THIS QUESTION OCCUPIED A MAJOR PART OF THE roundtable discussion on the glassy state in foods at the recent ISOPOW meeting. Questions about bound water are not unique to foods. The terms bound and free water are also used to describe the state of water in many porous substances which retain moisture. However, in porous media physics, as in food science, it is generally not sufficient to just give qualitative descriptions.
It is important to quantify how bound or how free the water is. Since Decagon serves both food scientists and porous media physicists, we think it is useful to apply knowledge from both areas to the quantification of the state of water in porous systems such as foods.
Water in porous systems may be bound in two ways; by lowering the energy state of the water in the system, and by reducing the rate of movement of water to interfaces. To measure the energy state of water we normally choose pure, free water as the reference state (zero energy). Forces of adhesion and cohesion (van der Waal-London forces) lower the energy state of adsorbed water compared to pure, free water.
Solutes dilute the water, increasing its entropy and therefore lowering its energy state. These two effects combine to lower the total free energy of the water. The lower energy (compared to pure, free water) of the water in the food binds it. In other words, work would need to be done on the water to remove it from the food. The energy per unit mass required to remove an infinitesimal quantity of water from the food and transport it to the pure, free reference state is called the water potential.
The water potential is therefore a quantitative measure of the binding energy of water in the food. As the water content decreases, the remaining water is more tightly bound, and the work required to remove water increases. One could say that all of the water in food is bound, since all of it is at water potentials below (more negative than) pure free water. The important issue is not whether water is bound, but how tightly it is bound.
Water potential describes the thermodynamic state of water in foods and other porous media, and is an equilibrium measure. A system is said to be in equilibrium when the water potential is the same at every location in the system. Food and other porous systems are often far from equilibrium, and this provides a second sense in which water can be bound. If the rate of movement of water in a system is so low that equilibrium can not be achieved within the normal lifetime of the food, then the water could be said to be bound. It is hypothesized that when foods enter the glassy state the movement of water is so slow that it is effectively bound.
Water activity is a direct measure of the energy state of the water in food. A well-known equation from thermodynamics relates the water activity and the partial specific Gibbs free energy or water potential (y) of a system as follows:
where Mw is the molecular weight of water, R is the gas constant, and T is the Kelvin temperature. The following table shows water activity, and associated free energy values in various units.
Most people are familiar with the colligative properties of solutions. One mole of an ideal solute in water lowers the freezing point of the water by 1.86 degrees C, raises the boiling point 0.5 degrees C and increases the osmotic pressure 22.4 atmospheres. People are often not aware, however, that these properties also apply to the adsorption of water in porous materials. These adsorptive forces are generally much larger than solute effects in intermediate moisture foods and other moderately dry porous media.
If the water potential of a cellulose or protein matrix were -14 kJ/kg, its water would not begin to freeze until its temperature reached about -10°C. Water potential (whether from matric forces or from solutes) can therefore be expressed in terms of freezing point depression. This is also shown in Table 1.
Table 1. Energy status of water expressed in various units and forms. The conversion from water activity to water potential assumes a temperature of 293K. Water potential is in kilojoules per kilogram. Osmolality is the concentration of an ideal solute which has the given water potential.
An interesting result of this reduction in freezing point from the binding of water is that the water in food and other porous media does not all freeze at a single temperature like pure free water does. Since the water potential of the water in the system ranges from its value in the unfrozen state to very low values, it freezes over a range of temperatures. An equilibrium exists between the frozen and unfrozen water in a frozen porous system.
There is always some unfrozen water in the system, and the unfrozen water content is determined by the temperature of the system (which sets its water potential or water activity) and the sorption isotherm of the matrix that holds the water. Figure 1 shows an example of this, where the unfrozen water content in a sample of wheat flour has been computed from the sorption isotherm and the freezing point equivalents of the water activity. If the pre-freezing water content of the sample is above 35 %, then the values shows would be the expected percentages of unfrozen water at the indicated temperatures.
Figure 1. Estimated unfrozen water content in a frozen wheat flour sample as a function of its temperature. It is assumed that the pre-freezing water content is above 35%.