What are the (most common) different ways to thicken sauces and gravies?

Question

I know of a few ways to thicken sauces and the like:

• Egg (protein)
• Flour (protein / starch)
• Starch (cornstarch etc)
• Reduction (reducing watery component)

Are these the only common ones, or what else should a cook keep in his repetoire?

Answer 1

Major ways of thickening a sauce -- or, for that matter, some other liquid:

• Starch (e.g., flour, cornstarch, tapioca, arrowroot)
• Emulsion (e.g., butter, cream, yolk, mustard, blending in lecithin-and-fat)
• Coagulated protein (e.g., blood, egg, pureed seafood)
• Gels/polysaccharides (gelatin, alginate, xanthan, pectin, agar agar, guar gum)
• Suspension (general puree of solids)

Note that some of the examples of one category can also fit into another.

Answer 2

What you listed are definitely the most common ways to thicken a sauce and most chefs stick to these basics.

However there ways I can think of to thicken sauce that you haven't mentioned.

• Some chef use blended cooked vegetables as bases for sauces(carrot, cauliflower, etc). I don't see why it can't work the other way if it is a flavour that works well with your current sauce then the blended vegetables can be added in to thicken that sauce.
• Gelatin found in roast liquids used to make gravies also participate in making the gravy thicker. I wouldn't recommend using powered gelatin though. You'll most likely end up putting too much and turn your sauce into savory jello. (Hmmmm lol)
• Xanthan gum as derobert mention in his comment can also be used to thicken sauces. It is a type of natural corn sugar polysaccharide. However note that it is safe to ingest up to 15 grams per day. Eating too much of it will cause bloating and gas.