## 4 Discussion
This indicates the protective role of nitrate as an antioxidant component in meat products. Various mechanisms of the antioxidant effect of nitrate are described in the literature, including binding of heme and the prevention of the release of the catalytic no-heme iron, the ability to bind heme and non-heme iron and inhibit catalysis, and the stabilization of lipids against oxidation [5]. However, at the end of the storage period, meat products containing various amounts of nitrate presented the same amount of TBARS similar to the findings by [PERSON] and [PERSON] [36]. [PERSON] et al. [37] noted that the addition of 40 and 120 mg kg\({}^{-1}\) of sodium(III) nitrite to frankfurters contributed to the effectively lowered amount of malondialdehydedehyde during eight weeks of chilled storage. Our previous study, in which we assessed the effect of a reduced nitrate addition in the production of heat-treated meat products [19], showed that the reduction of the nitrate addition by 50 mg kg\({}^{-1}\) in relation to the traditionally used level did not significantly change the amount of secondary lipid oxidation products measured with the TBARS. Apart from lipids, myoglobin can also undergo various processes resulting in changes in the quality characteristics of fermented meat products, including mainly the color and nutritional value in the context of heme iron content. In the current study, the reduction of nitrate did not lead to significant changes in total pigment content as well as heme iron content in fermented loins during six months of chilled storage. However, considering the storage time, at the end of production, all the samples were characterized by a significantly higher total pigment and iron content compared to the subsequent research periods. After one month, a significant reduction in the value of these components was observed. [PERSON] et al. [38] showed that the breakdown of heme pigments during storage might have led to the lower lightness value. This phenomenon has not been confirmed in the present study. The color parameters L* and a* observed in the present work in the fermented loins depended on the level of sodium nitrate addition, especially at the end of production. The reduction of nitrate decreased the value of the L* color parameter and redness (a* color parameter) which is associated with the nitrosation of the myoglobin to nitrosomyoglobin. This form of myoglobin results from the interaction between muscle-based myoglobin and nitric oxide, which is derived from nitrate [2]. During fermentation, nitric oxide is formed in numerous chemical reactions among which reaction with reductants (present in the meat or added) are of practical importance. Numerous nitrosation and nitrosylation reactions take place in the meat matrix, among which the formation of nitrosomyoglobin is of major importance [39]. In the present study, fermented loins without the addition of nitrate were characterized by the lowest a* color parameter. However, the redness values obtained in the study were significantly higher compared to those obtained with the fermented loin without the addition of nitrates [34]. In contrast, the results presented by [PERSON] et al. [19] showed no effect from the inclusion of different amounts of sodium nitrite on the lightness (L*) and redness (a*) values of cooked meat products. Interestingly, in Parma ham, which does not contain added nitrates, a stable red color compound is formed, in which iron is substituted by zinc in the myoglobin molecule. In the study by [PERSON] et al. [40], nitrite-free dried hams developed a stable color through the endogenous synthesis of red Zn-protoporphyrin IX, according to a natural, enzyme-dependent process conducted at a warm maturation temperature. [PERSON] et al. [41] documented the ability of the muscle ferrochelatase to catalyze the iron displacement from the heme moiety, a key step prior to the eventual insertion of zinc, leading to Zn protoporphyrin IX.