Vicente Bravo

Multifunctional properties of phosphate-solubilizing microorganisms grown on agro-industrial wastes in fermentation and soil conditions

One of the most studied approaches in solubilization of insoluble phosphates is the biological treatment of rock phosphates. In recent years, various techniques for rock phosphate solubilization have been proposed, with increasing emphasis on application of P-solubilizing microorganisms. The P-solubilizing activity is determined by the microbial biochemical ability to produce and release metabolites with metal-chelating functions. In a number of studies, we have shown that agro-industrial wastes can be efficiently used as substrates in solubilization of phosphate rocks. These processes were carried out employing various technologies including solid-state and submerged fermentations including immobilized cells. The review paper deals critically with several novel trends in exploring various properties of the above microbial/agro-wastes/rock phosphate systems. The major idea is to describe how a single P-solubilizing microorganism manifests wide range of metabolic abilities in different environments. In fermentation conditions, P-solubilizing microorganisms were found to produce various enzymes, siderophores, and plant hormones. Further introduction of the resulting biotechnological products into soil-plant systems resulted in significantly higher plant growth, enhanced soil properties, and biological (including biocontrol) activity. Application of these bio-products in bioremediation of disturbed (heavy metal contaminated and desertified) soils is based on another important part of their multifunctional properties.

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.

Xylitol production from olive-pruning debris by sulphuric acid hydrolysis and fermentation with Candida tropicalis

Abstract The debris of olive pruning is a renewable, low-cost and widely available agricultural waste. Its biochemical conversion by hydrolysis and fermentation was undertaken in the present study. Diluted acid hydrolysis was conducted in a heterogeneous stirred tank reactor at 90°C and at a low sulphuric acid concentration (0.0–1.0 N) for 300 min. To increase the d-xylose/d-glucose ratio into the hydrolysate, in another experiment amorphous cellulose and extracts were removed by means of a pretreatment in an extruder with 1 N H2SO4 at 70°C before the acid hydrolysis. The fermentation of hydrolysates was performed under microaerobic conditions in a batch bioreactor at 30°C and pH 5 with Candida tropicalis NBRC 0618. The controlled fermentation parameters included maximum specific growth rate, biomass productivity, rate of the specific substrate uptake, rates of specific ethanol and xylitol production, and overall yield of ethanol and xylitol. In the presence of 1.0 N H2SO4, the fermentation of the pretreated hydrolysate led to specific xylitol production rates and overall xylitol yield (0.1 g g-1 h for t=25 h; 0.49 g g-1, respectively) higher than those achieved without pretreatment (0.03 g g-1 h for t=25 h; 0.39 g g-1, respectively). Under these conditions, 53 g xylitol kg-1 of dry olive-pruning debris was obtained from the pretreated culture, whereas without pretreatment 70 g ethanol and 34 g xylitol were recovered.

Influence of the concentrations of d-xylose and yeast extract on ethanol production by Pachysolen tannophilus

Abstract To determine the most favorable conditions for the production of ethanol by Pachysolen tannophilus, this yeast was grown in batch cultures with various initial concentrations of two of the constituents of the culture medium: d -xylose (so), ranging from 1 g·l−1 to 200 g·l−1, and yeast extract (lo), ranging from 0 g·l−1 to 8 g·l−1. The most favorable conditions proved to be initial concentrations of So=25 g·l−1 and lo=4 g·l−1, which gave a maximum specific growth rate of 0.26 h−1, biomass productivity of 0.023 g·l−1·h−1, overall biomass yield of 0.094 g·g−1, specific xylose-uptake rate (qs) of 0.3 g·g−1·h−1 (for t=50 h), specific ethanol-production rate (qE) of 0.065 g·g−1·h−1 and overall ethanol yield of 0.34 g·g−1; qs values decreased after the exponential growth phase while qE remained practically constant.

The influence of temperature upon the hydrolysis of cellobiose by β-1,4-glucosidases from Aspergillus niger

We have made experimental studies into the enzymatic hydrolysis of cellobiose within the temperature range of 40 degrees C to 70 degrees C at pH 4.9, by using beta-1,4-glucosidase from Aspergillus niger. At 70 degrees C there was significant enzyme deactivation, which could be fitted to a potential deactivation model with values of n equal to 1.09 and k(d) to 0.1564 (g/l)(-0.09) min(-1), whereas the rate of hydrolysis could be fitted to the Michaelis-Menten equation. Between 40 degrees C and 60 degrees C we noted a substrate inhibition and that the CEC compound formed contributed to glucose production. The apparent activation energies had values of 4.66, 8.45, 4.82, and 3.99 kJ/mol for the kinetic constants k(a) and k(a2) the Michaelis constant and the substrate inhibition constant, respectively.

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.

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.

The effect of pH on kinetic and yield parameters during the ethanolic fermentation of D-xylose with Pachysolen tannophilus

We have studied the ethanolic fermentation of D-xylose with Pachysolen tannophilus in batch cultures. We propose a model to predict variations in D-xylose consumed, and biomass and ethanol produced, in which we include parameters for the specific growth rate, for the consumption of D-xylose and production of ethanol either related or not to growth.The ideal initial pH for ethanol production turned out to be 4.5. At this pH value the net specific growth rate was 0.26 h−1, biomass yield was 0.16 g.g−1, the cell-maintenance coefficient was 0.073 g.g−1.h−1, the parameter for ethanol production non-related to growth was 0.064 g.g−1,h−1 and the maximum ethanol yield was 0.32 g.g−1.

The Influence of pH upon the Kinetic Parameters of the Enzymatic Hydrolysis of Cellobiose with Novozym 188

We have studied experimentally within the pH range of 3.65−5.5 at 50 °C the hydrolysis of cellobiose with Novozym 188, a commercial product with high β‐1,4‐glucosidase activity derived from Aspergillus niger. We used wide variations in the conversion to be able to apply the integral method and thus determine that there is substrate and mixed product inhibition. Whether the SES triple compound contributes to the formation of glucose does not influence the fitting of the experimental results to the theoretical model to any significant extent. We have established how pH affects the kinetic parameters and ascertained that pH 4.3 is the optimum for the conversion of cellobiose into glucose.