Franco Magno

Effect of nitrogen supplementation and saccharomyces species on hydrogen sulfide and other volatile sulfur compounds in Shiraz fermentation and wine

A Shiraz must with low yeast assimilable nitrogen (YAN) was supplemented with two increasing concentrations of diammonium phosphate (DAP) and fermented with one Saccharomyces cerevisiae and one Saccharomyces bayanus strain, with maceration on grape skins. Hydrogen sulfide (H(2)S) was monitored throughout fermentation, and a total of 16 volatile sulfur compounds (VSCs) were quantified in the finished wines. For the S. cerevisiae yeast strain, addition of DAP to a final YAN of 250 or 400 mg/L resulted in an increased formation of H(2)S compared to nonsupplemented fermentations (100 mg/L YAN). For this yeast, DAP-supplemented fermentations also showed prolonged formation of H(2)S into the later stage of fermentation, which was associated with increased H(2)S in the final wines. The S. bayanus strain showed a different H(2)S production profile, in which production was inversely correlated to initial YAN. No correlation was found between total H(2)S produced by either yeast during fermentation and H(2)S concentration in the final wines. For both yeasts, DAP supplementation yielded higher concentrations of organic VSCs in the finished wines, including sulfides, disulfides, mercaptans, and mercaptoesters. PCA analysis indicated that nitrogen supplementation before fermentation determined a much clearer distinction between the VSC profiles of the two yeasts compared to nonsupplemented fermentations. These results raise questions concerning the widespread use of DAP in the management of low YAN fermentations with respect to the formation of reductive characters in wine.

Steady state voltammetry at microelectrodes for the hydrogen evolution from strong and weak acids under pseudo-first and second order kinetic conditions

Abstract The reaction of hydrogen evolution involving discharge of protons arising from strong and weak acids, namely perchloric, nitric, acetic, ethylenediaminetetra-acetic and dihydrogen phosphoric acids, was investigated in the concentration range 0.1–10 mM by steady state voltammetry at inlaid platinum micro-disc electrodes (of 10–12.5 μm radius). The second-order CE mechanism for the weak acids was rationalised by digital simulation, demonstrating the effects of the various kinetic, thermodynamic and diffusion parameters on the reduction process. In particular, it was found that proton reduction from acetic acid is associated with a completely mobile equilibrium over the whole concentration range studied, but in buffer solutions containing a large excess of the conjugate base the CE process becomes affected kinetically. For the other two weak acids the chemical step controls the kinetics at any concentration employed.

Amperometric Mediated Carbon Nanotube Paste Biosensor for Fructose Determination

Abstract A new mediated carbon nanotube paste (CNTP) amperometric biosensor for fructose is described. The biosensor is formed by a CNTP electrode modified with an electropolymerized film of 3,4‐dihydroxybenzaldehyde (3,4‐DHB) and is based on the activity of a commercial available D‐fructose dehydrogenase (FDH) immobilized on an immobilon membrane placed on the top of the electrode surface. Analytical parameters such as enzyme immobilization, pH, temperature, and probe lifetime were studied and optimized. The biosensor response current was directly proportional to D‐fructose concentration from 5 × 10−6 to 2 × 10−3 mol/L with a detection limit of 1 × 10−6 mol/L and a good reproducibility (RSD = 1.8%, n = 5). The biosensor was used for the determination of fructose content in three honey samples and validated with a commercial spectrophotometric enzymatic kit.

Performance of a numerical method based on the hopscotch algorithm and on an oblate spheroidal space coordinate-expanding time grid for simulation of v

Abstract The application of a simulation procedure which combines the Hopscoth alogrithm with a hyperbolic space grid in solving the initial value problems at an inlaid disk electrode for electrochemical processes of any reversibility degree is discussed. The importance of the resolution of the space and time grid in connection with the use of the Hopscotch algorithm and of the Butler-Volmer equation to gain good accuracy, is stressed. The improvements introduced to define analytically the diffusion layer at different mass transfer conditions and to save computing time with the use of an exponentially expanding time grid and a cubic-spline interpolation make the simulation algorithm suitable to regression procedures for parameter estimation.

On the reaction kinetics of electrogenerated superoxide ion with aryl benzoates

Abstract The reactions between electrogenerated superoxide ion O 2 ⨪ and two benzoate esters in N, N-dimethylformamide containing tetrabutylammonium perchlorate have been investigated. By using cyclic voltammetric techniques an e.c.e. type mechanism was found to be operative; this was confirmed by controlled potential electrolysis experiments. Secondorder rate constants for nucleophilic displacement by O 2 ⨪ on p -chlorophenyl benzoate and phenyl benzoate were estimated to be 25.0±5 and 3.0±0.3 M −1 s −1 respectively by fitting potential step reduction data to i k / i d vs. log kt working curves.

Electrochemical behaviour of the bromide ion at a platinum electrode in acetonitrile solvent

Summary The voltammetric behaviour of the bromide ion at a platinum electrode in acetonitrile solvent has been investigated. The occurrence of three subsequent anodic processes has been established. Experimental evidence of the electrochemical formation of the Br + species is gained and suggestions about the reaction mechanism are advanced.

Electrochemical behaviour of diphenyl sulfide in aceto-nitrile medium at a platinum electrode

Summary The electrochemical behaviour of PhSPh at a platinum electrode in CH 3 CN solvent has been studied. PhSPh undergoes three irreversible consecutive oxidation steps. The anodic products have been identified as sulfonium salts. Suggestions are advanced for the mechanisms of the anodic reactions.

Electrochemical oxidation of phenyldisulfide in acetonitrile medium

Summary The electrochemical oxidation of phenyldisulfide at a platinum electrode has been studied in acetonitrile with NaClO 4 and fluoroborate salts as supporting electrolytes. The cations initially generated are shown to react in different ways depending on the nature of the supporting electrolyte. With NaClO 4 , evidence of ClO 2 , Cl − and PhSO 3 Na was obtained. With fluoroborate salts it is suggested that cations electrochemically generated are involved in chemical reactions with themselves and the solvent.

Redox properties of the nickel(II),(I),(0)-triphenylphosphine system in acetonitrile

The cathodic behaviour of electrochemically generated nickel(II) has been investigated in acetonitrile in the presence of triphenylphosphine at a platinum electrode. An appropriate combination of voltammetric, spectrophotometric and NMR findings has allowed us to establish that Ni(II) is present in solution as [Ni(PPh3)2 (CH3CN)42+.]. For the reduction of this species an EECE mechanism is proposed which is consistent with the data. It undergoes an irreversible two-electron reduction giging the Ni(0) complex [Ni(PPh3)4] which reacts quickly with the depolarizer. In this last homogeneous redox reaction the not previously reported Ni(I) complex [Ni(PPh3)4+] is obtained. The degree of reversibility of the redox processes involved has been discussed taking into account the structure, the coordination number and the nature of the ligands in both the redox partners.

Overall calibration procedure via a statistically based matrix-comprehensive approach in the stir bar sorptive extraction–thermal desorption–gas chromatography–mass spectrometry analysis of pesticide residues in fruit-based soft drinks

Stir bar sorptive extraction (SBSE)-thermal desorption (TD) procedure combined with gas chromatography mass spectrometry (GC-MS) and the statistical variance component model (VCM) is applied to the determination of semi-volatile compounds including organochlorine and organophosphorus pesticides in various synthetic and real fruit-based soft drink matrices. When the matrix effects are corrected using isotopically labelled or non labelled internal standard, but matrix/calibration run-induced deviations are still present in the measurements, the adoption of a variance component model (VCM) in the quantitative analysis of various matrices via an overall calibration curve is successful. The method produces an overall calibration straight line for any analyte accounting for the uncertainty due to all the sources of uncertainty, namely matrix-induced deviations, calibration runs performed at different times, measurement errors. Small increases in the detection limits and in uncertainty in the concentration values obtained in the inverse regression face favourably the decrease in times and costs for routine analyses.