The antioxidant ethoxyquin and its analogues: A review

This review records original research carried out in our Institute since 1983 on the antioxidant ethoxyquin (1,2 dihydro-6-ethoxy-2,2,4-trimethylquinoline)(EQ, I) used for the protection of the valuable polyunsaturated fatty acids in fish meal. The residual lipids in South African fish meals have a high degree of unsaturation, which renders the meals prone to spontaneous combustion. Addition of EQ is therefore essential. Accurate physical constants have been determined for EQ, while over the years a number of analogues of EQ have been prepared and their antioxidant efficacy in fish meal compared to that of EQ. It became clear that the efficacy of an antioxidant intended for fish meal must be determined in fish meal and not in an accelerated test in fish oil. Only one analogue could compete with EQ in efficacy and price namely hydroquin (1,2-dihydro-2,2,4-trimethylquinoline)(VII). A patent on this antioxidant in fish meal has been taken out. On storage of fish meal the extractable EQ content diminishes and eventually reaches zero. Other workers have demonstrated that in fish meal EQ is converted into two oxidation products: a dimeric oxidative coupling product, 1,8'-di-(1,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline) (EQ-dimer, XII) and a quinolone (2,6-dihydro-2,2,4-trimethyl-6-quinolone)(XIII). Both compounds were shown in our Institute to be potent antioxidants in their own right. The "half life" of the dimer was considerably greater than that of EQ, implying that even if the EQ content of a meal were to drop to negligible levels there would still be sufficient EQ-dimer to protect the meal lipids against oxidation. This may explain why EQ is such a strong antioxidant. A method for assessing the purity of EQ and the analysis of EQ is described. It entails the use of methoxyquin (1,2-dihydro-6-methoxy-2,2,4-trimethylquinoline) (MQ, II) a close analogue of EQ. This method can also be employed for the simultaneous determination of the quinolone and using MQ-dimer for the analysis of the EQ-dimer.