Effect Of Acyl Content On Rheological And Gelling Properties Of Gellan

Gellan is the extracellular bacterial polysaccharide produced by Pseudomonas elodea ATCC 31461. The acyl groups have important effects on the property of gellan. Gellan is classified by the different content of acyl groups as high acyl gellan and deacylated gellan. The two types of gellan were widely investigated and had some disadvantages during application, such as gelation at high temperature of high acyl gellan so as to a short treating time and low viscosity so as to low suspension of deacylated gellan in the progression of manufacture. Partial deacylated gellan could supplement the limitations of high acyl and deacylated gellan, but there was no published method of accurate measurement of acyl groups. Preparing a series of gellan with different contents of acyl groups presents a bottleneck. To addressed the problem, our study established a HPLC-based method to accurately measure the contents of acyl groups. A series of gellan with different contents of acyl groups were prepared, and the properties of rheological and gelling properties which provided the essential theories for application were investigated. Our findings were as following:1. Established a method to measure the contents of glyceoyl and acetyl of gellan with high performance liquid chromatography(HPLC), separately. Glyceoyl and acetyl groups were dissociated from gellan by hydrolization and acidized to glyceric acid and acetic acid(or acid radical ions). Then alcohol precipitation was applied to remove the gel. The contents of the two organic acids were assayed by HPLC and calculated according to the contents of internal standard, propionic acid. Chromatographic column was HPX-87 H, mobile phase was 5 mmol·L-1 sufuric acid, column temperature was 35°C, flow velocity was 0.6 m L·min-1, measured under 220 nm. The data showed that the method could quantify the two organic acids simultaneously within 20 min with excellent sensitivity. The contents of glyceroyl and acetyl produced in our laboratory were 9.94% and 3.29%, respectively. The recovery was 97.76% and 93.36%, and RSD% was respectively 0.59% and 1.30% with excellent accuracy and repeatability.2. On the basis of the HPLC method, the preparing techniques and methodology of partial deacylated gellan was explored. By treatment with 10 mmol·L-1 Na OH at 40-70°C, reaction time was varied with various temperature. We found that the glyceoyl was easier to dissociate comparing to the acetyl. However, the G/A value was larger at low temperature than which at high temperature.3. Rheology and gelation properties of gellan with different contents of acyl groups were investigated. We found that the rheological pattern of gellan without ion accord with Herschel-Bulkley model with distinct shear thinning property. In the process of temperature scanning, there was no thermal hysteresis. The structure transition temperature(Tc) was decreased with the decreased contents of acyl groups. The up-regulation of system modulus was consistent with increased contents of acyl groups. Thermal hysteres was appeared with the addition of K+. The gel formation temperature(Tg) was decreased, melting temperature(Tm) was increased and thermal hysteresis became distinct with the addition of K+. At 25°C, the increment of modulues were consistent with decreased contents of acyl groups comparing to the system without ion. A series of phenomenon showed that acyl groups could stabilize the structure of gellan and the contents of acyl grps were the main factors to affect the viscosity and Tc or Tg. But the acyl groups hindered the interaction of cations and gellan, so the cations had less impact on gellan with more acyl groups. With addition of ions, the gellan with high contents of acyl groups was characterized by poor thermal stability and excellent frequency stability comparing to which with low acyl contents. The study on the gel texture demonstrated that the failure stress and modulue was enhanced, and the failure strain was decreased with the reduced acyl contents. The data showed that gel with lower acyl contents had greater gel strength, brittleness, hardness, and lower elasticity.4. Studies on the structure transition temperature of the mixed gum showed two transition temperature which was respectively the same as the Tc of the two samples formed the mixed gum, which demonstrated that the gellan would form gel network with the same type of gellan and the two networks would interpenetrate. So, the properties of two gum in the mixed system should be taken into consideration, which adversed to real application. The gellan prepared by deacylation possessed single structure transition temperature, which would be easily operated compared to the mixed gum.