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During ethanol fermentation, ethanol accumulates in the medium and becomes a significant stress factor for yeast. Trehalose and glycogen are major carbohydrate storages and presumably protect cells from such stress conditions. In this study, batch cultivations of wild and adapted Saccharomyces cerevisiae were performed to evaluate the fermentation kinetics and carbohydrate storages, interrelatedly. Trehalose and glycogen were determined by both High Performance Liquid Chromatography (HPLC) and Fourier Transform-Infrared (FT-IR) spectroscopy for comparison. The obtained maximum trehalose and glycogen (7.5 and 10.5 %) contents of the adapted strain was remarkably higher compared to the wild strain (5 and 6.5 %). The adapted S. cerevisiae displayed an enhanced fermentation stresses coping ability resulting in a higher ethanol productivity (2.03 vs 1.65 g ethanol/h) as well as higher ethanol titer (7.7 vs 6.4 %). This study contributes to the understanding of the cellular response of S. cerevisiae cells to fermentation stresses in relation to culture history together with the evaluation of fermentation performance, as well as the process kinetics. This study also confirms that FT-IR spectroscopy of whole S. cerevisiae cells for determination of storage compounds without pretreatment has valuable advantages compared to HPLC with intensive sample handling.

The aim of the research was to optimize the composition of the VHG (Very High Gravity) media by selecting the proportions of various plant materials, complementary in terms of the composition of substrates affecting the fermentation process and efficiency. The addition of 25, 33 and 50% of molasses in VHG mash resulted in higher ethanol concentration above 112 g L−1. The highest ethanol concentration of 129.96 ± 0.58 g L−1 and high actual fermentation yield as compared to theoretical one of 90.47 ± 0.40% was found for VHG mash made from all four plant materials used in the same proportions (25%). Raw spirit obtained by distillation of this medium was also characterized by the lowest content of acetaldehyde (332.8 ± 8.3 mg L−1 EtOH) and esters (94.3 ± 1.6 mg L−1 EtOH) in comparison with the spirit samples obtained from mash with a similar degree of attenuation. Addition of molasses to VHG mashes resulted also in lowering the concentration of higher alcohols in distillates to 3815 mg L−1 EtOH as compared to spirits obtained from substrates using starchy materials only. Use of only a complementary raw material composition ensuring that VHG mashes are rich in nutrients resulted in intensification of the alcoholic fermentation process.

To decrease alkaline dosage and increase the enzymatic saccharification of sugarcane bagasse (SB), dilute mixed alkali salts soaking (0.5% K2CO3, 0.068% K2SO3) in an autoclave at 100–125 °C for 10–60 min was attempted to pretreat SB. The increased pretreatment severity at elevated temperatures resulted in the increase of delignification and enzymatic saccharification. It was found the optimum pretreatment temperature and pretreatment time were 120 °C for 40 min. Furthermore, FTIR, XRD and SEM characterized the porosity, morphology, and crystallinity changes of the pretreated SB samples. After the enzymatic in situ saccharification of 50 g/L SB in dilute K2CO3/K2SO3 (0.35% K2CO3, 0.048% K2SO3) media for 72 h, the reducing sugars and glucose could be obtained at 32.94 and 20.18 g/L, respectively. Finally, the SB-hydrolysates containing 20 g/L glucose were used for ethanol fermentation in the presence of dilute alkali salts. After 48 h, the ethanol yield was 0.394 g ethanol/g glucose, which represents 77.25% of the theoretical yield. In conclusion, an effective process about the biomass pretreatment, enzymatic saccharification and ethanol fermentation was successfully demonstrated in “one-pot” media containing low loading of alkali salts K2CO3/K2SO3.

Bioethanol is a renewable biofuel that has a strong inhibitory effect on cells in bioethanol fermentation by yeast. In this study, carbon nanotube (CNT)-mixed polydimethylsiloxane (PDMS) membranes were used for ethanol recovery from model solutions as well as for fermentation by self-flocculating yeast. Imbedding CNTs into the PDMS membrane led to enhanced ethanol recovery, with a maximum total flux of 128.7 g/m2 h and an ethanol titer of 615.1 g/L in permeate. The CNTs can provide a flexible route for ethanol transport through the inner tubes or along the smooth surface. In fed-batch fermentation incorporating pervaporation, 112.3 g/L of ethanol was produced with an overall ethanol productivity and yield of 2.23 g/L h and 0.45 g/g, respectively. The membrane produced a highly concentrated condensate containing 400.3–487.5 g/L of ethanol. Furthermore, as yeast flocs can be throttled down in the bioreactor, self-flocculating yeast can be used to prevent membrane fouling induced by cell adsorption on the membrane. Therefore, the CNT-mixed membrane coupled with ethanol fermentation by self-flocculating yeast not only reduces ethanol-mediated inhibition of cells but also saves the production cost because of the reduced fouling risk. Thus, this combination approach has potential in industrial bioethanol production for long-time operation.

For the purpose of designing an automatic repeated batch ethanol fermentation process, a self-settlement feature of flocculating yeast was used along with the development of a relevant kinetic model to describe such an operation. The frequency of repeated batch operation was controlled by the measured fermentation redox potential. As the redox potential profile changed its slope from zero or from negative to positive, repeated operation was triggered. The ethanol productivity varied between 3.26 ± 0.22 and 6.72 ± 0.24 g/L h for ∼200 g glucose/L case, and between 3.88 ± 0.37 and 4.68 ± 0.15 g/L h for ∼240 g glucose/L cases during the course of repeated batch fermentation. Although the biomass built up as fermentation proceeded, the yeast viability decreased, consequently limiting the number of batch operation that could be repeated. The number of repeated fermentation depended on the initial glucose concentration used. The proposed model predicted that the maximum repeatable batch operation when flocculating yeast was used was 14 and 8 for ∼200 and ∼240 g glucose/L case, respectively. Although the biomass increased significantly comparing with single batch, the cell viability decreased along with the batch number. To overcome this problem, a partial removal of yeast during the repeated batch fermentation would extend the operation.

The fermentability of four different side streams produced in sulfite pulping has been compared in ethanol production with Saccharomyces cerevisiae. The results show that the fermentability of the different side streams varies, depending on where in the process they are produced, and the additional treatment applied to them. Side streams spent sulfite liquor, spent sulfite liquor derivative and spent sulfite liquor after ethanol fermentation that were fermentable benefited from the main cooking process, during which 90% of the sulfite was removed, whereas the side stream produced in the first cooking step, containing 11.0 g/L sulfite, was unfermentable. The fermentation of the side streams resulted in lower yields and productivity than fermentation in a defined medium. Furthermore, the fermentability of the side streams was improved after over-liming, evaporation, and laccase treatment. Over-liming was the most efficient means of detoxifying the side-streams, resulting in better fermentability. Sulfite treatment, however, had a counterproductive effect on fermentation due to the toxicity of this chemical to yeast metabolism. When the side-streams were detoxified by over-liming, loss of sugars was observed. Laccase treatment was less efficient, but it should be further explored as it offers a sustainable method of detoxifying side streams in situ.

When Saccharomyces cerevisiae was cultivated under ∼200 g glucose/l condition, the time point at which glucose was completely utilized coincided with the moment at which the slope of a redox potential profile changed from negative or zero to positive. Based on this feature, a redox potential-driven glucose-feeding fermentation operation was developed, and resulted in a self-cycling period of 14.25 ± 0.4 h. The corresponding ethanol concentration was maintained at 88.4 ± 1.0 g/l with complete glucose conversion, and the cell viabilities increased from 80% in the transition period to 97.2 ± 1.1%, implying the occurrence of yeast acclimatization. In contrast, a pre-determined 36-h manually adjusted period was chosen to oscillate yeast cells under ∼250 g glucose/l conditions, which resulted in 106.76 ± 0.7 g ethanol/l and 15.19 ± 1.3 g glucose/l remaining at the end of each cycle. Compared to the equivalent batch and continuous ethanol fermentation processes, the annual ethanol productivity of the reported fermentation operation is 2.4% and 13.2% greater, respectively in ∼200 g feeding glucose/l conditions.

The aim of the work was to study the properties of the bacterial cellulose membrane (BCM) and the feasibility of using it as a new, environmentally friendly support carrier for yeast cell immobilization. It was observed that the morphology of BCM varied with different cultivation methods and the scanning electron microscopy (SEM) images confirmed that the yeast cells were entrapped in the porous network of BCM obtained from the static culture and stabilized by the cross-linked fibrils. Particularly, the research confirmed the effectiveness of yeast immobilization in BCM reflected by the high yield of alcohol (9.7% v/v, a 21.25% increase of those using free cells) and the high stability. The specific rate of ethanol production by the immobilized cells in BCM was 2.1 g g−1 h−1, 31.3% greater than that of the suspended cells. Results implied that applying BCM as the support carrier had little adverse effects on cell viability and proliferation. Instead, it facilitated the product leakage and nutrients transportation through the porous network.

Supplementation effect of compatible solute ectoine on the ethanol fermentation by Zymomonas mobilis CICC10232 was examined in the presence of high glucose concentration. Addition of ectoine promoted the cell growth as well as volumetric ethanol productivity in the presence of 250 g/l of glucose. At the end of ethanol fermentation, cell dry weight resulted in 1.8 and 1.5 g CDW/l in the presence and absence of 1 mM ectoine, respectively. Volumetric ethanol productivity in the presence of 1 mM ectoine became 1.1 g/l/h, which resulted in 57.1% higher than that in the absence of ectoine, 0.7 g/l/h. In addition, fermentation time was shortened by 24 h when 1 mM ectoine existed in the medium. In the presence of 250 g/l of glucose together with various concentrations of ectoine, relative enzyme activities of glucokinase (GK), glucose-6-phosphate dehydrogenase (G-6-PDH), and alcohol dehydrogenase (ADH) increased by 29.9, 11.6, and 7.7% in the presence of 0.5, 0.5, and 0.25 mM ectoine compared to those without ectoine supplemented, respectively. The promoting function of ectoine on ethanol fermentation may be related to the protection of these enzyme activities under osmotic stress.

“Propagation” or “conditioning” of active dry yeast (ADY) for the production of fuel ethanol is thought to reduce lag times in fermentation and reduce overall fermentation times. The objectives of this study were to determine the optimal time that ADY should be conditioned prior to fermentation and to determine how well yeast and bacterial contaminants present in ADY are propagated. A new term, metabolic acclimatization (MAcc), was introduced to describe events occurring during conditioning. It was observed that MAcc is not necessary prior to alcoholic fermentation. Lag times were reduced but overall fermentation times were unaffected. In fact, fermentation rates were slower than control conditions where ADY was directly inoculated into fermentors. In addition, if MAcc systems were to be used, it is recommended that only batch MAcc systems be practiced. Continuous MAcc (continuous yeast propagation) leads to increased bacterial numbers and organic acid levels, which become stressful to yeast.

Ethanol fermentation under non-sterilized condition was developed in this paper. An acid-tolerant mutant of Zymomonas mobilis, which can begin fermentation at the low pH of 4.5, was selected. At this low pH, potential contamination of ethanol production was controlled at the normal level without autoclaving or filter sterilization. Compared with the filter sterilization process, the same ethanol yield of 73.1 g/l indicated that no contamination occurred in the non-sterilized process. Compared with the autoclaved process, ethanol yield increased from 70.3 to 73.2 g/l and residual sugar concentration decreased from 5.3 to 1.3 g/l with 40 h fermentation. The theoretical yield of ethanol from glucose was 0.488 g/g in this new ethanol process.

Whole cells of Zymomonas mobilis, immobilised on pectin beads, were stablised by curing in a mixture of 0.2 M CaCl2 � 2H2O and Na2B4O7 � 10H2O for 24 h at 4�C. Ethanol fermentation was studied using different glucose concentrations (50-200 g/litre) by these immobilised beads. The kinetics of this process was studied in a glucose step-feeding system with different total glucose concentrations (150 and 200 g/litre) in a specially designed shake flask. Immobilised whole cells were also used for ethanol fermentation in an expanded bed bioreactor and its performance characteristics were compared with those of a regular batch mechanically stirred tank bioreactor. Fermentation rates and ethanol yields were higher in the expanded bed bioreactor system than in the batch mechanically agitated stirred tank bioreactor. The stabilised pectin beads were damaged less in the expanded bioreactor than in the regular batch stirred tank reactor. Copyright ? 1996 Elsevier Science Ltd. All rights reserved.

Saccharomyces cerevisiae was immobilized on γ-alumina particles with binder polymer by a spray-drying process. Batch fermentations of sucrose to ethanol were performed using the immobilized cells. The optimum pH was 4, and this helped to minimize microbial contamination and facilitate electrostatic attraction between the γ-alumina particles and the yeast cells. Addition of yeast extract resulted in high ethanol conversion. The reasons for differences between cellulose acetate phthalate (CAP) and styrene-maleic acid co-polymer (SMC) as binders on ethanol conversion were not apparent. The release of binder with the SMC from the γ-alumina was less than that from the CAP. Presoaking of γ-alumina particles in resin for binder solution before the immobilization using a spray-dryer was effective in achieving high ethanol conversion.

Various simultaneous starch saccharification and ethanol fermentation (SSSF) processes were developed and evaluated on batch, semibatch and continuous modes of fermentation in laboratory and pilot scale fermentors using Zymomonas mobilis . Compared with a two-step process involving separate saccharification and fermentation stages, the SSSF reduced the total process time by half. In order to produce ethanol from sago starch economically, the immobilization of amyloglucosidase and Z. mobilis cells was studied. Among the various immobilization methods tested, a co-immobilized system using chitin and sodium alginate appeared most promising with respect to ethanol productivity and operational stability. In order to scale-up the SSSF process, large scale SSSF processes were studied in batch and semibatch fermentation modes. In pilot scale SSSFs, the kinetic results were found to be similar to those from laboratory scale fermentation. Finally, a mathematical model was described for the SSSF process. Simulations of glucose concentration and other fermentation parameters agreed closely with experimental results.

Continuous lactic acid fermentation was carried out to produce an appropriate feed solution for rapid ethanol fermentation in soy sauce production. Lactic acid bacteria were immobilised physically within porous alumina ceramic beads and the fermentation substrate was a filtered extract of soy sauce mash. Lactic acid concentration was controlled by dilution rate. The volumetric production rate was dependent on the dilution rate and was much higher than that in batch fermentation. The lactic acid productivity was mathematically modelled from the basic material balance and the rate equation. The simulated results agreed with the experimental ones.