Comprehension des mecanismes d'interactions polymere-plastifiant : application a l'enrobage alimentaire par des biopolymeres
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Abstract EN:
Mechanical and oxygen barrier properties of starch and pea proteins films were studied as a function of sorbitol, a plasticizer. Starch and proteins films were first studied separately and then bilayers films were realized. Two types of behavior were observed on starch films. At low sorbitol contents (<21% sorbitol and 8,7% water, <15% sorbitol and 14% water), yield at break and oxygen permeability are decreased. This behavior called antiplasticization is related to the formation of strong interactions between sorbitol and most mobile segments of starch macromolecules. Those interactions reflect the lowering of mobility of the system : they are revealed by the disappearance of starch [beta] relaxation and the lowering of T2* of the most mobile segments, measured by 1H-RMN. Those interactions are also responsible of the reduction of intermolecularly spaces, where oxygen molecules can diffuse ; thereby the oxygen permeability is decreased. At high sorbitol contents (>21% sorbitol and 8,7% water, >15% sorbitol and 14% water), starch films are plasticized. This plasticization is not due to the decreased Tg bellow the room temperature, but is attributed to the formation of clusters of sorbitol. Those clusters allowed general motions in the system : therefore yield at break is increased. In contrast, oxygen permeability is increasing very slowly instead of the apparition of clusters. Sorbitol has a classical plasticizer effect on proteins mechanical properties. Properties of bilayer films are controlled by starch films properties and variations observed seem to be due to sorbitol migration between the two layers. Special interactions, which occured at starch -protein interface, could be at the origin of some unexplained mechanical properties, but those interactions were not seen. Oxygen barrier properties of starch films are interesting, especially at low sorbitol content (P=0,14. 10-16 cm3/cm. S. Pa at 8,8% de sorbitol). That is why some coatings experiments on phospholipidids particles have been carried out at the pilot scale. The rate of oxydation is lower in coated products, proving some ability of these type of coating to protect against oxydation.
Abstract FR:
Les proprietes mecaniques et barrieres a l'oxygene de films d'amidon et de proteines de pois ont ete etudiees en fonction du taux de sorbitol, molecule plastifiante. Ces films ont ete etudies en monocouche puis des films bicouche amidon-proteines ont ete realises. Sur les films d'amidon, deux domaines de comportement ont pu etre distingues en fonction du taux de sorbitol. Pour les faibles concentrations en sorbitol (<21% sorbitol et 8,7% eau, <15% sorbitol et 14% eau), le pourcentage d'allongement ainsi que la permeabilite a l'oxygene diminuent. Ce comportement appele antiplastification, est attribue a la formation de liaisons entre le sorbitol et les parties les plus mobiles de l'amidon. Ces liaisons provoquent une diminution de la mobilite du systeme, revelee par la disparition de la relaxation [bêta] de l'amidon, et la diminution de temps de relaxation transversale moyen de la phase la plus mobile, mesuree par rmn. Ces liaisons ont egalement comme consequence de diminuer les espaces intermoleculaires par lesquels la molecule d'oxygene peut diffuser, la permeabilite a l'oxygene est donc diminuee. Aux forts pourcentages en sorbitol (>21% sorbitol et 8,7% eau, >15% sorbitol et 14% eau), le film d'amidon possede les caracteristiques d'un materiau plastifie. L'origine de ce comportement est attribuee a la formation de clusters de sorbitol et non a un abaissement de la tg en dessous de celle de l'experience. Ces clusters permettent d'augmenter la mobilite generale du systeme et donc le pourcentage d'allongement. En revanche, ils facilitent peu la diffusion de l'oxygene a l'interieur de la matrice, l'augmentation de permeabilite est faible. Les films bicouches ont des proprietes controlees par celles des films d'amidon. Les variations observees sont dues en grande partie a des migrations de sorbitol entre les deux couches. Des interactions particulieres, a l'interface amidon-proteines pourraient etre a l'origine de resultats non expliques, mais n'ont pu etre mises en evidence. Les films d'amidon ayant revele des proprietes barrieres a l'oxygene interessantes (p=0,14. 10##1#6 cm#2/s. Pa a 8,8% de sorbitol), des essais de pelliculage de granules lipidiques par une couche d'amidon ont ete realises a l'echelle pilote. La cinetique d'oxydation des granules pellicules est plus lente montrant l'efficacite du pelliculage a retarder l'oxydation.