UHPLC-DAD-ESI-HRMS/MS profile of phenolic compounds in northern red oak (Quercus rubra L., syn. Q. borealis F. Michx) seeds and its transformation during thermal processing

Abstract

This paper describes for the first time a complete characterization and quantification of the phenolic compounds in raw and heat-treated northern red oak ( Quercus rubra L.) seeds using UHPLC-DAD-ESI-HRMS/MS. A total 42 phenolic compounds belonging to five different groups of compounds, including ellagitannins, gallotannins, phenolic glycosides, hydroxybenzoic acid derivatives (HBA derivatives) and ellagic acid derivatives (EA derivatives), were identified, and most of them were reported in Q. rubra seeds for the first time. Significant variability of phenolic composition existed among raw and roasted samples under different temperature and time conditions. The total content of the phenolics, ellagitannins, gallotannins and phenolic glycosides was drastically reduced by roasting, regardless of the thermal processing conditions. In turn, an increase the roasting temperature from 135° to 200°C led to an increment in the total content of HBA and EA derivatives. Among the roasted seeds, the highest total phenolics content was found in the samples thermally treated at 185 °C for 25 min, while the lowest amounts were observed in those roasted at 135 °C for 80 min. Ellagitannins were the dominant class of phenolic compounds in the raw samples, while EA derivatives were more abundant in heat-treated seeds. Interestingly, the second most abundant phenolic group in both unroasted and roasted samples were phenolic glycosides. The predominant individual phenolic compounds, in raw seeds, were cretanin, oenothein B and rugosin E isomers. Since, among the phenolics found in the raw seeds, cretanin had the highest thermal stability, while dimeric ellagitannins and high molecular weight gallotannins exhibited the least thermal stability. The roasted samples contained mainly compounds formed by further decomposition of abovementioned compounds. • A total of 42 phenolics were found in Quercus rubra L. seeds for the first time. • In raw oak seeds, the dominant phenolics were cretanin and dimeric ellagitannins. • Ellagitannins and gallotannins showed higher heat sensitivity than other phenolics. • Roasting effectively increased the content of low-molecular-weight phenolics. • The raw and roasted Q. rubra seeds are a valuable source of bioactive phenolics.