Temperature Variation During The Preparation Process And Immobilization Of Microbial Cells In Large-diameter Cryogels

Supermacroporous cryogel is a novel chromatographic media for bioseparation, which is widely used in separation and purification of bioproducts. It provides advantages of high selectivity, fast separation, low mass transfer resistance and simple processing steps. In these years, due to the porous structure, the cryogel receives more attention as carriers for immobilizing cells.Temperature is one of the most important factors in the preparation process of large-diameter cryogels. An online temperature measuring system was established to monitor the temperature variation during the formation of a large-diameter cryogel with 7% (w/w) acrylamide solution in the stainless steel mold equipped with 9 thermocouples in its different positions. The diameter and height of the mold are 140 and 20 mm, respectively. The solution at these monitoring positions began to crystallize when the temperature decreased for about 20 to 40 minutes. The crystallization temperature of the solution was about -4.5 to -6.4℃, and its freezing point was about -2.4℃. The crystal growth duration at different positions were approximately from 2 to 18 minutes. The heat transfer characteristics in the cryogel mold based on the temperature curves obtained at various axial and radial positions were discussed. The basic properties of the large-diameter cryogel, i.e., pressure drop, residence time distribution (RTD), and height equivalent to a theoretical plate (HETP), were also measured. The results shows that the temperature during the formation of a large-diameter cryogel varies irregularly. The HETP and permeability of the obtained cryogels ranges from 0.21-0.41 cm and 3.54-4.17×10-11 m2, respectively.The cryogel columns of 10 mm diameter containing 10%, 30% and 50%(weight ratio of cells to the total mass of monomers) yeast cells were obtained under the appropriate condition. The yeast content, besides freezing rate and terminal freezing temperature, has a great influence on the porous structure of cryogel columns: the porosity and connectivity increased with the increase of yeast content, but the pore sizes and the mechanism intensity received the negative effect. For cryogel columns with 10% and 30% yeast cells, the basic properties are similar to common cryogel columns. When the yeast content is high, it exacerbates the basic properties of cryogel columns. Adenosine triphosphate(ATP)conversion ratio is proportional to the yeast content in cryogels and the fermentation time. The structure of cryogel column has a certain extent influence on fermentation process. The highest ATP conversion ratio is about 42% at present conditions, measured in cryogel containing 30% yeast cells.Large-scale preparation of the cryogel immobilized yeast cells has also been conducted in this work. The large-diameter cryogels with 1%, 5% and 10% yeast cells were prepared successfully under the the freezing rate of 0.167℃/min and the terminal freezing temperature of -20℃in a stainless steel mold with a height of 20 mm and a diameter of 143 mm. The porous structures of the cryogels do not change obviously for the yeast content in range of 1%~ 10%. The basic properties of the large-diameter cryogels immobilized yeast cells deteriorate at high yeast contents.