Spoilage of oil can be detected by Oleic acid.
Rancid is the unpleasant smell or taste usually from decomposition or chemical change of fats and oils. Rancidity is determined by measuring the Peroxide Value, Acid Value, p-Anisidine (p-AV) testing, and Refractive Index (RI) of the oils and fats. The presence of peroxide gives the initial evidence of rancidity in unsaturated fats and oils ( value should be >15 milli-equivalents (meq)/kg). Refractive index (RI) value shall be specific for a specific fat or oil i.e. the RI for rice bran oils is between 1.4600 and 1.4700. The p-Anisidine (p-AV) value indicates the presence of aldehydes in the lipids that are the secondary indicators of oxidation.
Free Fatty Acids (FFA) value determines the number of fatty acids that are liberated from their triglyceride structure after spoilage or decomposition. Free fatty acids are produces due to microbial activity. Therefore, FFA is the indicator of the degree of spoilage. The acid value is a measurement of these free fatty acids in the fats and oils and the value should not be more than 2%. FFA is calculated as a percentage of free oleic acid (a monounsaturated omega-9 fatty acid) of total sample weight. Therefore, spoilage of oil can be detected by the presence of Oleic acid.