PREVIOUSLY ASKED IN:
WBPSC Miscellaneous Preliminary 2019
Answer
starch
Explanation
Rice water is extremely rich in starch, which is a complex carbohydrate. The iodine test is a standard chemical test used to determine the presence of starch. When an iodine solution (potassium iodide) is introduced to starch, the iodine molecules slip inside the amylose coil of the starch, creating a complex that reflects a distinct blue-black color. Fats and proteins do not produce this reaction.
Key Points
- > Starch is a complex carbohydrate and the primary energy storage molecule in plants.
- > The natural color of iodine solution is yellowish-brown, which instantly turns blue-black upon contacting starch.
- > To test for the presence of proteins, the Biuret test is commonly conducted.
- > Fats are tested using the Sudan III stain or a simple translucent spot test on paper.
- > Starch consists of two distinct polysaccharide fractions: amylose and amylopectin.
Additional Information
Food Component Chemical Tests
| Food Component | Test Name | Reagents Used | Positive Result Color |
|---|---|---|---|
| Starch | Iodine Test | Iodine Solution | Blue-Black |
| Protein | Biuret Test | CuSO4 + NaOH | Violet / Purple |
| Reducing Sugars | Benedict's Test | Benedict's Solution | Brick Red / Orange |
| Lipids (Fats) | Emulsion Test | Ethanol + Water | Cloudy white emulsion |
Memory Tips
- Iodine-Starch Complex: Always remember I-S-B (Iodine + Starch = Black/Blue).