Formation of the Glassy Phase in Frozen Foods

During the freezing of foods, ice is formed as pure water goes through the two-step (nucleation and propagation) crystallization process. As temperature decreases and water is removed from a food in the form of ice, the solutes present in the UFP are freeze-concentrated. An equilibrium freezing temperature exists for each ice/UFP ratio, which is a function of the solute concentration. This equilibrium thermodynamic process can be modelled on a phase diagram as an equilibrium freezing (liquidus) curve (see figure below), which extends from the melting temperature (Tm) of pure water (0oC) to the eutectic temperature (Te) of the solute, the point at which the solute has been freeze-concentrated to its saturation concentration.

As temperature is lowered, it is highly unlikely that solute will crystallize at Te, due to high viscosity from concentration of solute and low temperature, so that freeze-concentration proceeds beyond Te in a non-equilibrium state. The highly-concentrated UFP can then go through a viscous liquid/glass state transition, driven by the reduction in molecular motion and diffusion kinetics as a result of both the very high concentration and low temperature.

A glass is defined as a non-equilibrium, metastable, amorphous, disordered solid of extremely high viscosity (ie., 10 exp10 to 10 exp14 Pa.s), also a function of temperature and concentration. The glass transition curve extends from the glass transition temperature (Tg) of pure water (-134oC) to the Tg of pure solute. The equilibrium phase diagram and the kinetically-derived state diagram can be modelled together on a supplemented state diagram. The supplementary state diagram showing the solid/liquid coexistence boundaries and glass transition profile for a binary sucrose/water system is shown in the figure below. Below and to the right of the glass transition line, the solution is in the amorphous glass state, with or without ice present depending on temperature and freezing path followed, while above and to the left of the glass transition line, the solution is in the liquid state, with or without ice depending on temperature.

Graph of glass transition

As an example, assume a sucrose solution with an initial concentration of 20% at room temperature (point A). The initial Tg of this solution at room temperature before phase separation is marked as point B (if the solution could be undercooled to this temperature without ice formation). However, upon slowly cooling of the system somewhat below its equilibrium freezing point (due to undercooling), nucleation and subsequent crystallization begins at point C and initiates the freeze-concentration process, removing water in its pure form as ice. As ice crystallization proceeds, the continual increase in solute concentration (removal of water) further depresses the equilibrium freezing point of the UFP in a manner which follows the liquidus curve (shown as path C) while the Tg of the UFP moves up the glass transition line (path B; due to increased concentration) with a rapid increase in viscosity in a non-Arrhenius manner, particularly in late stages of the freezing process.

Co-crystallization of solute at the Te is unlikely and thus freeze-concentration continues past Te into a non-equilibrium state since the solute becomes superstaurated. When a critical, solute-dependent concentration is reached, the unfrozen liquid exhibits very resisted mobility and the physical state of the UFP changes from a viscoelastic liquid to a brittle, amorphous solid glass.

The intersection of the non-equilibrium extension of the liquidus curve, beyond Te, and the kinetically-determined glass transition curve, point D in the above figure, represents the solute-specific, maximally freeze-concentrated Tg of the frozen system, denoted Tg', where ice formation ceases within the time-scale of the measurement. The corresponding maximum concentrations of water and sucrose "trapped" within the glass at Tg' and unable to crystallize are denoted the Wg' and Cg' , respectively. It is worth noting that this unfrozen water is not bound in an "energetic" sense, rather unable to freeze within practical time frames.

At the Tg', the supersaturated solute takes on solid properties because of reduced molecular motion, which is responsible for the tremendous reduction in translational, not rotational, mobility. It is this intrinsic slowness of molecular reorganization below Tg' that the food technologist seeks to create within the concentrated phase surrounding constituents of food materials.

However, warming from the glassy state to temperatures above the Tg' results in a tremendous increase in diffusion, not only from the effects of the amorphous to viscous liquid transition but also from increased dilution as melting of small ice crystals occurs almost simultaneously (Tg' = Tm'). The time-scale of molecular rearrangement continually changes as the Tg is approached, so that food technologists can also gain some enhanced stability at temperatures above Tg' by minimizing the delta T between the storage temperature and Tg' , either by reduced storage temperatures or enhaced Tg' through freezing methods or formulation. Hence, knowledge of the glass transition provides a clear indication of molecular diffusion and reactivity, and therefore, shelf-stability.