António M. Peres

Thermodynamic properties of sugars in aqueous solutions: correlation and prediction using a modified UNIQUAC model

Abstract A modified UNIQUAC model with linear temperature-dependent parameters is proposed to describe the thermodynamics properties (water activity, osmotic coefficient, vapor pressure, boiling temperature, freezing point and solubility data) of three binary systems: D-glucose/water, D-fructose/water and sucrose/water. New UNIQUAC interaction parameters are presented, estimated using the above mentioned binary data, collected from the literature. Good predictions of vapor-liquid and solid-liquid equilibrium data for ternary and quaternary mixtures of D-glucose, D-fructose, sucrose and water components are obtained. This approach uses the symmetric convention for all the components, pure fused sugar at solution pressure and temperature being its reference state. Therefore, it allows straightforward calculations of solid-liquid equilibria of sugars in mixed solvents. This constitutes an important advance over the approach based on the knowledge of the dilution enthalpy data available in the literature.

A modified UNIFAC model for the calculation of thermodynamic properties of aqueous and non-aqueous solutions containing sugars

A UNIFAC based model is proposed to describe experimental thermodynamic properties of aqueous and non-aqueous solutions containing common sugars, e.g., d -glucose, d -fructose and sucrose. A new group ‘OHring’ is introduced due to proximity effects. New temperature-independent UNIFAC interaction parameters are estimated based on the binary data available for both d -glucose and d -fructose and only on the solubility data for sucrose. The UNIFAC model used in this work is able to accurately predict the water activity, boiling temperature and freezing point of aqueous sucrose solutions. However, the predictions of the osmotic coefficients for the sucrose/water system present relative deviations up to 9%. Satisfactory predictions of vapor-liquid and solid-liquid equilibrium data for ternary and quaternary mixtures of d-fructose, sucrose and water components are also obtained. This approach uses the symmetric convention for all the components, i.e. the pure fused sugar at solution pressure and temperature is its reference state. Therefore, it allows to carry out in a straightforward way, calculations of solid-liquid equilibria of sugars in mixed solvents. It is shown that the new UNIFAC model is able to qualitatively predict the ternary solubility data of d -glucose d -xylose, d -mannose and sucrose in mixed solvent mixtures of ethanol/water, methanol/water and ethanol/methanol. A comparison with a modified UNIQUAC—essentially correlative model—and a UNIFAC—correlative and predictive model—approaches available in the literature is also given: the performance of the model proposed in this work is in general inferior to that of the UNIQUAC model, however it gives always a better agreement with the experimental data when compared with another UNIFAC based model available in the literature.

Comparison of adsorption equilibrium of fructose, glucose and sucrose on potassium gel-type and macroporous sodium ion-exchange resins.

Adsorption equilibrium of fructose, glucose and sucrose was evaluated on sulfonated poly(styrene-co-divinylbenzene) cation-exchange resins. Two types of resins were used: potassium (K+) gel-type and sodium (Na+) macroporous resins. Influence of the cation and effect of the resin structure on adsorption were studied. The adsorption isotherms were determined by the static method in batch mode for mono-component and multi-component sugar mixtures, at 25 and 40 degrees C, in a range of concentrations between 5 and 250 g L(-1). All adsorption isotherms were fitted by a linear model in this range of concentrations. Sugars were adsorbed in both resins by the following order: fructose > glucose > sucrose. Sucrose was more adsorbed in the Na+ macroporous resin, glucose was identically adsorbed, and fructose was more adsorbed in the K+ gel-type resin. Data obtained from the adsorption of multi-component mixtures as compared to the mono-component ones showed a competitive effect on the adsorption at 25 degrees C, and a synergetic effect at 40 degrees C. The temperature increase conducted to a decrease on the adsorption capacity for mono-component sugar mixtures, and to an increase for the multi-component mixtures. Based on the selectivity results, K+ gel-type resin seems to be the best choice for the separation of fructose, glucose and sucrose, at 25 degrees C.

Single-cultivar extra virgin olive oil classification using a potentiometric electronic tongue

Label authentication of monovarietal extra virgin olive oils is of great importance. A novel approach based on a potentiometric electronic tongue is proposed to classify oils obtained from single olive cultivars (Portuguese cvs. Cobrançosa, Madural, Verdeal Transmontana; Spanish cvs. Arbequina, Hojiblanca, Picual). A meta-heuristic simulated annealing algorithm was applied to select the most informative sets of sensors to establish predictive linear discriminant models. Olive oils were correctly classified according to olive cultivar (sensitivities greater than 97%) and each Spanish olive oil was satisfactorily discriminated from the Portuguese ones with the exception of cv. Arbequina (sensitivities from 61% to 98%). Also, the discriminant ability was related to the polar compounds contents of olive oils and so, indirectly, with organoleptic properties like bitterness, astringency or pungency. Therefore the proposed E-tongue can be foreseen as a useful auxiliary tool for trained sensory panels for the classification of monovarietal extra virgin olive oils.

Phase equilibria of d-glucose and sucrose in mixed solvent mixtures: Comparison of UNIQUAC 1-based models

Abstract Solid-liquid equilibrium measurements of ternary sucrose-mixed solvent mixtures at 25, 40, and 60 °C are carried out, using a simple and accurate analytical method. A search for a reliable activity coefficient model for sugar liquid mixtures is performed: three UNIQUAC based models are investigated—modified UNIQUAC, entropic free-volume and original Flory-Huggins. New UNIQUAC interaction parameters are estimated based on the aqueous and non-aqueous experimental data presented in this work together with those available in the literature. The solubility calculations were carried out using an equation based on fusion enthalpy data for the sugars. Although the UNIQUAC-based models investigated give in general a quite satisfactory representation of different thermodynamic properties of aqueous solutions containing d -glucose, d -fructose, and/or sucrose, it is shown that only the modified UNIQUAC model is able to accurately describe the ternary solubility data of d -glucose and sucrose in mixed solvent mixtures. Various UNIQUAC-based procedures were applied to ternary sucrose-mixed solvent mixtures. A modified UNIQUAC model was able to describe the ternary solubility data of d -glucose and sucrose in mixed solvents.

New improved method for fructooligosaccharides production by Aureobasidium pullulans

Fructooligosaccharides are prebiotics with numerous health benefits within which the improvement of gut microbiota balance can be highlighted, playing a key role in individual health. In this study, an integrated one-stage method for FOS production via sucrose fermentation by Aureobasidium pullulans was developed and optimized using experimental design tools. Optimization of temperature and agitation speed for maximizing the FOS production was performed using response surface methodology. Temperature was found to be the most significant parameter. The optimum fermentation conditions were found to be 32 °C and 385 rpm. Under these conditions, the model predicted a total FOS production yield of 64.7 gFOS/gsucrose. The model was validated at optimal conditions in order to check its adequacy and accuracy and an experimental yield of 64.1 (±0.0) gFOS/gsucrose was obtained. A significant improvement of the total FOS production yields by A. pullulans using a one-stage process was obtained.

Sensory intensity assessment of olive oils using an electronic tongue

Olive oils may be commercialized as intense, medium or light, according to the intensity perception of fruitiness, bitterness and pungency attributes, assessed by a sensory panel. In this work, the capability of an electronic tongue to correctly classify olive oils according to the sensory intensity perception levels was evaluated. Cross-sensitivity and non-specific lipid polymeric membranes were used as sensors. The sensor device was firstly tested using quinine monohydrochloride standard solutions. Mean sensitivities of 14±2 to 25±6 mV/decade, depending on the type of plasticizer used in the lipid membranes, were obtained showing the device capability for evaluating bitterness. Then, linear discriminant models based on sub-sets of sensors, selected by a meta-heuristic simulated annealing algorithm, were established enabling to correctly classify 91% of olive oils according to their intensity sensory grade (leave-one-out cross-validation procedure). This capability was further evaluated using a repeated K-fold cross-validation procedure, showing that the electronic tongue allowed an average correct classification of 80% of the olive oils used for internal-validation. So, the electronic tongue can be seen as a taste sensor, allowing differentiating olive oils with different sensory intensities, and could be used as a preliminary, complementary and practical tool for panelists during olive oil sensory analysis.

Representation of solubilities of amino acids using the UNIQUAC model for electrolytes

Abstract A molecular thermodynamic framework for the representation of the solubilities of several binary systems, amino acids—water (nine systems) and peptides—water (five systems), has been developed and tested. The proposed model takes into account simultaneously the chemical and physical equilibrium. The latter is described by the UNIQUAC model combined with a Debye—Huckel term. Molal activity coefficient data from the literature were used in order to estimate the new UNIQUAC parameters. These parameters, together with solubility data, were the basis for the correlation of the solubility product of the different amino acids considered in this work. With this approach, very satisfactory results have been obtained for the representation and prediction of the solubilities of amino acids in water as a function of both temperature and pH.

Oleuropein/ligstroside isomers and their derivatives in Portuguese olive mill wastewaters

Olive mill wastewaters (OMW) are a potential source of biophenols, but they have a complex composition with many unknown phenolics. The analysis of purified methanol extracts from two Portuguese OMW by electrospray mass spectrometry in the negative mode showed [M-H]- ions at m/z 539 and m/z 523, corresponding respectively to oleuropein and ligstroside isomers which contain the glucose unit linked to its aromatic moiety. Also, the fragmentation pathway of the [M-H]- ions at m/z 863, 685 and 847 indicated the presence of a diglucoside derivative of the oleuropein isomer and of mono- and diglucosides of the ligstroside isomer, respectively. Moreover, the two OMW samples contained an elenoic derivative of the ion at m/z 685 and a degradation product (m/z 453) of the [M-H]- ion at m/z 523. Future studies focusing on the abundance of these compounds on OMW, as well as their bioactivities, will determine their possible industrial exploitation.

Voltammetric aptasensors for protein disease biomarkers detection: a review

An electrochemical aptasensor is a compact analytical device where the bioreceptor (aptamer) is coupled to a transducer surface to convert a biological interaction into a measurable signal (current) that can be easily processed, recorded and displayed. Since the discovery of the Systematic Evolution of Ligands by Enrichment (SELEX) methodology, the selection of aptamers and their application as bioreceptors has become a promising tool in the design of electrochemical aptasensors. Aptamers present several advantages that highlight their usefulness as bioreceptors such as chemical stability, cost effectiveness and ease of modification towards detection and immobilization at different transducer surfaces. In this review, a special emphasis is given to the potential use of electrochemical aptasensors for the detection of protein disease biomarkers using voltammetry techniques. Methods for the immobilization of aptamers onto electrode surfaces are discussed, as well as different electrochemical strategies that can be used for the design of aptasensors.