Thursday, March 8, 2018

Physicochemical Properties of Gellan Gum

Gelling characteristics and texture properties of gellan gum

Gelation of gellan solutions occurs abruptly upon heating and cooling of gellan gum solutions in the presence of cations. Such sol-gel transitions are considered as phase transition. The gelation of gellan gum is a function of polymer concentration, temperature, and presence of monovalent and divalent cations in solution. At low temperature gellan forms an ordered helix of double strands, while at high temperature a single-stranded polysaccharide occurs, which significantly reduces the viscosity of the solution. The transition temperature is approximately 35 °C, but can range from 30–50 °C. Below transition temperature, a stiff structure is obtained (setting point), and results in gel formation. The mechanism of gelation involves the formation of double helical junction zones followed by aggregation of the double helical segments to form a three-dimensional network by complexation with cations and hydrogen bonding with water. Addition of monovalent or divalent cations during cooling markedly increases the number of salt bridges at junction zone, thereby improving the gelling potential of gellan gum. Various studies have been carried out to study the effect of different factors on the gel strength. Some of the important factors affecting gel strength are discussed bellow. 

Acetyl content

Acetyl content is the most important factor affecting the gel strength. Gellan gum with different acetyl content gives gels with different properties. Native gellan gum provides soft, elastic, thermoreversible gels, and is very weak because of bulky acetyl and glyceryl groups that prevent close association between gellan polymer
chains in bulk-helix formation, and hinder compact packing of the cross-linked double helix. Deacetylated
gellan gum forms firm, brittle and thermoreversible gel because of the absence of acetyl and glyceryl groups.


Type and concentration of ions

Ions have an impact on gel strength and brittleness. Gellan does not form gel in deionised water, but the addition of salts of calcium, potassium, sodium, and magnesium causes an increase in these two properties.
Notably, divalent cations are more effective in achieving this; even in gellan gels of very low concentration (0.2%, by mass per volume), a high strength was achieved with a maximum at about 0.004 % (by mass per volume) calcium and 0.005 % (by mass per volume) magnesium. Similar gel strength can be achieved with 0.16 % sodium or 0.12 % potassium (by mass per volume).

Gellan gels with KCl or NaCl had lower gel strength, even at high salt concentration (1 %, by mass per volume).A concentration of 0.1–0.2 % gellan is suitable for many food systems. It is important economically that strong gels can be obtained at low concentration of gellan, with the incorporation of trace amount of salt.


Gel pH

Sanderson and Clark  showed the gel strength to be enhanced within pH range of 3.5 to 8, which corresponds to the natural pH range of most foods. Change in pH does not alter the setting point of the gel, but affects melting temperature in some cases. For example, gels prepared with very low levels of monovalent ions melt at around 70 °C at neutral pH, but at pH=3.5 the melting temperature is slightly increased. This trend is not seen for divalent ions.


Presence of hydrophilic ingredients 

Addition of hydrophilic ingredients like sucrose (at about 10 %, by mass per volume) tends to decrease the ion concentration required for optimal gellan gel strength. Kasapis et al.used transmission electron microscopy to examine the changing nature of a polysaccharide network with increasing levels of sugar. Mixtures of deacylated gellan (<1 %) with low (0–20 %) and high (80–85 %) levels of sugar were prepared and studied. Micrographs of the high sugar/gellan gels produced clear evidence of reduced crosslinking in the polysaccharide network, which exhibits a transition from rubber to glass-like consistency upon cooling.
Tang et al. (49) studied the effects of fructose and sucrose on the gelling temperature, clarity, and texture properties of gellan gels cross-linked with calcium or sodium ions. They reported the gelling temperatures of gellan solutions to generally increase on the addition of sucrose,whereas addition of fructose up to 35 % (by mass per volume) had no effect. Incorporation of fructose and sucrose markedly increased the gel clarity. Effect of sucrose on gel strength was found to be dependent on cation concentration. At low cation concentrations, sucrose strengthened the gels; but at high cation concentrations, sucrose weakened them.


Temperature stability and flexibility of the melting point

Gellan gum is stable at higher temperatures and maintains its strength at 90 °C, whereas xanthan gum
looses 74 % of its original strength after heating up to 90 °C . According to Sanderson and Clark, the
melting temperature can be below or above 100 °C, depending on the conditions of gel formation. The most
important factor responsible for the flexibility of the melting point is concentration of cations in the gels because monovalent and divalent cations markedly increase the number of junction zones in gels and make
them more resistant to temperature. Modification of the melting point can successfully replace other conventional thickeners/stabilizers, while used in much lower concentration.


Effect of the presence of other hydrocolloids on textural properties of gellan gum

Various studies to find out the changes in the textural properties of gellan gum when mixed with other
food hydrocolloids have been carried out.


Sodium alginate

Sodium alginate dissolved in calcium chloride solution at 90 °C shows weak gel properties similar to those of ordered xanthan. The solutions show a sharp increase in rigidity on cooling, and convert to permanent gels on
storage at low temperature. The gels attain maximum hardness at about 40 % calcium conversion (for alginate
with a polyguluronate content of 58 %), and their elasticity can be readily controlled by adjustment of Ca2+concentration around this optimum value. Papageorgiou et al. (50) observed that incorporation of moderate concentrations of gellan (0.1–0.3 %, by mass per volume, in combination with 2 %, by mass per volume, alginate
and 5 mM trisodium citrate, sequestrant) increased the strength of the gels, but did not significantly change their elasticity, indicating that the gellan acts as strong 'filler’ in an alginate matrix.



Lau et al. (51) carried out texture profile analysis on mixed gellan-gelatin gels to assess the effect of the ratio
of the two components and calcium ion concentration. Hardness, brittleness, cohesiveness and springiness were
measured. The results suggested that there was a weak positive interaction between gellan and gelatin when no
calcium was added; at higher concentrations, gellan formed a continuous network and gelatin the discontinuous phase. Hardness was dependent on the concentration of gellan gum in the mixture, whereas brittleness, springiness and cohesiveness were very sensitive to low levels of calcium (0–10 mM), but less sensitive to higher calcium concentrations and gellan/gelatin ratio. 


Carrageenan and xanthan

Rodríguez-Hernández and Tecante (52) studied texture properties of gellan-carrageenan and gellan-xanthan mixtures in order to determine the contribution of both polysaccharides to the viscoelastic behaviour of the
mixture. Admixtures having a constant total concentration of 0.5 % (by mass) with different proportions were
prepared in the presence of 0.01 mol/kg CaCl2. It was observed that gel strength of 0.5 % gellan alone was the
highest, and gel strength of the two-component gels decreased as the proportion of gellan was reduced. Mixed
gels having a gellan concentration equal to or lower than 50 % mass of the total concentration were less stiff
and brittle, hence were more elastic.


Effect of chelatants on textural properties of gellan gum

Camelin et al. (53) studied the effect of various concentrations of sequestrants (sodium citrate, sodium metaphosphate, and EDTA) on gellan gel setting temperature and rheological properties. Addition of EDTA between 0 and 0.8 % (by mass per volume) progressively decreased the setting temperature. Citrate and metaphosphate decreased this parameter when added up to 0.4 or 0.6 %, depending on gellan gum concentration, eventually resulting in the absence of gel formation at room temperature for the 1.5 % gellan solution containing 0.4 % citrate. This effect was accompanied by a significant decrease of gel strength, and might be attributed to the binding of divalent cations required for chain association during gelatinisation by chelatants.