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Carbohydrates are organic compounds containing carbon, hydrogen, and oxygen. Simple sugars, pectin, gums, cellulose, starches and hemicelluloses are the main food carbohydrates that are mainly used as sweeteners, thickeners, and gelling agents and for stabilizing purposes in food. Monosaccharides are simple sugars that have multiple hydroxyl groups and can be joined together by glycosidic bonds. Disaccharides contain more than one sugar and polysaccharides are more of complex sugars containing several units linked together.
Hydrocollids or polysaccharide chains contan hydroxyl groups that bind water. Carrageenans are hydrocolloides, that are sulphated polysaccharides of galactans, four types , kappa I, Kappa II, lamba and iota-carrageenan (repeating unit of disachharide, 1,3-linked beta-D glactose, and 3,6- anthydro-D-galactose ) extracted from differet species of seaweed. Different seaweeds produce different carrageenans. These are highly used in food industry as thickening, gelling and stabilizing agents. This gelling is dependent on coil-to- helix transition governed by temperature and amount of cation in solution ( keppeler et al , 2009).
Structural and chemical properties of carrageenan.
Polysaccharides are rich in OH groups that make them hydrophilic which allows them to establish an intra H-bond network implying the local stiffness of the molecules; from this rigidity, they get a high thickener character. Hydrolysis can occur only when the glycoside bond at the cleavage site is pointing toward the acid/base catalyst. In carrageenans, the alternation of Î²-1,4 and Î±-1,3 linkages results in two successive Î²-1,4 bonds alternatively pointing up and down, respectively ( Michel et al, 2001; Bhatia et al, 2010). The hydroxyethylated kappa-carrageenan prepared can be useful as a substitute for hydroxyethyl cellulose as a thickener (e.g. for latex paint). The hydroxyethylated carrageenan displayed improved stability (Guiseley, 1978).
View Within Article All carrageenans are water soluble which depends on levels of sulphate groups and cations.They are composed of alternating 3-linked beta-D-galactopyranose(G units) and 4-linked alpha-D-galactopyranose (D-units) or linked 3,6-anhydrogalactose (DA-units) forming disaccharide repeating unit of carrageenans.beside galavtose and sulphate, other carbohydrates are also present in carrageenans (e.g. xylose, glucose and uronic acids) ( Nieto, n.d).
Various carbohydrates other than carrageenan undergo hydroxyalkylation to alter their properties. It has now been found that hydroxyalkylation of kappa-carrageenan to introduce hydroxyalkyl groups each having two to three carbon atoms, which effectively forms aqueous gels having decreased melting point ( 10Â°-20Â° C) compared to the unmodified kappa-carrageenan and displaying increased compliancy as measured by increased penetration
values. It is observed in many experiments that the properties of the lambda-carrageenan after hydroxyalkylation are unchanged, while the properties of the kappa-carrageenan changes. With the help of a reducing agent (e.g. sodium borohydride) aldehyde end group of carrageenan can be bloacked when dissolving in aqueous alkali solution, resulting is reduction of aldehyde to an alcohol group. Kappa-carrageenan dissolves in strong aqueous alkali in alkali metal hydroxide when hydroxyalkylation is carried out in aqueous solution. The reaction continues until the required substitution is attained, the solution can be neutralized by the addition of a suitable acid such as hydrochloric or acetic acid, or the alkali is removed by dialysis or other procedure, and the modified kappa-carrageenan is recovered by conventional procedure such as gelling by cooling, or precipitation by mixing with it a water-miscible organic solvent such as methanol, ethanol, propanol, acetone, etc. ( guiseley, 1978)
The effects of various sugars and polyols, such as ethylene glycol, glycerol, ribose, glucose, fructose, mannose, galactose, sucrose, maltose and raffinose, on the gel-sol transition of kappa-carrageenan gels were studied .The hydrogen bonding between hydroxyl groups in kappa-carrageenan and polyol or sugar is newly created, and that it stabilizes the structure of junction zones.( nishinari et al 1992). Carrageenans consists of alternating galactopyranosyl units dimer linked by alternating Î²- (1ïƒ 4) and ( Î±-( 1ïƒ 3) glycosidic bonds (Deslandes et al, 1988). The sugar units are sulphated either at C-2, C-3 or C-6 of galactose or C-2 of the anhydrogalactose units (fig: 1.1).kappa ( I) had least amount of sulphate ester as compared to other carrageenans which is responsible for solubility in water. As seen in figure (fig:1.1) the iota carrageenan molecules are arranged in a right- handed double helix with the strands parallel. The helix is fully stabilized by inter chain hydrogen bonds through unsubstituted positions, O-6 and O-2, of the complementary D-galactose units. The sulphate hemiesters project outwards from the main axis of the helix ( Genugel & Genuvisco, ). In k-
carrageenan the inter chain packing is more disordered than that of iota carrageenan forming an anlougous double helix. During heating, the gels become increasingly disordered and the strands disperse into random coil form, and upon cooling they re- aggregate to form gels. The ions that are responsible for gelation are K, Rb, Cs, NH, or Ca ions). Lamba carrageenan molecules are flat and ribbon life and these do not gel or form helixes due to the absence of 3, 6- anhydride residue which is generated by hydroxide reaction through catalysing cyclization reaction .This hydroxyl reaction occurs when 6-sulphated-alpha-galactose units are present which upon heating gives three ionized OH groups .Most of these carrageenan are used commercially for many purposes for galation, viscosity, bio-adhesion, film formation ,protein reactivity, etc.
Fig. 1.1Â Cyclization reaction catalyzed by alkaline treatment applied to carrageenans ( Gabriele et al, 2009).
Îº, Î¹, Î», Î¼, Î½, Î¸
Sulphate ester (O-SO3âˆ’)
3-Linked Î²-d-galactopyranose 2-sulphate
Îº, Î¹, Î¼, Î½
3-Linked Î²-d-galactopyranose 4-sulphate
4-Linked 3,6-anhydro-Î±-d-galactopyranose 2-sulphate
4-Linked Î±-d-galactopyranose 2,6-disulphate
Table.1. Different sugar units found in carrageenans ( Gabriele et al, 2009).
Most carrageenans are soluble in hot milk and water at high temperatures although few can be affected by calcium ions present in milk (above its gel melting ). 40° and 70° C are the soluble amplitude, depending on the solution concentration and the presence of cations, while in cold water, only lambda carrageenan and the sodium salt of kappa and iota carrageenan are soluble. K-carrageenans solution on cooling form thermo reversible gels due to the formation of double helix structure by carrageenan polymers. Three dimensional gel structure is formed on further cooling, and the curve quality and position of ester sulphate in the chain have effect on gelling properties which happened due to certain cations ( e..g potassium and calcium). All carrageenans have the ability to form gels by cooling a solution of the carrageenan in hot milk. Even lambda-carrageenan, which does not gel in water regardless of the cations present, will form a gel at levels of 0.2% or more by weight of the milk. This gelation is ascribed to the formation of carrageenan-casein bonds. The strength of milk gels is
enhanced by the addition of soluble calcium and potassium salts in a manner quite similar to that in water gels. Sulphate groups present is carrageenans are strongly unstable free acid anionic and stable due to cations .Carrageenan solutions are quite stable at neutral or alkaline pHs. At lower pHs their stability decreases, especially at high temperatures. As the pH is lowered hydrolysis of the carrageenan polymer occurs, resulting in loss of viscosity and gelling capability. However, once the gel is formed, even at low pHs (3.5 to 4.0), hydrolysis no longer occurs, and the gel remains stable. For practical applications, it is important to be aware of the limitations of carrageenan under acid conditions (either solution or gel). Therefore, processing of carrageenan solutions at low pHs and high temperatures for a prolonged period of time should be avoided.
Viscosity can be measured using viscometer at a required temperature. The amount of Sulphate and cations in water solution determine the viscosity or strength of gel formed by carrageenans ( Campo et al, 2009). Increase in viscosity can occur by two different methods, interaction between the linear chains , with a decrease of the free space or increase of the excluded volume, an dformation of physical gel caused by corsslinking between chains.The carrageenans solution gelifies along with increase in viscosity when the concentration of salt is high and temperature is low ( due to strongly gel-inducing cations k ions and calcium ions), whereas at high temperature, calcium ions lowers the viscosity ( Stanlet, n.d). Alkali modification of the carrageenan during processing increases the gel strength of the product by removal of these kinks through conversion of 6-sulfated residues to the 3,6-anhydride. Increased hydrophobicity from the added anhydride residues may also contribute to gelation.
The 2-sulfate groups on the 3,6-anhydride residues act as wedging groups to prevent the tightly-packed aggregation which is responsible for the rigidity of kappa gels. Whereas Viscosity decreases when cations are present at high levels, but increase it at lower concentrations. Îº-carrageenan hydrogel with a very high absorptivity in saline through polyacrylamide "crosslinking" followed by alkaline hydrolysis . The hydrogel displayed water absorbance properties similar to those of Îº-carrageenan copolymerized with acrylic acid. By using Î³-irradiation as an initiator and "crosslinking" agent at the same time, the swelling behaviour of these synthesized hydrogels could also be improved.( Deslandes et al, 1988).
Fig. 6.Â Structure of partially hydrolyzed Îº-carrageenan hydrogel copolymerized with acrylamideImage
Fig.Â 6.Â Viscosity of fluid gels, during their production process, as a function of the applied cooling rate. All systems are subjected to a constant shear rate of (a) 50Â sâˆ’1, (b) 100Â sâˆ’1 and (c) 300Â sâˆ’1.
Carrageenans are extensively used in food industry for many purposes such as gelling and stability. The polymer concentration effect on the mechanism of decomposition is provided, as is a mathematical description of the relationship between the increase in viscosity, due to the particle formation, and the concentration of the Îº-carrageenan.