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 (beta 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, 7hydroxymatairesinol, 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. (Authors abstract)

Lignans are phytoestrogens present in seeds, vegetable oils, cereals, legumes, fruits and vegetables as aglycones, glycosides, esterified glycosides or as bio oligomers. Frequently occurring lignans are lariciresinol, pinoresinol, secoisolariciresinol, syringaresinol, matairesinol, 7hydroxymatairesinol, sesamin, sesamolin and sesamol.  The aim of this study was to investigate the thermal stability of water soluble lignans in flax seeds, sesame seeds and rye. Due to the lack of appropriate standard compounds for glycosides an analytical method was developed to estimate the SDG content and to quantify the amount of aglycones. The content of lignans of some raw, heat treated and commercial products as well as home baked bread were determined to obtain information on the concentration of the remaining lignans. LC–MS measurement after non-destructive extraction of defatted flax seeds with ethanol/water did not indicate free aglycones, secoisolariciresinol diglucoside (SDG) or other glycosides. However, after enzymatic hydrolysis of the extract, small concentrations of secoisolariciresinol (0.1 mg/100 g), lariciresinol (0.2 mg/100 g) and pinoresinol (0.7 mg/100 g) were found. This is due to the presence of small concentrations of glycosides of the above mentioned aglycones.  Though SDG is the main glycoside, also glycosides of lariciresinol, pinoresinol and isolariciresinol were detected in far lower concentrations.  SDG and the aglycones were stable even if the samples are heated to 250C for 3.5 min. (Editors comments)