Structural changes of sinapic acid and sinapine bisulfate during autoclaving with respect to the development of colored substances

Structural changes in sinapic acid during autoclaving were studied using spectral analysis, thin‐layer chromatography, high‐performance liquid chromatography, nuclear magnetic resonance (NMR), and mass spectroscopy. Color properties of sinapic acid and its derivatives were studied by determining the transmittance spectrum, calculating the Commission Internationale de l’Eclairage 1931 tristimulus values and converting to Hunter L a b values. It was found that the colorless sinapic acid aqueous solution (100 µg/mL) turned yellow after 15 min in an autoclave at 121°C and 0.1 MPa. Filtering the yellow aqueous solution through a 0.45‐µm filter removed a brown solid consisting of at least three undetermined colored substances and left a yellow liquid. A newly developed yellow substance, syringaldehyde, was identified in the liquid phase by comparing the NMR and mass spectra of the unknown with those of authentic syringaldehyde. Thomasidioic acid was also found in the liquid phase. Under the same autoclaving conditions, sinapine bisulfate showed no evidence of any structural or color changes.