Rancidity in fats develops through oxidation of double bonds in unsaturated fatty acids, and the earliest chemical products of that oxidation are peroxides, so the value that tracks peroxides is the one used to catch rancidity as it develops.
- Saponification value: This tells us how large the fatty acid chains in the oil are on average. It stays essentially the same whether the oil is fresh or rancid, so it cannot be used to detect spoilage.
- Iodine value: This value tracks how many carbon-carbon double bonds are present, giving a measure of how unsaturated (and therefore how susceptible to future rancidity) the oil is. It is a predictor of rancidity risk, not a direct measurement of rancidity that has already happened.
- Peroxide value: As oxidation proceeds, oxygen adds across the double bonds to form peroxide intermediates well before the oil starts to smell or taste off. Measuring the peroxide value catches this oxidation at an early stage, which is why it is the standard indicator of rancidity.
- Acid value: This reflects free fatty acid content released mainly by hydrolysis (breakdown of the ester bond by water), a separate spoilage route from oxidative rancidity.
Because peroxides form directly and early as oxidative rancidity sets in, the correct answer is Peroxide value.