Hongzhi Ling

Statistical optimization of xylitol production from corncob hemicellulose hydrolysate by Candida tropicalis HDY-02.

The statistical experimental designs were adopted to optimize the culture medium in xylitol production by Candida tropicalis HDY-02 with corncob hemicellulose hydrolysate as substrate. In the first step, Plackett-Burman design was used for screening the important variables. KH(2)PO(4), yeast extract, (NH(4))(2)SO(4) and MgSO(4)·7H(2)O were found to significantly affect xylitol yield. In the second step, central composite design (CCD) was used to determine the optimum level of each of the significant variables. A second-order polynomial was determined by the multiple regression analysis of the experimental data. The interactive effects of yeast extract and MgSO(4)·7H(2)O on xylitol yield of C. tropicalis HDY-02 were determined to be significant. The validation experimental was consistent with the prediction model. The optimum combinations for xylitol yield were 5 gl(-1) (NH(4))(2)SO(4), 1.3 gl(-1) KH(2)PO(4), 4.6 gl(-1) yeast extract and 0.6 gl(-1) MgSO(4)·7H(2)O. Under these optimal conditions, the continuous fed-batch experiments could produce xylitol of 58 gl(-1) with a yield of 0.73 g g(-1) xylose.

Strain Isolation and Study on Process Parameters for Xylose-to-Xylitol Bioconversion

ABSTRACT A xylitol producing yeast was isolated from soil and identified as Candida tropicalis W103 by 18S r DNA gene sequence analysis and physiological characteristics. The optimal fermentation conditions for Candida tropicalis W103 were: 35°C, pH 4.5 and 120 g l−1 initial xylose concentration. However, the maximum yield and productivity of xylitol were obtained at KLa 16.5 h−1 and 18.3 h−1, respectively. A two-stage aeration strategy (0–24 h, KLa 18.3 h−1; after 24 h, shift KLa to 16.5 min−1) was applied in the fermentation to get higher xylitol yield and productivity. After 60 h in batch fermentations, both the xylitol concentration and xylose consumption reached the maximum, obtaining 87.1 g l−1 of xylitol with 1.45 g l−1 h−1 productivity and 0.72 g g−1 xylose yield. Fed-batch fermentation with 120 g l−1 initial xylose and regulating xylose concentration to 40–55 g l−1 during 24–96 h was performed to reach a higher productivity of 1.82 g l−1 h−1, and xylitol concentration of 218.7 g l−1.

Isolation of a novel strain of Monoraphidium sp. and characterization of its potential for α-linolenic acid and biodiesel production

α-Linolenic acid (ALA) is an essential fatty acid which cannot be synthesized de novo in mammals and must be ingested regularly in the diet. In this study, a microalgal strain named HDMA-11 was isolated from Lake Ming, China, and was found to accumulate a high ALA content (39.2% of total lipids). Phylogenetic neighbor-joining analysis indicated that HDMA-11 belongs to the genus Monoraphidium (Selenastraceae, Sphaeropleales) and its 18S ribosomal DNA sequence seemed to be a new molecular record of a Monoraphidium species. The fatty acid profiles, biomass productivity and lipid content of HDMA-11 were also investigated in autotrophic conditions. The high levels of polyunsaturated fatty acids in HDMA-11, especially ALA, make it suitable as a source of nutritional supplementation for human health. Furthermore, HDMA-11 exhibited good properties for biodiesel production, characterized by high lipid content (28.5% of dry weight), moderate biomass productivity (31.5 mg L-1 day-1) and a promising lipid profile.