Continuous ethanol fermentation in tower reactors with cell recycling using flocculent Saccharomyces cerevisiae

High-solid enzymatic hydrolysis of sugarcane bagasse and ethanol production in repeated batch process using column reactors

Alkaline sulfite pretreated sugarcane bagasse was enzymatically hydrolyzed in a packed-bed column reactor and a bubble column reactor was evaluated to produce ethanol from the hydrolysate. Initial solid loadings of 9-16% were used in column reactor in the hydrolysis step, and the use of lower value (9%) resulted in 41 g L^-1 of glucose in the hydrolysate, corresponding to 87% of cellulose hydrolysis yield. This yield was reduced to 65% for a solid loading of 16%, corresponding to a glucose concentration of 54 g L^-1. Subsequently, Saccharomyces cerevisiae and Scheffersomyces stipitis were used for ethanol production in medium based on hydrolysate previously obtained, using different aeration flowrates (0.3, 0.5 and 0.7 vvm). In simple batch fermentation using S. cerevisiae, higher ethanol yield (0.40 g.g^-1) and productivity (1.58 g.L^-1.h^-1) were achieved using 0.5 vvm. When S. stipitis was used in simple batch co-fermentations, the maximum ethanol productivities were obtained using 0.5 and 0.7 vvm (0.64 and 0.63 g.L^-1.h^-1, respectively). Successive repeated batches resulted in average ethanol concentration of 38 g.L^-1 and fermentation efficiency of 82%, when using S. cerevisiae. For S. stipitis, those values were, respectively, 36 g.L^-1 and 50%, with volumetric productivity increased along the cycles. Thus, the potential of the bioreactors as simple systems for use in the biological steps of biorefineries was demonstrated.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02932-3.

Study of biorefineries based on experimental data: production of bioethanol, biogas, syngas, and electricity using coffee-cut stems as raw material

Energy-driven biorefineries can be designed considering biotechnological and thermochemical conversion pathways. Nevertheless, energy and environmental comparisons are necessary to establish the best way to upgrade lignocellulosic biomass and set the requirements of these processes in different scenarios. This paper aims to evaluate experimentally a biorefinery producing energy vectors using coffee-cut stems (CCS) as feedstock. The obtained yields were the basis for energy and environmental analysis, in two different biorefinery scenarios: (i) production of bioethanol and biogas and (ii) production of syngas and electricity. The energy results indicated that the overall energy efficiency calculated in the first scenario was only 9.15%. Meanwhile, the second biorefinery configuration based on thermochemical routes presented an energy efficiency value of 70.89%. This difference was attributed to the higher consumption of utilities in the biorefinery based on biotechnological routes. The environmental results showed that the impact category of climate change for the first biorefinery (i.e., 0.0193 kg CO₂ eq./MJ) had a lower value than that of the second process (i.e., 0.2377 kg CO₂ eq./MJ). Thus, the biorefinery based on the biotechnological route presented a better environmental performance. Additionally, the results for both biorefineries allowed concluding that the inclusion of by-products and co-products in the calculation of the environmental analysis can dramatically affect the results.

High Gravity and Very High Gravity Fermentation of Sugarcane Molasses by Flocculating Saccharomyces cerevisiae: Experimental Investigation and Kinetic Modeling

Substantial progress has been made in ethanol fermentation technology under high gravity (HG) and very high gravity (VHG), which offer environmental and economic benefits. HG and VHG processes increase the productivity of ethanol, reduce distillation costs, and enable higher yields. The aim of the present study was to evaluate the use of sugarcane molasses as the medium component along with flocculating yeasts for fermentation in a fed-batch process employing this promising technology. We evaluated fed-batch fermentation, HG, and VHG involving a molasses-based medium with high concentrations of reducing sugars (209, 222, and 250 g/L). Fermentation of 222 g/L of total reducing sugars achieved 89.45% efficiency, with a final ethanol concentration of 104.4 g/L, whereas the highest productivity (2.98 g/(L.h)) was achieved with the fermentation of 209 g/L of total reducing sugars. The ethanol concentration achieved with the fermentation of 222 g/L of total reducing sugars was close to the value obtained for P'_max (105.35 g/L). The kinetic model provided a good fit to the experimental data regarding the fermentation of 222 g/L. The results revealed that sugarcane molasses and flocculating yeasts can be efficiently used in HG fermentation to reduce the costs of the process and achieve high ethanol titers.

Mathematical modeling of the ethanol fermentation of cashew apple juice by a flocculent yeast: the effect of initial substrate concentration and temperature

In this work, the effect of initial sugar concentration and temperature on the production of ethanol by Saccharomyces cerevisiae CCA008, a flocculent yeast, using cashew apple juice in a 1L-bioreactor was studied. The experimental results were used to develop a kinetic model relating biomass, ethanol production and total reducing sugar consumption. Monod, Andrews, Levenspiel and Ghose and Tyagi models were investigated to represent the specific growth rate without inhibition, with inhibition by substrate and with inhibition by product, respectively. Model validation was performed using a new set of experimental data obtained at 34 °C and using 100 g L^-1 of initial substrate concentration. The model proposed by Ghose and Tyagi was able to accurately describe the dynamics of ethanol production by S. cerevisiae CCA008 growing on cashew apple juice, containing an initial reducing sugar concentration ranging from 70 to 170 g L^-1 and temperature, from 26 to 42 °C. The model optimization was also accomplished based on the following parameters: percentage volume of ethanol per volume of solution (%V _ethanol/V _solution), efficiency and reaction productivity. The optimal operational conditions were determined using response surface graphs constructed with simulated data, reaching an efficiency and a productivity of 93.5% and 5.45 g L^-1 h^-1, respectively.

Bioethanol production by recycled Scheffersomyces stipitis in sequential batch fermentations with high cell density using xylose and glucose mixture

Here, it is shown three-step investigative procedures aiming to improve pentose-rich fermentations performance, involving a simple system for elevated mass production by Scheffersomyces stipitis (I), cellular recycle batch fermentations (CRBFs) at high cell density using two temperature strategies (fixed at 30°C; decreasing from 30 to 26°C) (II), and a short-term adaptation action seeking to acclimatize the microorganism in xylose rich-media (III). Cellular propagation provided 0.52gdrycellweightgRS(-1), resulting in an expressive value of 45.9gdrycellweightL(-1). The yeast robustness in CRBF was proven by effective ethanol production, reaching high xylose consumption (81%) and EtOH productivity (1.53gL(-1)h(-1)). Regarding the short-term adaptation, S. stipitis strengthened its robustness, as shown by a 6-fold increase in xylose reductase (XR) activity. The short fermentation time (20h for each batch) and the fermentation kinetics for ethanol production from xylose are quite promising.