Amelia C. Rubiolo

Determination of kinetics parameters for free and immobilized β-galactosidase

Abstract The kinetic parameters of soluble and immobilized β-galactosidase of Kluyveromyces fragilis were determined using integrated reaction rate equations. Three reaction rate models were used to correlate experimental values by means of a multi-response non-linear computer program. The deviations of the fitted models to the data were compared using a residual sum of squares and F-test for nested models. The Michaelis-Menten equation predicted the best values with product inhibition by galactose and provided the most reasonable approach to describe the enzymic hydrolysis of lactose for low and high conversion in a range of initial concentrations from 2·5 to 15·0%.

A method for evaluating lactose hydrolysis in a fixed bed reactor with β-galactosidase immobilized on chitosan

Abstract A model for evaluating lactose hydrolysis with immobilized enzyme in a packed bed reactor was developed. It is assumed that the conditions were plug flow and Michaelis-Menten kinetics with product competitive inhibition. The effects on the reaction rate of the external mass transfer resistance were considered by estimating the total fluid mass transfer coefficient. Experimental results were determined using β -galactosidase of Kluyveromices fragilis immobilized on chitosan beads in a packed bed reactor under special conditions, in order to calculate the parameters for different flow ranges and substrate concentrations. The model with these values allowed us to fit the reactor behavior for a wider range of work.

Thermal conductivity of unfrozen and frozen strawberry and spinach

Abstract The objective of this work was to develop an appropriate measurement system to collect accurate data and obtain thermal conductivity values for some fruits and vegetables and for temperatures below the freezing point, since there is limited information available, particularly for vegetables such as spinach, for which the values obtained experimentally are dissimilar to those predicted from composition. The effects of various experimental conditions were also considered, to optimize the line heat source technique used for the measurements. Fresh and frozen strawberry (Tioga variety) and spinach samples as well as blanched spinach were tested in the temperature range from −21 to +28 °C. The differences between the thermal conductivity values obtained for a specified temperature range and those cited in the literature showed that the data acquisition system was good and the line heat source technique worked satisfactorily.

Crosslinking kinetics of cation-hydrocolloid gels

Abstract The gel formation of hydrocolloid (sodium alginate and kappa-carrageenan) is the result of the molecule aggregation in presence of an effective cation. The gelling process was considered as a process of diffusion–reaction between the effective cation and the hydrocolloid to produce a porous solid structure. A simple mathematical model for predicting the reaction interface position in alginate or alginate-carrageenan gelation was developed. The model represents appropriately the physicochemical phenomenon and allows to determine the time for complete crosslinking of the hydrocolloid and the amount of incorporated ions.

Prediction of the fixed-bed reactor behaviour using dispersion and plug-flow models with different kinetics for immobilised enzyme

Abstract The behaviour of fixed-bed reactors, which have an immobilised enzyme on the packing surface, was studied considering steady-state conditions and external mass transfer resistance in the fluid around catalyst spherical particles. Solutions were obtained by integration of the plug-flow model equation and by the orthogonal collocation method of the second order differential equation of the axial dispersion flow model. Both models were analysed for lactose hydrolysis with β-galactosidase immobilised on chitosan using different kinetic reaction mechanisms after determining the specific parameters. The calculated results show the importance of the hydrodynamic and kinetic reaction parameters for error reduction in the prediction of the experimental behaviour.

A model for using the diffusion cell in the determination of multicomponent diffusion coefficients in gels or foods

Abstract A model for a multicomponent system using the cell for determining apparent diffusion coefficients in gels and foods was developed. In this case, the generalized Ficks law form was used as a constitutive equation for the diffusive molar flux of solutes. Using the method of eigenvectors and eigenvalues, the mass transfer equations were rearranged to simplify the mathematical treatment. Simple expressions for determining the diffusion coefficients of two or more solutes which diffuse in a rigid medium were obtained. In this case, experiments to study the diffusion of NaCl and KCl in cheese were shown and the corresponding effective diffusion coefficients were obtained.

Evaluation of stress-strain for characterization of the rheological behavior of alginate and carrageenan gels

The stress-strain of samples deformed until failure and the relaxation response after 50% deformation of the initial height under constant stress were obtained. Uniaxial compression and stress-relaxation tests enabled satisfactory differentiation of the mechanical resistance of gels with different alginate and carrageenan concentrations. Higher values for initial force at the beginning of the relaxation test were associated with higher calcium uptake by the gels. An increment of failure stress during the uniaxial compression tests for higher concentration of calcium in the gel structure was also observed. The maximum amount of cation uptake was higher than the theoretical value for saturation of all the carboxylic groups available in alginate molecules due to structural rearrangements. Stress-relaxation tests indicated that the residual stress of the gel increased with k-carrageenan concentration.

Screening and selection of wild strains for L-arabinose isomerase production

The majority of L-arabinose isomerases have been isolated by recombinant techniques, but this methodology implies a reduced technological application. For this reason, 29 bacterial strains, some of them previously characterized as L-arabinose isomerase producers, were assayed as L-arabinose fermenting strains by employing conveniently designed culture media with 0.5% (w/v) L-arabinose as main carbon source. From all evaluated bacterial strains, Enterococcus faecium DBFIQ ID: E36, Enterococcus faecium DBFIQ ID: ETW4 and Pediococcus acidilactici ATCC ID: 8042 were, in this order, the best L-arabinose fermenting strains. Afterwards, to assay L-arabinose metabolization and L-arabinose isomerase activity, cell-free extract and saline precipitated cell-free extract of the three bacterial cultures were obtained and the production of ketoses was determined by the cysteine carbazole sulfuric acid method. Results showed that the greater the L-arabinose metabolization ability, the higher the enzymatic activity achieved, so Enterococcus faecium DBFIQ ID: E36 was selected to continue with production, purification and characterization studies. This work thus describes a simple microbiological method for the selection of L-arabinose fermenting bacteria for the potential production of the enzyme L-arabinose isomerase.

Response Surface Methodology To Optimize β‐Galactosidase Immobilization Using a Combination of Hydrocolloids as the Supporting Matrix

Response surface methodology (RSM) was applied to optimize the composition of a mixture of hydrocolloids (low‐viscosity sodium alginate, high‐viscosity sodium alginate, and κ‐carrageenan) used in the immobilization of β‐galactosidase for application in the hydrolysis of lactose. A five levels, a three‐factor design was adopted. The activity and stability of the immobilized enzyme and the strength of the supporting matrix were optimized by using a mathematical model applied in the range of process conditions. Requirements for multifactor response surface designs were satisfied, and the correlation coefficient, R2, was larger than 0.850, ensuring a good adjustment of the model to the experimental values. Best results were obtained for values of 1.00% low‐viscosity sodium alginate, 1.40−1.60% high‐viscosity sodium alginate, 0.10−0.30% κ‐carrageenan, and 10−12% enzyme.

Calcium Uptake during Immersion of Strawberries in CaCl2 Solutions

The present study studied calcium uptake in strawberries during immersion in CaCl(2) solutions by analyzing total calcium and cell-wall calcium contents and the relationships between cell-wall calcium content, pectin content, and the degree of pectin esterification, considering different temperatures and concentrations of the immersion solution. In addition, calcium uptake at different zones of the strawberries was studied. Calcium uptake increased significantly when the concentration of the immersion solution increased from 1% to 10% (v/v), and calcium uptake was higher at 38 degrees C at some immersion times, but there were no significant differences between 1 and 24 degrees C. Most of the calcium incorporated by the strawberries was retained in the alcohol insoluble solids and it may be related to calcium bounded to the cell wall. Calcium content did not change during immersion in CaCl(2) solution in the internal or in the central zones; however, it increased significantly in the external zone.