14 The physical basis of crispness: Hydration and molecular mobility.

M. LE MESTE, G. Roudaut, and C. Dacremont. ENSBANA, 1 Esplanade Erasme, 21000 Dijon, France.

The crispness of low moisture cereal foods is highly sensitive to changes in moisture. Hydration during storage generally results from i) a migration of water from a region, in a “multi-domain” food, were the chemical potential of water is, or become, higher than in the cereal part; ii) an equilibration between the cereal part and the air in contact with the food. The amount of moisture exchanged per unit of time is controlled by both thermodynamics and kinetics. Even in low moisture solids, water mobility is relatively high and thus migration occurs when a thermodynamic driving force is active. Only, barriers with a low permeability (i.e., a low solubility of water) to water may prevent water migration. Our results show that, upon hydration, crispness of amorphous cereal based foods or model systems is lost while the samples are still in the glassy state. The localized motions remaining in the glassy materials have been evidenced with mechanical and dielectric spectroscopies. A mobility map has been established for dry bread. Sensory evaluation of cereal food and of starch based model systems, have demonstrated a hardening effect of water. A discussion of the molecular mechanisms involved will be proposed.