## 1 Introduction  
The use of nitrites and nitrates is a part of the meat processing tradition associated with their multidirectional impact on the quality of meat products including microbiological and chemical safety. The main purpose of using nitrites or nitrates is related to their preservative effect by inhibiting the growth of bacteria (especially _Clostridium botulinum_ and spore germination). They also delay the progress of oxidative reactions in the main components of meat (proteins and lipids) and participates in the formation of the characteristic color (called the \"cured meat\" color) and taste of meat products [1]. When added to meat, the nitrates must first be reduced to nitrite in order to become active curing agents. The reduction of nitrates to nitrites occurs with the participation of bacteria possessing nitrate reductase activity [2]. The most known and most effective microorganisms in reducing nitrates are staphylococci and microococci; however, lactic acid bacteria can also reduce nitrates to nitrites influencing the course of the fermentation process in meat products. Moreover, according to [PERSON] [2], nitrate reductase activity is normally present in animal physiology and the presence of microorganisms in this process is not essential. At the optimum pH of meat (5.6 to 5.8), nitrites decrease by conversion to the compounds that can act as oxidizing, reducing or nitrosylating agents (nitrous acid, nitric oxide and nitrates) [3]. The production of nitric oxide from nitrite is required for the reaction with myoglobin, the protein responsible for the color of the meat. The formation of nitrosyl myoglobin generates the cured meat product color. The effect of nitrates on aroma is also well-known. It is related to the content of aldehyde compounds (producing green odor notes) formed during lipid oxidation reactions and key sulfur odor compounds responsible for meaty odors [4]. The literature also describes the mechanisms of the antioxidant properties of nitrites [5]. Despite the beneficial properties of nitrites and nitrates in meat products, the processing of meat using these additives is closely related to the harmful effects of high consumption of processed meat, as reported by the International Agency for Research on Cancer [6]. Their involvement in the formation of nitroso-compounds such as carcinogenic N-nitroosamines is widely known [7]. N-nitrosamines can form through nitrosation reactions. The nitric oxide, formed by the reduction of nitrites, can react with secondary amines to form nitrosamines. Several conditions favor the formation of these compounds, such as acid pH with the amine protonated, a large amount of residual nitrite and a longer storage time [8]. The negative health effects of nitrates and nitrites are commonly described in the literature [9; 10; 11]. On the other hand, in recent years, various benefits of the supply of nitrates with the diet have been indicated, in particular on the function of the cardiovascular system [12; 13]. These benefits are related to the supply of nitric oxide, which have a positive effect on the cardiovascular system. The role of nitrites/nitrates in this context is multidirectional; their protective effect in lowering blood pressure and inflammation, improving exercise capacity and mitochondrial function, lowering triglycerides and reducing heart attacks and strokes is indicated. Although the scientific evidence shows the positive effects of nitrates in our body, there is still huge pressure from consumers to choose nitrate-free meat products. On the other hand, despite the increased risk of cancer associated with eating large amounts of processed meat [6], the consumption and production of meat products worldwide is increasing due to the globalization [14]. According to Commission Regulation (EU) No. 1129/2011 [15], nitrates (designated as E251 and E 252, respectively) and nitrites (E249, E250) are included in the list of additives approved for use in meat processing.