Characterization of Yellow and Red Pigments Associated with Soybean Reddening During Processing

Discoloration of soybean-based foods, particularly reddening during processing, remains poorly understood. In this study, yellow and red pigments associated with soybean reddening were extracted, purified, and characterized. The pigments showed distinct UV–Vis spectra, with a shoulder near 420 nm for the yellow pigment and an absorption maximum near 510 nm for the red pigment. Heat and pH stability experiments showed that the yellow pigment decreased during thermal treatment, accompanied by an increase in the red pigment. Mass spectrometric analyses revealed a consistent 2 Da mass difference and a common fragment ion, indicating a close structural relationship between the pigments. Both pigments were detected in soybean, soymilk, and tofu, and the red pigment increased during processing. The pigments did not correspond to known major soybean constituents. These findings suggest that soybean reddening involves the conversion of a yellow pigment into a structurally related red pigment during soybean processing.