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*Corresponding author: [email protected]
4
Seaweed Polysaccharides: Structure, Extraction and Applications
Oya Irmak Şahin
Chemical Engineering Department, Faculty of Engineering, Yalova University, Yalova, Turkey
Abstract
For the last decades, there has been a profound interest in metabolites from marine sources. In marine habitat, seaweed is a precious source of biologically active compounds. Polysaccharide-rich seaweeds contain these metabolites, such as agar, carrageenan, fucoidan, alginate and ulvan, in their cell walls and intracellular structures. These sulfate-structured polysaccharides have drawn attention in recent years in the food, pharmaceutical, cosmetics and clinical fields. This chapter presents a general information about chemical structure, extraction procedures, properties and a brief report for the potential applications of these polymers.
Keywords: Seaweed, macroalgae, polysaccharide, agar, carrageenan, fucoidan, laminaran, alginate
4.1 Introduction
Marine environment has various living organisms where it is the place of the first living organism that appeared almost 3,500 million years ago. Marine organisms have typical metabolic and physiological properties which give them the growth and reproductive ability in extreme conditions such as high salinity and temperature. Seaweeds are the 90% of the marine abitat, and half of these seaweeds are responsible for the global photosynthesis [1, 2].
Although, creating an acceptable and easily identifiable classification system for algae is becoming more difficult due to the emergence of new species and classes. Seaweeds, also known as macroalgae, belong to the domain Eukarya and kingdoms Plantae and Chromista. Due to being a multicellular photosynthetical organism, seaweeds can be classified after the pigmentation of the organism as, red (Rhodophyta), brown (Heterokontophyta) and green (Chlorophyta) [3].
Seaweed polysaccharide structure and chemical and functional properties are influenced by many physical, biological and environmental factors, such as harvest time, the algal species, growth media composition, extraction methods, etc. [4]. All polysaccharide types have an application in human area, especially in biological and life sciences. Seaweed polysaccharides can be classified into two, due to its place in seaweeds, as cell-wall and storage polysaccharides. Most seaweed polysaccharides are cell-wall polysaccharides, except the storage carbohydrates, are located in the cell plastids. Also, they may be classified as food grade and non-food grade polysaccharides among their usage today.
Seaweed polysaccharides, like agar [5, 6], alginates, and carrageenans [7, 8] are economically and industrially the most important products from seaweeds, commonly in food industry [9, 10]. Seaweed polysaccharides have a relation with wide range of technologies including food, pharmaceutical, biomedical, textile, paper and biodegradable packaging materials. Non-food grade class polysaccharides are fucoidan, laminaran and ulvan, which have application areas as pharmaceutical, cosmeceutical and medical [11].
4.1.1 Agar
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.
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
