peanut brittle experiments

What Is The Glass Transition Temperature?

Wondering why your brittle remains soft, instead of getting crunchy? Or want to know how candy floss works? Then you might come across an important, but quite a complex concept: the “glass transition temperature”.

It can be a tricky concept to wrap your mind around. The glass transition temperature describes when a material transitions from a hard, glassy material, into a viscous, but pliable product. In other words, it can explain why a previously crunchy peanut brittle turned chewy, helping you become a better candy maker.

Glass is a liquid that seems solid

Most of us are familiar with the three basic states of matter: gas, liquid and solid. You probably know that: frozen ice is a solid, room temperature water is a liquid, and evaporated water is a gas.

What sets these three phases apart from each other is how freely the atoms and/or molecules within the material are able to move:

  • In a solid, they are tightly organized in a crystalline structure. They are pretty much stuck in place.
  • In a liquid, they can move around but do stay together.
  • In a gas, molecules are limited by physical boundaries but move around freely within that space.

So what’s a glass? Well, it doesn’t neatly fall into any of these three distinct categories. Glassy materials look like a solid. They hold their shape, they crack, and don’t flow. But, on an atomic scale, they look like a liquid! That is, the atoms aren’t structured in a crystalline format. Instead, they are organized more randomly, just like a liquid would be. We call this an amorphous structure.

hard candy

Hard candy is a common glass in food

A good example of a glass in food is hard candy. This type of candy is very hard and brittle, but has an amorphous structure within. It doesn’t contain crystals. Honeycomb is an example of hard candy, as are some types of pulled candy.

Another example of a glass is cotton candy. Cotton candy melts in your mouth and isn’t hard and brittle. But, that’s only because the threads making up cotton candy are very thin. Every thread is a glass though.

A transition from viscous to hard

For a liquid to turn solid, the molecules of that liquid need to be able to organize themselves tightly into an organized crystalline structure. However, if a liquid is very viscous, it may be hard for the molecules to organize themselves this way. Instead, the molecules are ‘locked’ into place, without actually being able to form that organized structure. When this happens, a material turns into a glass. This transition, from viscous liquid into glass is the glass transition.

The viscosity of a liquid describes how it flows. Water is a liquid with quite a low viscosity, it flows quite easily. Honey has a higher viscosity, it’s thicker and clearly flows differently. A highly concentrated sugar syrup is even more viscous.

Which depends on temperature

Whether something is a gas, a liquid, or a solid, depends, among other factors, on the temperature. (Which is one of the reasons it’s so important to measure the temperature when making candy.) The temperature at which a component changes phase is called a phase transition temperature. For example, at the melting point a material changes from solid to liquid. At its boiling point a liquid transitions into a gas.

The transition from glass to viscous liquid isn’t a formal phase transition. The glass transition temperature works a little differently. Nevertheless, it does also depend on temperature and is governed by similar principles.

When a product is held ABOVE its glass transition temperature it is a viscous LIQUID. When it is held BELOW its glass transition temperature, it’s a GLASS.

Even though the name may make you think otherwise, the glass transition temperature isn’t one specific temperature. Generally speaking, the glass transition occurs over a range of temperatures. For instance, it might occur from 40-45°C (104-133°F).

Also, how and if it occurs depends on how fast you cool down the product. A lot of foods only form a glass if you cool it down quickly. If not, the molecules may be able to organize themselves differently, not giving the glass a chance to follow.

Lastly, it strongly depends on the composition of the product, which we’ll discuss below.

peanut brittle experiments

An example: peanut brittle

Peanut brittle is made by cooking a sugar syrup to a high temperature and then cooling it down again. While it’s hot, the mixture is liquid. You can easily pour it out of a pot, onto a tray. But, as it cools down, it becomes more viscous. At some point, it even turns solid, you’ve passed through the glass transition. Whereas you could bend warm peanut brittle, cooled down brittle will simply snap.

More or less moisture makes a difference

What can make the concept of glass transition temperature very hard to grasp is the fact that it depends very strongly on the exact composition of the product. For food especially, the amount of water plays a key role.

To explain, we’re going to dive deeper into the world of candy science.

To make candy you generally start by bringing a solution of sugar and water to a boil. You boil this sugar solution to a specific temperature. That temperature is a measure of the amount of water that is still left in the syrup. The higher the temperature, the lower the amount of water in that solution. (We discuss the science behind this in more detail here.)

The amount of water that’s left in the syrup has a big impact on the viscosity of the syrup. The less water is left, the more viscous, thus thicker, the syrup becomes.

Recall that especially very viscous liquids have a tendency to form a glass. It is harder for the molecules in such a liquid to move around and form nicely organized crystalline structures. As such, the higher the temperature of your cooked syrup has been, the more likely it is to be able to form a glass.

When there still is a lot of moisture in the candy, the glass transition temperature of the candy will be well below room temperature. Even if a candy cools down, it won’t form a glass since the glass transition temperature will be too low.

The lower the moisture content of your candy though, the higher the glass transition temperature. As such, it becomes more likely that your candy passes through the glass transition when cooling down. To make hard candy you generally need to boil your sugar syrup to temperatures above at least 132°C (270°F) to make a glass. The exact temperature depends on the composition of the candy though.

sucrose state diagram
You can visualize the behavior of sugar in a diagram. Note that the glass transition temperature goes up for an increasing amount of sugar (the remainder being water). Learn more about these diagrams here.

Generally speaking, the relationship between water content and glass transition temperature is as follows:

The LOWER the water content, the HIGHER the glass transition temperature.

Why making candy on a hot humid day is hard

Recipes for candy often state that you shouldn’t attempt them on a hot humid day. One of the reasons for doing so has to do with the glass transition temperature.

In order for a hard candy to become glassy, it needs to:

  • cool down quickly
  • have a low moisture content.

If the glass transition temperature of your candy is 27°C (80°F) hot days can make it impossible for the candy to cool down enough to dip below its glass transition temperature. Keep in mind that only when the product is below the glass transition temperature will it turn glassy!

High humidity doesn’t help either. On a hot humid day, the air contains a lot of moisture. Hard candy on the other hand contains very little moisture. By placing the candy in a humid environment, some of the moisture from the air will be pulled in by the candy. As a result, the glass transition temperature dips. If the glass transition temperature dips below the storage temperature, the candy no longer becomes hard and brittle. Instead, it will be rubbery and slightly flexible.

This is also why you should always store these types of candy in an airtight container. Even on non-humid days the air likely contains more moisture than the candy, and moisture will slowly move into your candy. As a result, your previously hard and brittle candy turns soft over time!

It’s also why cotton candy melts in your mouth almost instantaneously. Cotton candy is made of a lot of thin glassy strands. Once they touch your saliva, the moisture content increases rapidly, turning the glass, into a sugar solution!


TU Eindhoven, The glass phase: a physics mystery, link

Tom Husband, The sweet science of candymaking, Oct-2014, link

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