Effect of thermal heating on some lignans in flax seeds, sesame seeds and rye

Consumption of lignan rich food is presumed to have positive effects on human health. As numerous foods are consumed mainly in processed form it is important to investigate the changes of the lignan content during processing. To this end, unheated and heated sesame seeds, sesame products, rye grains, rye flour, rye bread and flax seeds were extracted by sonication with ethanol/water (70:30, v:v) or sodium methoxide. The extracts were additionally hydrolysed enzymatically (β-glucuronidase/arylsulphatase, cellulase), the compounds separated on a reversed phase column by gradient elution and detected by UV/ESI-MS in the negative ionisation multiple reaction monitoring mode (MRM). Secoisolariciresinol, lariciresinol, pinoresinol, 7-hydroxymatairesinol, syringaresinol, isolariciresinol, secoisolariciresinol diglycoside, lariciresinol monoglycoside, pinoresinol mono-, di- and triglycoside, sesaminol, sesaminol triglycoside, sesamolinol and sesamolinol diglycoside were identified. Moderate heating at 100°C did not degrade the lignan aglycones and glycosides in dry foods. In contrast, heating was responsible for the better extractability of the lignans. If samples with high moisture content were heated, the degradation of the lignans in sesame seeds and rye was observed already at 100°C. Higher roasting temperatures caused degradation of aglycones and glycosides. Especially at 250°C, lignans were degraded rapidly in sesame seeds and rye but not in flax seeds.