High viscosity xanthan and process for preparing same

Details High viscosity xanthan and process for preparing same High viscosity xanthan and process for preparing same

2000-09-19

2002-05-21

Prats, Francisco (Department: 1651)

Chemistry: molecular biology and microbiology

Micro-organism, tissue cell culture or enzyme using process...

Preparing compound containing saccharide radical

C435S101000, C422S026000, C426S521000, C536S114000, C536S123000

Reexamination Certificate

active

06391596

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a high viscosity xanthan and processes for the preparation of the high viscosity xanthan. One preferred embodiment is directed to dilution of a xanthan fermentation broth (solution) to produce a high viscosity xanthan. In another preferred embodiment pasteurization of a xanthan fermentation broth (solution) by a direct steam injection system provides a xanthan having increased viscosity compared to its native form. This invention is further related to a high viscosity xanthan produced by the processes of this invention.
2. Related Background Art
The fermentation of carbohydrates to produce the biosynthetic water-soluble polysaccharide xanthan gum by the action of Xanthomonas bacteria is well known. The earliest work was conducted by the United States Department of Agriculture and is described in U.S. Pat. No. 3,000,790. Xanthomonas hydrophilic colloid (“xanthan”) is an exocellular heteropolysaccharide. The heteropolysaccharide has a backbone chain of (1→4)-&bgr;-glucose residues substituted by short, lateral chains linked to alternate monomeric residues of the main chain (Milas and Rinaudo, Carbohydrate Research, 76, 189-196, 1979). Xanthan has a wide variety of industrial applications including use in oil well drilling muds, as a viscosity control additive in secondary recovery of petroleum by water flooding, as a thickener in foods, as a stabilizing agent, and as a emulsifying, suspending and sizing agent (Encyclopedia of Polymer Science and Engineering, 2nd Edition, Editors John Wiley & Sons, 901-918, 1989). Xanthan can also be used in cosmetic preparations, pharmaceutical vehicles and similar compositions.
Xanthan is produced by aerobic submerged fermentation of a bacterium of the genus Xanthomonas. The fermentation medium contains carbohydrate (such as sugar), trace elements and other nutrients. Once fermentation is complete, the resulting fermentation broth (solution) is heat-treated. It is well established that heat treatment of xanthan fermentation broths and solutions leads to a conformational change of native xanthan at or above a transition temperature (“T
m
”) to produce a higher viscosity xanthan. Heat treatment also has the beneficial effect of destroying viable microorganisms and undesired enzyme activities in the xanthan. Following heat-treatment, the xanthan is recovered by alcohol precipitation. However, heat treatment of xanthan fermentation broths (solutions) also has disadvantages, such as thermal degradation of the xanthan. Heating xanthan solutions or broths beyond T
m
or holding them at temperatures above T
m
for more than a few seconds leads to thermal degradation of the xanthan. Degradation of xanthan irreversibly reduces its viscosity. Accordingly, heat treatment is an important technique with which to control the quality and consistency of xanthan.
Xanthan quality is primarily determined by two viscosity tests that are well known to those skilled in the art, i.e., the Low Shear Rate Viscosity (“LSRV”) in tap water solutions and the Sea Water Viscosity (“SWV”) in high salt solutions. Pasteurization of xanthan fermentation broths at temperatures at or above T
m
have been found to result in the recovered xanthan having a higher viscosity as indicated by higher LSRV and SWV values.
The processes occurring during thermal treatment of xanthan fermentation broth (solution) are not well understood. It is believed that two processes occur during the thermal treatment of xanthan fermentation broth. Firstly, the thermal heating induces a conformation change which unwinds the double stranded native xanthan into a disordered xanthan which on cooling renatures to form an ordered xanthan. The renatured ordered xanthan has a higher viscosity compared to the native ordered xanthan which has not been pasteurized. Secondly, a competing degradation process may occur during the pasteurization (heat-treatment) which cleaves the native xanthan strands into shorter pieces resulting in a substantial decrease in viscosity. Studies have shown that the processes resulting in the desired conformational change occur rapidly, on the order of seconds, once T
m
is reached (Norton et al., Journal of Molecular Biology, 175, 371-394, 1984). Conversely, the thermal degradation process is slow, on the order of minutes at typical operating temperatures, but accelerates with increasing temperature. Below a temperature of 70° C., thermal degradation appears to be negligible. Maximum xanthan viscosity is achieved by rapidly heating the xanthan broth at or above T
m
followed by rapid cooling. It is generally important to minimize the time that the fermentation broth temperature exceeds 70° C. in order to preserve the integrity of the final xanthan.
The relative viscosity at 25° C. of both native unpasteurized xanthan solutions and solutions which were previously pasteurized at various temperatures have been measured (Milas and Rinaudo, Polymer Bulletin, 12, 507-514, 1984). For solutions previously heated well above their T
m
, a greater than three fold increase in the relative viscosity of the xanthan was observed. For solutions previously heated to temperatures near their T
m
, partial increases in the relative viscosity, reflecting the fraction of molecules undergoing the conformational transition as determined from optical rotation measurements, were observed. These viscosity results, in addition to the observed hysteresis of the optical rotation curve upon an initial heating and cooling cycle of a xanthan solution, are consistent with the following model:
A maximum viscosity will be obtained when all xanthan molecules have been transformed from the native to the renatured conformation. However, if T
m
is high (≧100° C.), pasteurization of the xanthan broth above T
m
can result in degradation of the xanthan molecules and a resulting decrease in product viscosity (lower LSRV and SWV values).
One of the variables T
m
depends upon is the ionic strength of the fermentation solution. The T
m
of aqueous xanthan solutions as a function of added sodium chloride has been measured (Milas and Rinaudo, Carbohydrate Research, 76, 189-196, 1979). The T
m
was found to vary linearly with the log of the total ionic strength (the ionic strength contains contributions from both the added sodium chloride and the xanthan molecules). The T
m
of xanthan has also been monitored in aqueous potassium chloride solutions using differential scanning calorimetry (Norton et al., Journal of Molecular Biology, 175, 371-394, 1984). A linear dependence of T
m
on the log of the ionic strength was also found.
A problem that has been identified with current recovery processes used to produce xanthan is the difficulty in achieving uniform heating of the broth. In particular, during pasteurization of the fermentation broth (solution) it is difficult to heat the broth in a uniform, controlled manner. As a result, some variability in the viscosity of xanthan recovered at the completion of the process may be observed. This may also give rise to reproducibility problems for xanthans having certain desired characteristics. Heating in a uniform and controlled manner is also dependent in part on the characteristics of the solution being heated. For example, xanthan fermentation broths are very viscous yet pseudoplastic (shear-thinning). When high concentration xanthan solutions are heated in conventional heat exchangers, non-uniform heating will occur. It is difficult to achieve a uniform temperature and residence time for every element of fluid passing through conventional heat exchange equipment during pasteurization of xanthan. Thus, during the heating of a xanthan fermentor broth, the conductive and convective heat transfer are poor and the formation of unwanted temperature gradients in the broth are difficult to prevent. This results in xanthan viscosities far below what is theoretically achievable. For example, this occurs in a xanthan solution passing through a heated tube. Because of the high viscosity of xanthan solutions, tu

Affiliated with

Also associated with

Rating

High viscosity xanthan and process for preparing same does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with High viscosity xanthan and process for preparing same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High viscosity xanthan and process for preparing same will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2835372