Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a process of recovering
polysaccharides in granular, free-flowing form produced
by bacteria of the species Xanthomonas or Arthrobacter,
wherein the polysaccharides are precipitated by adding a
precipitating liquid to the fermentation medium, are then
separated, washed with an organic washing liquid which is
preferably also used as the precipitating liquid,
separated and subsequently dried.
Processes of this type are known and described, e.g.
in Austrian Patent No. 330,121 issued on June 10, 1976.
Polysaccharides fermentatively formed by Xanthomonas
and Arthrobacter are exo-polysaccharides; the Xanthomonas
products are designated as xanthane and are an industrial
product which is used as a regulator of the flowing
properties of aqueous systems due to its rheological
behavior and as a stabilizer for emulsions and
suspensions. Its high thermostability, pronounced pH
resistance and unusually good compatibility with salts
have opened a broad industrial application spectrum for
xanthane. Xanthane formed by Xanthomonas campestris, for
instance, is admitted as a thickener and stabilizer for
foodstuffs.
Xanthane is produced industrially by aerobic
fermentation of a glucose-containing fermentation broth.
After depletion of the carbon source, the xanthane-
containing mash is pasteurized and the xanthane is
precipitated by adding an organic liquid, usually a low
alcohol like isopropanol. When using isopropanol, the
precipitation concentration ranges between 45 and 60~ by
weight of alcohol. The precipitation concentration is
influenced by the electrolyte content of the mash and can
be reduced by the addition of salts prior to
precipitation. The precipitated and separated xanthane is
subsequently taken up, but not dissolved, in a suitable
organic liquid for further purification. The washing
liquid is usually identical with the liquid used for
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precipitation, thus mostly isopropanol. After separating
the washed and thus purer xanthane, this is dried and
ground.
In its usual commercial form, xanthane is a powdered,
free-flowing product. As the free-flowing property of the
powder is determined by the structure of the individual
particles, particles of granular appearance are sought,
while fibrous, fluffy particles cause difficulties during
grinding, packing and finally during metering.
AT-PS 330 121 discusses the problem of obtaining
xanthane with granular and not fibrous particles and
proposes as a solution of this problem to use a washing
liquid whose organic portion does not exceed a certain
range to be determined by experiments, as this would
result in a fibrous structure, and does not fall short of
this range, as this would yield a tacky product. When
using isopropanol/water as a washing liquid, the
reference states that the isopropanol content should be
by 8 to 25% by weight higher than the isopropanol content
just barely sufficient for dissolving the xanthane; a
range of 9 to 28% by weight is indicated for tert-
butanol/water. The usual azeotropic distillates, i.e.
87.8% isopropanol and 96.5% tert-butanol, are explicitly
mentioned in AT-PS 330 121 as washing liquids unsuitable
for obtaining a granular structure: these alcohols must
always be diluted with water.
This results, among others, in the fact that the
washing liquid can subsequently not be used as the
precipitation liquid because its alcohol concentration is
too low; precipitation liquid and washing liquid must be
upgraded.
It was now surprisingly found that it is also
possible in the recommendable use of high-percentage
organic washing liquids to obtain a granular, free-
flowing final product in spite of the primarily fibrousstructure of the xanthane formed.
According to the present invention, there is provided
a process of recovering polysaccharides in granular,
free-flowing form produced in a fermentation medium by
bacteria of the species Xanthomonas or Arthrobacter,
wherein the polysaccharides are precipitated in the form
of fibers by the addition of a precipitating liquid to
the fermentation medium, and the polysaccharide fibers
are separated and washed with an organic washing liquid.
The process of the invention is characterized in that the
polysaccharide fibers are coarsely separated from the
washing liquid after washing, are kept in motion with the
addition of water to cause agglomeration of the
polysaccharide fibers, and the agglomerated
polysaccharide fibers are dried to obtain the desired
polysaccharides in granular, free-flowing form.
Preferably, up to about 40% by weight, in particular
8 to 30% by weight, more particularly 10 to 15% by
weight, of water based on their dry weight are added to
the polysaccharide fibers, based on the dry weight of the
polysaccharide fibers. The agglomerated polysaccharide
fibers are preferably dried with continued motion.
In other words, the xanthane fibers are moistened
prior to drying, the most uniform, possible distribution
of the amount of water added being aimed at, to be
achieved by intimate mixing.
Not all the water should be added at once in order to
prevent the formation of lumps; the water is preferably
added within a period of up to one hour, in particular of
10 to 30 minutes, preferably continuously.
It is assumed that in the process according to the
invention, the xanthane fibers are first surface-
solubilized and subsequently agglomerated; this results
in a granular, free-flowing and easily manipulable final
product.
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Keeping the material in motion prevents the formation
of larger aggregates.
Since the washing agent still present evaporates in
the course of subsequent drying, i.e. the H20
concentration increases locally, the material is
conveniently also kept in motion during drying.
The moistening of the washed material is most
conveniently effected in the drier itself, in particular
in a drum drier at rotating drum.
A preferred manner of carrying out the washing and
drying treatment according to the invention is described
as follows:
The precipitated xanthane is taken up in isopropanol
recovered by industrial distillation (about 87%) and
washed with stirring at room temperature. The dwell time
amounts to 10 to 30 minutes. The amount of isopropanol is
up to 100 times, preferably 45 to 55 times, the amount of
xanthane. After this washing operation, the xanthane is
of fibrous structure
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and is introduced into a vacuum drum drier after
separating the washing solution. This drier is equipped
with a spinning device for the intensive mixing of the
product. Prior to drying, water is sprayed onto the
5 fibrous xanthane with intimate mixing. In order to
prevent the possible formation of lumps, the water must
be finely distributed. This is achieved by gradual
addition via jets during mixing. The addition period
ranges from 10 to 30 minutes. The amount of water
10 metered in amounts to 8 to 30 percent, preferably 10 to
15 percent, of the dry substance present in the drier.
If the volume of water metered in is too low, the
product remains fibrous. Too much water causes the
formation of grains difficult to comminute. The
15 subsequent drying operation with continued intimate
mixing is effected in the usual gentle manner in vacuo
and is to assure an adequately low residual isopropanol
content in the final product (500 ppm). Prior to the
addition of water, so much washing liquid is separated,
20 e.g. squeezed, from the polysaccharide particle/washing
agent phase that a solids content of at least about 40
percent by weight, preferably 50 to 55 percent by
weight, is obtained. The drying operation itself is
effected within a temperature range of 50 to 120~C,
25 preferably of 60 to 80~C, and preferably with
application of negative pressure. In particular, a
residual vacuum of about 100 mbar is adjusted.
By the intensive motion of the material with the
addition of water by means of the spinner, the fibers
30 are transformed to spherical particles. Since the
volatile isopropanol first evaporates during drying, the
higher water content of the drying material facilitates
the surface-solubilizing of the xanthane fibers. This
causes the formation of xanthane particles of denser
35 granular structure.
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As the isopropanol does not have to be diluted for
washing, the advantage of this manner of proceeding,
among others, resides in the fact that the spent washing
solution does not have to be regenerated, but can
5 economically directly be used for the precipitation of
the xanthane from the mash. This saves operating and
installation costs for washing liquid regeneration. Only
a single isopropanol purification and recovering
facility is required. The economical use of the washing
solution for the precipitation of the xanthane from the
mash is made possible by the high isopropanol
concentration of the washing solution. At a given amount
of mash and a given precipitation concentration, the
amount of alcohol to be regenerated depends on the
15 isopropanol concentration in the precipitating agent.
Since the increase of the isopropanol concentration in
the precipitating agent causes a decrease of the amount
of isopropanol to be regenerated, the regenerating cost
is reduced.
The general process is described on the basis of
the following practical example.
Example:
A fermentation broth was obtained by aerobic
fermentation of a glucose-containing nutrient solution
25 (40 g/l) inoculated with Xanthomonas campestris. The
fermentation was interrupted at a residual sugar content
of 0.1 percent. The fermentation broth was pasteurized
and the xanthane was subsequently continuously
precipitated by the addition of isopropanol of about 86
30 percent by weight. The precipitation was carried out in
a vessel of a volume of 100 1 equipped with turbine
agitator by parallel pumping in of mash and of
isopropanol of about 86 percent by weight. At the same
time, the precipitated xanthane with the suspending
35 alcohol-water mixture was pumped off. The ratio of
amount of mash to amount of alcohol is determined by the
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precipitation concentration which ranges at about 50
percent by weight of isopropanol. In order to assure an
intimate contact between mash and isopropanol, the
turbine agitator is operated at 1,000 rpm. The xanthane
5 fibers are subsequently separated from the supernatant,
suspended in 50 times the amount of isopropanol (86
percent by weight) and washed for 10 minutes with
intensive stirring (turbine agitator at 1,000 rpm).
Precipitation and washing are carried out at about 20~C.
10 The washed fibrous xanthane is separated via a fine mesh
screen and manually pressed out.
a) Further treatment not according to the
invention:
30 kg of moist product (this amount corresponds to
15 about 15 kg of dry xanthane substance) are introduced
into a pilot scale vacuum drum drier. The drum drier is
equipped with a spinner for mixing the dry matter. After
applying the vacuum, the temperature is increased with
the spinner running at 300 rpm until a final temperature
20 of 80~C and a residual vacuum of 100 mbar is obtained.
The dried xanthane has a dry matter content of about 93
percent and is of fibrous or fluffy structure. The
portion to be ground (particle size 95 percent 180 ~m)
has an apparent density of 0.41. The ground product has
25 the appearance (at 50-fold enlargement) shown in Fig. 1.
b) Further treatment according to the invention:
The drum drier is charged as indicated under a).
Then the spinner is turned on and operated at 300 rpm
and about 2 liters of water are sprayed onto the moving
30 xanthane via a nozzle within about 10 minutes. The
drying process is as described in a). The xanthane is
obtained in the form of round grains and small granules.
The residual moisture content amounts to about 7
percent, the residual isopropanol content amounts to
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less than 500 ppm. After grinding, the power consists of
granules, has a higher apparent density than the product
dried without the addition of water and flows more
freely. The apparent density after grinding (grain size
5 95 percent 180 ~m) amounts to about 0.7 (mean value).
The ground product has the appearance (at 50-fold
enlargement) shown in Fig. 2.
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