Tuesday, December 18, 2018

Types of Gellan Gum

Types of Gellan Gum 

Native gellan gum

Native gellan gum consists of a backbone of repeating unit of b-1,3-D-glucose, b-1,4-D-glucuronic acid, b-1,3-D-glucose, a-1,4-L-rhamnose, and two acyl groups, acetate and glycerate, bound to glucose residue adjacent to glucuronic acid. 

Deacetylated gellan gum

The acetyl groups in native gellan gum are removed by alkaline treatment to produce deacetylated gellan gum.
Acyl substituents affect the rheology, and deacetylation of native gellan results in a change from soft, elastic thermoreversible gels to harder, more brittle gels with higher thermal stability.

Steps involved in deacetylation of native gellan gum are as follows. The fermentation broth was immersed in boiling water bath for 15 min, cooled and pH increased to 10.0 using 1.0 M NaOH. The broth was then kept at 80 °C for 10 min and the pH was brought down to 7.0 using 1.0 M HCl. Cell mass from the broth was separated by centrifugation at 8000 rpm for 30 min at 4 °C. The supernatant was then added into three volumes of ice-cold alcohol to precipitate the deacetylated gellan. The precipitated gellan gum was then dried to constant mass in hot air oven at 80 °C for 12 h.

There are two types of deacetylated gellan gum differentiated on the basis of degree of deacetylation: 
high acyl gellan gum (partially deacetylated) and low acyl gellan gum (highly deacetylated).

Clarified gellan gum

Clarified gellan gum results from filtration of hot, deacetylated gellan gum for enhanced removal of cell protein residue. Clarification of gellan gum is of value especially when the gum is used as agar substitute.
Dreveton et al. described the method for clarification of gellan gum. Initially, 0.1 % solutions of gellan gum were prepared by mechanical stirring at 40 °C for 16 h in deionised water. Then the solutions were heated at 95 °C for 30 min. These heated solutions were then centrifuged at 13 000 × g for 30 min. The supernatants obtained were heated to 95 °C and then totally clarified  by filtration (0.7 m). Clarified gellan gum is suitable for some confectionary products where clarity is a crucial quality issue. It is also used as gelling agent for microbial growth media. Fig. 3 illustrates the repeating unit of chemical structure of acetylated and deacetylated gellan gum.