Barbara Rogé

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.

Hydration and self-association of caffeine molecules in aqueous solution : Comparative effects of sucrose and β-cyclodextrin

The UV-spectra of pure caffeine were measured and two quite differentiated hyper- or hypo-chromic effects were observed as concentration was increased. The first one was explained as due to caffeine-water molecule interaction and the second as originating from dimer formation and staking of caffeine molecules. The effects of sucrose and beta-cyclodextrin on the hydration and the self-association of caffeine were also examined by UV spectroscopy. Sucrose was found to enhance the self-association of caffeine molecules by attracting and structuring water molecules around itself. The caffeine-caffeine hydrophobic interactions were promoted in such hydrophilic environment and so was the stacking. The molecular aggregation leads to reducing the electronic mobility and so is the case for the mesomeric effect in the heterogeneous cycle. This could explain the hypo-chromic phenomenon observed when sucrose concentration was increased. Beta-cyclodextrin shows a distinct behaviour because of its ability to form inclusion complexes with various hydrophobic guest molecules. This ability enhances the solubility of caffeine molecules throughout the inclusion interactions and prevents the caffeine self-association.

Model equations for the calculation of the volume and surface of a sucrose crystal: a tool for accurate crystal growth rate determination

Usually, crystal growth rate is expressed as a linear velocity deduced from the direct observation of the growth of individual crystals. In the general case of crystals grown in a pilot crystallizer or in an industrial pan, it is more convenient to express the overall growth rate in unit mass per unit time per unit crystal surface area. In the absence of direct measurement of crystal dimensions, such an expression necessitates the use of a shape factor relating the crystal surface to the volume (or mass). We have used a model of a sucrose crystal with a simplified geometric shape to approach the practical problem of determination of crystal growth rates without the need to adopt empirical shape factors from the literature. The geometric model allowed establishing of model equations for the calculation of a sucrose crystal surface and volume and applying these equations to the determination of crystal growth expressed as mass unit per unit time and unit surface area of the growing crystal. The proposed model was validated by direct measurement of the dimensions of hundreds of crystals of different origins. Comparison of the experimental sucrose crystal growth rates obtained by the end-to-end method and the rates calculated with the proposed model show a good fit. This work aims at making crystal growth rate determination less dependent on empirical shape factors and more accurate.