Mechanism of Cr(VI) biosorption by flocculating yeast / 生物工程学报

The flocculating yeast strain SPSC01 is a fusant strain of Saccharomyces cerevisiae and Schizosaccharomyces pombe. The use of SPSC01 to absorb Cr(VI) from Cr(VI) containing aqueous solution would greatly reduce the cost of post-adsorption separation, since the superior flocculating property of SPSC01 would allow easy separation of the Cr(VI)-biomass from the solution. In order to investigate the effects of flocculating proteins on Cr(VI) reduction and absorption by SPSC01, the absorption behaviors of SPSC01 and its parental strains were compared. The results showed that Cr(VI) removal rate of SPSC01 was almost the same as that of S. pombe, which also has flocculating ability, but was faster than that of S. cerevisiae, which has no flocculating ability. When the system reached equilibrium, the amount of total Cr adsorbed by S. pombe, SPSC01 and S. cerevisiae were 68.8%, 48.6% and 37.5%, respectively. This showed that flocculation was beneficial to Cr(VI) reduction and adsorption, and suggested that focculating proteins may play a role in enhancing the Cr(VI) adsorption capacity of SPSC01 and S. pombe. We investigated the mechanism of Cr(VI) adsorption by SPSC01 using chemical modification and FTIR. The results indicated that the major functional groups (amino, carboxyl and amide) of surface proteins may contribute to the absorption of Cr(VI).

Ethanol fermentation from Jerusalem artichoke tubers by a genetically-modified Saccharomyces cerevisiae strain capable of secreting inulinase / 生物工程学报

Ethanol fermentation from Jerusalem artichoke tubers by recombinant Saccharomyces cerevisiae strains expressing the inulinase gene (inu) from Kluyveromyces marxianus was investigated. The inu native and pgk promoters were used to drive the expression of the inu gene, and the inulinase was expressed as an extracellular enzyme. All positive clones (confirmed by PCR) were able to express inulinase as measured by enzyme activity in the culture supernatant, among which two clones HI6/6 and HPI6/3 were selected, and their inulinase activity and ethanol fermentation performance were compared with their wild type. The inulinase activities of 86 and 23.8 U/mL were achieved, which were 4.6-fold and 1.5-fold higher than that of the wild type. Furthermore, ethanol fermentation was carried out with the recombinants and medium containing 200 g/L raw Jerusalem artichoke meal, and ethanol concentrations of 55 g/L and 52 g/L were obtained, with ethanol yields of 0.495 and 0.453, respectively, equivalent to 96.9% and 88.6% of the theoretical value.

Impact of fermentation system initial status on oscillations in very high gravity ethanol continuous fermentation process and analysis of fermentation efficiency improvement / 生物工程学报

Prior research reported the oscillatory behavior characterized by long period and high amplitude during high gravity continuous ethanol fermentations at the dilution rate of 0.027 h(-1). In this paper, high gravity continuous ethanol fermentations using Saccharomyces cerevisia at different dilution rates were carried out. Similar oscillations were observed when the dilution rate was switched to 0.04 h(-1). Both oscillatory and steady processes can be achieved at dilution rates of 0.027 or 0.04 h(-1), which depends on the initial status of the fermentation system. However, compared to steady process at the same dilution rate of 0.04 h(-1), the average residual sugar concentration was lowered by 14.8% for the oscillatory process, while the average ethanol concentration and productivity were increased by 12.6% and 12.3%, respectively. Further investigation revealed that besides the lag time, oscillatory processes were different from steady ones in kinetics because a higher specific growth rate can be achieved at the same residual sugar and ethanol concentrations (increased by 53.8% in average).

Consecutive very-high-gravity batch ethanol fermentation with self-flocculation yeast / 生物工程学报

In order to save energy consumption for the downstream processes, consecutive very-high-gravity batch fermentation was developed for ethanol production with the self-flocculating yeast Saccharomyces cerevisiae flo. The fermentation system exhibited a high ethanol productivity of 8.2 g/(L x h) with average ethanol concentration around 120 g/L. However, deterioration of the sedimentation performance of yeast flocs was observed as the consecutive fermentation process was prolonged, which significantly extended the time required for yeast flocs to separate from fermentation broth, and exaggerated the inhibition of high ethanol concentration on the yeast flocs, making them quickly lost viability and the fermentation system interrupted after 11 consecutive batches. Experimental results illustrated that decrease of the size of yeast flocs was the main reason, which could be prevented by stimulating the propagation of the yeast flocs. Thus, yeast was purged from the fermentation system at the end of each batch, and the concentration of yeast flocs within the fermentor was maintained at a relatively low level to stimulate their propagation. Although the ethanol productivity was decreased to 4.0 g/(L x h), the size of yeast flocs was stabilized after 10 consecutive batches and maintained for another 14 batches without further decrease, indicating the fermentation system could be operated reliably.

Influence of floc size distribution on the ethanol tolerance of a self-flocculating yeast strain SPSC01 / 生物工程学报

Ethanol tolerance of self-flocculating yeast SPSC01 was studied in a 3-L bioreactor under fed-batch culture. Yeast floc populations with the average sizes around 100, 200, 300, and 400 microm were obtained by adjusting the mechanical stirring rates of the fermentation system. When subjected to 20% (V/V) ethanol shock for 6 h at 30 degrees C, the remained cell viability was 3.5%, 26.7%, 48.8% and 37.6% for the aforementioned four floc populations, respectively. The highest ethanol yield 85.5% was achieved for the 300 microm flocs, 7.2% higher than that of the 100 microm flocs. The amounts of trehalose and ergosterol (including free ergosterol and total ergosterol) were positively correlated with the average size distributions from 100 to 300 microm. However, in the 400 microm flocs, the content of trehalose and ergosterol decreased, which coincided with its reduced ethanol tolerance compared to that of the 300 microm flocs. Furthermore, when subjected to 15% (V/V) ethanol shock at 30 degrees C, the equilibrium nucleotide concentration and plasma membrane permeability coefficient(P') of the 300 microm flocs accounted for only 43% and 52% respectively of those of the 100 microm and 200 microm populations. The effect of floc size distribution on the ethanol tolerance of the self-flocculating yeast strain SPSC01 was closely related to plasma membrane permeability. An optimal floc size distribution with the highest ethanol tolerance and ethanol production level could be obtained by controlling mechanical stirring speed of the bioreactor, which provides basis for the process optimization of fuel ethanol production using this self-flocculating strain.