Western countries are consuming “organically” grown foods with increasing quantity and frequency (Jensen et al. 2011). The term organic production is applied to unprocessed agricultural crop products or minimally processed foods, which have not received excessive amounts of chemicals such as synthetic pesticides and fertilizers, without genetically modified organisms (GMOs) (EU 1995; Luttikholt 2007). Organic food is a small but increasing part of the food industry in the European Union. In Portugal, a recent study from Agriculture Ministry (GPP 2011) reported an increase in organic production. The total area dedicated to organic production was 210,981 ha in 2010 against 120,729 ha in 1994; in relation to vegetable sector, in 2010, the area occupied by organic mode was 737 ha against no more than 163 ha in 1994 (GPP 2011).
Organic production, as an alternative to conventional agricultural, relies on the incorporation of organic material in soil, normally by the use of animal manure as fertilizer (EU 1995). Animal manure is a good source of macronutrients and micronutrients, particularly nitrogen. The presence of nitrates is one of the consequences of the mechanism in which plants absorb the nitrogen element, in the form of NO3−, from fertilizers or organic materials (Gangolli et al. 1994), which are essential for the process of protein synthesis. Nitrates and nitrites are natural constituents of plant material, and they are normally present in high levels, particularly in green vegetables (Correia et al. 2010). Also, despite nitrate being an important component of plant material, it has the potential to accumulate in tissues, and thus, nitrate from fertilizers could accumulate in vegetables in large scale. Therefore, keeping nitrate concentrations below legal limits is a constant struggle for producers and farmers (Katan 2009). Nitrate is nontoxic below maximum residue levels (MRLs), but if it reaches above this level, it could be dangerous due to its reduction in nitrites, which can react with amines and amides to produce “N-nitroso” compounds responsible for gastric cancer (Santamaria 2006; Savino et al. 2006). High levels of nitrates in children stomach are responsible for methemoglobinemia (blue baby syndrome) (Greer and Shannon 2005; Chan 2011). Several factors influence the accumulation of nitrates in plants, including lack of sunlight or water, variety, maturity, high levels of fertilizers, nitrate levels in the soil, and quality of irrigation water. Excessive use of nitrogen fertilizers should be avoided so as to reduce nitrate buildup in soil and vegetables (Santamaria 2006). In order to maximize the health benefits from eating vegetables, measures should be taken to reduce the nitrate and nitrite exposures (Correia et al. 2010). Vegetables must be stored and processed properly to prevent bacterial contamination and hence reduction of nitrate to nitrite (Leszczyńska et al. 2009).
The preference for organic products is increasing all over Europe, due to the absence of chemical contaminants within this mode of production (EC 1991; EU 1995). In principle, organic products should contain fewer nitrates than their counterparts from conventional methods (Woese et al. 1997; González et al. 2010); however, some authors (De Martin and Restani 2003; Guadagnin et al. 2005) showed that content of nitrate in vegetables could be independent from the agricultural production system, and often organic vegetables could present very high nitrate average levels. Also many studies have demonstrated that organically grown crops have similar levels of nitrates and nitrites to their conventionally counterparts, and therefore doubts still persist. Pussemier et al. (2006) and Rembialkowska (2007) have published studies in which these contrasts are discussed. Therefore, despite of this growing interest in organic production, there is insufficient information to state categorically that the risk of nitrate or nitrite accumulation in organic production does not differ significantly from the risk associated with conventional practices. The results from different studies are inconsistent and doubts still persist. Thus, studies that can evaluate the growing conditions on the levels of nutrients and toxicants are urgently required. Therefore, the main objectives of this study were to assess the information on the nitrate and nitrite average levels on six different fresh baby-leaf salads produced and largely marketed in Northern Portugal, and to assess and determine the effect of production system (conventional and organic) on their accumulation in nitrates and nitrites and to estimate the toxicological risk associated with the consumption of baby-leaf salads containing nitrate and nitrite. The main aim of this study is to verify whether there is significant difference in nitrate or nitrite loads between organically and conventionally cultivated salads and whether such influence may increase the risk of disease occurrence for consumers.