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Method to Determine Rancidity of Oil

Peroxide Value and Anisidine Value

Edible oil can be oxidized due to temperature and time. It generates radical substance and rancid odour. Hence oil is unable to be consumed. Peroxide Value is considered miliequivalent oxygen each kilogram sampel. The greater number of peroxide value, the more rancidity oil will become. Here's why peroxide value should be lower so that oil can be consumed. 

Peroxide value method is conducted through these steps. Sample is dissolved with solvent 30 mL (acetic acid and iso octane 3:2). Sample is added 0,5 mL KI saturated. It's carried out so that oxidative substance make bond with iodine. The mixture is added with 30 mL water in order that to stop reaction. The mixture is added with starch indicator. It produces dark purple. The mixture is titrated with sodium thiosulphate 0,01 M. The white mixture is produced. The principle of this method is iodometry titration (AOCS Cd 8b-90).

Besides peroxide value, anisidine value is also parameter to determine rancidity of oil.  This method applies spectroscopy UV-VIS method. The light of absorbance by imine chromophore which is generated through condensation between p-anisidine and aldehyde (Ye et al, 2020). Oil is dissolved with iso-octane. The oil solution is taken 1 mL and added 5 mL p-anisidine 1%. The way to generate p-anisidine 1%. P-anisidine is illustrated below. P-anisidine 1 g is dissolved in 100 mL acetic acid. The mixture is measured through spectrophotometer UV-VIS at 350 nm. p-anisidine is convention 100 times of optical density oil. It measures secondary oxidation compounds which is given through decomposition of peroxides. The compounds are 2-alkenals and 2,4-dialkenals (Gupta 2017).  


In addition, totox is also method to determine rancidity of oil. Totox is  total twice of peroxide value and anisidine value. In other condition, peroxide value is steady however ansidine value is getting bigger. It's been occured since the secondary oxidations have been given from radical agent which is existed inside oil. 

Reference 

AOCS Cd 8b-90

Gupta, M, 2017, Practical Guide to Vegetable Oil Processing, 2nd Edition, London : AOCS Press

Ye, L, Harris, M, Budge, S, Ritter, J, 2020, Flavour Decreasing Contribution to p-Anisidine Value over Shelf Life may Invalidate the Current Recomended Protocol for Flavoured Fish Oils, J.Am.Oil.Chem.Soc., 97(12): 1335-1341 

   

   

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