Improvement of an Analytical Method for Six Types of Vitamin K, Including Long-Chain Menaquinones, via Column-Switching High-Performance Liquid Chromatography and Analysis of the Vitamin K Contents of Various Cheeses and Fermented Soybean Products

In this study, a column-switching high-performance liquid chromatography with fluorescence method to facilitate the simultaneous analysis of both the primary vitamin K types, including phylloquinone (PK), menaquinone-4 (MK-4), and MK-7, as well as long-chain menaquinones such as MK-6, MK-8, and MK-9 was enhanced. The six types of vitamin K were detected with high-sensitivity within 50-min runtime. Moreover, precision and specificity of the developed method were evaluated. Relative standard deviation of retention time and peak area was less than 0.3% and 3.00%, respectively; the limits of detection and quantification for different vitamin K types were 0.17–0.60 and 0.42–1.71 ng/mL, respectively. Various cheeses and fermented soybean products were analyzed using the improved method. In cheeses, almost all samples contained PK, MK-4, MK-8, and MK-9. In particular, MK-8 and MK-9 were dominant and highly intercorrelated ( r = 0.97); the level of MK-9 was approximately 2.5 times higher than that of MK-8. In fermented soybean products, natto, douche, and doenjang contained high total vitamin K contents: 406.71 ± 6.83, 100.30 ± 0.80, and 307.95 ± 7.11 µg PKeq/100 g, respectively. In particular, these samples had a high content of MK-7, as well as high levels of MK-8. The present study provides the first comprehensive comparison of long-chain vitamin K composition across diverse Asian fermented soybean products, as well as Japanese natto.