Color changes and red pigment formation during sun-drying process of Umeboshi, salt-pickled Japanese apricot fruit

Salt-pickled Japanese apricot (Prunus mume, umeboshi) develops red coloration during sun-drying, although the mechanism remains unclear. This study suggests that proanthocyanidins (PAs) play a central role in this phenomenon. In a model system simulating umeboshi processing, polyphenol fractions from ume flesh (UFP) were heated with citric acid and sodium chloride, resulting in the formation of cyanidin and a broad unidentified high-molecular-weight peak (≥5 kDa). Experiments using authentic standards showed that procyanidin B2 produced cyanidin together with a broad peak, whereas procyanidin A2 yielded different reaction products including a presumed cyanidin–(epi)catechin complex without the broad peak. These findings indicate that cyanidin formation originates from B-type PAs, while under high concentrations of citric acid and salt, A-type PAs—characteristic of ume—create a unique chemical environment that promotes distinct PA-derived reactions. Such structural diversity of PA-derived pigments likely contributes to the red coloration observed in salt-pickled ume.