By Harold McGee
A single microscopic fragment of a tomato cell wall or muscle fiber is built up of many thousands of submicroscopic molecules. Not all of the large molecules in those fragments can be teased away from each other so that they are individually dispersed in water. But those that can be extracted in this way—starch, pectins, such proteins as gelatin—are very useful thickening agents. Because single molecules are so much smaller and lighter than intact starch granules and cell fragments, they don’t settle out and separate. And they are too small and too widely separated to block the passage of light rays: so unlike suspensions, molecular dispersions are usually translucent and glassy-looking. In general, the longer the molecule, the better it is at obstructing water movement, because long molecules more readily get tangled up in each other. So a small quantity of long amylose starch molecules will do the same thickening job as a large quantity of short amylopectin, and long gelatin molecules thicken more efficiently than short ones. Thickening with molecules often requires heat, either to liberate the molecules from the larger structures— starch molecules from their granules, gelatin molecules from meat connective tissue—or to shake out compactly folded molecules—egg proteins—into their long, extended, tangly form.