EFFECT OF LIGHT ON THE KINETICS OF OXIDATION REACTIONS IN VEGETABLE OILS

Abstract

The article deals with the effect the lighting conditions of storage have on the oxidation stability of vegetable oils of various unsaturation degrees and made by different technologies (raw-pressed and unrefined sunflower oil and flaxseed oil) according to the main oxidation parameters: peroxide value (PV), acid value (AV), and colour value. It has been experimentally established that in natural lighting, at the end of the shelf life, the PV of flaxseed oil increased most significantly, from 0.55·10-3 to 11.2·10-3 mmolO2/kg (20 times). The PV of sunflower oils varied less: in unrefined oil, it changed from 0.48·10-3 to 4.5·10-3 mmolO2/kg (9 times), in raw-pressed, from 0.3·10-3 to 4.4·10‑3 mmolO2/kg (14.7 times). Under artificial UV radiation, the PV of sunflower oil increased from 0.3 to 0.55 mmol O2/kg after 2 hours of the experiment, and remained practically unchanged. In the case of flaxseed oil, after a slight increase from 0.55 to 0.7 mmol O2/kg, within the same period of time, there was then a decrease in the PV to 0.45 mmol O2/kg, which may indicate a relative instability of hydroperoxides that, due to their degradation, converted into more stable secondary compounds. The analysis of the oil oxidation kinetics by the PV has shown that the average rates of peroxide compounds accumulation in unrefined, raw-pressed, and flaxseed oils was 47·10-4, 48·10-4, 127·10-4 mmol ½О/kg·hour, respectively. At the same time, under natural light, the true rate of change of the AV in the oils studied was uneven over time. At the beginning of the experiment, it increased (especially noticeably in the case of flaxseed oil), at the end of the shelf life (the 5th week), it was slowed down significantly, decreasing to negative values, and in flaxseed oil, it had zero value, which indicates a constant value of AV in the experimental setting.