Carbohydrates - Biodegradable Polymers - page 173

lular eliminase-type enzymes (lyases) which are cleaving the sequence β - d -

glucopyranosyl - 4) - d-glucuronopyranosyl in the tetrasaccharide repeat unit

of gellan. In most of the bacterial isolates, the lyases are predominantly endoen-
zymes. The enzymes are highly specifi c toward gellan and rhamsan but do not
degrade most of the other bacterial exopolysaccharides, which are structurally
related to gellan [116, 117]. Hashimoto et al. cloned a 140 kDa gellan lyase from
Bacillus sp GL1. The recombinant enzyme is most active at pH 7.5, 50 ° C and stable
below 45 °C. The recombinant lyase is active on gellan, especially in the deacylated
form, but is inert against gellan-related polysaccharides such as S-88, welan,
rhamsan, and S - 198 [118] .
A thermostable gellan lyase, with a residual activity of 100% after 24 h incubation
at 60 ° C and a half - life time of 60 min at 70 ° C, was isolated form a thermophilic
strain Geobacillus stearothermophilus . The strain produces the thermostable gellan
lyase extracellularly during exponential phase and the enzyme is not present in
culture liquid without gellan. The enzyme has an optimal activity at 75 °C in a very
large pH area between 4 and 8.5 [119].
The easiest and most common modifi cation of gellan is the complete deacyla-
tion. The native gellan gum is dissolved in 0.2 M potassium hydroxide and stirred
at room temperature for 12 h under atmosphere of nitrogen. After neutralization
with 0.1 M hydrochloric acid and fi ltration through Celite 545, the deacylated
gellan can be precipitated in the presence of 0.05% KCl by the addition of two
volumes ethanol [111].
In a recent study, acrylate and maleate esters were synthesized. The esterfi cation
is possible using homogenous (acrylic acid in water) or heterogeneous (acryloyl
chloride or maleic anhydride in N,N -dimethylformamide or aceton) reaction con-
ditions. These macromonomeres can be polymerized under mild conditions and
lead to biodegradable thermo- and pH-stimulable hydrogels with adjustable
crosslink density [120].
Self - crosslinking of aqueous gellan with 1 - ethyl - 3 - [3 - (dimethylammino)propyl] -
carbodiimide leads to thermally stable hydrogels. Based on X-ray data, the struc-
ture of the gel is proposed to be a bundle of 48 gellan double helices aligned in
parallel. The rigid bundles formed by associated gellan double helices constitute
the junction zones which sustain the overall gel structure displaying solid-like
properties. Relatively large cavities supported by rigid bundles inside the gels
absorb water quickly as indicated by kinetic water uptake data [121].
Gellan was approved for use in food in Japan in 1988 and in the United States
in 1992. Today gellan is used in food products, which require a highly gelled
structure such as meat and vegetable aspic, jams, and jellies. As an example,
reduced calorie jams can be prepared with only 0.15% of clarifi ed gellan, and a
matrix containing 0.7% gellan does not melt on sterilization. In addition, gellan
is used to provide body and mouth-feel as a substitute of gelatin and to speed up
the set time of jellies as a substitute of starch.
Additionally, gellan is used as solid culture media for growth of microorganisms
and plants, as matrix in gel electrophoresis and to immobilize cells. Gellan
has some promising properties in the area of controlled drug release. Sustained

β - (1 →