Identification of Bitter Off-Taste Compounds in the Stored Cold Pressed Linseed Oil

Abstract

Whereas freshly pressed linseed oil provides a delicate nutty flavor, a lingering bitter off-taste is developing upon storage at room temperature. Using a sensory guided fractionation approach, the key bitter compound was identified in stored linseed oil, and its structure was determined as the methionine sulfoxide-containing, cyclic octapeptide cyclo(PLFIM OLVF) by means of FTIR, LC-MS, NMR spectroscopy, and amino acid analysis. Although this peptide is known in the literature as cyclolinopeptide E, the bitter taste activity of this compound has not previously been described. Sensory evaluation revealed a recognition threshold concentration of 12.3 micromol/L water.

Bitter off‐taste in stored cold‐pressed linseed oil obtained from different varieties

“…The bitterness increased during storage, as has been demonstrated in [10], while by direct sensory evaluation only mean values for the whole storage period have been reported with no indication of an increase of this effect [9]. However, this development was hard to detect by direct sensory evaluation, because there is only a small interval of linearity of perceived bitterness.…”

“…After that period an increase of CLE was observed, possibly due to oxidation of co-extracted CLE', a further cyclolinopeptide with the same structure as CLE except that it has a non-oxidized methionine amino acid in the ring. An external calibration line was set up for the range from 3-900 mg/mL CLE with a coefficient of determination R 2 of 0.998 using isolated CLE from stored linseed oil obtained in a previous work [10]. The mean relative standard deviation for five consecutive HPLC determinations of one oil sample was 65.3% at a level of 1 mg/mL CLE in oil.…”

“…1). This bitter compound has been identified recently [10] as the major bitter principle in cold-pressed linseed oil. Beside this cyclic octa-peptide, several other cyclic peptides have been identified in seeds of linseed plants [12][13][14][15][16].…”

“…In the literature, the bitter off-flavor in stored linseed oil was mentioned only in a few references [9,10] and has been determined by direct sensory evaluation [9] and taste dilution analysis [10] of the bitterness. The bitterness increased during storage, as has been demonstrated in [10], while by direct sensory evaluation only mean values for the whole storage period have been reported with no indication of an increase of this effect [9].…”

“…After preparing linseed oil by pressing, the shelf life of the oil obtained is limited to only about 3-6 months in sealed bottles or containers, due to its high amount of unsaturated fatty acids and mainly depending on storage and packaging conditions [9]. While freshly pressed linseed oil provides a delicate nutty, mild roasted flavor, an unpleasant bitter off-taste starts to develop already after 1 day of storage [10].…”