Rapeseed oil is the most popular vegetable oil in Poland. It is a valuable source of bioactive compounds such as polyphenols, phospholipids, and sterols. Additionally, it contains a high amount of essential fatty acids. Unfortunately, oils containing a high amount of unsaturated acids are easily oxidized. Therefore, it is desirable to protect the oil from oxidation process. One of the ways to protect the oil from oxidation is supplementation with antioxidants.

For this reason, the aim of this work were (1) the enzymatic synthesis of ethyl sinapate and (2) the fortification of refined rapeseed oil with two concentrations 0.02 and 0.5% of the synthesized ester. Then in vitro gastrointestinal digestion was performed on the two-step model. Antioxidant activity of oil samples before and after digestion was analyzed by three spectrophotometric methods: 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Folin–Ciocalteu. The addition of synthesized ethyl sinapate caused an increase in antioxidant activity measured by each method, while increasing the ester concentration increased the antioxidant activity from two to ten times. An increase in antioxidant activity also was observed after two-step digestion. The enzymatically synthesized ethyl sinapate improved the antioxidant potential of refined rapeseed oil. Therefore it could probably be used as an efficient antioxidant in the oil industry.