Dry-cured ham, produced through long-term curing, fermentation and ripening, is rich in bioactive substances and has good sensory characteristics. In the traditional production process of dry-cured ham, nitrate or nitrite is often added to obtain a stable color. However, nitrite reacts with secondary amines to produce carcinogenic nitrosamines, which have attracted extensive attention. Research has found that zinc protoporphyrin Ⅸ contributes mainly to the red color of Parma ham, cured without added nitrate or nitrite. This paper summarizes the rescent progress in research on the red pigment in dry-cured ham with or without added nitrate, focusing on the formation mechanism of Zn protoporphyrin Ⅸ in nitrate-free dry-cured ham. The purpose of this review is to provide a scientific theoretical basis for the color regulation of dry-cured ham.

In order to explore the chemical nature of red pigment in Nuodeng ham, the red pigment was extracted with 75% acetone aqueous solution and purified using a C₁₈ solid phase extraction column, and its chemical structure was characterized by using ultraviolet-visible (UV-Vis) spectroscopy, fluorescence spectroscopy and ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The UV-Vis spectrum showed a strong absorption peak at 416 nm and two weakly symmetrical absorption peaks at 546 and 584 nm, which are characteristic of metalloporphyrins. When it was excited at 420 nm, the red pigment exhibited a strong fluorescence emission peak at 590 nm and a weak fluorescence emission peak at 644 nm, which highly coincided with those of Zn protoporphyrin Ⅸ (ZnPP) standard, indicating that the metal ions in the porphyrin ring were zinc ions. ZnPP was detected by UPLC-MS/MS in both positive (m/z 625.1779 [M+H]⁺) and negative ion (m/z 623.1614 [M-H]^–) modes, and the stretching vibrations of characteristic functional groups on the porphyrin ring such as –CH₃, –CH₂–, C=O, –C–OH in carboxyl groups, C–H on olefin and porphyrin ring C=N were observed in the FTIR spectrum. In addition, the ¹H-NMR results of the red pigment were consistent with the structure of the target compoundZnPP. Therefore, the red pigment in Nuodeng ham was identified as ZnPP. The results of this study provide a theoretical basis for the color regulation of Nuodeng ham.