A glass is a solid, brittle material that has an amorphous, liquid-like structure without obvious fluidity. A glass forms when a typical liquid, with a disordered molecular structure, is cooled to a temperature generally about 100dC (for many pure liquids) bellow its equilibrium crystalline melting temperature (Tm) or freezing point, at a rate sufficiently high to avoid crystallisation of the liquid.

This solidification process, known as vitrification, results in the "freezing in" or immobilisation of the disordered structure of the liquid state such that the resulting glassy solid is spatially homogeneous, but without any long-range lattice order, and is incapable of exhibiting any long range, cooperative relaxation behaviour (e.g. translational mobility) on a practical time scale. The temperature at which this transition occurs is called the glass transition temperature (Tg). Above the glass transition temperature, the system is in the rubbery state (capable of flow in real time) and bellow Tg, the system is in the glassy state. Thermodynamically the glass transition is defined as a second order transition, that is, it does not happen instantaneously but over a finite period of time.

Glass transition temperature can be measured in a number of different ways. One of the most used methods for detection is differential scanning calorimetry (DSC). This method is based on the measurement of the increase in specific heat of the material as it passes from the glassy to the rubbery state. Dynamic methods can also be used. The dynamic mechanical thermal analysis (DMTA) measures the viscoelastic properties of the materials as a function of frequency of oscillatory deformation. In the glassy state, the material will exhibit solid-like characteristics and have a measurable shear modulus. The glass transition can be detected as the temperature where the storage modulus starts to fall rapidly with increasing temperature from the very high value in the glassy state. A peak in the ratio of the loss to the storage modulus is also found at the glass transition temperature.

Since it is a kinetic rather than equilibrium phenomena, glass transition temperature will depend on the time or frequency scale of measurement. This material-specific change in physical state is also temperature and composition dependent.

The glass transition concept was developed on the basis of the polymer science. Only in recent years it has been applied to the food science, after the recognition that most foods or biological materials are non equilibrium or metastable systems and glass formation is a plausible mechanism through which such non equilibrium systems can be stabilised. This concept has been used to predict product quality, safety and storage stability.

It has been proposed that under certain conditions, as freezing or removal of water, foods may reach a state of virtual immobilisation of the disordered structure of the liquid state, where all diffusion-limited processes are stopped, thus preventing any physico-chemical deterioration changes; This is the glassy state. As a system moves from the glassy to the rubbery state, the viscosity drops dramatically, allowing greater polymer chain and reactant mobility. The free volume, defined as the amount of space associated with a system which is not taken up by polymer chains themselves, increases, what should allow faster diffusion reactions. In addition, rotational and protons mobility was also found to be higher in the rubbery state compared to the glassy state. Finally, it has been recognised that bound water molecules are more easily de-sorbed and used in biochemical and microbial metabolisms in the rubbery state. All these factors contribute to explain the lowered stability of food systems in this physical state.

Based on these concepts, it has been proposed that the glass transition may be the explanation for cessation of reactions at the monolayer determined from moisture sorption isotherms. It may be possible that the observed monolayer is not actually a monolayer, but rather a moisture content at which the glass transition is observed at a particular temperature.

The glass transition temperature is dependent on the water content of the food material, similarly to the influence of plasticisers on synthetic polymers. The effect of plasticisers on synthetic polymers is explained in terms of two mechanisms: (i) the plasticiser molecules screen off attractive forces between polymer chains and/or (ii) the plasticiser molecules enlarge the spaces between polymer chains allowing chain segments greater freedom of movement. As a consequence, Tg will decrease.

Water is the most important plasticiser for hydrophilic food components. It has become well documented that plasticisation by water depresses the Tg of completely amorphous or partially crystalline food ingredients, and that this Tg depression may be advantageous or disadvantageous to product processing, functional properties and storage stability.

Temperature and moisture limits for product stability can be estimated from state diagrams. A typical state diagram is shown in the figure bellow:


As it has already been stated, the only way to obtain a glass, at a determined concentration of solids, is by cooling at a sufficiently high rate, so that crystallisation and ice crystals formation will not occur. It is possible to observe that when temperature is decreased at a sufficient low rate, ice formation occurs bellow freezing temperature. This will cause the cryo-concentration of the solution, as the ice crystals will function as an additional solute. This way, the glass transition temperature of the solution will increase and the melting temperature will decrease. On continued slow cooling, as temperature approaches to the intersection of Tm and Tg, ice growth becomes more difficult, due to the increase in viscosity. Eventually a point will be reached when further cryo-concentration of the solution becomes impossible and the material surrounding the ice crystals passes into the glassy state. This is the freeze concentrated glass region. The glass transition temperature denoted by Tg' is the glass transition temperature of the unfrozen portion of a maximally freeze concentrated unfrozen matrix. The concentration of the unfrozen matrix (Cg') is equal to that of solutions with Tg = Tg'. The glass transition temperature within the freeze concentrated glass region is maintained constant at Tg'. If the storage temperature is bellow Tg', the food system will be stable. However, if the temperature is higher than Tg', the continued slow growth of ice crystals and other undesirable changes may occur.

It should be stated that this is a simplified version of a state diagram since pressure was assumed constant and time dependence was neglected. These supplemented state diagrams are known as dynamics maps.

Typical processes affected by the physical state are ice formation, stickiness, collapse and crystallisation, as shown in the figure. Namely crystallisation, collapse and stickiness, which are observed to occur within the rubbery state are properly described by the WLF equation :

    logAt = - C1(T-Tg)/(C2 + (T - Tg))

where At is a rate constant (viscosity, diffusion, etc.) at a given temperature T and at a reference temperature Tg and C1 and C2 are constants.

Specially interesting is the case of freeze drying a food material. When a food material is freeze dried water is rapidly removed by sublimation of ice. The solutes will be transformed to an amorphous state and the resulting freeze dried solid will retain the shape in which it was frozen, thus forming a cake. This kind of systems have a high glass transition temperatures and so, at usual atmospheric temperatures they will be in the glassy state. An increase in temperature or water content (moisture) may lead to changes in the physical behaviour of the food system. If the system reaches the rubbery state, the increasing mobility and diffusion will cause the reaction rates to increase as well. This includes enzyme reactions, non enzymatic browning and release of volatiles. Besides, loss of texture and collapse of the structure is to be expected. Therefore, conditions that cause a glass-rubber transition of food material are to be avoided as they will cause a decrease of the food quality. 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.

Next, a brief comparison between the concepts of water activity and glass transition is made.

  • Aw vs. Glass Transition