Alberto J. Moya

Influence of temperature on the fermentation of d-xylose by Pachysolen tannophilus to produce ethanol and xylitol

The influence of temperature between 283 and 313 K on the fermentation of d-xylose with Pachysolen tannophilusATTC 32691 to produce ethanol and xylitol was studied. All experiments were made in a batch-culture reactor keeping the aeration level constant and the pH of the culture medium at 4.5. In each experiments the maximum specific net growth rate ( µm), biomass productivity (b), the specific rates of xylose uptake (qs) and ethanol and xylitol production (qE and qXy) and overall yields in biomass (Y G ), ethanol (Y G ) and xylitol (Y G/s ) were determined. A fitting of the experimental values of µm−T, within the wide temperature range studied lead to the equation µm = 2.2 × 10 9 e −6839/T − 9.1 × 10 22 e −16 702/T

The influence of pH and aeration rate on the fermentation of D-xylose by Candida shehatae

The effects of the initial pH and air supply on the production of ethanol from D-xylose using the yeast Candida shehatae in a batch reactor were investigated. The initial pH was altered within the range of 2.5-6.5 and the specific aeration rate from 0.0-0.3 vv-1 min-1. The results showed that the most favorable initial pH for ethanol production was 4.5 and aeration via the stirring vortex of the bioreactor was sufficient. Under these conditions, the maximum specific growth rate (mu(m)) was 0.329 h-1; biomass production rate (b), 0.024 kg m-3 h-1; overall biomass yield (YGx/s), 0.036 kg kg-1; the specific uptake rate of D-xylose (qs), 2.0 kg kg-1 h-1; and the specific ethanol production rate (qE), 0.72 kg kg-1 h-1 (both at 20 h culture time). The average xylitol yield (Yxy/s) was 0.078 kg kg-1 and the overall ethanol yield (YGE/s), 0.41 kg kg-1. Both qs and qE diminished once the exponential growth phase was over.

Oligosaccharides and monomeric carbohydrates production from olive tree pruning biomass.

Using the severity factor, it has been possible to study cellulose and hemicellulose fractional conversion, sugar yields change and oligosaccharides variation through olive tree pruning biomass pretreatments with acid or liquid hot water under pressure. The temperatures tested were in the range 180-230°C, operation time varying between 0 and 30min and acid concentration used did not exceed 0.05M. Complete hemicellulose solubilization in autohydrolysis was achieved using severity factors (logR0) close to 3.9 (most sugars are like oligomers), while if sulfuric acid 0.025M is employed, this parameter could be smaller (≥3.4). With these treatments, we have obtained cellulose conversions between 30 and 42% from liquid hot water experiments, 40-51% with sulfuric acid 0.025M and 42-57% when the acid concentration was 0.05M. The best results in terms of maximum yield in total sugars, d-glucose and d-xylose, with a low amount of acetic acid and hydroxymethylfurfural, was obtained at 200°C, 0min (what means that there is no time of temperature maintenance, only heating and cooling) and H2SO4 0.025M.

The production of xylitol from d-xylose by fermentation with Hansenula polymorpha

Abstract We have analysed the influence of the initial pH of the medium and the quantity of aeration provided during the batch fermentation of solutions of d-xylose by the yeast Hansenula polymorpha (34438 ATCC). The initial pH was altered between 3.5 and 6.5 whilst aeration varied between 0.0 and 0.3 vvm. The temperature was kept at 30 °C during all the experiments. Hansenula polymorpha is known to produce high quantities of xylitol and low quantities of ethanol. The most favourable conditions for the growth of xylitol turned out to be: an initial pH of between 4.5 and 5.5 and the aeration provided by the stirring vortex alone. Thus, at an initial pH of 5.5, the maximum specific production rate (μm) was 0.41 h−1, the overall biomass yield (Yx/s G) was 0.12 g g−1, the specific d-xylose-consumption rate (qs) was 0.075 g g−1 h−1 (for t = 75 h), the specific xylitol-production rate (qXy) was 0.31 g g−1 h−1 (for t = 30 h) and the overall yields of ethanol (YE/sG) and xylitol (YXy/sG) were 0.017 and 0.61 g g−1 respectively. Both qs and qXy decreased during the course of the experiments once the exponential growth phase had finished.

Detoxification of Rice Straw and Olive Tree Pruning Hemicellulosic Hydrolysates Employing Saccharomyces cerevisiae and Its Effect on the Ethanol Production by Pichia stipitis

The aim of this work was to study the ability of Saccharomyces cerevisiae (bakers yeast) to metabolize a variety of aromatic compounds found in rice straw (RSHH) and olive tree pruning (OTHH) hemicellulosic hydrolysates, obtained by acid hydrolysis at different sugar and toxic compound concentrations. Initially, the hydrolysates were inoculated with S. cerevisiae (10 g L(-1)) and incubated at 30 °C under agitation at 200 rpm for 6 h. The results showed that this yeast was able to utilize phenolic and furan compounds in both hemicellulose hydrolysates. Next, the treated hydrolysates were inoculated with Pichia stipitis NRRL Y-7124 to evaluate the effect of biotransformation of aromatic compounds on ethanol production, and better fermentation results were obtained in this case compared to untreated ones. The untreated hemicellulose hydrolysates were not able to be fermented when they were incubated with Pichia stipitis. However, in RSHH treated hydrolysates, ethanol (Y(P/S)) and biomass (Y(X/S)) yields and volumetric ethanol productivity (Q(P)) were 0.17 g g(-1), 0.15 g g(-1) and 0.09 g L(-1) h(-1), respectively. The OTHH-treated hydrolysates showed less favorable results compared to RSHH, but the fermentation process was favored with regard to untreated hydrolysate. These results showed that the fermentation by P. stipitis in untreated hydrolysates was strongly inhibited by toxic compounds present in the media and that treatment with S. cerevisiae promoted a significant reduction in their toxicities.

Ethanol and xylitol production by fermentation of acid hydrolysate from olive pruning with Candida tropicalis NBRC 0618.

Olive tree pruning biomass has been pretreated with pressurized steam, hydrolysed with hydrochloric acid, conditioned and afterwards fermented using the non-traditional yeast Candida tropicalis NBRC 0618. The main aim of this study was to analyse the influence of acid concentration on the hydrolysis process and its effect on the subsequent fermentation to produce ethanol and xylitol. From the results, it could be deduced that both total sugars and d-glucose recovery were enhanced by increasing the acid concentration tested; almost the whole hemicellulose fraction was hydrolysed when 3.77% was used. It has been observed a sequential production first of ethanol, from d-glucose, and then xylitol from d-xylose. The overall ethanol and xylitol yields ranged from 0.27 to 0.38kgkg(-1), and 0.12 to 0.23kgkg(-1) respectively, reaching the highest values in the fermentation of the hydrolysates obtained with hydrochloric acid 2.61% and 1.11%, respectively.

Comparative study of the fermentation of D-glucose/D-xylose mixtures with Pachysolen tannophilus and Candida shehatae.

Abstract We have performed a comparative analysis of the fermentation of the solutions of the mixtures of D-glucose and D-xylose with the yeasts Pachysolen tannophilus (ATCC 32691) and Candida shehatae (ATCC 34887), with the aim of producing bioethanol. All the experiments were performed in a batch bioreactor, with a constant aeration level, temperature of 30 °C, and a culture medium with an initial pH of 4.5. For both yeasts, the comparison was established on the basis of the following parameters: maximum specific growth rate, biomass productivity, specific rate of substrate consumption (qs) and of ethanol production (qE), and overall ethanol and xylitol yields. For the calculation of the specific rates of substrate consumption and ethanol production, differential and integral methods were applied to the kinetic data. From the experimental results, it is deduced that both Candida and Pachysolen sequentially consume the two substrates, first D-glucose and then D-xylose. In both yeasts, the specific substrate-consumption rate diminished over each culture. The values qs and qE proved higher in Candida, although the higher ethanol yield was of the same order for both yeasts, close to 0.4 kg kg−1.

Monomeric carbohydrates production from olive tree pruning biomass: Modeling of dilute acid hydrolysis

Statistical modeling and optimization of dilute sulfuric acid hydrolysis of olive tree pruning biomass has been performed using response surface methodology. Central composite rotatable design was applied to assess the effect of acid concentration, reaction time and temperature on efficiency and selectivity of hemicellulosic monomeric carbohydrates to d-xylose. Second-order polynomial model was fitted to experimental data to find the optimum reaction conditions by multiple regression analysis. The monomeric d-xylose recovery 85% (as predicted by the model) was achieved under optimized hydrolysis conditions (1.27% acid concentration, 96.5°C and 138 min), confirming the high validity of the developed model. The content of d-glucose (8.3%) and monosaccharide degradation products (0.1% furfural and 0.04% 5-hydroxymethylfurfural) provided a high quality subtract, ready for subsequent biochemical conversion to value-added products.

Improving bioethanol production from olive pruning biomass by deacetylation step prior acid hydrolysis and fermentation processes.

In order to produce bioethanol from olive tree pruning biomass, deacetylation was performed employing sodium hydroxide. Optimal conditions were determined using experimental design techniques. The highest acetic acid removal (3.8g/dm(3)), obtained by response surface methodology, was at optimum pretreatment conditions of temperature 60°C, 0.8% NaOH and residence time 60min. After oxalic acid hydrolysis of pretreated biomass, the hydrolysates were directly used for ethanol production without further detoxification process. Ethanol yields ranged from 0.19 to 0.45g/g, reaching the maximum yield value when pretreatment was carried out at 130°C with 100mM oxalic acid, involving a combined severity factor (CSF) of 1.05. The highest ethanol concentration obtained from pretreated biomass was 6.2g/dm(3) at 150°C, using 75mM of oxalic acid (CSF=1.53).

ACID HYDROLYSIS OF THE WASTE FROM THE PRUNING OF THE OLIVE TREE

ABSTRACT We have hydrolysed olive-tree prunings with both hydrochloric and sulphuric acid, after grinding and size grading them, and analysed the influence of both the type and concentration of acid used on the sugar yields in the hydrolyzates. We found that both D-glucose (YG) and total reducing sugar (YT) yields were always higher with hydrochloric acid when both acids were used at the same concentration. When the concentration of HCl was altered between 0.1 and 1 N, the temperature kept constant at 90°C and the operating time at 240 min, the hemicellulose content decreased concomitantly with a rise in acid concentration, and YT values reached 0.33 whilst YG reached 0.095 kg.kg−1.