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The presence of naturally occurring polysaccharide in the seaweed genera of Gigartina, Chondrus, Iridaea, Eucheuma and Kappaphycus, namely carrageenan, which fills the voids within the cellulose structure, is used to maintain the seaweed's survival rate by helping to cope with wave motion and vibrant current in the ocean (Nussinovitch, 1997). Today, the structure, conformation, and physical as well as biological properties of carrageenan, has been subjected to numerous studies as they able act as natural stabilizer, gelling agent or thickening agent which is stable in the presence of other chemical compound (Sauza, et al., 2011; Stortz, 2005). Differences in chemical composition and conformation yields a wide range of rheological properties, ranging from a viscous thickener to thermally reversible gels which ranges in texture from elastic to brittle (Stortz, 2005).
2.3.1 History of Structure Study of Three Main Type of Carrageenan
Carrageenan has been used widely in food application a few hundred years ago around the world including China, Japan and Europe. Ireland was the first country described to have used carrageenan as thickening and stabilizing agents in food around 1810 (Stortz, 2005). Carrageenin, as it was first called of carrageenan, was discovered by the British pharmacist Stanford in 1862 which represent the gelatinous material extracted by water from C. crispus (Pereira, et al., 2009). The dried type of C. crispus seaweed was imported to USA from Ireland until in 1835, Dr J. V. C. Smith have verified the plants growing at Massachusetts coast were same as the C. crispus. In 1844, Schmidt reported the first isolation of the mucilage of C. crispus and began the journey of chemical studies on carrageenan. Scientists have used about 40 years to determine the type of polysaccharide present in carrageenan, which is galactose. The first structure study was carried out around 1940 (Stortz, 2005).
Carrageenan can be classified into three main commercial classes which include kappa (), lambda (), and iota () carrageenans. Each type of carrageenan has true family of structure which justified at 1960. Fractional of crude carrageenan was first done by Smith and Cook in 1953 by mean of potassium chloride. The carrageenan that can precipitate with potassium ion and form gel is known as -carrageenan whereas the rest that remains soluble is known as -carrageenan. This study also indicates that -carrageenan is sensitive toward potassium ion. The complete structure of -carrageenan was discovered after the determination of 3,6-anhydro-D-galactose which is the main component of carrageenan whereas complete structure of -carrageenan was determined by David Rees in 1960s. -carrageenan, the structure of third carrageenan was elucidated in 1967 (Stortz, 2005; FAO, 1990).
2.3.2 Chemical Structure and Gelling Mechanisms of Carrageenan
Carrageenan represents a generic name for a family of linear heteropolysaccharides which the galactans with high sulphate content derived from Rhodophyceae. The main chain consists of alternating copolymer of -1,3-D-galactopyranosyl and -1,4-D-galactopyranosyl residue (Funami, et al., 2007; Dunstan, et al., 2001; Falshaw, et al., 2001). Naturally derived carrageenan seldom have a regularly repeating structure and is more complicated than theoretical description as the galactose units usually interspersed with sulphate hemiester groups, pyruvic acid ketals, methylation of some hydroxyl groups, side chain, and have the presence of 3,6-anhydro ring as a cyclic ether of majority -galactose units (Pereira, et al., 2009; Usov, 1998). All types of carrageenan are different in degrees and positions of ester sulphate groups as well as the degree of 3,6-anhydro-D-galactose and hence, yields different structures of the galactan and rheological properties of solution and gel. (Distantina, et al., 2011)
Carrageenan is well known in thermo-reversible gels. The gelling mechanisms is involve in two steps which highly dependent on temperature and the concentration of alkali-metal ion (Falshaw, et al., 2001). The first step involves carrageenan in the alkali hot solution undergoes a disordered-ordered transition which random coiling in the solution. The second step is the dissolution of carrageenan aggregate into double helices structure and stabilized though hydrogen bond as cooling gel (Nickerson, et al., 2004). The presence of the ester sulphate will disturb the bonding and acts as a kink to prevent the double helix structure forming (Goycoolea & Chronakis, 1998). The detail of gelling mechanisms is under study (Souza, et al., 2001), theoretically, however, research believes that the negative charged sulphate interacts with cations which potassium and calcium ions ion for -carrageenan and -carrageenan respectively to stabilize the structure by forming the 3-6-anyhydrogalactose bridge to straighten the chain. This leads the great regularity in the polymer and enhancing the gel strength (Goycoolea & Chronakis, 1998; FAO, 1990). Only the ester sulphate located at 6-D galactose is able to eliminate, therefore it is known that the higher the sulphate content, the lower the solubility temperature and hence lower the gel strength. (Nazarudin, et al., 2011)
2.3.3 Overview of Kappa () carrageenan
-carrageenan is mainly extracted from Kappaphycus alvarezii and has the precursor of -carrageenanan (McHugh, 2003). It has lowest sulphate content compare to - and - carrageenan It constitutes by -D-galactose sulphated in C4 linked to 3,6-anhydrogalactose (Vera, et al., 2011). Pure -carrageenan has a brittle and firm texture. However, strong and rigid thermo-reversible gel will form with the presence of alkaline-potassium cationoweHhhh. (Cardoso & Sabadini, 2010) It is freeze-thaw unstable and significant syneresis. Syneresis is due to the further form the helixes structure in the gel leading contracts of the internal network and thus causing weeping of liquid (Pilkington, et al., 1999). Hence, the most traditional application of -carrageenan is to produce water dessert gels and cake glazes with the texture modified with -carrageenan or locust bean gum to improve elasticity and cohesiveness (Imerson, 2000).
Figure AA: (a) The structural disaccharide unit of -carrageenan (b) A schematic representation of the -carrageenan gelation mechanisms: (i) random coil formation, (ii) helix formation. (iii) aggregation of helixes (Daniel-da-Silva, et al., 2008)
2.3.4 Main Application of Kappa-Carrageenan and Their Current Study
This study is evaluating the yield and content of refined carrageenan from in vitro cultivated K.alverazii which represent their commercial value. The reactivity between carrageenan and milk protein (primarily the casein micelle) is the best known synergistic carrageenan interaction in the food industry, is the unique properties to differentiate carrageenan from other hydrocolloids (Imerson, 2000). This interaction is to produce viscosity and gelation enhancement of milk product during manufacture and storage such as ice creams, chocolate milk, flans, puddings and creamy milk desserts (Imerson, 2000; ; Spagnuolo, et al., 2005). The presence of visual phase separation between casein micelles and polysaccharides which known as biopolymer incompatibility can be inhabit and stabilizing by -carrageenan. The synergistic interaction which the -carrageenan acts as stabilising agent with casein micelles can explain by two existing theories. The first theory is based on the electrostatic interaction between the negatively charged sulphate group of -carrageenan and positively charged region of -casein, and hence, resulting in a stable three dimensional network. The second theory is based on the formation of a weak -carrageenan gel in the aqueous phase, which holds the casein micelles suspended (Blakemore & Harpell, 2010; Mounsey, et al., 2006; Spagnuolo, et al., 2005)
-carrageenan is acts as kettle fining agents in the beer, wine or vinegar production to produce clear and transparent product via the precipitation of the proteinaceous impurities after the yeast fermentation (Chichester, 1962; Therkelsen, 1993; Dale, et al., 1995). Meanwhile, this reaction also able to improve the foam lasing and increase the stability of the product. The generally accepted mechanism of clarification is the electrostatic interaction between negative charged sulphate group in -carrageenan and positive charged protein. -carrageenan in the liquid medium is responsible to react with the soluble protein and connecting several molecules to form a large insoluble complex and stay in the solution, or, it is react with the insoluble particles and causes the flocculation, and resulting the sedimentation (Palmer, 2012; Ward, 2012)
Chibata, 1979, was the pioneer who introducing the idea of -carrageenan immobilization for cell entrapment (Tampion & Tampion, 1987). The strongest gel and beads form from -carrageenan have the sufficient and superior mechanical strength for packing in columns, and permeable to most substrate and serve as suitable carrier for yeast immobilisation for continuous fermentation in beer production. -carrageenan is a well-known thermo-reversible gel. This property is highly depending on the concentration of potassium ion as the concentration is used to alter gelation temperature. Higher concentration of potassium ion leads the higher the gelling temperature of -carrageenan. This unique characteristic is used to avoid the severe temperature fluctuation which can affect the growth of yeast cell. Therefore, the concentration of potassium ion used to alter the gelling temperature such as, the gel state of -carrageenan is formed under fermentation condition whereas the liquid state of -carrageenan is formed when mixing with the yeast cells without detrimental effects (McHugh, 2003; Neufeld, et al, 2001; Pilkington, et al., 1999). Apart from beer fermentation, -carrageenan also use to entrap Lactobacillus acidophilus in the fermentation of tomato juices to increase the cell survival rate as well as the overall palatability (Tsen, et al., 2008).
Beside food application, carrageenan wisely use in non-food products such as preparation of cosmetic. For example, carrageenan use in mascara is to prepare gel-in-water-in-oil emulsion which to improve lash lengthening and curling (Pallela & Kim, 2011) . Their gelling ability can be applied in numerous cosmetic and skin care products to impart texture and consistency such as LANCÔME clarifying exfoliating gel (LANCÔME, 2011), shu uemura depsea hydrability intense moisture filling mask (shu uemura, 2011), Nano white hydrating whitening mask and many others. The gel formed able to stabilize solid particles and give suspension properties to keep particles from settling. Moreover, the gel formed has the bio-adhesion compatibility which has better connection between the carrageenan product and skin. Apart from this, skin lotion, hair care ingredient, eye makeup constituents, shaving foams, air freshener gels are the examples of personal care which containing carrageenan. (Pallela & Kim, 2011)
Recent study of -carrageenan is involve in medical application. Current study shows that the insulin which is entrapped in lectin-functionalized carboxymethylated -carrageenan can be orally delivered and is protected from hydrolysis in acidic environment in starch and proteolysis by enzyme. The function of lectin is to increase the interaction between the microparticles and intestinal walls and hence increase the absorption of insulin. The gelling property of carrageenan favours the drug entrapment and the natural sulphate group located at the carboxymethylated -carrageenan have the ionic interaction with the amino groups of the amino acid residue in insulin and hence, enables the drug to be released easily and prolongs insulin residence time compared to the treatment via parenteral route which is bulk release of insulin. This also leads to better basal and prolonged hypoglycemic control. (Leong, et al., 2011; Leong, et al., 2011; Bies, et al., 2003; Clark, et al., 2000;)