Ethanol fermentation from Jerusalem artichoke tubers was performed at elevated temperatures by the consolidated bioprocessing strategy using Saccharomyces cerevisiae MK01 expressing inulinase through cell surface display. No significant difference was observed in yeast growth when temperature was controlled at 38 and 40 °C, respectively, but inulinase activity with yeast cells was substantially enhanced at 40 °C. As a result, enzymatic hydrolysis of inulin was facilitated and ethanol production was improved with 89.3 g/L ethanol produced within 72 h from 198.2 g/L total inulin sugars consumed. Similar results were also observed in ethanol production from Jerusalem artichoke tubers with 85.2 g/L ethanol produced within 72 h from 185.7 g/L total sugars consumed. On the other hand, capital investment on cooling facilities and energy consumption for running the facilities would be saved, since regular cooling water instead of chill water could be used to cool down the fermentation system.

Yeast flocculation is an important property for the brewing industry as well as for ethanol fermentation to facilitate biomass recovery by sedimentation from the fermentation broth, which is cost-effective. In this study, a new flocculating gene FLO10spsc of 4,221 bp homologous to FLO10 was identified in the industrial flocculating yeast SPSC01. Sequence analysis indicated that the N- and C-terminus of the deduced protein of this new FLO gene are 99 % identical to that of FLO10, but more intragenic repeats are included. The study on the function of FLO10spsc by its integrative expression in the non-flocculating industrial yeast indicated severe inhibition in the flocculation of the transformant by mannose and maltose, moderate inhibition by sucrose and glucose and no inhibition by xylose and galactose, and thus the NewFlo type was established. Meanwhile, the flocculation of the transformant was stable when the temperature was below 50 °C and the pH was in the range of 4.0–6.0. Furthermore, the medium containing 250 g/l glucose was completely fermented within 48 h by the transformant, with about 110 g/l ethanol and 5.5 g(DCW)/l biomass produced, and no significant difference in ethanol fermentation performance was observed compared to its wide-type strain. Therefore, the FLO gene and corresponding transformation strategy provide a platform for engineering yeast strains with the flocculation phenotype to facilitate biomass recovery.

Compared with steady state, oscillation in continuous very-high-gravity ethanol fermentation with Saccharomyces cerevisiae improved process productivity, which was thus introduced for the fermentation system composed of a tank fermentor followed by four-stage packed tubular bioreactors. When the very-high-gravity medium containing 280 g l−1 glucose was fed at the dilution rate of 0.04 h−1, the average ethanol of 15.8% (v/v) and residual glucose of 1.5 g l−1 were achieved under the oscillatory state, with an average ethanol productivity of 2.14 g h−1 l−1. By contrast, only 14.8% (v/v) ethanol was achieved under the steady state at the same dilution rate, and the residual glucose was as high as 17.1 g l−1, with an ethanol productivity of 2.00 g h−1 l−1, indicating a 7% improvement under the oscillatory state. When the fermentation system was operated under the steady state at the dilution rate of 0.027 h−1 to extend the average fermentation time to 88 h from 59 h, the ethanol concentration increased slightly to 15.4% (v/v) and residual glucose decreased to 7.3 g l−1, correspondingly, but the ethanol productivity was decreased drastically to 1.43 g h−1 l−1, indicating a 48% improvement under the oscillatory state at the dilution rate of 0.04 h−1.

An innovative consecutive batch fermentation process was developed for very high gravity (VHG) ethanol fermentation with the self-flocculating yeast under high biomass concentration conditions. On the one hand, the high biomass concentration significantly shortened the time required to complete the VHG fermentation and the duration of yeast cells suffering from strong ethanol inhibition, preventing them from losing viability and making them suitable for being repeatedly used in the process. On the other hand, the separation of yeast cells from the fermentation broth by sedimentation instead of centrifugation, making the process economically more competitive. The VHG medium composed of 255 g L−1 glucose and 6.75 g L−1 each of yeast extract and peptone was fed into the fermentation system for nine consecutive batch fermentations, which were completed within 8–14 h with an average ethanol concentration of 15% (v/v) and ethanol yield of 0.464, 90.8% of its theoretical value of 0.511. The average ethanol productivity that was calculated with the inclusion of the downstream time for the yeast flocs to settle from the fermentation broth and the supernatant to be removed from the fermentation system was 8.2 g L−1 h−1, much higher than those previously reported for VHG ethanol fermentation and regular ethanol fermentation with ethanol concentration around 12% (v/v) as well.

Many fermentation products are produced under microaerobic or anaerobic conditions, in which oxygen is undetectable by dissolved oxygen probe, presenting a challenge for process monitoring and control. Extracellular redox potentials that can be detected conveniently affect intracellular redox homeostasis and metabolism, and consequently control profiles of fermentation products, which provide an alternative for monitoring and control of these fermentation processes. This article reviews updated progress in the impact of redox potentials on gene expression, protein biosynthesis and metabolism as well as redox potential control strategies for more efficient production of fermentation products, taking ethanol fermentation by the yeast Saccharomyces under microaerobic conditions and butanol production by the bacterium Clostridium under anaerobic conditions as examples.

•Continuous cultivation of Rhodosporidium toruloides under carbon limitation.•Continuous cultivation of Rhodosporidium toruloides under nitrogen limitation.•Culture process parameters determined.•Established relationship between specific lipid production rate and dilution rate.Microbial lipids are potential alternative feedstock for biofuel and oleochemical industries. The oleaginous yeast Rhodosporidium toruloides AS 2.1389 is an excellent lipid producer. To attain parameters for the understanding of the lipid production process, we performed continuous cultivation experiments under either carbon or nitrogen limitation. The maintenance coefficient and maximum cell mass yield for this yeast were determined as 5.7 mg glucose/g cell/h and 0.42 g cell/g glucose, respectively, under carbon limitation. Under nitrogen limitation, the highest lipid yield of 0.19 g/g was observed at the dilution rate of 0.02 h−1 while the highest specific lipid formation rate of 0.058 g/g cell/h at the dilution rate of 0.08 h−1. A kinetic model of lipid formation under steady state conditions was developed, parameters estimated, and optimal continuous cultivation conditions forecasted. These data should be very helpful to develop and design more efficient bioprocesses for microbial lipid production.

In this article, effect of zinc supplementation on acetone–butanol–ethanol (ABE) fermentation by Clostridium acetobutylicum was studied. It was found that when 0.001 g/L ZnSO4·7H2O was supplemented into the medium, solventogenesis was initiated earlier, with 21.0 g/L ABE (12.6 g/L butanol, 6.7 g/L acetone and 1.7 g/L ethanol) produced with a fermentation time of 40 h, compared to 19.4 g/L ABE (11.7 g/L butanol, 6.4 g/L acetone and 1.3 g/L ethanol) produced with a fermentation time of 64 h in the control without zinc supplementation, and correspondingly ABE and butanol productivities were increased to 0.53 and 0.32 g/L/h from 0.30 and 0.18 g/L/h, increases of 76.7% and 77.8%, respectively, but their yields were not compromised. The reason for this phenomenon was attributed to rapid acids re-assimilation for more efficient ABE production, which was in accordance with relatively high pH and ORP levels maintained during the fermentation process. The maximum cell density increased by 23.8%, indicating that zinc supplementation stimulated cell growth, and consequently facilitated glucose utilization. However, more zinc supplementation exhibited an inhibitory effect, indicating that zinc supplementation at very low levels such as 0.001 g/L ZnSO4·7H2O will be an economically competitive strategy for improving butanol production.Highlights► Acetone–butanol–ethanol (ABE) fermentation by Clostridium acetobutylicum. ► Zinc supplementation stimulated cell growth and initiated solventogenesis earlier. ► Both butanol and ABE productivities were significantly improved.

Scenedesmus obliquus belongs to green microalgae, which is attracting attention as a feedstock for biofuels production and biorefinery as well as in bioremediation of environmental pollutants, making its genetic modifications for more efficient growth and accumulation of aimed metabolites significant. However, the genetic transformation system of S. obliquus is still not well established. In the current work, S. obliquus was transformed via electroporation using a plasmid containing chloramphenicol resistance gene (CAT) as a selectable marker and the green fluorescent protein gene (gfp) as a reporter. Using the optimized transformation conditions, the transformation efficiency was 494 ± 48 positive transgenic clones per 106 recipient cells, which is more efficient comparing with those reported in other microalgal transformation studies. Green fluorescence was observed after six months of cultivation, and CAT-specific products were also detected in the transformants by PCR, Southern blot and RT-PCR analysis. This is the first report on establishing such an efficient and stable transformation system for S. obliquus, a prerequisite for both functional genomic studies and strain improvement for other biotechnology applications of this important microalgal species.Highlights► Genetic transformation method of Scenedesmus obliquus was established and high transformation efficiency was obtained. ► Stable transformants maintaining the plasmid for more than 6 months were obtained. ► It was proved that GFP and CaMV35S promoter are usable marker and promoter for genetic engineering of S. obliquus.