Which test indicates the presence of starch and causes a color change to dark purple or black?

Which test indicates the presence of starch and causes a color change to dark purple or black?

• A. Benedict's solution
• B. Lugol's iodine
• C. Biuret's solution
• D. Sudan IV

Answer: (B)

Starch detection relies on the starch-iodine reaction. Iodine fits into the helical structure of starch (especially amylose) and forms a starch-iodine complex that has a dark purple to black color. That deep color signals the presence of starch. Other tests detect different biomolecules: Benedict's solution responds to reducing sugars and changes color with heat, Biuret's reagent produces a violet color in the presence of peptide bonds in proteins, and Sudan IV stains lipids by coloring fat droplets red. So the test that indicates starch with a dark purple/black change is the iodine-based Lugol’s test.

Starch detection relies on the starch-iodine reaction. Iodine fits into the helical structure of starch (especially amylose) and forms a starch-iodine complex that has a dark purple to black color. That deep color signals the presence of starch. Other tests detect different biomolecules: Benedict's solution responds to reducing sugars and changes color with heat, Biuret's reagent produces a violet color in the presence of peptide bonds in proteins, and Sudan IV stains lipids by coloring fat droplets red. So the test that indicates starch with a dark purple/black change is the iodine-based Lugol’s test.