Optimization of Xylitol Production by Candida tropicalis in Two-stage Fed-batch Culture

Two-stage fed-batch culture of Candide tropicalis that was designated primarily to cultivate the cell in the glucose medium (1st stage) and then produced the xylitol from xylose medium (2nd stage) was developed to improve a xylitol yield and productivity. In the growth stage, glucose was automatically supplied to the fermentor by pH-stat mode when the pH was up 5.7, When a feeding medium was added in order to reach the glucose and yeast extract concentrations up to 100 and 40 g/L, respectively, a high cell concentration and a relatively low ethanol concentration were obtained in 18.5 h culture. In the production stage, initial xylose concentration of 150 g/L was the most favorable for obtaining the final xylitol concentration and productivity. The addition of mineral salts was also enhanced a xylitol production. But the aeration rate was not significantly affected a xylitol production. When the addition of 16 g yeast extract and 232.5 g xylose powder at the production stage was used, xylitol yield and productivity were significantly increased. With these conditions, xylitol concentration, yield and productivity of 108.9 g/L, 74%) and 3.3 g/L·h, respectively, were obtained in a final volume of 1.58 L. The further addition of 16 g yeast extract and 232.5 g xylose powder increased the working volume partly (1.67 L) and resulted in a relatively high xylitol concentration, yield and productivity of 193 g/L, 70% and 3.6 g/L·h, respectively.

Candida tropicalis의 2단계 유가식 배양에 의한 Xylitol 생산의 최적화

C. tropicalis ATCC 20336을 이용하여 높은 수율과 생산성으로 xylitol을 생산하기 위하여 glucose 배지에서 짧은 시간에 고농도로 세포를 배양한 후에 xylose를 첨가하여 xylitol로 전환하는 2단계 유가식 발효공정에 관한 연구를 수행하였다. Cell growth-stage에서의 배지공급 방법으로는 glucose가 모두 고갈된 후에 pH가 5.7 이상으로 올라가는 것을 신호로 glucose를 첨가하는 pH-stat 방법으로 18.5시간 배양에 의하여 67.9 g/L의 세포 농도를 얻었으며, ethanol 생성은 1.0 g/L로 매우 낮았다. Production-stage에서 최적의 초기 xylose 농도는 150 g/L이었으며, 이때에 2 g/L ($NH_4)_2SO_4$, 1 g/L $K_2HPO_4$, 0.2 g/L $MgSO_47H_2$O로 구성된 mineral salt를 2회 첨가에 의하여 xylitol의 생성이 향상되었다 그러나 0.2 vvm에서 1.0 vvm 사이의 통기조건에서는 xylitol의 생산에 큰 영향을 미치지 못하였다. 반면에 production-stage에서 yeast extract를 10 g/L가 되도록 첨가한 경우에 xylitol 수율과 생산성이 큰 폭으로 증가하였다. 즉 xylose 농도가 약 150 g/L이 되도록 232.5 g의 xylose를 2회 첨가한 경우에 배양액의 최종 부피는 1.67 L이 되었고, 이때의 xylitol 농도는 193 g/L이었으며, 수율은 70%이고 생산성은 3.55 g/L.h이었다.