Marek Sikorski

Characterization of Edible Oils Using Total Luminescence Spectroscopy

Total luminescence spectroscopy was used to characterise and differentiate edible oils and additionally, to control one of the major problems in the oil quality—the effect of thermal and photo-oxidation. We studied several vegetable oils available on the Polish market, including soybean, rapeseed, corn, sunflower, linseed and olive oils. Total luminescence spectroscopy measurements were performed using two different sample geometries: front-face for pure oil samples and right-angle for transparent samples, diluted in n-hexane. All the samples studied as n-hexane solutions exhibit an intense peak, which appears at 320 nm in emission and 290 nm in excitation, attributed to tocopherols. Some of the oils exhibit a second long-wavelength peak, appearing at 670 nm in emission and 405 nm in excitation, belonging to pigments of the chlorophyll group. Additional bands were present in the intermediate range of excitation and emission wavelengths; however, the compounds responsible for this emission were not identified. The front-face spectra for pure oils included chlorophyll peaks for most samples, and some additional peaks in the intermediate range, while the tocopherol peaks were comparatively less intense. The results presented demonstrate the capability of the total luminescence techniques to characterise and differentiate vegetable oil products, and additionally, to characterize the effect of thermal and photo-oxidation on such products. In the photo-oxidation experiments, special attention was paid to possible involvement of singlet oxygen. Experiments were done to monitor the highly specific O2 (1Δg) → O2(3Σ−) singlet oxygen emission at 1270 nm. Thus, total luminescence spectroscopy presents an interesting alternative to time-consuming and expensive techniques such as gas or liquid chromatography, mass spectrometry and other methods requiring wet chemistry steps.

Simultaneous analysis of riboflavin and aromatic amino acids in beer using fluorescence and multivariate calibration methods

The study demonstrates an application of the front-face fluorescence spectroscopy combined with multivariate regression methods to the analysis of fluorescent beer components. Partial least-squares regressions (PLS1, PLS2, and N-way PLS) were utilized to develop calibration models between synchronous fluorescence spectra and excitation-emission matrices of beers, on one hand, and analytical concentrations of riboflavin and aromatic amino acids, on the other hand. The best results were obtained in the analysis of excitation-emission matrices using the N-way PLS2 method. The respective correlation coefficients, and the values of the root mean-square error of cross-validation (RMSECV), expressed as percentages of the respective mean analytic concentrations, were: 0.963 and 14% for riboflavin, 0.974 and 4% for tryptophan, 0.980 and 4% for tyrosine, and 0.982 and 19% for phenylalanine.

Photophysical properties of lumichromes in water.

The photophysics of lumichrome, 1-methyllumichrome, and lumiflavin in water solutions have been investigated. Fluorescence lifetimes of 2.7 and 2.2 ns were observed for lumichrome and 1-methyllumichrome, respectively, the corresponding triplet state lifetimes of 17 and 18 micros have been obtained from the transient absorption spectra. Evidence for long lived species with absorption maxima near 450 nm and lifetimes of ca. 400 micros has been found in the transient absorption spectra of both lumichromes. Quantum yields for the sensitised production of singlet oxygen, phi(Delta), are 0.36 and 0.41 for lumichrome and 1-methyllumichrome, respectively, in D(2)O.

Photooxidation of Sulfides to Sulfoxides Mediated by Tetra-O-Acetylriboflavin and Visible Light

Chemoselective photooxidation: Tetra-O-acetylriboflavin and blue light mediate the selective oxidation of sulfides to sulfoxides without overoxidation to sulfones. Reaction is very effective in ethanol/water (95:5,v/v) mixture reaching a quantum yield of Φ=0.60. The photooxidation reaction scope includes aliphatic and aromatic sulfides including sterically hindered substrates.

Spectroscopy and Photophysics of Iso- and Alloxazines: Experimental and Theoretical Study

We present a systematic study of the effect of methyl substitution on iso- and alloxazines in acetonitrile solutions. Substitution patterns have profound effects on both spectral and photophysical properties, with fluorescence quantum yields varying by more than an order of magnitude. TD-DFT calculation were used for the first time to correlate electronic structure changes with the substitution patterns, with good agreement between calculated and theoretical band positions and oscillator strengths. Both n−π* and π −π* states in these compounds are predicted, with the oscillator strengths indicating that only the π −π* states should be observable in the absorption spectra. Substitution patterns are shown to be responsible for energy order inversion between these states.

Efficiency of singlet oxygen generation by alloxazines and isoalloxazines

Some spectroscopic properties of alloxazines and the lifetimes of their singlet and triplet states in acetonitrile are reported. In addition, the efficiencies of singlet oxygen production by energy transfer from the excited states of a range of substituted alloxazines and lumiflavine have been determined in aerated acetonitrile solution. Triplet state quantum yields have also been measured and it has been shown that the efficiency of singlet oxygen production from the triplet state is equal to unity within experimental error in all cases.

The kinetics of fast fluorescence quenching processes

Abstract The results of fluorescence quenching experiments which bear on the kinetics of fast bimolecular processes are reviewed and analyzed critically in light of ongoing developments of the theory of these processes. A set of recommendations for ‘best’ experimental procedures and data analysis methods is proposed.

Near and mid infrared spectroscopy and multivariate data analysis in studies of oxidation of edible oils

Infrared spectroscopic techniques and chemometric methods were used to study oxidation of olive, sunflower and rapeseed oils. Accelerated oxidative degradation of oils at 60°C was monitored using peroxide values and FT-MIR ATR and FT-NIR transmittance spectroscopy. Principal component analysis (PCA) facilitated visualization and interpretation of spectral changes occurring during oxidation. Multivariate curve resolution (MCR) method found three spectral components in the NIR and MIR spectral matrix, corresponding to the oxidation products, and saturated and unsaturated structures. Good quantitative relation was found between peroxide value and contribution of oxidation products evaluated using MCR--based on NIR (R(2) = 0.890), MIR (R(2) = 0.707) and combined NIR and MIR (R(2) = 0.747) data. Calibration models for prediction peroxide value established using partial least squares (PLS) regression were characterized for MIR (R(2) = 0.701, RPD = 1.7), NIR (R(2) = 0.970, RPD = 5.3), and combined NIR and MIR data (R(2) = 0.954, RPD = 3.1).

Analysis of Olive Oils by Fluorescence Spectroscopy: Methods and Applications

Fluorescence spectroscopy is a well established and extensively used research and analytical tool in many disciplines. In recent years, a remarkable growth in the use of fluorescence in food analysis has been observed (Christensen et al., 2006; Sadecka & Tothova, 2007; Karoui & Blecker, 2011). Vegetable oils including olive oil constitute an important group of food products for which fluorescence was successfully applied. Fluorescence is a type of photoluminescence, a process in which a molecule, promoted to an electronically excited state by absorption of UV, VIS or NIR radiation, decays back to its ground state by emission of a photon. Fluorescence is emission from an excited state, in which the electronic spin is equal to that in the ground state, and typically equal to zero. Such transitions are spin allowed, and occur at relatively high rates, typically 108 s-1 (Lakowicz, 2006).

Riboflavin Interactions with Oxygen—A Survey from the Photochemical Perspective

In this short review we provide some insights to the main processes that riboflavin is involved in upon absorption of a photon. We describe riboflavin properties in its interactions with oxygen, comparing them to the properties of some other singlet oxygen sensitizers. Data are provided on riboflavin photosensitizing properties in vivo and in vitro, and its properties as an endogenous singlet oxygen sensitizer are discussed. We additionally report flavin catalytic role in organic synthesis and photochemical reactivity in solutions of riboflavin and some of its derivatives.