Sinapic acid and its derivatives are main phenolic compounds present in rapeseed. Identification and quantitative determination of those compounds were carried out in crude extracts as well as extracts after basic and acidic hydrolysis. The total phenolic content in the crude extracts, expressed as sinapic acid equivalent, ranged from 1577 to 1705 mg/100 g for cv. Visby and Bellevue, respectively. An increase in total phenolics contents was observed in extracts after hydrolysis in comparison to crude extracts. The predominant phenolic acids in rapeseed samples were trans-sinapic and cis-sinapic acids. The highest concentration of sinapic acid methyl ester (2533–2702 mg/100 g) was detected in the extract after basic hydrolysis. In turn, two sinapic acid derivatives, i.e., kaempferol 3-dihexoside-7-sinapoyl-hexoside and 3-hexoside-7-sinapoyl-hexoside, were detected in the hydrolysed extracts. Total peroxyl radical-trapping potential (TRAP) of the investigated crude extracts amounted to 20.21 and 27.05 μM Trolox Eq/g in the case of cv. Visby and Bellevue, respectively. The increase in antioxidant activity of the analyzed extracts after basic and acidic hydrolysis was probably a result of the release of free phenolic compounds from their conjugates (esters, glycosides).
Practical applications: The content and qualitative composition of the phenolic compounds in rapeseed have much influence on the stability, sensory, and nutritional characteristics of the products. During oil production process phenolic compounds may be transferred into final products and inhibit lipid oxidation. Rapeseed oil, among all commercially available oils, has the highest content of phenolic compounds, especially sinapic acid and their derivatives. That is why the antioxidant activity of phenolic compounds was investigated in the presented study. Determination of polar phenolics before and after hydrolysis indicates which compounds are responsible for antioxidant effect in oils and which are available for the human organism after digestion in the digestive tract.