Synthesis, characteristics and evaluation of antioxidant activity of [1-(tannin-ether)-ethyl]stearate

The [1-(tannin-ether)-ethyl]stearate (TEES) was synthesized by two-stage process successfully. 1-Chloroethyl tannin ether (CTE), as an intermediate, was initially prepared with tannic acid (TA) and paraldehyde. Then, the TEES was synthesized by sodium stearate and CTE in the presence of FeCl₃-PEG-400 as phase-transfer catalyst. Synthetic conditions were optimized. The structural characteristics of TEES were analyzed by FTIR, ¹H NMR and UV-vis techniques. And the thermal stability of TEES was investigated. Moreover, the antioxidant activity of TEES for linseed oil was evaluated and compared with other substance such as TA and butylated hydroxyanisole (BHA). The results showed that the yield reached 88.19 wt% (theoretical value: 88.80 wt%, relative deviation: 0.80 ± 0.34%) under the optimized condition, in which the ratio of TA: FeCl₃: PEG-400 was 1 g: 0.09 g: 0.693 mL, the reaction temperature and time was 75 °C and 240 min, respectively. The antioxidant activity of TEES was higher than TA and comparable to BHA in linseed oil. The POV of oil samples with TEES, TA and BHA were 63.4, 201.3 and 84.2 meq/kg after 20 days, respectively. The reason of this was relate to the better oil solubility of TEES and its unique structure. More importantly, the interaction between the TEES and SC was weaker than that of TA and SC by the fluorescence experiment.