Thickening Agents: Starches

On heating, starch granules absorb water and begin to swell. Thickening is caused by: (i) the absorption of water, (ii) the restriction of water flow by the swollen granules, and (iii) the leaching of starch molecules from the swollen granules into solution. Overheating, or adding acid, may thin the sauce by breaking up the granules or the leaked starch molecules.

flour, the most common starch, needs to be preheated with fat to remove the ‘floury’ taste. The relatively high protein content in flour reduces its relative starch concentration and thus its effectiveness to thicken. These proteins can also cause lumps, and the formation of a skin. Other grain starches, like cornflour, thicken more efficiently and without forming lumps, due to the absence of protein. However, they impart a fairly strong flavour.