**2.1. Carrageenan**

Carrageenan is a sulfated polysaccharide extracted from red algae. Marine organisms from *Rhodophycaea* family like *Hypnea*, *Euchema*, *Chondrus*, *Crispus*, and *Gigartina* are the main type of red seaweeds manufactured for carrageenan sources. Different types of red seaweed is used to extract different carrageenan, namely, kappa (κ), iota (ı), lambda (λ), nu (η), mu (μ), ksi (ξ), and theta (φ). The structures of the three most prevalent and commercialized carrageenans are shown in **Figure 1**. Examples of some different sources of carragenans are *Euchomadenticulatum* (spinosum) for ı‐carrageenan, *Kappaphycusalvarezi* (cottoni) for κ‐carrageenan, and *Gigartinar‐ adula* and *Chonduscrispus* for extraction of both ı‐ and κ‐carrageenans [3]. All types of carra‐ geenans are water‐soluble.

Carrageenans contain alternate units of D‐galactose and 3, 6‐anhydro‐galactose linked glycosidically. As can be seen in **Figure 1**, κ‐carrageenan has only one sulfate group per disaccharide chain, two for ı‐carrageenan, whereas λ‐carrageenan got three. This resulted in anionic polysaccharide that is often neutralized by cations like sodium, potassium, calcium, magnesium, and ammonium. Interesting to note that the structure of λ‐carrageenan does not have 3,6‐anhydro‐bridge like in the κ‐ and ı‐carrageenans. This structure gives κ‐ and ır‐ carrageenans gelling ability in response to thermal condition. The location of ester sulfate group affects the solubility and gel strength of carrageenan, while existence of 3,6‐anhydro‐ bridge results in polysaccharide gelation [5]. Besides galactose and sulfate units, other carbohydrate residues that commonly exist in carrageenan are xylose, glucose, and uronic acids [6]. Carrageenans are used in a variety of commercial applications as gelling, thickening, and stabilizing agents, especially in food products and sauces. Aside from these functions, carrageenans are being explored in experimental medicine, pharmaceutical formulations, cosmetics, and industrial applications.

**Figure 1.** Structure of carrageenan [4].

#### **2.2. Alginate**

Polysaccharides are polymeric carbohydrate molecules consisting of long chains of monosac‐ charide units bound by glycosidic linkages. The fact that these polymers are extracted from natural resources has led to the impression of good biocompatibility and biodegradability. Chemically, nearly all materials from plants are carbohydrate in nature and composed of repeating unit of monosaccharides. Thus, they are nontoxic. Its biocompatible nature is also attributed to the structural similarity of glycosaminoglycans (GAGs), which is a vital compo‐ nent of extracellular matrix in tissue. There is an emerging interest in reducing the amount of undisposable plastic waste that often leads to serious environmental problem. Polysaccharides are potential alternative for replacing conventional petroleum‐based plastics which are able to biodegrade naturally in soil. Polysaccharides are famous for their used in the food and dairy industries. However, its unique structure and versatile modification can be explored for other

Polysaccharide can be categorized into structural and storage polysaccharides. Examples of structural polysaccharides are cellulose in plant and chitin in the shells of crustacean, while storage polysaccharides include starch and glycogen. Polysaccharides are present in most living organisms. In fact, polysaccharides comprise about 70% of the dry weight of the total biomass [2]. Although polysaccharide is advantageous as biomaterials as they are more ecofriendly than petro‐polymers, there are still critical drawbacks that need special attention to make it an ideal choice. Polysaccharide exhibits poorer mechanical properties than the conventional plastics. Some polysaccharides also have strong hydrophilic behavior that may cause early rupture. Thus, polysaccharide composites have been extensively studied in regard

Several types of polysaccharide were widely studied over the past decades due to their potential in numerous research areas. Some of the polysaccharides being explored as bioma‐

Carrageenan is a sulfated polysaccharide extracted from red algae. Marine organisms from *Rhodophycaea* family like *Hypnea*, *Euchema*, *Chondrus*, *Crispus*, and *Gigartina* are the main type of red seaweeds manufactured for carrageenan sources. Different types of red seaweed is used to extract different carrageenan, namely, kappa (κ), iota (ı), lambda (λ), nu (η), mu (μ), ksi (ξ), and theta (φ). The structures of the three most prevalent and commercialized carrageenans are shown in **Figure 1**. Examples of some different sources of carragenans are *Euchomadenticulatum* (spinosum) for ı‐carrageenan, *Kappaphycusalvarezi* (cottoni) for κ‐carrageenan, and *Gigartinar‐ adula* and *Chonduscrispus* for extraction of both ı‐ and κ‐carrageenans [3]. All types of carra‐

Carrageenans contain alternate units of D‐galactose and 3, 6‐anhydro‐galactose linked glycosidically. As can be seen in **Figure 1**, κ‐carrageenan has only one sulfate group per

to counter this problem and obtain additional properties for specific application.

terials are carrageenan, alginate, chitosan, starch, and cellulose.

important fields.

66 Composites from Renewable and Sustainable Materials

**2. Types of polysaccharide**

**2.1. Carrageenan**

geenans are water‐soluble.

Alginate, or also called alginic acid, can be derived from both algal and bacterial sources. Current commercial alginates are mostly from the cell walls of brown algae (*Phaeophyceae*) [7] such as *Laminaria hyperborea*, *Laminaria lessonia*, *Macrocystis pyrifera*, and *Ascophyllum nodosum*. They are harvested to be converted into raw material commonly known as sodium alginate. On the other hand, alginates that are synthesized by bacterial biosynthesis obtain more defined chemical structures and physical properties than that of seaweed‐derived alginates [8]. These bacterial alginates can be produced from *Azotobacter* and *Pseudomonas*. Other common forms of alginates are potassium alginate and calcium alginate. Alginates are anionic polysaccharides that can form viscous gum when bound with water. They are composed of linear unbranched copolymers containing blocks of (1,4)‐linked β‐D‐mannuronic acid (M) and α‐L‐guluronate (G) residues, covalently linked in different sequences or blocks. The blocks can be consecutive MMMMM or GGGGG, or alternating GMGMGM. The amount of G and M blocks and the length depends on the alginate origin. The gel formation of alginate occurs when two G blocks of adjacent chains chelate with cations like Ca2+ with their carboxylic groups [9].

Alginate is also another popular material used in foods as a thickening agent, gelling agent, emulsifier, stabilizer, and texture improver. It can be added to color paste for textile printing and act as binder of flux in welding rod production. Alginates are also established as bioma‐ terials in the pharmaceutical industry where they can be compounded into tablets to accelerate disintegration of tablet for faster release of drugs. In cosmetic field, alginate can help to retain the color of lipstick on lip surface by forming gel network.
