Mohamed Mathlouthi

FTIR and laser-Raman spectra of oligosaccharides in water: characterization of the glycosidic bond

Changes in conformation of oligosaccharides, and the constraints imposed by hydrogen bonding with the solvent, were studied by means of vibrational spectroscopy (FTIR-ATR and laser-Raman). Oligosaccharides differing in positions of the glycosidic bond, such as trehalose, sucrose, maltose, melibiose, lactose, maltotriose, raffinose, and stachyose, were investigated. FTIR spectra of oligosaccharides in aqueous solution at different concentrations allow differentiation of these molecules according to the types of glycosidic bonds present and the changes in conformation of their constituent monosaccharides. Characteristic spectral ranges influenced by the glycosidic linkage position, overall hydration, and concentration of the aqueous solution were found. Tentative assignment of the observed IR and Raman bands was achieved.

Water content, water activity, water structure and the stability of foodstuffs

Abstract Determination of water content, whatever the accuracy of the analytical method, is not sufficiently informative in relation to the stability of the investigated food product. Water activity (aw) brings a supplement of information as it accounts for the availability of water for degradation reactions. The understanding of why certain products are more stable than others at the same aw needs an elucidation of water structure. Of particular importance are the interactions (hydrophilic, hydrophobic) between water and the components of the foodstuff and the effect of the soluble molecules of the food on the hydrogen bonding in solvent water. Studying water in foods should start with an anlytical determination of water content for commercial and legal reasons which are evident. This has to be completed with the measurement of the thermodynamic activity of water in the food. Such a parameter (aw) should hold an important place in the identification of the food product, especially as regards its shelf life. A further step in unveiling the behaviour of water in foods consists in determining water molecules in the molecules in the studied food matrix. The tripartite (analytical, thermodynamical and structural) approach to water in foods will be examined based on examples of sugars and sugar rich products.

Water vapour sorption isotherms and the caking of food powders

Abstract Water vapour sorption isotherms describe the relationship between water content and water activity. Depending on the nature of food powder (crystalline or amorphous), the shape of isotherm is different. Mostly food powders have complex structures, including potentially crystallisable solutes such as sugars, which show changes in crystallinity during the adsorption of water. Even for such an apparently simple system as crystalline sugar, numerous factors affect the adsorption of water vapour and, as a consequence, the storage stability. The presence of a thin film of saturated solution at the surface of the crystal, grain size distribution and the inclusion of mother liquor droplets in the crystal are some of the factors which perturb the equilibrium relative humidity of sugar and its aptitude to caking. These conditions were carefully studied at the level of the laboratory and in a pilot silo. Conditions of “decaking” (recovering a flowing sugar after caking) were also established. In the case of noncrystalline powders, water activity, together with glass transition temperature, is important to determine if it is necessary to interpret the origin of the formation of bridges between food powder particles and the caking phenomenon.

Optimization of sugarcane bagasse conversion by hydrothermal treatment for the recovery of xylose

This work aims at the valorization of sugarcane bagasse by extracting xylose which is destined to the production of xylitol after purification and hydrogenation. Our approach consists in applying the principle of biorefinery to sugarcane bagasse because of its hemicellulose composition (particularly rich in xylan: (92%)). Optimizing of the thermal treatment was investigated. A treatment at 170 degrees C for 2 h was found optimal, with higher solubilzation of hemicellulose than that at 150 degrees C and lower degradation of sugar monomers than 190 degrees C. Recovery of xylose was high and the purity of xylose solution (78%) allows expecting an easy purification and separation of xylose before hydrogenation. Analysis of thermal hydrolyzates shows the presence of xylan oligomers and polymers with large distribution of DPs. This fraction should be submitted to enzymatic treatment to recover more xylose monomer.

Laser-raman spectra of d-glucose and sucrose in aqueous solution

Abstract Laser-Raman spectra of d -glucose in water at various concentrations were recorded below 1700 cm −1 . Assignments of the frequencies of d -glucose were proposed, based on earlier work on the vibrational spectra of this sugar, and determination by other techniques of the anomeric composition of aqueous solutions of d -glucose. The proportions of the anomers found from the ratio of the Raman intensities for the same models of vibrations were similar to those found by other techniques. Assignments of the frequencies for sucrose were proposed, in the light of previous results on d -glucose and d -fructose, and the effect, on their Raman spectra, of the condensation of these two monosaccharides was pointed out.

Laser-raman study of solute-solvent interactions in aqueous solutions of d-fructose, d-glucose, and sucrose

Abstract The solute-solvent interactions of d -fructose, d -glucose, and sucrose in aqueous solution were studied by comparison of characteristic, Raman of the water and the sugar components. Shifts in frequency and intensity were observed in both the bending and the stretching regions of CH2 and H2O. The ratios of integrated, Raman intensities I(CH2)/I(H2O) of the CH2 peak and the H2O bending band, and I(CH)/I(OH) of the C-H stretching line to O-H stretching band were determined. Their evolutions in terms of mass-concentration display discontinuities at specific concentrations for each of the three sugars. These breaks were interpreted as changes in the hydrogen bonding of the various species.

F. T.-I.R. and laser-Raman spectra of cytosine and cytidine

Fourier-transform infrared (F.t.-i.r.) and laser-Raman spectra of cytosine and cytidine in the solid state have been recorded and assignments of the frequencies made. Comparison of the observed frequencies for cytosine with those for cytidine permits identification of the bands characteristic of the sugar on the one hand, and of the pyrimidine base on the other.

X-ray diffraction study of the molecular association in aqueous solutions of d-fructose, d-glucose, and sucrose

Abstract X-ray diffractograms of aqueous solutions of d -fructose, d -glucose, and sucrose, and of two amorphous “solid” forms of these carbohydrates are recorded in the angular range of O 2-16°. The formation, in these diffractograms, of one or two intensity maxima as the concentration is varied is interpreted in terms of modification of the molecular association. A model is proposed that takes account of the state of organization of the solutions of the three sugars as a function of the concentration. Sucrose shows an “anomaly”, as it is the only sugar to establish intramolecular bonds. The number of these intramolecular bonds also depends on the concentration.

Laser-raman spectra of d-fructose in aqueous solution

Abstract Laser-Raman spectra of d -fructose in water at different concentrations were recorded, and assignments of the frequencies were proposed, based on earlier work on the Raman spectra of other sugars, and determination by other techniques of the composition of aqueous solutions of d -fructose as regards different isomers. It was found that the frequencies of vibration of the furanoid are higher than those of the pyranoid ring. The proportions of the furanoses and pyranoses, found from the ratio of the Raman intensities for the same modes of vibration, were similar to those found by other techniques. Shifts of intensities and frequencies were observed in the region of OH and CH bands, and were assigned to probable association between molecules of d -fructose and water.

Physicochemical and psychophysical characteristics of binary mixtures of bulk and intense sweeteners

Abstract Correlating psychophysical characteristics with physicochemical properties of sweeteners is of relevance to the understanding of the origin of sweetener synergy, an essential parameter for the food manufacturer. Psychophysical evaluation was carried out on bulk sweeteners (sucrose and maltitol) and intense sweeteners (aspartame, sodium cyclamate, acesulfam-K, alitame) in mixtures. The concentrations of mixtures were calculated to be equisweet to 10% sucrose and sweetness intensity was evaluated by reference to sucrose solutions using a “sip and spit” method. While a positive synergistic phenomenon is observed for sugar/sodium cyclamate and maltitol/acesulfamK mixtures, a significant suppression effect is obtained when aspartame is added to sugars. Additivity is observed for sucrose/alitame and sucrose/acesulfamK mixtures. The origin of these differences lies in the influence of the two molecules on water structure and in the nature of their hydration. From physicochemical properties (intrinsic viscosity, Huggins coefficient, apparent specific volume, hydration number, surface tension and contact angle), alitame and aspartame seem characterised by hydrophobic hydration; sodium cyclamate, as well as the bulk sweeteners, appear more compatible with water structure and possess hydrophilic hydration. ACK is differentiated from other sweeteners by a negative hydration. Synergy occurs when components with identical types of hydration are mixed. This phenomenon is accompanied by an increase in the mobility of water molecules in the proximity of bulk sweeteners (maltitol and sucrose) and a reduction of volume of the hydrated solute molecule. Inversely, suppression and additivity occur when constituents of the mixture possess different natures of hydration, as in sucrose/aspartame mixtures, and when physicochemical properties show a reduction of the mobility of water around the sweeteners. For suppression effects, an increase in volume of the hydration sphere is also observed. Interpretation of the sweetness of mixtures of sugars and artificial sweeteners, in terms of their compatibility with water structure, is of relevance at an economic level in food formulations.