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      Figure 4.1 Chemical structure of agarose and agaropectin.

The genus Gracilaria is widespread all over the world and contains many important agar producing species. However, a low-quality gel is obtained from the agar produced from Gracilaria spp. This is because of its high sulfate content. The helix in the sulfate agar causes mixing, i.e., it prevents the formation of the gel web [18, 19].

      Extraction of agar is usually carried out with hot water. Hot water allows a concentrated filtrate to form in the subsequent processing steps, allowing the gel to form; this gel is then subjected to various processes, dried and ground [18–21]. In order to form a gel from agars, they must be stable under variables and factors such as temperature, humidity and chemicals. Agar yield and quality depends on species, season, environmental factors, stages of growth, extraction method, type of solvent, extraction time and temperature [14, 19–23]. In addition, the gel properties of the agar may vary with the growth conditions of algae such as chemical composition of the growth media.

      About 90% of agar is used for food and only 10% is used for industrial purposes. The agar’s gel-forming feature is ten times higher than gelatin, so it has a wide range of uses, especially in the production of foodstuffs. It is used as a protective additive used in meat and fish products, puddings and desserts, bakery products and marmalades. In addition to its use in food industry, it is used as a medium for growth of microorganisms such as bacteria and yeast. Agar is also used in pharmaceuticals, cosmetics, biological and medical research for their functionality of decreasing blood glucose levels, preventing aggregation of red blood cells, and absorption of ultraviolet radiation [24–27]. Agar type polysaccharides have also anti-inflammatory, antitumor and antioxidant effects [28–30]. Furthermore, in the pharmaceutical industry agar has been used as a smooth laxative.

      4.1.2 Carrageenan

      Carrageenan is, also a sulfated polysaccharide, consisting of bounds which α-1,3 and β-1,4 linked d-galactopyranose, also with other carbohydrate residues (xylose, glucose, etc.) and substituents (methyl ethers, pyruvates, etc.) [30]. There are 10 different structure (Figure 4.2) based on sulfate group numbers and positions, but the mostly known are these six groups, namely iota-(ι), kappa-(k), lambda-(l), mu-(m), nu-(n), and theta-(q) carrageenan [31]. These sulfate group positions and contents effect the functional and behavioral properties of carrageenan [24, 26, 30–34]. Commercially important carrageenan types are iota-(ι), kappa-(k) and lambda-(l) carrageenan.

      Extraction of carrageenan is based on alkaline extraction, filtration or centrifugation of both are used for recovering. There are two extraction pathways after alkaline treatment for obtaining semi-refined or refined carrageenan forms. For the refined carrageenan, the carrageenan is precipitated with alcohol and in the second method (semi-refined carrageenan), it is dissolved in an alkali solution, like potassium chloride to form a strong gel which is then dried [35, 36].

Protein reactivity of carrageenan is an important property to be discussed for which application will be used of. Carrageenan is negatively charged, which means they are able to combine with positively charged particles, for example positively charged proteins, as in the three-dimensional gel network with the protein in milk (casein). Their main applications are in the food industry, especially in dairy products. As an emulsifier, carrageenan addition is necessary in dairy products to prevent separation of whey in cheese and ice creams and coffee whiteners. Carrageenan’s interactions with galactomannans are an important reason that it finds use in the production of fruit sorbet, poultry and meat products. Also, in pet foods semi-refined carrageenan are using for their gel forming structure. Carrageenan is found application areas in the structure of air freshener gels and toothpastes; also, biotechnological applications of carrageenan has been demonstrated as a medium for immobilizing enzymes or whole cells [32–39].

      Figure 4.2 Chemical structure of carrageenan.

      4.1.3 Alginate (Alginic Acid, Algin)

      Alginate is named after the salts of alginic acid, all derivatives of alginic acid and itself. They are found in cell walls of brown algae. Phaeophyceae, Laminaria, Ecklonia, Ascophyllum, Durvillaea, Lessonia, Macrocystis, Sargassum and Turbinaria species are the sources for marine alginate [26, 27].

      Alginate is a linear polysaccharide with an anionic polymeric and hydrocolloidal structure (Figure 4.3) and are composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G). The chemical structure of the alginate varies from one genus to another brown seaweeds and also, the physical properties of alginates vary according to the ratio of mannuronic acid to guluronic acid, monomer sequence, and molecular weight of the chains. The more guluronic acid content means the more high-quality gelling property for alginate isolated from any seaweed. The physical properties of alginates also control the drug release rate from gels, and the phenotype and function of cells encapsulated in alginate gels as well as gelation [10, 35].

Alginate extraction can be examined under three steps: pre-extraction, neutralization and precipitation. By the precipitation stage, pathways are divided into two as seen in Figure 4.4. Sodium alginate is the main commercial form of alginate, with forms of soluble alginates such as alginic acid and alginic acid esters. Isolation and extraction process of alginates’ major disadvantage is the difficulty of the process from contaminated seaweed, where there are impurities in the final product because of the presence of cytotoxic materials in the contaminated seaweeds where further purification steps needed [36].

      Figure 4.3 G blocks, M blocks and alternating blocks of alginate.

      Figure 4.4 Isolation and extraction of pathways of alginate.

      Alginate have several applications due to its gel-forming, thickening, stabilizing properties, bioactive and biodegradable functions and low toxicity [37]. For these reasons, it is widely used for food, textile, cosmetics, painting or dye and pharmaceutical industries. In food industry they have usage areas as thickeners, gelling agents, and as stabilizers of water-in-oil emulsions, suspensions like fruit juices. Especially in dairy products, alginates are using to obtain in non-sticky, non-softened and stable texture. Alginate matrices such as alginate-pectin or alginate-chitosan may also be used as an encapsulation agent for probiotics, proteins, pigments and volatile compounds [40–42]. Because of good film-forming properties for shelf life extension, reducing the browning rate, inhibiting the yeast and mold growing and maintaining the textural and color attributes they have been used in edible coating formulations [43–46]. Besides from the gelling property, they have applications to provide stable, longer lasting beer foam, to clarify the wine, also in restructured or re-formed food products [47–51].

In textile industry, alginate is also using for the thickening property for

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