p-Anisidine value (p-AV)
The AOCS standard method (Cd 18-90) was used to measurep -AV (AOCS, 2011). First, ap -anisidine solution was prepared by dissolvingp -anisidine (25 g) in acetic acid (100 ml). Oil samples (about 0.5 g) were weighed into a 25 ml volumetric flask and isooctane was added to fill the flask. The sample solution (5 ml) was transferred into a test tube, thep -anisidine solution (1 ml) was added, and the solution was briefly vortexed. After 10 min, the absorbance was measured in a quartz cuvette with the same spectrometer used for PV at 350 nm. Isooctane (5 ml) and p -anisidine solution (1 ml) added in a test tube was used as a blank. When p -AV was over the measurement limit, the concentration of the sample was reduced to half and the value was measured again. p -AV of three experimental replicates were determined.