Table 1. Major pathways for Zinc protoporphyrin IX (ZnPP) formation in meat and meat products

Pathway type	Sample description	Processing conditions	Mechanism	Key insights	References
Enzymatic	Dry-cured Serrano hams with varied post-mortem pH values	Aging for 12 months at 3°C–25°C, 60%–85% RH	Ferrochelatase (FECH) activity enables Fe removal and Zn insertion into heme, modulated by pH and salt.	ZnPP content was higher in low post-mortem pH (≤5.4) hams; salting time affected salt but not ZnPP content.	Bou et al. (2020)
Enzymatic	Fresh meat from multiple animal sources (pork, chicken, turkey, lamb, beef, veal, horse, porcine liver)	Anaerobic incubation at 26°C for 7 d	Zinc chelatase activity promotes endogenous ZnPP formation, especially in liver and horsemeat.	ZnPP formation varied by meat type; liver and horsemeat exhibited high potential.	De Maere et al. (2017)
Enzymatic	Pork meat extracts	Anaerobic incubation at 30°C for 72 h	Fe(II)-Zn(II) substitution in myoglobin drives ZnPP formation without myoglobin degradation.	ZnPP increased during storage; intact myoglobin showed enzymatic substitution potential.	Khozroughi et al. (2017)
Enzymatic	Ultrasound-treated pork liver extracts	Ultrasound treatment (400 W, 24 kHz) followed by incubation at 37°C	Ultrasonic cavitation enhances FECH activity, promoting ZnPP production.	33% increase in ZnPP yield observed with ultrasound; prolonged exposure may degrade enzymes.	Abril et al. (2021)
Enzymatic	Dried pork liver samples	Drying at moderate (10°C–20°C) vs. extreme (−10°C, 70°C) temperatures	Moderate drying preserves enzyme activity for optimal ZnPP formation.	Extreme drying conditions reduced ZnPP yield; moderate drying enabled efficient nitrite-free curing.	Abril et al. (2022)
Non-enzymatic	Parma ham and experimental pork models	Anaerobic incubation at 35°C, pH 5.5 for 10 d	Weak heme stability in hemoglobin drives ZnPP complex formation with apo-hemoglobin.	Hemoglobin crucial for nitrite-free color development; myoglobin played a minor role.	Zhai et al. (2022)
Non-enzymatic	Parma ham	Frozen at −20°C, water extraction	ZnPP binds to hemoglobin and myoglobin, forming stable complexes.	Non-enzymatic pathways crucial for color stability in nitrite-free Parma ham.	Wang et al. (2021)
Non-enzymatic	Pork loin and liver	Anaerobic incubation at 35°C, pH 5.5	Ferriheme dissociates, reducing to ferroheme; ZnPP forms without nitrites/nitrates.	Stable red color developed naturally; alternative to synthetic colorants.	Zhai et al. (2023)
Non-enzymatic	Dry cured Parma hams	Aging at low temperature (3°C–4°C)	Enzyme-independent ZnPP formation observed under cold conditions.	Low temperature reduced enzymatic activity but allowed non-enzymatic ZnPP synthesis.	Parolari et al. (2016)
Non-enzymatic	Nitrite-free dry fermented sausages	Aging at pH>4.9	Iron-to-zinc substitution in protoporphyrin IX (PPIX) occurs non-enzymatically, influenced by pH and maturation time.	ZnPP formation correlated with product redness in nitrite-free sausages.	De Maere et al. (2016)
Bacterial	Pork muscle inoculated with Pseudomonas fluorescens	Anaerobic incubation at 30°C for 120 h	Bacterial FECH catalyzes Zn(II) insertion into PPIX, but muscle matrix limits efficacy.	ZnPP concentration in liquid media was 2.5 times higher than in meat muscle.	Khozroughi et al. (2018)
Bacterial	Parma ham inoculated with Leuconostoc strains and sausage models	Fermented at 20°C for 30 d	Bacterial FECH promotes ZnPP formation by incorporating Zn(II) into PPIX in aqueous systems.	Leuconostoc mesenteroides achieved CIE a* comparable to nitrite-cured sausages.	Wu et al. (2023)
Bacterial	High ZnPP-forming food-grade lactic acid bacteria (LAB) inoculated in pork	Incubation at 18°C for 14 d	Bacteria-induced ZnPP enhances red color in nitrite-free meat products.	Heat-stable red color achieved, indicating potential for commercial applications.	Asaduzzaman et al. (2020)
Bacterial	LAB inoculated in minced meat	Anaerobic incubation at 25°C for 7 d	LAB produces ZnPP through Zn insertion into PPIX under salt conditions.	Promising food-grade bacteria for replacing nitrite in meat products.	Kauser-Ul-Alam et al. (2021)
Bacterial	Bacterial isolates (non-food grade) from homogenate	Anaerobic incubation at 25°C for 5 d	Resident bacteria facilitate ZnPP formation, with optimal pH shifted to 5.5 in their presence.	Serratia liquefaciens showed the highest ZnPP formation; Carnobacterium divergens offered stable ZnPP production.	Wakamatsu et al. (2020)