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Agar is a polysaccharide mixture obtained by water extraction from red algae species. The largest amount of agars in the world are produced from Gracilaria, Gelidium, Pterocladia, Acanthopeltis and Ahnfeltia algae. Agar consists of two basic structures, these are high-gelling (70%) natural polymer, agarose and low-gelling (30%) sulfated polysaccharide agaropectin (Figure 4.1). Agarose, naturally, is in a neutral and linear form of repeating units of the disaccharide agarobiose (d-galactose and 3,6-anhydro-lgalactopyranose). Agar has hot water solubility, a gel forming structure at 32–40 °C and does not melt below 85 °C [12–16]. As seen in Figure 4.1, their common feature is that they all consisted of D-galactose and 3,6-anhydro-L-galactose monomers of galactose. The agar structure also contains sulfate, pyruvate and methoxy groups. The amount of molecules in the agar structure varies depending on macroalgae biomass and subsequent processing procedures [17].

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.