All thickeners essentially do one thing: they increase the viscosity of a liquid by a set amount. So why are there so many different types of thickeners?
The properties of thickeners fall into two categories: aesthetic and functional.
Aesthetic properties are the properties of thickeners that determine how the end consumer (the diner eating the food) perceives them.
Mouthfeel is the broad, difficult to define category that people use to describe a wide range of sensations. For example, many people perceive xanthan gum as having a "snotty" texture in high concentrations while locust bean gum is more likely to be described as "thick" and "sticky." The two most important measurable mouthfeel sensations are viscosity and creaminess.
Viscosity is a physical property defined by the amount of energy it takes to travel through a liquid. That is, how hard do you have to push with your tongue to move it through the liquid in your mouth? Viscosity in turn affects the speed at which a liquid can flow, and particularly interesting to chefs, the shape that a thickened sauce will take on the plate.
Creaminess has to do with the particle size of a hydrocolloid. Larger particles are perceived as more grainy, while smaller particles are considered more creamy. For example, traditional white flour can be used to thicken stews, but is generally perceived as grainy when used in higher concentrations. In response, manufacturers have developed the Ultra-Sperse and Ultra-Tex lines of products as thickeners that can be used more easily than flour and have a creamy mouthfeel.
Flavor release is related to viscosity, but the two are not always directly correlated. Flavor release is exactly what it sounds like: how much does the addition of a thickener affect the ability to perceive the flavors in the original liquid? Thickeners with better flavor release are generally preferred.
And let's not forget the final aesthetic quality: appearance. Most thickeners will adopt the colors of the liquid they are used in, but some will be more opaque than others. This property is known as clarity.
Functional properties are those properties of hydrocolloids that define how chefs are able to use them. Much of the food industry, when developing and testing hydrocolloids, focuses on functional properties rather than aesthetic qualities. That's because many thickeners already exist for established recipes. New innovations push boundaries by applying thickeners to recipes where they have not previously been tried.
The most important functional concern is the temperature at which hydrocolloids disperse, hydrate, and thicken. 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. Two quick rules of thumb: (1) xanthan gum is the easiest and most consistent hydrocolloid to use at a range of temperatures and (2) methylcellulose, particularly hydroxypropyl methylcellulose F50, is one of the few hydrocolloids that thickens with increased temperature and looses its thickness when chilled.
The next functional property worth discussing is the scaling or strength of a thickener. That is, how much of a thickener does it take to thicken a liquid to a desired level? Xanthan gum is well-known for being a powerful thickener, but that also makes it difficult to use. A slight mismeasurement can result in an overthickened product that can't be reversed. On the opposite end of the spectrum, gum Arabic is a very weak thickener. It would take many hundreds of times as much gum Arabic to result in the same thickness as a product thickened with xanthan.
The final functional properties all deal with how a thickener behaves in the presence of other things. For example, will the thickener function when used in conjunction with sugar, alcohol, and acid?
These properties become much more relevant in the case of gels, as gels can break or "weep" liquid in the presence of impurities. But since many gelling agents can also be used to give thickness, these ingredients should be considered when creating new recipes.
Thickeners can also demonstrate synergies with other hydrocolloids as well as in the presence of ions. For example, guar gum thickens more in the presence of xanthan gum, while carraggeenan is stronger in the presence of calcium.