F. Camacho

A new kinetic model proposed for enzymatic hydrolysis of lactose by a β-galactosidase from Kluyveromyces fragilis

We study the enzymatic hydrolysis of lactose by a commercial enzyme from a selected strain of Kluyveromyces fragilis. The variables analyzed were: temperature (25–40 ◦ C), enzyme concentration (0.1–3.0 g l −1 ), lactose concentration (0.0278–0.208 M), and initial galactose concentration (0.0347 M). On the basis of the data analyzed, both published and in the present work, we propose a Michaelis–Menten kinetic model with inhibition by the product (galactose), which reveals that the substrate (lactose) and the product (galactose) present similar affinity for the active site of the enzyme.

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.

Application of the sodium dithionite oxidation to measure oxygen transfer parameters

Abstract The oxidation of sodium dithionite with molecular oxygen in alkaline solutions takes place in two steps: in the first, dithionite oxidizes to sulphite and sulphate, S 2 O 4 2− + O 2 + 2OH − →SO 3 2− + SO 4 2− + H 2 O which occurs in the fast reaction regime; within this step the oxygen concentration in the bulk liquid is effectively zero and the kinetics can be followed by the decrease in the solutions reducing power or by the consumption of base required to keep the pH constant. These results allow us to determine the specific interfacial area. In the second step, sulphite oxidizes to sulphate: No 2 a = − 1 2 d [ SO 3 2 ] d t = − 1 4 d c d t which takes place in the hydrodynamic regime without catalytic agents deliberately added and allows us to determine the volumetric oxygen transfer coefficient. However, when this method is applied using air as the gaseous phase, it leads to values of both parameters of around 90% of those that were obtained using pure oxygen. These results may be explained by the wide range of the bubble size distribution. This implies that the actual average driving-force of the oxygen transfer using air is even lower than that obtained with the approximation of perfect mixing used.

Correlation of base consumption with the degree of hydrolysis in enzymic protein hydrolysis

It is fairly easy to control the enzymic hydrolysis of proteins in alkaline conditions by measuring the base consumption required to keep the pH constant in the reactor. Unfortunately, however, base consumption is not related in any simple way to the degree of hydrolysis reached at any given moment and to establish this relationship it is essential to find out the mean pK of the alpha-amino groups released during the hydrolytic process. We have shown here that the correct mean pK value varies according to the pH of the working conditions and that the relationship between these values may depend upon the kind of protein and protease used. We have put forward a method for determining this relationship experimentally by using a given protein-protease system, consisting of an alkaline titration of the raw protein and when partially hydrolysed. We have tested the results predicted by our theoretical model by applying it to the hydrolysis of whey proteins with a bacterial protease from Bacillus licheniformis at 50 degrees C, pH 8.0. This model can easily be applied to any hydrolytic process involving the appearance of functional groups that are partially protonizable under the working conditions in question in order to follow the kinetics of the reaction via the consumption of the neutralizing agent required to keep pH constant.

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.

Influence of enzymes, pH and temperature on the kinetics of whey protein hydrolysis / Influencia de los enzimas, pH y temperatura en la cinética de la hidrólisis de las proteínas del lactosuero

The influence of pH, temperature and the mixture of enzymes (MKC Protease 660 L and PEM 2500 S) on the enzymatic hydrolysis of whey proteins was studied. The experiments show that all results were reproducible via a kinetic model that supposes the rapid and irreversible binding of part of the proteases to an inhibitor in the substrate, followed by a zero-order hydrolysis with respect to the substrate which occurs simultaneously with a second order enzymatic denatural ization produced by an attack of the free proteases upon those bound to the substrate-enzyme complex. Use of the optimum operating temperature of 60 °C and pH 8-10 led to a greater degree of hydrolysis. However, increasing the pH to these levels means that the salt content, on neutral izing the hydrolysate, is somewhat high and this is often unsuitable for the preparation of special diets. In the experiments performed with mixtures of enzymes, two contrasting phenomena occurred; there appears to be synergism between the proteases, which is preceded by a loss in enzymatic activity greater than that which can be accounted for by the presence of the inhibitor in the whey proteins.

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.

A simple method for obtaining kinetic equations to describe the enzymatic hydrolysis of biopolymers

A procedure to obtain the overall rate of hydrolysis of biopolymers is proposed, based on the fitting of the experimental data x = f(t) to cubic spline functions and from these, by differentiation, to obtain dx/dt. The values of these dx/dt slopes are an exclusive function of the conversion, x, when E 0 , S 0 , pH and temperature are constant. The fitting of dx/dt versus x leads to equations of the type dx / dt = a.x exp(-b. x) for the glucoamylase-starch system, where b = 8.75 and a = f(E 0 , T).