What is Glass Transition?
Glass transition (Tg) is a physical property of materials. It is the temperature range where food polymers such as proteins, starch and non-starch polysaccharides such as cellulose, glucan and arabinoxylans undergo a phase change from rigid/glassy to soft.
Put simply, glass transition is a physical transformation from a “glassy” state of low molecular mobility (highly ordered, brittle or rigid molecular structure) to a “rubbery,” flexible or workable state of higher molecular mobility and kinetic energy.1
How does it work?
Although not commonly used or fully understood, glass transition plays a critical role in the processing (operations involving heat application and/or addition/removal of water), storage and shelf-life of cereal-based products such as bread, cakes, cookies and pasta.
Glass transitions are mediated by water, temperature and presence of low molecular weight solute molecules. The following diagram helps visualize the glass transition of food materials such as bread, flour or cookies (naturally rich in protein and starch).1,2,3
It is important to note that the words “rubbery” and “glassy” are only descriptors of the physical state of the food system rather than the texture or eating quality of the food material.
In order for a material to go from a glassy state to a rubbery state (or vice versa), it must first pass through glass transition temperature (Tg), a temperature value (or range) at which a given food material undergoes a large change in its modulus (stress force/strain ratio).2 Below its glass transition temperature, a food system or material is in a glassy state, while above such temperature, it is in a rubbery state.
Factors that affect Tg:2,3
- Water activity (free water): The higher the water activity and/or moisture content, the lower the glass transition temperature (Tg). Presence of water depresses the glass transition temperature of food materials.
- Presence of soluble compounds such as alcohol, polyols, sugars, etc: Presence of water binding molecules decreases water activity and increases Tg. This is why high-sugar cake batters run the risk of being underbaked as part of the starch does not get fully gelatinized and set of crumb is incomplete.
- Molecular weight of polymers: In general, the Tg of a polymer increases with its molecular weight. This is why starch and gluten proteins in the dough require a considerable amount of heat application for proper baking and a sufficient heat removal for proper freezing and final temperature.
Glass transition mediated by water
Sprouting or malting grain. Dry, friable and hard barley or wheat kernels are in the glassy state before they are steeped in water. Once they absorb water, the kernel polymers soften due to their transition into the rubbery state. In this case, the kernels have undergone a glass transition in which the water uptake mediated the glass transition without change in temperature (excess moisture decreased Tg required to ambient levels to achieve such phase change).
Dough mixing. In hydration processes such as dough mixing, starch and gluten proteins attract and hold water molecules by hydrophilic interactions (e.g. hydrogen bonds). In this case, water acts as a plasticizer, providing molecular mobility and allowing a glass transition to occur. Under this scenario, the flour (the glassy material) transforms into a workable material called dough.
Glass transition mediated by temperature
Baking and cooling. The baking step is a complex process that causes both a drying (water removal) and a heating effect. At a given dough temperature (which depends on the formulation), a portion of the starch fully gelatinizes and proteins coagulate. The set of crumb is evidenced by the formation of a “rubbery” starch gel/coagulated protein matrix that is porous and somewhat flexible.
If the starch in the baked matrix is allowed to cool down, it will retrograde into a rigid material (representing a glass transition from a rubbery or soft state to a solid-like consistency).
Freezing: Bread (high in moisture and water activity) is in a flexible or rubbery state at room temperature. Once frozen to extend its shelf-life, bread loses its flexibility and becomes a glassy material. In this case, the temperature change was the driving factor for the glass transition.
Glass transition can be assessed using various techniques, the two most important ones are:
- Differential scanning calorimetry (DSC) – measures the differential heat flow between a polymer sample and an inert reference at atmospheric pressure.
- Dynamic mechanical thermal analysis (DMTA)- by sinusoidally varying stress applied to a polymer, this test measures the polymer deformation as a function of time, temperature and frequency around its Tg.
- BeMiller, J.N. “Polysaccharides: Properties.” Carbohydrate Chemistry for Food Scientists, 3rd edition, AACCI and Elsevier Inc., 2019, pp. 103–109.
- Ahmed, J., and Rahman, S. “Glass Transition in Foods.” Engineering Properties of Foods, 4th edition, CRC Press, Taylor & Francis Group, LLC, 2014, pp. 93–111.
- Delcour, J.A., Hoseney, R.C. “Glass Transition and Its Role in Cereals.” Principles of Cereal Science and Technology, 3rd edition, Cereals & Grains Associations, AACC International, Inc., 2010, pp. 89–96.