Development of a model for the ethanol concentration limit as a function of temperature and initial substrate concentration using the yeast Saccharomyces cerevisiae.

In this study, a model was developed to simulate the effect of temperature ( T $T$ ) and initial substrate concentration ( S 0 ${S}_{0}$ ) on the ethanol concentration limit ( P max ${P}_{\max }$ ) using the yeast Saccharomyces cerevisiae. To achieve this, regressions were performed using data provided by other authors for P max ${P}_{\max }$ to establish a model dependent on T $T$ and S 0 ${S}_{0}$ capable of predicting results with statistical significance. After constructing the model, a response surface was generated to determine the conditions where P max ${P}_{\max }$ reaches higher values: temperatures between 28°C and 32°C and an initial substrate concentration around 200 g/L. Thus, the proposed model is consistent with the observations that increasing temperatures decrease the ethanol concentration obtained, and substrate concentrations above 200 g/L lead to a reduction in ethanol concentration even at low temperatures such as 28°C.

Improvement of ethanol production by extractive fed-batch fermentation in a drop column bioreactor.

The use of fed-batch extractive fermentation can overcome inhibitory effects caused by the substrate and ethanol to the yeast cells, since it allows regulate the substrate concentration and remove the product as it is produced. The present study describes the modelling and experimental validation of ethanol production in fed-batch extractive fermentation with in situ ethanol removal by oleic acid in a non-conventional drop column bioreactor (DCB) operated under industrial conditions. The model developed using the hybrid Andrews-Levenspiel equation and ethanol distribution coefficient parameter (KDE) provided an excellent description of the fed-batch extractive ethanol fermentation process with oleic acid. Furthermore, extractive fed-batch fermentation allowed the feed up to 306.6 kg m-3 of substrate (total reducing sugars), with total ethanol concentration in extractive fermentation in the ranging 100.3-139.8 kg m-3 (12.7-17.7 ºGL), 19.9-67.2% higher when compared with the conventional process without ethanol removal. Moreover, this process has the advantage of less effluent generated and energy consumption for ethanol recovery when compared to the conventional process.

Improving EGSB reactor performance for simultaneous bioenergy and organic acid production from cheese whey via continuous biological H2 production.

OBJECTIVES: To evaluate the influence of hydraulic retention time (HRT) and cheese whey (CW) substrate concentration (15 and 25 g lactose l-1) on the performance of EGSB reactors (R15 and R25, respectively) for H2 production. RESULTS: A decrease in the HRT from 8 to 4 h favored the H2 yield and H2 production rate (HPR) in R15, with maximum values of 0.86 ± 0.11 mmol H2 g COD-1 and 0.23 ± 0.024 l H2 h-1 l-1, respectively. H2 production in R25 was also favored at a HRT of 4 h, with maximum yield and HPR values of 0.64 ± 0.023 mmol H2 g COD-1 and 0.31 ± 0.032 l H2 h-1 l-1, respectively. The main metabolites produced were butyric, acetic and lactic acids. CONCLUSIONS: The EGSB reactor was evaluated as a viable acidogenic step in the two-stage anaerobic treatment of CW for the increase of COD removal efficiency and biomethane production.

Size product modulation by enzyme concentration reveals two distinct levan elongation mechanisms in Bacillus subtilis levansucrase.

Two levan distributions are produced typically by Bacillus subtilis levansucrase (SacB): a high-molecular weight (HMW) levan with an average molecular weight of 2300 kDa, and a low-molecular weight (LMW) levan with 7.2 kDa. Previous results have demonstrated how reaction conditions modulate levan molecular weight distribution. Here we demonstrate that the SacB enzyme is able to perform two mechanisms: a processive mechanism for the synthesis of HMW levan and a non-processive mechanism for the synthesis of LMW levan. Furthermore, the effect of enzyme and substrate concentration on the elongation mechanism was studied. While a negligible effect of substrate concentration was observed, we found that SacB elongation mechanism is determined by enzyme concentration. A high concentration of enzyme is required to synthesize LMW levan, involving the sequential formation of a wide variety of intermediate size levan oligosaccharides with a degree of polymerization (DP) up to ∼70. In contrast, an HMW levan distribution is synthesized through a processive mechanism producing oligosaccharides with DP <20, in reactions occurring at low enzyme concentration. Additionally, reactions where levansucrase concentration was varied while the total enzyme activity was kept constant (using a combination of active SacB and an inactive SacB E342A/D86A) allowed us to demonstrate that enzyme concentration and not enzyme activity affects the final levan molecular weight distribution. The effect of enzyme concentration on the elongation mechanism is discussed in detail, finding that protein-product interactions are responsible for the mechanism shift.

Soluble malate dehydrogenase of Geophagus brasiliensis (Cichlidae, Perciformes): isolated isoforms and kinetics properties

Kinetic properties and thermal stabilities of Geophagus brasiliensis skeletal muscle unfractionated malate dehydrogenase (MDH, EC 1.1.1.37) and its isolated isoforms were analyzed to examine a possible sMDH-B* locus duplication in a fixation process influenced by genetic drift. Two optimal pHs were detected: 7.5 for AB1 unfractionated muscle phenotype and its B1 isoform, and 8.0 for AB1B2 unfractionated muscle phenotype, A and B2 isoforms. While G. brasiliensis A isoform could be characterized as thermostable, the duplicated B isoform cannot be assumed as thermolabile. Km values for isolated B2 isoforms were 1.6 times lower than for B1. A duplication event in progress best explains the electrophoretic six-band pattern detected in G. brasiliensis, which would be caused by genetic drift.