1. Sugar competes with starch for water by dissolving in water. The less water available for starch, the less gelatinization, so less thickening. Acid breaks down starch molecules. Less starch available, less gelanization, so less thickening power. Adding fat decreases viscosity which then decreases gelatinization and hence less thickening.
2. Flour has less amylopectin than pure starch. When less amylopectin is present, the gel formed is stronger and the thickening power is ...
This solution discusses the thickening power of starch, sugar, acid and fat.