Studies on Citrus Carotenoids

Part 6. On the redder carotenoid group contained in peel of Citrus Unshiu var. Benidobashi

Abstract

The unknown carotenoid group existing in amount of 20% or more and contributing to the bright red color of the Unshiu var. Benidobashi peel have been examined. The unknown craotenoid group occurred between the monol monoepoxid and diol on the TLC, was seperated into three carotenoids of UL, U2 and U3 in order of non-polarity. All threse carotenoids showed orange to pink color on the TLC and U1, U2 and U3 were present in the ratio of 60.0%, 28.8% and 11.2% respectively.
Every unkown carotenoids had one of hydroxy group in β-ionone ring and one of aldehyde group in chromophoric double bonds system. The properies of these unknown carotenoids were compared with unusual citrus carotenoids and ketonic carotenoids such as semi-β-carotenone, capsanthin, capsorubin, reticulataxanthin and citraurin. Investigations of spectral absorption maximum, shape of spectral absorption curve, NaBH₄ reduction on aldehyde group and changes of polarity with methylation resulted in that U1 was considered as citraurin or citraurin-like carotenoid, U2 was identical to the reticulataxanthin and U3 was the cis-from of U1.