-linking) or acidic gels (formed by acid precipitation).

1.4.1.1 Ionic Alginate Gels

The ability of ALG to form ionic gel in the presence of multivalent cations is mostly desired in food industries. The process of binding of ALG to divalent cation is very specific, and the affinity of ALG toward cations is in the order Mn < Zn, Ni, Co < Fe < Ca < Sr < Ba < Cd < Cu < Pb [7, 8], and it depends on the number of G blocks present in the structure [9]. The cooperative binding of G block and divalent cations results in gelation of ALGs. The use of highly toxic cations such as Pb, Cu, and Cd is limited for practical applications, but less toxic cations like Sr and Ba have been reported to be used in cell immobilization applications at limited concentrations [10]. Calcium being nontoxic is widely accepted to form ionic ALG gels. Calcium-ALG gel is the most commonly used ALG gel. Interactions between Ca ions and G residues result in gelation of ALG, which leads to chain–chain association and to the formation of junction zones. The two G chains bind on opposite sides with the addition of Ca ions to the ALG polymer, which results in a diamond-shaped structure with a hydrophilic cavity. The oxygen atoms from the carboxyl groups form multicoordination with the Ca ions in the hydrophilic cavity. This tightly bound complex forms a junction zone that is shaped like an “egg box” (Figure 1.3). In this egg box, a 3-D network is formed by the binding of each cation with four G residues [11]. In case of Ca ALG gels, there should be 8 to 20 adjacent G residues in order to form a stable junction [12]. Although it is generally observed that most divalent cations form ALG gels by the “egg-box” formation, it is still not known if other divalent cations follow the same mechanism for gel formation [13–16]. Binding of Ca ion enhances with increasing content of G residues in the chains, while poly-M blocks and alternating MG blocks have lower affinity toward the ion. Generally, by raising the ALG G block content or molecular weight, more strong and brittle ALG gels may be achieved [4]. The affinity of ALG toward Ca ions increases with increasing content of the ion in the gel due to an autocooperative zipper mechanism. This first stage of dimerization is followed by a second stage of lateral association of the dimers at higher Ca²⁺ concentrations. Isolated and purified G blocks have been shown to act as gel modulators, forming higher-order junction zones composed of two or more chains.

Figure 1.3 Egg-box structure formation during the ionic gelation of sodium ALG [17].

Studies have shown previously that there could be different block sequence than G blocks to which cations can bind in ALG. For example, binding studies have recognized that Ca is able to bind to G and MG blocks, Ba can bind to G and M blocks, and Sr can bind to G blocks only [8, 12]. Trivalent cations such as Al³⁺ and Fe³⁺ can also be used to gel ALG. In fact, they generally have an increased affinity of binding with ALGs as compared to divalent cations. They form a more compact gel network by binding in a 3-D structure due to their ability to bind with three carboxyl groups from different ALG biopolymers at the same time [18].

The ionic gels are widely used in various industries; like in the food industry, these are used in encapsulation of bioactives, in pharmaceuticals for maki...

Table of contents

• Cover
• Title page
• Copyright page
• Dedication
• Preface
• Part 1: Alginates—Introduction, Characterization and Properties
• Part 2: Alginates in Biomedical Applications
• Part 3: Alginates in Food Industry
• Part 4: Alginates Future Prospects
• Index
• End User License Agreement