Starch is often used in fabric weaving to avoid breaking of the thread. It increases the strength of thread and also decreases the friction during weaving. This starch is must to remove before any kind of dyeing or chemical washing of garment. A number of desizing process (removal of starch /size) Â are in practice in industry.
Starch forms a very dark blue-black compound with iodine. The indicator is not disturbed by the presence of iodide (I-). Starch forms an un-stable complex (blue colored) in low concentrations of Iodine. However, this complex is highly stable in high concentrations of Iodine solution (It can’t be decolorized and it spoils the reaction medium). Starch indicator solutions may be synthesized in the laboratory. Alternatively, commercially available preparations are also available.
When a garment is desized, then to check the extent to which starch is removed, a few drops of starch indicator are dropped on the garment. If it changes its color to blue-black, that indicates that desizing process is incomplete and starch is still present there.