Etude de la stabilité physique de mélanges biopolymère-sucre amorphes

L'instabilité des produits vitreux, attribuée depuis une vingtaine d'années à la transition vitreuse, peut également se produire à l'état vitreux. Ces modifications sont donc probablement fortement influencées par la mobilité moléculaire résiduelle du matériau vitreux. L'objectif principal de cette étude est de comprendre les bases physico-chimiques de la stabilité des produits vitreux afin d'assurer par la suite, un meilleur contrôle de leur qualité. Les matériaux d'étude sont l'amidon et le saccharose seuls et en mélange (0-20% de saccharose /matière sèche) faiblement hydratés (0-13.5% d'eau /matière humide). Une fois les matériaux caractérisés, leur évolution structurale au cours de leur conservation a été suivie par analyse enthalpique différentielle. Les relaxations secondaires ont été étudiées par spectroscopie mécanique et diélectrique, dans le but d'appréhender l'influence de la composition des verres sur ces mouvements locaux. Enfin, une étude de la mobilité à l'échelle des protons, sur des verres d'amidon, a été réalisée. Le saccharose semble diminuer la mobilité de l'amidon vitreux probablement par modification de la structure du verre, tandis que l'eau plastifie le verre d'amidon.For twenty years, the loss of glass stability has been attributed to the glass transition which is regarded as the only predictive parameter of stability. Recently, studies suggested that the loss of stability could also occur in the glassy state. These changes are probably dependent on local motions. The main purpose of this study is to better understand the physical basis of the stability of glassy materials for a better control of quality. This study is focused on local molecular mobility of glasses. All the studies were carried out on gelatinized waxy maize starch at different sucrose content (0 to 20 % solids) equilibrated between 0 to 13.5% water (wet basis). The structural evolution of the materials during storage was followed with differential scanning calorimetry (DSC). Then, secondary relaxations were studied by dynamic mechanical (DMTA) or dielectric spectroscopy, to investigate the influence of the glass composition on local motions. Finally, a study of protons mobility (NMR) was performed. Sucrose could reduce starch motions probably by glass structure changes ; however, water increases molecular motions of glassy starch by plasticization.DIJON-BU Sciences Economie (212312102) / SudocSudocFranceF

Influence of water, temperature and sucrose on dynamics in glassy starch-based products studied by low field 1H NMR

International audienceIn this study, the influence of both water content and temperature on the mobility of glassy starch-based matrices (starch alone or mixed with sucrose at the ratio 0% to 20% db) were studied using time domain 1H NMR. The 2nd moment M2, and transversal relaxation time T2∗ were used to study the molecular mobility of the rigid and mobile fraction of the NMR signal, respectively.The molecular mobility of the protons constituting the samples increased with water content (up to 13% wb) at all temperatures. For a given water content, both rigid and mobile protons exhibited a lower mobility in the presence of sucrose. When mobility characteristic parameters, M2 second moment values and spin–spin relaxation times T2∗, were normalized by Tg, i.e plotted versus Tg/T, the effect of water was no longer visible whereas the effect of sucrose remained marked. The temperature increase induced an increase in both rigid and mobile protons mobility. Whereas no clear change could be observed on the rigid protons mobility around the glass transition temperature the mobile fraction exhibited a marked mobility change in that temperature range

Influence of sucrose and water content on molecular mobilityin starch-based glasses as assessed through structureand secondary relaxation

International audienceMolecular mobility is known to be a key parameter in controlling the physical properties of materials and thus their quality and performance. Beyond glass transition related changes, attention should be called to the impact of local motions remaining in the glassy state. Gelatinized waxy maize starch at different sucrose contents (0–20% solids) was equilibrated between 0 and 14% water and sorption isotherms determined at 25°C. The effect of water and sucrose content on the molecular mobility of glassy starch was investigated by differential scanning calorimetry through enthalpy relaxation studies and dynamical mechanical thermal analysis. The existence of sucrose–starch interactions was suggested by the sorption isotherms not following the expected additivity of the single component sorption curves. Contrary to the glass transition or associated α relaxation, water and sucrose affected differently the secondary relaxations. Indeed, the β relaxation observed around −15°C was shifted to lower temperature upon increasing hydration, and to higher temperature when sucrose content increased, suggesting a hindering of these local motions. Enthalpy relaxation of the ternary mixtures was studied following aging up to 668 h at Tg −15°C. Ternary mixtures exhibited an enthalpy relaxation upon aging lower than starch alone as a sign of lower polymer mobility in the presence of small molecules, contrary to the free volume theory. Relaxation kinetics were characterized with the Cowie–Ferguson model and compared to literature data. The extent of the enthalpy relaxation appeared to be controlled by the distance between the aging temperature and the β relaxation temperature