By Harold McGee
Compared to the other ways of thickening, simple pureeing tends to produce a coarse sauce that more readily separates into solid particles and thin fluid. The solid fragments of plant cell walls are clumps of many thousands of carbohydrate and protein molecules. If those molecules were dispersed separately and finely throughout the fluid— as gelatin or starch molecules are in other sauces—they would bind many more water molecules, get tangled up in each other, and be far too small for the tongue to detect as particles. But plant-cell fragments range from 0.01 to 1 millimeter across; they give a grainy impression on the tongue and they’re far less efficient than individual molecules at binding water or interfering with fluid flow. And because the fragments are usually denser than the cell fluids, they end up sinking and separating from the fluids. Heating without stirring tends to speed this separation, because the free water is able to flow and rise from the bottom of the pot through the thicker particle phase, and accumulate above it.