Oxidative rancidity is caused by oxidation process of oil. During this process, oxygen molecules will be bounded on double bind of unsaturated fatty acids. The double binds of this unsaturated fatty acid which have been oxidized will be broken to form short chained fatty acid, aldehyde, and ketone. The oxidative rancidity rate depends on the quantity of unsaturated fatty acids contain and double binds contain in the oil. The higher unsaturated fatty acid contains and higher the double bind contains, faster the rate of oxidative rancidity.
A phrase ‘auto oxidation’ refers to oxidative rancidity in which the oxygen comes from the material itself, whether atmospheric oxidation occurs when the oxygen came from atmosphere.
Oxidative rancidity mechanism is described below:
Oxidation occurred on unsaturated bind of fatty acid. At room temperature till 1000C, every single unsaturated bind can absorb 2 oxygen atoms to form unstable peroxide compound. This peroxide can decompose unsaturated radical which is still intact so that 2 peroxide compounds formed. Labile peroxide can form isomeric compound (dihydroxy compound or derivate of alfa-hydroxy ketone). Isomeric compound will be decomposed to aldehyde compounds with lower molecular weight as epyhydryn aldehyde and ketone compound.
Hydrolytic rancidity is caused by the hydrolytic process. This process happened because of either water contain of the oil itself or water from the air, during this process, oil’s components decomposed to glycerol and free fatty acid. Hydrolytic triglyceride occurs in 3 steps. At the first step, triglyceride is broken into diglyceride. At the 2nd step the diglyceride is broken into monoglyceride. At the final step the monoglyceride is broken into glycerol and free fatty acid.
Enzymatic rancidity is caused by enzyme’s action. Water and impurities such as protein and carbohydrate will be good medium for microorganism growth. Microorganism then will produce lipase enzyme which can decompose oil or fats becomes glycerol and free fatty acids.
Source: Iskandar, Alex.“Pengambilan Minyak Kelapa dari Santan dengan Cara Fermentasi Menggunakan Campuran Biak Murni Zygosccharomyces rouxii CBS-732 dan Saccharomyces cereviceae St.-381”.Jurusan Teknik Kimia Fakultas Teknik Universitas Diponegoro. Semarang.1996