As a chef, there are two types of properties to consider when choosing a gelling agent: aesthetic and functional.
Gel Aesthetic Properties
Aesthetic properties are the properties of gelling agents that determine how the end consumer (the diner eating the food) perceives them.
Gel mouthfeel, brittleness, elasticity and stiffness
Mouthfeel is the broad, difficult to define category that people use to describe a wide range of sensations. With thickeners, we talked about viscosity and creaminess, and while these do matter to gels, we'll talk more about brittleness, elasticity, and stiffness.
In edible gels, brittleness and elasticity are usually related. Elasticity is defined by the ability of a material to return to its original shape after it is deformed. Consider the difference between Play-Doh (not elastic) and a water balloon (elastic). Brittleness is defined by the amount an object will deform before it breaks. Think about the difference between a new rubber band and one that has been dried out for a few months. The dry rubber band snaps after much less pulling than the new one.
While there are inelastic materials that have a brittleness independent of their elasticity (like metals, for example), edible gels are almost all elastic, so you can think of elasticity and brittleness as opposites when dealing with edible gels.
Be sure not to confuse elasticity and brittleness with stiffness. Although these qualities are perceived in similar ways by the diner, chefs should understand the distinction. Whereas elasticity and brittleness are innate properties of a hydrocolloid, stiffness is defined by the concentration of hydrocolloid to water. For example, a cube of gelatin is made with the same ingredient (gelatin) as a gummy candy, but the two have very different textures that depend on the how much gelatin is used.
Gel Appearance: clarity
And let's not forget the final aesthetic quality: appearance. Some gels are more transparent than others. This property is known as clarity. For example, gelatin forms a clear gel while agar forms a semi-transparent gel.
Gel Functional Properties
The whole point of gelling agents is to create textures from ingredients that are normally liquids. In order for chefs to create novel textures, they have to understand the capabilities and limitations of the gelling agents they are using.
Dispersion and Hydration Temperature
The most important functional concern is the temperature at which hydrocolloids disperse and hydrate. In the cornstarch example, note that cornstarch will disperse in cold water but does not hydrate until heated. If, on the flip side, you tried to disperse corn starch in hot water, some of the cornstarch would rapidly hydrate and form protective shells around powdered cornstarch within, resulting in lumps of the thickener in your liquid. In addition, cornstarch will adopt very different textures depending on the temperature you choose to serve it at.
We extensively discuss the temperature properties of many hydrocolloids in our hydrocolloid guide.
Gelling and Melting Temperature
Gelling agents have a few special temperature considerations. All gels have a temperature at which they will gel and melt. Let's look at the example of plain gelatin.
Gelatin should be dispersed in room-temperature or cold water to prevent lumps. It hydrates at temperatures near the boiling point of water. Once the gelatin liquid has been heated, it forms a gel at refrigerator temperatures. The gel begins to melt at the upper range of reasonable room temperature.
And some strange things can happen at other temperatures as well as we explain below.
Gel syneresis and freeze-thaw stable gels
If you were to freeze gelatin and then place it in the refrigerator, much of the liquid held in the gel would leak out. This is called "syneresis" and it's generally considered an undesirable quality. Not all gels exhibit syneresis after freezing, but usually those gels must be stabilized with a combination of hydrocolloids. A gel that does not exhibit syneresis after freezing is called "freeze-thaw stable."
Thermoreversible and thermoirreversible gels
Freeze-thaw stability should not be confused with thermoirreversibility. If a gel is thermoreversible, it can be converted back into a liquid form through a simple temperature change. Gelatin, for example, is thermoreversible. Egg white, on the other hand is thermoirreversible. Once an egg white has set, there is no way to change it back into liquid egg whites using temperature alone.
Some gels demonstrate a property known as hysteresis. Explained simply, hysteresis is a range of temperatures in which a gel is likely to stay in its current state. When we think of solids and liquids, we think of a static temperature at which a solid turns into a liquid and vice versa. For example, water turns into ice at 32°F/0°C and ice turns into water at the same temperature. But, some gels gel at a temperature different from the temperature at which they melt. For example, agar agar melts at 185°F/85°C and forms a gel at 88°F/32°C. The state of agar in between this range depends on the state that it is currently in.
When we talk about using blenders with hydrocolloids, we usually mean using these machines to help hydrate a hydrocolloid. But some hydrocolloids display a unique characteristic enabled through a blender. If you were to blend a gel made of gelatin or carrageenan, the force of the blades (called shear) would turn the gellified gelatin into a liquid. If the temperatures were right, that liquid would slowly convert back into a gel. This shear thinning property is called thixotropy. Under static conditions, these hydrocolloids produce solid gels (or thick viscous fluids) that will start flowing and become less viscous when agitated, stirred, shaken or blended. If let to rest, these fluids would return to their original solid state or thick viscosity. The time it takes for this transformation depends on the hydrocolloid. It could be instant or it could take a few hours.
Other hydrocolloids such as agar agar, become fluid when blended but do not return back to their original solid gel state. When blending an agar agar gel, it breaks down into tiny pieces but will not revert back into a solid. We refer to this by saying that agar agar forms a fluid gel under shear. A fluid gel is still a gel, but because of the smaller particle size, it behaves in many ways like a thickened fluid. Agar agar and gellan gum are the most common hydrocolloids used to make fluid gels.
Fluid gels have both the properties of solids and liquids. Gels are usually solidified liquids that hold its shape thanks to their molecular structure. On the other hand, liquids are fluid and do not hold their shape given their weak molecular structure. But fluid gels can hold their shape while they are also fluid and can be spread or reshaped into various forms.
The below recipe for Blackberry in Textures by Chef Russell Karath exhibits a tuile formed by dehydrating a fluid gel in the foreground.
Gel Flavor Release
Some gels have better flavor release than others. In general, the softer the gel the better the flavor release. High-fat gels and starches tend to have a slow and long-lasting flavor release while gelatins, pectins and gellan gels tend to have a fast and short-lived flavor release.
The final functional properties all deal with how a hydrocolloid behaves in the presence of other things. For example, will a gel form when used in conjunction with sugar, alcohol, and acid?
Thickeners can also demonstrate synergies with other hydrocolloids as well as in the presence of ions. For example, neither locust bean gum nor xanthan gum form gels alone, but will form a gel when you combine the two hydrocolloids. Another example: carraggeenan is stronger in the presence of calcium, which is why it is often used in calcium-rich dairy applications.