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. 


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Sunday, April 15, 2018

Functional Properties of Gellan

As mentioned before, gellan gum has a number of functional properties that can be readily modified. These include:

Versatile texture, which is one of its most important features. This is usually defined in terms of hardness (measure of rupture strength), modulus (measure of gel firmness) brittleness (strain required to break the gel), and elasticity (measure of rubberiness).

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Thursday, April 5, 2018

Sunday, April 1, 2018

Rheology of the Fermentation Broth

Rheology of the Fermentation Broth 

The rheology of the fermentation fluid during gellan gum production exhibits strongly pseudoplastic behaviour, even at 0.1 % (by mass per volume). 
The initial broth viscosity is that of Newtonian fluid with a viscosity close to that of water, but the broth rapidly becomes non-Newtonian with strong shear thinning properties.
This pseudoplastic behaviour during the exopolysaccharide accumulation phase is also common in the production of other microbial polymers and described by power law model:
· h = t/g = kg n–1 

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Friday, March 30, 2018

Fermentative Production of Gellan Gum

Fermentative Production of Gellan Gum 


The growth media suitable for the production of different exopolysaccharides by microorganisms vary widely,
and this probably reflects the differing role of each exopolysaccharide in nature.
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Monday, March 26, 2018

Biosynthetic Pathway of Gellan Gum

Biosynthetic Pathway of Gellan Gum 

Many researchers have investigated the pathway for the synthesis of repeating tetrasaccharide units of gellan
gum by Sphingomonas paucimobilis. The route of gellan synthesis, role of enzymes involved and some process conditions supporting optimum production of gellan gum have been described.


The route of gellan synthesis

Vartak et al. studied gellan gum biosynthesis in two strains of Sphingomonas paucimobilis, the wild type
and a polyhydroxybutyrate (PHB) deficient mutant. Enzyme analysis suggested that in both strains glucose utilization was initiated by the action of glucokinase and glucose dehydrogenase. No exogenous gluconate utilization was observed.


Sphingomonas paucimobilis catabolizes glucose via the Embden-Meyerhof pathway (glycolysis), or the pentose
phosphate pathway. The Embden-Meyerhof pathway apparently does not have a role in glucose degradation,
because no phosphofructokinase activity, a key enzyme in glycolysis, has been detected. Fig. 1 illustrates the proposed scheme for glucose catabolism in Sphingomonas paucimobilis. According to the proposed pathway glucose has to enter the cell before it is degraded. Either of the following steps is used for glucose uptake: 



Mutant strain lacking G6PD (glucose-6-phosphate dehydrogenase) showed no difference in rates of glucose
utilization, gellan production or CO2 production suggesting that this enzyme is not essential for glucose
metabolism in Sphingomonas. This indicates that either the main route of glucose utilization involves glucose dehydrogenase or gluconate kinase, or the absence of G6PD induces a compensatory increase in these enzymes. As yet, however, there is no clear indication of which mechanism occurs.  


Martins and Sá-Correia (19) proposed a possible pathway for the synthesis of repeating tetrasaccharide
unit of gellan gum. They assumed that gellan synthesis requires activated precursors before the repeating unit is
assembled, similar to other exopolysaccharides synthesized in the cell wall of microorganisms. These gellan  precursors were detected by enzyme assays, and were found to be nucleotide diphosphate sugars, viz. UDP-
-glucose, TDP-rhamnose and UDP-glucuronic acid. The proposed biosynthetic pathway is shown in Fig. 2.

Glucose-6-phosphate seems to occupy a key position from which two routes commence, one leading to
uridine-5-diphosphate glucose (UDPG) and the other leading to thymine-5-diphosphate glucose (TDPG). In
turn, UDPG induces D-glucose and D-glucuronic acid synthesis and TDPG leads to the synthesis of rhamnose.
The combination of these three compounds presumably results in the synthesis of gellan (22). However, the reactions leading to binding of these three monomers have not been clearly elucidated. 

Specific activities of gellan synthetic enzymes

Conditions that favour gellan gum formation might be expected to increase the levels of the enzyme responsible for the formation of precursors. Phosphoglucose isomerase (PGI) and phosphoglucose mutase (PGM)
possess the highest activities in cell-free extracts (in vitro) as they play multiple roles in the cell metabolism.
The enzymes UDPG phosphorylase (UGP) and TDPG phosphorylase (TGP) appeared to have values of specific activities lower than PGI and PGM. TDPR synthetase (TRS) and UGD are the least active and the most thermosensitive enzymes above 30 °C. They are essential for synthesis of rhamnose (TRS) and glucuronic acid (UGD).The activity of these enzymes presumably limits gellan synthesis, especially at temperatures higher than 30 °C
 Additionally, it has been found that isocitrate isomerase, which is involved in CO2 production through the  carboxylic acid cycle, has very high specific activity in vitro. This might represent an unfavourable reaction
for industrial purposes, because glucose is not channelled towards gellan gum production. Apart from the enzymes mentioned, there must be other enzymes that influence gellan synthesis after the formation of gellan
precursors.

Genetic engineering of the gellan pathway

The most exciting prospects for gellan modification and increasing production yield are found in genetic engineering. Some attempts have been made to increase the relatively low conversion efficiency of gellan from
glucose in S. paucimobilis ATCC 31461. 
By site-specific mutagenesis, the G6PD gene encoding glucose-6-phosphate dehydrogenase was inactivated, envisaging diversion of the carbon flow toward gellan synthesis, apparently without the expected results.

Identification of a few genes and elucidation of crucial steps of the gellan biosynthesis pathway indicated
some possibilities of exerting control over gellan production at any of the three levels of its biosynthesis:
(i)at the level of synthesis of sugar-activated precursors,
(ii) at the level of the repeat unit assembly and of gellan,
(iii) at polymerization and export. By modifying expres- sion of any of the individual, or of a group of these
genes the conversion efficiency and gellan gum yield can be increased.
In spite of recent advances in the elucidation of the gellan biosynthetic pathway, a better knowledge of the
poorly understood steps and of the regulation and bottlenecks of the pathway is crucial for the eventual success of the metabolic engineering of gellan production. 






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Friday, March 23, 2018

Composition of Different Types of Gellan Gum


Composition of Different Types of Gellan Gum

The repeating unit of gellan polysaccharide is composed of b-D-glucose (D-Glc), L-rhamnose (L-Rha), andD-glucuronic acid (D-GlcA). 

The composition is approximately: glucose 60 %, rhamnose 20 % and glucuronic acid 20 %. 

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Thursday, March 22, 2018

Strains Producing Gellan Gum

Sphingomonas is a group of Gram-negative, rod-shaped, chemoheterotrophic, strictly aerobic bacteria
containing glycosphingolipids (GSLs) in their cell envelopes, and they typically produce yellow-pigmented colonies.

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Tuesday, March 20, 2018

Gellan Gum History

Gellan gum is the generic name for extracellular polysaccharide produced by bacterium Pseudomonas elodea.

Kaneko and Kang discovered the polymer in the laboratory of the Kelco Division of Merck and Co., California, USA in 1978. 

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Sunday, March 18, 2018

Gellan Gum Introduction

Gellan Gum Introduction

Microbial exopolysaccharides have found a wide range of applications in the food, pharmaceutical and
other industries due to their unique structure and physical properties. 

Some of these applications include their use as emulsifiers, stabilizers, binders, gelling agents,coagulants, lubricants, film formers, thickening and suspending agents . 

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