Novel caffeic acid amide antioxidants: Synthesis, radical scavenging activity and performance under storage and frying conditions

Twelve novel dihydro-caffeic acid amides were synthesised in good yields and fully characterised by 1H NMR, 13C NMR, and MS. Their radical scavenging activities were assessed by DPPH assay. Additionally, their abilities to protect polyunsaturated oils under accelerated storage and frying conditions were evaluated. All the new compounds possessed significantly higher radical scavenging activities than α-tocopherol and butylated hydroxytoluene (BHT). The radical scavenging activity of N-decyl-N-(3,5-dimethoxy-4-hydroxybenzyl)-3-(3,4-dihydroxyphenyl) propanamide was 1.7 and 4 times higher than α-tocopherol and BHT, respectively. At the end of the storage period, the respective amounts of hydroperoxides in canola oil triacylglycerols (CTAG) fortified with α-tocopherol and BHT was 6.1 and 1.4 times higher, respectively, than CTAG containing the amide. The frying test showed that CTAG containing N-decyl-N-benzyl-3-(3,4-dihydroxybenzyl) propanamide was 1.3, 1.4, and 1.6 times more stable compared to oil fortified with dihydro-caffeic acid, α-tocopherol and BHT, respectively, as assessed by the amounts of the total polar compounds. Moreover, these compounds were remarkably thermally stable, making them suitable for frying applications.