Total tocopherols (TT) content
A calibration curve of α-tocopherol in toluene (0 – 240 µg/ml) was
prepared. Five ml of toluene were added to 100 ± 10 mg of the oil sample
in a 10-ml volumetric flask. Three and one-half ml of 2,2’-bipyridine
(0.07% w/v in 95% aqueous ethanol) and 0.5 ml of
FeCl3.6H2O (0.2% w/v in 95% aqueous
ethanol) were successively added. The solution was made up to 10 ml with
95% aqueous ethanol. After standing for 1 min, the absorption at 520 nm
was read using a blank solution as a reference, prepared as above but
omitting the oil. All the operations during color development were
carried out under subdued light. Results were expressed in mg of
α-tocopherol per kg of oil (Wong et al., 1988).