Juan José Calvente

Influence of Olive Ripeness on the Concentration of Green Aroma Compounds in Virgin Olive Oil

Changes have been demonstrated in the concentration of those volatile compounds responsible for green sensory notes in virgin olive oils obtained from four different varieties at three stages of ripeness. The information corresponding to each volatile compound for each stage of ripeness, after fuzzy filtering of the quantitative data, has been established and the relationship between volatile compounds and green sensory attributes has also been demonstrated by means of principal components analysis, correlation and stepwise linear regression analysis.

Sensory Authentication of European Extra‐Virgin Olive Oil Varieties by Mathematical Procedures

Multidimensional scaling and a fuzzy filter qualifier were used together for authenticating and characterising four olive oil varieties-Picual and Arbequina (Spain), Coratina (Italy) and Koroneiki (Greece)-representative of the most important producer countries. Twenty four samples were evaluated by five panels of different nationalities-British, Greek, Italian and Spanish-these being representatives of traditional and potential consumers of virgin olive oil. Each sample was initially characterised by 74 sensory attributes. The paper analyses the most outstanding attributes and their evaluation intensities that characterise the varieties and gives the sensory profiling of each variety. Green attributes characterise Koroneiki variety, Arbequina variety is characterised by sweet-fruity sensory attributes, the bitter-pungent sensory attributes can identify Coratina while a strong fruity odour explains the sensory characteristics of Picual variety.

SNIFTIRS studies of the electrochemical double layer: Part II. Au(111) electrode in solutions with specifically adsorbed nitrate ions

Abstract In-situ IR reflection spectroscopy has been used to probe the structure of the double layer on an Au(111) electrode in solutions containing specifically adsorbed nitrate ions. The adsorbed nitrate ion is found to be bonded to the gold surface through one of its oxygen atoms. Both adsorbed nitrate ions, as well as those in the solution are found to possess C 2 v symmetry. Nitrate ions in the double layer form contact ion-pairs with hydronium ions, possibly with an extra water molecule incorporated in the ion-pair moiety. The extent of ion-pairing is found to depend on the applied potential. An indication of the onset of water chemisorption on the gold surface is also found with the present technique. On the basis of combined ATR and SNIFTIRS (subtractively normalized interfacial Fourier transform IR spectroscopy) experiments it is shown that the width of the deconvoluted nitrate band can be used to estimate the vicinity of the potential of zero charge (pzc). It was found that the pzc in 0.01 M HNO 3 is close to that observed in 0.01 M HClO 4 , and that it shifts by ca. −100 mV per pH unit due to the specific adsorption of nitrate.

Folding and Unfolding in the Blue Copper Protein Rusticyanin:Role of the Oxidation State

The unfolding process of the blue copper protein rusticyanin has been studied from the structural and the thermodynamic points of view at two pH values (pH 2.5 and 7.0). When Rc unfolds, copper ion remains bound to the polypeptide chain. Nuclear magnetic resonance data suggest that three of the copper ligands in the folded state are bound to the metal ion in the unfolded form, while the other native ligand is detached. These structural changes are reflected in the redox potentials of the protein in both folded and unfolded forms. The affinities of the copper ion in both redox states have been also determined at the two specified pH values. The results indicate that the presence of two histidine ligands in the folded protein can compensate the change in the net charge that the copper ion receives from their ligands, while, in the unfolded protein, charges of aminoacids are completely transferred to the copper ion, altering decisively the relative stability of its two-redox states.

Quantitative characterization of desorptive stripping voltammograms complicated by surface dimerization reactions. Application to the reductive desorption of thiols from mercury

Abstract The influence of analyte adsorption and of a follow-up surface dimerization reaction on desorptive stripping voltammograms has been investigated. A modified version of the spline orthogonal collocation technique has been developed to compute the voltammetric response under Langmuirian reversible conditions. Simple analytical expressions derived for surface-confined redox processes are shown to reproduce the stripping voltammograms only in the presence of significant ( θ * R >0.01) analyte adsorption. In the absence of analyte adsorption, asymmetrical waves are obtained as an intrinsic characteristic of the stripping protocol. Empirical equations relating the stripping peak potential and the voltammetric half-height width with scan rate and adsorbed product coverage are presented. Voltammetric scan rate dependence is shown to provide a quantitative estimate of the analyte adsorption extent, whereas its dependence on adsorbed product coverage allows one to identify the presence of surface dimerization processes. Maximum surface excesses and dimerization equilibrium constants can readily be obtained by plotting the reciprocal half-height width versus the surface concentration of stripped species. In the absence of analyte adsorption, a complementary convolutive analysis is also proposed. Application is made to the cathodic stripping of three mercaptocarboxylic acids differing in their hydrocarbon chain-length. The extent of oxidized product dimerization is shown to display a strong dependence on the solution pH, so that higher peak current sensitivities are found in acidic solutions, where oxidized products are present as dimers.

Limiting partial molar volumes of electrolytes in dimethylformamide–water mixtures at 298.15 K

Limiting partial molar volumes of electrolytes in several water-dimethyl sulfoxide mixtures, up to xDMSO= 0.1587, have been determined from density measurements at 298.15 K. A reference electrolyte assumption (TATB) has been used in order to obtain single-ion partial molar volumes of transfer from water to DMSO–water mixtures. The influence of DMSO on ΔtV′0i of alkali metals and halide ions is negligible in the highly water-rich region (up to xDMSO≈ 0.04) except for the I– ion. However, when the DMSO composition is greater than xDMSO≈ 0.04, the variation of ΔtV′0i for alkali metals is the reverse to that of halide ions. The results are discussed on the basis of ion–solvent and solvent–solvent interactions using Conways model.

SNIFTIRS studies of the electrical double layer at gold electrodes

Abstract Studies of double layer structure at gold electrodes using subtractively normalized interfacial FTIR spectroscopy are presented. It is shown that, when a hemispherical window is used to introduce the infrared radiation into the electrochemical cell, a significant increase in sensitivity results. Data are presented illustrating the reorientation of solvent dipoles in the double layer. In addition, results obtained for the accumulation of perchlorate and nitrate ions at single crystal gold electrodes are discussed. In the former case, analysis of the integrated intensity of the perchlorate band at 1109 cm −1 is used to show the potential region over which the surface excess follows the Gouy–Chapman theory and the potential region in which perchlorate ion is specifically adsorbed.

Proton transfer voltammetry at electrodes modified with acid thiol monolayers.

By combining a description of the potential profile at electrodes coated with acid thiol monolayers with a quadratic relationship between activation energy and electrode potential, a rather simple expression for proton transfer voltammograms is derived. Our electrostatic analysis shows that proton transfer can only produce narrow voltammetric peaks when the immobilized acid groups lie close to the metal substrate. Quantitative fits of experimental voltammograms obtained with an Au(111) electrode modified with a 11-mercaptoundecanoic monolayer at pH 8.5 reveal that less than 1% of the carboxylic groups in the monolayer participate in the potential induced proton transfer process and that these groups lay close to the metal surface. A preliminary analysis of the kinetic parameters suggests that the interfacial electric field facilitates an intrinsically slow proton exchange between a proton donor and acceptor pair that are not in close contact with each other at the interface.

Reorientation of Thiols during 2D Self-Assembly: Interplay between Steric and Energetic Factors

Reorientation of thiols during their 2D self-assembly is well established; however, little is known about its energetics and the factors that control its onset. We have developed a new strategy to determine the critical reorientational surface concentration (crsc) of thiols at the substrate/solution interface, which makes use of a cathodic stripping protocol. Its application to distinct homologous series of alkylthiols shows that the magnitude of the crsc and its variation with the molecular size is strongly dependent on the nature of the terminal group. Methyl-terminated alkylthiols reorient close to the saturation coverage of the lying-down phase, thus following their molecular size trend; whereas reorientation of alkylthiols bearing a negatively charged end group starts well below the monolayer coverage of the lying-down phase, with its onset being almost independent of the molecular size. Hydroxy-terminated alkylthiols show an intermediate behavior. A theoretical approach is developed to determine the reorientation equilibrium constant from the crsc value. The standard free energy of reorientation has been found to vary linearly with the alkyl chain length, and to increase upon replacing the terminal methyl group by a negatively charged one. A quantitative correlation between the reorientation equilibrium constant and the hydrophobicity of the molecule has been established. Overall, these findings have allowed us to disentangle the role of steric and energetic factors in the onset of the reorientation process of alkylthiols, demonstrating that their interplay can be finely tuned by varying either the alkyl chain length or the nature of the terminal group.

Influence of temperature on the reduction kinetics of Zn2+ at a mercury electrode

The reduction of Zn 2 at a mercury electrode has been studied by the a.c. impedance technique in the 263/318 K temperature range. Kinetic data have been analyzed in terms of a mechanism consisting of two consecutive charge transfer steps. The apparent activation enthalpies and charge transfer coefficients of both steps are found to be temperature independent. Differential capacity curves and potentials of zero charge of the NaClO4 supporting electrolyte solutions were measured at each temperature of interest. Double layer effects on the Zn 2 reduction rate constants were analyzed within the framework of the unequal distances of closest approach theory, and the corrected rate constants were found to be independent of supporting electrolyte concentration when allowance for changes in the Zn 2 activity coefficient was made. The implications of the activation parameter values on the nature of the rate determining steps are also discussed. # 2002 Elsevier Science B.V. All rights reserved.