The ability to thicken a soup, gravy, sauce, or beverage is a critical tool for any cook. The trick to creating a successful thickened sauce, soup, or gravy is through incorporation of a complex carbohydrate and/or protein that binds water, yielding a more viscous liquid. The feeling that comes with tasting and eating a thick gravy or salad dressing is called mouthfeel. In order to understand how thickening works, you need to learn some physics about liquids.
Water is considered a Newtonian fluid because there is a proportional relationship between viscosity and the force applied to the fluid. The more viscous the Newtonian fluid, the more force required to get it to flow. Gravies and other thickened foods are non‐Newtonian fluids; these fluids require a larger force to start movement, but once in motion non‐ Newtonian fluids move with a greater ease.
Most of you have experience with ketchup. What a great example of a fluid that does not easily start to move but, once flowing, flows much faster than you often desire! What does a non‐Newtonian fluid have to do with mouthfeel? The stickiness and viscosity of a non‐Newtonian fluid are amazingly detected by the human mouth and give foods containing these fluids a distinctive, pleasurable character.