It seems that the stars are aligned or something for me to continue my discussion on water and material science. Not only did Dario comment on my Water Activity post asking me to discuss the relationship between glass transitions and water activity, but in a recent Friday Sprog post on Janet’s blog, she discussed phases of matter and a commenter talked about materials that was both solid and liquid. These materials are known as amorphous materials – there seems to confusion as to whether they are solid or liquid because they can have the behavior of both phases. Amorphous materials do not have the regular, ordered pattern of crystalline solids and they typically have more structure than a liquid.
Many foods are amorphous. In fact, very few foods are true crystalline solids or true liquids. Those that are; salt, sucrose, oils; are generally pure compounds. In most foods, water is present with solutes; salts and sugars; and polymers; proteins, complex carbohydrates, and lipids. These interfere with the standard behavior of pure water.
To get you started Janet has a great post about the typical three phases of matter that you meet in high school chemistry class, namely solid, liquid and gas. In that post she represents a great phase diagram:
This is a great representation for materials such as water that have a crystalline solid, ice, and when pure easily convert into a liquid. The point of change between ice and water is the melting point if you increase the temperature or the freezing point if you decrease the temperature.
As you add solutes to water the temperature of freezing/melting changes. Freezing occurs at a lower temperature. This is one of the colligative properties and is the reason why, as long as you don’t live anywhere really cold like Minnesota, a dilute solution of salt can be used as an antifreeze, as it freezes at a lower temperature than pure water. Here is a phase diagram from Spark Notes showing both the pure solution and the solution with dissolved solutes:
This is true of whatever solute you use, so sugar would have a similar influence on the freezing temperature. Thus, food freezes at a lower temperature than pure water. If you have ever seen the Alton Brown episode on Thanksgiving the turkey guy talks about the different temperatures and definitions of fresh, hard chilled and frozen turkey. They are also given here. For instance:
Any turkey labeled “fresh” in a food store has never been cooled to a temperature lower than 26°F, which is the point that turkey meat begins to freeze.
And:
[…]in order to be considered “frozen” a turkey must be cooled to a temperature of 0°F or below.
Obviously, hard chilled turkeys are stored between 0 and 26F.
So where does that get us?
Pure water has three phases: ice, water and steam (or solid, liquid and gas for other materials) and the phase change temperatures are influenced by the presence of solutes and polymers present in the liquid. Additionally, as a food freezes, the water typically freezes out, leaving behind a more and more concentrated solute liquid phase. This can result in an change in pH, viscosity amongst other properties. In some instances, the solute-liquid phase might never solidify. If a food is rapidly cooled to below the freezing temperature, a glass may be formed. A glass is an amorphous solid that has a disordered structure while behaving like a highly viscous solid. In some frozen foods, water will freeze to from crystalline ice and the solute-liquid phase will form a glass.
In my next post, I will discuss in detail the effect of sucrose on water and water activity. This is interesting because it is why we can make different candy types.
Lab Cat 