Glass Transition Theory

Crystalline matter can originate from melt in which the first phase transitions take place and are characterized by an exchange of latent heat.

Amorphous substances can also crystalize from melt or from a solution. During a fast cooling they will be converted to a metastable glass state. Glass state is in fact a liquid state of which the viscosity is so high that the material acts as a solid without being a crystal. This vitification results in a reduction of transitional mobility of the components, which is the core element to make the link with the stability of foods. Thus the processes that are controlled by diffusion will be inhibited as a result of the drastic loss of molecular mobility.  However even in the glass state, molecular mobility is still occurring (especially on atomic level: vibrations, rotations). This so called relaxations will result in additional changes in a particular state.

Depending of the way of cooling various types of glass can be formed. These phase transitions are associated with temperatue changes. The glass transition does not occur at one particular temperature , but occurs over a temperature range. Still binary water-solid mixtures are being characterized by a particular glass temperature.

The glass state is induced by the process of vitrification and it can only happen in a particular temperature. It depends upon the circumstances (cooling, agitation, etc), this will have an impact on the moment when the glass transition will take place. Glass transition temperature is the temperature at which the food becomes glass.

If the system is given sufficient time, allowing a maximal amount of ice being formed, the remaining aquous solution will be able to reach the highes possible concentration of dissolved substances.

Change from liquid to crystal has a first order phase transition, meaning that there is a change in the enthalpy, but it does not occur due to change on temperature.eg. From liquid to ice, from liquid to vapor.

Second order phase transition, is when you have a change in enthalpy together with a change in temperature, these are metastable conditions.

The transition of the complementary  liquid state to th glass state is a transition zone in which the food acts as a rubber, hence indicates the rubber state. The rubber state is not completly liquid, nor completely solid, but an intermediate between both thus shows particular mobility. Both the rubber state and the glass state are essential and characteristic elements of the glass transition theory.

Summarizing: a liquid solution of a particular substance can be cooled down (o concentrated) until the product starts to act leathery, rubbery material which upon further cooling (or concentration) will be converted into glass, in which the molecular mobility is restricted significantly. Both states are NON EQUILIBRIUM STATES and consequently also time dependent. Glass and rubber transitions are also dependent on the speed at which water is removed from the system, by evaporation (drying, extrusion) or crystallization (freezing.

The glass and the rubber state describe the state of the non-aqueous fraction of the food. The role of the water in a food according to the glass transition theory becomes clear if the parallel with polymer science is made. Water will act as plasticizer and will reduce the interaction between the non-aqueous food components which will result in lower glass transition temperature. This process is called plasticization. The impact of moisture content on the properties of the food as function of the glass transition theory is shown in the state diagram.

What happens sequentially?

According to polymer science, you can have following phases: solution, eg. Sucrose solution, when in concentrate the solution I can have crystalline sucrose (equilibrium condition), when I have crystalline solution I can heat it and melt it (equilibrium condition). However rubber and glass transition are not stable, non-equilibrium conditions, glass transition is time dependent phenomenon, it depends upon circumstances of the food (time dependent, and also depends upon circumstances). When we have a solution we can concentrate it by evaporating the water and a particular point we expect the sucrose start to crystalize, when we remove the water very gently (evaporation) we will not obtain crystals, we will obtain over saturated solution. When you put it on fridge you will obtain a crystal (solid as a rock). What we did is turned the solution into a vitrified substance, into a vitrified glass. So by concentrating and by cooling you can turn it into a glass (vitrified).

The rubber is the intermediate state between the liquid solution and the vitrified state. There are a lot of foods that are in rubber state (dried meat), semi liquid sate.

State diagram:

As the glass and rubber state are non-equilibrium condition diagrams, the state diagram describes the properties of food as function of the moisture content.  The state diagram is a kind of map on which the changes in phase behavior of a particular substance is given as function of temperature and moisture content at a constant pressure.

From the state diagram it can be concluded that the glass transition depends largely upon the solute concentration. Because of its plasticizing effect the water will reduce largely the glass transition temperature. The impact is more intense at low moisture content.  The glass transition temperature of moisture rich foods are very low and typically lower than the freezing temperatures used in industry.  the glass transition temperature of binary mixtures can be estimated based on the empirical Gordon Taylor equation ( but it has some problems so in the literature it can be found various temperatures for the same food).

The glass transition temperature is also dependent upon the type of molecule which is dissolved in the water. Aqueous mixtures of low molecular weight sugars are characterized by a much lower glass transition temperature of aqueous mixtures of poly-saccharides as similar moisture content. Foods containing typically complex bio-polymers are characterized by higher glass transition temperatures, which are reached during normal freezing temperatures used in industry (eg: bread). Foods richer in low molecular weight compounds can be in the glass state upon concentration or drying (milk powder). Moreover there is a clear relationship between the polymerization degree of sugars and the glass transition temperature. It is clear that such a behavior can be related to the number of interactions ad the intensity of the interactions between water and the solutes. Apart from water also glycerol can act as plasticizer.

Factors determining Tg

• Moisture content

• Temperature

• Type of solute

Impact of temperature and moisture content on molecular mobility

The relaxation time (level of molecular mobility) is directly proportional to the viscosity of the aquous phase of the food. When viscosity is low the relaxation time will also be low. They are also dependent on temperature (according to Arrhenius kinetics).  

Arrhenius kinetics is not valid in rubber state (relation between temperature and molecular mobility is not valid here). Impact of temperature is much higher here. There is an Inverse relationship between relaxation time and molecular mobility.

When food is in rubber state, small variations in temperature will have a big variation on the stability of food when they are in the rubber state. When increase temperature the relaxation time is smaller, thus molecular mobility becomes lower. The course you get depends on the measuring technique. The temperature dependency of the relaxation time is extremely big in rubber state, much bigger than in glass state.

Moisture content and molecular mobility

Water is a plasticizer so it will increase molecular mobility thus reduces relaxation time. Also small variations on water content will have drastic variations on relaxation time, thus on stability of food.  Small variations on water content will have big impact on water activity.