Question

Explain in detail how glass transition temperature and molecular mobility relate to the following:

a. A stable drying process

b. A stable freezing process

c. A stable freeze drying process

Answer 1

a. A stable drying process to achieve glass transition temperature is helpful in storing food materials. It has been found that the associated water with food materials is desorbed easily and is used in the microbial processes which eventuallly degrades food while storing for longer periods of time. When water is removed rapidly during a stable drying process so as to prevent ordering of material below glass transition temperature, a practical state of immoblilization of dissordered state is achieved where all diffusion related processes are halted. Thus the processes related to desorption of water and deteroration process is eliminated and food may now be stored for long period.

b. A stable freezing process is the methods of cooling a material at a certain rate to avoid formation of ice crystals and crystallization in the disordered state of material. When a material is initially cooled below its glass transition temperature formation of ice crystals is observed in certain portion of the matrix, which acts as another solute in the system. We have a combination of frozen and unfrozen material. The glass transition temperature of the unfrozen material is found to be a constant and cooling the material below this new temperature allows the food material to be stored without getting deteorated over long periods of time. Any changes greater than this new temperature point allows for greater diffusion and mobility of water in free space available and thereby enhance degradation.

c. A stable freeze drying process involves rapidly removal of water from food by first sublimation of ice. An amorphous state of solute is seen with the disordered state maintained as was in the case of frozen material forming a cake. This kind of systems have a high glass transition temperatures. At usual atmospheric temperatures they will be in the glassy state. An increase in temperature or amount of water in the material allows for changes in the physical behaviour of the food system in consideraton. As the system reaches the rubbery state, the increasing mobility and diffusion will cause the reaction rates to increase as well. Prcesses include but not limited to enhanced enzymatic and non-enzymatic reactions, loss of texture and collapse of the structure is to be expected. Food materials are thus not allowed to reach this new rubbery glass transition temperature as it leads to increase in rate of deteoration of material a lower quality of food formation. This is specially important in the case of freeze-dried foods, which are highly hygroscopic materials. The temperature during storage must be maintained lower than the glass transition temperature and the diffusion of water through packaging foil must be accurately controlled.