Dominique Champion

Direct investigation of viscosity of an atypical inner membrane of Bacillus spores: A molecular rotor/FLIM study

We utilize the fluorescent molecular rotor Bodipy-C12 to investigate the viscoelastic properties of hydrophobic layers of bacterial spores Bacillus subtilis. The molecular rotor shows a marked increase in fluorescence lifetime, from 0.3 to 4ns, upon viscosity increase from 1 to 1500cP and can be incorporated into the hydrophobic layers within the spores from dormant state through to germination. We use fluorescence lifetime imaging microscopy to visualize the viscosity inside different compartments of the bacterial spore in order to investigate the inner membrane and relate its compaction to the extreme resistance observed during exposure of spores to toxic chemicals. We demonstrate that the bacterial spores possess an inner membrane that is characterized by a very high viscosity, exceeding 1000cP, where the lipid bilayer is likely in a gel state. We also show that this membrane evolves during germination to reach a viscosity value close to that of a vegetative cell membrane, ca. 600cP. The present study demonstrates quantitative imaging of the microscopic viscosity in hydrophobic layers of bacterial spores Bacillus subtilis and shows the potential for further investigation of spore membranes under environmental stress.

Study of α- and β-relaxation processes in supercooled sucrose liquids

The relaxation map of highly concentrated sucrose water mixtures was built using mechanical and impedance spectroscopies. Data of α- and β-relaxation processes obtained with both techniques complete calorimetric and rheological measurements. The temperature evolutions of the relaxations were extrapolated using the measured data and the equations commonly used to describe the relaxations: Arrhenius and WLF behaviours for respectively the β- and α -relaxations. The temperature/frequency domain when α and β processes merge for 99% sucrose solution is discussed with respect to scenery proposed in the literature.

Effect of the state of water and relative humidity on ageing of PLA films

Various types of food are now commercialized in packaging materials based on poly(lactic acid) (PLA) due to its eco-friendly nature. However, one of the main limitations related to PLA is its reactivity with water. For food applications, it is of critical importance to better understand the hydrolysis of PLA driven by water molecules either in liquid or in vapour state. This work focuses on the modifications of PLA induced by water when simulating contact with semi-dry foods (aw≈0.5), high moisture foods (aw≈1) and liquid foods (aw≈1). This study undoubtedly shows that both the chemical potential of water and its physical state influence the hydrolytic degradation of PLA films. From a practical point of view, PLA packaging is very well suited for semi-dry foods, but is highly sensitive to high moisture and liquid foods.

Water in Dairy Products | Analysis and Measurement of Water Activity

Water activity of a system (aw) is a thermodynamic concept used to characterize the contained water. It has been considered, in spite of some limitations, as the most important parameter in food technology for the past 50 years. It can be measured by the ratio (p/p0) of the water vapor pressure in the atmosphere at equilibrium with the material to the saturated vapor pressure of pure water at the same temperature, or by the physical properties related to it (freezing point depression, mechanical/electrical properties). Although the physical/chemical mechanisms responsible for the depression of aw as compared to that of pure water are more or less identified, the expressions describing the relations between composition and aw remain mainly empirical for real foods. Water activity is only one of the several water-related factors controlling quality characteristics of food,with molecular mobility being equally important. It is, however, an essential parameter to consider in situations involving microbial activity (shelf life, production of aroma, etc.). It is also an essential tool in food technology, permitting to predict the transfer of water between two compartments of a product or between a food product and its environment during storage or a drying operation. Thus, aw controls the water content of the product, and in turn the quality characteristics such as texture of cheese, and the kinetics of physical (crystallization of lactose) or chemical/enzymatic reactions.

How does hydration affect the mechanical properties of wine stoppers

Data related to the comparison of the mechanical properties of the different stoppers used in the wine industry are scarce. This study aims at comparing the effect of hydration (from 0 to 100xa0% relative humidity at 25xa0°C) on the mechanical properties of four widely used types of stoppers: natural corks, agglomerated corks, technical stoppers and synthetic (co-extruded) stoppers. For both natural and agglomerated corks, the Young’s modulus was significantly and similarly affected by hydration, with a constant plateau value up to 50xa0% relative humidity (RH) and a mean value around 22 and 14xa0MPa, respectively. For higher RH, the increase in water content leads to a decrease in the material rigidity (Young’s modulusxa0<10xa0MPa), which is attributed to water clusters formation between polymer chains. Technical stoppers revealed a similar profile, but with a much smaller impact of the water content and with overall lower Young’s moduli values, around 5xa0MPa, throughout the RH range. The stiffness of synthetic closures was not affected by hydration, in agreement with the hydrophobic behavior of polyethylene. Differential scanning calorimetry and dynamic mechanical thermal analysis allowed us to identify a glass transition temperature (Tg) in cork (around 0xa0°C), and another one in agglomerated cork and technical stoppers (close to −45xa0°C, corresponding to additives). All together, for the first time the data highlight the comparative mechanical properties of such materials of the wine industry, and the progressive loss of the “cork-like” behavior of cork composites when other components are mixed with cork.

Measurement of white wines resistance against oxidation by Electron Paramagnetic Resonance spectroscopy

Free radical theory of aging hypothesizes that oxygen-derived radicals are responsible for the storage-related flavor instability in wine. In an optimal situation, a balanced-distribution exists between oxidants and antioxidants among wines intrinsic/extrinsic metabolites. Based on the kinetic study of POBN-1-hydroxyethyl spin adduct (POBN-1-HER) formation in wines initiated via the Fenton reaction, a novel tool based on EPR spin trapping methodology was developed to quantify wines resistance against oxidation. Antioxidant capacities of wines were evaluated according to POBN-1-HER maximum signal intensity (Imax POBN-1-HER) and rate formation (rPOBN-1-HER) kinetic parameters. Low Imax POBN-1-HER and rPOBN-1-HER values suggest that endogenous antioxidants in wine are able to quench a substantial amount of radicals capable to take part in deleterious oxidative reactions. This will be very valuable in understanding aging potential and will provide an avenue to better control the process by knowing how it might be possible to change wines resistance against oxidation.