Mobility and reactivity in low moisture and frozen foods

摘要

The glass transition temperature,T_g,is oftne considered as a reference temperature for the stability of low moisture and frozen foods.Firstly,many degradation reactions are assumed to occur only above the T_g.Secondly,the rate of a reaction measured at a temperature T is considered as being controlled by the T-T_g.The objectives of this paper are to demonstrate that in amorphous materials and depending on the reaction considered, the reference temperature may or may not be related to the main relaxaiton process,exhibiting a sensitivity to temperature which differs from that of collective motions associated with the glass transition.This demonstration will be illustrated by two examples.Firstly,the rate of an enzymatic reaction in frozen sucrose solutions will be related to the diffusion of a probe (similar in size to the substrate of the enzyme)and to the viscosity of the reaction medium,Secondly,the movements responsible for the loss in crispness in cereal based foods and models systems will be discussed.NMR (Nuclear Magnetic Resonance),ESR (Electron Spin Resonance),dielectric and mechanical spectroscopies,and DSC have been used to evaluate molecular mobility,translational diffusion coefficients were obtained from the FRAP (Fluorescence Recovery After Photobleaching) and concentration profile methods.