Step 1: Understanding the Question:
The question asks which chemical test is used to detect rancidity in oils.
Step 2: Key Concept:
Oxidative rancidity happens when the unsaturated double bonds in fatty acid chains react with atmospheric oxygen to form hydroperoxides as intermediate breakdown products, before these go on to form the smaller aldehydes and ketones responsible for the rancid smell.
Step 3: Detailed Explanation:
The peroxide value test directly titrates and quantifies these hydroperoxide intermediates, so a rising peroxide value tracks oxidative rancidity as it develops. Saponification value only reflects the average chain length and molecular weight of the fatty acids present, iodine value only reflects how many double bonds are available to react, a risk factor rather than actual spoilage, and acid value mainly reflects hydrolytic breakdown of the ester bonds rather than oxidation. Only the peroxide value measures the actual oxidative degradation product.
Step 4: Final Answer:
Peroxide value.