Produce cell-free xanthan gum from glucose in immobilized cell fermentation, and cut costs.

Current commercial production of xanthan gum, using a conventional fermentation process, suffers from several limitations. The process is energy-intensive and costly. There is low product concentration and reactor productivity because of the high viscosity of the broth. The cells in the broth hinder filterability.

Scientists at The Ohio State University have developed a cell immobilization technique for use in a centrifugal packed-bed reactor (CPBR). This approach overcomes the previously mentioned limitations by producing a cell-free xanthan broth. Researchers estimate that the savings in energy and total production costs could be more than 50%. In the CPBR, Xanthomonas campestris cells are immobilized in a rotating fibrous matrix. The medium broth is continuously circulated through the fibrous matrix, making possible the intimate contact of gas and liquid with the immobilized cells. This circumvents mixing and aeration problems that occur in viscous xanthan fermentation.

Conventional free-cell batch fermentation in a stirred-tank reactor usually has a low cell density of less than 2 g per L, low xanthan productivity of ~0.5 g per L and a low final xanthan concentration of ~25 g per L. To study the feasibility and performance of long-term xanthan production in the CPBR, researchers operated the fermentation in a repeated-batch mode. High cell density (~7 g per L), high xanthan productivity (3 g per L based on fibrous bed volume and 1 g per L based on total liquid volume) and a high final xanthan concentration (up to 75 g per L) were achieved in the CPBR.

The fermentation process was stable enough for continuous operation for one month, achieving an average 85% xanthan yield from glucose. The scientists used ultrafiltration as an energy-efficient method to concentrate the cell-free xanthan broth to about 150 g per L before further purification occurred with alcohol precipitation.

The permeate or filtrate can be recycled and reused in the fermentation, saving the raw material costs and reducing the amount of spent medium. The CPBR and ultrafiltration process can be used in the large-scale production of high-quality, low-cost xanthan gum, broadening its numerous applications in the food, pharmaceutical and oil recovery industries.

Further information. Shang-Tian Yang, Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Ave., Columbus, OH 43210; phone: [phone]; fax: [phone]; email: [e-mail].